Opioids

in the

Hippocampus

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES • Public Health Service • Alcohol, Drug Abuse, and Mental Health Administration
Opioids in the Hippocampus

Editors:

Jacqueline F. McGinty, Ph.D.

East Carolina University School of Medicine

Greenville, NC

David P. Friedman, Ph.D.

Division of Preclinical Research

National Institute on Drug Abuse

NIDA Research Monograph 82

1988

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES

Public Health Service

Alcohol, Drug Abuse, and Mental Health Administration

National Institute on Drug Abuse

5600 Fishers Lane

Rockville, MD 20857
NIDA Research Monographs are prepared by the research divisions of the

National Institute on Drug Abuse and published by its Office of Science. The

primary objective of the series is to provide critical reviews of research problem

areas and techniques, the content of state-of-the-art conferences, and

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dissemination to the scientific and professional community.

Editorial Advisors

MARTIN W. ADLER, Ph.D.

Temple Universlty School of Medicine

Philadelphia, Pennsylvania

SYDNEY ARCHER, Ph.D.

Rensselaer Polytechnic lnstitute

Troy, New York

RICHARD E. BELLEVILLE, Ph.D.

NB Associates, Health Sciences

RockviIle, Maryland

KARST J. BESTEMAN

Alcohol and Drug Problems Association

of North America

Washington, D. C.

GILBERT J. BOTVIN, Ph.D.

Cornell University Medical College

New York, New York

JOSEPH V BRADY, Ph.D

The Johns Hopkins University School of

Medicine

Baltimore, Maryland

THEODORE J. CICERO, Ph.D.

Washington University School of

Medicine

St Louis, Missouri

MARY L. JACOBSON

National Federation of Parents for

Drug Free Youth

Omaha, Nebraska

REESE T. JONES, M.D.

Langley Porter Neuropsychiatric lnstitute

San Francisco, California

DENISE KANDEL, Ph.D

College of Physicians and Surgeons of

Columbia Universlty

New York, New York

HERBERT KLEBER, M.D.

Yale University School of Medicine

New Haven, Connecticut

RICHARD RUSSO

New Jersey State Department of Health

Trenton, New Jersey

NIDA Research Monograph Series

CHARLES R. SCHUSTER, Ph.D.

Director, NIDA

THEODORE M. PINKERT, M.D., J.D.

Acting Associate Director for Science, NIDA

Parklawn Building, 5600 Fishers Lane, Rockville, Maryland 20857
Opioids in the Hippocampus
ACKNOWLEDGMENT

This monograph is based upon papers and discussion from a workshop

review on opioid peptides in the hippocampus which took place on

November 10, 1986, at the Society for Neuroscience meeting, in

Washington, DC. The workshop was sponsored by the Office of

Science and the Division of Preclinical Research, National Institute on

Drug Abuse.

COPYRIGHT STATUS

The National Institute on Drug Abuse has obtained permission from

the copyright holders to reproduce certain previously published

figures and tables, as noted in the text. Some of these have been

modified for use in this monograph. Further reproduction of the

copyrighted material is permitted only as part of a reprinting of the

entire publication or chapter. For any other use, the copyright

holder’s permission is required. All other material in this volume

except quoted passages from copyrighted sources is in the public

domain and may be used or reproduced without permission from the

Institute or the authors. Citation of the source is appreciated.

DHHS publication number (ADM)88-1568

Alcohol, Drug Abuse, and Mental Health Administration

Printed 1988

NIDA Research Monographs are indexed in the Index Medicus. They

are selectively included in the coverage of American Statistics Index,

Biosciences Information Service, Chemical Abstracts, Current

Contents, Psychological Abstracts, and Psychopharmacolgy Abstracts.

iv
Preface

The discovery in the middle 1970s of opiate receptors and endogenous opioid peptides was clearly one of the landmark events in the

history of neuroscience. Studies designed to localize and characterize both the receptors and the peptides and to reveal their

functional significance have grown at nearly an exponential rate

since that time. Not surprisingly, a good deal of that effort has

focused on the hippocampal formation. Its relatively simple, laminated, and well-described structure have made it a key model for

studying many aspects of neural processing in the forebrain. Because different functional components exist almost exclusively in

separate structural compartments, the hippocampus presents an excellent opportunity to correlate structure with function. As a result, it was not long after the initial discoveries of brain opioid

systems that effects of oploids on hippocampal electrophysiology

were reported.

The importance of the hippocampus far exceeds its use as a model,

however, because the hippocampus acts as one important gateway

by which sensory information that has been processed by the

neocortex gains access to the limbic system. The hippocampus

therefore plays a key role in memory, even showing the ability to

control its own input and responses based on previous experience.

In addition, the hippocampus appears to be a highly excitable structure with an unusual vulnerability to seizures. For a variety of

reasons, then, the hippocampus has drawn the attention of

neuroscientists.

Those who wished to understand the role of opioids and their receptors in neural processing have been among the group studying

the hippocampus, and this monograph summarizes their first decade

of work. As the reports in this volume show, this decade has indeed been a profitable one. The neural elements containing opioids

and the particular forms of the peptides they contain are now

v
known. The distributions of particular subclasses of opioid receptors have been described on a regional and laminar basis, and the

anatomical relationships of these receptors to the peptides that

presumably bind to them are also known. At the cellular level,

there is a good understanding of how opioids may control the excitability of some kinds of hippocampal neurons, and the receptors

that mediate these effects have in some cases also been described.

Tools have been developed that now allow us to examine within

specific hippocampal pathways the metabolic control of both opioid

peptide content and the genetic message leading to the synthesis of

the propeptides from which hippocampal enkephalins and dynorphins

are derived. In addition, the relations of opioid peptides to other

transmitters and neurally active substances that may be contained

in those pathways are also being described. Even some of the possible behavioral effects of opioids acting in the hippocampus have

been carefully scrutinized.

This progress, while impressive, serves to highlight what we still

need to learn. Most importantly, the physiological conditions that

lead to the release of opioid peptides from axonal terminations and

the effects elicited under those conditions remain to be revealed.

These questions must be considered the central mysteries about opioids in the hippocampus. In addition, many more specific questions

still need to be answered. For example, although we know the regional and laminar distribution of opioid peptides and their recepton, these do not always coincide. Thus, there are places with

peptide but no receptors and vice versa. The meaning of this mismatch is still unclear. Moreover, we have not directly demonstrated the neural elements on which the receptors reside.

Most of the studies of cellular electrophysiology have concentrated

on the pyramidal cells of the CA1 region of the hippocampus. We

need to extend our understanding to other cell types and hippocampal regions. CA3 and the dentate gyrus are both coming under

scrutiny, but further efforts are required. The dentate gyrus is

especially crucial because it is the major recipient of the neocortical input that is relayed through the entorhinal cortex via the

perforant path, and because it is the origin of one of the major

opioid-containing pathways in the hippocampus, the mossy fiber

system.

Our models of metabolic control are still new, and future needs require that they become even more sophisticated and sensitive so

vi
that they may reveal the control mechanisms operating under

physiological conditions. Finally, the role of the hippocampus in

the myriad of opioid- and opiate-modulated behavioral effects requires much study, because there is still so much we do not know.

The first 10 years of study of opioids in the hippocampus have produced a solid base of knowledge and any number of interesting

leads for the future. For the most part, however, the “simple”

questions have been answered. Many of the gaps in our knowledge,

therefore, represent opportunities for the study of difficult problems. The chapters that follow do an admirable job in explaining

for us the current state of our knowledge and where future challenges lie.

David P. Friedman, Ph.D.

Division of Preclinical Research

vii

Contents

Page

Preface. . . . . . . . . . . . . . . . . . . . . . . . . v

What We Know and Still Need To Learn About Opioids in

the Hippocampus. . . . . . . . . . . . . . . . . . . . . 1

Jacqueline F. McGinty

Localization and Seizure-Induced Alterations of Opioid Peptides

and CCK in the Hippocampus. . . . . . . . . . . . . . . .12

Christine M. Gall

The Anatomical Relationship of Opioid Peptides and Opiate

Receptors in the Hippocampi of Four Rodent Species . . . . . .33

Miles Herkenham and Stafford McLean

Seizure-Induced Alterations in the Metabolism of Hippocampal

Opioid Peptides Suggest Opioid Modulation of Seizure-Related

Behaviors. . . . . . . . . . . . . . . . . . . . . . . .48

Jau-Shyong Hong, Jacqueline F. McGinty, L. Grimes,

T. Kanamatsu, J. Obie, and C.L. Mitchell

Opioids in the Hippocampus: Progress Obtained From In Vivo

Electrophysiological Analyses . . . . . . . . . . . . . . . . 67

Steven J. Henriksen, James B. Wiesner, and Guy Chouvet

Opioid Receptor Mechanisms in the Rat Hippocampus . . . . . . 94

Charles Chavkin, John F. Neumaier, and

Elizabeth Swearengen

ix
Page

Behavioral Significance of Opioid Peptides in Relation to

Hippocampal Function . . . . . . . . . . . . . . . . . . 118

Michela Gallagher

Effects of Mu Opioid Receptor Activation in Rat Hippocampal

Slice. . . . . . . . . . . . . . . . . . . . . . . . . 133

Menahem Segal

List of NIDA Research Monographs . . . . . . . . . . . . . 146

x
What We Know and Still Need To

Learn About Opioids in the

Hippocampus

Jacqueline F. McGinty

The study of opioid peptides and their receptors in the hippocampus

integrates two major areas of fascination for neuroscientists. The

current level of interest in hippocampal organization and functioning is demonstrated by the presentation of greater than 250 abstracts on the subject at the 1986 Society for Neuroscience

meeting. This number is comparable to the number of abstracts at

this meeting which focused on opiates and opioid peptides. A

significant number of those abstracts represent the most current

work of the participants in the “Opioids in the Hippocampus”

workshop presented at that meeting. This volume summarizes those

presentations, which include data on the anatomical, physiological,

pharmacological, and behavioral aspects of the proenkephalin and

prodynorphin systems in the hippocampal formation (HF, a term

which includes the hippocampus proper and the dentate gyrus; see

below). The authors have integrated the information accumulated

on opioid peptides during the last dozen years with the body of

knowledge accumulated on the neurobiology of the hippocampal

formation during the last century. Together the data point to the

possibility that opioid peptides are intimately involved in the

molecular mechanisms underlying learning and memory and seizure

susceptibility in the hippocampus.

In order to grasp the significance of the distribution of opioid

peptides and receptors in the HF, it is necessary to have a basic

appreciation of hippocampal structure and circuitry. The HF is a

curved structure which must be sectioned in the frontal plane rostrally and in the horizontal plane caudally in order to retain the

characteristic “laminar” relationship between the major cellular

1
layers throughout the structure. Figure 1 illustrates these relationships in a frontal section through the dorsal HF. The basic route

of information flow through the HF, which is most relevant to the

location of opioid-containing neurons, is via a trisynaptic excitatory

chain. This chain originates in the entorhinal cortex, which projects highly processed, polymodal sensory information from the entire cerebral cortex through the perforant pathway to the granule

cells in the dentate gyrus of the HF.

FIGURE 1. A schematic illustration of a coronal section through

the dorsal HF of the rat

NOTE: The trisynaptlc excitatory chain consists of the perforant path arising in

the entorhinal cortex, the granule cell-mossy fiber pathway innervating

the CA4-3 cell fieid, and the Schaffer collateral pathway innervating the

CA1 pyramidal cells.

SOURCE: Modified from Henriksen et al. 1982, Copyright 1982, The New York

Academy of Sciences.

The granule cells are the second link in the chain: they project

their axons, the mossy fibers, to the proximal apical dendrites of

the pyramidal cells in the stratum lucidum of the CA3 (inferior)

2
region of the HF. The CA3 pyramids (the third link) not only project their axons out of the HF but also send a branch (the

Schaffer collateral) to the pyramidal cell dendrites in the stratum

radiatum of the CA1 (superior) region of the HF. In turn, the CA1

pyramids project axons out of the HF primarily through the fornix.

Inhibition mediated by gamma-aminobutyric acid (GABA)- and/or

peptide-containing interneurons (basket cells) whose axons surround

the granule and pyramidal cells is a major control over HF excitability. Without this tonic inhibition, seizures would be free to

march through the uninhibited hippocampus via the trisynaptic

excitatory chain.

In studying the electrophysiological responsiveness of the HF, one

must not assume that the entire structure is homogeneous from the

dorsal through the ventral pole. Recently, it has become clear, for

instance, that the perforant pathway from the entorhinal cortex

into the HF is neurochemically as well as topographically heterogeneous, and that the threshold for excitability in the ventral pole

is lower than that in the dorsal pole.

In the early 1980s, proenkephalin- and prodynorphin- (but not proopiomelanocortin-) containing neurons were found in the HF of rats

by immunocytochemistry (Gall et al. 1981; Gall, this volume;

McGinty et al. 1933; McGinty et al. 1994). Three different systems

in the rat HF contain enkephalin immunoreactivity: two of the

three major excitatory pathways, the perforant and mossy fiber

systems (figure 2), and interneurons scattered throughout all layers

of the HF (Gall et al. 1981). Subsequent to Gall’s observations, a

large concentration of prodynorphin peptides was discovered

exclusively in the mossy fiber path which, under normal conditions,

contains less proenkephalin than prodynorphin (Weber et al. 1982;

McGinty et al. 1983; Gall, this volume: Herkenham and McLean, this

volume). More recently, Gall (this volume) has observed some

species differences in the enkephalin distribution in the HF, particularly a lack of enkephalin immunoreactivity in the perforant

pathway in primates.

The realization that both proenkephalin and prodynorphin are contained in the mossy fiber path raised a number of interesting questions about opioid peptide interactions in the hippocampus. For

example, are prodynorphin and proenkephalin both expressed in the

same dentate granule cells? We explored this question with double

antibody immunocytochemistry at the light microscopic level in

3
FIGURE 2. A schematic illustratio depicting the three major

neuronal types in the trisynaptic chain, the dentate

granule cells, and the CA3 and CA1 pyramidal cells

NOTE: Proenkephalin- containing fibers in the perforant pathyway innervate the

proenkephalin- and prodynorphin-containing dentate granule cells. The

granule cells givee rise to mossy fibers.

SOURCE: Modified from Altman et al. 1973, Copyright 1973, Academic Press.

colchicine-treated rats (McGinty 1985). Colchicine injected directly

into the HF causes granule cell death, but when injected into the

lateral ventricle, it causes axoplasmic transport blockade and can

be used to reveal immunoreactive cell bodies throughout the central

nervous system. In preparations where granule cells were visible,

we were unable to find any granule cells which stained with antisera against products of both prodynorphin (dynorphin B) and

proenkephalin (BAM22P). More recent in situ hybridization histochemical data have demonstrated that most if not all dentate

4
granule cells contain prodynorphin messenger RNA (mRNA) (Morris

et al. 1988; Young, unpublished observations), but relatively few

granule cells in a resting or nonstimulated state have been observed

to contain proenkephalin mRNA (Young, unpublished observations).

These results do not rule out the possibility that granule cells are

capable of expressing both prodynorphin and proenkephalin peptides

under certain physiological conditions. In fact, as pointed out by

Gall (this volume) and Hong et al. (this volume), mossy fibers may

be capable of synthesizing and releasing different combinations of

neuroactive substances in response to changes in physiological

activity.

Another question pertaining to the cellular localization of opioid

peptides is whether or not enkephalins or dynorphins in granule

cells and mossy fibers are colocalized with and released with other

synaptically related molecules such as zinc, excitatory amino acids,

or, in some species, cholecystokinin (Gall, this volume). Chavkin et

al. (1983) have demonstrated that opioid peptides are released from

hippocampal slices in a Ca+-dependent manner by K

+

 depolarization

or by the excitatory amino acid analogue kainic acid. Kainic acid

also releases opioid peptides (Hong et al., this volume) and zinc

(McGinty et al., in press) from mossy fibers in vivo. If other such

substances are released along with opioids when mossy fibers are

activated, investigations of their physiological effects should be

pursued by testing their interactions on pyramidal and granule cell

excitability in addition to examining the effects of each agent

alone. Chavkin et al. (this volume) and Segal (this volume)

introduce their most recent experiments in which the actions of

enkephalins in the hippocampus are blocked with an excitatory

amino acid antagonist (see below).

Two electrophysiological observations in rats initially attracted

attention to the nature of opiate actions in the hippocampus

(reviewed by Henriksen et al. (this volume) and Chavkin et al. (this

volume)). First, intracerebroventricular (ICV) infusions of betaendorphin or met5

-enkephalin into unanesthetized rats were found

to induce electroencephalographic seizure activity in the hippocampus and amygdala in the absence of frank motor convulsions

(Urca et al. 1977; Henriksen et al. 1978). These “limbic seizures”

were accompanied by a frozen posture interrupted by “wet dog

shakes,” a behavior which also occurs during opiate withdrawal in

rodents. According to Hong et al. (this volume), the occurrence of

wet dog shakes appears to depend on the actions of enkephalin on

5
or in the granule cell-mossy fiber pathway. Second, Nicoll and

colleagues (1977) demonstrated a unique excitatory effect of

electrophoresed enkephalin on individual hippocampal pyramidal

cells. Only inhibitory actions had been recorded in other regions

of the mammalian brain (Nicoll et al. 1977). Intracellular recordings (Zieglgänsberger et al. 1979), however, were unable to demonstrate the changes in pyramidal cell membrane polarization which

should have occurred if opiate receptors on pyramidal cells were

mediating the excitation. Zieglgänsberger and colleagues reconciled

these two apparently discrepant findings by hypothesizing that

inhibitory GABA-containing neurons intervened between inhibitory

enkephalin-containing neurons and hippocampal pyramidal cells. In

this scenario, when enkephalin is released, it would cause inhibition

of GABA release and a resulting disinhibition of pyramidal cells.

Recently it has been learned that enkephalin analogues do decrease

the firing rates (Lee et al. 1980) of and hyperpolarize the membranes of CA1 intemeurons by increasing potassium conductance

(Nicoll and Madison 1984). The study of the effects of opioid peptides in the dentate gyrus has a much shorter history; progress in

this area is reported by Henriksen et al. (this volume) and Chavkin

et al. (this volume).

To date, the enkephalin-GABA-pyramidal cell circuitry proposed by

electrophysiologists in CA1 has neither been demonstrated nor

refuted by morphological data. A demonstration of monosynaptic

connections between enkephalin- and GABA-containing cells would

require immunostaining for GABA (or glutamic acid decarboxylase,

which synthesizes GABA from glutamate) and enkephalin simultaneously at the electron microscopic level. There is not yet, however,

even a published account of enkephalin immunoreactivity alone in

the hippocampus at the ultrastructural level. In other sytems

(teleost and avian retina), it has been shown that exogenously

applied enkephalins inhibit the K

+

-evoked release of GABA from

amacrine cells and that enkephalin is localized in some amacrine

cells which take up [3H]GABA (Chin and Lam 1982).

Until the localization of enkephalin and GABA at the ultrastructural

level is performed together successfully, we will not know what

class of hippocampal neurons have oploid receptors on their plasma

membranes, and, as a result, we will not know by what mechanism

enkephalin excites pyramidal cells. Although receptor autoradiography has provided us with detailed maps of the distributions

of the mu, delta, and kappa opioid receptor subtypes within the

8
different fields and layers of the HF (Herkenham and McLean, this

volume), the resolution of these receptor maps does not yet allow

us to discern on what cell types the receptors are located. We

still do not know if GABA-containing cells and/or pyramidal cells

possess opioid receptors. What we do know is that the pyramidal

cell layer, which is only sparsely innervated by opioidergic terminals in many species, contains the densest concentration of opioid

receptor subtypes: whereas the mossy fiber layer, which contains

the densest concentration of opioidergic terminals, has few, if any,

opiate receptors. In other species, the distribution of opioidpeptide-containing terminals remains constant, but the distribution

of opioid receptors is different. Herkenham and McLean (this

volume) discuss the implications of this receptor-peptide

“mismatching” in several species and provide some indication of

which receptor subtypes may be in a position to mediate opioid

peptide actions in different hippocampal regions.

The actions of hippocampal dynorphins on opioid receptor sub

populations are also the subject of vigorous investigation. In

contrast to the seizurogenic effects of met5-enkephalin and betaendorphin after ICV infusion, and their excitatory effects on single

pyramidal cells, dynorphins not only do not produce seizures

(Henriksen et al. 1982), but they also appear to have naloxons

reversible excitatory as well as non-naloxone-reversible inhibitory

actions at the single-cell level (Henriksen et al. 1982; Gruol et al.

1983; Moises and Walker 1985). Dynorphin effects, however, are

mixed or are not very potent on pyramidal cells in the CA3 region

of the hippocampus, which contains the most dynorphin-immunostained terminals. The paper by Chavkin et al. (this volume) explores the current status of this paradox by addressing what oploid

receptor subtype(s) are activated by dynorphins in various regions

of the hippocampus. Chavkin and colleagues replace the earlier

hope that each class of opioid peptide would act via one class of

opioid receptor with evidence that each peptide probably acts to

different degrees on whatever receptor subtype(s) are present.

The presence of opioid peptides in excitatory pathways of the HF

has led to explorations of the alterations in the metabolism of

endogenous opioid peptides in response to hippocampal hyperexcitation. Hong et al. (1979) first demonstrated that kainic acid

administration caused a seizure-linked depletion followed by a

robust increase in met5-enkephalin levels in the hippocampus.

Since then, Hong’s group, in collaboration with my own, has shown

7
that recovery of mossy fiber and perforant path proenkephalin and

prodynorphin peptide and mRNA levels in response to seizures

elicited by kainic acid, electroconvulsive shock, or kindling occur

with very different time courses. These data complement the

findings of Gall (this volume), who has shown that a lesion of the

dentate hilus in mice also causes seizures that release opioids and

cholecystokinin from the mossy fibers. In all of these seizure

models, each peptide has a different rate of return to normal levels

in the postictal period. Taken together, these studies indicate

differential regulation of opioid peptide gene expression in hippocampal pathways and differential control of release of opioid

peptides from the axons comprising these pathways. They have

provided us with models in which to study the physiological consequences of short- and long-term alterations in peptide content and

metabolism in the HF and hold promise for uncovering the role(s)

of opioid peptides in hippocampal excitability.

Finally, in what way can the actions of hippocampal opioid peptides

be associated with the role of the hippocampus in overall brain

function? Chavkin et al. (this volume) and Segal (this volume)

report their most recent data on the mediation of a specific component of opioid excitation of pyramidal cells in CA1 by receptors

for the excitatory amino acid N-methyl-D-aspartate (NMDA).

Activation of NMDA receptors occurs in long-term potentiation, the

putative substrate for hippocampal learning and memory processing.

Indeed, Segal (this volume) describes exciting new data showing

that mu opioid receptor activation enables the expression of longterm excitatory postsynapatic potentials evoked by associating

afferent stimulation with depolarization of CA1 pyramidal cells, an

illustration of a so-called Hebbian synapse. Thus, by enhancing the

activity of excitatory neurotransmitters, possibly by decreasing

GABA-mediated inhibitory tone at specific synapses, opioid peptides

may be essential to the very fabric of hippocampal synaptic plasticity in which memory is stored through changes in the efficacy of

transmission among elements.

Based solely on the fact that opioid peptides are generously rep

resented in important hippocampal circuits, one would anticipate

that they would play a significant role in hippocampal-associated

behaviors. Gallagher (this volume) reviews the evidence that exogenous opioid agonists diminish, and opioid antagonists improve,

behavioral performance on memory tasks known to be dependent on

integrity of the HF. However, it is not clear from these data that

8
opioid peptidergic circuits in the hippocampus are involved in these

behaviors or that the tasks described lead to the release of endogenous opioids. A stronger indication that endogenous hlppocampal

opioids are involved in spatial learning is provided by the data of

Collier and Routtenberg (1984), showing that naloxone can block

the forgetting of a learned spatial task, the disruption of which

was induced by Stimulation of the dentate gyrus, presumably leading

to the release of opioid peptides. Gallagher also presents a line of

reasoning based on data from her laboratory indicating that cholinergic afferents from the septal nuclei to the hippocampus, which

are important for the acquisition and retention of spatial tasks, are

under opioid control. Finally, preliminary data from Gallagher’s

laboratory are presented which positively correlate changes in

hippocampal opioid peptide levels with the acquisition and performance of a spatial task in a radial arm maze.

The authors contributing to this monograph hope that they were

successful in synthesizing in a thoughtful and useful way the most

current multidisciplinary data on the cellular localization and

actions of opioid peptides in the HF. In addition, we hope we have

achieved our goal of highlighting the most exciting and fruitful

areas for future research, which will address not only the role of

opioid peptides in the hippocampus but in the overall scheme of

brain functioning as well.

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10
Nicoll, R., and Madison, D. The action of enkephalin on interneurons in the hippocampus. Soc Neurosci Abstr 10:660, 1984.

Urea, G.; Frenk, H.; Liebeskind, J.; and Taylor, A. Morphine and

enkephalin: Analgesic and epileptic properties. Science 197:83-

88, 1977.

Weber, E.; Roth, K.A.; and Barchas, J.D. lmmunohistochemical distribution of neo-endorphin/dynorphin neuronal systems in rat

brain: Evidence of colocalization. Proc Natl Acad Sci USA

79:3062-3066, 1982.

Zieglgänsberger, W.; French, E.D.; Siggins, G.R.; and Bloom, F.E.

Opioid peptideS may excite hippocampai pyramidal neurons by inhibiting adjacent inhibitory interneurons. Science 205:415-417,

1979.

AUTHOR

Jacqueline F. McGinty, Ph.D.

Department of Anatomy and

Cell Biology

East Carolina University

School of Medicine

Greenville, NC 27858

11
Localization and Seizure-Induced

Alterations of Opioid Peptides and

CCK in the Hippocampus

Christine M. Gall

INTRODUCTION

As a consequence of its very simple laminated anatomy, the hippocampal formation has come to occupy a central position in neuroscience research as a model system for the analysis of cellular

operations in the forebrain. As discussed in the various contributions to this volume, this has included use of the hippocampus as

a vehicle for analyses of the role of opioid peptides in neuronal

processing and behavior. Evaluation of the function of specific

neuropeptides in the hippocampus, as elsewhere, must begin with

consideration of such fundamental issues as their distribution, the

extent to which this distribution is consistent across species, and

the ontogeny of the peptides in the context of the morphological

and physiological development of the structure. Optimally, one

would also like to identify physiological processes which influence,

and/or are lnfluenced by, the peptides under investigation. In the

following pages, I will discuss the above points with regard to enkephalin, dynorphln, and cholecystokinin (CCK) in the hippocampus.

LOCALIZATlON Of OPIOID PEPTIDES IN THE HlPPOCAMPUS

As described in my original report (Gail et al. 1981 b) and verified

by subsequent immunocytochemical and biochemical studies,

enkephalin immunoreactivity is localized within three distinct axonal

systems in the rat hippocampal formation: the mossy fiber axons

of the dentate gyrus granule cells which innervate pyramidal cells

of region CA3; a thin band of axons distributed along the border of

the stratum radiatum and the stratum lacunosum-moleculare of

region CA1; and perforant path axons which arise from the lateral

12
entorhinal/perirhinal cortices, distribute within the Stratum

iacunosum-moleculare, and innervate the distal third of the dentate

gyrus molecular layer. The latter system is most dense in the

temporal hippocampus. Enkephalin immunoreactivity is also

observed within sparsely distributed perikarya throughout the

hippocampus proper and (in untreated rats) within only a small

number of morphologically characteristic dentate gyrus granule

ceils. Dynorphin exhibits a much more restricted distribution in

the rat hippocampus, being localized within the mossy fiber axonal

system (McGinty et al. 1983) and very few scattered, multipolar

neurons. Initially, the reported codistribution of [leu5]enkephalin

and dynorphin immunoreactivities within the mossy fiber system

aroused concern that some of the enkephalin immunostaining may

have been attributable to cross-reactivity of the enkephalin antisera

with dynorphin (McGinty et al. 1993). However, subsequent studies

have demonstrated that the dentate gyrus granule cells synthesize,

and the mossy fiber axons contain, methionine enkephalin as well

as the proenkephalin A fragments methionine-enkephalin-ArgPhe,

methionine-enkephaiin-ArgGlyLeu, BAM 18, and BAM 22 (McGinty et

al. 1984; White et al. 1987).

The localization of enkephalin and dynorphin within the mossy fiber

system is invariably conserved across all species thus far examined,

including rat, mouse, hamster, vole, European hedgehog, gerbil,

guinea pig, cat, tree shrew, and Old World monkey (Gall et al.

1981b; Fitzpatrick and Johnson 1981; Tielen et al. 1982; StengaardPedersen et al. 1983; Gall and Lauterborn, unpublished observations). ln contrast, subtle interspecies differences are evident in

the distribution of enkephalin within strata lacunosum and moleculare of the hippocampus proper, and these seem associated with the

more striking differences in the localization of enkephalin within

the perforant path afferents to the dentate gyrus. For example, in

stratum lacunosum-moleculare of the rat ventral hippocampus,

terminal-like immunoreactive puncta are most densely distributed

within a zone extending from the subiculum into the adjacent

region of CA1, and within region CA3 in a field continuous with

the dentate gyrus outer molecular layer (OML) (figures 1 and 2).

ln contrast, there is very little immunostaining in the hamster

stratum lacunosum-moleculare of CA3, but there is a rather dense

patch of immunoreactive puncta within this lamina at the CA2 limit

of region CA1. In the hamster, there is also very little immunoreactivity within the dentate gyrus OML in the guinea pig,

enkephalin immunoreactivity is only prominent in the extreme

13
temporal tip of the CA3 stratum moleculare and in the dentate

gyrus OML (Tielen et al. 1982), while in the hedgehog (StengaardPedersen et al. 1983) and the tree shrew (Fitzpatrick and Johnson

1981), enkephalin immunoreactivity has not been observed in either

of these laminae.

FIGURE 1. Illustrations of the distribution of enkephalin-like and

dynorphin-like immunoreactivity

NOTE: Enkephalln (ENK)-like (drawing and left micrograph) and dynorphin

(DYN)-like immunoreactivity (tight micrograph) in a horizontal section

through the ventral rat hippocampus (peroxidase antiperoxidase technique). Abbreviations: sl-m, stratum lacunosum-moleculare; sg, stratum

granulosum: mf, mossy fibers; oml, outer molecular layer: sp, stratum

pyramidale.

In the Old World monkey Macaca fascicularis, enkephalin-immunoreactive mossy fibers are present within the stratum lucidum, but

there is no clear lamination of enkephalin-positive elements in the

molecular layers of the hippocampus proper. Only a few scattered

immunoreactive axons and terminal puncta are seen within the stratum radiatum and, with somewhat greater density, within the stratum moleculare. Similarly, in the dentate gyrus molecular layer,

enkephalin is localized within a few broadly distributed varicose

axons: no prominent or laminated immunoreactive afferent systems

are seen (figure 2).

14
FIGURE 2. Differences in the distribution of enkephalin immunoreactivity in the ventral dentate gyrus of the rat, the

monkey, and the cat

NOTE: Light-field (A and C) and dark-field (D and E) photomicrogmphs illustrating differences in the distribution of enkephalin immunoreactivity in

the ventral dentate gyrus of the rat (A), the monkey (B), and the cat

(C), and the localization of fine punctate immunoreactivity overlying the

stratum pyramidale (sp) of the cat hippocampal region CA1. In A, B,

and C, the arrowheads bracket the dentate gyrus molecular layer which

can be seen to contain one distal outer molecular layer band of immunoreactivity in the rat (A), two bands of immunoreactivity in the cat (C),

and no Laminated immunoreactive systems in the monkey. The mossy A

ber system, in the hilus below the stratum granulosum (sg), is clearly

immunoreactive in all three animals (A, B. and C). Abbreviations: a.

alveus; F. hippocampal fissure: so, stratum oriens: sr, stratum radiatum.

In contrast to all other animals studied, the cat has two distinct

and dense bands of enkephalin immunoreactivity occupying the middle and outer thirds of the dentate gyrus molecular layer across its

15
full septotemporal extent (figure 2). The molecular layer is considered to be composed of three laminae defined by the laminated

termination of afferent systems: the inner, middle, and outer

molecular layers receive hippocampal commissural/associational,

medial entorhinal, and lateral entorhinal afferents, respectively. As

such it seems that, in the cat, afferents from both the medial and

lateral entorhinal cortex have relatively high enkephalin content.

In regard to the distibution of opioid receptors in the hippocampus

(Herkenham and McLean, this volume), it is noteworthy that neither

enkephalin nor dynorphin immunoreactivtty is localized to a significant number of axons or cells within the stratum pyramidale of the

hippocampus proper in most animals. The only known exception to

this generality is, again, the cat. In region CA1 of the cat hippocampus, fine enkephalin-immunoreactive terminal-like puncta are

distributed quite precisely over the stratum pyramidale and the

adjacent portion of the stratum oriens in a manner reminiscent of

the distribution of opioid ligand binding observed in other species

(figure 2).

DISTRIBUTION OF CCK IN THE HIPPOCAMPUS

Significantly more interspecies variability has been noted in the

hippocampal distribution of a third, nonopioid peptide, cholecystokinin octapeptide (CCK). CCK is of interest when considering

hippocampal opioids in that this peptide is codistrlbuted with enkephalin and dynorphin in the mossy fiber system of some species,

is reportedly antagonistic to opioid-mediated processes in other

systems (Fads et al. 1983), and is influenced by the same physiological events which after hippocampal opioid synthesis (vide infra).

CCK is reliably localized within scattered perikarya throughout the

hippocampus proper and the dentate gyrus hilus, and within coarse

pericellular axons in the stratum pyramidale. There are species

differences in the density and distribution of CCK in the commissural/associational afferents to the dentate gyrus inner molecular

layer, and the medial perforant path afferents to the dentate gyrus

middle molecular layer (Stengaard-Pedersen et al. 1983; Gall 1984a;

Gall et al. 1988). More pertinent here, CCK immunoreactivity has

been localized within dentate gyrus granule cells and the axons of

the mossy fiber system of guinea pig, European hedgehog

(Stengaard-Pedersen et al. 1983; Gall 1984a), mouse (Gall et al.

1986), and M. fascicularis (Gall and Lauterborn, unpublished), but is

16
absent from this system in rat (Greenwood et al. 1981), gerbil,

rabbit, and cat (Gall, unpublished).

The extent to which the three mossy fiber peptides are colocalized

within individual mossy fiber terminal boutons, as opposed to being

localized within separate elements within this system, is not as yet

known. In a double-labelling study, McGinty (1985) found immunoreactivity for dynorphin B and the proenkephalin A product BAM 22

in separate granule cells in colchicine-treated rats. However,

enkephalin synthesis and concentration In the dentate gyrus granule

cells has been demonstrated to be exquisitely sensitive to recent

physiological activity. In the normal state, without physiological

stimulation of synthesis, only a fraction of the granule cells which

are able to synthesize enkephalin contain detectable immunoreactivity (see below). Consequently, the absence of dynorphin/

enkephalin colocalizatlon In the normal rat, or enkephalin/CCK

colocalization in the normal mouse, does not necessarily mean that

these peptides arise from biochemically distinct perikarya. lt is

possible that individual cells do not synthesize these peptides in

detectable quantities all the time or at the same time. There is, in

fact, evidence that the synthesis of mossy fiber peptides is independently, or possibly even reciprocally, regulated in ontogeny

and adulthood, as will be discussed next.

ONTOGENY OF ENKEPHALIN, DYNORPHIN, AND CCK IMMUNOREACTlVITY IN THE HIPPOCAMPUS

Data on the ontogeny of opioid peptide immunoreactivity in the

hippocampus, and most particularly within the hippocampal mossy

fiber system, is directly relevant to the possible functions of the

opioids in hippocampal physiology. In young rats, enkephalin

immunoreactivity within the lateral perforant path afferents to the

ventral dentate outer molecular layer is evident at the time of

afferent ingrowth—4 days postnatal (dpn). In contrast, enkephalin

is not detectable within the mossy fibers until about 12 dpn and, in

both the rat and the mouse, is first seen in a few large, wellelaborated mossy fiber boutons (Gall 1984b; Gall et al. 1984). The

mossy fibers are known to grow into the stratum lucidum of region

CA3 by 3 dpn (Zimmer and Haug 1978), to have formed morphologically recognizable and functionally competent synapses by 5 dpn

(Bliss et al. 1974; Sterling and Bliss 1978), and, only after synaptic

contacts have been established, to slowly elaborate the large terminal boutons characteristic of the adult (Amaral and Dent 1981).

17
FIGURE 3. The hilus lesion and seizure-induced changes in mossy

fiber enkephalin immunoreactivity

NOTE: Photomicrographs illustrating (A) the extent of the electrolytic hilus

lesion found to initiate recurrent limbic seizuree activity (Nissl stain,

coronal section from rat), and (Band C) the increase in enkaphalin

lnmunostaining in the mossy fiber system (region CA3b) 4 days after a

contralateral hilus lesion (C), relative to that seen in a paired, untreated

rat (B). Abbreviations: h, hilus; mf, mossy fibers: sg, stratum granulosum: sl, stratum lucidum: sp, stratum pyramidale.

Considered in this context, it is clear that enkephalin is first

detectable within the mossy fibers fairly late in the morphological

development of the system and well after the establishment of

synaptic transmission.

Dynorphin and, in the mouse, CCK both appear earlier in the

development of the mossy fiber system, and closer to the apparent

18
onset of synaptic function. In the rat and the mouse, both peptides are first detectable on 6 dpn in small, indistinct puncta

within the dentate gyrus hilus and the stratum lucidum (Gall 1984b).

The immunoreactive elements within the stratum lucidum enlarge

with age, and it is only at later time points (12 to 14 dpn) that

both immunoreactivities are clearly localized within the large,

irregularly shaped terminal boutons characteristic of the mossy

fibers in the adult. Thus, in contrast to the development of enkephalin within this system, both dynorphin and CCK emerge very

soon in the wake of synaptogenesis, suggesting that these peptides,

but not enkephalin, may be present and active from the onset of

synaptic transmission at the mossy fiber synapse.

EVIDENCE FOR THE REGULATlON OF HIPPOCAMPAL NEUROPEPTlDE LEVELS BY PHYSIOLOGICAL ACTIVITY

Evidence that opioid peptide levels in the hippocampus and elsewhere are regulated to a significant extent by physiological activity

is shown in studies of the effects of experimentally induced seizures. Hong and coworkers first demonstrated that, following recurrent seizures induced by either repeated electroconvulsive shock

(Hong et al. 1979) or intracerebroventricular (ICV) injection of

kainic acid (Hong et al. 1980), the level of enkephalin immunoreactivity in the hippocampus increases threefold as measured by

radioimmunoassay (RIA) (Hong, this volume). These data provide an

interpretation for the observation I had made that a small electrolytic lesion placed in the dentate gyrus hilus induced a large bilateral increase in mossy fiber enkephalin immunoreactivity (Gall et

al. 1981 b). Since that time, a number of other seizure-producing

treatments have been demonstrated to alter both enkephalin and

dynorphin levels in the hippocampus. Work in my laboratory has

focused on the effect of limbic seizures induced by either focal

lesion placement in the dentate gyrus hilus or, for the purpose of

comparison, ICV injection of kainic acid, on enkephalin, dynorphin,

and CCK systems in rat and mouse hippocampi. Both treatments

induce dramatic increases in enkephalin immunoreactivity in the

hippocampus which are most prominently localized within the

hippocampal mossy fiber system (Gall et al. 1981a). Although

enkephalin immunoreactivity is clearly altered in other brain areas

following these treatments, our work has centered on peptide

changes in the mossy fibers due to the codistribution of three distinct peptide families within this one axonal system; the relatively

high sensitivity of mossy fiber peptide levels to seizure activity;

19
and the anatomical suitability of this compact, well-characterized,

and strictly unilateral axonal system to both biochemical and

immunocytochemical analysis.

In the rat, placement of a small unilateral electrolytic lesion in the

hilus of the dentate gyrus (figure 3) induces synchronous, bilateral

epiieptiform activity in the hippocampus, which is first observed 1

to 2 hours postlesion and continues, with periodic full paroxysmal

discharges, for approximately 10 hours thereafter. Throughout this

period, the animals exhibit intermittent convulsive behavioral seizures reminiscent of those described to accompany kindled limbic

seizures by Racine (1972). As such, three to seven episodes of

stage 4 or 5 convulsive seizure (rearing with facial and forelimb

clonus, rearing and falling), and more numerous displays of associated subconvulsive seizure behavior (forelimb clonus and wet dog

shakes) are typically observed. This lesion directly severs a narrow

rostrocaudal span of the strictly ipsilateral, and highly topographic,

mossy fiber system within the lesioned hippocampus; the majority of

the mossy fibers within the ipsilateral hippocampus and all of the

mossy fibers within the contralateral hippocampus remain fully intact. Kaink acid induces more continuous seizure activity lasting 3

to 6 hours after an ICV injection, does not induce mossy fiber

degeneration, but does result in the death of neurons within region

CA3 of the rostral, ipsilateral hippocampus.

Enkephalin levels undergo a biphasic fluctuation following seizures

induced by either treatment (Pico et al. 1985; Gall et al., in press:

Kanamatsu et al. 1986b). In rats sacrificed 6 to 12 hours following

a hilus lesion, enkephalin immunostaining is completely depleted

from the mossy fiber system bilaterally. By 18 to 20 hours,

immunoreactivity has returned to near-normal levels, and by 24

hours, immunostaining is clearly greater than that observed in

paired controls. Enkephalin immunoreactivity in the mossy fibers

continues to increase until it reaches an apparent maximum at 3 to

5 days postlesion, and thereafter decreases to normal by about

postlesion day 10. At its maximal elevation, hippocampal enkephalin

content reaches over fourfold above normal as measured by RIA.

The time course for the fluctuations in hippocampal enkephalin

content induced by the hilus lesion is comparable to that observed

following ICV or intrastriatal injection of kalnic acid in our

immunocytochemical material and in the reports of Hong et al.

(1980) and Kanamatsu et al. (1988b). We find enkephalin levels are

also transiently elevated in other areas following either the hilus

20
FIGURE 4. Seizure-induced changes in mossy fiber enkephalin,

dynorphin, and CCK immunoreactivities in mouse

hippocampus

NOTE: Photomicrographs of coronal sections through the mouse hippocampus

illustrating the distribution and density of peroxidase antiperoxidase

immunostaining for enkephalin (A and B), dynorphin (C and D), and

CCK (E and F) in untreated control mice (A, C, and E) and paired mice

sacrificed 4 days after the contralateral placement of a seizureproducing hilus lesion (B, D, and E). Abbreviations: iml, inner molecular layer; sg, stratum granulosum; sp, stratum pyramidale. Arrows

indicate mossy fiber system.

lesion or kainic acid, including the temporoammonic/lateral perforant path, entorhinal cortex, lateral septum, and amygdala.

The effect of hilus-lesion-induced seizure activity on the regulation

of mossy fiber enkephalin content is somewhat selective in that

neither dynorphin nor CCK immunoreactivities are increased by this

treatment. As mentioned above, dynorphin and, in the mouse, CCK

21
immunoreactivities are codistributed with enkephalin in the mossy

fiber axonal system. Twelve hours after the placement of a unilateral hilus lesion in rats or mice, immunostaining for both dynorphin A- and dynorphin B-like immunoreactivity is greatly reduced

in the mossy fiber system bilaterally. At 2 and 4 days postlesion,

dynorphin levels remain below normal, but the decreases seen at

these time points are not large and are highly variable: in some

animals with elevated mossy fiber enkephalin content, dynorphin

immunostaining was not clearly lower than that seen in paired controls. Dynorphin immunoreactivity was never observed to be elevated in the mossy fibers at these or longer intervals following

hilus lesion placement.

In contrast to the effect of hilus-lesion-induced seizure activity on

enkephalin and dynorphin, this treatment caused a complete and

prolonged bilateral depletion of CCK which was restricted to the

mossy fiber system. As mentioned above, in the mouse hippocampus, CCK is localized within the commissural/associational

afferents to the dentate inner molecular layer, within local circuit

neurons and their axonal arbors in the stratum pyramidale, as well

as within the mossy fibers. Six hours after hilus lesion placement

in the mouse, CCK is depleted from the mossy fibers bilaterally,

while immunostaining appears normal in other areas of the hippocampus. At 4 days postlesion (the time of maximal enkephalin

elevation), the pattern of CCK immunoreactivity remains unchanged:

no immunoreactivity is apparent within the mossy fiber system,

while other CCK systems stain normally. Low doses of kainic acid

(0.05 to 0.1 μg, ICV) induce precisely the same alterations in

mouse mossy fiber peptide immunoreactivity as those seen following

the hilus lesion: an initial depletion of immunostaining for all

three peptides; a return and supranormal elevation of enkephalin

immunoreactivity which appears maximal 3 to 5 days posttreatment;

a persistent depletion of CCK restricted to the mossy fibers; and a

partial reduction in immunostaining for dynorphin (Gall 1984c).

Data on the duration of altered peptide immunoreactivity in the

mouse following either the hilus lesion or kainic acid treatment

serve to further dissociate the effects of seizures on mossy fiber

enkephalin and CCK, and to indicate that aberrant peptide levels

induced by seizures can become permanent. Unlike the rat, the

duration of elevated mossy fiber enkephalin in the mouse is highly

variable: while enkephalin immunostaining has returned to normal

in most animals sacrificed 2 weeks posttreatment, in four mice

22
which initially exhibited particularly severe seizure behavior,

enkephalin immunostaining was still dramatically elevated in the

mossy fibers after 2 to 6 months’ survival. The absence of CCK

from the mossy fibers generally outlasted the enkephalin elevation.

In a number of the experimental mice with normal enkephalin

immunostaining 14 to 20 days posttreatment, CCK immunoreactivii

had not yet returned to the mossy fiber system. Among the mice

allowed 2 to 6 months’ survival, all mice with normal enkephalin

immunoreactivity also exhibited normal CCK immunoreactivity in the

mossy fibers. Those few animals with a persistent elevation in

mossy fiber enkephalin exhibited absolutely no immunostaining for

CCK in this system (Gall et al. 1986).

Because enkephalin, dynorphin, and CCK are similarly depleted from

the mossy fiber system during the recurrent seizure episode, one

might argue that the dentate gyrus granule cells, which synthesize

and release these peptides, are similarly stimulated by the epileptiform event. However, the postictal levels of the three mossy

fiber peptides are altered in distinctly different fashions. Either

the physiological activity or peptide depletion clearly stimulates a

large increase in enkephalin content. The same processes lead to

an equally dramatic, and somewhat more persistent, loss of mossy

fiber CCK. Dynorphin levels are less severely affected, seeming

only partially depressed in most experimental animals.

ALTERATIONS IN ENKEPHALIN SYNTHESIS FOLLOWING SEIZURE

There is now substantial evidence that the increase in enkephalin

content observed in the hippocampus and elsewhere (Yoshikawa et

al. 1985) following recurrent seizures is due to increased synthesis.

Work done in my laboratory in collaboration with Drs. Jeff White

and Jeff McKelvy (of State University of New York at Stony

Brook) has specifically examined the effect of recurrent seizure

activity induced by unilateral hilus lesion on enkephalin metabolism

in the dentate gyrus granule cells which give rise to the mossy

fiber axonal system. As reviewed earlier, following hilus lesion

placement in the rat, enkephalin immunoreactivity increased within

the mossy fiber axons bilaterally from 12 hours to 3 days postlesion. In order to determine the effect of this treatment on

enkephalin synthesis within the hippocampus contralateral to the

lesion, we injected 35S-methionine by osmotic minipump into the

dentate gyrus during the interval from 24 to 28 hours postlesion.

The incorporation of the isotope into chromatographically identified

23
methionine-enkephalin, methionine-enkephalin-ArgPhe, methionineenkephalin-ArgGlyLeu, and BAM 18 purified from region CA3 was

measured at 30 hours postlesion. We found that there was a 14-

fold greater incorporation of the radiolabel into methionineenkephalin in the mossy fiber system of hilus-lesioned rats relative

to paired controls, while there was no change in the ratio of the

four proenkephalin A peptides (White et al. 1987). This indicates

that hilus-lesion-induced seizure activity causes a large increase in

proenkephalin A synthesis without a fundamental change in posttranslational processing.

In a separate series of rats, we conducted Northern blot analysis to

determine the effect of hilus-lesion-induced seizure activity on

preproenkephalin messenger RNA (mRNA) content in the dentate

gyrus contralateral to lesion placement (figure 5). We detected an

increase in the single species of RNA hybridizing to our 32Plabelled RNA probe as early as 3 hours posttesion, less than 2

hours after the onset of seizure activity. Preproenkephalin mRNA

levels reached a maximal increase of about 24-fold above normal at

30 hours postlesion. By 4 days, message levels were only slightly

higher than those observed in untreated rats, and by 10 days the

experimental and control values were equivalent (White et al. 1987;

White and Gall 1987). As expected if these increases in enkephalin

message were induced by seizure activity rather than some other

aspect of treatment, we found that maintenance of animals under

anesthesia with the anticonvulsants sodium pentobarbital and phenytoin from lesion until sacrifice did, in fact, significantly block

the hilus lesion induction of increased preproenkephalin mRNA in

the dentate gyrus (figure 5).

The observation that recurrent seizure activity induces increased

enkephalin immunoreactivity and synthesis is consistent with the

results of other laboratories using different experimental manipulations including kainic acid injection (Kanamatsu et al. 1986b),

repeated electroconvulsive shock (ECS) (Kanamatsu et al. 1986a),

and chemical (pentylenetetrazol)- and amygdala-stimulation-induced

kindling (Vindrola et al. 1984; ladarola et al. 1986). Together,

these treatments have elicited enkephalin increases in the hippocampus, entorhinal cortex, amygdala, ventral pallidum, septum,

hypothalamus, and substantia nigra, although there are differences

in the reported distribution and magnitude of increased enkephalin

immunoreactivity with different modes of seizure induction. Following either the hilus lesion or kainic acid, the greatest increase

24
FIGURE 5. Bar graph illustrating the time course of increased

preproenkephalin A mRNA in the rat dentate gyrus

contralateral to a hilus lesion

NOTE: Each bar represents the mean densitometric value from autoradiograms of

Northern blots for at least three animals at each time point. Crosshatched bars indicate rats anesthetized during surgery with ketamlne/

xylazlne; black bar indicates rats maintained under pentobarbital/

phenytoin anesthesia from lesion until sacrifice.

in enkephalin immunoreactivity is localized in the mossy fiber system. In contrast, following ECS, elevated hippocampal enkephalin

levels are greatest in the perforant path afferents from the entorhinal cortex. ECS induces the largest increases in enkephalin

content in the hypothalamus, whereas enkephalin levels are unaffected in this area following kainic acid. lt is noteworthy that

all effective paradigms include recurrent seizure activity:

Kanamatsu et al. (1986a) did not observe significantly increased

hippocampal enkephalin content following less than six consecutive

ECS treatments. In both the hilus lesion (White and Gall 1987) and

kainic acid (Kanamatsu et al. 1986b) paradigms, increased synthesis,

as indicated by increased preproenkephalin mRNA levels, begins

25
early in the period of recurrent seizure activity and, in the hilus

lesion model, continues to increase for several hours after the

termination of behavioral seizures.

The effect of seizures on both dynorphin and CCK is significantly

less consistent across paradigms. While repeated ECS leads to increased dynorphin immunoreactivity in the substantia nigra, medial

basal hypothalamus, and ventral pallidum (Kanamatsu 1986a), ECS,

the hilus lesion, low doses of ICV kainic acid (present data), and

stage 5 amygdaloid kindling (ladarola et al. 1986) effect a reduction

in dynorphin in the mossy fibers lasting several days at least.

High doses of kainic acid injected into the striatum reportedly

induce a rapidly transient increase in mossy fiber dynorphin

(Kanamatsu et al. 1986b). While both amygdaloid kindling (ladarola

et al. 1986) and kainic acid (Meyer et al. 1986) have been reported

to increase CCK immunoreactivity in the substantia nigra and

hippocampus of rats, we have found that both the hilus lesion and

kainic acid eliminate CCK from the mouse mossy fiber system.

These results are not directly contradictory, however, in that CCK

immunoreactivity is not localized within the mossy fibers of the

rat.

CONCLUDING COMMENTS

lt is now clear that two separate opioid peptide families, represented by enkephalin and dynorphin, are localized and, to a large extent, codistributed within the hippocampal formation. While both

peptides are consistently observed within the dentate gyrus granule

cells and their mossy fiber axons in a number of different mammals, there are substantial interspecies differences in the localization of enkephalin within the entorhinal afferents to the dentate

gyrus and hippocampus proper.

Data on the ontogeny of and the effect of seizures on enkephalin,

dynorphin, and CCK in the mossy fiber system suggest that these

peptides are both independently regulated and differentially regulated by physiological activity, even though all three synthetic

systems may coexist within individual neurons. In the untreated

mouse, far more mossy fiber boutons contain dynorphin and CCK

immunoreactivity than enkephalin immunoreactivity. Following recurrent seizures, the reverse is true: a high density of boutons

contain enkephalin, while fewer dynorphin- and no CCK-immunoreactive boutons are found. The seizures seem to push the mossy

26
fiber system from “State-A” (enkephalin poor) to “State-B”

(enkephalin rich). Put in these terms, these findings encourage

speculation that individual granule cells might also have a State-A

in which they actively synthesize dynorphin and, in some animals,

CCK; and a State-B in which they actively synthesize enkephalin

and not the other peptides. As suggested earlier, this would

account for the localization of dynorphin B and the proenkephalin

A product BAM 22 within separate granule cells (McGinty 1985),

without leading to the conclusion that there are distinct populations of granule cells as characterized by their invariant synthesis

of either enkephalin or dynorphin.

Taking this hypothesis one step further, the presence of dense

enkephalin immunoreactivity in a subset of mossy fiber boutons in

the untreated animal may reflect the differences in the recent

physiological activity of their parent neurons. The enkephalinimmunoreactive neurons, in distinction from the larger group of

granule cells, may have moved into the enkephalin-synthesizing

State-B as a consequence of physiological events occurring days

earlier.

The indication from the seizure data that activity induces enkephalin synthesis may provide some interpretation for the ontogenetic

data as well. As reviewed above, dynorphin and CCK are present

within the hippocampal mossy fibers during the period of synaptogenesis and well before the morphological elaboration of the mossy

fiber boutons. In contrast, enkephalin does not appear in this system until much later. By the size of the mossy fiber boutons first

seen to contain enkephalin, one would estimate that this peptide

does not appear until about a week after synaptogenesis. lt is possible that enkephalin does not appear within the mossy fibers until

the physiological activity of the granule cells or their afferents is

sufficiently mature to induce synthesis.

The validity of the above suggestions is difficult to assess at the

moment. Most importantly, we do not know whether nonepileptiform physiological activity influences the synthesis of enkephalin in

the dentate gyrus granule cells. We do, however, have evidence for

the colocalization of enkephalin and CCK in other mammalian brain

areas (Gall et al. 1987) and for the independent regulation of colocalized enkephalin and CCK in invertebrates (Petrics-Gesser and

Larsson 1985). There is also a growing body of evidence that colocalized peptides and classical neurotransmitters are differentially

27
and probably independently regulated by physiological activity in

peripheral systems (Black 1984).

Returning from the cellular to the system level, the potential consequence of opposite fluctuations in enkephalin and CCK content in

the mossy fiber system following seizures is very intriguing. CCK

has been demonstrated to be antagonistic to endogenous-opioidmediated processes in other systems (Faris et al. 1988). CCK has

also been suggested to have anticonvulsant properties (Kádár et al.

1984) while enkephalin has been demonstrated to induce limbic

seizures when injected into the hippocampus (Elazar et al. 1979).

These factors lead one to speculate that the shift in CCK/

enkephalin balance in the mossy fiber system following seizures

may be of consequence to the future seizure susceptibility of the

structure.

In closing, it might be appropriate to ask if the hippocampus has

served, and continues to hold promise, as a fruitful model system

for the study of opioid peptides in the brain. I suggest that the

answer is yes, but in ways not anticipated 5 years ago. Although

the specific contribution of the endogenous opioid peptides to

synaptic physiology in the hippocampus and elsewhere remains

largely unresolved, we have identified systems and paradigms with

which we can study the physiological regulation of opioid gene

expression in the brain, and possibly resolve mechanisms through

which experience leaves its trace in the operation of neuronal

circuitry.

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31
AUTHOR

Christine M. Gall, Ph.D.

Department of Anatomy and Neurobiology

University of California

Irvine, CA 92717

32
The Anatomical Relationship

of Opioid Peptides and Opiate

Receptors in the Hippocampi

of Four Rodent Species

Miles Herkenham and Stafford McLean

lNTRODUCTlON

A commonly held assumption in neurobiology is that drugs and

neurotransmitters bind to postsynaptic receptors located at sites of

termination of central nervous system (CNS) pathways containing

the relevant transmitter. One expectation, therefore, is that autoradiographic maps of receptor distributions should closely resemble

(or match) the immunohistochemical maps of terminals of the related transmitters. This paper reports a test of that assumption in

the rodent hippocampus, a cortical structure whose laminar organization has been well characterized for a number of neurochemicai

systems, including opioid peptides and opiate receptors.

Currently, three families of opioid peptides have been identified:

the proenkephalin gene yielding the enkephalins and other carboxy

terminal extended forms, the prodynorphln gene providing the

dynorphins, and the proopiomelanocortin gene providing betaendorphin. Extensive work has documented enkephalin and dynorphin but not beta-endorphln immunoreactive systems in the rodent

hippocampus.

The existence of three major opiate receptor subtypes in the brain

is now accepted: the mu receptor exhibiting highest affinity for

morphine and related alkaloids, the delta receptor binding enkephalin better than morphine, and the kappa receptor having a high

affinity for ethytketocyclazocine and dynorphin (Lord et al. 1977;

Chang et al. 1979; Chavkin et al. 1982; Corbett et al. 1982).

33
In this study, the distributions of mu, delta, and kappa opiate

receptors were determined in the hippocampi of four species of

rodent (squirrel, guinea pig, rat, and hamster) and compared with

the distributions of immunoreactive dynorphln and enkephalin in

each species. Although distributions of the peptides were fairly

similar across species, the distributions of each of the receptor

types showed marked species variations. Moreover, the terminal

and receptor localization patterns did not typically correspond to

each other, leading to the conclusion that different determinants

underlie the distributions of each, After most explanations for

these mismatches are considered and found lacking, it is proposed

that transmitters like the opioid peptides act in a parasynaptic

fashion to diffuse from release sites and bind to receptors on noncontiguous cells.

lMMUNOHlSTOCHEMlSTRY

The distribution of enkephalin and dynotphin in the anterior hippocampus is relatively invariant across the four rodent species. Most

striking is the intense immunoreactivity in the mossy fiber system

(figure 1 on pp. 36 and 37). In all species, mossy fibers immunoreactive for both peptides course from the dentate hilus into the

stratum lucidum of the CA3 and CA2 regions. Here, the immunoreactivity is punctate and dense, indicative of a terminal zone.

in addition, isolated dynorphin-positive fibers are present in the

stratum radiatum, and sparse enkephalin fibers are found throughout

the CA fields. The dentate molecular layer contains occasional

dynorphin- and enkephalin-positive processes which often can be

traced back to immunoreactive granule cells.

At these rostral hippocampal levels, a dense plexus of enkephalin

fibers in the hamster and rat straddles the hippocampal fissure,

occupying superficial parts of both the dentate molecular layer and

stratum lacunosum-moleculare of CA1 (figure 1). This immunoreactivity represents the enkephalinergic component of the perforant path, which originates in the entorhinal cortex (Fredens et al.

1984).

RECEPTOR AUTOGRAPHY

The distributions of the opiate receptor subtypes mu, delta, and

kappa exhibit a variable relation to the distributions of endogenous

34
opioid peptides and considerable diversity among species. However,

within a species, mu and kappa receptors are similarly distributed

and, except for the rat, have a pattern which is generally the inverse of the delta pattern.

Squirrel

The dentate gyrus contains the greatest density of both mu and

kappa receptors (figure 1). Another peak of dense mu and kappa

receptors is found in the stratum lucidum of the CA3/CA2 fields.

The pattern of delta receptors in the squirrel is strikingly different

from the kappa and mu distributions. First, the stratum lucidum is

notable for its lack of labelling. Second, delta sites are conspicuously absent from all cell layers. Third, in contrast to the low

levels of mu and kappa receptors in the CA1 field, delta receptors

are dense there.

Guinea Pig

In the guinea pig, the stratum lucidum has the densest kappa labelling. The distribution of mu receptors is similar to that of kappa

receptors, but there are far fewer mu sites.

The delta binding pattern differs from the mu and kappa patterns.

Dense binding is present in CA1 in all layers but the pyramidal cell

layer and the superficial part of the stratum lacunosum-moleculare.

Within the CA3 Reid, the region of the mossy fiber terminal zone

has the lowest delta receptor density in the anterior hippocampus.

The dentate gyrus molecular layer is characterized by few delta

receptors, though a band of higher density is found in its most

superficial position, bordering the hippocampal fissure. The hilar

region is moderately labelled and shows elevated binding subjacent

to the granule layer.

Hamster

The hamster has kappa receptors densely and selectively distributed

in the two locations that also contain intense peptide immunoreactivity, namely the dentate hilus and the stratum lucidum (figure

1). In addition, a moderate kappa receptor density fills the remaining dentate gyrus. The mu pattern is similar, but the density

is much lower throughout. Mu and kappa binding is not detected

35
FIGURE 1. Distribution of opioid peptides and receptors in the

hippocampi of four rodent species

NOTE: Immunoreactivity was performed using the avidin-biotin technique, and

the receptor distributions were derived from autoradiographic film

images. For immunohistochemistry, frozen and cut fixed sections were

Incubated in antisera against dynorphin B (1:2,000) (E. Weber) or leuenkephalin (1:1,000) (Immunonuclear). For receptor autoradiography,

fresh frozen brains were cut and thaw-mounted onto slides. For mu

binding, slides were incubated for 90 minutes at 25 °C in tris buffer

with 2 nM [

3H]D-Ala2

, MePhe4,Gly-ol5 enkephalin. For delta binding,

slides were Incubated in tris buffer and 30 nM oxymorphone with 4.8 nM

36
[

3H]O-Ala2

,D-Leu5-enkephalin. For kappa binding, slides were transferred to MOPS buffer with 1 μM BIT and 1 μm FIT and incubated for

30 minutes at 25 °C. BIT and FIT are, respectively, selective mu and

delta opiate receptor irreversible acylating agents (Rice et al. 1953).

The sections were then incubated for 3 hours at 0 °C in phosphate

buffer containing 0.4 M NaCl and 7 nM [

3H]bremazocine. Abbreviations:

H, hilus; SG, stratum granulosum; HF, hippocampal Assure; SLM, stratum

lacunosum-moleculare; SR, stratum radiatum; SP, stratum pyramidale; SO,

stratum oriens; SL, stratum lucidum. Asterisks mark locations of SP.

Arrowheads mark location of HF. Calibration bar measures 450 μM for

squirrel and guinea pig and 250 μM for rat and hamster.

SOURCE: McLean et al. 1987.

37
in the CA1 field. In contrast, delta receptors show virtually the

opposite pattern; they are lowest in the dentate gyrus and densest

in hippocampal regio superior, where they occupy all layers but the

pyramidal. In CA3, the stratum lucidum is very delta receptor

sparse.

Rat

The rat differs from the previous three species in two ways (figure

1). First, the mu, kappa, and delta receptor distributions are all

similar to each other. Second, the zones of high density are located in and adjacent to the pyramidal and granule cell layers. The

pyramidal layer is enriched with all three subtypes, though the receptor band actually occupies the deep half of the pyramidal layer

and extends into the stratum oriens. Mu and kappa receptors have

a denser and wider band in CA2/CA3 than in CA1 , and they show

an abrupt shift in density at the border. Delta sites display no

major density shifts throughout the CA fields. Mu sites are selectively sparse in the stratum lucidum (figure 1). The three receptor

subtypes occupy the dentate gyrus in moderate densities. Mu and

kappa sites show a peak of higher density in the vicinity of the

granule cell layer.

DISCUSSION

The results of the present study show that there is no obvious

relationship between the locations of opioid peptides and opiate

receptors in the hippocampus, and this “mismatch” is the main focus

of the present study. In essence, the organization of the enkephalins and dynorphins is fairly consistent among the four rodent

species examined, whereas the distribution of the opiate receptor

subtypes is variable (figure 1). While both opioid and peptide

terminals and opiate receptors display patterns that closely conform

to the regional and laminar morphology of the hippocampal formation, the organizations of these pre- and postsynaptic elements are

clearly different from each other.

This does not mean that very good correspondence between opioid

peptides and receptors cannot be obtained. In the guinea pig and

hamster, mu but especially kappa receptors are densely located in

precisely the same locations (dentate hilus and stratum lucidum)

that show intense opioid peptide immunoreactivity. Similarly, in

the hamster and rat, the receptors in the outer third of the

38
dentate molecular layer and the adjacent stratum lacunosummoleculare are aligned with enkephalinergic fibers, presumably

terminals of the lateral perforant pathway. However, the dense

distributions of kappa and delta receptors in the dentate molecular

layers of the squirrel and guinea pig, respectively, are not matched

by a corresponding density of peptides. Unlike the other rodents

studied, the rat shows binding peaks for all three receptor subtypes

in the regions of the cell body layers where peptides are not detected; conversely, kappa but especially mu receptors are practically

absent in the peptide-rich mossy fiber terminal zone. Thus, individual instances of matches of peptides and receptors are present,

but no consistent pattern of correspondence occurs across species.

In fact, for all species examined, the delta receptors have unique

patterns which typically show an inverse relationship with the pep

tide densities and distributions.

Physiology

Administration of opiate alkaloids and peptides both in vivo (Nicoll

et al. 1977; Chou and Wang 1977; Henriksen et al. 1982) and in

slice preparations in vitro (Dunwiddie et al. 1980; Haas and Ryall

1980; Lee at al. 1980; Dingledine 1981) consistently produces increased hippocampal pyramidal cell activity. It has been concluded

that the opiates, rather than acting directly on pyramidal cells, act

instead to remove a tonic inhibitory input to the pyramidal cells

via occupation of receptors on interneurons (Zieglgänsberger et al.

1979; Dunwiddie et al. 1980; Lee et al. 1980).

Within the CA1 field of the rat, microiontophoretic mapping studies

have demonstrated that the stratum pyramidale and stratum oriens

are sites most sensitive to the administration of enkephalins

(Dingledine 1981). This is consistent with the elevated levels of all

three opiate receptor subtypes in these layers.

Opiate receptor localization and opiate physiology create expectations about opioid peptide terminal locations, but these are not met

by the relatively restricted localization of the peptides in the

mossy fiber system, which is known to convey excitatory impulses

to CA3 pyramidal cells, possibly using glutamate as the transmitter

(Storm-Mathisen and lversen 1979; Altschuler et al. 1985). In the

stratum lucidum, mossy fibers which contain the highest density of

opioid peptides in the hippocampus (Chavkin et al. 1985; Zamir et

al. 1985) terminate selectively on pyramidal cell dendrites (Hamlyn

39
1962). Because opiates appear to exert their inhibitory effects on

interneurons, opioid-containing processes should terminate on inter-’

neurons rather than pyramidal cell dendrites. However, there is no

evidence that any mossy fibers make selective inhibitory contacts

with interneurons in the stratum lucidum of CA3. Thus, there is

no currently known aspect of mossy fiber anatomy or physiology

that fits with our knowledge of the major opiate actions in the

hippocampus.

Mismatch Implications

The mismatches between opiate receptor locations and sites of release of opioid peptides at nerve terminals noted here for the

hippocampus are part of a much larger and more general phenomenon that occurs for ail well-characterized neurotransmitter/receptor

systems. We have pointed out (Herkenham and McLean 1988;

Herkenham 1987) that there is a failure of correspondence, a mismatch, between the locations of nerve terminals and receptors

throughout the central and even peripheral nervous system for all

neuropeptides, monoamines (norepinephrine, dopamine, and serotonin), and “fast” transmitters (gamma-aminobutyric acid, acetylcholine, and glutamate). Explanations for the mismatches can be

put into four categories:

(1)

(2)

(3)

(4)

Typically, it has been assumed that mismatches represent

technical failures and/or incomplete knowledge of the full

endowment of transmitters or receptors within a family of

chemically related substances.

It has also been pointed out that we should not expect a

match, since transmitters and receptors represent pre- and

postsynaptic elements of different neurons which may express

their features throughout their processes and not just at the

sites of contact (Kuhar 1985).

Biochemical possibilities for the discrepancies include occupied

receptors (not available for binding by virtue of prebound endogenous ligand), spare receptors, and nonfunctional receptors.

More recently, we have stressed the likelihood that many receptors, especially those to which the endogenous ligand binds

with high affinity, are not located at synapses and may be

40
sites of action of transmitters released from a distance

(Herkenham and McLean 1986; Herkenham 1987).

The present results can be used to test the various explanations for

the mismatches. Addressing the issues in the first category, we

have used the most sensitive and optimal histological techniques

currently available and have provided quantitative analysis of the

receptors using previously validated binding conditions. We have

used markers that recognize the predominant forms of endogenous

hippocampal opioid peptides and all of the major opiate receptor

subtypes, so that it is unlikely that undiscovered ligands or receptors exist which would significantly alter the presently described

relationships. Even if a low-affinity synaptic receptor is discovered

that is selectively localized to terminal zones, we would still have

to consider the roles of the more widely distributed and mismatched

high-affinity sites.

Consideration of hippocampal anatomy sheds light on the second

category. Given the horizontal stratification of the receptors and

the vertical alignment of hippocampal dendrites, it is clear that, if

receptors are localized on hippocampal neurons, then they are confined to restricted zones along the length of the dendritic tree. lf

some proportion of the receptors is located on afferent axons

rather than on dendrites, then this population of receptors appears

to be concentrated at axon terminals, as the major afferent fiber

tracts do not show commensurate receptor densities. Thus, cells

and fibers both show discrete receptor localization, making unlikely

the suggestion that receptors in one hippocampal location are cytoplasmic and in transit to another location (Stengaard-Pedersen

1983), or that they reflect the ceil’s inability to concentrate binding sites.

Transmitters, of course, are localized by immunocytochemistry

throughout cell bodies, axons, and terminals, and this can be a

basis for a mismatch if the sites of terminal release cannot be da

termined. Opioid peptide immunohistochemistry indicates that the

major hippocampal terminal zone lies in the stratum lucidum,

whereas scattered fibers with varicosities suggest additional release

sites in the dentate and CA fields. Another source of opioidpeptide-containing terminals is the perforant path (Gall et al. 1981;

Stengaard-Pedersen et al. 1981; Tielen et al. 1982; Fredens et al.

1984), but this provides enkephalin fibers predominantly to more

caudal (temporal) parts of the hippocampus rather than to the

41
rostral (septal) part examined in this study. Perforant path terminals are localized to the superficial parts of the hippocampai and

dentate molecular layers. Thus, for the hippocampus, the major

sites of peptide release are known, and these taken together are

the basis for the present claim that there is no correlation between

their locations and the distributions of the receptors.

Each of the various biochemical explanations for the mismatch in

the third category can be put in perspective by the present results.

The possibility of receptor occupation by the endogenous ligand is

prevented by preincubating sections in buffer containing high concentrations of Na+

, which causes dissociation of endogenous agonists from the receptors (Garzón et al. 1984). A more compelling

argument is that we show instances of dense mu and kappa receptors in the stratum lucidum, providing an excellent correspondence

of dense receptors in a location where endogenous enkephalin and

dynorphih are apparently released in high concentrations.

The possibility of spare receptors is an intriguing one that deserves

further study. Spare receptors can be responsible for an amplification of biological response, which might be useful in locations

where ligands exist in low concentrations. If spare receptors

“explain” some of the hippocampal mismatches, then they must exist

in high proportions in some hippocampal regions and not in others.

As argued elsewhere for the substance P (Mantyh et al. 1984,

Herkenham and McLean 1986; Herkenham 1987) and alpha-1-

adrenergic systems (Johnson and Minneman 1985), regional

correlations between receptor density and amplitude of biological

response have not documented differential distributions of spare

receptors. However, this line of inquiry could be fruitful in the

opiate system.

Finally, within this third category, receptors are seen by some

workers as either nonfunctional or “merely” drug-binding sites.

This is an implausible view in light of the precision with which

they are distributed, the metabolic expense and synthetic machinery

required to maintain them, and the proven functionality in physiological studies showing responses to drug application localized in

sites, such as CA1, where the concept of nonfunctional receptors

might be most appealing because of a paucity of the endogenous

ligand. In fact, within CA1 of the rat, the discrete distribution of

opiate receptors in stratum pyramidale and adjacent stratum oriens

42
corresponds precisely with D-ala2

-D-leu5-enkephalin (DADLE)-

sensitive sites most effective in enhancing the population spike

(Dingledine 1981).

The fourth category of explanations is centered around the possibility that many receptors are not localized to synapses. There is

ample evidence for this hypothesis. In a recent electron micrographic autoradiography study, Hamel and Beaudet (1984) found that

only a small percentage (7 percent) of mu opiate receptors in rat

striatum are located at junctional contacts satisfying the usual

criteria for synapses. Most sites were at nonjunctional, hence nonsynaptic, contacts between neuronal elements. lt is well known

that cell-to-ceil communication in the autonomic nervous system

occurs without synapses (Merillees et al. 1983). Presynaptic modulation of neurotransmitter release by ehdogenous ligand in the CNS

is likely to be nonsynaptic, insofar as electron micrographic studies

almost never show axoaxonic synapses at nerve terminals. Diffusion of transmitter to distant sites has been documented in the

peripheral nervous system (Jan and Jan 1983; Kravitz et al. 1983).

The possibility that nonsynaptically located opiate receptors are

targets of opioid peptides released from distant sites may shed light

on a fundamental question: Why does the CNS utilize so many opioid peptides and receptor subtypes? One hypothesis, promoted by

several laboratories and based on relative affinities and potencies in

bioassays, is that a given peptide is targeted to a particular type

of opiate receptor. An alternative hypothesis is that many forms

of opioid peptides can interact with each of the receptor subtypes:

thus, source and availability of ligand, ligand stability, receptor

microenvironment, as well as kinetics of the receptor/ligand interaction may play important roles in intercellular information

transfer.

The cerebrospinal fluid (CSF) is a possible conduit for diffusion of

endogenous opioid peptides to targets, such as the hippocampus,

that are close to the ventricles. Ventricular administration of

beta-endorphin causes hippocampal seizure activity by diffusion of

this endogenous ligand into the hippocampal neuropil (Henriksen et

al. 1978). Seizure activity itself causes release of an endogenous

opioid material into the CSF, and this material diffuses to distant

neuronal sites to alter the threshold for subsequent seizures

(Tortella and Long 1985). The sources and nature of the endogenous neurochemicais and the precise targets for the opiate effects

43
are currently unknown, and speculation about the role that hippocampal opiate receptors might play in generation or inhibition of

seizures is beyond the scope of the present data. However, we can

speculate that dense high-affinity opiate receptors in the hippocampus may be localized to participate in cell-to-cell communication

transmitted nonsynaptically at very low ligand concentrations. This

mechanism is consistent with proposed roles of opiates as neuromodulators of pyramidal cell activity.

.

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1979.

AUTHORS

Miles Herkenham, Ph.D.

Stafford McLean, Ph.D.

Unit on Functional Neuroanatomy

National Institute of Mental Health

Building 36, Room 2D-10

Bethesda, MD 20892

47
Seizure-Induced Alterations in the

Metabolism of Hippocampal Opioid

Peptides Suggest Opioid Modulation

of Seizure-Related Behaviors

Jau-Shyo Hong, Jacqueline F. McGinty, Laura Grimes,

Tomoyuki Kanamatsu, Johnny Obie, and Clifford L. Mitchell

INTRODUCTlON

Endogenous opioid peptides appear to play an important role in the

regulation of brain excitability, especially in the seizure-sensitive

hippocampus. Proenkephalinderived peptides have a unique excitatory action when iontophoretically applied to pyramidal cells of the

hippocampus, whereas in most other brain areas these peptides

manifest inhibitory actions (Nicoil et al. 1977). Electroencephalographic (EEG) recordings have indicated that intracerebroventricular

(ICV) or intrahippocampal injections of enkephalins or betaendorphin have either proconvulsant (Elazar et al. 1979; Henriksen

et al. 1978; Urca et al. 1977) or anticonvulsant (Tortella and Cowan

1982) properties depending on experimental conditions (Frenk 1983).

Unlike proenkephalin- or proopiomelanocortin-derived peptides,

however, prodynorphin-related peptides have a more complex

electrophysiological profile. Intracerebroventricular dynorphin does

not induce epiieptiform activity in the rat, and single-unit and field

potential studies show mixed effects on CA3 neuronal excitability

with more inhibitory responses than are seen with the enkephalins

(Henriksen et al. 1982; Walker et al. 1982; Gruol et al. 1983;

Henriksen et al., this volume; Chavkin et al., this volume). Sub

cutaneous or ICV injection of the specific kappa agonist U50 488

provides protection against certain electroconvulsive shock (ECS)-

induced seizures in rats, suggesting an anticonvulsant role of

endogenous kappa agonists such as dynorphin (Tortella et al. 1986).

Seizure activity exerts profound effects on the metabolism of opioid

peptides in certain seizure-sensitive brain regions such as the

hippocampus, amygdala, and substantia nigra. We have employed

48
different models of seizures, such as ECS (Hong et al. 1979;

Yoshikawa et al. 1985; Kanamatsu et al. 1988a), amygdaloid kindling

(McGinty et al. 1988), or kainic acid (Hong et al. 1980; Kanamatsu

et al. 1986b), to study the interrelationship between brain opioid

peptides and seizures. It is the purpose of this chapter to review

our progress in this area.

ELECTROCONVULSIVE SHOCK

Effects of Repeated ECS on the Metabolism of Enkephalins

lt has been suggested that endogeneous opioid peptides may mediate

some ECS-elicited behavioral alterations, including analgesia, retrograde amnesia, changes in seizure threshold, and postictal depression, since these behaviors can be prevented or altered by

pretreatment with opiate antagonists (Carrasco et al. 1982; Holaday

. and Belenky 1980; Tortella and Cowan 1982; Urca et al. 1981; Urca

et al. 1988). The increase in met5-enkephalin-like immunoreactivity

(ME-LI) in certain limbic areas of the rat brain, such as the septum, amygdala, nucleus accumbens, hypothalamus, and hippocampus

(figure 1 and table 1), produced by repeated ECS treatment, also

suggests a role for ME in the behaviors listed above. In contrast,

beta-endorphin content in the hypothalamus was not altered by repeated ECS, suggesting specific ME involvement (Hong et al. 1979;

Kanamatsu et al. 1986a). In order to determine whether the increase in ME-LI elicited by repeated ECS was due to an increase in

the biosynthetic activity of enkephalin-containing neurons, we

measured the abundance of messenger RNA (mRNA) coding for preproenkephalin A, using both in vitro cell-free translation and blot

hybridization using a complementary DNA (cDNA) clone coding for

preproenkephalin A derived from rat brain. The results demonstrated that repeated ECS increased the abundance of mRNA coding

for preproenkephalin A in the hypothalamus (Yoshikawa et al. 1985)

and hippocampus (Li et al., in preparation). This finding suggests

that ECS increases the metabolic rate of ME-LI and also may trigger a large release of this opioid peptide from nerve terminals.

This possibility was further supported by our recent report that

repeated ECS downregulates the opioid receptors in rat brain

(Nakata et al. 1985). Ten consecutive daily ECS treatments caused

a reduction of mu and delta receptor binding in the hypothalamus,

hippocampus, and caudate nucleus, but not in the frontal cortex and

brain stem. These changes of opioid receptor binding were not observed in rats receiving a single ECS. Scatchard analysis revealed

49
FIGURE 1. Hippocampal met5

-enkephalin and dynorphin A (1-8)

levels 1 day after 1 to 10 daily ECS treatments

NOTE: (A) Met5-enkephalin levels were significantly increased 1 day after 6 or

10 daily ECS treatments (p<0.05). (B) Dynotphin A levels were significantly decreased 1 day after 3, 6. or 10 ECS treatments (p<0.01).

SOURCE: Kanamatsu et al. 1986a. Copyright 1986, Society for Neuroscience.

that ECS-induced reduction of opioid receptor binding was due to a

decrease in the binding but not to a change in the binding affinity.

Similar to the changes in ME-LI, time-course studies showed that,

7 days after the end of 10 consecutive ECS treatments, both mu

and delta receptor binding remained lower than those of sham controls, but the effect disappeared in 2 to 3 weeks. These observations are consistent with the hypothesis that ECS treatments increase the release and biosynthesis of opioid peptides in certain

brain regions, which in turn downregulates the opioid receptors

(Nakata et al. 1985).

50
TABLE 1. ECS-induced changes in hippocampal opioid peptide

levels

ECS Treatment Dynorphin A Met5-enkephalin

5 Minutes After:

1 ECS -40% ±0%

1 Day After:

1 ECS

3 ECS

6-10 ECS

+10% ±0%

-30% ±0%

-70% +40%

2 Weeks After:

6-10 ECS ±0% ±0%

Effects of Repeated ECS on the Metabolism of Dynorphins

We have found that the metabolism of prodynorphin-derived peptides is also very responsive to repeated ECS treatments in different brain regions (Kanamatsu et al. 1986a). Ten daily ECS

treatments caused a significant increase in dynorphin A (1-8)-like

immunoreactivity (DN-LI) in most limbic-basal ganglia structures

including the hypothalamus, striatum, and septum. These results

are very similar to those for ME-LI. However, the ECS treatments

decreased DN-LI in the hippocampus by 64 percent, while ME-LI

increased 40 percent (figures 1 and 2 and table 1). A possible

anatomical basis for this was revealed by immunocytochemistry,

which showed that dynorphin A (1-8) immunostaining was depleted

in the mossy fiber pathway, and enkephalin immunostaining was increased in the perforant pathway. The differential responsiveness

of enkephalin- and dynorphin-containing neurons in the hippocampus can be observed in other seizure models including kainic

acid and amygdaloid kindling (see below). These observations

suggest that different mechanisms may regulate these two opioid

peptide systems in the hippocampus, and that they play different

51
FIGURE 2. Hippocampal opioid peptide levels 1 day and 28 days

after amygdaloid kindled seizures

NOTE: (A) ME-LI is increased significantly (p<0.05) 1 day after reaching two

consecutive stage 5 seizures only. (B) DN-LI is decreased significantly

(p<0.001) 1 day after reaching stage 5 seizures only.

SOURCE: McGinty et al. 1986, Copyright 1986, Elsevier Scientific Publishers

Ireland Ltd.

roles in mediating hippocampal excitability by interacting with different receptor subtypes.

Effects of a Single ECS on the Metabolism of Enkephalins and

Dynorphins

Recently, we examined the effects of a single ECS on brain opioid

peptides. Time-course studies revealed that, within 15 minutes

after a single ECS, there was a 40 percent decrease of DN-LI in

the hippocampus, but no change was found in the other brain

regions examined, including the hypothalamus, striatum, and brain

stem (Takeuchi et al., in preparation). Furthermore, levels of MELI, substance P, and cholecystokinin (CCK) (1-8) were not altered

in the hippocampus, suggesting that hippocampal dynorphincontaining neurons are most sensitive to ECS, and changes in

52
dynorphin metabolism are most correlated with ECS-elicited changes

in seizure threshold or retrograde amnesia.

KINDLING-INDUCED SEIZURES

Effects of Amygdaloid Kindling on the Metabolism of Enkephalins

and Dynorphins

Kindling is an experimental model of temporal lobe epilepsy in

which initially subthreshold stimulations result in progressively more

intense seizure activity (stages 1 to 4), until a generalized (stage 5)

seizure occurs (Goddard 1967; Racine 1972). Recent studies have

suggested that two dynorphin-rich structures, the hippocampal formation and the substantia nigra, are involved in the development of

kindling and expression of a fully kindled seizure, respectively

(McNamara et al. 1966). There is, however, no evidence that

alterations in opioids or other neurotransmitters or receptors are

responsible for the permanence of the kindling phenomenon

(Woodbury 1984).

Several lines of evidence have indicated involvement of endogenous

opioid peptides in modulating amygdaloid-kindled seizures. Naloxone, a stereoselective opioid antagonist, has been reported to facilitate amygdaloid kindling in rats (Hardy et al. 1980). In addition,

the duration of postictal depression can be altered by opioid agonists and antagonists (Frenk et al. 1979). Enkephalin has been

reported to be increased in whole rat brain (Vindrola et al. 1981),

and dynorphin has been observed to be increased in rabbit hippocampus 1 day after reaching stage 5 seizures (Przewlocki et al.

1983). In order to construct a time course of opioid peptide

changes during and after amygdaloid kindling, we examined rats 1

day after reaching subconvulsive stage 3 seizures, and 1 day and 4

weeks after two consecutive stage 5 seizures (table 2). One day

after stage 3 seizures, no significant change in opioid peptide levels

was observed by radioimmunoassay (RIA) or immunocytochemistry

(ICC). In contrast, 1 day after two consecutive stage 5 seizures,

there was a 50 percent increase in ME-LI in the projection from

the entorhinal cortex to hippocampal pyramidal cells (the temporoammonic component of the perforant pathway) and a 70 percent decrease in DN-LI in the mossy fibers of the hippocampus (McGinty

et al. 1986). The only other change was an increase in the

53
TABLE 2. Kindling-induced changes in hippocampal opioid

peptides

Site Dynorphin A Met5-enkephalin

Amygdala

1 Day After:

Stage 3

Stage 5

28 Days After:

Stage 5 ±0%

Prepiriform

5 Minutes After

Stage 2

Stage 5

1 Day After:

Stage 2

Stage 5

6 Weeks After:

Stage 2

Stage 5

±0%

-71%

20%

±0% +34%* +167%***

-23% +30%* +237%***

+13%

-12%

±0%

±0%

±0%

+43%

HPC ERC

+41%** +221%***

±0% +84%*

±0% ±0%

±0% ±0%

*p<0.05

**p<0.01

***p<0.001

NOTE: HPC=hippocampus: ERC=entorhinal cortex.

immunostaining of dynorphin in the substantia nigra pars reticulata,

a structure implicated in the regulation of kindled seizure thresholds (McNamara et al. 1984). Normal levels of both peptides in the

hippocampus and substantia nigra returned by 4 weeks after completion of kindling. A recent study by Naranjo et al. (1986)

indicates that the increase in the levels of hippocampal enkephalin

after amygdaloid kindling is due to an increase in the biosynthesis

54
of this peptide. Therefore, while opioid peptides are not involved

in the permanence of the kindling phenomenon, kindling-induced

seizures stimulate different changes in the metabolism of dynorphin

and enkephalin, suggesting separate regulation of each peptide in

states of hyperexcitation.

Effects of Prepiriform Cortex Kindling on the Metabolism of

Enkephalin and Dynorphin

In order to further examine the role of opioid peptides In the

development and expression of seizures, we have determined the

levels of opioid peptides in different brain regions kindled by stimulation of the deep prepiriform cortex (DPC) once every hour until

stage 2 or stage 5 seizures occurred. The DPC has recently been

identified as a critical epileptogenic site in the brain (Piredda and

Gale 1965). In contrast to the large decrease in hippocampal

dynorphin levels 1 day after reaching stage 5 amygdaloidkindled

seizures, DN-LI was slightly but significantly elevated in DPG

kindled hippocampus 5 minutes after stage 2 seizures, and slightly

depressed 5 minutes after stage 5 seizures (table 2). However,

there was a large increase in ME-LI in the entorhinal cortex (ERC)

and a moderate increase in the hippocampus (table 2) among other

areas 5 minutes or 24 hours after reaching stage 2 seizures, and 5

minutes after two consecutive stage 5 seizures (Zhao et al., in

preparation). Twenty-four hours after reaching stage 5 seizures,

the ME-LI increase in the ERC had moderated, and the elevation in

the hippocampus had disappeared. These data suggest that the

development of prepiriform-kindled seizures has more of an effect

on enkephalin in the ERC than on dynorphin in the hippocampus.

These data are supported by the observation that an ICV injection

of beta-fulnaltrexamine (a long-lasting mu receptor antagonist),

reduced the number of DPC stimulations required to reach stage 5

seizures by about 50 percent (Lee et al., unpublished observations).

These observations are consistent with previous reports which promote an anticonvulsant role of opioid peptides in electrically

stimulated seizures (Hardy et al. 1980; Frenk et al. 1979).

KAINIC-ACID-INDUCED SEIZURES

Kainic acid (KA) is an excitatory amino acid which causes hippocampal epileptiform activity and motor seizures. When applied

intracerebrally, it is most toxic to hippocampal pyramidal cells in

fields CA3 and CA4 (Nadler et al. 1978), which are innervated by

55
enkephalin- and dynorphincontaining mossy fibers. Several laboratories have employed this neurotoxin to provide a model for human

temporal lobe epilepsy (Ben-Ari 1985; Nadler 1981). Administration

of KA to rats by various routes elicits a repertoire of epileptiform

behaviors that depends upon the dose and the particular route of

administration. Systemic injection and injection into the amygdala

both produce a syndrome consisting mostly of wet dog shakes

(WDS) followed by gnawing, sniffing, and masticatory movements,

followed by repetitive head bobbing, rearing, forelimb clonus, and

sudden loss of upright posture (Ben-Ari et al. 1980). Hippocampal

injections of KA produce WDS, bilateral proptosis, forelimb clonus,

and episodes of hyperactivity characterized by circling away from

the injected side (Schwartz et al. 1978). Lower doses of intraventricular KA cause WDS, but higher doses of KA produce tonicclonic seizures (Lanthorn and lsaacson 1978). Usually, the seizures

last 3 to 6 hours depending on the doses used.

Effects of KA on the Metabolism of Hippocampal Enkephalins and

Dynorphins

We have employed KA as a tool to perturb the metabolism of opioid

peptides in the hippocampus so we could study in detail the molecular mechanisms regulating the biosynthesis, processing, and release

of these peptides. Initially we found that, 2 days after a single

intraventricular or intrastriatal dose of KA, the level of ME-LI increased significantly in the hippocampus and remained elevated for

2 weeks, as demonstrated by RIA (Hong et al. 1980). Subsequently,

we found that leu5-enkephalin-like immunoreactivity detected by

RIA and ICC was increased in mossy fibers and perforant path fibers, whereas the level of DN-LI was unchanged in mossy fibers 4

days after KA administration (McGinty et al. 1983). These alterations were our first indication that the metabolism of dynorphin

and enkephalin could be differently regulated, not only in the

same brain structure but in the same pathway.

Recently, we assayed hippocampal opioid peptides by RIA and ICC

at early time points after a single intrastriatal injection of KA

(Kanamatsu et al. 1986b; figure 2 and table 3). Three to six hours

after KA administration, when recurrent seizures were still apparent, DN-LI in mossy fibers was depleted 70 percent, and ME-LI in

mossy fibers and in the perforant path was depleted by 30 to 40

percent. By 24 to 48 hours, the levels of both peptides had risen

above normal, with DN-LI peaking at 48 hours at 171 percent of

56
normal, and ME-LI at 270 percent of normal. However, by 72

hours, ME-LI levels had risen to 300 percent of normal, especially

in the mossy fibers, while DN-LI in the mossy fibers had returned

to normal. The reduction in DN-LI and ME-LI within 3 to 6 hours

of KA administration suggests an increase in the release of these

peptides during the seizure period.

TABLE 3. Kainic-acid-induced changes in hippocampal opioid

peptide levels

Kainic Acid

(Intrastriatal) Dynorphin A Met5

-enkephalin

3 hours

8 hours

24 hours

48 hours

72 hours

-40% -30%

-70% -40%

+35% +95%

+65% +170%

±0% +200%

To determine whether the increase in hippocampal ME-U elicited

by KA reflected changes in ME biosynthesis, levels of mRNA coding

for preproenkephalin (mRNAenk) and cryptic ME-LI cleaved by enzyme digestion from preproenkephalin were measured (figure 3).

Following the convulsant period (8 hours), mRNAenk was 400 percent of control, and by 24 hours, cryptic ME-LI was 300 percent of

control. Increases in native and cryptic ME-LI and mRNAenk were

also noted in the entorhinal cortex but not in the hypothalamus or

uninjected strlatum. Our data suggest that KA-induced seizures

cause an increase in ME release, followed by a compensatory increase in ME biosynthesis in the hippocampus and entorhinal

cortex.

The finding that hippocampal mRNAenk was increased following KA

administration is consistent with our previous finding that protein

synthesis inhibition by cycloheximide attenuated the late increase in

hippocampal ME levels of KA-treated animals without affecting ME

levels of controls (Hong et al. 1980). Work is under way in our

laboratory to determine the effects of KA on preprodynorphin

mRNA biosynthesis. It is likely that our mRNA studies will indicate an increase in KA-induced dynorphin biosynthesis because

57
FIGURE 3. Hippocampal opioid peptide and mRNA levels after

intrastriatal kainic acid administration

NOTE: (A) Levels of cryptic and native met5-enkephalin and mRNA coding for

preproenkephalin 3 to 72 hours after kainic add. (B) Levels of

dynorphin A (1-8) 3 to 72 hours after kainic add.

SOURCE: Kanamatsu et al. 1986b, Copyright 1986, Society for Neuroscience.

cycloheximide, coadminlstered with intra-amygdaloid KA, has been

shown to block the late increase in hippocampal dynorphin immunoreactivity, indicating that proteln synthesis is necessary for the

effect to occur (Larson et al. 1963).

Possible Roles of Hippocampal Opioid Peptides in KA-Induced WDS

Several studies indicate that release of enkephalins from neurons

either within or afferent to the hippocampal formation is critical to

KA-induced WDS. ICV injections of enkephalin (Leybink et al.

1978; Wei et al. 1977; Drust and Connor 1983), beta-endorphln

(Bloom et al. 1976), or kainic acid (Lanthorn and lsaacson 1978)

cause naloxone-reversible WDS, suggesting that both agents produce

WDS by actions at opiate receptors. Electrolytic lesions of the

CA3 but not the CA1 region of the hippocampus attenuate WDS

produced by ICV injections of KA (Isaacson and Lanthorn 1981),

58
suggesting that WDS-producing agents act on CA3 neurons rather

than on their CA1 targets. In view of the release of both enkephalin and dynorphin when hippocampal mossy fibers are activated

(Chavkin et al. 1983), we have carded out a series of experiments

to distinguish the possible roles of opioid peptides In the expression

of WDS (Grimes et al., in press).

First, we have confirmed that pretreatment with naloxone attenuates KA-elicited WDS. We injected into rats 2.5, 5.0, or 10.0

mg/kg of naloxone intraperitoneally. Five mg/kg of naloxone reduced the number of KA-induced WDS by approximately 50 percent.

To determine which opioid peptide participates in KA-induced WDS,

we injected ICV antisera against either met5-enkephalin or dynorphin A (1-8) 4 minutes before KA was administered subcutaneously.

Antisera against met5-enkephalln, but not dynorphin A (1-8), significantly attenuated WDS. These data indicate that enkephalin, but

not dynorphin, may be associated with KA-induced shaking

behavior.

The cellular basis for KA- and opioid-induced WDS was explored by

injecting D-ala2-D-leu5-enkephalin (DADLE) and dynorphin A (1-8)

into ventral or dorsal hippocampus. Ten μg of DADLE injected

into ventral, but not dorsal, hippocampus elicited 50 to 100 WDS

within a 20-minute period, whereas the same dose of dynorphin

elicited less than 10 WDS.

Finally, colchicine injections into ventral hippocampus abolished

KA-induced WDS but not seizures. We injected colchicine, a cytotoxin which selectively destroys dentate granule cells, into dorsal

and/or ventral hippocampus to determine which part of the hippocampus is important for the expression of KA-induced WDS. Colchicine caused equivalent decreases (81 percent) of ME-LI in dorsal

and ventral hippocampus; however, the concentration of ME-Li was

nearly four times greater in ventral than in dorsal hippocampus.

Therefore, colchicine caused a greater absolute ME-LI decrease in

the ventral than in the dorsal region of this structure. Immunocytochemical and histological data showed that the enkephalin decrease was largely attributable to a destruction of dentate granule

cells and the concomitant reduction in staining of the mossy fiber

axons.

59
Following KA administration, WDS were not observed in rats with

colchicine-induced lesions of the ventral hippocampus. Lesions

restricted to the dorsal hippocampus caused a nonsignificant attenuation of KA-induced WDS. These studies suggest that the ventral

hippocampus is an important site for KA-induced WDS. The far

greater concentration of enkephalin in ventral than in dorsal hippocampus offers an explanation of why wlchicine lesions of this region were sufficient to eliminate KA-induced WDS, and why the

action of anti-met5-enkephalin was most effective in reducing KAinduced WDS in the ventral hippocampus.

In contrast to the effects on WDS, none of the wlchicine lesions

had effects on KA-induced behaviors such as facial and forelimb

clonus, rearing, and hypersalivation. These findings do not rule out

the possibility that enkephalin plays a role in hippocampal seizures,

but they do suggest that enkephalin released from mossy fibers is

not essential for seizure expression.

SUMMARY

The evidence accumulated so far indicates that seizure activity exerts profound changes on the metabolism of opioid peptides in the

hippocampus. Our data consistently show a large transient decrease

in dynorphin and a modest decrease in enkephalin in the hippocampus following either a single ECS or KA injection. These initial

reductions, which are indicative of increased release, may trigger

the biosynthetic process of hippocampal opioids and result in an

overproduction of the peptides seen in the rebound phase. However, the amount and timing of the rebound in enkephalin and

dynorphin levels in response to repeated ECS, amygdaloid kindling,

or KA differ drastically: a rapid and sustained increase in ME-LI

follows all three treatments, in contrast to a slow recovery after a

large and sustained decrease in DN-Ll induced by repeated ECS and

amygdalold kindling. These results, which are unique to the hippocampus, suggest that differential mechanisms are operative in

regulating the metabolism of these two opioid peptides in the

hippocampus. lt is likely that a well-coordinated regulation of

hippocampal function can be achieved through the differential release of enkephalin and dynorphin and their subsequent interactions

at different subtypes of opioid receptors following seizure

activities.

60
From a functional point of view, our data provide a neurochemical

correlate of previous reports that brain opioid peptides may mediate

ECS-induced behavioral alterations, such as changes in seizure

threshold, postictal depression, and retrograde amnesia. The robust

changes in the levels of opioid peptides in kindled rats, plus shortening of the kindling process by pretreatment with mu opioid

antagonists, strongly suggest the involvement of brain opioid pep

tides in the development of kindling. Finally, these studies show

clear evidence that enkephalin in the hippocampus is important in

KA-induced WDS, a component of the opiate withdrawal syndrome

in rodents (Isaacson and Lanthom 1981). Further studies should

help distinguish the regulatory mechanisms responsible for changes

in opioid peptide metabolism during states of hyperexcitability in

the hippocampai formation.

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ACKNOWLEDGMENTS

Supported in part by U.S. Public Health Service grant DA 03982

(JFM).

AUTHORS

Jau-Shyong Hong, Ph.D.

Laboratory of Behavioral and

Neurological Toxicology

National Institute of Environmental

Health Sciences

P.O. Box 12233

Research Triangle Park, NC 27709

Jacqueline F. McGinty, Ph.D.

Department of Anatomy

East Carolina University

School of Medicine

Greenville, NC 27858

Laura Grimes, Ph.D.

Curriculum in Toxicology

University of North Carolina

at Chapel Hill

Chapel Hill, NC 27514

Tomoyuki Kanamatsu, Ph.D.

Department of Physiology

Toho University

Tokyo, Japan

Johnny Obie, B.S.

Laboratory of Behavioral and

Neurological Toxicology

National Institute of Environmental

Health Sciences

P.O. Box 12233

Research Triangle Park, NC 27709

65
Clifford L. Mitchell, Ph.D.

Laboratory of Behavioral and

Neurological Toxicology

National Institute of Environmental

Health Sciences

P.O. Box 12233

Research Triangle Park, NC 27709

66
Opioids in the Hippocampus:

Progress Obtained From In Vivo

Electrophysiological Analyses

Steven J. Henriksen, James B. Wiesner, and

Guy Chouvet

lNTRODUCTlON

The earliest evidence implicating a role for endogenous opioids in

hippocampal function derived from a survey of the actions of

electrophoretically applied opioid peptides on various central nervous system (CNS) structures (Nicoll et al. 1977). The unique excitatory actions of opioid peptides on the spontaneous discharge of

presumed hippocampal pyramidal neurons, when compared to inhibition of neuronal discharge observed for most other brain areas,

provided the impetus for an increasingly detailed analysis of the

actions of opioids in this structure (see Henderson (1983), Corrigall

(1983), and Siggins and Gruol (1986) for reviews). In parallel

studies, potential clinical relevance of hippocampal opioids was provided by a series of investigations conducted in unrestrained, freely

moving animals. Similar studies conducted in several laboratories

demonstrated that minute doses of intracerebroventricularly or

intracerebrally administered opioid peptides elicited nonconvulsive

epileptiform seizures in rats (Urca et al. 1977; Henriksen et al.

1978; Tortella et al. 1978; Elazar et al. 1979; Cain and Corcoran

1984). Significantly, these electrographic seizures were found to

have their origin in limbic structures, the major apparent focus

being the hippocampus (Henriksen et al. 1978; Henriksen et al.

1983b), and could be elicited with doses of opioids that were devoid

of analgesic or other overt behavioral signs (Henriksen et al. 1978;

Frenk 1983). In the past 5 years, with more precise localization of

hippocampal opioids and their receptor subtypes (McGinty, this

volume; Gall, this volume; Herkenham and McLean, this volume), a

systematic and more relevant functional analysis of hippocampal

opioids has been possible. The following essay will review the

67
critical hippocampal studies employing in vivo methodologies and

will summarize the most recent evidence concerning the potential

roles opioids may play in hippocampal function and pathology.

ENCEPHALOGRAPHIC INVESTIGATIONS

As with the study of many other candidate neurotransmitters in the

CNS, electrophysiological observations of opioid effects on neurons

preceded precise immunohistochemical localization of hlppocampal

opioids and/or their receptors. Early studies employing intracerebroventricular injections of the opioid peptides beta-endorphin

and leucine-enkephalin suggested a proconvulsant action for these

substances. Figure 1 illustrates an example of the onset of such an

event following beta-endorphin administration in a rat.

The multichannel encephalographic record shows the abrupt initiation of high-voltage paroxysmal discharges recorded from numerous

sites. Systematic bipolar electrode recordings of subcortical brain

loci strongly suggested, however, that the hippocampus and amygdala were the primary origins of this activity (Henriksen et al.

1978). Subsequent investigations using carbon-14-labelled 2-

deoxyglucose ([14C]2-DG) in conjunction with intracerebroventricular beta-endorphin injections demonstrated conclusively that

the brain areas showing major increases in 2-DG uptake following

beta-endorphin coincided with the sites where the greatest

electrical abnormalities were recorded (figure 2).

In contrast, most other brain areas examined showed significant

decreases in 2-DG uptake during endorphin-induced seizures. These

early studies suggested that opioid peptides could produce pathological alterations in limbic neural activity and that the hippocampus

could play a major role in the elaboration of these epileptiform

events. ln an attempt to generalize these results to other species,

similar experiments employing a variety of other animals have been

conducted using beta-endorphin (Henriksen and Ehlers, unpublished

results). lntracerebroventricular injection of opioid peptides to

squirrel monkeys, cats, or rabbits have failed to evoke the type of

paroxysmal electrical discharge observed in rats (figure 3).

Although intermittent isolated high-voltage sharp waves were frequently elicited, nothing akin to the frank ictal seizure episodes

were observed, even at dose levels producing behavioral analgesia

68
FIGURE 1. Multitrace EEG record of beta-endorphin-induced

epileptiform activity in the rat

NOTE: Beta-endorphin was administered intracerebroventricularly in a total dose

of 3 nM at the end of the baseline trace. The top three traces are bipolar recordings of cortical EEG; the fast three traces are bipolar

recordings taken from deep electrodes in dorsal and ventral hippocampus.

and catatonia. In fact, in certain animal species exhibiting spontaneous or stimulus-induced seizures, opioid peptides and certain

opiate alkaloids have proved to be anticonvulsant (Meldrum et al.

1979; Bajorek and Lomax 1982). Moreover, more recent studies in

rat have demonstrated pro- as well as anticonvulsant properties of

opioid peptides depending on the seizure model employed and the

specificity of the ligand for particular opioid receptor subtypes

(Tortella et al. 1981; Tortella et al. 1985; Tortella et al. 1987;

69
FIGURE 2. EEG traces and [

14C]2-DG autoradiographic micrographs

of the response to intracerebroventricularly administered beta-endorphin (3 nM) in a representative rat

NOTE: (A) Electrographic tracing of ictal seizures episode elicited by beta-endorphin (BE). (B) [

14C]2-DG autoradiographs of representative sections of control rat brain

and the brain of a rat exhibiting seizure shown in figure 2A following B-E.

Henriksen et al. 1978; Snead and Beardon 1980; Frenk 1983). The

role of hippocampal opioids in seizure activity has been further

complicated by the observation that spontaneous and artificially

70
FIGURE 3. lntracerebroventricular beta-endorphin in the squirrel

monkey

NOTE: EEG traces recorded from cortical and subcortical sites following the

administration of 200 μg of beta-endorphin intracerebroventricularly.

Note high-voltage slow cortical EEG associated with fast hippocampal

rhythms.

induced seizure episodes in experimental animals are associated with

changes in the postseizure content of opioids in this structure

(Hong et al. 1980; McGinty et al. 1983; Hong et al., this volume;

Gall, this volume). Finally, postictal behavioral as well as electrographic activity in several seizure models have been shown to be

sensitive to naloxone, indicating that endogenous opioids potentially

released during seizures may serve to decrease the probability of

subsequent seizures and the aberrant postictal and/or interictal

behaviors (Frenk et al. 1979; Caldecott-Hazard et al. 1984; Bajorek

et al. 1986).

To summarize, a decade of data obtained using electroencephalographic tools has suggested a multitude of intriguing relationships.

First, it appears that ventricularly administered opioids can produce

naloxone-reversible synchronized epileptiform events in rat hippocampi, and at least some paroxysmal activity in other species as

well. This indicates that opioids can act at receptors in the

hippocampus to facilitate specific patterns of excitation/inhibition

at the cellular level which result in the complex EEG waveforms

described above. Second, hippocampal opioids may also function to

71
attenuate and/or block the recurrence of ictal episodes by causing

a selective depression of cellular excitability. Last, the specific

opioid receptor subtypes, the specific distribution of endogenous

opioid ligands, and the localization of opioids within the intrinsic

circuitry of the various subfields of the hippocampal formation may

be the critical factors underlying the observed EEG actions of opioids and their antagonists.

CELLULAR lNVESTIGATlONS

Early in vivo extracellular studies of opioid effects on hippocampal

neurons were consistent with the EEG studies described above.

That is, electrophoretic application of endorphin peptides elicited

increases in spontaneous discharge of isolated hippocampal neurons

in contrast to the decreases in activity observed in other brain

areas (Hill et al. 1977; Nicoll et al. 1977; Zieglgänsberger et al.

1979). In addition, prolonged ejection of beta-endorphin from adjacent micropipettes elicited paroxysmal unfiltered potentials that

FIGURE 4. Effects of electrophoretically applied beta-endorphin on

hippocampal CA 1 neuron discharge and local EEG

NOTE: Upper trace: DC unfiltered recording (low gain) from microelectrode tip.

Beta-endorphin (150 nA) ejected during DC deflections. (Note sharp

waves emerging following beta-endorphin application.) Bottom trace:

Ratemeter record of individual CA1 neuron. (Note increase in spontaneous rate associated with beta-endorphin application.)

72
were synchronous with EEG-recorded epileptifom activity (French

and Siggins 1980) (figure 4). These data provided the first cellular

evidence substantiating the excitatory action of opioids on hippocampal neurons and demonstrated that this global effect could

occur as a direct result of the local application of opioid peptide,

rather than being mediated indirectly through distant, extrahippocampal processes.

These cellular electrophysiological findings were made in the absence of firm evidence of either a significant concentration of opiate receptors (identified by the use of tritiated naloxone), or of

substantial amounts of opioid peptide in the hippocampus. However,

with the demonstration first of enkephalin-like immunoreactivity in

the hippocampus (Gall et al. 1981) and, later, of dynorphin-like

immunoreactivity (McGinty et al. 1983; McGinty 1985), with a clear

difference in the distribution of these two peptides in the hippocampus, an impetus was provided to investigate the potential roles

of the endogenous opioids in those hippocampal areas of preferred

opioid distribution (Gall, this volume; McGinty, this volume).

Early immunohlstochemical investigations of endogenous oploids in

rodents demonstrated broad distribution in most limbic structures

but sparse labelling of cells in the hippocampus (Sar et al. 1978;

Bloom and McGinty 1981). More recent radioimmunoassay studies,

however, have revealed a significant concentration of leu5-

enkephalin (ENK)-immunoreactivity (IR) in the dentate-CA3 fields

with only half as much ENK-IR in CA1 and the subiculum (Bayon et

al. 1983).

The mossy fiber pathway originating in dentate granule cells and

projecting through the hilus to innervate CA3 pyramidal cell dendrites was originally thought also to contain primarily enkephalin

(Gall et al. 1981). The enkephalin antisera used in these earlier

immunochemical studies, however, also cross-reacted with dynotphin.

Subsequent histochemical studies using more specific antisera

(McGinty et al. 1983) have shown that mossy fibers primarily possess immunoreactivity for dynorphin(s) and a much lower level of

proenkephalin-derived opioids (Khachaturian et al. 1983; McGinty

et al. 1984, White et al. 1987). This pathway was previously

thought to be exclusively excitatory, releasing an acidic amino acid

such as glutamate or aspartate. The presence of prodynorphinderived peptides and an excitatory amino acid in the same pathway

has aroused speculation that these substances, and perhaps others

73
(Gall, this volume), may be cotransmitters within the same mossy

fiber terminals.

In addition, more recent hlstochemical studies of the rat CA1

hippocampal field demonstrate scattered dynorphin-IR and

enkephalin-IR cells and fibers (Bayon et al. 1983; Gall et al. 1981;

McGinty et al. 1984). Enkephalin-IR and proenkephalin-IR (but not

dynorphin-IR) are also observed in the rat entorhinal cortex in

cells that project to the hippocampus and dentate gyrus (McGinty

1985). Thus, opioid immunoreactivity in the three opioid peptidergic systems of the hippocampal formation represents the presence

of three different prohormonal systems: (1) predominantly prodynorphin-derived peptides in the dentate-CA3 mossy fiber pathway;

(2) predominantly proenkephalinderlved peptides in scattered interneurons in CA1 and CA3; and (3) proenkephalinderived peptides in

the entorhinal to dentate gyrus perforant pathways.

The localization of the opioid peptides in nerve cells and fibers of

the hippocampus suggest that they are likely transmitter candidates.

This possibility is supported by recent biochemical studies showing

that at least five related prodynorphin/neoendorphin derived peptides can be released from hippocampal slices by high K+

 in a

Ca2+dependent manner (Chavkin et al. 1983a). These include

dynorphin A(1-17), dynorphin A(1-8), dynorphin B, alphaneoendorphin, and beta-neoendorphin. Proenkephalin derived

peptides are also released in this preparation (Chavkin et al. 1983a;

Chavkin et al. 1983b). In light of acknowledged distributional

differences in endogenous hippocampal opioids, it is of heuristic

value to discuss separately the three major hippocampal cellular

fields-CA1, CA3, and dentate gyrus-and their respective responses

to exogenously applied opioids.

NEUROPHARMACOLOGY OF OPIODS IN AREA CA1

Due primarily to their dorsal location in the hippocampal formation

and resultant ease in isolation and recording stability, cells in the

CA1 field were early targets for analysis of opioid effects. In

CA1, enkephalin immunoreactlvity is distributed as a band of reactive vaticosities within the border of the stratum radiatum and

stratum lacunosum-moleculare.

When opioids, including the enkephalins beta-endorphin and dynorphin A, are ejected from micropipettes in the vicinity of CA1

74
pyramidal neurons in vivo, the consensus effect is to increase the

spontaneous discharge of these cells (see Henderson (1988) and

Siggins and Gruol (1986) for reviews). These effects are naloxone

reversible and therefore presumed to be mediated by opiold receptors. As previously demonstrated (see figure 4), large lontophoretic

currents of beta-endorphin during single-unit recording of CA1

neurons can trigger repetitive large DC potentials suggestive of

epileptiform activity (French and Siggins 1980). The single-unit

excitation and epileptiform activity are intrinsic to the hippocampus, since they can also be evoked by superfusion of opioids

into intraocular transplants of hippocampus (Taylor et al. 1979).

Considerable effort has been expended to analyze and comprehend

the mechanisms underlying the paradoxical excitatory effects of

opioids on hippocampal neurons (see Siggins and Gruol (1988) for a

review). In vivo studies of hippocampal neurons using gammaaminobutyric acid (GABA) blockade by bicuculline and blockade of

transmitter release by Mg2+ ions indicate that the excitatory

FIGURE 5. A model for opioid disinhibition

NOTE: Left panel: Poststimulus histograms before and during electrophoretic

application of met-5-enkephalin on a CA1 neuron. During control, antidromic activation is followed by a prolonged inhibitory sequence. This

inhibition (mediated by GABA interneurons) is blocked by met-5-

enkephalin electrophoresis. Right panel: Proposed model of disinhibition

(see text).

75
responses of hippocampal CA1 pyramidal neurons may be viewed as

indirect, resulting from a primary inhibitory effect of the opioid on

neighboring inhibitory interneurons, leading to excitation of pyramidal cells by dlsinhibitlon (Zieglgänsberger et al. 1979; Lee et al.

1980). Figure 5 illustrates the proposed model for this interaction.

Despite this evidence, extracellular studies cannot reveal the exact

mechanisms behind this unique opioid excitation. Several laboratories have therefore conducted intracellular studies using in vitro

preparations such as hippocampal slices or explant cultures.

Such intracellular studies of pyramidal cells of the hippocampus

CA1 have shown little or no direct effects on membrane potential

or input resistance of morphine or several opioid peptides (leu5-

enkephalin; met-5-enkephalin; D-ala2,D-leu5 enkephalin; D-ala2,Dleu5

-enkephalin amide; beta-endorphin) in concentrations up to

50 μM (Dingledlne 1981; Gahwiler and Herding 1981; Gruol et al.

1988; Haas and Ryall 1980). The lack of a direct membrane effect

on pyramidal cells is predicted by the disinhibition hypothesis.

Unfortunately, there has not been complete agreement as to the

mechanism by which the opioids alter synaptic potentials. Intracellularly, most studies on the hippocampal slice indicate that the

enkephalins and beta-endorphin primarily reduce the size of presumed GABAergic recurrent and feedforward inhibitory postsynaptic

potentials (IPSPs) in both the CA1 and CA3 fields, and perhaps in

the dentate gyrus as well (Gruol et al. 1988; Masukawa and Prince

1982; Nicoll et al. 1980; Siggins and Zieglgänsberger 1981). This

also supports the hypothesis of a disinhibltory mechanism. Disinhibltion appears to be exerted presynaptically to the pyramidal

cell since pyramidal cell responses to GABA, the likely transmitter

for these IPSPs, are not reduced by the opioids (Dingledine 1981;

Nicoll 1982).

However, in other hippocampal slice studies (Dingledine 1981; Haas

and Ryall 1980), only enhanced excitatory postsynaptic potentials

(EPSPs) or, conversely, a reduction of EPSPs (as well as IPSPs and

depolarizing glutamate responses) were observed in about one-half

of the pyramidal neurons studied (Siggins and Zieglgänsberger 1981).

lt is still uncertain whether the observed hippocampal opioid effects

are due to direct hyperpolarization of interneurons, to a true

modulation of transmitter release, or to a more remote action.

However, the bulk of the extracellular data (Dunwiddie et al. 1980;

Lee et al. 1980; Nicoll 1982; Zieglgänsberger et al. 1979) and

76
intracellular data (see Robinson and Deadwyler (I 981) and text

above) still support a disinhibitory mechanism of action for the

single-unit excitatory and epileptigenic effect of these opioid

peptides in the hippocampus (see Corrigall (1983) for a review).

Still, facilitation of excitatory transmission has not been ruled out

entirely. Some discrepancies between laboratories might be ascribed

to (1) different methods of afferent stimulation in vitro and difficulties in selectively stimulating pure excitatory or inhibitory

pathways; (2) the use of different opioid agonists acting on different opiate receptors; (3) the method of drug administration; and/or

(4) the fact that the hippocampal pyramidal cell is usually the

subject of these intracellular recordings, while the primary site of

opioid action appears to be presynaptic to this cell. However, a

preliminary intracellular study by Nicoll and Madison (1984) demonstrated that enkephalin analogues hyperpolarize hippocampal interneurons by increasing their potassium conductance, thus directly

supporting the disinhibiiion hypothesis. in another recent in vitro

extracellular investigation, Raggenbass et al. (1985) studied the

action of a variety of opioids selective for certain opioid receptor

subtypes on nonpyramidal neurons in the CA1 region. These

authors found that the mu-selective agonist [D-Ala2-MePhe4-Glyol5

-enkephalin (DAGO), but not the kappa-selective agonist

U50,488H, decreased the spontaneous discharge of presumed inhibitory interneurons in a naloxone-reversible fashion. These cellular

data support earlier field-potential studies (see below) suggesting

that the indirect, excitatory action of opioids in all hippocampal

cell fields studied may be mediated by a mu- and perhaps a deltaopioid receptor (Chavkin et al. 1985; Chavkin et al., this

.

Another important electrophysiological tool used successfully to

assess the effects of opioids in the hippocampus is the measurement

of cellular excitability following afferent pathway stimulation. Because of the precise laminar arrangement of neurons, dendrites,

and axons in the hippocampus, large synaptic potentials can be recorded with electrodes situated in selected strata of the hippocampal formation. These potentials are believed to represent local

synchronous synaptic input evoked by afferent stimulation

(Anderson et al. 1971). Although few studies have investigated the

effects of opioids on these stimulated potentials in vivo, the existing data support the proposition that opioids elicit disinhibitory

effects when tested in this assay system. Early studies by

Linseman and Corrigall (1982) demonstrated that systemic morphine

77
potentiated the population field potentials evoked by stimulation of

the afferent pathways to both CA1 and the dentate gyrus in the

freely moving rat. Dunwiddie et al. (1980) described similar results

for d-met-5-enkephalin amide in the CA1 region in urethaneanesthetized rats and, further, demonstrated that the action of the

opioid could be mimicked by the GABA antagonist picrotoxin. In

addition to the in vivo studies described above, numerous in vitro

investigations have examined the effects of opioids in the CA1 area

of the hippocampal slice preparation (Dingledine 1981; Dunwiddie et

al. 1980; Gawiler and Herding 1981; Gruol et al. 1983; Haas and

Ryall 1980; Masukawa and Prince 1982; Nicoll et al. 1980; Roblnson

and Deadwyler 1981; Siggins and Zieglgänsberger 1981; and many

more). In reviewing these data, two flndings emerge as wmmon

denominators. First, superfusion or local application of opioid

peptides increases the excitability of neurons in CA1 as documented

by an increase in the evoked population spike and field EPSP.

Second, opioids frequently give rise to multiple afterpotentials,

which follow the primary population spike and are likely to be the

result of the opioid-mediated decrease in recurrent inhibition (see

Corrigall (1985) for a review).

In an attempt to define more precisely the opioid receptor type

mediating these effects on evoked field potentials, two pharmacological strategies have been employed. First, several investigators

have used highly selective opioid receptor agonists to determine the

specificity of opioid action in the hippocampus (Iwama et al. 1986;

Vidal et al. 1984; Wiesner and Henriksen 1987a; Raggenbass et al.

1985; Chavkin et al., this volume). Second, Chavkin et al. (1985)

employed selective receptor inactivation in conjunction with selective opioid agonists to further characterize the opioid receptor sub

types mediating the alteration in cellular excitability. Roth lines of

investigation have revealed that, throughout the hippocampus, the

opioid-induced increase in cellular responsiveness is mediated by

both mu and delta opioid receptors. The kappa-selective opiates

U50,488H and ethylketocyclazocine are inactive when tested in

physiologically meaningful dose ranges (Chavkin et al., this volume).

In summary, the findings from in vivo investigations of opioid

neuropharmacology in the CA1 region of the hippocampus are consistent with the pharmacological actions of opioids observed in both

freely moving animals and in vitro preparations. These effects are

also consistent (Dingledine 1981) with the observed anatomical distribution of opioids in this hippocampal subfield (Gall, this volume;

78
McGinty, this volume) as well as the regional distributions of opioid

receptor subtypes as assessed by ligand-binding studies (Herkenkam

and McLean, this volume). The main effect of exogenously applied

opioids is to decrease tonic and recurrent inhibitory processes in

presumed GABAergic interneurons, leading to an increase in spontaneous discharge rate of individual CA1 pyramidal neurons and an

increase in neuronal excitability assessed by afferent stimulation.

Despite the relative unanimity regarding the major pharmacologic

actions of opioids in CA1, other questions regarding the role of

opioids in hippocampal function remain: (I) There is no known

neurophysiological event altered by the systemic or local administration of the opioid antagonist naloxone (but see Gallagher (this

volume)); (2) controversy still attends the explanation of the cellular mechanism underlying disinhibition in CAI; and (3) substantial

quantitative differences are seen in opioid-induced effects from in

vivo compared to the in vitro preparation. These unresolved

issues, not discussed in detail here, suggest that the hippocampal

slice preparation may have severely compromised intrinsic inhibitory

processes that may in turn taint opioid responsiveness (Dunwiddie

et al. 1980; lwama et al. 1986; Durand et al. 1981), and/or anesthetics employed by neuropharmacologists in vivo may alter opioid

neuropharmacology or cellular responsiveness.

NEUROPHARMACOLOGY OF OPIOIDS IN AREA CA3

Far fewer studies of opioids in the hippocampus have been directed

toward pyramidal cells in subfield CA3. CA3 neurons, however,

provide a unique assay for opioid function in the hippocampus because, in addition to scattered enkephalinergic terminal and cell

bodies located in the zone stratum radiatum, CA3 neurons receive a

massive projection of prodynorphinderived peptides carried by the

mossy fibers of the dentate granule cells (Gall et al. 1981; McGinty

et al. 1983; Gall, this volume; McGinty, this volume). CA3 neurons

therefore have the potential for being acted upon by endogenous

opioids indirectly through the mechanism described for subfield

CA1 , as well as directly through prodynorphinderived peptides released from the mossy fiber terminals. Results from in vivo studies

of CA3 neuron responsiveness to exogenously applied opioids reflect

these complexities. Met-5

-enkephalin and its stable analogues uniformly increased the spontaneous discharge of CA3 neurons in in

vivo electrophoretic experiments (Henriksen et al. 1982a; Henriksen

et al. 1983a; Moises and Walker 1985). These effects were naloxone

reversible and similar to the responses seen for cells in the CA1

79
region of the hippocampus. They likely reflect the same disinhibitory processes proposed for those neurons (see above). In

contrast, prodynorphinderived opioids have less consistent effects

when applied to CA3 neurons in vivo. Henriksen et al. investigated

the effects of several prodynorphin-derived peptides (dynorphin A(1-17), dynorphin B, alpha-neoendorphin, dynorphin A(1-8),

and des-tyr-dynorphin A(2-17)) on spontaneous discharge of CA3

neurons as well as on cellular excitability (Henriksen et al. 1982a;

Henriksen et al. 1983a; Henriksen et al. 1983b). We observed that,

in halothane-anesthetized rats, dynorphin A(1-17), dynorphin A(1-8),

and dynorphin B, but not dynorphin A(2-17), had predominantly

excitatory (77 percent) effects on CA3 neurons. These effects were

naloxone reversible.

Figure 6 illustrates the prolonged excitatory effect of electrophoretically applied prodynorphin products on a CA3 neuron. Given

the results obtained in CA1, where mu agonist activity for dynorphin A(1-17) has been observed in vitro (Chavkin et al. 1985), and

the fact that the effect in CA3 can be blocked by the synaptic

uncoupling agent magnesium, the excitatory effects of dynotphin on

CA3 neurons probably reflects a disinhibiiory action of the peptide

similar to that observed for enkephalin in both cell fields. However, variable percentages of CA3 neurons respond to dynorphin

(and other kappa-receptor-selective opiates) with a decrease in

FIGURE 6. Excitatory effects of prodynorphin-derived peptides on

a CA3 neuron

NOTE: Electrophoretic application of dynorphin A(1-17), alpha-neoendorphin. and

dynorphin A(1-8). Note apparent relative potencies.

80
spontaneous discharge (Brookes and Bradley 1984, Moises and

Walker 1985; Henriksen et al. 1982a; Henrlksen et al. 1983b).

Figure 7 is an example of dynotphin- and ethylketocyclazocine (Win

38-197)-induced inhibltion of a CA3 neuron with partial naloxone

reversal. lt appears therefore that the direct effect of dynorphin

on CA3 neurons may be inhibitory, consonant with the direct

actions of opioid peptides in many other brain areas. Moreover,

the more selective the drug is for kappa opioid receptors, the

higher the percentage of inhibition of CA3 neuron one obtains,

suggesting that the observed inhibitory effects may be mediated by

kappa receptors. Figure 8 is an example of inhibition of a CA3

neuron elicited by the highly selective kappa ligand, U50,488H.

FIGURE 7. Inhibitory response to dynorphin

NOTE: Top: Ratemeter records from a CA3 hippocampal neuron during electrophoretk administration of dynorphin (dyn A(1-17)) and ethylketocyclazocine (Win 35-197). Bottom: Naloxone antagonism of dynorphininduced inhibition. Note that, even with low ejection current (5 nA) of

naloxone, there is a substantial decrease in the apontaneous firing rate,

between the ejectlng pulses, underlying a possible nonspecific effect for

naloxone.

81
FIGURE 8. Ratemeter record of a CA3 neuron during iontophoretic

applications of kappa agonist U50,488H

NOTE: Note the somewhat higher than usual spontaneous firing rate, whkh

permits observation of U50,488H Inhibition.

SOURCE: Siggins et al. 1986, Copyright 1982, Raven Press.

Preliminary intracellular studies of dynorphin effects on CA3

pyramidal neurons in the hippocampal slice are relatively consistent

with these extracellular findings: depending on the particular cell

study, either slow hyperpolarizing or depolarizing responses are

seen with low concentrations of dynorphin A and dynorphin B

(Gruol et al. 1983). The changes in membrane potential were occasionally associated with changes in discharge activii in a direction

opposite to that expected (Gruol et al. 1983), thus raising the

possibility of an indirect action of dynorphin. The depolarizing

responses were also not of the type expected of mossy fiber activation, being slow and not always capable of activating spikes. Similar mixed or weak effects in CA3 were seen in vitro with respect

to the action of dynorphins on extracellular field potentials evoked

by mossy fiber stimulation, although inhibitory action was sometimes observed (also see lwama et al. (1986)).

Preliminary studies in our laboratory on the effect of locally

applied dynorphin A(1-17) or naloxone on mossy fiber evoked field

potentials in vivo have yielded other intriguing results. Using pairs

of stimulus pulses to assess the action of opioids on sequential

82
recurrent inhibition and/or potentials, we have observed that

neither dynorphin nor naloxone has effects on either the primary

or secondary evoked population spike. The second evoked response,

however, is followed occasionally by an afterpotential. This afterpotential is potentiated by electrophoretically applied dynorphin

A(1-17) and inhibited by electrophoretically applied naloxone (see

figure 9).

These results imply that an opioid peptide released by mossy fiber

stimulation, although not altering the primary excitatory effects on

CA3 neurons, may serve as a modulatory agent enhancing s&sequent inputs to the CA3 area in a frequency-dependent fashion.

FIGURE 9. Effect of iontophoretically applied naloxone on CA3

field potentials evoked by mossy fiber stimulation

NOTE: Field potentials in (A) represent CA3 overlaid double pulse stimulation

responses (interstimulus interval was 185 msec). Note emergence of

potential at approximately 8 msec during second stimulus. In (B), note

decrease In this potential. In (C), recovery follows naloxone

electrophoresis. Response is plotted at left and represents the mean of

10 responses at each point.

83
NEUROPHARMACOLOGY OF OPIODS IN THE DENTATE GYRUS

As demonstrated by immunocytochemical studies (Gall et al. 1981;

McGinty et al. 1983), the fascia dentata of the hippocampus contains endogenous opioid peptides from various anatomical sources.

Three known sources of opioids observred in the dentate gyrus are

(1) an enkephalinergic pathway arising from the entorhinal cortex;

(2) intrinsic enkephalinergic local circuit neurons; and (3) the

dentate granule cells themselves, which contain predynorphinderived peptides (Christian et al. 1985; Linseman and Corrigall 1982;

Wiesner et al. 1988; Wiesner and Henriksen 1987a; Wiesner and

Henriksen 1987b). The few studies that have examined the

physiological effects of opioids in the dentate gyrus in vitro (Haas

and Ryall 1988; Tielen et al. 1981) or in vivo have focused on field

potentials evoked by stimulation of the afferent perforant path.

These studies have generally found exogenously applied opioids to

exert an “excitatory” effect in that the opioids enhanced evoked

potentials, indicating an increased response of the granule cells to

input from the perforant path. In this respect, the effect of opioids in the dentate gyrus appears to be similar to the effect of

opioids in the hippocampal subfields CA1 and CA3, where evoked

potentials are also enhanced (Corrigall 1983). This parallelism of

pharmacological effect is reinforced by the finding that, in a

fashion similar to that previously discussed for CA1 and CA3, opioids applied to the dentate gyrus decrease sequential inhibition as

determined by a “paired-pulse” paradigm (Wiesner et al. 1988).

While a disinhibftory action of opioids like that seen in the CA1

and CA3 subfields might be expected in the dentate gyrus, similar

effects of opioids on individual dentate granule cells have only

been recently reported (Wiesner et al. 1986; Wiesner and Henriksen

1987b; Chavkin et al., this volume). Using a prototypic opioid agonist (D-ala2

-met-5-enkephalin amide (DAMEA)), we have assessed

the effects of electrophoretic application on granule cell spontaneous discharge and responsivity to afferent input. Although

DAMEA has been used as a standard opioid agonist in other studies, we have determined that selective mu agonists, but not delta

or kappa agonists, produce comparable results (Wiesner and

Henriksen 1987a) (but see Chavkin et al. (this volume)).

To evaluate recurrent (or feedforward) inhibition characteristics of

hippocampal circuitry, a paired-pulse paradigm was employed before

and during each opioid application. This paradigm involved pairs of

stimulus pulses (a “conditioning” pulse followed by a “test” pulse)

84
with varying interpulse intervals. Under control conditions, the

test response is normally inhibited at short interpulse intervals

(less than 50 msec) but facilitated at longer interpulse intervals (50

to 140 msec). During the DAMEA application, the inhibition characteristic of short interpulse intervals was decreased, resulting in

significantly greater test responses at 20- and 40-msec interpulse

intervals. The decreased inhibition in the presence of opioids resembles that which is induced by certain other opioid peptides

(Wiesner and Henriksen 1987a) and is reversed by naloxone.

Recordings of single units within the granule cell layer allowed

testing of their response to opioid peptides. Electrophoretic application of DAMEA consistently inhibited the spontaneous activity of

dentate units while concurrently increasing the amplitude of evoked

population spikes (figure 10). When unit responses to perforant

path stimulation could be identified superimposed on the field

potentials (5 to 15 msec poststimulus), an increased number of

evoked action potentials was seen during DAMEA application, suggesting that evoked unit activity was increased even though spontaneous cellular activity was suppressed.

The inhibition of spontaneous unit activity (and the concurrent

increase in population spike amplitude) by DAMEA was antagonized

by naloxone. The results of this study again confirm that exogenously applied opioids can enhance the excitatory response of the

dentate gyrus to perforant path stimulation, and that this effect is

associated with a disinhibition of the evoked response as determined by paired-pulse stimulation. In addition, the results have

demonstrated a concurrent inhibition of spontaneous granule cell

activity by an enkephalin analogue. Although this inhibitory effect

of enkephalin is typical of that seen in most areas of the brain

(North 1979), it is surprising in light of the opioid effects reported

in other hippocampal areas and presents an interesting paradox with

respect to the “excitatory” effect of enkephalin on responsiveness

to perforant path stimulation. It is possible that these divergent

opioid effects may be causally related, although it seems likely that

the effects may be mediated through separate mechanisms. While

these mechanisms remain obscure, the enhanced response to perforant path input may result from a disinhibitory effect of opioids;

this effect could be opposed by a separate, perhaps more direct

inhibitory opioid effect. Alternatively, it is conceivable that the

decreased spontaneous firing rate may predispose the cell to an

increased synchrony of response to incoming stimuli.

85
FIGURE 10. Concurrent effects of DAMEA on population spike

amplitude (PS amp) and on spontaneous tiring rate of

a single unit of the granule cell layer

SOURCE: Wiesner and Henriksen 1987a, Copyright 1987, Elsevier Ltd.

These results underscore the importance of caution in interpreting

effects of drugs as “excitatory” or “inhibitory” based on studies of

evoked potentials alone. Evoked activity such as that reflected by

dentate field potentials must be considered a specific measure of

transmission within the individual neural pathway involved.

CONCLUSION

lt is clear from the foregoing review of in vivo opioid neuropharmacology that opiold peptides exist in the hippocampus and

can dramatically influence neuronal excitability. The question still

remains as to what degree endogenous opioids are functionally involved in hippocampal physiology. However, in light of the data

described, the presence of multiple opiate receptors differentially

distributed in the hippocampus could explain the complex cellular

and EEG effects of the various opioid peptides. These multiple

receptors and their distributions could therefore clarify a possible

role of opioid peptides in normal hippocampal cellular activity as

well as in limbic epilepsy and drug abuse. Hence, opioid peptides

of the beta-endorphin-enkephalin class and morphine-like opiate

86
alkaloids, which act with some degree of selectivity on mu and delta receptors, appear to be predominantly excitatory in the hippocampus. The dynorphins exhibit both excitatory and inhibitory

effects, perhaps via actions at both mu and kappa receptors. lt

seems likely that the predominant receptors involved in opioid effects in the CA1 cell field of the hippocampus and the dentate

gyrus are of the mu and delta subtypes, whereas kappa receptors

also may be involved in CA3 opioid response. lt seems apparent

that a multidisciplinary approach will continue to be required before the potential roles of the hippocampal opioids will be finally

characterized.

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AUTHORS

Steven J. Henriksen, Ph.D.

James B. Wiesner, Ph.D.

Division of Preclinical Neuroscience

and Endocrinology

Department of Basic and Clinical Research

Research Institute of Scripps Clinic

La Jolla, CA 92037

Guy Chouvet, Ph.D.

Groupe de Recherche de Neurochimie

Functionnelle

INSERM

St. Genis-Laval, France

93
Opioid Receptor Mechanisms in the

Rat Hippocampus

Charles Chavkin, John F. Neumaier, and

Elizabeth Swearengen

INTRODUCTION

When the actions of opiates were measured on individual neurons,

the predominant effects seen throughout the nervous system were

membrane hyperpolarization and decrease in firing rate (Nicoll et

al. 1977; Duggan and North 1984; Henderson 1983). An opioidinduced increase in excitability shown by hippocampal CA1 pyramidal cells was therefore viewed as an anomaly. The ionic basis for

opioid actions outside the hippocampus has been shown to be an

increase in membrane conductance to potassium following activation

of mu or delta opioid receptors, or alternatively, a decrease in a

voltage-dependent calcium conductance following activation of

kappa opioid receptors. Investigation of the response to opioids

within the CA1 region of the hippocampus revealed that the direct

action of opioids was also an increase in potassium conductance

(North 1986). To account for opioid-Induced excitation in the

hippocampus, several groups have proposed that opioids hyperpolarize a population of interneurons and thereby inhibit the

release of an inhibitory transmitter that normally controls the

excitability of the CA1 pyramidal cells (Henderson 1983). In this

article, we review some Of the recent evidence supporting this

hypothesis and present data suggesting that opioids act via a

similar cellular mechanism in the dentate and CA3 regions of the

hippocampus.

IONIC BASIS OF OPIOID ACTlON

Wen and Macdonald characterized the opioid receptors and ionic

transduction mechanisms involved in mediating the actions of

94
opioids by studying the electrophysiological effects of opioids

applied to fetal mouse dorsal root ganglion (DRG) cells in culture

(Wen and Macdonald 1983; Wen and Macdonald 1984; Macdonald

and Wen 1986). In this preparation, opioids decrease calciumdependent action potential duration (Werz and Macdonald 1983), as

was reported previously for cultured chick DRG cells (Mudge et al.

1979). In addition, kappa-specific opioids also selectively reduced

the voltage-sensitive calcium conductance (Werz and Macdonald

1984; Werz and Macdonald 1985); however, the mechanism of action

for kappa receptors was different from that for mu or delta receptors. Intracellular recordings showed that electrodes filled with

cesium acetate (blocking potassium conductances) completely inhibited the effects of [Leu5

]enkephalin (delta agonist) and morphiceptin (mu agonist), whereas cesium did not affect the reduction in

calcium conductance caused by dynorphin-A(1-17), dynorphinA(1-8), dynorphins, and alpha-neoendorphin (kappa agonists). The

authors therefore concluded that kappa receptors primarily control

a calcium channel in mouse DRG cells, whereas delta or mu receptor activation decreases calcium conductance by directly increasing

potassium conductance (Werz and Macdonald 1984; Wen and

Macdonald 1985).

Opioid receptor-linked conductance changes were also measured in

guinea pig myenteric neurons. Neurons in the myenteric ganglia

respond to application of mu-selective opioids with a 3-to 7-mV

hyperpolarization accompanied by a decrease in membrane resistance

(Morita and North 1982). Opioids also prolong the calciumdependent hyperpolarization that normally follows a burst of action

potentials (IK(Ca)) (Tokimasa et al. 1981). These effects were

shown to be mediated by mu receptor activation by the sensitivity

of a variety of opioids to antagonism by beta-funaltrexamine

(beta-FNA) (Cherubini and North 1985), a mu-receptor-alkylating

agent (Portoghese et al. 1980; Takemori et al. 1981). Several

observations demonstrated that the ionic basis of the hyperpolarization is an opioid-induced increase in potassium conductance:

(1) the amplitude of the hyperpolarization depends on the external

potassium concentration: (2) the reversal potential of the opioidactivated current is about -100 mV, a value close to the potassium

equilibrium potential of these cells (Morita and North 1982;

Tokimasa et al. 1981); and (3) potassium channel blockade by

intracellular cesium injection blocks the mu receptor-mediated

hyperpolarization (Cherubini and North 1985).

95
Kappa agonists (e.g., dynorphin-A(1-17), tifluadom, and U50,488H)

affect guinea pig myenteric neurons differently than delta and mu

agonists. Rather than membrane hyperpolarization, cells respond

with a reduction in the duration of the calcium-dependent action

potential (Cherubini and North 1985). Kappa agonist action is

distinguishable from mu receptor activation in the same preparation

by the insensitivity of kappa agonists to beta-FNA treatment, and

by either the intracellular injection of cesium or the addition of

low concentrations of barium to the super-fusion buffer to block

potassium channels (Cherubini and North 1985; Cherubini et al.

1985). The results of experiments using these techniques suggest

that kappa receptors are present on myenteric neurons and are

coupled to calcium channels. Thus, kappa and mu receptors in this

tissue, as in mouse DRG neurons, are apparently linked to different

transduction mechanisms, although both may reduce calcium influx

during the action potential.

Analysis of opioid effects on locus coeruleus neurons in rat brain

slices demonstrates that mu receptor activation causes a membrane

hyperpolariration of these neurons as well (Pepper and Henderson

1980; Williams et al. 1982; Williams et al. 1984). Experimental

manipulations similar to those done for the guinea pig myenteric

neurons show that the membrane hyperpolarization is due to an

increase in potassium conductance. In addition, quinine (200 μM),

which blocks specific potassium channels in vertebrate and invertebrate neurons (Walden and Speckmann 1981; Yoshida et al.

1986), also blocks the opioid-activated potassium conductance in

locus coeruleus neurons (North and Williams 1985). These data

indicate that mu opioid receptors are linked to similar transduction

mechanisms not only in different tissues but in different species as

well.

Delta receptors have been characterized in neurons of the submucous plexus of the guinea pig ileum. These neurons were hyperpolarized following application of delta agonists, [Met5

]enkephalin,

[Leu5]enkephalin, [D-Ser2,L-Leu5,L-Thr6]enkephalin (DSLET), or

[D-Ala2,D-Leu5]enkephalin (DADLE) (Surprenant and North 1985;

Mihara and North 1986). Opioid effects on the submucous plexus

neurons were found to be mediated by delta receptors as shown by

the apparent dissociation constants of naloxone and ICI-l 74864 calculated by Schild analysis (Schild 1947; Schild 1949) and by the

insensitivity of the cells to either mu or kappa agonists. The conductance change was very similar to that controlled by mu receptor

96
activation in the myenteric plexus (i.e., hyperpolarization that

reversed at the potassium equilibrium potential and appeared to

result from an outward potassium current).

Based on studies of the submucous plexus in vitro and of DRG cells

in culture, the properties of the delta-receptor-activated potassium

channel were indistinguishable from the mu receptor channel. The

similarity In mechanism may reflect a close evolutionary relationship between the mu and delta receptors and suggests that there

may be other common features. In contrast, kappa receptors have

been shown to control calcium conductance. However, the net effect on cell excitability may be the same regardless of which

mechanism is activated. Increased potassium conductance reduces

excitability of the postsynaptic cell and decreases transmitter release at the nerve terminal. lnhibition of calcium conductance

also reduces postsynaptic cell excitability by reducing the calcium

component of the action potential. Reduction of transmitter release

may result if kappa receptors are also located on the nerve

terminals.

ENDOGENOUS OPIOlD IN THE HIPPOCAMPUS

Interest in the actions of opioids in the CA3 region of the hippocampus was greatly stimulated by the immunocytochemical localization of proenkephalin- and prodynorphinderived opioids in the

dentate gyrus and mossy fiber pathway of the rat hippocampus (Gall

et al. 1981; Gall 1984; McGinty et al. 1983; McGinty et al. 1984).

Neurochemical and immunohistochemical studies of the hippocampus,

dentate gyrus, and entorhinal cortex suggest the presence of two

separate opiold peptide projection systems in this complex (Chavkin

et al. 1985k Gall, this volume: McGinty, this volume).

To identify the molecular forms and quantify the regional contents

of the dynorphin-A and [Leu5]-enkephalin immunoreactivity, peptide

extracts of microdissected hippocampal regions were resolved by reversed phase high-pressure liquid chromatography (HPLC) and

molecular exclusion HPLC (Chavkin et al. 1983b). These studies

demonstrated that the dynorphin-A and [Leu5]enkephalin immunostaining corresponded to authentic dynorphin-A(1-17) and

[Leu5]enkephalin. The probable neurotransmitter roles of

[Leu5]enkephalin and dynorphin-A were further supported by our

demonstration of calcium-dependent release of these peptides after

in vitro depolarization of hippocampal tissue (Chavkin et al. 1983a).

97
Release of putative transmitters from hippocampal slices was induced by cellular depolarization using 50 mM KCI, veratridine, or

kainic acid added to the super-fusion buffer. The effluents were

assayed using specific radioimmunoassays for [Leu5]enkephalin and

each of the five known prodynorphin derived opioids. Although

transmitter release induced by chemical depolarization may not

precisely mimic endogenous release stimuli, the results of these

experiments show that each of the endogenous dynorphin oploids is

present in the hippocampus and has the potential of being released

by physiological stimuli.

Thus, certain minimal criteria necessary to establish a neurotransmitter role of the oploid peptides in the hippocampus have

been satisfied. Anatomical methods have identified the distribution

of the peptides within the hippocampus, and depolarization-induced

release of opioids has been shown. Opioid binding sites are present

in this region as demonstrated by membrane binding and slice autoradiography (Herkenham and McLean, this volume). Further characterization of the neurotransmitter role of the opioids in the

hippocampus requires that the actions of opioids be more completely

understood, that the cellular distribution and types of the receptors

mediating those effects be defined, and that endogenous opioid release be physiologically demonstrated. Significant progress has

been made toward these goals.

OPIOID ACTIONS IN THE CA1 REGION

Numerous groups have studied the electrophysiological effects of

opioids in the CA1 region of the rat and guinea pig hippocampi

(see Henderson (1983), Duggan and North (1984) for review). Most

opioids increase pyramidal cell firing rates and increase the electrical excitability of the CA1 region. The latter effect is most

clearly demonstrated by electrical stimulation of afferent fibers

(Schaffer collaterals) in the stratum radiatum. The effect of opioids under these conditions is to consistently increase the sensitivity of these cells to electrical stimulation measured by a decrease

in the stimulation intensity required to evoke a response. Another

consistent opioid effect is the generation of secondary responses

which follow the primary population spike (figure 1). Despite these

easily demonstrated effects on excitability, intracellular recordings

have shown that opioids do not change intrinsic membrane properties of CA1 pyramidal cells (Dingledine 1981; Siggins and

Zieglgänsberger 1981).

98
FIGURE 1. CA1 pyramidal cell population responses evoked by

afferent stimulation of Schaffer collaterals

NOTE: The stimuiating electrode was placed in the center of the stratum radiatum, and the recording electrode was placed at least 1 mm away in

the CA1 pyramidal cell layer. The figure is marked as follows: (a)

control, (b) 2 mM pentylenetetrazole, and (c) 1 μM normorphine.

Arrows mark the primary population spike, and asterisks mark the

afterpotential. Drugs were added to the superfusion buffer bathing the

tissue and allowed to reach steady state concentration before making

experimental measures (10 minutes). Drug effects were completely

reversible within 10 to 60 minutes. The size and number of afterpotentials are dose dependent.

The mechanism of the opioid effect on CA1 pyramidal ceil excitability has been studied by several groups and is thought to be

largely due to reduction of inhibitory input (e.g., excitation

resulting from disinhibition) as proposed by Zieglgänsberger et al.

(1979). Extracellular recording of CA1 interneurons in vitro support this hypothesis by showing that opioids directly reduce firing

rates and excitability (Lee et al. 1980). As predicted by the disinhibition hypothesis, DADLE and other opioids applied to the

hippocampal slice were found to directly hyperpolarize intemeurons

by increasing their potassium conductance, consistent with the

effects of delta receptor activation in myenteric neurons (Nicoll

and Madison 1984). The resulting reduction in excitability and

firing rate of the intemeurons could decrease gamma-aminobutyric

acid (GABA) release and reduce inhibitory postsynaptic potential

(IPSP) amplitude recorded in pyramidal ceils.

99
Raggenbass et al. (1985) also studied the effects of opioids on

nonpyramidal ceils in the CA1 region of the rat hippocampus.

[D-Ala2,MePhe4,Gly-ol5]enkephalin (DAGO) (a mu-selective agonist)

but not U50,488H (a kappa-selective agonist) suppressed the firing

of these cells. This action was direct, since elevated magnesium

and reduced calcium in the superfusion medium did not Mock the

effects of DAGO. The authors concluded that the mu receptor type

was activated, as the effect was shown to be produced by DAGO

but not U50,488H and was Mocked by naloxone. Presumably, the

inhibitory transmitter affected by opioids is GABA, aithough a

direct neurochemical measure of the effect of [D-Ala2]enkephalinamide on endogenous GABA release from hippocampal slices failed

to find any reduction in stimulated release (Fan et al. 1982).

To further test the disinhibition hypothesis of opioid action, it is

necessary to investigate the actions of opioid agonists in relation

to GABA and excitatory amino acid receptor antagonists. if opioids

increase CA1 pyramidal excitability by directly inhibiting GABA release from interneurons, then GABA antagonists should mimic opioid

effects. This is the case; a variety of GABA receptor antagonists

such as pentylenetetrazole, bicuculine, and picrotoxin produce multiple population bursts in response to afferent stimulation (Herron

et al. 1988; Hablitz and Langmoen 1986; Ashwood and Wheal 1986).

GABA antagonists reduce the IPSP and increase the excitatory

postsynaptic potential (EPSP), resulting in sufficient depolarization

to unmask N-methyl-D-aspartate (NMDA) receptors (Herron et al.

1986; Hynes and Dingledine 1984; King and Dingiedine 1985). NMOA

receptors are a specific subset of excitatory amino acid receptors

named for their most potent agonist (Watkins and Evans 1981).

During moderate stimulation of synaptic afferents, NMDA receptors

are not activated in the hippocampus. D-2-amino.-5-phosphonovalerate (D-APV), a selective NMDA receptor antagonist, has no

effect by itself on synaptic responses (Koerner and Cotman 1982;

Collingridge et al. 1983; Ashwood and Wheal 1986). However,

several NMDA receptor antagonists display potent anticonvulsant

activity in a variety of in vivo models of epilepsy (Croucher et al.

1982; Meldrum et al. 1983), and D-APV was shown to completely

block CA1 pyramidal cell afterpotentials produced by pentylenetetrazole (Herron et al. 1988).

We compared CA1 pyramidal cell population responses evoked in the

absence and presence of the GABA receptor antagonist pentylenetetrazole with responses evoked by the mu-selective opioid receptor

100
agonist normorphine (figure 1) (Swearengen and Chavkin 1987). In

both cases, drug application resulted in the appearance of a second

population burst (afterpotential) following the primary population

spike. As predicted, D-APV partially blocked normorphine-induced

afterpotentials in CA1 pyramidal cell population responses (figure

2), but did not affect the opioid-induced increase in sensitivity of

FIGURE 2. Effect of D-APV on stimulus-response characteristics of

normorphine-induced afterpotentials

NOTE: The stimulation and recording protocol was identical to that described

in figure 1. Application of normorphine (10 μM) to the tissue elicited a

single afterpotential as in figure lc. in the absence of normorphine,

the afterpotential was completely absent (open Mangles). The maximum

afterpotential amplitude was dependent on the concentration of normorphine in the superfusion buffer (closed triangles). Addition of D-APV

to the superfuslon buffer partially blocked the stimulation-dependent

rise in afterpotential amplitude (squares, 16 μM D-APV circles, 50 μM

D-APV

101
the primary population spike to afferent stimulation (data not

shown). Thus, the mechanism of opioid action must include, but

not be limited to, an unmasking of NMDA receptors on the CA1

pyramidal cells.

To further define the types of receptors that mediate opioid action

in the CA1 region, we used a selective receptor inactivation paradigm (Chavkin et al. 1985a). In that study, we measured the ability

of opioids to increase CA1 pyramidal cell sensitivity to electrical

stimulation after pretreatment of the rat hippocampai slice in vitro

with the irreversible mu-selective antagonist beta-FNA. Beta-FNAtreated slices were insensitive to the effects of normorphine and

[N-MePhe3,D-Pro4] morphiceptin (PL017). Moreover, the effects of

both dynorphin-A and dynorphin-B were almost completely blocked,

suggesting that these agonists activate mu receptors in CA1. In

contrast, the responses to DADLE and DSLET were not significantly

changed by beta-FNA treatment. Our interpretation of these results is that both mu and delta opioid receptors are involved in the

specific response to opioids in the CA1 region.

Additional receptor characterization was achieved by Schiid analysis

(Neumaier and Chavkin 1988; Neumaier et al. 1988). As expected

for compounds acting through mu opioid receptors, normorphine,

PLO17, and dynorphin-A(1-17) were found to be highly sensitive to

naloxone antagonism in CA1 (apparent dissociation constants were

between 1 and 3 nM for each) (figure 3) (Neumaier and Chavkin

1986; Neumaier et al. 1986). in contrast, the naloxone sensitivity

of DSLET was significantly lower than that of PLO17 (4.5 nM,

P<0.05, n=8).

These results support the conclusion that opioids in the CA1 region

of the rat hippocampus activate mu and delta but not kappa receptors, and that at moderate concentrations the dynorphin opioids can

activate mu receptors (Chavkin et al. 1985a). Although the population of opioid receptors acted on by DSLET is different from that

acted on by normorphine, PLO17, and dynorphin-A, the extracellularly measured response to DSLET is very similar to that found

with mu-selective agonists (figure 4a, b, and c).

In analyzing opioid effects in the CA1 region, it is important to

address the disagreement among the published reports on the

effects of dynorphin-A(1-17). We consistently see excitatory

actions of dynorphin-A in the rat hippocampal slice preparation

102
FIGURE 3. A comparison of the apparent dissociation constants of

naloxone at receptors activated by opioid agonists in

the CA1 and dentate regions

NOTE: Schild analysis of naloxone-induced shifts of agonist dose-response

curves yielded apparent dissociation constants (Ki values) for naloxone

at the receptor activated by each agonist used. Each value is the geometric mean plus or minus SEM of two to eight experiments. The muselective agonists normorphine and PL017 were used to determine the

naioxone Ki at mu receptors; these values were not significantly different from one another (ranked sum test). The Ki

 associated with DSLET

was significantly greater than that for PL017 (p<0.05) in both CA1 and

dentate regions. The dynorphin-A(1-17)-related value in CA1 was not

significantly different from that for the mu agonists; however, in the

dentate gyrus, the dynorphin-related value was significantly larger than

that for PL017 in dentate (p<0.01) and dynorphin in CA1 (p<0.05).

(Chavkin et al. 1985a; Neumaier and Chavkin 1986; Neumaier et al.

1986). Others report that dynorphin-A(1-17) is excitatory at low

103
FIGURE 4. DSLET causes afterpotentials in CA1 and dentate

regions

NOTE: Extracellular recordings were made from CA1 pyramidal cells (a, b, and

c) and dentate granule cells (d, e, and f) as described in the preceding

text, in both regions, control responses consisted of a stimulus artifact

followed by a fiber potential and a single synaptic population spike (a,

d). DSLET was applied using micropressure pipetes placed very close

to the recording site. The addition of DSLET (1 second, 40 psi, 100

μM) caused a secondary afterpotential in a dose-dependent manner in

both region (b, e). The DSLET effect reversed completely after a

washout of 1 to 3 minutes (c,, data not shown for dentate) and was

completely blocked by naloxone (40 nM) added to the superfusion bath

(f, data not shown for CA1).

concentrations (0.1 nM) but inhibitory at higher concentrations

(Vidal et al. 1984). It is crucial to note that, following in vivo

iontophoretic application of dynorphin-A(1-17), inhibition of extracellularly recorded single-unit firing rate was not reversed by

naloxone (Walker et al. 1982; Henriksen et al. 1982; Moises and

Walker 1995). Des-tyr-dynorphin-A(2-17), a dynorphin analogue

lacking opioid receptor affinity (Chavkin and Goldstein 1981), had

similar inhibitory effects that were insensitive to naloxone (Walker

et al. 1982). Thus, low to moderate concentrations (0.1 nM to 1.0

μM) of dynorphin-A(1-17) can activate mu receptors, producing

typical excitatory effects, while at higher concentrations pyramidal

104
cell activity is depressed by a nonspecific and naloxone-insensitive

mechanism.

In summary, current evidence indicates that the excitatory effects

of opioids in the CA1 pyramidal cell region are due to the presence

of functional mu and delta opioid receptors on nonpyramidal cells.

The question of whether single interneurons express both mu and

delta receptors has not yet been resolved. The data suggest that

CA1 intemeurons respond to opioids by increasing potassium conductance; the resulting, hyperpolarization may reduce GABA release

in the region. This reduction in inhibition of the pyramidal cell

leads to activation of NMDA receptors which mediates at least one

component of pyramidal cell excitation.

OPOID ACTlONS IN THE CA3 REGION

In the CA3 region, [Leu5]enkephalin and DADLE consistently

increase pyramidal cell excitability both in vivo (Henriksen et al.

1982; Moises and Walker 1985) and in vitro (Masukawa and Prince

1982; Gruol et al. 1963). In contrast, dynorphin-A(1-17) has

variable effects on CA3 pyramidal cell excitability (Gruol et al.

1963; Chavkin and Bloom 1986; lwama et al. 1986).

Henrisen et al. (1962) studied the effects of iontophoretic and

micropneumatic application of the dynorphins on hippocampal

pyramidal cell firing rates in vivo. The excitatory dynorphin

effects appear to be mediated by an opioid receptor, because the’

nonopioid analogue des-tyr-dynorphin-A(2-17) was inactive, and the

excitatory effects were blocked by systemically or iontophoretically

applied naloxone. In comparison, Bradley and Brookes reported that

the inhibitory responses to iontophoretically applied dynorphinA(1-13) in vivo were blocked by naloxone (Bradley and Brookes

1984; Brookes and Bradley 1984). Interpretation of this study is

difficult, however, because the exact region of the hippocampus

(e.g., CA1 , CA3, or dentate) was not identified. In addition, these

findings are not consistent with those of Walker et al. (1982) who

reported that, in the CA3 region, dynorphin-A(1-17) had only

naloxone-insensitive inhibitory effects. In their hands, des-tyrdynorphin also reduced the CA3 spontaneous firing rate, but the

observed inhibition was not naloxone reversible.

105
The results obtained in vitro have also been controversial. We

have compared the effects of dynorphin-A(1-17) and [Leu5]enkephalin on CA3 pyramidal cell excitability using extracellular field

potential recording methods in the hippocampal slice (Gruol et al.

1963). [Leu5]enkephalin consistently enhanced the pyramidal ceil

responses. However, in different slices, dynorphin-A(1-17) either

increased or decreased pyramidal sensitivity to mossy fiber stimulation. One example of a clear excitatory effect is shown in figure

5: dynorphin-A(1-17)-induced afterpotentials were completely

blocked by 20 nM naloxone. The basis for this inconsistency is not

known, but it may be due to differences in viability among the

slices.

lwama et al. (1966) similarly reported mixed effects of dynorphinA(1-17) on guinea pig hippocampal slices. In that study, the

depressions produced by dynorphin-A(-17) were also produced by

very high concentrations (100 μM) of U50,488H and bremazocine

and were not reversed by naloxone (100 μM). The authors suggest

that the inhibitory action may be mediated by kappa receptors.

However, the low potency of kappa agonists and the insensitivity of

the effect to naloxone antagonism indicate that the observed

depression was not likely to have been mediated by an opioid

receptor.

To further investigate the actions of dynorphin-A(1-17) in the CA3

region, we have recorded intracellularly from hippocampal pyramidal

cells in vitro (Gruol et al. 1963). Dynorphin-A(1-17) added to the

superfusion buffer produced variable effects on the spontaneous

activity, burst discharges, and membrane potential. Neither orthodromically evoked synaptic potentials nor resting input resistance

were significantly altered by dynorphin-A(1-17). In this system,

[Leu5]enkephalin reduced IPSPs with no change in membrane potential or input resistance, in accord with previous studies in CA1

(Masukawa and Prince 1982). The predominant effect we observed

using dynorphin-B(1-13) was a moderately long duration (0.1 to 10.0

seconds) depolarizing response of 5 to 20 mV that was completely

blocked by naloxone (Chavkin and Schwartzkroin, unpublished

observations). No consistent effect of dynorphin-B was seen on the

other components of the spontaneous or synaptically activated responses including IPSPs evoked by mossy fiber stimulation or the

after hyperpolarization following intrinsic bursts.

106
FIGURE 5. Dynorphin-A(1-17) causes after potentials in CA3

NOTE: Guinea pig CA3 pyramidal cells were electrically stimulated In the

mossy fiber tract (150 μA), and their activity was recorded extracellularly in the stratum pyramidale. Control responses showed no

afterpotentials (a). Following application of dynorphln-A(1-17) (1

second, 40 psi, 100 (μM), multiple afterpotentials were generated (b).

These afterpotentials disappeared completely following 1 to 3 minutes of

washout (c) and were completely blocked by 20 nM naloxone (d).

Schild analysis of the shift in dynorphln-A(1-17) dose-response curves

with bath superfusion of naloxone at 10, 20, and 40 nM yielded an

apparent dissociation constant of 4.8 nM (data not shown).

The basis for the different findings reported by different laboratories is unclear but may be due to differences in tissue preparation or method of peptide application. It is also possible that the

potent nonspecific actions of the dynorphin peptides may obscure

107
the specific, opioid-receptor-mediated actions under the experimental conditions used. We expect that the dynorphins have presynaptic disinhibitory effects in CA3 based on the typical inhibitory

actions of opioids seen in other brain regions. However, neither

the receptor types present nor the ionic basis for the opioid

actions in the CA3 region have yet been established.

OPIOID ACTlONS IN THE DENTATE REGION

Since dynorphin and enkephalin peptides are present in the dentate

granule cells and their processes, it is possible that these peptides

are released from dentate granule cell collaterals to modulate

neuron excitability within the dentate gyrus. In addition, the proenkephalin-derived opioids present in the perforant pathway are

likely to be released at terminals in the dentate molecular layer.

Intracellular and extracellular recordings as well as behavioral data

indicate that opioids have effects on the dentate granule cells

(Haas and Ryall 1980; Tielen et al. 1981; Linseman and Corrigall

1982; Collier and Routtenberg 1984). Using rat and guinea pi

hippocampal slices, Tielen et al. (1961) found that [D-Ala2,Met5]enkephalin added to the superfusion buffer caused a naloxone-sensitive depression of the evoked response at peptide concentrations

between 0.3 and 1.0 μMI.

We find that extracellular recording in the granule cell layer

following orthodromic perforant path stimulation reveals a

stimulus-response relationship that differs from that found in CA1

or CA3 (figure 6). Whereas the stimulus-response cures always

plateau in CA1 and CA3, a biphasic relationship is often observed

in the dentate region. The biphasic pattern may reflect the recruitment of inhibitory input to granule cells at high stimulus

intensities. Both the inhibition and the excitation have been shown

to be sensitive to opioids (Tielen et al. 1981; Neumaier et al. 1986).

Our findings suggest that the main effect of opioids on the primary

population spike in the dentate gyrus was to transform the

stimulus-response relationship from a biphasic to a sigmoidal shape

without an apparent increase in sensitivity to electrical stimulation

of the primary spike (figure 6). This is different from the opioidinduced shift to the left of the stimulus-response curves seen in

CA1 and CA3 and may reflect differences in the synaptic circuitry.

An additional naloxone-sensitive effect of opioids in the dentate

gyrus was the dose-dependent induction of afterpotentials (Neumaier

108
FIGURE 6. Stimulus-response curves in the hippocampus are altered

by opioids

NOTE: Stimulus-response curves were generated by measuring the amplitude of

the primary spike while varying the intensity of electrical stimulation.

In each region, a control curve was constructed, an opioid was introduced, and then a new stimulus-response curve was measured. In CA1

and CA3, normorphine (5 μM) added by bath superfusion caused a shift

to the left in the stimulus-response relationship with no change in the

maximum response. In the dentate gyrus, dynorphin-A(1-17) added by

pressure micropipette (1 second, 40 psi, 100 μM) did not shift the curve

to the left, but changed the curve from a biphasic to a sigmoidal shape.

and Chavkin 1988; Neumaier et al. 1988). The effect was reversed

1 to 5 minutes after drug application by micropipette; it was also

completely blocked by the addition of naloxone (10 to 100 nM) to

the superfusion buffer (figure 4). We quantified opioid action by

measuring the amplitude of the afterpotential recorded in the

109
granule cell layer. Schild analysis of normorphine sensitivity to

naloxone antagonism yielded an apparent dissociation constant of

1.6 nM (0.9 to 2.6, n=4). This value is very close to the apparent

affinity of naloxone for the mu opioid receptor (Lord et al. 1977).

Similarly, the apparent dissociation constant of naloxone derived

from the shift in PL017 (a mu-selective agonist) was 1.0 nM (0.6 to

1.7, n=4). These values agree well with the results from similar

experiments using the same drugs in CA1 (figure 3) and suggest

that normorphine and PL017 act through the mu receptors in the

dentate region of the hippocampus.

Using the extracellular recording paradigm described above, we

measured the effects of dynorphin-A(1-17) following application by

micropipette. Dynorphin-A(1-17) also caused the appearance of an

afterpotential in the dentate granule cell layer response (Neumaier

and Chavkin 1986; Neumaier et al. 1988). The dynorphin effect was

reversed by naloxone but required a higher concentration than that

required to antagonize completely the actions of normorphine

or PL017. The apparent dissociation constant of naloxone in these

experiments was 12.3 nM (8.4 to 18.1, n=8), which was significantly

larger than that associated with PL017 (p<0.01, ranked sum test).

The resistance of dynorphin-A(1-17) effects to naloxone in the dentate region suggests that these effects are mediated by kappa receptors (Neumaier and Chavkin 1986; Neumaier et al. 1986). In

similar experiments (figure 4d, e, and f), we found that DSLETinduced afterpotentials were inhibited by naloxone with an apparent

dissociation constant of 5.2 nM (n=3). Although this value is lower

than that typically seen with delta receptor activation (Lord et al.

1977), the naloxone sensitivity of DSLET was significantly higher

than that found for PL017 in the same assay. The receptor types

activated by DSLET are not yet known. We can speculate that the

intermediate naloxone sensitivity indicates that DSLET activates

both mu and delta receptors under these assay conditions.

CONCLUSIONS

An image of endogenous opioid action in the hippocampus is emerging. Disinhibition of the principal cell type is the most consistent

effect of pharmacologically applied opioids in each of the three

major divisions of the hippocampal formation. This was most

clearly demonstrated in the CA1 region, and recent studies have

documented similar actions in CA3 and the dentate gyrus. Additional experimental details are necessary to define precisely the

110
cell type expressing the opioid receptors, the location of the receptors on the cell, and the transmitter(s) whose release is controlled by opioid receptor activation. while similarity among the

regions is evident, differences have also been noted in the responses to opioid application; thus, differences in the synaptic

circuitry are likely.

The analysis of opioid action in the hippocampus continues to require an understanding of the conditions under which endogenous

opioids are released and the cellular targets of the released opioid.

In addition, we will need to compare the actions of endogenously

released enkephalins and dynorphins with the effects of pharmacological activation of opioid receptors. Ultimately, this information

will provide a clearer view of the role of endogenous opioids in

controlling the excitability of hippocampal neurons and information

transfer through this brain region.

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ACKNOWLEDOMENTS

Portions of this work were supported by a Pharmaceutical

Manufacturers Association Foundation research starter grant and

grants GM-07266 and NS-23483 from the Public Health Service.

116
AUTHORS

Charles Chavkin

John F. Neumaier

Elizabeth Swearengen

Department of Pharmacology

University of Washington

Seattle, WA 98195

117
Behavioral Significance of

Opioid Peptides in Relation

to Hippocampal Function

Michela Gallagher

INTRODUCTION

The first reference to an action of opiates on learning and memory

can be found in a report published in 1920 (Macht and Mora 1920).

This archival study would seem a fitting background for considering

some behavioral implications of the hippocampal opioid peptides

described in previous chapters. These early investigators reported

that low (subanalgesic) doses of morphine impaired a “memory

habit” in rats tested on a circular maze. This observation echoes

two prominent themes in contemporary behavioral studies on hippocampal function. On the one hand, the results from a variety of

research studies point to the role of the hippocampus in memory

(Mishkin 1982; Olton 1983; Zola-Morgan et al. 1988). At the same

time, much evidence indicates that rat hippocampal neurons process

spatial information such as that required in many maze-learning

tasks (O’Keefe and Nadel 1978).

This inquiry will begin by considering the effects of altering opioid

peptide function on learning and memory processes that are associated with the hippocampus. A second line of inquiry will focus on

the possibility of oploid peptide regulation of afferents to the

hippocampal formation. Primarily, my remarks will address evidence

that septohippocampal choiinergic (acetylcholine (ACh)) function is

under opiold peptide control. Finally, some preliminary evidence

will be discussed indicating that training procedures, which engage

hippocampal function, might be used to monitor opioid peptide

activity in this structure.

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OPlOlD PEPTlDE REGULATlON OF MEMORY FOR SPATIAL

INFORMATlON

Ample evidence is now available indicating that opiate treatments

alter learning and memory across a wide array of tasks and species

(see recent review by Gallagher (1985)). With one notable exception discussed later in this chapter, the results from this work are

quite uniform. in general, low doses of opiate agonists and pep

tides have detrimental effects, whereas opiate antagonist administration, by itself, augments the effects of training on acquisition

and retention of new learning. Drug treatments can produce these

effects when administered either prior to training or during an

interval after the training experience is completed. When examined, the effects of opiate treatments are found to exhibit stereospecificity and to be shared by a series of compounds with either

agonist or antagonist properties.

Quite a few studies in this line of work have used testing procedures that are also sensitive to treatments which affect the hippocampal formation. For example, several studies have examined the

effects of opiate antagonists on memory-based performance using

spatial tasks that are sensitive to hippocampal damage. In one

such study, Collier and Routtenberg (1984) tested rats on a radial

arm maze task in which animals were trained to remember the

location of a food reward over a delay period. Either drug or

vehicle injections were given prior to sessions when presentation of

a food reward at one location on the maze was followed 20 minutes

later by a retention test, when animals were again rewarded if they

returned to the same location on the maze. The rewarded location

varied among the eight arms of the maze across test sessions. Administration of naloxone was found to facilitate accurate performance: following drug treatment, animals committed fewer errors

after the delay in returning to the previously rewarded location.

Rats with hippocampal damage fail to perform accurately on this

maze task (Kesner and DiMattia 1984). Interference with normal

hippocampal function produced by intracranial electrical stimulation

likewise impairs performance (Bierley et al. 1983; Collier and

Routtenberg 1984). Indeed, in their study, Collier and Routtenberg

(1984) found that electrical stimulation of the dentate gyrus after

the training trial impaired subsequent retention. In addition, this

effect of hippocampal stimulation was prevented by prior systemic

administration of naloxone. At the time, these investigators

119
speculated that the disruptive effects of dentate stimulation on

memory might be due to the release of opioid peptides from dentate

granule cells.

In our laboratory, we have developed a somewhat different version

of the radial arm maze task to examine the effects of opiate

treatments on memory-based performance using spatial information.

Animals are trained to visit each arm of the maze once in a session in one spatial environment. The experiment is then conducted

by examining acquisition of criterion performance on the maze in a

series of new spatial environments. In each new environment,

probe trials are conducted to confirm that the accurate performance which develops over several training trials depends on spatial

cues specifically located around the maze in the new room. In

each new spatial environment, a treatment is given-either drug or

vehicle. Injections are administered after removing the animal from

the maze 5 or 8 hours prior to testing subsequent performance. We

have found highly reliable and robust improvement in acquisition of

criterion performance on this task when opiate antagonists are

given. The data in table 1 are collected from several experiments

and show this effect with systemic administration of several different opiate antagonist agents (Fanelli et al. 1985; Gallagher et al.

1983; Gallagher et al. 1985a).

In these studies using rats, opiate antagonists do not disrupt functions that presumably depend, at least in part, on hippocampal circuitry. Indeed, there is no evidence from this work that activation

of these or other opioid peptides supports or improves the behaviors monitored in these experiments; when opiate antagonists are

given, the results uniformly reveal improved performance. Based on

these findings, we might even speculate that activation of hippocampal opioid peptides might restrain or diminish the learning/

memory capacities that depend on hippocampal processing; but, of

course, none of these experiments have shown that antagonists act

by modifying opioid peptide activity in the hippocampal formation.

This is especially important to note because, clearly, the behaviors

examined in these studies are not uniquely sensitive to hippocampal

manipulations. Other sites within functionally related neural

systems may serve as targets for the effects of opiate antagonist

treatments. For example, such a target site may be provided by

opioid peptides/binding sites located in the septal region.

120
TABLE 1. Rats tested on the radial arm maze in novel spatial

environments

Saline Vehicle Condition Opiate Antagonist Condition

Trials to Errors to Trials to Errors to

N Criterion Criterion Criterion Crieterion

8 5.4 12.0 3.3* a

1 0 4.9 3.6* b

1 0 4.8

12.2

5:8

10.6

2 7 12.0

3.5* a

3.7*c

*Significantly different from vehicle.

aDrug treatment with naloxone (2 mg/kg).

bDrug treatment with diprenorphine (1 mg/kg).

cDrug treatment with naltrexone (1 mg/kg).

SOURCE: Adapted from Gallagher et al. 1983; Gallagher et al. 1985a.

4.4*

5.8*

3.7*

5.0*

OPIOID PEPTlDE REGULATlON OF AFFERENTS TO THE

HIPPOCAMPUS FORMATlON

In a series of investigations dating from the late 1970’s, microinjections of agonists, morphine and opioid peptides, into the medial

septal area (MSA) in rats were reported to inhibit ACh activity in

the hippocampus (Botticelli and Wurtman 1982; Moroni et al. 1977;

Moroni et al. 1978; Wood et al. 1979). For example, either morphine or beta-endorphin administration into the medial septum

decreases ACh turnover in the hippocampal formation (Moroni et al.

1977). Inhibition of ACh by opioids is blocked by prior systemic

administration of opiate antagonist, as well as by acute transection

of the fimbria/fornix (Botticelli and Wurtman 1982; Moroni et al.

1977). Such findings led to the proposal that opiate-sensitive

mechanisms in the vicinity of ACh perikarya inhibit septohippocampal ACh neural activity. Endogenous opioid peptides may

also regulate normal septohippocampal ACh function. For example,

systemic administration of naltrexone prevents inhibition of ACh

activity in the hippocampus that can be induced by an episode of

stress (Lai et al. 1988). lt is notable that this effect of opiate

antagonist treatment exhibits some selectivity within the basal

forebrain ACh system. While the effect of stress on inhibition of

121
ACh activity in the hippocampus is prevented by naltrexone, the

inhibition of ACh function induced in prefrontal cortex by the same

behavioral manipulation remains unaffected by opiate antagonist

treatment (Lai et al. 1988).

The MSA, which has served as a target site for opiate microinjections, contains a relatively sparse innervation by opioid

peptides. Dynorphin-like immunoreactivity is absent in the medial

septal nucleus: scattered fibers are seen in the lateral septum

(Khachaturian et al. 1982). Beta-endorphin innervation of the

septal region appears to be confined to the ventral lateral septal

nucleus in a region that does not encroach on the MSA (Finley et

al. 1981). In a report by Gall and Moore (1984), perikarya exhibiting enkephalin-like immunoreactivity were found in the lateral

septum, predominantly in the intermediate lateral septai nucleus.

Enkephalin-immunoreactive fibers/terminals were observed in two

zones: one located dorsolateral to the enkephalin cell bodies at

rostral levels and midlevels of the septum, and another positioned

along the lateral borders of the medial septal nucleus throughout

its rostral/caudal extent.

The distribution of opioid peptides is interesting to consider

alongside the pattern of opiate binding in the septum. Two zones

of opiate-binding sites are apparent in autoradiograms of the septal

region that correspond rather well with the localization of immunoreactive fibers/terminals reported by Gall and Moore (1984). In our

analysis of autoradiograms generated from sections through the

rostral/caudal extent of the region, this correspondence is apparent

at all levels of the septum (unpublished observations). These autoradiograms of [

3H]naloxone binding show a pattern of distribution

that is also similar to that depicted in a recent report using DAla2

-MePhe4-Met(o)5-ol-enkephalin (FK) to selectively label mu

binding sites (McLean et al. 1988). In the present context, the

distribution of opioid fibers/terminals and binding sites in proximity

to the MSA is interesting, especially because this area may overlap

the topography of choline acetyltransferase (ChAT)-immunoreactive

neurons that project to the hippocampal formation. Following

injection of retrograde tracer into the hippocampai formation, cells

that double-labelled for ChAT immunoreactivity are notably absent

along the midline (Amarai and Kurz 1984). Rather, these identified

ACh projection neurons at the level of the septum are positioned

laterally within the medial septai nucleus, and their distribution is

122
reported to extend Into the adjacent lateral septum. These anatomical details are consistent with the hypothesis that opiatesensitive sites within the septal region are in a position to regulate

septohippocampal ACh function.

Recent work in our laboratory has shown that microinjections of

opiate treatments into the medial septum can alter performance on

learning and memory tasks (Bostock and Gallagher, in press;

Gallagher et al., in press). One testing procedure used in this

work is the radial arm maze task already described. In an experiment in which the same animals were tested in four different spatial environments under vehicle and varying doses of naloxone,

posttraining microinjection into the medial septum appreciably

improved performance (table 2).

TABLE 2. Effects of postraining naloxone administration into the

medial septum on performance in new environments

(n=10)

Treatment Trials to Criterion Errors to Criterion

Vehicle 5.1 ± 0.4 14.2 ± 2.1

5 Naloxone ng 5.4 ± 0.4 13.5 ± 1.7

50 Naloxone ng 4.1 ± 0.4* 0.3 ± 1.8**

500 Naloxone ng 5.0 ± 0.4 9.0 ± 1.3*

*Different from vehicle treatment condition; correlated t test, p<0.05.

**Different from vehicle treatment condition: correlated t test, p<0.01.

NOTE: Values given are means ± SEM.

In related experiments (Bostock et al., in press), memory-based performance on this task was found to be impaired by medial septal

administration of beta-endorphin over a range of doses previously

shown to inhibit hippocampal ACh activity (5 to 500 ng) (table 3).

As shown in table 3, the effects of beta-endorphin microinjections

into this area have no detrimental effect on behavior, however,

once animals have already learned to perform well in a given

123
spatial environment. These results suggest that opiate-sensitive

sites located outside the hippocampal trisynaptic circuit alter the

ability to retain new spatial information. However, these effects

may be due, at least in part, to changes in hippocampal function

via regulation of septohippocampal activity.

TABLE 3. Effects of beta-endorphin on maze performance

Treatment

Errors to

Achieve

Criteriona

Errors at

Criterionb

Vehicle 13.9 ± 2.0 1.3±0.17

Beta-endorphin 32.0 ± 6.3* 0.9 ± 0.27

*Significantly different from vehicle, p<0.05.

a

These animals (n=12) received vehicle or peptide (500 ng) on the first 4 days

of testing in a new spatial environment.

b

These animals (n=9) received vehicle or peptide (500 ng) on 4 consecutive

days after reaching criterion in a spatial environment.

NOTE: Values given are means ± SEM.

Although opiate antagonists have proved to be quite effective in

altering behavioral capacities that are associated with hippocampal

function, the hippocampus per se is not necessarily a target for

these effects. In this context, it may be important to note that

other afferents to the hippocampal formation, such as the ascending

noradrenergic system, may also be under opioid peptide control.

Here, in contrast to the interactions described between septohippocampal ACh neurons and opiate-sensitive mechanisms, regulation of the norepinephrine (NE) system may occur at the level of

both the NE cell bodies and the NE terminals (Montel et al. 1974;

Nakamura et al. 1982; Pickel et al. 1979; Williams and North 1964).

Inhibition of the evoked release of NE by opiate agonists has most

recently been reported in hippocampal slices from several species

including rats (Jackisch et al. 1984; Jackisch et al. 1966). Recent

behavioral studies have shown that intact function in the dorsal

noradrenergic pathway is necessary for obtaining the memoryenhancing effects of opiate antagonists (Fanelli et al. 1965;

124
Gallagher et al. 1985b). Thus, removing the NE system from opioid

peptide inhibition at the level of the locus coeruleus (LC) and/or in

terminal fields of the hippocampal formation may also influence

these pharmacological effects.

OPIOID PEPTlDES IN THE HIPPOCAMPAL FORMATION

Turning to the opioid peptides that are localized within the hippocampal formation, there is little known about the conditions under

which these opioid peptides are normally activated or the effects

they exert on behavior. One interesting correlation has emerged

recently between the effects of treatments aimed at blocking opioid

peptide activity, including anti-Met-enkephalin antiserum administered into the hippocampus, and behavior in two different strains

of mice (Castellano 1961; Van Abeelen and Gerads 1966; Van

Abeelen and Van den Heuvel 1962). The DBA/2 and C57BL/6 mice

exhibit opposing responses to each of the treatments used. For example, the DBA/2 mice, like rats, show increased retention after

posttraining administration of naloxone. This same strain shows

increased measures of exploration in a novel environment after

intrahippocampal injection of either naloxone or Met-enkephalin

antiserum. This finding is also reminiscent of the effects of systemic naloxone administration in rats, where increases in stimulusdirected behavior in a novel complex environment are found

(Arnsten and Segal 1979). In contrast, the C57BL/6 mice yield an

opposite pattern of results on each of these measures. These

studies suggest that hippocampal opioid peptides are activated when

mice explore a novel environment, but there are quite distinctive

effects on behavior in these two genotypes.

Using a somewhat different approach, we have begun to examine

whether exposing animals to training experiences that should engage hippocampal neurons will alter opioid peptide activity in rats.

In this work, we have an additional interest in determining whether

hippocampal inputs such as those arising from noradrenergic and

cholinergic neurons can be shown to regulate opioid peptide activity. This interest arises from our focus on learning and memory,

as well as from previous work reporting that lesions of the fimbria/

fornix alter levels of hippocampal opioid peptides (Hoffman 1964;

Hoffman and Zamir 1964; Hong and Schmid 1961).

Initially, we used classical conditioning procedures to examine

whether exposing animals to training would alter opioid peptide

125
activity within the hippocampus (Meagher et al. 1966). There is

substantial evidence that hippocampal neural activity is engaged

during such associative learning. When classical conditioning

procedures are used, conditioned responses are observed in a large

proportion of pyramidal neurons In the hippocampus of rabbits

(Berger and Thompson 1976). There is also evidence for neural

correlates of associative learning in the dentate gyrus, as well as

in the hippocampus, of rats (Deadwyier et al. 1979; Deiacour 1984;

Rose 1983; Segal 1973; Segai et al. 1972). In our initial experiment,

the animals that served as subjects received unilateral lesions of

the dorsal noradrenergic bundle 3 to 4 weeks prior to the experiment. These rats were sacrificed either 30 minutes or 3 hours

after a conditioning session in which a tone conditioned stimulus

(CS) and footshock unconditioned stimulus (UCS) were either contingently presented (paired groups, 10 trials) or noncontingently

presented (unpaired groups, 10 presentations of each stimulus).

Subsequent radioimmunoassay for dynorphin A(1-8) in the hippocampal formation revealed a significant effect of training and of

time of sacrifice but no effect of the NE lesion. Whereas dynorphin A(1-8)-like immunoreactivity (dynA(1-8) LI) was comparable

for paired and unpaired groups sacrificed 3 hours after training

(expressed as picomoies per gram of tissue, wet weight, 10.61 ± 0.41

and 9.64 ± 0.61, respectively), levels were significantly higher in

the paired group relative to the unpaired group when rats were

sacrificed 30 minutes after training (14.36 ± 0.61 and 10.36 ± 1.01,

respectively). Radioimmunoassay of Met5-enkephalin LI activity

yielded no significant difference among these same treatment

conditions.

These results indicate that training procedures similar to those that

differentially affect the develorment of conditioned neural activity

in the hippocampai formation may have distinctive effects on the

content of dynA(1-8) LI presumably within the granule ceil/mossy

fiber pathway in the hippocampus. The relatively higher level of

dynA(1-6) LI in animals that received paired CS/UCS presentations

might conceivably reflect lower utilization of opioid peptide relative

to those animals that received shock noncontingently. Although

unilateral NE lesions were not found to have a significant impact

on these results, we are investigating the possible effects of bilateral NE denervation on opioid peptides. This approach may be

particularly important when considering peptides associated with the

granule cell/mossy fiber pathway because the dentate, as compared

126
to the hippocampus, receives a significant bilateral NE innervation

(Loy et al. 1960).

Another condition that engages the activity of large populations of

hippocampai neurons is found when rats explore their environments:

hippocampai neurons have place fields that depend on the spatial

location of the animal (O’Keefe and Nadei 1976). In addition,

Sharpe et al. (1965) have found that exposing rats to a new spatial

environment facilitates neural responses evoked in the perforant/

dentate pathway. This neural enhancement grows over several

days’ exposure to a complex spatial environment. The registration

of spatial information in the activity of the hippocampal formation

is particularly interesting because, as previously cited, lesions of

this structure profoundly affect the ability of rats to perform

accurately on spatial tasks.

in another preliminary study, however, we have not found any reliable effect on the opioid peptide content of the hippocampai formation when rats are trained on a spatial learning task. in this

study, Long-Evans rats were trained for 4 days (four trials per day)

to locate a camouflaged escape platform in a Morris water maze.

These place-trained animals and their yoked controls, who spent an

equivalent amount of time swimming in a tank, were sacrificed 15

minutes after completion of their last session along with experimentally naive subjects. Levels of dynA(1-8) LI and Met5

-

enkephaiin LI in the hippocampus were not significantly different

for animals across these conditions. This negative finding is

somewhat more interesting when it is considered that hippocampai

choiinergic function was affected in these same subjects. The

contralateral hippocampus was used to measure sodium-dependent

high-affinity choline uptake in these rats. This measure of ACh

function in the hippocampai formation was selectively decreased in

these same animals by place training relative to their yoked controls (Decker et al., in press). Thus, we have not yet found a

functional relationship between a marker for septohippocampai ACh

activity and altered levels of hippocampai Met5-enkephalin LI, in

contrast to the relationship found in previous reports that fimbria/

fornix transections elevate Met5-enkephalin LI in the hippocampus

(Hoffman 1964; Hoffman and Zamir 1964; Hong and Schmid 1961).

Additionally, plasma corticosterone was measured in our rats at the

time of sacrifice. Whereas a training-induced elevation in 11-

hydroxycorticosteroid was observed relative to behaviorally naive

rats, this index of pituitary-adrenal activation was not accompanied

127
by altered opioid peptide immunoreactivity in the hippocampai

formation.

CONCLUSION

Although opiate antagonists are found to improve behaviors that

are highly sensitive to hippocampal damage, we have cautioned that

opiate-sensitive sites outside the hippocampus may be involved.

Possible routes for opiate antagonist improvement of learning and

memory include interactions with the basal forebrain choiinergic

and ascending noradrenergic systems, both of which provide afferents to the hippocampai formation.

As research on opioid peptides enters its second decade, we are

just on the threshold of exploring the functional properties of

these neuropeptides in the hippocampai formation. No conclusions

can yet be drawn about their biological significance. At present,

we know almost nothing about the conditions that favor their release or the physiological responses evoked by endogenous opioid

peptides-ones that might be revealed in the presence of appropriate antagonists. The view expressed here is that currently available information about natural stimuli that engage hippocampal

neural activity in intact animals may prove a useful and necessary

adjunct to research using in vitro preparations.

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ACKNOWLEDGMENTS

This work was supported by National Institute of Mental Health

grant MH 35554 and a Research Scientist Development Award (KO2

MH00406) to Michela Gallagher.

AUTHOR

Michela Gallagher, Ph.D.

Department of Psychology

University of North Carolina

at Chapel Hill

Davie Hall 013A

Chapel Hill, NC 27514

132
Effects of Mu Opioid Receptor

Activation in Rat Hippocampal

Slice

Menahem Segal

INTRODUCTION

In the hippocampus, the major action of opioid peptides is to

enhance excitability of both pyramidal and granule cells (Corrigall

1983; Henderson 1983; Zieglgänsberger et al. 1979). Although this

fundamental observation is now generally accepted as true, our

knowledge of opioid action even at this simple level of description

is still incomplete. The specific actions of different classes of

opioids in each region of the hippocampus, for example, have not

yet been systematically studied. Most electrophysiological experiments have been conducted in the CA1 region of the dorsal hippocampus, where recording is easiest; yet there are, in the adjacent

CA2 region, three times as many mu- and delta-binding sites as in

CA1 (Crain et al. 1986; Herkenham and McLean, this volume).

Similarly, the ventral hippocampus is far richer in opioid peptides

than the dorsal hippocampus.

The opioid-induced excitation of pyramidal and granule cells is

reportedly due to disinhibition resulting from the hyperpolarization

of inhibitory gamma-aminobutyric acid (GABA)-containing interneurons (Nicoll et al. 1980; Zieglgänsberger et al. 1979). The evidence for this relies on occasional recording from interneurons in

hippocampal slices (Nicoll et al. 1980) and is also supported by in

vitro experiments from other brain structures. In the nucleus locus

coeruleus, particularly, it has been shown convincingly that opioids

cause an increase in some types of potassium currents, which act

to decrease neural excitability (Williams et al. 1982). In other systems, opioid peptides are found to cause other effects, perhaps on

calcium currents (Werz and Macdonald 1982).

133
Whether or not disinhibition is the only mechanism that mediates

opiate action on hippocampal pyramidal ceils and dentate gyrus

granule cells is not clear; there is evidence that opioid peptides

may enhance excitatory postsynaptic potentials (EPSPs) directly,

even when inhibitory postsynaptic potentials (IPSPs) are not affected by these drugs (Dingledine 1981; Gähwiler 1983; Haas and Ryall

1981; Lynch et al. 1981). Furthermore, it is not clear that excitation is the only effect of opiold peptides on hippocampal pyramidal

ceils; there have been several reports of inhibitory actions of some

peptides and opioid analogues on hippocampal activity (Henriksen et

al. 1982; lwama et al. 1986; Moises and Walker 1985). Clearly,

additional work is still needed to sort out the differences among

conflicting reports. The use of selective agonists for mu, delta,

and kappa receptors (Bostock et al. 1984; Brookes and Bradley 1984;

Chavkin et al. 1985; Neumaier and Chavkin 1986; Henderson 1983;

lwama et al. 1986; Tielen et al. 1981; Valentino and Dingledine

1982; Vidal et al. 1984) will certainly facilitate classification of

different response types, although particular caution must be exercised in using ligands in the proper concentration range, especially

when only a relative selectivity has been demonstrated.

Another puzzling observation is the lack of evidence of any opioid

role following activation of the mossy fiber system (the pathway

that connects the dentate gyrus to the hippocampus proper) which

is known to contain dynorphin A. No naloxone-sensitive response

is seen in CA3 neurons following stimulation of this pathway

(Chavkin and Bloom 1986; Moises and Walker 1985; Henriksen et al.,

this volume). Because the mossy fiber system is presumed to contain an excitatory amino acid (EAA) neurotransmitter in addition

to dynorphin A, a potential role of the opioids in the hippocampus

may be in modulating the effects of other transmitters (Segal 1977).

There is already evidence that mu, delta, and kappa agonists can

modulate the release of acetylcholine or norepinephrine from

hippocampal slices (Jackisch et al. 1986). Moreover, in the hippocampus, the enzyme acetylcholinesterase, which is responsible for

the rapid breakdown of acetylcholine, is not only colocalized with

opiate receptors but also has peptidase activity of its own. lt

may therefore be involved in the inactivation of opioid peptide

activity in places where acetylcholine and opioids overlap.

Opioid peptides may also modulate EAA-induced neurotoxicity. Injection of kainic acid into the hippocampus, the cerebral ventricles,

134
or parenterally, produces marked and specific changes in the hippocampus involving initial seizure activity followed by neuronal death.

Concurrent with these seizures are marked initial reductions and

later increases in opioid peptide concentrations in the hippocampus

(Kanamatsu et al. 1986, Gall, this volume). Interestingly, region

CA2, the region with the highest concentration of opioid-binding

sites, is also the region most resistant to the toxic effects of

kainic acid. Although it still has to be shown that the intrinsic

opioids play a role in protecting CA2 from the destructive effects

of EAA neurotoxins, this finding is consistent with such an effect.

Most importantly, the functional significance of opioid peptides in

the normal operation of the hippocampus remains elusive. In part,

the problem may be divided according to the two main hippocampal

peptide types. lf dentate gyrus granule cells do indeed release

dynorphin from terminals as a neurotransmitter or cotransmitter,

and if enkephalin is similarly released by the terminals of perforant

path fibers originating in the entorhinal cortex, then correlation of

the physiological activity of these classes of neurons with the effects on their targets should yield information on the involvement

of these peptides in the hippocampus. To date, however, it has

proved quite difficult to record from identified opioid cells. As a

result, most information concerning opioid involvement in hippocampal function has been accumulated by applying agonists and

antagonists to the postsynaptic targets of the mossy fiber and

perforant pathway systems. Some assessment of opioid concentration in the hippocampus during structured behavioral tests have

also been utilized to analyze opioid actions on learning (Gallagher,

this volume).

The experiments described below address some of the questions

raised here, and the findings are used to propose a mode of action

for and possible function of the opioid peptides acting at mu receptors in the hippocampus. Specifically, the findings address the

possibility of differences in excitability between regions CA1 and

CA2 in response to the application of the mu agonist D-Ala2-

MePhe4-Gly-ol5-enkephalin (DAGO) (Jackisch et al. 1986), differences in opioid action along the septotemporal axis of the hippocampus, the involvement of other neurotransmitters and extrinsic

afferents in the observed effects of opioids in the hippocampus, and

some of the functional implications of activating mu receptors in

the hippocampus.

135
METHODS

Experiments were conducted with transverse slices of rat hippocampus as described elsewhere (Segal 1986). An intracellular

recording was made with glass micropipettes containing K-acetate,

and an extracellular recording was made with low-resistance (2- to

3-M) micropipettes containing the superfusion medium. Signals

were amplified with an Axoclamp II amplifier and recorded continuously on a chart recorder. Drugs were applied with the microdrop

technique on the slice, near the recording site. Only one cell (or

extracellular recording position) was recorded from a given slice.

DAGO (a gift from Dr. R. Simantov of the Weizmann Institute,

Rehovot, lsrael) was applied at a microdrop concentration of 1 μM;

the final concentration at the receptor site is unknown but is

probably 10 to 100 times smaller than at the microdrop.

REGIONAL EFFECTS OF DAGO

The first series of experiments was conducted in order to compare

reactivity of CA1 and CA2 neurons to topical application of DAGO.

Systematic experiments were done with eight CA1 and seven CA2

neurons. Resting membrane potential (range -65 to -80 mV) and

spike height (80 to 110 mV) were not different between the two

cell groups. The Input resistance of CA2 cells was slightly lower

than that of CA1 cells (range 12 to 60 mohm). CA2 ceils possess a

unique rectifytng property that was not found in CA1 neurons

(Segal, in preparation). Topical application of DAGO to CA1 cells

produced no significant change in the resting properties of the

recorded neuron. The only marked effect, seen by others with mu

agonists in CA1 cells (Bostock et al. 1984), was a reduction in the

two-component slow IPSP evoked in response to stimulation of the

stratum radiatum. Concurrent with this, the initial EPSP evoked by

the same stimulation was enhanced. In general, the EPSP slope

was unchanged, indicating that the reduction in the IPSP size can

account for the changes in EPSPs. Similar responses were detected

in all CA2 cells tested, with no qualitative differences from the

CA1 responses. in four of the seven CA2 cells examined, application of DAGO produced a mild (2 to 2.5 mV) hyperpolarization

(figure 1). This response lasted 1 to 2 minutes and recovered,

while the effect of DAGO on the EPSP-IPSP sequence recovered

only 10 to 20 minutes after drug application. It appears that

DAGO may have two independent effects on CA2 cells, only one of

136
FIGURE 1. Effects of DAGO on properties of a CA2 neuron

recorded intracellularly

NOTE: (A) Specimen record of membrane potential -65 mV) and responses to

hyperpolarizing current pulses and afferent stimulation before and after

a microdrop application of DAGO. The hyperpolarizing current pulses

are applied every 5 seconds (bottom record current) and alternate with

afferent stimulation. The latter produces a biphasic response consisting

of an EPSP followed by a slow IPSP. DAGO produces a 2-mV slow

hyperpolarization that develops over 15 seconds after drug application.

(B) An illustration of the EPSP-IPBP sequence before (top) and after

(bottom) application of DAGO. Time scale: 10 seconds for A, 100 msec

for B, 5 mV for both. (C) Input-output relations of the size of the

EPBP and IPSP as a function of simulation intensity before (control,

CON) and after (DAGO) application of the drug.

which is shared by CA1 neurons. The pharmacological nature of

the hyperpolarizing response has yet to be determined.

EXTRINSIC MODULATION OF REACTIVITY TO DAGO

Stimulation of the excitatory afferents residing in the stratum radIatum can generate a population action potential (population spike)

in the recorded hippocampus. This population spike results from

concurrent activation of many cells in the recorded area. Normally, a single population spike is generated and is followed by an

137
inhibitory period corresponding to the duration of the IPSP. Topical application of opiates has been shown by several investigators

to produce a larger population spike, and this is commonly followed

by a secondary afterdischarge (Corrigall and Linseman 1980). We

could replicate these responses following application of DAGO

(figure 2). A secondary afterdischarge, not seen normally in our

preparation, amounted to 48 percent of the initial population spike

after application of DAGO (n=12 slices).

FIGURE 2. Effect of prior fornix transection on the efficacy of

DAGO-induced secondary afterpotential (P2)

NOTE: (A) Examples shown are taken from two slices recorded in the same

experiment, of normal and fornix (Fx)-transected rat hippocampi.

Complete, bilateral Fx transection was performed 1 to 2 weeks before

recording. This transection results in nearly total loss of hippocampal

content of acetylcholinesterase. Neither of the two slices produce a

secondary potential under the control condition, and both produce one in

the presence of DAGO, but the ratio of the second to the first (P2/Pl)

is larger In the normal slice. (B) Data from 12 normal slices and 11 Fxlesioned slices are averaged. Error bars are standard errors of the

mean.

In one of several types of experiments designed to examine the

modifiability of the response to DAGO, we used fornix-transected

rats. Fornix transection depletes the hippocampus of most of its

cholinergic, serotonergic, and noradrenergic afferents. In addition,

it disrupts the feedback circuit between the hippocampus and the

septal nuclei. lt has also been reported that fornix transection, or

septal lesion, produces a transient rise in concentrations of opioid

138
peptides in the hippocampus (Hong and Schmid 1981). In 11 slices

taken from 4 fornix-transected rat hippocampi, we found that

DAGO produced only a small secondary afterdischarge, about 10

percent of the initial population spike amplitude. These results

suggest one of three possibillties: (1) That fornix transection

reduces the ability to evoke an afterdischarge (this is unlikely

since, in these same slices, a tetanic stimulation is capable of

producing an afterdischarge (Segal, unpublished)); (2) that intrinsic

opioid-containing neurons are regulated by extrinsic afferents, and

the absence of these afferents causes increased activity of opioidcontaining neurons, enhanced production of opioid peptides, and

consequent subsensitivity of postsynaptic opiate receptors; and (3)

that DAGO produces at least part of its observed effects by acting

on afferent terminals arriving via the fornix, and in their absence

the afterdischarge is not recorded. Further experiments are needed

to examine these possibilities.

THE NATURE OF THE SYNAPSE ACTlVATED BY DAGO

The enhancement of EPSPs and the blockade of IPSPs result in the

appearance of a secondary afterdischarge. This enhanced excitability can also result from activation on the postsynaptic neurons of

additional receptors that are normally masked by the presence of

inhibitors. lt has been suggested that these additional voltagedependent responses are mediated by activation of N-methyl-Daspartate (NMDA)-like receptors (Collingridge et al. 1983). If that

is the case, one should be able to block the afterdischarge by

application of an NMDA receptor antagonist. Indeed, application of

P-amino phosphorovaleric acid (2-APV) did block the afterdischarge

while having only a small effect on the DAGO-enhanced primary

population spike (figure 3).

The possibility that opioid peptides uncover a voltage-dependent

NMDA-like receptor has fascinating implications; these receptors

are probably involved in plastic properties of the hippocampus,

including its ability to exhibit long-term potentiation (Kelso et al.

1986; Wigstrum and Gustafsson 1983). The effect of DAGO on the

ability of hippocampal neurons to undergo long-term synaptic

changes was examined in a test condition where synaptic input was

associated with postsynaptic depolarization. A similar condition was

considered recently to be an illustration of a Hebbian synapse

(Kelso et al. 1986), that is, an excitatory synapse that, when activated when the cell was depolarized, demonstrated subsequent

139
FIGURE 3. Effects of NMDA antagonist 2-APV on a secondary

population spike evoked in the presence of DAGO

NOTE: On the left are specimen extracellular records taken (top to bottom) before and after application of DAGO. and after application of 2-APV in

the presence of DAGO. Note the total disappearance of the secondary

population spike in the presence of 2-APV. On the right Is an inputoutput curve of the size of the initial population spike under the three

recording conditions (control, DAGO, and DAGO + 2-APV).

enhancement of its excitation. Initially, we stimulated the stratum

radiatum to record a stable intracellular EPSP followed by the

typical IPSP. The cell was then depolarized by passage of DC current pulse concurrent with the application of the afferent stimulation. Under these conditions, we were unable to detect subsequent

long-term changes in the magnitude of the EPSPs elicited by

stratum radiatum stimulation. We then applied DAGO (five cells)

and found that subsequent association between depolarizing current

pulses and EPSPs now produced long-term enhancement of the

EPSPs (figure 4). This enabling function of DAGO might provide a

clue to a role of endogenous opioid peptides in the hippocampus;

activation of opioid neurons may enhance local plasticity. This

possibility will be examined in future experiments.

140
FIGURE 4. Enabling action of DAGO on potentietion of EPSPs in a

hippocampal neuron

NOTE: (A) Schematic presentation of the stimulatlon paradigm: Top row is

afferent stimulation; bottom row is intracellular current injection; a to e

refer to averaged PSPs depicted in B. Afferent stimulation was applied

once every 5 seconds. (a) Before association of the afferent stimulation

with the depolarizing current pulses: (b) 1 to 5 minutes after a series of

five large depolarizing current pulses, which produce a series of action

potentials, applied simultaneously with the afferent stimulation; (c) 5

minutes after application of DAGO; (d) as in b; and (e) 10 minutes later.

(B) Series of averages of 10 to 20 PSPs recorded in the conditions seen

in A. (C) Individual PSPs on an expanded scale taken before (during

period b) and after (period c) application of DAGO.

DISCUSSION

The present experiments address some puzzles accumulated over the

past few years in the study of opioid peptides in the rat hippocampus. The apparent involvement of extrinsic afferents coming

via the fomix-fimbria in the action of DAGO may indicate that, in

order to appreciate the extent of opioid effects, one has to activate these extrinsic afferents.

141
This is difficult to achieve in the artificial situation inherent in

the slice preparation, where the extrinsic afferents are not functional. Hence, the disinhibiiory action of DAGO and the other opioid peptides might reflect one of several cases of presynaptic

action of these peptides on hippocampal circuits. The reduced efficacy of DAGO in the fornix-transected hippocampus is consistent

with the reported effects of opioids on release of acetylcholine and

noradrenaline in the hippocampus. Acetyicholine has several loci of

action in the hippocampus and elsewhere: it causes a decrease in

evoked PSPs, depolarizes postsynaptic cells in a slow manner, produces a fast activation of interneurons, and causes an increase in

spontaneous PSPs. lt is possible that the opioid peptides modulate

only some of these effects, i.e., those taking place near the pyramidal cell layer. lf this is indeed the case, it introduces a measure

of specificity into a neural pathway-the cholinergic septohippocampal connection-which is inherently diffuse.

The enabling action of DAGO on activation of an NMDA receptor is

of important functional significance. In previous experiments on a

Hebbian synapse, it was necessary to block inhibition using GABA

antagonists (Kelso et al. 1986). The meaning of such experiments

was not clear because the brain is not known to contain an intrinsic GABA-antagonist-like compound. If an opioid-containing neuron

can function to enable an NMDA receptor, then the interpretation

of such experiments is more straightforward. One has to find the

conditions that will activate opiold-containing neurons and demonstrate that such conditions will also facilitate plasticity in the

hippocampus and then learning by the animal.

SUMMARY

The effects of the mu opioid agonist DAGO were examined in the

hippocampal slice preparation. The effects of the drug on active

and passive properties of neurons in CA1 and CA2 were examined.

Intracellular responses of these neurons to afferent stimulation

were examined before and after DAGO application. As seen with

other mu agonists, DAGO reduced IPSPs and enhanced the duration

of EPSPs but not their rise time. in CA2 cells, DAGO also produced a transient hyperpolarization. Further experiments examined

the interactions of the opioid peptides with other neurotransmitter

systems in the hippocampus. The effects of DAGO were reduced by

prior fornix transection. Some effects of DAGO on generation of a

142
secondary afterdischarge are abolished by an NMDA receptor antagonist, 2-APV. The significance of these results to the understanding of functioning of the hippocampal circuit is discussed.

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AUTHOR

Menahem Segal, Ph.D.

Center for Neurosciences

Weizmann institute of Science

Rehovot 76100, Israel

145
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59 CURRENT RESEARCH ON THE CONSEQUENCES OF MATERNAL

DRUG ABUSE. Theodore M. Pinkert, M.D., J.D., ed.

GPO Stock #017-024-01249-1 $2.50

80 PRENATAL DRUG EXPOSURE: KINETICS AND DYNAMICS.

C. Nora Chiang, Ph.D., and Charles C. Lee, Ph.D., eds.

GPO Stock #017-024-01257-2 $3.50

61 COCAINE USE IN AMERICA: EPIDEMIOLOGIC AND CLINICAL

PERSPECTIVES. Nicholas J. Kozel, M.S., and Edgar H. Adams, M.S.,

eds.

GPO Stock #017-024-01258-1 $5

149
82 NEUROSCIENCE METHODS IN DRUG ABUSE RESEARCH. Roger

M. Brown, Ph.D., and David P. Friedman, Ph.D., eds.

GPO Stock #017-824-01260-2 63.50

63 PREVENTION RESEARCH: DETERRING DRUG ABUSE AMONG

CHILDREN AND ADOLESCENTS. Catherine S. Bell, M.S., and Robert

Battjes, D.S.W., eds.

GPO Stock #017-624-01263-7 $5.50

64 PHENCYCLIDINE: AN UPDATE. Doris H. Ciouet, Ph.D., ed.

GPO Stock #017-024-01281-5 $6.50

65 WOMEN AND DRUGS: A NEW ERA FOR RESEARCH. Barbara A.

Ray, Ph.D., and Monique C. Braude, Ph.D., eds.

GPO Stock #017-024-01283-1 83.25

66 GENETIC AND BIOLOGICAL MARKERS IN DRUG ABUSE AND

ALCOHOLISM. Monique C. Braude, Ph.D., and Helen M. Chao, Ph.D.,

eds.

GPO Stock #017-624-01291-2

$3.50

NCADI out of stock

68 STRATEGIES FOR RESEARCH ON THE INTERACTIONS OF DRUGS

OF ABUSE. Monique C. Braude, Ph.D., and Harold M. Ginzburg, M.D.,

J.D., M.P.H., eds.

GPO Stock #017-624-01296-3

$6.50

NCADI out of stock

69 OPIOID PEPTIDES: MEDICINAL CHEMISTRY. Rao S. Rapaka,

Ph.D.: Gene Barnett, Ph.D.; and Richard L. Hawks, Ph.D., eds.

GPO Stock #017-024-01297-1 $11

70 OPIOID PEPTIDES: MOLECULAR PHARMACOLOGY, BIOSYNTHESIS, AND ANALYSIS. Rao S. Rapaka, Ph.D., and Richard L.

Hawks, Ph.D., eds.

GPO Stock #017-024-01298-0 $12

71 OPIATE RECEPTOR SUBTYPES AND BRAIN FUNCTION. Roger M.

Brown, Ph.D.; Doris H. Clouet, Ph.D.; and David P. Friedman, Ph.D.,

eds.

GPO Stock #017-024-01303-0 $6

150
72 RELAPSE AND RECOVERY IN DRUG ABUSE. Frank M. Tims,

Ph.D., and Carl G. Leukefeld, D.S.W., eds.

GPO Stock #017-024-01302-1 $6

73 URINE TESTING FOR DRUGS OF ABUSE. Richard L. Hawks,

Ph.D., and C. Nora Chiang, Ph.D., eds.

GPO Stock #017-624-01313-7 $3.75

74 NEUROBIOLOGY OF BEHAVIORAL CONTROL IN DRUG ABUSE.

Stephen I. Szara, M.D., D.Sc., ed.

GPO Stock #017-024-01314-5 $3.75

75 PROGRESS IN OPIOID RESEARCH. PROCEEDINGS OF THE 1986

INTERNATIONAL NARCOTICS RESEARCH CONFERENCE. John W.

Holaday, Ph.D.; Ping-Yee Law, Ph.D.: and Albert Hetz, M.D., eds.

GPO Stock #017-824-61315-3 $21

76 PROBLEMS OF DRUG DEPENDENCE, 1986. PROCEEDINGS OF

THE 48TH ANNUAL SCIENTIFIC MEETING, THE COMMllTEE ON

PROBLEMS OF DRUG DEPENDENCE, INC. Louis S. Harris, Ph.D., ed.

GPO Stock #017-024-61316-1 $16

78 THE ROLE OF NEUROPLASTICITY IN THE RESPONSE TO DRUGS.

David P. Friedman, Ph.D., and Doris H. Clouet, Ph.D., eds.

GPO Stock #017-624-61330-7 $6

79 STRUCTURE-ACTIVITY RELATIONSHIPS OF THE CANNABINOIDS.

Rao S. Rapaka, Ph.D., and Alexandros Makriyannis, Ph.D., eds.

GPO Stock #017-624-01331-5 $6

IN PRESS

77 ADOLESCENT DRUG ABUSE: ANALYSES OF TREATMENT

RESEARCH. Elizabeth R. Rahdert, Ph.D., and John Grabowski, Ph.D.,

eds.

80 NEEDLE-SHARING AMONG INTRAVENOUS DRUG ABUSERS:

NATIONAL AND INTERNATIONAL PERSPECTIVES. Robert J. Battjes,

D.S.W., and Roy W. Pickens, Ph.D., eds.

81 PROBLEMS OF DRUG DEPENDENCE, 1987. PROCEEDINGS OF

THE 49TH ANNUAL SCIENTIFIC MEETING, THE COMMITTEE ON

PROBLEMS OF DRUG DEPENDENCE, INC. Louis S. Harris, Ph.D., ed.

U. S. GOVERNMENT PRINTING OFFICE: 1988-212-148

151
DHHS Publication No. (ADM)88-1568

Alcohol, Drug Abuse, and Mental Health Administration

Printed 1988