bluuebunny/crossref_metadata_2025 huggingface dataset:
N-methyl-D-aspartate antagonists prevent interaction of binocular maps in Xenopus tectum
Authors: Scherer WJ , Udin SB
Published: March 2018

Glutamate receptors appear to play a key role in several forms of experience-dependent modification of both the strength of synapses and synaptic connectivity. In developing Xenopus frogs, the connections made by isthmotectal axons relaying visual input from the eye to the ipsilateral tectum are markedly influenced by the visual activity of contralateral retinotectal axons, and normal binocular visual input is necessary in order for the ipsilateral visuotectal map to come into register with the contralateral map. We have tested whether NMDA receptors play a role in establishment of the topographic matching of binocular maps during development. We have examined the effects of chronic treatment of tectum with either the receptor agonist NMDA or the antagonists APV or CPP applied throughout early postmetamorphic life using subpial implants of drug-impregnated elvax. Both antagonists blocked the matching of the ipsilateral map to the contralateral map, while NMDA permitted such matching. Our data therefore indicate that NMDA receptors are involved in the experience-dependent establishment of matching binocular maps during development.
Disappearance and reformation of synaptic vesicle membrane upon transmitter release observed under reversible blockage of membrane retrieval
Authors: Koenig JH , Ikeda K
Published: March 2018

The temperature-sensitive mutant of Drosophila, shibire(ts-1), which is normal at 19 degrees C, but in which endocytosis is reversibly blocked at 29 degrees C, was used to deplete synapses of vesicles by inducing transmitter release while membrane retrieval was blocked. When the synapse was kept at 29 degrees C for 8 min, complete vesicle depletion occurred. However, no compensatory increase in the terminal plasma membrane, either as invaginations or evaginations, was observed. Also, no internalized membranous compartment, such as cisternae or coated vesicles, appeared. No invaginations or out-pocketings were seen along the axon between release sites, and no evidence for elongation of the whole axon was found. Thus, the vesicle membrane compartment became unobservable as a result of transmitter release. Depleted synapses were observed by electron microscopy at various times after lowering the temperature, so that the process of synaptic vesicle reformation could be observed. In the first 2–3 min at 19 degrees C, gradually enlarging uncoated invaginations of the plasma membrane were observed. Between 5– 10 min at 19 degrees C, these invaginations pinched off to form large cisternae. Newly formed synaptic vesicles were observed associated with these cisternae by an electron-dense material. Between 10–20 min at 19 degrees C, the number of synaptic vesicles increased, while the size of the cisternae decreased. Within 30 min, the full complement of vesicles had reappeared. No involvement of the coated vesicle pathway in synaptic vesicle reformation was observed. The data suggest that synaptic vesicle membrane is dissembled at the time of transmitter release and then is reassembled at sites along the plasma membrane and internalized in the form of large cisternae, from which new vesicles are formed.
Differential effects of nerve transection on the ACh and GABA receptors of chick ciliary ganglion neurons
Authors: McEachern AE , Jacob MH , Berg DK
Published: March 2018

Chick ciliary ganglion neurons have nicotinic acetylcholine receptors (AChRs) that mediate chemical transmission through the ganglion, and GABAA receptors of unknown significance. Previous experiments examining the role of cell-cell interactions in regulating neuronal AChRs have shown that postganglionic axotomy of ciliary ganglia in newly hatched chicks causes a 10-fold decline in total AChRs within 5 d compared with unoperated contralateral ganglia and that preganglionic denervation causes a 3-fold decline within 10 d. Many of the AChRs are known to be intracellular; of those present on the cell surface, only a small fraction appears to be functionally available normally. In the present experiments, the effects of the operations on functional AChRs and GABAA receptors in the plasma membrane of the neurons were examined by removing the ganglia 5 d after axotomy or 10 d after denervation, dissociating them into single cells, and immediately measuring their ACh and GABA sensitivities with intracellular recording techniques. The ACh sensitivity of axotomized ciliary ganglion neurons was reduced 10- fold compared with neurons from unoperated contralateral ganglia of the same chicks. The reduction could be largely accounted for by a decrease in the maximum response and did not arise from a change either in the dose-response curve or the acetylcholinesterase activity of the neurons. Autoradiographic studies using a radiolabeled anti-AChR monoclonal antibody also demonstrated a substantial decrease in the total number of surface AChRs associated with axotomized neurons. In contrast, axotomy had no unilateral effect on the GABA response.(ABSTRACT TRUNCATED AT 250 WORDS)
Preserved spatial coding in hippocampal CA1 pyramidal cells during reversible suppression of CA3c output: evidence for pattern completion in hippocampus
Authors: Mizumori SJ , McNaughton BL , Barnes CA , Fox KB
Published: March 2018

Medial septal modulation of hippocampal single-unit activity was examined by assessing the behavioral and physiological consequences of reversibly inactivating the medial septum via microinjection of a local anesthetic (tetracaine) in freely behaving rats trained to solve a working memory problem on a radial maze. Reversible septal inactivation resulted in a dramatic, but temporary (15–20 min), impairment in choice accuracy. In addition, movement-induced theta (theta) modulation of the hippocampal EEG was eliminated. Septal injection of tetracaine also produced a significant reduction in location-specific firing by hilar/CA3c complex-spike cells (about 50%), with no significant change in the place-specific firing properties of CA1 complex-spike units. The mean spontaneous rates of stratum granulosum and CA1 theta cells were temporarily reduced by about 50% following septal injection of tetracaine. Although there was a significant reduction in the activities of inhibitory interneurons (theta cells) in CA1, there was no loss of spatial selectivity in the CA1 pyramidal cell discharge patterns. We interpret these results as support for the proposal originally put forth by Marr (1969, 1971) that hippocampal circuits perform pattern completion on fragmentary input information as a result of a normalization operation carried out by inhibitory interneurons. A second major finding in this study was that location specific firing of CA1 cells can be maintained in the virtual absence of the hippocampal theta-rhythm.
Progressive restriction of synaptic vesicle protein to the nerve terminal during development of the neuromuscular junction
Authors: Lupa MT , Hall ZW
Published: March 2018

Antibodies to synaptic vesicle (SV) proteins and to neurofilament (NF) proteins were used to investigate presynaptic differentiation and its relation to the formation of acetylcholine receptor (AChR) clusters at developing mouse neuromuscular junctions. At all times during development, SV proteins and NF proteins were segregated into neighboring, but separate regions of the presynaptic neurite. At embryonic day (ED) 14 SV proteins were present throughout preterminal neurites but at later ages became progressively restricted to the distal parts of the neurites. NF proteins occupied a more proximal region that extended distally during development until NF proteins occupied the entire axon up to the terminal. The restriction of SV proteins exclusively to the terminal did not occur until the second postnatal week. At the time of their first appearance (ED 14), up to 50% of AChR clusters were not associated with neurites; precise colocalization required 12–36 hr to develop. These findings demonstrate a progressive restriction of both pre- and postsynaptic components to the synapse during development.
Interaction of phencyclidine with voltage-dependent potassium channels in cultured rat hippocampal neurons: comparison with block of the NMDA receptor-ionophore complex
Authors: ffrench-Mullen JM , Rogawski MA
Published: March 2018

Whole-cell voltage-clamp recording techniques were used to investigate the blockade of voltage-dependent K+ channels by phencyclidine (PCP) in cultured rat hippocampal neurons. All recordings were carried out in the presence of tetrodotoxin (1–2 microM) to eliminate Na+ currents. Step depolarization from a holding potential of -40 mV activated a slowly rising, minimally inactivating K+ current (IK). PCP (0.5–1000 microM) caused a reduction in the maximum conductance of IK [IC50(+30 mV), 22 microM] without altering its voltage dependency. The PCP block of IK diminished at depolarized potentials. Analysis according to the scheme of Woodhull (1973) suggested that block occurs via binding to an acceptor site (presumably within the channel pore) that senses 40–50% of the transmembrane electrostatic field. PCP had no effect on the kinetic properties of IK and the block failed to show use dependency, suggesting that PCP may bind to the IK channel via a hydrophobic mechanism not requiring open channels. For comparison, we also investigated the effect of PCP on the transient K+ current, IA, activated by step depolarization following a 200 msec prepulse to -90 mV (20 mM tetraethylammonium was present in the bathing solution to reduce IK). In contrast to the potent blocking action of PCP on IK, the drug only affected IA at high concentrations [IC50(+30 mV), 224 microM]. At concentrations causing substantial block (300–500 microM), PCP produced an acceleration in the IA inactivation rate, and, for brief (5–6 msec) depolarizing steps, the suppression of IA was use dependent. These observations suggest that PCP block of IA requires open channels. PCP reduced inward current responses induced by the excitatory amino acid agonist N-methyl-D-aspartate (NMDA) at substantially lower concentrations than those required for its effects on K+ channels [IC50(-60 mV), 0.45 microM]. The PCP-like dioxadrol stereoisomer dexoxadrol (10 microM) blocked NMDA-evoked inward current responses, while its behaviorally inactive enantiomer levoxadrol did not. Dexoxadrol and levoxadrol also blocked IK in a stereoselective fashion (IC50's, 73 and 260 microM, respectively), whereas the sigma ligands (+)- and (-)-SKF 10,047 and (+)-3-[3-hydroxyphenyl]-N-(1- propyl)piperidine [(+)-3-PPP] had little effect on the current (IC50's, greater than 300–500 microM). We conclude that PCP causes a selective, voltage-dependent block of IK in hippocampal neurons via a PCP- and not a sigma-type acceptor site.(ABSTRACT TRUNCATED AT 400 WORDS)
Resolution of metabolic columns by a double-label 2-DG technique: interdigitation and coincidence in visual cortical areas of the same monkey
Authors: Friedman HR , Bruce CJ , Goldman-Rakic PS
Published: March 2018

A double-label modification of the 2-deoxyglucose (DG) technique uses both 3H-2-DG and 14C-2-DG and allows for metabolic activity engaged by 2 distinct experimental conditions to be dissociated throughout the brain of a single subject. In the present study, we used this double- label method to examine the relationship between metabolic columns subserving the 2 eyes in cortical visual areas V1 and V2. The left and right eye's ocular dominance columns were separately activated in the same monkey to demonstrate that the double-label 2-DG method can resolve metabolic differences at the level of the cortical column. In 2 monkeys, 14C-2-DG was injected first and one eye was occluded while the other eye was visually stimulated for 30 min. Then 3H-2-DG was injected, and the occluder was switched so that the alternate eye was stimulated with the same pattern for 30 min. Autoradiographs depicting the 2 labels separately were obtained by exploiting the differential sensitivity of X-ray film and Ultrofilm to 3H and 14C emissions and by applying a radioactivity-subtraction algorithm to pairs of digitized images of the same section (Friedman et al., 1987). Columnar regions of increased activity were evident throughout V1, excepting the representations of the optic disk and monocular crescent. Superimposition of the 3H and 14C images from the same sections demonstrated that columns of increased 3H label were interdigitated with columns of increased 14C label in V1. In contrast, bands of increased 3H-2-DG uptake in extrastriate area V2 were largely coincident with the bands of increased 14C-2-DG uptake. These results illustrate the value of the double-label 2-DG technique for studying fluctuations of metabolic activity under different experimental conditions in the same subject. In the present example, the demonstration that ocular dominance columns are interdigitated in V1, whereas metabolically active bands are coincident in V2, would not have been fully appreciated by comparing 2-DG labeling across separate animals.
Phosphorylation-dependent inhibition by synapsin I of organelle movement in squid axoplasm
Authors: McGuinness TL , Brady ST , Gruner JA , Sugimori M , Llinas R , Greengard P
Published: March 2018

Synapsin I, a neuron-specific, synaptic vesicle-associated phosphoprotein, is thought to play an important role in synaptic vesicle function. Recent microinjection studies have shown that synapsin I inhibits neurotransmitter release at the squid giant synapse and that the inhibitory effect is abolished by phosphorylation of the synapsin I molecule (Llinas et al., 1985). We have considered the possibility that synapsin I might modulate release by regulating the ability of synaptic vesicles to move to, or fuse with, the plasma membrane. Since it is not yet possible to examine these mechanisms in the intact nerve terminal, we have used video-enhanced microscopy to study synaptic vesicle mobility in axoplasm extruded from the squid giant axon. We report here that the dephosphorylated form of synapsin I inhibits organelle movement along microtubules within the interior of extruded axoplasm and that phosphorylation of synapsin I on sites 2 and 3 by calcium/calmodulin-dependent protein kinase II removes this inhibitory effect. Phosphorylation of synapsin I on site 1 by the catalytic subunit of cAMP-dependent protein kinase only partially reduces the inhibitory effect. In contrast to the inhibition of movement along microtubules seen within the interior of the axoplasm, movement along isolated microtubules protruding from the edges of the axoplasm is unaffected by dephospho-synapsin I, despite the fact that the synapsin I concentration is higher there. Thus, synapsin I does not appear to inhibit the fast axonal transport mechanism itself. Rather, these results are consistent with the possibility that dephospho- synapsin I acts by a crosslinking mechanism involving some component(s) of the cytoskeleton, such as F-actin, to create a dense network that restricts organelle movement. The relevance of the present observations to regulation of neurotransmitter release is discussed.
Localization of potential serotonergic facilitator neurons in Aplysia by glyoxylic acid histofluorescence combined with retrograde fluorescent labeling
Authors: Hawkins RD
Published: March 2018

A variety of evidence suggests that 5-HT participates in presynaptic facilitation of the siphon sensory cells contributing to dishabituation and sensitization of the gill- and siphon-withdrawal reflex in Aplysia. Most recently, Glanzman et al. (1989) have shown that the 5-HT neurotoxin 5,7-DHT markedly reduces both the synaptic facilitation and behavioral dishabituation produced by tail shock. To provide more direct evidence for a role of 5-HT, I have used histological techniques to try to locate individual serotonergic facilitator neurons. I first used a modification of the glyoxylic acid histofluorescence technique to map serotonergic and dopaminergic neurons in the CNS of Aplysia. Intracellular fluorescent labeling combined with histofluorescence indicates that the previously identified L29 facilitator neurons are not serotonergic. Nerve transection experiments suggest that most of the perisomatic 5-HT histofluorescence in the abdominal ganglion (the location of the siphon sensory cells) comes from neurons whose cell bodies are located in the pedal or cerebral ganglia. As there are at least 500 serotonergic neurons in those ganglia, I combined retrograde fluorescent labeling with histofluorescence to identify a small subset of those neurons which send processes to the abdominal ganglion and are therefore potential serotonergic facilitators. In the following paper, Mackey et al. (1989) show that stimulation of 2 of those neurons in the cerebral ganglia (the CB1 cells) produces presynaptic facilitation of the siphon sensory cells contributing to dishabituation and sensitization of the withdrawal reflex.
Identified serotonergic neurons LCB1 and RCB1 in the cerebral ganglia of Aplysia produce presynaptic facilitation of siphon sensory neurons
Authors: Mackey SL , Kandel ER , Hawkins RD
Published: March 2018

Several lines of evidence suggest that 5-HT plays a significant role in presynaptic facilitation of the siphon sensory cells contributing to dishabituation and sensitization of the gill- and siphon-withdrawal reflex in Aplysia. Most recently, Glanzman et al. (1989) found that treatment with the 5-HT neurotoxin, 5,7-DHT markedly reduced both synaptic facilitation and behavioral dishabituation. To provide more direct evidence for a role of 5-HT, we have attempted to identify individual serotonergic facilitator neurons. Hawkins (1989) used histological techniques to locate several serotonergic neurons in the ring ganglia that send axons to the abdominal ganglion and are therefore possible serotonergic facilitators. These include one neuron in the B cluster of each cerebral ganglion, which we have identified electrophysiologically and named the CB1 cells. Both glyoxylic acid histofluorescence and 5-HT immunofluorescence indicate that the CB1 neurons are serotonergic. In a semiintact preparation, the CB1 neurons respond to cutaneous stimulation which produces dishabituation and sensitization (such as tail shock) with an increase in firing, which may outlast the stimulation by 15 min. Intracellular stimulation of a CB1 neuron in a manner similar to its response to tail shock produces facilitation of the EPSPs from siphon sensory neurons to motor neurons, as well as broadening of the action potential in the sensory neurons in tetraethylammonium solution. These results strongly suggest that the identified serotonergic CB1 neurons participate in mediating presynaptic facilitation contributing to dishabituation and sensitization of the gill- and siphon-withdrawal reflex in Aplysia.
Adenylate cyclase system is essential for long-term facilitation at the crayfish neuromuscular junction
Authors: Dixon D , Atwood HL
Published: March 2018

Long-term facilitation (LTF), a form of synaptic plasticity demonstrated at the crayfish neuromuscular junction, is induced by tetanic stimulation and persists for hours. LTF can be divided into 2 phases: a tetanic phase, which occurs during stimulation, and a long- lasting phase, which persists after stimulation. Activators and potentiators of cAMP (forskolin and 3-isobutyl-methyl-xanthine) produce facilitation of excitatory postsynaptic potentials, which attain approximately the amplitude of the long-lasting phase of LTF but last for a shorter time. Localized presynaptic injection of a protein inhibitor (“Walsh inhibitor”) specific for the cAMP-dependent protein kinase blocks the long-lasting phase of LTF at synapses near the injection site with no apparent effect on the tetanic phase. Normal LTF develops and persists at synapses of the same axon distant from the injection site. Localization of the injected inhibitor was confirmed by fluorescent tagging. Localized injection of SQ22,536, an adenylate cyclase inhibitor, also blocks the second phase of LTF near the injection site, but not at distant synapses. These experiments establish a role for adenylate cyclase activation in the long-lasting phase of LTF. The phosphatidylinositol second-messenger system is not important in LTF as inhibition of phospholipase C by injection of RA233, which blocks facilitatory effects of serotonin, does not affect any aspect of LTF.
Axonal guidance and the development of muscle fiber-specific innervation in Drosophila embryos
Authors: Johansen J , Halpern ME , Keshishian H
Published: March 2018

The outgrowth of peripheral nerves and the development of muscle fiber- specific neuromuscular junctions were examined in Drosophila embryos using immunocytochemistry and computer-enhanced digital optical microscopy. We find that the pioneering of the peripheral nerves and the formation of the neuromuscular junctions occur through a precisely orchestrated sequence of stereotyped axonal trajectories, mediated by the selective growth cone choices of pioneer motoneurons. We have also examined the establishment of the embryonic muscle fibers and, using intracellular dye fills, have identified cells that are putative muscle pioneers. The muscle fibers of the bodywall have completed their morphogenesis prior to the initiation of synaptic contacts, and owing to the timing of neurite outgrowth from the CNS, synaptogenesis is synchronous at muscle fibers throughout the bodywall. At each muscle fiber the innervating axons make their initial contacts on a characteristic surface domain of the target cell's membrane. Through stereotyped growth cone-mediated trajectories the motoneurons actively establish the basic anatomical features of the mature neuromuscular junction, including the stereotyped, muscle fiber-specific branch anatomy. These events occur without significant process pruning or apparent synapse elimination. Our results suggest that the basic elements of the mature neuromuscular innervation, including the details of the ending trajectory on the target cell's surface, are formed by the precise navigation and presumed recognition by the motoneuron growth cones of muscle membrane surface features.
Multiple electrosensory maps in the medulla of weakly electric gymnotiform fish. II. Anatomical differences
Authors: Shumway CA
Published: March 2018

Both wave- and pulse-type species of weakly electric gymnotiform fish have 3 topographic maps of electroreceptive information in the electrosensory lateral line lobe (ELL). These maps receive identical input from trifurcating axons of phase- and amplitude-coding primary afferents (Carr et al., 1982; Heiligenberg and Dye, 1982). Physiological experiments in the ELL of the wave-type fish Eigenmannia show that the amplitude-coding pyramidal cells differ among maps with respect to receptive field size, sensitivity, rate of adaptation, and temporal-frequency response (Shumway, 1989). This study investigated morphological correlates of the physiological differences among maps. Estimates of primary afferent convergence in Eigenmannia, based on map size, cell counts, and areas of terminal fields from intracellularly filled P-type primary afferents, suggest a 2-fold increase in convergence in the lateral map relative to the centromedial map. Similar differences in convergence between maps are found in the wave- type species Apteronotus leptorhynchus and the pulse-type fish Hypopomus occidentalis. The lateral and centrolateral maps in Hypopomus, however, show an even greater difference in convergence. Comparison of the efferent projections of pyramidal cells among the different maps of Eigenmannia indicates that cells from the 3 maps terminate in the same laminae of the torus semicircularis, but the maps differ in the strength of projection to particular laminae. In both wave-type species, the abundance of a class of interneurons which receives descending input and inhibits pyramidal cells (interneurons of the ventral molecular layer) differs among maps; the centromedial map has 10 times fewer neurons of this type than the other 2 maps. Cytochrome oxidase studies in all 3 species demonstrated increased levels of activity in the lateral map, within the region receiving descending input from the cerebellum. These results suggest that the primary anatomical bases of the physiological differences among maps are differences in the amount of primary afferent convergence, coupled with differences in descending input.
The role of mononuclear phagocytes in wound healing after traumatic injury to adult mammalian brain
Authors: Giulian D , Chen J , Ingeman JE , George JK , Noponen M
Published: March 2018

We monitor cellular responses to a penetrating wound in the cerebral cortex of adult rat during the first weeks after injury. Two classes of activated mononuclear phagocytes containing acetylated low-density lipoprotein (ac-LDL) receptors appear within hours at the wound site. One type of cell surrounding the lesion edge had thin, delicate processes and is identical in appearance to ramified microglia found in developing brain. Within the lesion, round cells are recognized as blood-borne macrophages when labeled by intravenous injection of carbon particles. Thus, both process-bearing reactive microglia and invading macrophages respond to brain trauma. The greatest number of ac-LDL(+) or nonspecific esterase(+) mononuclear phagocytes appears 2 days after injury within the wound site and are associated with a peak production of the cytokine interleukin-1 (IL-1). Because intracerebral infusion of IL-1 is known to stimulate astrogliosis and neovascularization (Giulian et al., 1988), we examine the time course of injury-induced reactive astrogliosis and angiogenesis. A 5-fold increase in the number of reactive astroglia is found at 3 d and a marked neovascularization at 5 d after injury. During the first week, mononuclear phagocytes engulf particles and clear them from the wound site either by migrating to the brain surface or by entering newly formed brain vasculature. To investigate further the role of reactive brain mononuclear phagocytes in CNS injury, we use drugs to inhibit trauma-induced inflammation. When applied in vivo, chloroquine or colchicine reduce the number of mononuclear phagocytes in damaged brain, help to block reactive astrogliosis and neovascularization, and slow the rate of debris clearance from sites of traumatic injury. In contrast, the glucocorticoid dexamethasone neither reduces the number of brain inflammatory cells nor hampers such responses as phagocytosis, astrogliosis, neovascularization, or debris clearance in vivo. Our observations show that mononuclear phagocytes play a major role in wound healing after CNS trauma with some events controlled by secretion of cytokines. Moreover, certain classes of immunosuppressive drugs may be useful in the treatment of acute brain injury.
Histochemical visualization of neutral endopeptidase-24.11 (enkephalinase) activity in rat brain: cellular localization and codistribution with enkephalins in the globus pallidus
Authors: Back SA , Gorenstein C
Published: March 2018

We have developed a novel fluorescent histochemical method to localize the enzyme neutral endopeptidase-24.11 (NEP, E.C. 3.4.24.11, enkephalinase) in the rat brain in order to directly compare the relative distributions of the enzyme and its putative peptide substrate, the enkephalins. The method is based on the sequential cleavage of the synthetic peptide substrate, glutaryl-alanyl-alanyl- phenylanyl-4-methoxy-2-naphthylamide, by NEP and exogenous aminopeptidase M to yield free 4-methoxy-2-naphthylamine (MNA). In the presence of nitrosalicylaldehyde, free MNA is captured, yielding an insoluble yellow fluorescent precipitate which marks the site of NEP activity. The specificity of the method was demonstrated using the selective NEP inhibitors thiorphan, phosphoramidon, and JHF26. All NEP staining throughout the brain was abolished using a 50-nM concentration of these inhibitors. The enzyme was richly localized to many regions, including the cerebral cortex, caudate putamen, globus pallidus, hippocampus, substantia nigra, periaqueductal gray, several cranial nerve nuclei, nuclei of the reticular formation of the medulla. In most regions, reaction product was associated with cell bodies of varying size and morphology. In a number of regions, colchicine increased the amount of NEP staining, particularly in cell processes. The regional distribution pattern of the enzyme, however, did not change in response to colchicine and was similar to that of untreated animals. The histochemical localization of NEP was combined with fluorescent immunocytochemical visualization of the enkephalins in order to localize both in the same tissue section. In the globus pallidus, this combined fluorescent technique revealed numerous NEP-positive cell bodies surrounded by fiber pathways displaying intense enkephalin-like immunoreactivity. The source of the NEP in the globus pallidus was studied using the neurotoxic agent, N-methyl-D-aspartate (NMDA). A pronounced decrease in NEP cellular staining was observed within 7 d in response to NMDA, persisted for at least 16 weeks, and correlated with injury of pallidal neurons. There was no apparent change in enkephalin- like immunoreactivity in the globus pallidus in response to NMDA. These data provide evidence that NEP and enkephalin in the globus pallidus derive from different sources. This study supports the hypothesis that NEP localizes to enkephalin-rich regions of the rat brain, and that the enzyme may be involved in the inactivation of synaptically released enkephalins.
Prolonged enhancement of anterior thalamic synaptic responsiveness by stimulation of a brain-stem cholinergic group
Authors: Pare D , Steriade M , Deschenes M , Bouhassira D
Published: March 2018

This study describes the effects of brain-stem cholinergic laterodorsal tegmental (LDT) stimulation on the synaptic responsiveness of anterior thalamic (AT) neurons. A sample of AT cells, physiologically identified by their short-latency (less than 6.5 msec) response to mammillary body (MB) stimulation, was recorded in unanesthetized, chronically implanted cats and in urethane-anesthetized cats. In chronic experiments, LDT stimulation evoked a short-latency (10–20 msec) excitation in most AT cells. Moreover, brief LDT trains (3 shocks at 300 Hz, every 3 sec) enhanced the responsiveness of AT cells to both MB (orthodromic) and cortical (ortho- and antidromic) stimuli. This effect did not vary as a function of the interval between LDT conditioning and MB or cortical testing shocks, but as a function of the number of trials. The effects of LDT stimuli resisted reserpine treatment (0.75 mg/kg), suggesting that they were not dependent on the coactivation of monoaminergic fibers. Finally, LDT trains did not suppress inhibitory processes in AT neurons when conditioning-testing intervals were longer than 60 msec. Intracellular recordings performed in urethane-anesthetized cats revealed that LDT stimulation induced a short-latency depolarization which increased with membrane hyperpolarization and was associated with an increase in apparent membrane conductance. Usually, isolated LDT trains did not evoke lasting changes in membrane potential or conductance. However, when LDT trains were applied every 3 sec, they gradually decreased the apparent membrane conductance without altering the membrane potential. This conductance change had a time course similar to the LDT-induced potentiation of responsiveness observed in the chronic experiments. In some neurons, LDT conditioning trains also induced a marked increase in the probability of fast prepotentials being triggered by subthreshold MB or cortical orthodromic volleys. In order to distinguish the cumulative effects of repeated LDT trains from the possibly slow time course of LDT influences, we studied the effects of a unique 1 sec LDT train (at 30 Hz) on the synaptic responsiveness of AT cells recorded extracellularly in reserpine-treated, urethane- anesthetized animals. Such LDT trains induced a 2.9-fold increase in synaptic responsiveness, reaching its peak 40–50 sec after the LDT train and lasting up to 4 min. Iontophoretic application of the muscarinic blocker scopolamine blocked these long-lasting potentiating effects of LDT stimuli. Removal of cortical and basal forebrain inputs to the AT nuclear complex by appropriate transections did not abolish the potentiating effects of LDT trains.(ABSTRACT TRUNCATED AT 400 WORDS)
Effect of zinc on NMDA receptor-mediated channel currents in cortical neurons
Authors: Christine CW , Choi DW
Published: March 2018

Recent data have indicated that the divalent cation Zn2+ can selectively block central neuronal excitation mediated by N-methyl-D- aspartate (NMDA) receptors. The present experiments were conducted to determine the action of Zn2+ at the single-channel level. Outside-out membrane patches were prepared from cultured murine cortical neurons. Glutamate, 3 microM, in the presence of 5 microM glycine activated channels with a main conductance state of about 50 pS which were blocked in a voltage-dependent manner by Mg2+. Zn2+ appeared to have 2 effects on these NMDA receptor-activated channels. First, at concentrations as low as 1–10 microM, Zn2+ produced a concentration- dependent reduction in channel open probability, insensitive to membrane voltage between -60 and +40 mV; about 50% reduction in open probability was produced by 3 microM Zn2+. This reduction was mostly due to a decrease in opening frequency and only weakly mimicked by Mg2+. Second, at higher concentrations (10–100 microM) and negative membrane voltages, Zn2+ additionally produced an apparent reduction in single-channel amplitude, associated with an increase in channel noise, suggestive of a fast channel block. The amplitude reduction was voltage- dependent, with a delta of 0.51; amplitude distribution analysis suggested that this voltage dependence was primarily contributed by the “on” blocking rate constant, with little contribution from the “off” rate constant. The channel block produced by Zn2+ was faster than that of Mg2+, which at 100 microM and negative membrane voltages induces flickering of the NMDA receptor-activated channel without changing apparent channel amplitude.(ABSTRACT TRUNCATED AT 250 WORDS)
Conservative amino acid substitution in the myelin proteolipid protein of jimpymsd mice
Authors: Gencic S , Hudson LD
Published: March 2018

The capacity for synthesizing and maintaining a compact myelin sheath is destroyed in a number of inborn errors of myelin metabolism. One class of hypomyelinating mutations, which displays an X-linked pattern of inheritance, is distinguished by marked disturbances in oligodendrocyte differentiation. We have defined the molecular defect in one such mutant that lacks mature oligodendrocytes, the X-linked jimpy myelin synthesis deficient (jpmsd) trait in mice. The structure of the gene encoding the most abundant myelin protein, proteolipid protein (PLP), was determined by mapping and partially sequencing genomic clones from jpmsd and wild-type mice. Jpmsd mice have a single base change in PLP, a C----T transition in exon 6 that would substitute a valine for alanine in both PLP and its alternatively spliced isoform, DM20. The mutation was confirmed by polymerase chain reaction- amplifying exon 6 from genomic DNA and then either sequencing the amplified DNA or directly probing exon 6 with oligonucleotides designed to detect a single base mismatch. The conservative amino acid replacement in PLP/DM20 of jpmsd mice results in a pleiotropic phenotype similar to that observed for the allelic mutation jimpy, in which a splicing defect has radically altered the PLP/DM20 protein. The accelerated turnover of oligodendrocytes in both mouse mutants suggests a function for PLP/DM20 in oligodendrocyte differentiation distinct from the role of these proteolipid proteins as structural components of the myelin sheath.
NMDA receptors mediate poly- and monosynaptic potentials in motoneurons of rat embryos
Authors: Ziskind-Conhaim L
Published: March 2018

We determined the contribution of glutamate receptor subtypes to developing excitatory synaptic transmission in isolated spinal cord of rat embryos. Using electrophysiological and morphological techniques, we studied the pattern of development of synapses between dorsal root afferents and motoneurons in lumbar spinal cords of 15- to 21-d-old rat embryos. Motoneuron dendritic fields and afferent projections onto motoneurons were identified by labeling with HRP. Afferents first entered the gray matter at Day 15 of gestation, and by Day 16 they terminated close to motoneuron dendritic trees. Afferent axons projected onto motoneuron dendritic fields at Day 17, when boutons were detected on motoneuron dendrites that were crossed by afferent axons. To determine the time course of formation of functional sensorimotor synapses and their pharmacological properties, a dorsal root was stimulated while recording intracellularly from segmental motoneurons. At Day 16, excitatory postsynaptic potentials (EPSPs) with long latencies, slow rates of rise, and long durations were recorded. The amplitudes of these EPSPs increased with membrane depolarization and in the absence of extracellular Mg2+. These EPSPs were blocked by D-2- amino-5-phosphonovalerate (APV) and ketamine, which are selective antagonists of N-methyl-D-aspartate (NMDA) receptors. These findings suggest that initial synaptic transmission in embryonic motoneurons is mediated solely by NMDA receptors. Short-latency EPSPs with fast rates of rise were first recorded in most motoneurons by Day 17. These EPSPs were composed of fast- and slow-rising potentials. The slow component was blocked by APV, while the fast component was eliminated by 6-cyano- 7-nitroquinoxaline-2,3-dione and kynurenate. This indicates that the short-latency EPSPs are mediated by both NMDA and non-NMDA receptors. Dose-response curves of motoneurons to L-glutamate, NMDA, and kainate demonstrated that motoneurons are sensitive to these agonists prior to the formation of synapses between afferents and motoneurons. Motoneuron responses to NMDA and kainate increased immediately after the onset of short-latency EPSPs. This increased sensitivity could be due to extracellular factors influenced by growing sensory axons or intrinsic properties of differentiating motoneurons.
Expression of myosin heavy chain isoforms in stimulated fast and slow rat muscles
Authors: Ausoni S , Gorza L , Schiaffino S , Gundersen K , Lomo T
Published: March 2018

The expression of 4 myosin heavy chain (MHC) isoforms was analyzed in the rat soleus (SOL) and extensor digitorum longus (EDL) muscles after denervation and chronic electric stimulation. The stimulation frequencies used were 20 and 150 Hz and the amount of stimulation was either large (20 Hz), intermediate (150 Hz), or small (150 Hz). These patterns resemble some features of normal motor unit activity in SOL and EDL of freely moving rats (Hennig and Lomo, 1985). The relative expression of each MHC isoform depended strongly on the stimulation pattern. Furthermore, for any particular stimulation pattern, fibers in SOL and EDL expressed different MHCs. Coexistence of different MHC types in the same fiber was frequently observed in stimulated muscles. 20-Hz stimulation preserved normal expression of type 1-MHC in SOL but failed to induce type 1-MHC in type 2 fibers of the EDL, where type 2A- and 2X-MHC expression dominated and type 2B-MHC expression was completely suppressed. 150-Hz low-amount stimulation preserved nearly normal 2B-MHC expression in many type 2 fibers of the EDL but failed to induce type 2B-MHC expression in the SOL, where 2X-MHC became predominant. 150-Hz high-amount stimulation differed from 150-Hz small amount stimulation by suppressing almost all type 2B-MHC expression in EDL and by inducing considerable type 2A-MHC expression in the SOL. Scattered fibers in EDL that were probably the original type 1 fibers responded differently from both type 2 fibers in the EDL and from type 1 fibers in the SOL to stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)
Corticotropin-releasing factor produces fear-enhancing and behavioral activating effects following infusion into the locus coeruleus
Authors: Butler PD , Weiss JM , Stout JC , Nemeroff CB
Published: March 2018

The present series of experiments tested the hypothesis that the behavioral activating and anxiogenic effects produced by intraventricular administration of corticotropin-releasing factor (CRF) may be mediated by noradrenergic neurons in the brain-stem locus coeruleus (LC). Results showed that infusion of CRF into the LC (100 ng) significantly increased nonambulatory spontaneous motor activity measured in photocell cages; ambulatory (i.e., locomotor) activity was not altered. In the modified Porsolt swim test, which examines arousal and agitation in a stressful situation, significant behavioral activation (i.e., decreased floating) was seen following infusion of CRF (10 ng) into the LC; a 500 ng dose of CRF was necessary to produce similar effects following infusion into the lateral ventricle. The results of these 2 tests suggest that the behavioral activating effects of CRF in the LC may be related to arousing or stress-related effects, rather than to increased locomotor activity per se. Anxiogenic activity was assessed in animals placed in an open field containing a small, darkened compartment. Infusion or CRF into the LC (1–100 ng) significantly increased the time spent in the compartment and decreased the amount of time spent exploring the outside of the compartment or venturing into the inner squares of the open field, all indices of anxiogenic behavior. Biochemical studies showed that bilateral infusion of CRF into the LC produced significant increases in the concentration of the norepinephrine metabolite 3,4-dihydroxyphenylglycol in such forebrain projection areas of the LC as the amygdala and posterior hypothalamus. These data, taken together, suggest that CRF produces its behavioral activating and anxiogenic effects, at least in part, by increasing the activity of LC noradrenergic neurons.
The spatial distribution of excitatory and inhibitory inputs to ganglion cell dendrites in the tiger salamander retina
Authors: Lukasiewicz PD , Werblin FS
Published: March 2018

In response to focal stimuli, ganglion cell dendrites receive excitation over a relatively narrow extent of the inner plexiform layer (IPL). This excitation is embedded in 2 wider lateral inhibitory regions. Here we estimate the lateral dimensions of the inhibitory regions. Ganglion cells were whole-cell patch-clamped and dendrites were identified and located in retinal slices using Lucifer yellow in the pipettes. The spatial distribution of ganglion cell dendritic sensitivity was measured with puffs of transmitter substances applied at different distances along the dendrites. All ganglion cell dendrites were sensitive to glutamate, GABA, and glycine across their full extent. The responses to puffs decreased with lateral distance from the soma and were well fit by Gaussians. The responses to puffs of potassium showed a similar decrement with distance. Since potassium channels are probably uniformly distributed along the dendrites, the similarity in profiles suggests that receptor density is also uniform along the dendrites. The spatial distribution of responses of ganglion cells to excitatory and inhibitory synaptic inputs was measured by depolarizing local populations of bipolar terminals (and subsequently local populations of amacrine cells) with transretinal current (TRC). TRC-stimulating electrodes were displaced laterally, with respect to the ganglion cell soma, to generate response profiles. We estimated the dimensions of the inhibitory and excitatory signals received by the ganglion cells by removing the contributions of their dendrites, the stimulus, and other interneurons from the response profiles. The excitatory signal extended less than 100 microns, the approximate dimensions of the ganglion cell dendrites, and corresponds roughly to the width of the bipolar inputs. The GABAergic signal extended, on average, 253 microns and glycinergic signal extended, on average, 315 microns. These inhibitory signal dimensions correspond to the width of classes of amacrine cell processes measured in other studies.
GAP-43 in growth cones is associated with areas of membrane that are tightly bound to substrate and is a component of a membrane skeleton subcellular fraction
Authors: Meiri KF , Gordon-Weeks PR
Published: March 2018

To ascertain the subcellular localization of the growth-associated protein GAP-43 in growth cones, we isolated growth cones from the forebrains of neonatal rats. Anti-GAP-43 immunoreactivity in these isolated growth cones (IGCs) resembled that seen in cultured growth cones in 2 respects: First, in substrate-attached IGCs, as in cultured growth cones, immunoreactivity was intracellular, punctate, and extended throughout the IGCs and their processes. Second, when IGCs were dislodged from this substrate, patches of membrane that were highly immunoreactive for GAP-43 were left behind. Extracting IGCs in nonionic detergents revealed that almost all the particulate GAP-43 colocalized with a membrane skeleton fraction: It was not present in the cytoskeleton. The association of GAP-43 with the membrane skeleton was not due to nonspecific aggregation, nor was it calcium dependent. Examination of this fraction under the electron microscope showed it to consist of membrane fragments associated with amorphous material that could be visualized with tannic acid. Immunoelectron microscopy showed that anti-GAP-43 immunoreactivity was localized to this amorphous material. Fodrin, talin, and a-actinin immunoreactivity could be detected in the membrane skeleton fraction, and actin and tubulin were also identifiable from SDS gels. The association of GAP-43 with the membrane skeleton, which by analogy with other cell types is involved in the dynamic regulation of cell shape, implies that GAP-43 in growth cones may be involved in this function.
Microinjection of cocaine into the nucleus accumbens elicits locomotor activation in the rat
Authors: Delfs JM , Schreiber L , Kelley AE
Published: March 2018

Cocaine is believed to exert its psychostimulant effects through activation of the mesocorticolimbic system. Although the nucleus accumbens, in particular, has been hypothesized as the site of action of cocaine's stimulating effects, there is no direct evidence that microinjection of cocaine into this region produces behavioral activation. The present experiments investigated the locomotor response to microinjection of cocaine (0, 10, 30, 100 micrograms/0.5 microliter) into the nucleus accumbens in rats. Cocaine elicited a pronounced, dose- dependent motor activation of approximately 60 min duration. This stimulant effect was blocked by prior administration of a dopamine (DA) receptor antagonist, cis-flupenthixol. The response to cocaine was differentiated from nucleus accumbens microinjections of procaine and lidocaine, compounds that have potent local anesthetic effects but little affinity for the dopamine-uptake site. Neither procaine nor lidocaine (0, 10, 30, 100 micrograms/0.5 microliter) had any overall effect, although activity was somewhat decreased in the initial part of the test session and increased at the end, relative to control activity. Cocaine injected into the anterior dorsal or ventrolateral striatum (100 micrograms) also increased motor activity; procaine and lidocaine had no effect. Cocaine injected into the ventrolateral striatum significantly increased stereotypy. The amplitude of motor activation following cocaine injection into nucleus accumbens was much greater than that elicited at the other striatal sites. Further, observation of the time course of motor activation following cocaine injection into the anterior dorsal and ventrolateral striatum suggested that the motor effect was due to diffusion, most likely to the nucleus accumbens.(ABSTRACT TRUNCATED AT 250 WORDS)