bluuebunny/crossref_metadata_2025 huggingface dataset:
Opiates suppress a resting sodium-dependent inward current and activate an outward potassium current in locus coeruleus neurons
Authors: Alreja M , Aghajanian GK
Published: March 2018

The opioid peptide met-enkephalin (met-ENK) produces an outward current with an increase in input conductance in locus coeruleus (LC) neurons. This current has been attributed to an opening of potassium channels. However, the opioid-induced current tends to reverse at potentials more negative than the expected potassium reversal potential (EK) or does not reverse at all. Since lack of reversal can occur if there is a simultaneous increase in one conductance and a decrease in a second conductance, we tested the possible contribution of a second conductance to the opioid-induced outward current in LC neurons. Biochemically, opiates inhibit adenylate cyclase in LC neurons and cAMP- active agents produce a sodium-dependent inward current in these neurons. This current is also present at rest, as sodium substitution hyperpolarizes LC neurons. By inhibiting adenylate cyclase, could opiates be turning off this current? To evaluate this possibility, we used intracellular voltage-clamp technique in rat LC slices, and studied the effect of sodium substitution on the opiate response. Replacement of external sodium (80%) with Tris or choline caused (1) an outward current with a decrease in input conductance and (2) an approximately 50% decrease in the met-ENK-induced outward current with a shift in its reversal potential toward EK. Extracellular Ba2+, a K+ channel blocker, also partially reduced the opiate response, but it shifted its reversal potential away from EK. The met-ENK-induced outward current was almost totally abolished by combined sodium substitution and extracellular Ba2+ in an additive manner.(ABSTRACT TRUNCATED AT 250 WORDS)
Process outgrowth and synaptic varicosity formation by adult photoreceptors in vitro
Authors: Mandell JW , MacLeish PR , Townes-Anderson E
Published: March 2018

To assess the regenerative capability of the photoreceptor synapse, we have isolated and cultured photoreceptors from the mature salamander retina. Both rod and cone photoreceptors were able to regenerate processes within 3 d of plating. Cells extended numerous actin- containing filopodia as well as a few neuritic processes. The neurites contained microtubules and formed synaptic vesicle-filled varicosities, as shown by immunostaining for tubulin and synaptic vesicle proteins and by electron microscopy. Furthermore, regenerated varicosities were capable of depolarization-induced vesicle labeling, suggesting that they can recycle synaptic vesicles and release neurotransmitter by synaptic vesicle exocytosis. Differences were observed between rod and cone cell synaptic regeneration in vitro, which resembled structural differences between their synaptic terminals in situ: rod cells formed multiple synaptic vesicle-filled varicosities along neurites at a distance from the soma, whereas cone cells tended to accumulate synaptic vesicles within the soma. The regeneration of neurites and synaptic vesicle-filled varicosities was abolished by microtubule depolymerizing agents, suggesting a role for microtubule-based vesicle transport in the formation of varicosities. Finally, process outgrowth and varicosity formation were independent of cell-cell contact and, indeed, proceeded in the complete absence of other cells. These findings suggest not only that differentiated photoreceptors are capable of synaptic renewal but that the regeneration of presynaptic- like terminals is an intrinsic ability of rod and cone cells.
Development of individual geniculocortical arbors in cat striate cortex and effects of binocular impulse blockade
Authors: Antonini A , Stryker MP
Published: March 2018

This study analyzes the morphological changes in geniculocortical axons terminating in the primary visual cortex of the cat, during the period in which, in normal development, the terminals in layer IV undergo an eye-specific segregation. Geniculocortical afferent fibers were filled anterogradely by the Phaseolus lectin (PHA-L) injected into the main laminae of the LGN. After standard immunohistochemical procedures, single axons were serially reconstructed in two or three dimensions. Experiments were performed in normal kittens and in kittens in which retinal activity was continuously blocked by repeated intraocular injections of TTX. In normal kittens, arbors were reconstructed at four different ages (19, 23, 30–31, and 39 days postnatally) spanning the period during which the geniculocortical projection segregates into eye- specific columns in layer IV (LeVay et al., 1978). Results reveal that sparse but widely extending branches characteristic of young arbors are eliminated during normal development at the same time as selected portions of the arbor grow considerably in length and complexity. The terminal arborizations also subdivide into distinct patches of terminals, consistent with the segregation of left and right eye afferents. In TTX-treated animals, axonal arbors reconstructed at postnatal days 23, 29, and 39 show a complexity and extent of terminal arborization similar to that of normal animals, though more variable in size and degree of elaboration. No progressive changes are evident with age. Further, the majority of arbors reconstructed from TTX-treated animals lack the patchy organization typical of normal animals.
Interleukin-1 beta augments release of norepinephrine, dopamine, and serotonin in the rat anterior hypothalamus
Authors: Shintani F , Kanba S , Nakaki T , Nibuya M , Kinoshita N , Suzuki E , Yagi G , Kato R , Asai M
Published: March 2018

We investigated the effects of interleukin-1 beta (IL-1 beta), administered directly into the rat anterior hypothalamus (AHY), on monoamine release in the same region by using a brain microdialysis technique and an HPLC-electrochemical detection system. First, to study the local effects of IL-1 beta, we used a microdialysis probe equipped with a microinjection tube for administering IL-1 beta in the same region into which the probe had been inserted. IL-1 beta (1 ng) injected directly into the AHY elicited release of norepinephrine (NE), dopamine (DA), and 5-HT, as well as increases in their metabolites, 4- hydroxy-3-methoxyphenylglycol, 3,4-dihydroxyphenylacetic acid, 4- hydroxy-3-methoxyphenylacetic acid, and 5-hydroxyindole-3-acetic acid, in the AHY. Vehicle alone exerted no effect on monoamine release. Although the elevated levels of NE and DA persisted for more than 6 hr after injection of IL-1 beta, the elevated levels of 5-HT were transient. Second, in order to investigate whether this effect of IL-1 beta is a direct action in the AHY, we performed in vitro experiments using hypothalamus slices. IL-1 beta (0.1 and 1 nM) increased the levels of each monoamine released from hypothalamic slices in a dose- dependent manner. These findings suggest that IL-1 beta acts directly on the hypothalamus to induce release of NE, DA, and 5-HT. Third, the roles of prostaglandins (PGs) in NE release in the AHY elicited by direct injection of IL-1 beta were examined.(ABSTRACT TRUNCATED AT 250 WORDS)
Horizontal cells in the cone-dominated tree shrew retina: morphology, photoreceptor contacts, and topographical distribution
Authors: Muller B , Peichl L
Published: March 2018

While most mammalian retinas are rod dominated, in the tree shrew retina 95% of the photoreceptors are cones. We studied three shrew horizontal cells to look for features associated with this unusual photoreceptor arrangement. The morphology of horizontal cells was revealed by intracellular injections of Lucifer yellow, and their photoreceptor contacts were assessed by light and electron microscopy. Horizontal cell topography was studied in material stained with a neurofilament antibody and with toluidine blue. The tree shrew has two types of horizontal cell that are basically the same as A- and B-type horizontal cells of other mammals. All the photoreceptor contacts of the larger, axonless, A-type cell and the dendritic contacts of the smaller, axon-bearing, B-type cell are with cones. Both types contact nearly all the cones in their dendritic field and both types synapse with both red and blue cones. There is no anatomical evidence for chromatic selectivity. The sparsely branched B-type horizontal cell axon probably contacts rods as in other mammals. The unusual features of the A-type cell are the profuse dendritic terminal arborizations and the large dendritic field size. These features may be related to the abundance of cones but do not justify the conclusion for a special type of horizontal cell as has previously been supposed. Both types of horizontal cell have a central-peripheral density gradient; at any location B-type cells are up to three times as numerous as A-type cells. There are detailed features of the distributions that differ from those of other mammalian horizontal cells. The density maximum of B-type cells is in inferior retina and roughly coincides with that of the cones; the A-type maximum is located more superiorly. Neither maximum is colocalized with the ganglion cell peak in the central area. The mosaic of B-type cells is much more regular than that of A-type cells.
The direction change concept for reticulospinal control of goldfish escape
Authors: Foreman MB , Eaton RC
Published: March 2018

This is an analysis of whether biomechanical or kinematic variables are controlled by descending reticulospinal commands to the spinal cord during escape responses (C-starts) in the goldfish. We studied how the animal contracted its trunk musculature to orient an escape trajectory. We used trunk EMG recordings as a measure of the reticulospinal output to the musculature and we simultaneously gathered high-speed cinematic records of the resulting movements. We found that the escape trajectory is controlled by (1) the relative size of the agonist versus the antagonist muscle contractions on two sides of the body and (2) the timing between these contractions. We found no separate signal for forward propulsion (or force) apart from the initial stage 1 bending of the body. Rather, the neural specification of force is embedded in the commands to bend the body. Thus, our findings demonstrate the importance of the angular kinematic components, or direction changes, caused by the descending reticulospinal command. This new direction change concept is important for two reasons. First, it unifies the diversity of C-start movement patterns into a single and rather simple quantitative model. Second, the model is analogous to the systematic EMG and kinematic changes observed by others to underlie single joint movements of limbs in other vertebrates such as primates. As in these cases, the fish capitalizes on the mechanical properties of the muscle by setting the extent and timing of agonist and antagonist contractions. This, plus the fact that sensory feedback is likely to be minimal, may enable the animal to reduce the number of computational steps in its motor commands used to produce the escape response. Because horizontal body movements in fish are a fundamental vertebrate movement pattern produced by a highly conserved brainstem movement system, our findings may have general implications for understanding the neural basis of rapid movements of diverse body parts.
The layout of iso-orientation domains in area 18 of cat visual cortex: optical imaging reveals a pinwheel-like organization
Authors: Bonhoeffer T , Grinvald A
Published: March 2018

In this study we used optical imaging based on activity-dependent intrinsic signals to determine the distribution of cells responding to gratings of various orientations moving in different directions in area 18 of cat visual cortex. To test directional-selective clustering of neurons, we compared cortical activity maps obtained by stimulation with two gratings of identical orientation but moving in opposite directions. We found those maps to be almost identical, suggesting that neurons are not notably clustered into directionality columns. We also compared activity maps obtained with gratings of different orientations. Each of the orientation maps was similar to the 2- deoxyglucose maps previously reported. Having compiled the information obtained from the different orientations into one “orientation preference map,” we found, in contrast to earlier reports, that iso- orientation domains are not elongated parallel bands but are small patches organized in “pinwheels” around points that we refer to as “orientation centers.” We furthermore show that the only locations at which orientation preference changes rapidly are these orientation centers and not lines or loops. In addition, this report clarifies that our observations on the functional architecture of cat area 18, although at first sight at variance with earlier observations, are actually fully consistent with them. We therefore propose that in cat visual cortex pinwheel-like patterns of orientation preference form an irregular mosaic of modular units with an average density of 1.2 pinwheels per square millimeter.
Growth factors protect PC12 cells against ischemia by a mechanism that is independent of PKA, PKC, and protein synthesis
Authors: Boniece IR , Wagner JA
Published: March 2018

We have established an in vitro model of ischemia incorporating the combination of anoxia with glucose deprivation, which is toxic to PC12 cells. In this model, nerve growth factor (NGF), basic fibroblast growth factor (bFGF), and epidermal growth factor (EGF) improve PC12 cell survival. K252a, a specific inhibitor of NGF-induced trk p140 autophosphorylation, did not alter the neuroprotection provided by EGF or bFGF, yet it completely abolished the protection provided by NGF. Activation of protein kinase A (PKA) with dibutyryl-cAMP also protected during ischemia, although it was not additive with the effect provided by growth factors. Furthermore, growth factors protected a PKA- deficient mutant as effectively as the parental cell line; thus, activation of PKA is protective against ischemia but is not necessary for the action of peptide growth factors. Neither the stimulation of protein kinase C (PKC) with acute phorbol ester treatment nor the downregulation of PKC with chronic high-dose phorbol ester treatment resulted in an altered response to growth factors in either the PC12 wild type or PKA-deficient mutant. Thus, protection by peptide growth factors depends on neither PKA nor PKC. Furthermore, downregulation of PKC alone was protective, indicating that PKC may contribute to toxicity. Interestingly, treatment with the kinase inhibitor H-7 was neuroprotective and may have enhanced the neuroprotective effect of NGF. In contrast, staurosporine, a broadly acting kinase inhibitor, inhibited the neuroprotective effect of NGF, but not of EGF or FGF.(ABSTRACT TRUNCATED AT 250 WORDS)
Calcium signaling of glial cells along mammalian axons
Authors: Kriegler S , Chiu SY
Published: March 2018

Glial [Ca2+]i signaling was examined in a mammalian white matter lacking neuronal cell bodies and synapses. Rat optic nerves (postnatal days 2 and 7) were stained with calcium indicator dyes and confocal images of [Ca2+bdi were recorded at approximately 25 degrees C or approximately 37 degrees C. Glial cell bodies showed spiking or sustained [Ca2+], response to bath-applied glutamate (50–500 microM). The metabotropic glutamate agonist trans-ACPD elicited transient, sometimes spiking, [Ca2+], responses, whereas ionotropic agonists kainate and AMPA elicited a 6,7-dinitroquinoxaline-2,3-dione-sensitive, mostly sustained [Ca2+]i response. Transient and spiking glial [Ca2+]i responses also were elicited by adenosine and ATP (0.1–100 microM). Repetitive nerve stimulation (10–20 Hz) elicited [Ca2+bdi spiking in 15–25% of glial cells in postnatal day 7 nerves, with spiking typically occurring 15–60 sec after onset of nerve stimulation. At 37 degrees C, the frequency of glial [Ca2+]i spikes increased from approximately 0.06 Hz to approximately 0.11 Hz when axonal stimulation was increased from 10 to 20 Hz. This activity-dependent glial spiking was inhibited by TTX, could not be mimicked by increasing the bath K+ by 20 mM, and occurred when nerves were stimulated in the absence of bath calcium. Activity-dependent and glutamate-induced glial spiking could be mimicked by altering ionic gradients known to favor release of glutamate via glutamate transporters, including elevation of intracellular Na+ by veratridine concurrent with external K+ elevation. We suggest that glial [Ca2+]i spiking observed during electrical activity resulted from activation of glial receptors (e.g., metabotropic glutamate receptor, adenosine receptor) by substances (e.g., glutamate, adenosine) released from the optic nerve in a nonvesicular fashion, possibly through a reversal of sodium-coupled transporters when Na+ and K+ gradients are altered by prolonged nerve activity.
Dopamine transporter expression confers cytotoxicity to low doses of the parkinsonism-inducing neurotoxin 1-methyl-4-phenylpyridinium
Authors: Pifl C , Giros B , Caron MG
Published: March 2018

The uptake of 1-methyl-4-phenylpyridinium (MPP+), the active metabolite of the parkinsonism-inducing neurotoxin 1-methyl-4-phenyl-1,2,3,6- tetrahydropyridine (MPTP), was studied in various mammalian cell lines transfected, respectively, with the cloned human and rat dopamine transporters, and compared with rat striatal synaptosome preparations. Only in neuronally derived cell lines such as NG108–15, NS20Y, and SK-N- MC cells did MPP+ have a KM for the cloned transporters comparable to that of dopamine as seen in rat striatal synaptosomes. In non- neuronally derived cells such as COS-7, CHO, and Ltk- cells transiently or permanently expressing the transporters, the KM of MPP+ was at least 10-fold higher. The permanent expression of either the cloned human or rat dopamine transporters conferred to SK-N-MC cells susceptibility to the cytotoxic effects of low concentrations of MPP+. The extent of this effect was dependent on the expression level of the dopamine transporters and could be specifically antagonized by the catecholamine uptake inhibitor mazindol. There were no significant differences in the susceptibility to MPP+ of cells expressing similar levels of either the human or rat dopamine transporter. The demonstration for the first time of a quantitative relationship between the cellular expression of the plasma membrane transporter and the extent of the cytotoxic effects of MPP+ suggests that known differences in vulnerability of various brain regions to MPP+ cytotoxicity might be related to their actual content of dopamine uptake sites.(ABSTRACT TRUNCATED AT 250 WORDS)
Autoradiographic localization of serotonin receptor subtypes in cat visual cortex: transient regional, laminar, and columnar distributions during postnatal development
Authors: Dyck RH , Cynader MS
Published: March 2018

Postnatal changes in the distribution of 5-HT receptor subtypes in the visual cortex of cats were assessed both qualitatively and quantitatively using in vitro autoradiographic methods. The 5-HT 1A, 1C, 2, and 3 receptor subtypes and the 5-HT uptake (5-HTUp) site were visualized with 3H-8-hydroxy-2(di-n-propyl-amino)tetralin, 3H- mesulergine, (2,5-dimethoxy-4-125I-iodophenyl)-2-aminopropane, 3H- BRL43694, and 3H-cyanoimipramine, respectively. Although specific labeling of 5-HT3 receptors was not detected in the cat visual cortex at any age, each of the remaining 5-HT receptor subtypes exhibited unique temporal, regional, and laminar patterns of expression in visual cortical areas 17, 18, and 19 and lateral suprasylvian cortex (LS). 5- HT1A receptors were the earliest to demonstrate visual cortex-specific changes in expression. They exhibited peak levels of expression in all visual cortical areas, predominantly in supra- and infragranular layers, between postnatal day 10 (PD10) and PD30. Their levels in all areas declined progressively with increasing age. 5-HT1c receptors demonstrated their highest levels of expression in the deeper half of layer IV, but only in area 17, between PD40 and PD75. The most striking feature of their distribution throughout this period was that, in layer IV and extending into layer III, the 5-HT1c receptors were concentrated in columns that were 400 microns wide and had a center-to-center spacing of about 900 microns. This transient pattern of expression was not present beyond PD90. 5-HT2 receptors were most densely expressed in layer IV between PD40 and PD120, but they displayed a distinctly different distribution pattern. The densest binding of 5-HT2 receptors was limited to the upper half of layer IV and found in areas 17, 18, and LS. The distribution of layer 5-HT2 receptors along the dense band in layer IV of area 17 was discontinuous, exhibiting patches that were found in the same vertical columns as were the 5-HT1c receptors. Intermediate binding levels for the 5-HT2 receptors were found through layers I-III, the remainder of layer IV, and the subcortical white matter. The levels of 5-HT uptake sites increased gradually to reach adult levels by PD40, but with a distribution pattern that was basically homogeneous, both across cortical regions and across laminae.(ABSTRACT TRUNCATED AT 400 WORDS)
The contributions of NMDA, non-NMDA, and GABA receptors to postsynaptic responses in neurons of the optic tectum
Authors: Hickmott PW , Constantine-Paton M
Published: March 2018

Activation of the NMDA subtype of glutamate receptor has been implicated in activity-dependent development and plasticity in several systems, including the retinotectal system of amphibians. To gain a better understanding of the response properties of tectal neurons, with particular emphasis on the role of both non-NMDA and NMDA glutamate receptors, we have developed an in vitro slice preparation of the diencephalon and midbrain of frog (Rana pipiens) tadpoles. In these slices, we electrically stimulated the optic tract and recorded both mono- and polysynaptic responses in single tectal neurons using whole- cell voltage clamp or current clamp. By including biocytin in the recording electrode, we were also able to determine the location and morphology of many of these neurons. Using these techniques, we found that the current-voltage (I-V) relations for both mono- and polysynaptic responses of tectal neurons showed voltage dependence only in the presence of extracellular Mg2+. This dependence reflects the hyperpolarization-dependent block of the NMDA channel by Mg2+. Bath application of 6-cyano-7-nitroquinoxaline-2,3-dione, a non-NMDA glutamate receptor antagonist, reduced both mono- and polysynaptic responses of tectal neurons. Bath application of the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (DL-APV) strongly reduced polysynaptic responses. When neurons were depolarized by the voltage clamp, relieving the Mg(2+)-dependent block of the NMDA channel, DL-APV application also reduced monosynaptic responses. Application of the GABAA receptor antagonist (-)bicuculline methiodide significantly increased the polysynaptic responses of tectal neurons, reflecting block of inhibition. We further confirmed the presence of these three types of receptors by examining postsynaptic currents evoked by iontophoretic application of the three agonists, NMDA, (R,S)-alpha- amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), and GABA. These results confirm that the dominant excitatory transmitter in the tectum appears to be glutamate. Furthermore, the retinotectal synapses (i.e., monosynaptic currents) express functional NMDA receptors that are voltage dependent and are not responsible for the bulk of normal excitatory transmission. Polysynaptic responses, however, are mediated by both non-NMDA and NMDA receptors, and inhibition plays a significant role in sculpting these polysynaptic responses.
Redistribution of cytoskeletal proteins in mammalian axons disconnected from their cell bodies
Authors: Watson DF , Glass JD , Griffin JW
Published: March 2018

Mice of the strain C57/BL/Ola exhibit a delay of Wallerian degeneration, such that axons survive for several weeks after a nerve transection that separates the axons from the cell bodies. In this Ola strain we have examined the distribution of cytoskeletal proteins in a 5 mm segment of the sciatic nerve for as long as 2 weeks after proximal and distal transections that prevent entry or exit of proteins via axonal transport. By 7 d after transections, there was a marked accumulation of alpha- and beta-tubulin, actin, and nonphosphorylated neurofilament epitopes at the proximal and at the distal ends of the transected axons, and loss of these proteins from the center of the isolated nerve segment. Highly phosphorylated neurofilament epitopes did not redistribute along the nerve, but there was a gradual loss of phosphorylated neurofilament immunoreactivity. These observations indicate the potential for bidirectional transport of a substantial portion of certain cytoskeletal proteins within axons of the PNS.
Activation of protein kinase C by arachidonic acid selectively enhances the phosphorylation of GAP-43 in nerve terminal membranes
Authors: Schaechter JD , Benowitz LI
Published: March 2018

Arachidonic acid (AA), a cis-unsaturated fatty acid that activates certain subspecies of protein kinase C (PKC), has been proposed to act as a retrograde messenger in modifying the efficacy of synapses during long-term potentiation (LTP). One prominent PKC substrate of the nerve terminal membrane, GAP-43 (F1, B-50, neuromodulin), shows an increase in phosphorylation that correlates with the persistence of LTP. The present study investigated whether AA might exert its effects on presynaptic endings by modulating the phosphorylation of GAP-43 and other membrane-bound proteins. Using synaptosomal membranes from the rat cerebrocortex, in which in vivo relationships between protein kinases and their native substrates are likely to be preserved, we found that in the absence of Ca2+, AA exerted a modest effect on the phosphorylation of GAP-43 and several other proteins; however, when AA was applied in conjunction with Ca2+, GAP-43 showed a particularly striking response: at Ca2+ levels likely to exist at the nerve terminal membrane during synaptic activity (10(-7) to 10(-5) M), AA (50 microM) increased the sensitivity of GAP-43 phosphorylation to Ca2+ by an order of magnitude, and increased its maximal level of phosphorylation by 50%. At resting Ca2+ levels, AA potentiated the stimulation in GAP-43 phosphorylation produced by 4 beta-phorbol 12,13-dibutyrate, a diacylglycerol (DAG) analog. The stimulatory effect of AA and its synergistic interaction with Ca2+ were found to be mediated by PKC, since they were blocked by a specific peptide inhibitor of PKC, [Ala25]PKC(19–31), but were unaffected by an inhibitor of protein phosphatase activity or by scavengers of free radicals. Since GAP-43 has been implicated in the development and plasticity of synaptic relationships, the synergistic effects of AA and the intracellular signals Ca2+ and DAG on the phosphorylation of GAP-43 may serve as an AND gate to modify presynaptic function and/or structure in response to coincident pre- and postsynaptic activity.
Crayfish tonic inhibition: prolonged modulation of behavioral excitability by classical GABAergic inhibition
Authors: Vu ET , Krasne FB
Published: March 2018

Previous studies have indirectly implicated the two neurotransmitters 5- HT and GABA in mediating tonic inhibition of the crayfish lateral giant (LG) escape reaction. In this study, pharmacological agents were selectively delivered to restricted portions of the abdominal CNS (where LG escape circuitry resides) to assess directly the role of these two transmitters in tonic inhibition. Both 5-HT and GABA depressed monosynaptic, electrical transmission to the LG neurons, the command neurons for LG escape, and application of either transmitter resulted in a depolarizing conductance increase in the LG neuron. The effects of 5-HT persisted in preparations in which chemical transmission was effectively abolished, implying that there are 5-HT receptors on the LG neuron itself, along with the known GABA receptors. Restricted delivery of the GABA chloride channel blocker picrotoxin to only the abdominal CNS blocked the expression of tonic inhibition there (without interfering with the rostral generation of tonic inhibition). Therefore, if 5-HT mediated tonic inhibition, the effects of 5-HT on the abdomen should also be antagonized by picrotoxin. However, this was not the case, thus suggesting that 5-HT does not mediate tonic inhibition. The most likely neurotransmitter used for tonic inhibition is GABA acting via ligand-gated chloride channels. Thus, although this form of behavioral modulation can be tonically active for very long periods, it nevertheless appears to be mediated by a classical synaptic mechanism.
Extracellular ATP activates a cation conductance and a K+ conductance in cultured microglial cells from mouse brain
Authors: Walz W , Ilschner S , Ohlemeyer C , Banati R , Kettenmann H
Published: March 2018

Microglial cells have important functions during regenerative processes after brain injury. It is well established that they rapidly respond to damage to the brain tissue. Stages of activation are associated with changes of cellular properties such as proliferation rate or expression of surface antigens. Yet, nothing is known about signal substances leading to the rapid changes of membrane properties, which may be required to initiate the transition from one cell stage into another. From our present study, using the patch-clamp technique, we report that cultured microglial cells obtained from mouse or rat brain respond to extracellularly applied ATP with the activation of a cation conductance. Additionally, in the majority of cells an outwardly directed K+ conductance was activated with some delay. Since ADP, AMP, and adenosine (in descending order) were less potent or ineffective in inducing the cation conductance, the involvement of a P2 purinergic receptor is proposed. The receptor activation is accompanied by an increase of cytosolic Ca2+ as determined by a fura-2-based Ca(2+)- imaging system. This ATP receptor could enable microglial cells to respond to transmitter release from nerve endings with ATP as a transmitter or cotransmitter or to the death of cells with resulting leakage of ATP.
Seizures and brain injury in neonatal rats induced by 1S,3R-ACPD, a metabotropic glutamate receptor agonist
Authors: McDonald JW , Fix AS , Tizzano JP , Schoepp DD
Published: March 2018

The role of metabotropic excitatory amino acid receptors in seizures and brain injury was examined using the selective metabotropic agonist 1S,3R-ACPD [(1S,3R)-1-aminocyclopentane-1–3-dicarboxylic acid] in 7-d- old neonatal rats. Systemic administration of 1S,3R-ACPD produced dose- dependent convulsions (ED50 = 16 mg/kg, i.p.) that were stereoselective for the active metabotropic ACPD isomer, since 1R,3S-ACPD was less potent (ED50 = 93 mg/kg, i.p.). 1S,3R-ACPD-induced seizures were antagonized by systemic administration of dantrolene, an inhibitor of intracellular calcium mobilization, but not by the ionotropic glutamate antagonists MK-801 or GYKI-52466. As indexed by hemispheric brain weight differences 5 d postinjection, unilateral intrastriatal injection of 1S,3R-ACPD (0.1–2.0 mumol/microliters), but not 1R,3S- ACPD, produced dose-dependent brain injury (maximal effect of 3.4 +/- 0.5% damage). 1S,3R-ACPD brain injury occurred in the absence of prominent behavioral convulsions. Histologic and ultrastructural examination of 1S,3R-ACPD-injected rat brains revealed swelling and degeneration of select neurons at 4 hr postinjection, but little evidence of injured neurons 5 d later. 1S,3R-ACPD-mediated brain injury was not attenuated by systemic administration of the NMDA antagonist MK- 801 or the AMPA antagonist GYKI-52466. However, cointrastriatal injection of dantrolene reduced the severity of 1S,3R-ACPD injury by 88 +/- 7%. These studies indicate that seizures and neuronal injury can be elicited by the selective activation of metabotropic glutamate receptors in perinatal rats, and these effects of 1S,3R-ACPD involve the mobilization of intracellular calcium stores.
Two distinct rhythmic motor patterns are driven by common premotor and motor neurons in a simple vertebrate spinal cord
Authors: Soffe SR
Published: March 2018

Xenopus embryos show two distinct rhythmic motor patterns: swimming and struggling. Both can be generated by spinal cord circuitry and evoked by stimulation of a single skin sensory pathway (Soffe, 1991b). This presents a valuable opportunity to explore mechanisms for vertebrate motor pattern switching. Swimming and struggling have been compared using intracellular recording from spinal neurons in immobilized embryos. Underlying synaptic drive was similar; motoneurons and premotor interneurons were excited in phase with ipsilateral motor root discharge and inhibited in phase with contralateral motor root discharge. Excitation was stronger during struggling and associated with short bursts of impulses, contrasting with single spikes per cycle during swimming. Excitation was reduced in both patterns by local application of 1 mM kynurenic acid, indicating excitatory amino acid mediation. Inhibition was antagonized by 1 microM strychnine, indicating glycine mediation. Many motoneurons (76%) and premotor interneurons (68%) fired during both swimming and struggling, including examples of all three spinal premotor interneuron classes. Most of the remaining motoneurons (20%) and premotor interneurons (24%) fired only during struggling, providing roughly 30% more active neurons than during swimming. To investigate whether new neuronal classes become active during struggling, recordings were made from sensory neurons and sensory interneurons. Rohon-Beard sensory neurons did not fire during either swimming or struggling. Dorsolateral commissural sensory interneurons received rhythmic, strychnine-sensitive inhibition during both swimming and struggling and also did not fire. Neither of these neuronal classes is therefore recruited to the circuitry for struggling. Although behaviorally distinct, Xenopus embryo swimming and struggling motor patterns appear to employ similar synaptic drive. I propose that this reflects the common nature of much of the premotor circuitry that drives them. Extra neurons are recruited to this circuitry during struggling, but only from within classes that also participate in swimming.
Hippocampal changes in developing postnatal mice following intrauterine exposure to domoic acid
Authors: Dakshinamurti K , Sharma SK , Sundaram M , Watanabe T
Published: March 2018

Domoic acid (0.6 mg/kg) was injected intravenously through the caudal vein in pregnant female mice on the 13th day of gestation and EEG was monitored in the developing progeny during postnatal days 10–30. No clinical seizure activity was observed during this period. However, these mice demonstrated generalized electrocortical inhibition associated with diffuse spike and wave activity in their basal EEG records. Intrauterine domoic acid-exposed (IUD) mice had significantly reduced seizure thresholds to an additional dose of domoic acid, given postnatally. At the light microscopic level, hippocampus of IUD mice exhibited age related developmental neurotoxicity. No cellular damage was observed on postnatal day 1. On day 14, severe neuronal damage was observed in the hippocampal CA3 and dentate gyrus regions. On day 30, in addition to CA3 and dentate gyrus, CA4 was also involved. Brain regional GABA levels were significantly reduced and glutamate levels increased in IUD mice. Kainate receptor binding to hippocampal synaptosomal membranes from IUD mice at 30 d of age was significantly increased. There was also an enhanced 45Ca influx into cortical and hippocampal slices of these mice. These findings suggest that intrauterine exposure to domoic acid can induce hippocampal excitotoxicity by increasing the neuronal calcium influx through kainate receptor activation. Histological changes suggest progressive hippocampal damage in IUD mice, but without overt clinical seizures.
Robust survival of isolated bovine adrenal chromaffin cells following intrastriatal transplantation: a novel hypothesis of adrenal graft viability
Authors: Schueler SB , Ortega JD , Sagen J , Kordower JH
Published: March 2018

Previous investigations have demonstrated that adrenal chromaffin cells survive poorly when grafted into the striatum of rodents, nonhuman primates, and patients with Parkinson's disease. This poor survival has been attributed to the low levels of endogenous NGF within the striatum. However, chromaffin cells isolated from the nonchromaffin constituents of the adrenal medulla (fibroblasts and endothelial cells) have recently been demonstrated to survive grafting into a number of CNS sites. The present study determined whether nonchromaffin constituents of the adrenal medulla may be responsible for poor graft survival. We compared the survival of intrastriatally grafted isolated bovine chromaffin cells with that observed following implantation of either perfused adrenal medullary suspensions containing all adrenal medullary cell types or isolated chromaffin cells that were then reseeded with autologous fibroblasts and endothelial cells. Implants of perfused adrenal medullary cells survived poorly and most graft sites were infiltrated with macrophages. The chromaffin cells in this group that did survive appeared to be in the process of degeneration. In contrast, large numbers of isolated chromaffin cells survived for up to 2 months following transplantation. These cells maintained their endocrine phenotype and stained for all enzymatic markers of catecholamine synthesis as well as chromogranin A. Morphologically, these cells resembled chromaffin cells seen in situ and the perigraft region was essentially devoid of macrophages. When isolated chromaffin cells were reseeded with autologous fibroblasts and endothelial cells, the implants degenerated and few, if any, surviving chromaffin cells were observed. Interestingly, these latter grafts induced a host- derived sprouting response of tyrosine hydroxylase-immunoreactive fibers. These data demonstrate that large numbers of adrenal chromaffin cells can survive intrastriatal implantation in the absence of exposure to exogenous NGF. Rather, the nonchromaffin cells of the adrenal medulla (fibroblasts and endothelial cells) appear to compromise the viability of grafted chromaffin cells. Once they are eliminated from the graft, robust survival of chromaffin cells occurs. If clinical trials employing adrenal medullary grafts are still to be considered for the treatment of Parkinson's disease, isolation of the chromaffin cells should be considered to enhance graft viability.
Structural and functional specialization of A delta and C fiber free nerve endings innervating rabbit corneal epithelium
Authors: MacIver MB , Tanelian DL
Published: March 2018

An in vitro preparation of rabbit cornea was used to compare anatomical specialization and electrophysiological responses of A delta and C fiber sensory afferents which terminate as free nerve endings. Living nerve endings were visualized using epifluorescence microscopy and the vital dye 4-di2-ASP, and response properties were determined using microstimulation and recording of fiber discharge activity. Fiber type was determined based on conduction velocity measurement and preferred stimulus energy (modality) of each fiber. Four modality-specific fiber populations were identified: (1) slowly adapting C fiber cold receptors (conduction velocity of 0.25–1.6 m/sec), (2) C fiber chemosensitive units with mixed phasic and tonic activity (1.1–1.8 m/sec), (3) rapidly adapting mechanosensitive A delta fibers (1.5–2.8 m/sec), and (4) high- threshold mechano/heat (> 350 dyne or > 40 degrees C) phasic A delta afferents (3.5–4.4 m/sec). In addition to these physiological differences, anatomical specialization was also noted. A delta fiber nerve endings were distinguished from those of C fibers by thin, elongated sensory endings that ran parallel to the corneal surface; C fiber endings formed short, branching clusters that ran mostly perpendicular to the surface. The elongated structure of A delta nerve endings was associated with directional selectivity for mechanical stimuli. These results substantiate previous suggestions that free nerve endings can exhibit both structural and functional specialization.
Experience-dependent plasticity in the inferior colliculus: a site for visual calibration of the neural representation of auditory space in the barn owl
Authors: Brainard MS , Knudsen EI
Published: March 2018

The optic tectum (homolog of the superior colliculus) contains mutually aligned neural maps of auditory and visual space. During development, the organization of the auditory map is guided by spatial information provided by vision: barn owls raised wearing prismatic spectacles, which optically shift the visual field and the visual map in the optic tectum, develop an auditory map that is shifted by an approximately equivalent amount, such that alignment between the two maps is preserved (Knudsen and Brainard, 1991). In this study we investigated whether this shift in the auditory map is intrinsic to the optic tectum or whether it reflects plasticity at an earlier stage in the auditory pathway. Owls were raised wearing prismatic spectacles that displaced the visual field by 23 degrees to the left or right. This manipulation alters the normal correspondence between locations in the visual field and interaural time difference (ITD), the primary cue for the azimuth of a sound source. In normal owls and in owls with at least 150 d of prism experience, extracellular unit recordings were used to assess the representations of ITD at anatomically and physiologically defined sites in the optic tectum and in the two prior stages of the auditory pathway, the external and central nuclei of the inferior colliculus (ICx and ICc). In the optic tectum of normal owls, the values of ITD to which units responded most strongly (best ITDs) varied systematically with the azimuths of unit visual receptive fields (VRFs). In the prism- reared owls, best ITDs were shifted from normal toward the values of ITD produced by sounds at the locations of the units' optically displaced VRFs. In the ICx of prism-reared owls, the representation of ITD also was shifted from normal, by an amount and in a direction that could completely account for the shift in ITD measured in the optic tectum. At some sites in the ICx, the shift in ITD tuning was apparent within the first 7–8 msec of the response; shifted tuning at such short latencies argues that the altered representation of ITD in the ICx reflects plasticity in the ascending auditory pathway, and is not the result of descending activity from higher auditory centers. In the ICc, which immediately precedes the ICx in the ascending pathway, the representation of ITD was normal. The results indicate that the visual instruction of auditory spatial tuning of neurons in the optic tectum reflects plasticity at the level of the ICx, the site where the auditory map of space is first synthesized.
Stepwise repolarization from Ca2+ plateaus in neocortical pyramidal cells: evidence for nonhomogeneous distribution of HVA Ca2+ channels in dendrites
Authors: Reuveni I , Friedman A , Amitai Y , Gutnick MJ
Published: March 2018

Although cortical dendrites have classically been thought of as passive structures, recent evidence suggests that active conductances, including Ca2+ conductance, are also present in the dendritic membrane. To investigate this, we have recorded intracellularly in slices of rat neocortex bathed in 24 mM tetraethylammonium chloride and 1 microM TTX. Under these conditions, pyramidal neurons generated prolonged Ca2+ spikes. In computer simulations, the breakpoint voltage from which the plateau level began to repolarize was closely related to a specific region on the voltage/activation curve of the high-voltage-activated Ca2+ conductance underlying the spike. This modeling result was supported by the experimental observation that substituting Ba2+ for Ca2+ caused a hyperpolarizing shift in breakpoint voltage by 8–10 mV. Often there was stepwise repolarization from the Ca2+ spike to one or more additional plateau levels. In compartmental computer models, this could be simulated by two different mechanisms: (1) the presence of multiple, electrotonically separated sites of Ca2+ spike electrogenesis in the dendritic tree, and (2) the presence of Ca2+ channels with different voltage dependencies in the same compartment. In experiments, brief hyperpolarizing pulses could cut short the high-amplitude plateau without terminating the smaller “steps.” This result could be simulated by both computer models. However, only the multicompartmental model could simulate effects of prolonged depolarizing and hyperpolarizing currents on the breakpoint. Thus, the more depolarized the breakpoint, and hence the closer the spike initiation zone to the recording site, the less it was affected by the injected current. In experiments, the ratio of the breakpoint voltages for the different plateau levels was equal to the ratio of the highest repolarization rates. These data indicate that the breakpoint voltage and the time course of repolarization were the same at all the sites of Ca2+ electrogenesis. Our findings provide strong evidence that Ca2+ spike initiation occurs at electrotonically separated “hot spots” in the dendrites, and that voltage dependence of the Ca2+ channels that underlie the spikes is the same at all sites.
Developmental analysis of hippocampal mossy fiber outgrowth in a mutant mouse with inherited spike-wave seizures
Authors: Qiao X , Noebels JL
Published: March 2018

Neural firing patterns are an essential determinant of normal axon terminal growth and synaptic connectivity in developing afferent pathways, but the trophic role of synchronous activity in associative neural networks is less well defined. We examined the ontogeny of inherited synchronous hippocampal network discharges and mossy fiber innervation patterns at sequential stages of development in the stargazer (stg) mutant, a single-locus mouse mutation expressing generalized spike-wave epilepsy. Brief bursts of 6/sec repetitive discharges arise spontaneously on postnatal days 17–18 and persistently activate neocortical and hippocampal networks throughout adulthood. We found a striking pattern of mossy fiber recurrent axon collateral sprouting in the inner molecular layer of dentate gyrus in the adult stg hippocampus. Sprouting is not apparent until 4–6 weeks following seizure onset, but then steadily intensifies with continued synchronous activation. In the adult mutant, axon outgrowth is accompanied by a mild selective loss of hilar interneurons without gliosis. These data indicate that hypersynchronous stimulation during late postnatal brain development is linked, following a prolonged latent period, to significant fiber outgrowth and synaptic reorganization within the hippocampal formation. Since the pattern of synchronous activation in the stg mutant strongly resembles that seen in human spike-wave absence epilepsy, the synaptic plasticity described in this model has important implications for normal brain development in this common disorder.