N-methyl aspartate activates voltage-dependent calcium conductance in rat hippocampal pyramidal cells

Circuits and Synapses: Hypothesis, Observation, Controversy and Serendipity - An Opinion Piece

More than a century of dedicated research has resulted in what we now know, and what we think we know, about synapses and neural circuits. This piece asks to what extent some of the major advances - both theoretical and practical - have resulted from carefully considered theory, or experimental design: endeavors that aim to address a question, or to refute an existing hypothesis. It also, however, addresses the important part that serendipity and chance have played. There are cases where hypothesis driven research has resulted in important progress. There are also examples where a hypothesis, a model, or even an experimental approach - particularly one that seems to provide welcome simplification - has become so popular that it becomes dogma and stifles advance in other directions. The nervous system rejoices in complexity, which should neither be ignored, nor run from. The emergence of testable "rules" that can simplify our understanding of neuronal circuits has required the collection of large amounts of data that were difficult to obtain. And although those collecting these data have been criticized for not advancing hypotheses while they were "collecting butterflies," the beauty of the butterflies always enticed us toward further exploration.

Maintenance of conditioned place avoidance induced by gastric malaise requires NMDA activity within the ventral hippocampus

It has been reported that during chemotherapy treatment, some patients can experience nausea before pharmacological administration, suggesting that contextual stimuli are associated with the nauseating effects. There are attempts to reproduce with animal models the conditions under which this phenomenon is observed to provide a useful paradigm for studying contextual aversion learning and the brain structures involved. This manuscript assessed the hippocampus involvement in acquiring and maintaining long-term conditioned place avoidance (CPA) induced by a gastric malaise-inducing agent, LiCl. Our results demonstrate that a reliable induction of CPA is possible after one acquisition trial. However, CPA establishment requires a 20-min confinement in the compartment associated with LiCl administration. Interestingly, both hippocampal regions seem to be necessary for CPA establishment; nonetheless, inactivation of the ventral hippocampus results in a reversion of avoidance and turns it into preference. Moreover, we demonstrate that activation of dorsal/ventral hippocampal NMDA receptors after CS–US association is required for long-term CPA memory maintenance.

A Specific Role of Hippocampal NMDA Receptors and Arc Protein in Rapid Encoding of Novel Environmental Representations and a More General Long-Term Consolidation Function

Activation of the NMDA receptor (NMDAR) has been proposed to be a key event responsible for the structural changes that occur in neurons during learning and memory formation. It has been extensively studied yet no consensus has been reached on its mnemonic role as both NMDAR dependent and independent forms of learning have been observed. We investigated the role that hippocampal NMDAR have in rapid spatial learning and memory across training environments. Hippocampal NMDAR was blocked via intra-hippocampal injection of the competitive antagonist CPP. Groups of rats were pre-trained on a spatial version of the Morris water task, and then mass reversal training under NMDAR blockade occurred in the same or different training environments as pre-training. We measured expression of Arc protein throughout the main hippocampal subfields, CA1, CA3, and dentate gyrus, after mass-training. We observed that NMDAR blockade allowed for rapid spatial learning, but not consolidation, when the SUBJECTS used previously acquired environmental information. Interestingly, NMDAR blockade impaired rapid spatial learning when rats were mass-trained in a novel context. Arc protein expression in the dentate gyrus followed this pattern of NMDAR dependent spatial behavior, with high levels of expression observed after being trained in the new environment, and low levels when trained in the same environment. CPP significantly reduced Arc expression in the dentate gyrus. These results implicate dentate NMDAR in the acquisition of novel environmental information.

The structure and function of glutamate receptors: Mg2+ block to X-ray diffraction

Experiments on the action of glutamate on mammalian and amphibian nervous systems started back in the 1950s but decades passed before it became widely accepted that glutamate was the major excitatory neurotransmitter in the CNS. The pace of research greatly accelerated in the 1980s when selective ligands that identified glutamate receptor subtypes became widely available, and voltage clamp techniques, coupled with rapid perfusion, began to resolve the unique functional properties of what cloning subsequently revealed to be a large family of receptors with numerous subtypes. More recently the power of X-ray crystallography and cryo-EM has been applied to the study of glutamate receptors, revealing their atomic structures, and the conformational changes that underlie their gating. In this review I summarize the history of this field, viewed through the lens of a career in which I spent 3 decades working on the structure and function of glutamate receptor ion channels. This article is part of the Special Issue entitled 'Ionotropic glutamate receptors'.

The role of the tripartite glutamatergic synapse in the pathophysiology of Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia in individuals over 65 years of age and is characterized by accumulation of beta-amyloid (Aβ) and tau. Both Aβ and tau alter synaptic plasticity, leading to synapse loss, neural network dysfunction, and eventually neuron loss. However, the exact mechanism by which these proteins cause neurodegeneration is still not clear. A growing body of evidence suggests perturbations in the glutamatergic tripartite synapse, comprised of a presynaptic terminal, a postsynaptic spine, and an astrocytic process, may underlie the pathogenic mechanisms of AD. Glutamate is the primary excitatory neurotransmitter in the brain and plays an important role in learning and memory, but alterations in glutamatergic signaling can lead to excitotoxicity. This review discusses the ways in which both beta-amyloid (Aβ) and tau act alone and in concert to perturb synaptic functioning of the tripartite synapse, including alterations in glutamate release, astrocytic uptake, and receptor signaling. Particular emphasis is given to the role of N-methyl-D-aspartate (NMDA) as a possible convergence point for Aβ and tau toxicity.

Adenosine, glutamate and pH: interactions and implications

Adenosine's role in the nervous system is multifaceted. As the core molecule of adenosine triphosphate (ATP), adenosine exists in equilibrium with the adenine nucleotide pool and contributes to cellular energy charge, a quantification of relative amounts of available ATP, ADP, AMP and adenosine. Beyond participating in overall energy balance and thus in maintaining cellular homeostasis, adenosine critically influences dynamic signaling in the nervous system. In particular, adenosine has an effect on and is affected by excitatory synaptic transmission. This report describes the ubiquitous nature of adenosine's influence, outlines specific scenarios of clinical import and highlights emerging knowledge about the regulation of adenosine.

Prefrontal cortex and drug abuse vulnerability: translation to prevention and treatment interventions

Vulnerability to drug abuse is related to both reward seeking and impulsivity, two constructs thought to have a biological basis in the prefrontal cortex (PFC). This review addresses similarities and differences in neuroanatomy, neurochemistry and behavior associated with PFC function in rodents and humans. Emphasis is placed on monoamine and amino acid neurotransmitter systems located in anatomically distinct subregions: medial prefrontal cortex (mPFC); lateral prefrontal cortex (lPFC); anterior cingulate cortex (ACC); and orbitofrontal cortex (OFC). While there are complex interconnections and overlapping functions among these regions, each is thought to be involved in various functions related to health-related risk behaviors and drug abuse vulnerability. Among the various functions implicated, evidence suggests that mPFC is involved in reward processing, attention and drug reinstatement; lPFC is involved in decision-making, behavioral inhibition and attentional gating; ACC is involved in attention, emotional processing and self-monitoring; and OFC is involved in behavioral inhibition, signaling of expected outcomes and reward/punishment sensitivity. Individual differences (e.g., age and sex) influence functioning of these regions, which, in turn, impacts drug abuse vulnerability. Implications for the development of drug abuse prevention and treatment strategies aimed at engaging PFC inhibitory processes that may reduce risk-related behaviors are discussed, including the design of effective public service announcements, cognitive exercises, physical activity, direct current stimulation, feedback control training and pharmacotherapies. A major challenge in drug abuse prevention and treatment rests with improving intervention strategies aimed at strengthening PFC inhibitory systems among at-risk individuals.

Differential responses to NMDA receptor activation in rat hippocampal interneurons and pyramidal cells may underlie enhanced pyramidal cell vulnerability

Hippocampal interneurons are generally more resistant than pyramidal cells to excitotoxic insults. Because NMDA receptors play a crucial role in neurodegeneration, we have compared the response to exogenous NMDA in CA1 pyramidal cells and interneurons of the stratum oriens using combined whole-cell patch-clamp recording and ratiometric Ca2+ imaging. In voltage-clamp, current-clamp or in nominally Mg2+-free medium, NMDA (10 microM; 3-5 min exposure in the presence of tetrodotoxin) induced a markedly larger inward current and Ca2+ rise in pyramidal cells than in interneurons. Pyramidal cells also showed a more pronounced voltage dependence in their response to NMDA. We hypothesized that this enhanced response to NMDA receptor activation in pyramidal cells could underlie their increased vulnerability to excitotoxicity. Using loss of dye as an indicator of degenerative membrane disruption, interneurons tolerated continuous exposure to a high concentration of NMDA (30 microM) for longer periods than pyramidal cells. This acute neurodegeneration in pyramidal cells was independent of intracellular Ca2+, because high intracellular BAPTA (20 mM) did not prolong survival time. Thus, a plausible explanation for the enhanced sensitivity of pyramidal neurons to excitotoxic insults associated with cerebral ischemia is their greater response to NMDA receptor activation, which may reflect differences in NMDA receptor expression and/or subunit composition.

Effect of β-amyloid peptide on behavior and synaptic plasticity in terrestrial snail

A large body of evidence implicates beta-amyloid peptide (betaAP) and other derivatives of the evolutionarily highly conserved amyloid precursor protein (APP) in the pathogenesis of Alzheimer's disease. However, the functional relationship of APP and its proteolytic derivatives to synaptic plasticity is not well known. We demonstrate that 30 min exposure to the 25-35 fragment of betaAP do not markedly change the dynamics of synaptic responses in identified neurons of terrestrial snail while a significant decrease of long-term sensitization was observed after 180 min betaAP bath application. In the behavioral experiments, a significant reduction of sensitization, and decreased ability to develop food-aversion conditioning was observed after betaAP injection. Our results clearly demonstrate that the neurotoxic 25-35 fragment of betaAP may play a significant role in behavioral plasticity by chronically eliminating certain underlying forms of synaptic plasticity. The study also proposes a novel invertebrate model to Alzheimer's disease.

In vitro characterization of novel NR2B selective NMDA receptor antagonists

N-Methyl-D-aspartate (NMDA) subunit specific receptor antagonism has potential therapeutic application for multiple CNS pathologies. MERCK 1, MERCK 2, and MERCK 3 are novel NR2B subtype selective NMDA receptor antagonists. The affinity and the kinetic mechanism of inhibition by these antagonists and ifenprodil were investigated using the whole-cell configuration of the patch clamp technique, calcium flux, and radioligand binding on a mouse cell line L(tk-) expressing recombinant human heteromeric NMDA receptors consisting of NR1a/NR2B subunit combinations. The rank order of potency, as determined by electrophysiology, was ifenprodil<MERCK 2<MERCK 1<MERCK 3 with K(D)'s 79+/-8, 2.4+/-1.1, 1.3+/-0.9, and approximately 0.16+/-0.02 nM, respectively. The apparent dissociation rate constants among these compounds differed by as much as 394-fold whereas the apparent association constants varied less than 3-fold. Higher affinities were a result of slower drug dissociation kinetics of receptor unbinding. Maximal inhibition was not voltage-dependent and was not statistically different at saturating concentrations by these compounds. These results provide the first detailed functional analysis of the kinetic mechanism of MERCK 1, MERCK 2, and MERCK 3 inhibition of NMDA receptors.

Spinal Axonal Injury Induces Brief Downregulation of Ionotropic Glutamate Receptors and No Stripping of Synapses in Cord-Projection Central Neurons

Spinal cord injury often damages the axons of cord-projecting central neurons. To determine whether their excitatory inputs are altered following axonal injury, we used rat rubrospinal neurons as a model and examined their excitatory input following upper cervical axotomy. Anterograde tracing showed that the primary afferents from the cerebellum terminated in a pattern similar to that of control animals. Ultrastructurally, neurons in the injured nucleus were contacted by excitatory synapses of normal appearance, with no sign of glial stripping. Since cerebellar fibers are glutamatergic, we examined the expression of ionotropic receptor subunits GluR1-4 and NR1 for AMPA and NMDA receptors, respectively, in control and injured neurons using immunolabeling methods. In control neurons, GluR2 appeared to be low as compared to GluR1, GluR3, and GluR4, while NR1 labeling was intense. Following unilateral tractotomy, the levels of expression of each subunit in axotomized neurons appeared to be normal, with the exception that they were lower than those of control neurons of the nonlesioned side at 2-6 days postinjury. These findings suggest that axotomized neurons are only temporarily protected from excitotoxicity. This is in sharp contrast to the responses of central neurons that innervate peripheral targets, in which both synaptic stripping and reduction of their ionotropic glutamate receptor subunits persist following axotomy. The absence of an injury-induced trimming of afferents and stripping of synapses and the lack of a persistent downregulation of postsynaptic receptors might enable injured cord-projection neurons to continue to control their supraspinal targets during most of their postinjury survival. Although this may support neurons by providing trophic influences, it nevertheless may subject them to excitotoxicity and ultimately lead to their degenerative fate.

The Involvement of N-Methyl-D-Aspartate Receptors in Induction and Maintenance of Long-Term Potentiation in Rat Visual Cortex

Pyramidal neurons from layers II and III of rat visual cortex slices were studied with intracellular recordings. The involvement of N-methyl-D-aspartate (NMDA) receptors was investigated: (1) in the synaptic response to white matter stimulation; (2) in the induction of long-term potentiation (LTP); and (3) in the maintenance of LTP. Bath application of 25 microM of 2-amino-5-phosphonovalerate (APV), an NMDA receptor antagonist, caused a slight (< 10%) reduction of the amplitude of the synaptic response elicited by white matter stimulation. The APV-sensitive excitatory postsynaptic potential (EPSP) had a longer peak latency and duration than the APV-resistant EPSP. Bath application of 10 microM of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a non-NMDA glutamate receptor antagonist, revealed a CNQX-resistant EPSP in response to white matter stimulation which was APV-sensitive. The time course of the CNQX-resistant EPSP was similar to that of the APV-sensitive EPSP and its onset latency was similar to that of the synaptic response in normal medium. Bath application of the GABA-A antagonist bicuculline (0.1 to 0.5 microM) led to a progressive enhancement of the amplitude of the APV-sensitive EPSP. At bicuculline concentrations above 0.3 microM the amplitude of this EPSP increased with membrane depolarization as was the case for the CNQX-resistant EPSP implying that the NMDA receptors were located on the recorded neuron. The susceptibility of the cells to undergo LTP was tested at various concentrations of bicuculline. The effectiveness of bicuculline treatment was quantified by comparing the amplitudes of the synaptic response to just subthreshold stimuli at two post-stimulus delays: (i) at 22 ms, which corresponds to the time to peak of both the initial inhibitory postsynaptic potential and the APV-sensitive EPSP; and (ii) at 8 - 11 ms post-stimulus, which corresponds to the peak of the postsynaptic potential (PSP) in normal medium. Bath application of APV, 20 min after the conditioning tetanus, allowed the authors to measure the amplitude of the APV-sensitive EPSP in the potentiated response. In normal medium, the ratio of the late over the early PSP amplitude was 33.6 +/- 4.1% and tetanic stimulation failed to induce LTP. The conditions remained the same at bicuculline concentrations of 0.1 to 0.2 microM. At higher concentrations of bicuculline the amplitude ratio of late versus early PSP increased and tetanic stimulation induced LTP. In cells, in which bicuculline had caused small ratio increases, only the APV-sensitive EPSP underwent LTP. In cells in which bicuculline had caused large ratio changes, both the APV-resistant and the APV-sensitive EPSP showed LTP. Together with the previous finding that blockade of NMDA receptors prevents LTP (Artola and Singer, 1987) these results suggest that there is a threshold for LTP induction, which is only reached if NMDA receptor-gated channels are sufficiently activated. The data indicate further that the NMDA receptor-mediated EPSP is itself susceptible to LTP whereby its LTP threshold is lower than that of the APV-resistant EPSP. Given the different LTP thresholds of the APV-resistant and APV-sensitive EPSPs, the possibility is raised that their potentiation depends on different mechanisms.

Presynaptic GABAA and GABAB Receptor-mediated Phasic Modulation in Axons of Spinal Motor Interneurons

The lamprey spinal cord has been utilized to investigate the role of presynaptic inhibition in the control of the spinal motor system. Axons of the lamprey spinal cord are comparatively large because of their lack of myelination. Axons impaled with microelectrodes demonstrate depolarizing responses to the application of GABAA and GABAB receptor agonists, muscimol and baclofen. These depolarizing effects are counteracted by the specific GABAA and GABAB receptor antagonists, bicuculline and phaclofen. GABAA receptor activation leads to a gating of Cl- channels on the axons. However, the ionic mechanism leading to axonal depolarization following GABAB receptor activation is unknown. After initiation of fictive locomotion, these axons demonstrate oscillations in axonal membrane potential related to the locomotor cycle. During ficitive locomotion they depolarize in phase with the bursting of the ipsilateral ventral root of the same segment. These axonal membrane potential oscillations are due to a phasic GABAA and GABAB receptor-mediated gating of ion channels on the axonal membrane. Fictive locomotion in the lamprey spinal cord is largely unaffected by antagonism of one or other GABA receptor subtype alone, but is severely disrupted by simultaneous antagonism of both subtypes. In conclusions, we demonstrate, for the first time, an agonist-gated depolarization of a vertebrate presynaptic element measured by direct impalement of the axon under study. We also demonstrate that GABA-mediated presynaptic inhibition occurs in axons of spinal interneurons. It is not limited to the primary afferents as has previously been believed.

Enhanced NMDA receptor activity in retinal inputs to the rat suprachiasmatic nucleus during the subjective night

Circadian oscillator mechanisms in the suprachiasmatic nucleus (SCN) can be reset by photic input, which is mediated by glutamatergic afferents originating in the retina. A key question is why light can only induce phase shifts of the biological clock during a restricted period of the circadian cycle, namely the subjective night. One of several possible mechanisms holds that glutamatergic transmission at retinosuprachiasmatic synapses would be altered, in particular the contribution of glutamate receptor subtypes to the postsynaptic response. By studying the contributions of NMDA and non-NMDA glutamate receptors to the retinal input to SCN in whole-cell patch-clamp recordings in acutely prepared slices, we tested the hypothesis that NMDA receptor current evoked by optic nerve activity is potentiated during the subjective night. During the day the NMDA component of the EPSC evoked by optic nerve stimulation was found less frequently and was significantly smaller in magnitude than during the night. In contrast, the non-NMDA component did not show a significant day-night difference. When the magnitude of the NMDA component was normalized to that of the non-NMDA component, the day-night difference was maintained, suggesting a selective potentiation of NMDA receptor conductance. In addition to contributing to electrically evoked EPSCs, the NMDA receptor was found to sustain a small, tonically active inward current during the night phase. No significant tonic contribution by NMDA receptors was detected during the day. These results suggest, first, a dual mode of NMDA receptor function in the SCN and, second, a clock-controlled type of receptor plasticity, which may gate the transfer of photic input to phase-shifting mechanisms operating at the level of molecular autoregulatory feedback loops.

Intracellular survival pathways against glutamate receptor agonist excitotoxicity in cultured neurons. Intracellular calcium responses

Cultured rat cerebellar granule cells are resistant to the excitotoxic effects of N-methyl-D-aspartate (NMDA) and non-NMDA receptor agonists under three conditions: 1) prior to day seven in vitro when cultured in depolarizing concentrations of potassium [25 mM]; 2) at any time in vitro when cultured in non-depolarizing concentrations of potassium 5 mM[; and 3) when neurons, cultured in depolarizing concentrations of potassium 25 mM[ for eight days in vitro, are pretreated with a subtoxic concentration of NMDA. The focus of this paper is to determine: a) whether the resistance to excitotoxicity by NMDA and non-NMDA receptor agonists is due to a decreased intracellular calcium Ca++[i response to glutamate receptor agonists in cultured rat cerebellar granule cells; or b) whether Ca++[i levels induced by the agonists are similar to those observed under excitotoxic conditions. Granule cells, matured in non-depolarizing growth medium, treated with glutamate resulted in an increase in Ca++[i followed by a plateau that remained above baseline in virtually all neurons that responded to glutamate. The response was rapid in onset (< 10 sec) and the pattern of response heterogeneous in that cells responsive to glutamate increased their Ca++[i to different extents; some cells did not respond to glutamate. Kainate also produced significant elevations in Ca++[i. The Ca++[i response to glutamate in neurons matured in depolarizing (25 mM K+) growth medium for three days was rapid, transient and heterogeneous, which reached a plateau that was elevated above baseline levels; removing the glutamate markedly reduced the Ca++[i concentration. Activation of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptors by kainic acid produced similar changes in Ca++[i responses. At a time when cultured cerebellar granule cells become susceptible to the excitotoxic effects of glutamate acting at NMDA receptors (day in vitro (DIV) 8) in depolarizing growth medium, glutamate elicited Ca++[i responses similar to those observed at a culture time when the neurons are not susceptible to the excitotoxic effects of glutamate (DIV 3). Pretreatment of the cultured neurons with a subtoxic concentration of NMDA, which protects all neurons against the excitotoxic effects of glutamate, did not alter the maximal Ca++[i elicited by an excitotoxic concentration of glutamate.

Hippocampal astrocytes exhibit Ca2+-elevating muscarinic cholinergic and histaminergic receptors in situ

Recent findings suggest that astrocytes respond to neuronally released neurotransmitters with Ca2+ elevations. These Ca2+ elevations may trigger astrocytes to release glutamate, affecting neuronal activity. Neuronal activity is also affected by modulatory neurotransmitters that stimulate G protein-coupled receptors. These neurotransmitters, including acetylcholine and histamine, might affect neuronal activity by triggering Ca2+-dependent release of neurotransmitters from astrocytes. However, there is no physiological evidence for histaminergic or cholinergic receptors on astrocytes in situ. We asked whether astrocytes have these receptors by imaging Ca2+-sensitive dyes sequestered by astrocytes in hippocampal slices. Our results show that immunocytochemically identified astrocytes respond to carbachol and histamine with increases in intracellular free Ca2+ concentration. The H1 histamine receptor antagonist chlorpheniramine inhibited responses to histamine. Similarly, atropine and the M1-selective muscarinic receptor antagonist pirenzepine inhibited carbachol-elicited responses. Astrocyte responses to histamine and carbachol were compared with responses elicited by alpha1-adrenergic and metabotropic glutamate receptor agonists. Individual astrocytes responded to different subsets of receptor agonists. Ca2+ oscillations were the prevalent response pattern only with metabotropic glutamate receptor stimulation. Finally, functional alpha1-adrenergic receptors and muscarinic receptors were not detected before postnatal day 8. Our data show that astrocytes have acetylcholine and histamine receptors coupled to Ca2+. Given that Ca2+ elevations in astrocytes trigger neurotransmitter release, it is possible that these astrocyte receptors modulate neuronal activity.

Na(+) and K(+) concentrations, extra- and intracellular voltages, and the effect of TTX in hypoxic rat hippocampal slices

Severe hypoxia causes rapid depolarization of CA1 neurons and glial cells that resembles spreading depression (SD). In brain slices in vitro, the SD-like depolarization and the associated irreversible loss of function can be postponed, but not prevented, by blockade of Na(+) currents by tetrodotoxin (TTX). To investigate the role of Na(+) flux, we made recordings from the CA1 region in hippocampal slices in the presence and absence of TTX. We measured membrane changes in single CA1 pyramidal neurons simultaneously with extracellular DC potential (V(o)) and either extracellular [K(+)] or [Na(+)]; alternatively, we simultaneously recorded [Na(+)](o), [K(+)](o), and V(o). Confirming previous reports, early during hypoxia, before SD onset, [K(+)](o) began to rise, whereas [Na(+)](o) still remained normal and V(o) showed a slight, gradual, negative shift; neurons first hyperpolarized and then began to gradually depolarize. The SD-like abrupt negative DeltaV(o) corresponded to a near complete depolarization of pyramidal neurons and an 89% decrease in input resistance. [K(+)](o) increased by 47 mM and [Na(+)](o) dropped by 91 mM. Changes in intracellular Na(+) and K(+) concentrations, estimated on the basis of the measured extracellular ion levels and the relative volume fractions of the neuronal, glial, and extracellular compartment, were much more moderate. Because [Na(+)](o) dropped more than [K(+)](o) increased, simple exchange of Na(+) for K(+) cannot account for these ionic changes. The apparent imbalance of charge could be made up by Cl(-) influx into neurons paralleling Na(+) flux and release of Mg(2+) from cells. The hypoxia-induced changes in interneurons resembled those observed in pyramidal neurons. Astrocytes responded with an initial slow depolarization as [K(+)](o) rose. It was followed by a rapid but incomplete depolarization as soon as SD occurred, which could be accounted for by the reduced ratio, [K(+)](i)/[K(+)](o). TTX (1 microM) markedly postponed SD, but the SD-related changes in [K(+)](o) and [Na(+)](o) were only reduced by 23 and 12%, respectively. In TTX-treated pyramidal neurons, the delayed SD-like depolarization took off from a more positive level, but the final depolarized intracellular potential and input resistance were not different from control. We conclude that TTX-sensitive channels mediate only a fraction of the Na(+) influx, and that some of the K(+) is released in exchange for Na(+). Even though TTX-sensitive Na(+) currents are not essential for the self-regenerative membrane changes during hypoxic SD, in control solutions their activation may trigger the transition from gradual to rapid depolarization of neurons, thereby synchronizing the SD-like event.

Mature hippocampal astrocytes exhibit functional metabotropic and ionotropic glutamate receptors in situ

Astrocytes closely contact neurons where they respond to neuronally released glutamate in immature brain slices. In previous studies, neither metabotropic nor ionotropic glutamate receptor-mediated responses were detected by imaging Ca2+ in astrocytes from mature (P21-P42) animals, suggesting astrocyte glutamate receptors only contribute to hippocampus physiology during development. In contrast to Ca2+ imaging, published electrophysiological experiments suggest P30-P35 astrocytes have alpha-amino-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. For this study, we imaged astrocytes in P31-P38 hippocampal slices to determine if metabotropic and ionotropic glutamate receptor activation elevates intracellular calcium in mature astrocytes. Drugs were perfused while [Ca2+]i was monitored (confocal imaging) in cells loaded with Calcium Green 1-AM. Imaged cells were subsequently identified as astrocytes by GFAP/S-100 immunostaining. Astrocytic Ca2+ increased after glutamate application in the presence of a glutamate uptake inhibitor. An agonist at group I/II metabotropic glutamate receptors, (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (t-ACPD), elicited Ca2+ increases as did group I agonist 3,5-dihydroxyphenylglycine (DHPG), suggesting that mature astrocytes respond to glutamate via metabotropic glutamate receptors. AMPA also elicited Ca2+ elevations that were inhibited by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and occurred after treatment with omega-conotoxin MVIIC to block neurotransmitter release. These results demonstrate that astrocytes in mature hippocampus have functional ionotropic and metabotropic glutamate receptors that regulate astrocytic calcium levels. Glutamatergic regulation of astrocytic [Ca2+]i may be involved in synapse modeling, long-term potentiation, excitotoxicity and other events dependent on glutamatergic transmission in adult hippocampus.

Injection of MK-801 affects ocular dominance shifts more than visual activity

Kittens were given intramuscular injections of the N-methyl--aspartate (NMDA) antagonist MK-801 twice daily (morning and midday) during the peak of the period of susceptibility for ocular dominance changes. They were then exposed to light with one eye closed for 4 h after each injection. The ocular dominance of these kittens was shifted significantly less than that of kittens injected with saline and exposed to light over the same period at the same age. After recording a sample of cells for an ocular dominance histogram, the kittens were injected with the same dose of MK-801 that was used during rearing to observe its effect on the activity of single cells in the visual cortex. In the majority of cells (7/13) there was no significant change in activity. Positive evidence for a reduction in activity was seen in only a minority (3/13) of cells. In a separate series of experiments, dose-response curves were measured for cells in the visual cortex in response to iontophoresis of NMDA or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), and the effect of an injection of MK-801 on these curves was measured. MK-801, at doses similar to those used in the ocular dominance experiments, had a significant effect on the dose-response curves for NMDA, but little effect on the dose-response curves for AMPA, or the visual responses of the cells. We conclude that ocular dominance shifts can be reduced significantly by a treatment that has little effect on the level of activity of cells in the visual cortex but does specifically affect the responses of the cells to NMDA as opposed to the responses to AMPA.

Inhibition of major cationic inward currents prevents spreading depression-like hypoxic depolarization in rat hippocampal tissue slices

Hypoxia-induced spreading depression-like depolarization (hypoxic SD, or anoxic depolarization) is accompanied by the near-loss of membrane potential, severe reduction of membrane resistance, and influx of Na+, Ca2+, Cl- and water into neurons. The biophysical nature of these membrane changes is incompletely understood. In the present study we applied a pharmacological mixture (10 microM DNQX, 10 microM CPP, 1 microM TTX, and 2 mM Ni2+) to rat hippocampal tissue slices to inhibit major Na+ and Ca2+ inward currents. This inhibitory cocktail slightly depolarized CA1 pyramidal neurons and completely blocked all evoked potentials. In its presence severe hypoxia of up to 20 min duration failed to induce hypoxic SD and the accompanying intrinsic optical signal. Instead, only moderate, very slow negative shifts of the extracellular DC potential were observed. Following 10 min hypoxia and 1 hour wash-out of the inhibitors antidromic and orthodromic responses were still blocked but hypoxic SD with markedly delayed onset could be induced in most slices. In current-clamped CA1 pyramidal cells hypoxia induced a rapid, near-complete depolarization and decreased the input resistance by 89%. In the presence of the cocktail, however, hypoxia caused a gradual, partial depolarization, to about -40 mV; the membrane resistance decreased by only 37%. We conclude that simultaneous blockade of the known major Na+ and Ca2+ channels consistently prevents hypoxic SD. The hypothesis that SD initiation is the consequence of general loss of selective permeability or general membrane breakdown becomes unlikely. Instead, influx of Na+ and Ca2+ might play a crucial role in the generation of the rapid SD-like depolarization.

Glucocorticoid receptor activation lowers the threshold for NMDA-receptor-dependent homosynaptic long-term depression in the hippocampus through activation of voltage-dependent calcium channels

The effects of the glucocorticoid receptor agonist RU-28362 on homosynaptic long-term depression (LTD) were examined in hippocampal slices obtained from adrenal-intact adult male rats. Field excitatory postsynaptic potentials were evoked by stimulation of the Schaffer collateral/commissural pathway and recorded in stratum radiatum of area CA1. Low-frequency stimulation (LFS) was delivered at LTD threshold (2 bouts of 600 pulses, 1 Hz, at baseline stimulation intensity). LFS of the Schaffer collaterals did not produce significant homosynaptic LTD in control slices. However, identical conditioning in the presence of the glucocorticoid receptor agonist RU-28362 (10 microM) produced a robust LTD, which was blocked by the selective glucocorticoid antagonist RU-38486. The LTD induced by glucocorticoid receptor activation was dependent on N-methyl-D-aspartate (NMDA) receptor activity, because the specific NMDA receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (D-AP5) blocked the facilitation. However, the facilitation of LTD was not due to a potentiation of the isolated NMDA receptor potential by RU-28362. The facilitation of LTD by RU-28362 was also blocked by coincubation of the L-type voltage-dependent calcium channel (VDCC) antagonist nimodipine. Selective activation of the L-type VDCCs by the agonist Bay K 8644 also facilitated LTD induction. Both nimodipine and D-AP5 were effective in blocking the facilitation of LTD by Bay K 8644. These results indicate that L-type VDCCs can contribute to NMDA-receptor-dependent LTD induction.

Astrocytic neurotransmitter receptors in situ and in vivo

In the brain, astrocytes are associated intimately with neurons and surround synapses. Due to their close proximity to synaptic clefts, astrocytes are in a prime location for receiving synaptic information from released neurotransmitters. Cultured astrocytes express a wide range of neurotransmitter receptors, but do astrocytes in vivo also express neurotransmitter receptors and, if so, are the receptors activated by synaptically released neurotransmitters? In recent years, considerable efforts has gone into addressing these issues. The experimental results of this effort have been compiled and are presented in this review. Although there are many different receptors which have not been identified on astrocytes in situ, it is clear that astrocytes in situ express a number of different receptors. There is evidence of glutamatergic, GABAergic, adrenergic, purinergic, serotonergic, muscarinic, and peptidergic receptors on protoplasmic, fibrous, or specialized (Bergmann glia, pituicytes, Müller glia) astrocytes in situ and in vivo. These receptors are functionally coupled to changes in membrane potential or to intracellular signaling pathways such as activation of phospholipase C or adenylate cyclase. The expression of neurotransmitter receptors by astrocytes in situ exhibits regional and intraregional heterogeneity and changes during development and in response to injury. There is also evidence that receptors on astrocytes in situ can be activated by neurotransmitter(s) released from synaptic terminals. Given the evidence of extra-synaptic signaling and the expression of neurotransmitter receptors by astrocytes in situ, direct communication between neurons and astrocytes via neurotransmitters could be a widespread form of communication in the brain which may affect many different aspects of brain function, such as glutamate uptake and the modulation of extracellular space.

Control of Trypanosoma cruzi epimastigote motility through the nitric oxide pathway

Trypanosoma cruzi epimastigote motility can be enhanced by addition of L-arginine, to the culture. This effect is blocked by N-methyl-L-arginine, a competitive inhibitor of the nitric oxide synthase. N-methyl-D-aspartate and L-glutamate, two agonists of the NMDA/L-glutamate receptor, also enhanced motility. This stimulation is blocked by MK-801 a noncompetitive antagonist of the NMDA receptor. In addition, sodium nitroprusside, a guanylyl cyclase stimulator and 8-Br-cyclic GMP, and analog of cyclic GMP, also stimulated epimastigote motility. It is suggested that an increase of intracellular cyclic GMP levels mediated by nitric oxide may be responsible for the increase in epimastigote motility.

Spatiotemporal distribution of intracellular calcium transients during epileptiform activity in guinea pig hippocampal slices

Calcium ions are known to play an important role in epileptogenesis. Although there is clear evidence for increased neuronal calcium influx during epileptiform potentials, direct measurements of the corresponding intracellular calcium transients are rare and the origin of calcium influx is not known. Therefore the spatial and temporal distribution of intracellular calcium transients during epileptiform activity in guinea pig hippocampal slices was monitored with the use of the indicator Calcium-Green and a fast optical recording method. Two models of epilepsy (bicuculline and low Mg2+) were compared. In both models, single epileptiform events were evoked by electrical stimulation of the Schaffer collaterals in CA1 or of stratum pyramidale in area CA3. Intracellular calcium transients during epileptiform activity were approximately 5 times larger than during control stimulation. Calcium transients during epileptiform activity were present across at least the entire CA1 area, whereas presynaptic calcium transients from stimulated fibers were only seen at a distance up to 1 mm from the stimulation site. DL-2-amino-5-phosphonovaleric acid (APV), a specific antagonist of the N-methyl-D-aspartate (NMDA) receptor, abolished low-Mg2+ epileptiform activity and reduced bicuculline-induced epileptiform activity; it reduced calcium transients following stimulation of CA1 by only 29% (bicuculline) and 38% (low Mg2+). For comparison, calcium transients during control stimulation were 78% (bicuculline) and 69% (low Mg2+) smaller than epileptiform calcium transients. At a distance from the stimulation site, calcium transients and their NMDA-receptor-dependent components were largest in stratum pyramidale in the bicuculline model and in stratum oriens in the low-Mg2+ model. In both models, minimal onset latencies of calcium influx shifted with increasing distance to the stimulation electrode from stratum radiatum to stratum oriens. APV reduced the extent of spread of calcium transients in the low-Mg2+ model. In the bicuculline model, the spatial extent of spread of epileptiform calcium transients was not affected by application of APV; however, the mean velocity of spread was reduced from 0.20 to 0.12 m/s. In conclusion, the large size of calcium transients and of their NMDA-receptor-dependent components in stratum pyramidale or stratum oriens as well as shortest onset latencies of calcium transients at these sites suggest an important role of cell somata, basal dendrites, and possibly local circuit excitatory interactions for the generation and spread of epileptiform activity.

Hippocampal astrocytes in situ respond to glutamate released from synaptic terminals

A long-standing question in neurobiology is whether astrocytes respond to the neuronal release of neurotransmitters in vivo. To address this question, acutely isolated hippocampal slices were loaded with the calcium-sensitive dye Calcium Green-1 and the responses of the astrocytes to electrical stimulation of the Schaffer collaterals were monitored by confocal microscopy. To confirm that the responsive cells were astrocytes, the slices were immunostained for the astrocytic marker glial fibrillary acidic protein. Stimulation of the Schaffer collaterals (50 Hz, 2 sec) resulted in increases in the concentration of intracellular calcium ([Ca2+]i) in the astrocytes located in the stratum radiatum of CA1. The astrocytic responses were blocked by the sodium channel blocker tetrodotoxin, the voltage-dependent calcium channel blocker omega-conotoxin-MVIIC, and the selective metabotropic glutamate receptor antagonist alpha-methyl-4-carboxyphenylglycine (MCPG). These results suggest that the astrocytic responses were induced by stimulation of metabotropic glutamate receptors on the astrocytes by neuronally released glutamate. The astrocytic responses to neuronal stimulation were enhanced in the presence of the K+ channel antagonist 4-aminopyridine (4-AP). Inhibition of the astrocytic responses in the presence of 4-AP required the presence of both MCPG and the ionotropic glutamate receptor antagonist kynurenic acid. These results suggest that higher levels of neuronal activity result in stimulation of both metabotropic and ionotropic glutamate receptors on the astrocytes. Overall, the results indicate that hippocampal astrocytes in situ are able to respond to the neuronal release of the neurotransmitter glutamate with increases in [Ca2+]i.

Hippocampal astrocytes in situ respond to glutamate released from synaptic terminals

A long-standing question in neurobiology is whether astrocytes respond to the neuronal release of neurotransmitters in vivo. To address this question, acutely isolated hippocampal slices were loaded with the calcium-sensitive dye Calcium Green-1 and the responses of the astrocytes to electrical stimulation of the Schaffer collaterals were monitored by confocal microscopy. To confirm that the responsive cells were astrocytes, the slices were immunostained for the astrocytic marker glial fibrillary acidic protein. Stimulation of the Schaffer collaterals (50 Hz, 2 sec) resulted in increases in the concentration of intracellular calcium ([Ca2+]i) in the astrocytes located in the stratum radiatum of CA1. The astrocytic responses were blocked by the sodium channel blocker tetrodotoxin, the voltage-dependent calcium channel blocker omega-conotoxin-MVIIC, and the selective metabotropic glutamate receptor antagonist alpha-methyl-4-carboxyphenylglycine (MCPG). These results suggest that the astrocytic responses were induced by stimulation of metabotropic glutamate receptors on the astrocytes by neuronally released glutamate. The astrocytic responses to neuronal stimulation were enhanced in the presence of the K+ channel antagonist 4-aminopyridine (4-AP). Inhibition of the astrocytic responses in the presence of 4-AP required the presence of both MCPG and the ionotropic glutamate receptor antagonist kynurenic acid. These results suggest that higher levels of neuronal activity result in stimulation of both metabotropic and ionotropic glutamate receptors on the astrocytes. Overall, the results indicate that hippocampal astrocytes in situ are able to respond to the neuronal release of the neurotransmitter glutamate with increases in [Ca2+]i.

Electrophysiological correlates of prior training: an in vitro study of an area of the avian brain which is essential for early learning

Day-old domestic chicks will peck at any small, distinct object, such as a metal bead. One-trial passive avoidance learning can be established by coating the metal bead with methyl anthranilate (MeA) and allowing the birds to peck it once, after which they conspicuously avoid it. We have used birds successfully trained not to peck metal beads, and a control set of chicks where the training beads were innocuously dipped in water. Brain slices were prepared from both groups, containing the left, intermediate, medial part of the hyperstriatum ventrale (IMHV)--a region essential for this form of early learning. The electrophysiological properties of neurones in the IMHV were examined in vitro. Neurones recorded intracellularly in slices taken from MeA-trained birds had higher membrane resistances than did cells from water-trained controls. MeA training was also associated with an increased incidence of spontaneous, large EPSPs. Field responses to local electrical stimulation appeared to be somewhat greater in MeA-trained birds than in water-trained controls. In contrast, field potentials proved harder to potentiate with a burst of relatively high frequency stimulation in MeA-trained birds: the change in amplitude was less in MeA-trained birds, and there was less variability than in slices from water-trained controls.

Expression of NMDA receptor mRNAs in rat motoneurons is down-regulated after axotomy

The cytotoxic effects of glutamate via the N-methyl-D-aspartate (NMDA) receptor have been suggested to take part in the events leading to death of motoneurons after neonatal axotomy. By the use of in situ hybridization and immunohistochemistry we have investigated motoneuron mRNA expression of the NMDA receptor subunits NR1, NR2B and NR2D and of the NR1 subunit protein in two lesion models leading to partial motoneuron death: sciatic nerve transection early postnatally in the rat and ventral root avulsion in the adult rat. The results were compared with a lesion model with no subsequent death of motoneurons, i.e. sciatic nerve transection in the adult rat. All lesions were followed by down-regulation of the mRNAs for all studied subunits in severed motoneuron populations; down-regulation was detectable already at early stages postoperatively before any significant death had taken place. The strongest down-regulation was in fact seen in the lesion with the largest loss of motoneurons (ventral root avulsion). The reduction in the expression of NR1 mRNA was paralleled by a decrease in NR1 subunit protein. We conclude that down-regulation of NMDA receptor subunit expression is part of the acute response to axonal injury in motoneurons, whether or not neuronal death follows, and that the susceptibility of lesioned motoneurons to excitotoxic effects should be highest early after axonal injury.

Inhibitory mechanisms in epileptiform activity induced by low magnesium

In rat hippocampal slices epileptiform activity was induced by superfusion with Mg(2+)-free artificial cerebrospinal fluid (ACSF). Paroxysmal depolarization shifts (PDS) were evoked by electrical stimulation of Schaffer collaterals. To investigate the afterpotentials that follow PDS, intracellular recordings were made from CA1 pyramidal cells. The experiments revealed that several components are engaged in the generation of PDS afterpotentials in Mg(2+)-free ACSF. A long lasting component which determined the overall duration of the PDS afterhyperpolarization was blocked by intracellular application of ethylenebis(oxonitrilo)-tetraacetate (EGTA); concomitantly, the afterhyperpolarizations following depolarizing current injections were blocked. This indicated that the long lasting component was due to a slow Ca(2+)-activated K+ current. The block of Ca(2+)-activated K+ current uncovered a depolarizing PDS afterpotential with an N-shaped voltage dependence, suggesting that this depolarizing afterpotential component may be due to an N-methyl D-aspartate (NMDA) conductance. Intracellular injection of Cl- revealed that the PDS were followed by Cl- currents lasting about 500 ms. This component could be blocked by application of bicuculline suggesting that it is due to a synaptically GABA-mediated (i.e. gamma-aminobutyric acid) Cl- current. A comparison of PDS afterpotentials in Mg(2+)-free ACSF and those in other models of epileptiform activity suggests that similar sequences of inhibitory components are activated in spite of different pharmacological alterations of membrane conductances which induce the epileptiform discharges.

Clobazam, oxcarbazepine, tiagabine, topiramate, and other new antiepileptic drugs

Clinical investigators recently have studied at least 21 new antiepileptic drugs (AEDs) in people with epilepsy. This review briefly examines 15 of these new AEDs: clobazam (CLB), dezinamide, flunarizine (FNR), loreclezole, milacemide (MLM), MK-801, nafimidone, ORG-6370, oxcarbazepine (OCBZ), progabide (PGB), ralitoline, stiripentol, tiagabine (TGB), topiramate (TPM), and zonisamide (ZNS). CLB, PGB, and TGB represent agents that act on the GABA system, and MLM acts on the glycine system. MK-801 and ZNS (in part) are excitatory amino acid antagonists, and FNR is a calcium-channel antagonist. OCBZ is a keto analogue of carbamazepine, which is not metabolized to the epoxide and may have fewer side effects. The remaining agents are novel compounds with a variety of suspected mechanisms. TPM appears especially effective for intractable partial seizures but has a high incidence of cognitive side effects. None of these new AEDs is useful for all patients with inadequate seizure control or ongoing toxicity. The role of each will require further clinical study and experience.

GFAP-positive hippocampal astrocytes in situ respond to glutamatergic neuroligands with increases in [Ca2+]i

It is becoming increasingly clear that astrocytes play very dynamic and interactive roles that are important for the normal functioning of the central nervous system. In culture, astrocytes express many receptors coupled to increases in intracellular calcium ([Ca2+]i). In vivo, it is likely that these receptors are important for the modulation of astrocytic functions such as the uptake of neurotransmitters and ions. Currently, however, very little is known about the expression or stimulation of such astrocytic receptors in vivo. To address this issue, confocal microscopy and calcium-sensitive fluorescent dyes were used to examine the dynamic changes in astrocytic [Ca2+]i within acutely isolated hippocampal slices. Astrocytes were subsequently identified by immunocytochemistry for glial fibrillary acidic protein. In this paper, we present data indicating that hippocampal astrocytes in situ respond to glutamate, kainate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), 1-aminocyclopentane-trans-1,3-dicarboxylic acid (t-ACPD), N-methyl-D-aspartate (NMDA), and depolarization with increases in [Ca2+]i. The increases in [Ca2+]i occurred in both the astrocytic cell bodies and the processes. Temporally the changes in [Ca2+]i were very dynamic, and various patterns ranging from sustained elevations to oscillations of [Ca2+]i were observed. Individual astrocytes responded to neuroligands selective for both ionotropic and metabotropic glutamate receptors with increases in [Ca2+]i. These findings indicate that astrocytes in vivo contain glutamatergic receptors coupled to increases in [Ca2+]i and are able to respond to neuronally released neurotransmitters.

Voltage-gated Ca2+ channel blockers, omega-AgaIVA and Ni2+, suppress the induction of theta-burst induced long-term potentiation in guinea-pig hippocampal CA1 neurons

It is widely believed that a rise in post-synaptic calcium concentration ([Ca2+]i) is a necessary step in the induction of long-term potentiation (LTP) (Bliss and Collingridge, Nature, 361 (1993) 31-39). In this experiment, we examine the involvement of voltage-gated Ca2+ channels (VGCC) in the induction of AP5-sensitive LTP induced by theta-burst stimulation in guinea-pig hippocampal CA1 neurons. The VGCC blockers, Ni2+ (25 microM, T-channel blocker) or omega-AgaIVA (60 nM, P-channel blocker), which have no effect on synaptic transmission, suppress 60% or 78% of the theta-burst induced LTP, respectively. This implies that Ca2+ entry through VGCC is an important step in this form of LTP.

Effect of dizocilpine (MK-801) on analgesia and tolerance induced by U-50,488H, a kappa-opioid receptor agonist, in the mouse

The effect of dizocilpine (MK-801), an N-methyl-D-aspartate (NMDA) receptor antagonist, on the analgesic response to U-50,488H, a kappa-opioid receptor agonist, and tolerance to the analgesic effect of U-50,488H was determined in mice. The doses of MK-801 used were 0.03-0.30 mg/kg, whereas U-50,488H was administered at a dose of 25 mg/kg. Intraperitoneal (i.p.) administration of U-50,488H (25 mg/kg) produced analgesia as evidenced by the delay in the tail-flick latency in the mouse and lasted for a period of 240 min. MK-801 (0.03-0.30 mg/kg, i.p.) given 30 min prior to the injection of U-50,488H did not modify U-50,488H-induced analgesia. Twice daily administration of U-50,488H (25 mg/kg) for 9 days produced tolerance to its analgesic action. Administration of MK-801 (0.03 and 0.10 mg/kg) injected 30 min before each injection of U-50,488H prevented the development of tolerance to its analgesic effect. The higher dose, 0.3 mg/kg, of MK-801 had a minimal effect on U-50,488H tolerance. It is concluded that MK-801 in doses which do not affect U-50,488H-induced analgesia blocks the development of tolerance to its analgesic action in mice. These studies suggest that NMDA receptors play a crucial role in the development of tolerance to kappa-opioid agonist in mice.

Effects of chronic ethanol ingestion on long-term potentiation remain even after a prolonged recovery from ethanol exposure

Previous work in our laboratory has demonstrated that memory formation, a behavioral process thought to be at least in part attributed to hippocampal functioning, is severely attenuated following 5 months of chronic ethanol treatment (CET) and 2 months of recovery from CET. Additionally, 48 h following CET, a well-recognized physiological correlate of memory formation, long-term potentiation (LTP), is reduced in the hippocampus. We hypothesized that the reduction in LTP may in part contribute to the behavioral deficit in memory formation, which is a permanent consequence of CET. In order for the reduction of LTP to be involved with the permanent deficit in memory acquisition, it too must be present following a prolonged period of ethanol abstinence. The present study examined the permanent effect of CET on LTP. Animals were fed a nutritionally complete, ethanol containing diet for 28 weeks and then allowed a 5-7 month abstinence period. A control group was fed the same diet except sucrose was isocalorically substituted for ethanol. Neurophysiological methods measured the capacity of synaptic connections onto CA1 pyramidal cells to support LTP in response to a variety of conditioning trains. The magnitude of LTP was reduced in CET animals as compared with pair-fed controls. LTP induction is mediated by activation of the N-methyl-d-aspartate (NMDA) receptor complex and is modulated by activation of gamma-aminobutyric acid (GABA)ergic synaptic transmission. The effect of CET on LTP magnitude may be due to effects on the NMDA-induced induction itself or on the GABAergic modulation of induction.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium channel blockers verapamil and nimodipine inhibit kindling in adult and immature rats

The calcium channel blockers verapamil (VPM) and nimodipine (NMD) were administered to adult or immature (16-day-old) rats to determine their effects on amygdala-kindled seizures. The afterdischarge threshold (ADT) kindling rate and degree of postictal refractoriness were determined for two doses of VPM (0.5 and 5.0 mg/kg in rat pups and 2 and 10 mg/kg in adult rats) or 30 mg/kg nimodipine (NMD). Neither VPM nor NMD affected the ADT of the amygdala in adult or immature rats. VPM retarded the rate of kindling in both adult and immature rats in a dose-dependent manner; the number of stimulations required to progress through seizure stages were increased. NMD 30 mg/kg reduced the kindling rate and AD duration in both adult and immature rats. Neither drug was able to suppress recurrent seizures elicited by repetitive stimulation. These results suggest that verapamil, and possibly NMD may be of clinical utility in treatment of epilepsy, especially complex partial seizures.

Influence of the organic calcium antagonist verapamil on N-methyl-D-aspartate (NMDA) induced cortical field potentials (neocortical slice, guinea pig)

In the present experiments it was tested whether the organic calcium antagonist verapamil has an influence on N-methyl-D-aspartate (NMDA) induced cortical field potentials (CFP) in neocortical slices of guinea pigs. NMDA (1 mumol/l) was applied via a micropipette by pressure pulses. Verapamil (60 mumol/l) was administered by bath application or ejected locally (100 mumol/l) and simultaneously with NMDA. Neither the systemic administration nor the local application of verapamil had an influence on NMDA induced CFP. It is concluded that the antiepileptic effect of verapamil is not mediated via the NMDA receptor.

Contribution of NMDA receptors to tetanus-induced increase in postsynaptic Ca2+ in visual cortex of young rats

Mechanisms underlying the Ca2+ increase during tetanic synaptic inputs in layer II/III of visual cortical slices of young rats were investigated with microfluorometry using a Ca2+ indicator, rhod-2, and simultaneous recordings of field potentials evoked by white matter stimulation. Application of an antagonist for N-methyl-D-aspartate (NMDA) receptors, 2-amino-5-phosphonovalerate, did not significantly affect field potentials but reduced the tetanus-induced fluorescence rise to 56%, on average, of the control values. Application of a broad-spectrum antagonist for both NMDA and non-NMDA receptors, kynurenate, completely abolished the synaptically evoked component of field potentials and decreased the tetanus-induced fluorescence rise to 42%. Application of a non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione, in the Mg(2+)-free medium diminished the field potentials but did not decrease the tetanus-induced fluorescence increase. Nifedipine and diltiazem, L-type Ca2+ channel blockers, and Ni2+, a relatively selective blocker for T-type Ca2+ channels, did not affect the tetanus-induced fluorescence rise. These results indicate that NMDA receptors play a significant role in the increase of intracellular Ca2+ during tetanic synaptic inputs in the visual cortex of young rats.

Laminar difference in tetanus-induced increase of intracellular Ca2+ in visual cortex of young rats

Changes in intracellular Ca2+ evoked by electrical stimulation of the white matter were observed by means of microfluorometry with a Ca2+ indicator, rhod-2, in slice preparations of the visual cortex obtained from young rats. Tetanic stimulation at 5 Hz for 1 min induced a marked fluorescence increase, while single-shock stimulation did not induce a sizable increase in normal perfusate. The tetanus-induced increase took place in a column-like manner from layer VI near the stimulation site to layer II/III of the cortex, although it spread horizontally in layer II/III. The magnitude of fluorescence rise was largest in layer II/III of the cortex. Since N-methyl-D-aspartate (NMDA) receptors are known to exist only on neurons, the following results are taken to indicate that the fluorescent signal is derived mostly from postsynaptic neurons: Application of NMDA in the presence of tetrodotoxin induced a marked fluorescence increase with the same laminar bias as tetanic stimulation did, and the fluorescence increase by single-shock stimulation in Mg(2+)-free medium was almost completely blocked by an antagonist for NMDA receptors. These results support the hypothesis that input-associated entry of Ca2+ into postsynaptic neurons triggers processes for induction of long-term potentiation of synaptic efficacy.

Blockade of morphine-induced analgesia and tolerance in mice by MK-801

The effect of MK-801 on morphine-induced analgesia, tolerance and opioid binding sites was examined in mice. In analgesia studies, mice received either naloxone or MK-801. Controls were injected with saline. Mice were then injected with morphine 10 or 30 min following naloxone or MK-801, respectively, and tested for analgesia (tail flick assay) 45 min later. Pretreatment with naloxone or MK-801 blocked morphine-induced analgesia. In tolerance studies, mice were pretreated with either saline or MK-801. Thirty minutes later, mice were injected with either saline or morphine (acutely or chronically) and tested for analgesia 24 h later. Pretreatment with MK-801 partially or completely blocked the development of acute and chronic tolerance, respectively. In binding studies, MK-801 displaced [3H]naloxone poorly compared to naloxone or morphine. Together, these data suggest a role for NMDA receptors in morphine-induced analgesia and tolerance. The poor inhibition of the [3H]naloxone binding sites by MK-801 supports the possibility that MK-801 might not act directly on the opioid receptors, but rather, inhibits morphine-induced analgesia and tolerance by some other mechanisms.

Anticonvulsant properties of calcium channel blockers in mice: N-methyl-D-,L-aspartate- and Bay K 8644-induced convulsions are potently blocked by the dihydropyridines

Ten calcium channel blockers were evaluated in mice after intraperitoneal (i.p.) administration for prevention of seizures induced by various convulsants. The dihydropyridines (class II calcium antagonists, i.e., nisoldipine, nitrendipine, nicardipine, nifedipine, and nimodipine) selectively prevented seizures elicited by administration of pentylenetetrazol (PTZ), N-methyl-D,L-aspartate (NMDLA) and the dihydropyridine calcium channel agonist BAY K 8644. With regard to prevention of NMDLA-induced seizures and the subsequent mortality, these compounds were similar in potency to the noncompetitive NMDA receptor antagonist MK801. Unlike MK801 (IC50 = 0.014 microM), the dihydropyridines did not inhibit in vitro binding of MK801 to synaptic membrane fractions prepared from rat cerebrohippocampal tissue. The dihydropyridines did not influence seizures elicited by maximal electroshock (MES). Flunarizine (diphenyl-alkylamine, class IV) was selectively active in the MES test, considerably less potent against NMDLA-induced convulsions/mortality, exhibited weak noncompetitive NMDA antagonism in vitro (IC50 = 28 microM), and was inactive in the PTZ and BAY K 8644 testing paradigms. Diltiazem, a class III benzothiazepine, possessed relatively weak broad spectra of activity against MES, PTZ, NMDLA, and BAY K 8644 test situations. It was inactive in vitro as a noncompetitive NMDA antagonist. The class I compound verapamil (phenylalkylamine) displayed only moderate inhibition of NMDLA-evoked seizures/mortality. Prenylamine (class V) was moderately active against convulsions produced by MES and NMDLA while retaining a degree (IC50 = 16 microM) of noncompetitive NMDA antagonism. Lidoflazine (class VI) was inactive in all tests. The Ca2+ channel blockers and MK801 were inconsistent in their ability to prevent bicuculline (BIC)-elicited convulsions.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of L-type calcium channel antagonists on the serotonin-depleting actions of MDMA in rats

The calcium channel antagonists verapamil nifedipine and flunarizine all increased the threshold for convulsions induced by N-methyl-D-aspartate in rats. By contrast, only flunarizine blocked the long-term serotonin-depleting effects of 3,4-methylenedioxymethamphetamine. Flunarizine was also the only drug that antagonized methamphetamine-induced stereotypy. These findings suggest that calcium influx through L-type channels does not participate in the neurotoxic mechanism of MDMA, and that the neuroprotective actions of flunarizine are probably related to its anti-dopaminergic activity.