The effect of acidic amino acid antagonists on synaptic transmission in the hippocampal formation in vitro

Group III metabotropic glutamate receptors: pharmacology, physiology and therapeutic potential

Glutamate, the primary excitatory neurotransmitter in the central nervous system (CNS), exerts neuromodulatory actions via the activation of metabotropic glutamate (mGlu) receptors. There are eight known mGlu receptor subtypes (mGlu1-8), which are widely expressed throughout the brain, and are divided into three groups (I-III), based on signalling pathways and pharmacological profiles. Group III mGlu receptors (mGlu4/6/7/8) are primarily, although not exclusively, localised on presynaptic terminals, where they act as both auto- and hetero-receptors, inhibiting the release of neurotransmitter. Until recently, our understanding of the role of individual group III mGlu receptor subtypes was hindered by a lack of subtype-selective pharmacological tools. Recent advances in the development of both orthosteric and allosteric group III-targeting compounds, however, have prompted detailed investigations into the possible functional role of these receptors within the CNS, and revealed their involvement in a number of pathological conditions, such as epilepsy, anxiety and Parkinson's disease. The heterogeneous expression of group III mGlu receptor subtypes throughout the brain, as well as their distinct distribution at glutamatergic and GABAergic synapses, makes them ideal targets for therapeutic intervention. This review summarises the advances in subtype-selective pharmacology, and discusses the individual roles of group III mGlu receptors in physiology, and their potential involvement in disease.

Physiological roles and therapeutic potential of metabotropic glutamate receptors

Discovery of mGlu receptors has dramatically influenced our understanding of glutamatergic neurotransmission in the central nervous system. This receptor family provides a mechanism by which activation by glutamate can regulate a number of important neuronal and glial functions that are not typically modulated by ligand-gated ion channels. This includes modulation of neuronal excitability, synaptic transmission, and various metabolic functions. Because of the ubiquitous distribution of glutamatergic synapses, discovery of the mGlu receptors immediately raised the likelihood that mGlu receptors would participate in most, if not all, major functions of the CNS. In addition, the wide diversity and heterogeneous distribution of mGlu receptor subtypes could provide an opportunity for development of pharmacological agents that selectively target specific CNS systems to achieve a therapeutic effect. Over the past decade, an increasing number of agonists and antagonists selective for specific mGlu receptor subtypes have been developed. Use of these pharmacological tools along with genetic approaches has led to major advances in our understanding of the roles of mGlu receptors in regulating CNS systems and animal behavior. These studies suggest that drugs active at mGlu receptors may be useful in treatment of a wide variety of neurological and psychiatric disorders.

Metabotropic glutamate receptors: electrophysiological properties and role in plasticity

Electrophysiological research on mGluRs is now very extensive, and it is clear that activation of mGluRs results in a large number of diverse cellular actions. Studies of mGluRs and on ionic channels has clearly demonstrated that mGluR activation has a widespread and potent inhibitory action on both voltage-gated Ca2+ channels and K+ channels. Inhibition of N-type Ca2+ channels, and inhibition of Ca(++)-dependent K+ current, IAHP, and IM being particularly prominent. Potentiation of activation of both Ca2+ and K+ channels has also been observed, although less prominently than inhibition, but mGluR-mediated activation of non-selective cationic channels is widespread. In a small number of studies, generation of an mGluR-mediated slow excitatory postsynaptic potential has been demonstrated as a consequence of the effect of mGluR activation on ion channels, such as activation of a non-selective cationic channels. Although certain mGluR-modulation of channels is a consequence of direct G-protein-linked action, for example, inhibition of Ca2+ channels, many other effects occur as a result of activation of intracellular messenger pathways, but at present, little progress has been made on the identification of the messengers. The field of study of the involvement of mGluRs in synaptic plasticity is very large. Evidence for the involvement of mGluRs in one form of LTD induction in the cerebellum and hippocampus is now particularly impressive. However, the role of mGluRs in LTP induction continues to be a source of dispute, and resolution of the question of the exact involvement of mGluRs in the induction of LTP will have to await the production of more selective ligands and of selective gene knockouts.

Effects of the enantiomers of (+/-)-HA-966 on dopamine neurons: an electrophysiological study of a chiral molecule

The present study was conducted to evaluate the effects of the resolved enantiomers of (+/-)-1-1 hydroxy-3-aminopyrrolidone-2 ((+/-)-HA-966) on the electrophysiological properties of dopamine-containing neurons in the substantia nigra of the chloral hydrate anesthetized rat. Both (+)- and (-)-HA-966 produced a dose-dependent reduction in firing rate that eventually resulted in total cessation of spontaneous neuronal activity (ID50 = 5.7 and 57.8 mg/kg i.v., respectively). The inhibitory effects of both drugs were accompanied by a marked increase in the regularity of neuronal firing and a concomitant suppression of bursting activity. Although approximately 10-fold less potent than the (-) enantiomer, the inhibitory effects of (+)-HA-966 were completely antagonized by the centrally active, GABAB receptor antagonist, CGP-35348 (300 mg/kg i.v.). These data suggest that the complementary electrophysiological effects of the enantiomers of (+/-)-HA-966 on nigral dopamine neurons are mediated through a common mechanism of action possibly involving a novel interaction with GABAB receptors.

Actions of phosphomonoesters on CA1 hippocampal neurons as revealed by a combined electrophysiological and nuclear magnetic resonance study

Phosphomonoesters (PMEs), precursors of membrane phospholipids, are found in high levels in the developing brain and Alzheimer's disease brain. The present study details the neurophysiological and metabolic effects of acute PME elevation on the Fisher 344 rat in vitro hippocampal slice. Two abundant PMEs, phosphoethanolamine (PE) and L-phosphoserine (PS), reliably altered properties of synaptic transmission at the Schaffer collateral/commissural-CA1 cell synapse. Specifically, PE reversibly depressed the amplitude of population EPSPs at millimolar concentrations but had no effect at micromolar concentrations. PS had biphasic effects on population EPSPs, inducing first a reduction followed by an enhancement of response amplitude. In contrast to PE, the effects of PS were not reversible; population EPSPs were augmented during the wash of PS, and the CA3 region generated evoked (but not spontaneous) epileptiform discharges. 31P nuclear magnetic resonance spectroscopy revealed enhanced slice uptake of PS compared to PE. There was no significant effect of PE on slice high-energy phosphates but incubation with PS significantly lowered slice phosphocreatine (PCr) and ATP concentrations. These observations indicate that the slice uptake of PS could be energy requiring and the enhanced response amplitude observed at 5 mM PS also could produce a drain on high-energy phosphates. Possible modes of PME action on hippocampal physiology are discussed.

Presynaptic modulation of glutamate and dynorphin release by excitatory amino acids in the guinea-pig hippocampus

Excitatory amino acid agonists and antagonists were evaluated for their ability to affect the concomitant release of endogenous L-glutamate and dynorphin A(1-8)-like immunoreactivity from guinea-pig hippocampal mossy fiber synaptosomes. Previous work in this laboratory demonstrated that L(+)2-amino-4-phosphonobutyrate inhibits the potassium-evoked release of these endogenous neurotransmitters from guinea-pig but not rat hippocampal mossy fiber synaptosomes. Therefore, the present study was conducted to evaluate excitatory amino acid agonists as indices to the functional properties of this L(+)2-amino-4-phosphonobutyrate-sensitive glutamatergic autoreceptor on mossy fiber terminals. Low micromolar concentrations of quisqualate, but not kainate, N-methyl-D-aspartate, nor RS-alpha-amino-3-hydroxy-5-methyl-4-isoazole-propionic acid, significantly inhibited the potassium-evoked release of both L-glutamate and dynorphin A(1-8)-like immunoreactivity. Quisqualate-induced inhibition of L-glutamate release from mossy fiber terminals was antagonized by the non-N-methyl-D-aspartate antagonist 6-cyano-7-nitroquinoxaline-2,3-dione. In contrast, high concentrations of kainate enhanced the potassium-evoked release of L-glutamate and dynorphin A(1-8)-like immunoreactivity, and this potentiation was blocked by 6-cyano-7-nitroquinoxaline-2,3-dione. Kainate (1 mM) was the only agonist which significantly enhanced the basal release of L-glutamate, whereas the spontaneous efflux of dynorphin A(1-8)-like immunoreactivity was not affected by any of the agonists tested. The results presented in this paper suggest the existence of inhibitory and excitatory presynaptic glutamatergic autoreceptors that act to modulate the release of endogenous L-glutamate- and prodynorphin-derived peptides from guinea-pig hippocampal mossy fiber terminals. These inhibitory and excitatory autoreceptors, which are sensitive to quisqualate/L(+)2-amino-4-phosphonobutyrate or kainate, respectively, may play an important role in regulating synaptic activity at glutamatergic synapses throughout the central nervous system.

Modulation of memory processing by glutamic acid receptor agonists and antagonists

Recent hypotheses suggesting a critical role of glutamate receptors in hippocampal long-term potentiation and memory processing suggested a closer examination of this transmitter's effect on memory processing in an in vivo setting. New pharmacological antagonists allow for a separation and examination of various glutamate receptors and their role in memory processing. Mice were trained on a shock avoidance learning paradigm and injected intracerebroventricularly after training with agonists and antagonists of various classes of glutamate receptors. Retention was tested 1 week after training. N-Methyl-D-aspartate (NMDA) receptor agonists enhanced retention in a dose-dependent manner. The enhancement of retention by the non-NMDA agonist kainic acid and quisqualic acid was dose-dependent. L-Glutamic acid, but not D-glutamic acid, enhanced retention. Both NMDA and non-NMDA receptor antagonists produced dose-dependent impairment of retention for footshock training. Administration of the antagonists 24 h after training did not impair memory retention.

Nicotine effects in mouse hippocampus are blocked by mecamylamine, but not other nicotinic antagonists

Previous data indicated that bath-application of nicotine to mouse hippocampal slices resulted in a concentration-dependent increase in the amplitude of the orthodromic population spike and the appearance of multiple population spikes in the CA1 pyramidal cell layer. d-Tubocurarine (4-100 microM), alpha-bungarotoxin (10-160 microM), and atropine (40-200 microM) had similar effects, although for alpha-bungarotoxin these excitatory effects were transient. Mecamylamine (1.6-3.2 mM) inhibited the population spike, while hexamethonium (3.2 mM) had no effect. These cholinergic antagonists were tested for their ability to block excitatory effects of nicotine (800 microM) at antagonist concentrations which were at or near threshold for intrinsic effects. Of the 5 antagonists tested, only mecamylamine (400 microM) effectively inhibited the nicotine-induced increase of the population spike amplitude and the appearance of multiple population spikes. These results suggest that nicotine exerts electrophysiological effects via a subclass of nicotinic cholinergic receptors that is neither neuromuscular nor ganglionic in the classical sense; these brain nicotinic receptors are sensitive to mecamylamine, but not to hexamethonium, alpha-bungarotoxin, or D-tubocurarine.

DL-2-[3,4-3H]amino-4-phosphonobutyrate binding sites in the rat hippocampus: distribution and possible physiological role

Binding sites for the novel, glutamate-like radioligand DL-2-[3,4-3H]amino-4-phosphonobutyrate (DL-[3H]APB) on rat hippocampal synaptic membranes were identified and characterised. The existence of a single, saturable population of binding sites was demonstrated. These appeared to be indistinguishable, in terms of their pharmacological profile and ionic dependence, from those described previously in the striatum and whole brain. The distribution of these sites was also examined using a number of discrete neuronal lesions. A majority of sites (approx. 55%) were located on dentate gyrus granule cells. Smaller populations appeared to be situated on perforant path terminals and on pyramidal cells. However, L-APB was found to be ineffective as an inhibitor of basal and potassium evoked D-[3H]aspartate release from hippocampal slices. A presynaptic location can therefore presumably be ruled out. The likely postsynaptic location of DL-[3H]APB-binding sites in the hippocampus suggests that this site may be involved in synaptic neurotransmission. This possibility is discussed with regard to electrophysiological data concerning the synaptic pharmacology of neuronal connections within the hippocampus.

Synaptic pharmacology of the hippocampus

There is a wealth of information available regarding the complex synaptic pharmacology of the mammalian hippocampus. It is clear that many neurotransmitters are present in the hippocampus. Fig. 3 is an attempt to summarize in a schematic manner some of the synaptic connections of this structure and the neurotransmitters involved. The pyramidal cell and its inputs are shown and how various neurotransmitters modify its action in an excitatory (+) and inhibitory (-) manner. The hippocampus is an excellent model system for studying not only normal brain physiology, but also pathologic processes such as seizures, aging, etc. In view of the important role of the hippocampus in learning and memory and other brain functions, it is essential that the detailed synaptic mechanisms of the hippocampus be thoroughly understood. The present review is an attempt to integrate our knowledge of this structure into a rational basis for understanding how it functions and how it can be modified by pharmacological agents.

Adenosine antagonists increase spontaneous and evoked transmitter release from neuronal cells in culture

To examine the role played by endogenous adenosine in the modulation of transmitter release in the CNS, the effect of adenosine antagonists has been studied. Two systems have been used: EPSPs recorded from pyramidal cells in organotypic hippocampal cultures; and release of newly synthesized [3H]glutamate from cerebellar granule cells in dissociated culture. Bath application of 0.1-1 microM 8-phenyltheophylline (8-PT) reversibly increased both the number and size of spontaneous EPSPs and caused bursting activity in some cells. This effect was blocked by the glutamate antagonist gamma-D-glutamylglycine (DGG) (1 mM) but not by atropine (10 microM) or bicuculline (100 microM). Another adenosine antagonist isobutylmethylxanthine (IBMX, 10 microM) had a similar effect to 8-PT. Spontaneous activity in pyramidal cells and that induced by adenosine antagonists was blocked by the adenosine agonist 2-chloroadenosine (2-CA) (0.2-20 microM). 8-PT (10 microM) markedly potentiated K+-stimulated release of newly synthesized glutamate, and also enhanced basal glutamate release. The agonist (-)-phenylisopropyladenosine ((-)-PIA, 2 microM) which is relatively selective for A1 receptors, reduced by 19 +/- 5% the 8-PT-induced enhancement, and reduced K+-stimulated glutamate release in the absence of 8-PT to a similar extent. In contrast 5'-N-ethylcarboxamido adenosine (NECA, 2 microM), which is a relatively selective A2 agonist, slightly enhanced glutamate release. From these results it is likely that 8-PT potentiates glutamate release in both systems by blocking the effect of endogenous adenosine on presynaptic A1 receptors.

Exposure of hippocampal slices to quisqualate sensitizes synaptic responses to phosphonate-containing analogues of glutamate

Exposure of transverse slices of rat hippocampus to quisqualate (Quis) resulted in a marked increase in the potency of D- and L-2-amino-4-phosphonobutanoate (APB) and D- and L-2-amino-5-phosphonopentanoate (APV) for depression of extracellular synaptic field potentials recorded from CA1 pyramidal cells. L-APB depressed the amplitude of CA1 field potentials with an IC50 = 1800 microM before exposure to Quis. After a brief (4 min) exposure to sufficient Quis (16 microM) to depress the response by 50%, L-APB depressed these responses with an IC50 = 54 microM. These phosphonate-containing glutamate analogues transiently induced population-spiking after the tissue was pretreated with Quis. This suggests that APB and APV can act as agonists at micromolar concentrations.

Antagonism of lateral olfactory tract synaptic potentials in rat prepyriform cortex slices

Dose-response data were collected for the inhibition of the monosynaptic excitatory input onto prepyriform neurons from fibers of the rat lateral olfactory tract, using the potent antagonists of excitatory transmission, L(+)-2-amino-4-phosphonobutyrate (L(+)-AP4), kynurenate, N-(p-chlorobenzoyl)piperazine-2,3-dicarboxylate, and N-(p-bromobenzoyl)piperazine-2,3-dicarboxylate. Kynurenate and the piperazine derivatives blocked up to 80% of the synaptic response at doses of 1000 microM, with single-affinity dose-response curves. L(+)-AP4 blocked only 50% of the synaptic response at a dose of 1000 microM, with a multicomponent dose-response curve.

A comparison of 2-amino-4-phosphonobutyric acid (AP4) receptors and [3H]AP4 binding sites in the rat brain

The glutamate analogue 2-amino-4-phosphonobutyric acid (AP4) is a potent antagonist at several synapses where an excitatory amino acid appears to be the neurotransmitter. Previous studies identified a Cl-/Ca2+ dependent [3H]glutamate binding site in synaptic plasma membrane (SPM) preparations that was also labeled by [3H]AP4 and exhibited a pharmacology similar to the AP4 receptor. This report examines the pharmacological specificity in both biochemical and electrophysiological preparations in greater detail. Several compounds are identified which readily interact with the apparent binding site in membranes, but neither mimic nor inhibit the action of AP4 in electrophysiological studies. The rate of dissociation of [3H]AP4 from SPMs is shown to increase in the presence of added AP4 and increasing the osmolarity in the SPM binding assay decreases the level of observed [3H]AP4 binding. These findings indicate both a heterogeneous population of binding sites and the occurrence of transport. It is concluded that much of the AP4 binding observed in SPM preparations is to a site other than the AP4 receptor. The results provide a further pharmacological description of AP4 receptors which should facilitate the identification of the receptor in biochemical preparations.

Assessment of some transmitter actions in rat cerebellar slices

The effects of 100 microM norepinephrine (NE), GABA, aspartate, glutamate, and carbachol on the release of endogenous NE, GABA, aspartate, and glutamate from slices of rat cerebellum were examined. The 35 mM K+-stimulated release of NE was potentiated by GABA (136% of control), glutamate (123%), and carbachol (123%); aspartate had no effect. Glutamate increased the release of GABA to 250% of control levels, while neither NE nor carbachol exerted any effect. Glutamate and GABA increased aspartate release to 260% and 300% of control values, respectively. NE decreased the release of aspartate to 86% of control levels while carbachol had no effect. The stimulated release of glutamate was increased by GABA (166% of control) but was unaffected by NE and carbachol. All of these effects were observed only under depolarizing conditions and in the presence of extracellular Ca2+. These data suggest a cholinergic, GABAergic and glutamatergic control of the noradrenergic system in the cerebellum; the presence of a specific aspartergic system in the cerebellum; and a net excitatory action of GABA may be present within the cerebellum.

Opiate antagonists do not alter neuronal responses to stimulation of opioid-containing pathways in rat hippocampus

The opioid receptor antagonists, naloxone and beta-chlornaltrexamine, were used to determine whether activation of endogenous opioid peptide containing pathways produced pharmacologically reversible opioid actions. Extracellularly recorded responses of the hippocampal CA3 pyramidal cells were evoked by stimulation of the dynorphin-containing mossy fiber pathway. Neither naloxone nor beta-chlornaltrexamine pretreatment significantly changed the evoked response. However, both antagonists blocked the effect of applied dynorphin-A(1-17) on CA1 pyramidal cell evoked responses. Thus, our data demonstrate that if endogenous opioids are released from this pathway, the peptides cannot be responsible for the evoked response measured in hippocampal CA3 cellular field. With no direct evidence for endogenous opioid peptides acting through opioid receptors, the neurotransmitter role of dynorphins in rat hippocampus remains obscure.

Action of excitatory amino acids and their antagonists on hippocampal neurons

Intracellular recordings were obtained from guinea pig hippocampal neurons maintained in vitro. Current- and voltage-clamp techniques were used to study the effect of microiontophoresis of excitatory amino acid agonists. Modification of agonist responses by bath application of known concentrations of antagonist agents was also examined. All agonists used, glutamate, aspartate, N-methyl-D-aspartic acid (NMDA), and quisqualate, depolarized hippocampal neurons and caused repetitive firing. NMDA was also noted to induce burst-firing in some neurons. Quisqualate and NMDA were more potent than glutamate or aspartate. In slices perfused with a nominally calcium-free saline containing tetrodotoxin and manganese, quisqualate application produced a depolarization associated with a conductance increase. Under those conditions, NMDA-induced depolarizations caused apparent decreases as well as increases in conductance. The apparent decreases in conductance were observed in the voltage range of -40 to -70 mV, whereas increases in conductance were observed at membrane potentials more positive than -35 mV. Under voltage-clamp conditions, quisqualate produced an inward current whose amplitude increased with hyperpolarization and decreased upon depolarization, reversing near 0 mV. The conductance change induced by quisqualate was independent of voltage. NMDA application resulted in an inward current that was maximal around the resting potential and decreased with both hyperpolarization and depolarization. Response reversal was not observed with hyperpolarization to -100 mV but was apparent with depolarization beyond 0 mV. Conductance changes induced by NMDA were voltage dependent, and the application of this agent was associated with the appearance of a region of negative slope conductance in the current-voltage relationship. Apparent decreases in conductance in response to NMDA were reduced when the extracellular magnesium concentration was lowered. Response amplitudes were not affected. The NMDA receptor antagonist DL-2-amino-5-phosphonovalerate (DL-APV) was a potent and selective blocker of NMDA responses, whereas the antagonist DL-2-amino-4-phosphonobutyric acid (DL-APB) was less potent and did not select between NMDA and quisqualate responses. Analysis of iontophoretic dose-response curves indicated that DL-APV was a competitive antagonist. The results of these experiments indicate that hippocampal CA1 pyramidal neurons possess separate receptors for quisqualate and NMDA, with different pharmacological and electrophysiological profiles.

Peptides derived from kainic acid as antagonists of N-methyl-D-aspartate-induced neuroexcitation in rat brain

The effects of dipeptides and an amide chemically derived from kainic acid (KA) on the response of rat striatal slices to excitatory amino acids were studied. Some of the gamma-peptides of KA were found to antagonize the response to N-methyl-D-aspartate (NMDA) more than that to quisqualate and glutamate and not to have any effect on the response to kainate. The least potent antagonists among the tested compounds were the gamma-amide of KA and the peptides of KA with beta-alanine and gamma-aminobutyric acid, whereas the gamma-kainyl peptides of the alpha-amino acids glycine, tyrosine, glutamate and KA were more active. The latter are the best blockers of the response to NMDA among the tested compounds.

Monoamine, amino acid and cholinergic interactions in slices of rat cerebral cortex

Interactions of monoamine, amino acid and cholinergic transmitter systems were studied in slices of rat cerebral cortex using a superfusion procedure and measuring release of endogenous dopamine (DA), norepinephrine (NE), serotonin (5-HT), GABA, glutamate (GLU) and aspartate (ASP). Depolarizing concentrations of K+ were used to induce a Ca2+-dependent, Mg2+-inhibited release of the monoamines and amino acids. Submaximal release of the monoamines and amino acids was observed at 35 mM K+, which permitted studies of possible excitatory or inhibitory actions of the added agents. The 35 mM K+-stimulated, Ca2+-dependent release of GABA was inhibited 40, 30 and 25% by 100 microM NE, DA and 5-HT, respectively. The release of GLU was potentiated by NE and reduced by DA. Both DA and 5-HT inhibited the release of ASP. The Ca2+-dependent, K+-stimulated release of endogenous NE, DA and 5-HT was not altered by 100 microM GABA, GLU or ASP. However, 100 microM GLU did enhance the stimulated release of GABA. The cholinergic agonist, carbachol, enhanced the stimulated release of NE, 5-HT and GLU 10, 60 and 40%, respectively. On the other hand, carbachol attenuated the release of DA and GABA approximately 20%. One interpretation of the data is that the amino acid transmitter pathways in slices of the cerebral cortex of the rat can be controlled by monoaminergic and cholinergic systems while the monoamine afferents appear to have a cholinergic regulation but not a major direct amino acid transmitter influence.

Actions of kynurenic acid and quinolinic acid in the rat hippocampus in vivo

An iontophoretic study was made of the interaction of kynurenic acid with excitatory amino acids in the hippocampus and with the commissural input from the contralateral hippocampus in the rat. The results showed that kynurenic acid was an effective blocker of synaptic transmission in the hippocampus in vivo, adding further support to the idea that an excitatory amino acid is involved in neurotransmission in this structure. In addition there was an increase in the specificity of kynurenate as an antagonist of excitatory amino acids in the hippocampus compared with neocortex, with much more activity being shown toward the NMDA-preferring rather than the quisqualic acid-preferring receptor. Kynurenic acid was also able to distinguish partially between quinolinic acid and NMDA, supporting the possibility that two types of NMDA/quinolinate receptors exist in the hippocampus.

2-Amino-4-phosphonobutyric acid selectively blocks two-way avoidance learning in the mouse

There seems to be ample evidence supporting the view that glutamate plays a significant role in the mammalian brain as a neurotransmitter. It is considered to be a likely transmitter candidate in one or more hippocampal pathways. Recently it has been visualized in excitatory, possibly glutamatergic, neurons in the hippocampus. Glutamate has been proposed to mediate memory formation. We wanted to see if blocking glutamate action by a specific glutamate antagonist could result in reduction of learning ability. 2-Amino-4-phosphonobutyric acid (APB) is an analogue of glutamic acid and has been used as a glutamate antagonist in electrophysiological studies on invertebrate neuromuscular junction, retina and hippocampus. We tested the influence of APB on the acquisition of two way avoidance learning in the shuttle box and on learning in the water maze. Our results show that intraperitoneal injection of APB led to a reduction in avoidance learning, whereas learning in the water maze was unaffected.

An evaluation of selected brain constituents as putative excitatory neurotransmitters

Searching for the endogenous ligands of the 4 classes of excitatory amino acid receptors detected in the mammalian CNS, we have measured, using a 22Na+ efflux receptor assay, the excitatory activity of 42 brain constituents or analogs and established the receptor specificity of those substances which possess excitatory properties. Among the substances tested were methyltetrahydrofolate and N-acetylaspartylglutamate, two putative ligands of the kainate and glutamate receptors. These compounds were found to have very little or no excitatory activity, respectively. The 8 brain constituents possessing excitatory properties displayed a receptor specificity similar to either that of N-methyl-D-aspartate (e.g. quinolinate) or glutamate (e.g. cysteine sulfinate) but not of kainate or quisqualate. These results are discussed in relation with the problem of the identification of brain excitatory neurotransmitters.

Kynurenic acid as an antagonist of hippocampal excitatory transmission

Kynurenate, an endogenous tryptophan metabolite, was bath-applied to hippocampal slices while recording extracellular synaptic field potentials. Kynurenate antagonized the medial and lateral entorhinal projections to dentate granule cells, the Schaffer collateral projections to CA1 pyramidal cells, and inputs to the CA3 stratum radiatum of regio inferior with similar potencies. Concentration-response curves for these pathways paralleled theoretical antagonist curves with a Hill coefficient of 1, and the KdS were in the range of 130-400 microM. Projections to the stratum lucidum of regio inferior were much less sensitive to kynurenate. Inputs to CA3 pyramidal cells showed varying sensitivities to kynurenate, L-2-amino-4-phosphonobutanoic acid (L-APB), and (-)-baclofen depending on the placements of the stimulating and recording electrodes. When both electrodes were located in area CA3, outside the hilus of area dentata, all responses were insensitive to inhibition by L-APB. Under these conditions, responses recorded within the stratum radiatum were sensitive to inhibition by kynurenate and baclofen, while responses recorded within the stratum lucidum were insensitive to these drugs. When the stimulating electrode was placed within the hilus of area dentata, variable patterns of sensitivity to APB, baclofen, and kynurenate were observed from recording electrodes in area CA3. These results suggest that stimulation in the hilus, while recording in the stratum lucidum, produces responses that show composite effects resulting both from direct stimulation of mossy fibers and from stimulation of neuronal elements in the hilus which produce outputs to mossy fibers.

Differential blockade of hippocampal excitatory synaptic responses by an analogue of aspartate

L-Aspartic acid-beta-benzyl ester blocks the mossy fiber-CA3 and medial perforant path-dentate gyrus synaptic responses in the rat hippocampal slice with an IC50 of about 1 mM. Higher concentrations (IC50 greater than 10 mM) are necessary to block the Schaffer collateral/commissural-CA1, commissural/associational-CA3 and lateral perforant path-dentate gyrus synaptic responses. These results indicate that it is possible to differentiate between excitatory synaptic responses which have previously appeared pharmacologically similar.

Suprachiasmatic neurons in tissue slices from ovariectomized rats: electrophysiological and neuropharmacological characterization and the effects of estrogen treatment

Single-unit activity and unit responses to putative neurotransmitters were recorded from suprachiasmatic nucleus (SCN) neurons in brain tissue slices from ovariectomized rats either treated or untreated with estrogen. Altogether, 204 units were studied from estrogen-treated and untreated preparations, and at the resting state, 37% of these units fired regularly, 57% fired irregularly, and 6% were silent but evokable by electrical stimulation. Most of the irregular units fired continuously (n = 100), while the rest fired intermittently (n = 12) or phasically (n = 4). Neurons with different types of firing patterns also varied significantly in resting firing rate and in responsiveness to transmitters and to estrogen treatment. The average resting firing rate decreased significantly from regular, irregular and continuous, intermittent, to silent units. Acetylcholine (ACh) and/or serotonin (5-HT) injected directly into the perfusion chamber evoked responses from more irregular (69% of 61 units) than regular units (20% of 46 units). None of the 5 silent units tested was activated by ACh or 5-HT. Responses to ACh (predominantly inhibitory) and 5-HT (predominantly excitatory) seen here in vitro were opposite to those observed in vivo with iontophoretic application, and were not reversed or abolished by the blockade of synaptic transmission. Comparisons of data between the two types of preparations showed that only the responsiveness of the irregular units to ACh and to 5-HT were significantly different: both types of responsiveness were higher in estrogen-treated than in untreated preparations. No significant difference was found in the responsiveness of regular units, or in firing patterns or firing rate. Thus, the present in vitro studies have demonstrated that SCN contains a heterogeneous population of neurons distinguishable by their electrophysiological and neuropharmacological characteristics, and that estrogen has a specific action on specific types of SCN neurons.

A neural plasticity hypothesis of schizophrenia

An extensive research effort has failed, thus far, to conclusively identify a specific disease process (or processes) underlying the behavioral symptoms of schizophrenia. The present paper will entertain the hypothesis that the structural and functional plasticity of the brain can constitute a "nonspecific" biological etiology of schizophrenia. This plasticity need not be accompanied by infectious processes or gross alterations in neurotransmitter levels, enzyme activities, etc. that are specific to schizophrenia. The monkey isolation syndrome provides a precedent for a causal relationship between brain plasticity and pathological behavior. In a speculative manner, it will be demonstrated that neural plasticity concepts can be invoked to potentially explain several aspects of schizophrenia: the various types of behavioral symptoms exhibited by schizophrenics, the regional alterations in brain structure and function seen in chronic schizophrenics, the involvement of genetic and environmental etiological factors, the pharmacological support for the dopamine hypothesis, and the delayed onset of neuroleptic antipsychotic action. Considering the explanatory potential of neural plasticity concepts, a research program which focuses on these concepts seems warranted.

Localization of cysteine sulfinic acid uptake sites in rat brain by quantitative autoradiography

In vitro autoradiography was employed to localize and quantify Na-dependent binding sites of [35S]cysteic acid (CA), an analog of cysteine sulfinic acid (CSA). The heterogeneous anatomical distribution and pharmacological specificity of [35S]CA differs from that of the glutamate/aspartate marker D-[3H]aspartate, and appears to represent a specific uptake site for CSA. These results suggest that CSA may act as an excitatory transmitter in the central nervous system.

Modulation by dopamine of population responses and cell membrane properties of hippocampal CA1 neurons in vitro

Dopamine (DA) was applied to rat hippocampal slices maintained in vitro. Extracellular and intracellular recording techniques were used to study the effect of DA on population responses, membrane potentials, and membrane responses to hyperpolarizing current pulses in CA1 pyramidal cells. Temporary exposure of hippocampal slices to DA has a dual effect. The initial action of DA is to produce a suppression of the extra-cellularly recorded population responses. In individual neurons, this initial effect is seen as a membrane hyperpolarization accompanied by a decrease in the amplitude of responses to hyperpolarizing current pulses. The frequency of occurrence of spontaneous depolarizations and spikes is reduced. The early action of DA is followed by a profound potentiation of the population responses that can last for hours. This long-lasting potentiation of the population response, induced by DA, is depressed by spiroperidol, a DA antagonist. In individual neurons, the late effect of DA is a long-lasting membrane depolarization associated with an increase in the amplitude of responses to hyperpolarizing current pulses. During this late phase, spontaneous activity is increased, as are single cell responses to stimulation of afferents. The evidence presented here indicates that DA is able to induce a long-lasting modification of the excitability of CA1 hippocampal neurons. This modulation of excitability by DA may be similar in nature to previously described DA-modulatory actions in the peripheral nervous system.

Antagonist activity of phosphorus-containing glutamate analogues in the perforant path

Two analogues of the amino acid L-2-amino-4-phosphonobutanoic acid (L-APB) were synthesized in order to test the hypothesis that the dianionic nature of the side chain is responsible for antagonism of excitatory synapses in the hippocampal perforant path. These compounds, DL-2-amino-4-(methylphosphino)-butanoic acid (DL-AMPB) and O-methylphosphonyl-L-serine (O-MPLS), possess singly-charged side chains and yet display antagonistic activity, illustrating that a dianionic charge on the side chain is not necessary for antagonism. Comparing structure-activity relationships for DL-AMPB, O-MPLS, L-APB, and O-phospho-L-serine (O-PLS), patterns of synaptic activity emerged which suggest that substitution of a methyl group for one of the phosphoryl hydroxyl groups lowers ligand potency in both medial and lateral pathways. Also, the nature of the atom at the gamma-position appears to alter the potency and degree of pathway selectivity of these ligands, a methylene unit imparting more potency and selectivity than an oxygen atom.

2-Amino-4-phosphonobutyrate selectively blocks mossy fiber-CA3 responses in guinea pig but not rat hippocampus

The acidic amino acid antagonist D,L-2-amino-4-phosphonobutyrate (DL-APB) is a potent blocker of synaptic transmission at guinea pig but not rat mossy fiber-CA3 synapses in hippocampal slices. The L-isomer of APB is responsible for the potent inhibition at the guinea pig synapse. The L-APB analogue L-serine-O-phosphate (L-SOP) also is more potent against the guinea pig response. These differences may reflect a difference in a synaptic acidic amino acid receptor in these two species. Other acidic amino acid antagonists are less potent than APB or L-SOP and do not discriminate between the mossy fiber responses in the two species.

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

The depolarizing actions of N-methyl-DL-aspartate (NMA) and L-glutamate on pyramidal neurones were compared in a hippocampal slice preparation. Tetrodotoxin (1 microM) was added to the perfusion solution to suppress regenerative Na conductances. Depolarization evoked by ionophoretic application of NMA triggered slow, high-threshold regenerative spikes. These are considered to be Ca spikes since the amplitude and rate of rise could be reduced by verapamil, D-600, Co2+ and Mn2+, and increased by Ba2+. Multiple Ca-spike thresholds could be demonstrated in the same cell. In contrast, depolarizations evoked by L-glutamate only rarely triggered Ca-spikes. The minimum latency to the onset of depolarization evoked by NMA was less than 20 ms. The latency and amplitude of NMA-evoked responses were highly dependent on the position of the ionophoretic pipette; movements of the pipette by as little as 10-50 micron could markedly change the size of the response. Spatially separate hot spots for NMA and glutamate were not found. Depolarizations evoked by small to moderate ionophoretic currents of NMA were usually associated with an apparent rise in input resistance, as tested by the response to transmembrane current pulses. Ionophoresis of L-glutamate, or high NMA doses, however, usually caused a fall in input resistance. Both the depolarization and the conductance change evoked by NMA were highly voltage-dependent within the approximate range -50 to -80 mV; they could be increased by modest depolarization and reduced by hyperpolarization of the membrane. No reversal potential could be demonstrated in the hyperpolarizing direction. Rather, the NMA response approached zero asymptotically at sufficiently hyperpolarized membrane potentials. Subthreshold depolarizations and conductance changes elicited by NMA could be blocked by Co2+, Mn2+ and Cd2+, and reduced by D-600 and verapamil. These Ca2+ antagonists had little or no effect on resting membrane potential or input resistance, or on responses to L-glutamate. Ba2+ increased the amplitude of subthreshold NMA responses. Intracellular injection of Cs+ plus tetraethylammonium caused cells to fire large, prolonged (up to 15 s) Ca spikes, presumably because most K+ conductances were blocked. Under these conditions the effect of NMA was unchanged or enhanced. Raising [K+]o to 10.5 mM (from the normal 3.5 mM) caused a depolarization and fall in input resistance, but did not change the amplitude or voltage dependence of the NMA response. Reducing [Na+]o caused an initial increase, then usually a delayed decrease in the amplitude of the NMA response.(ABSTRACT TRUNCATED AT 400 WORDS)

Kynurenic acid inhibits synaptic and acidic amino acid-induced responses in the rat hippocampus and spinal cord

Kynurenic acid, a tryptophan metabolite, inhibits excitatory synaptic transmission in the rat hippocampal slice and the isolated immature rat spinal cord, but does not affect membrane potential or input resistance of hippocampal CA1 pyramidal cells. Kynurenic acid also antagonizes responses induced in the dentate gyrus by excitatory amino acids, particularly N-methyl-DL-aspartate and the endogenous excitant quinolinic acid. These results indicate that kynurenic acid antagonizes synaptic transmission probably by blocking postsynaptic transmitter receptors at putative amino acid mediated synapses.

Structure - function relationships for gamma-substituted glutamate analogues on dentate granule cells

We previously demonstrated in the Schaffer collateral-CA1 region of the hippocampus that bath-applied agonists could be distinguished from antagonists among a group of acidic amino acid analogues by extracellular recording techniques. Here we report the use of the extracellular signs of agonist activity for discerning agonists and antagonists among several gamma-substituted glutamate analogues tested in the perforant path. The two-pathway composition of the perforant path offers the advantage over CA1 in that pathway-specificity, a postulated characteristic of antagonists, may be tested. By extracellular recording, D- and L-homocysteic acid, L-serine-O-sulfate, and L-2-amino-4-(5-tetrazolyl)-butanoic acid [L-glutamate tetrazole] were identified as agonists, and all 4 analogues were more potent than L-glutamate for inhibiting synaptic field potentials. Two previously identified antagonists, L-2-amino-4-phosphonobutyric acid and L-O-phosphoserine, exhibited the pathway-specificity and inhibitory kinetics consistent with properties expected for antagonists; both compounds detected 3 perforant path components with the same rank in sensitivity, suggesting that they are acting on the same set of receptors.

Selective activation of synapses near the tip of drug-ejecting microelectrode, and effects of antagonists of excitatory amino acids in the hippocampus

A technique was developed to selectively facilitate transmission through synapses near the tip of a drug-ejecting micropipette by increasing Ca2+ concentration in a small space surrounding the tip. By means of this technique, we found that antagonists of excitatory amino acids, cis-2,3-piperidine dicarboxylic acid, gamma-D-glutamylglycine and glutamic acid diethylester, blocked excitation induced in CA3 neurons by glutamate and by mossy fiber stimulation in thin hippocampal sections of the guinea pig.

Antagonists of synaptic and amino acid excitation of neurones in the cat spinal cord

In the spinal cord of the anaesthetized cat microelectrophoretically administered (+/-)-cis-2,3-piperidine dicarboxylate (2,3-PDA), (+/-)-cis-2,5-piperidine dicarboxylate (2,5-PDA), gamma-D-glutamylglycine (gamma DGG), beta-D-aspartyl-beta-alanine (beta DAA), (+/-)-2-amino-4-phosphonobutyrate (2-APB), (+/-)-2-amino-5-phosphonovalerate (2-APV) and (+/-)-2-amino-7-phosphonoheptanoate (2-APH) were assessed as antagonists of chemical excitation of dorsal horn interneurones and Renshaw cells by N-methyl-D-aspartate (NMDA), L-aspartate, quisqualate (QUIS), kainate and L-glutamate, and of monosynaptic and polysynaptic excitation by impulses in primary afferent fibres of muscle and cutaneous origin. Whereas polysynaptic excitation of interneurones was readily and reversibly depressed by 2-APV, 2-APH, beta DAA, gamma DGG and 2,3-PDA, all of which also reduced excitation by NMDA (and L-aspartate) more than that by QUIS (and L-glutamate), no selective antagonism of monosynaptic excitation could be demonstrated. In particular, 2,3-PDA, which depressed excitation by kainate to a greater extent than that by either QUIS or NMDA, appeared to have no effect on monosynaptic excitation. The results support the involvement of L-aspartate as the transmitter of some spinal excitatory interneurones, but none of the antagonists tested were considered suitable for assessing the role of L-glutamate as the transmitter of some spinal primary afferent fibres.

A microperfusion chamber for brain slice pharmacology

The construction of a chamber for extracellular recording from submerged CNS tissue is described. Its operation is illustrated using hippocampal slices prepared from rat brains. The total volume of perfusing medium is less than 0.4 ml, and a drug solution can be uniformly distributed throughout this volume in less than 1 min. Drug-laden medium can also be rapidly replaced with fresh medium. The perfusing medium is continuously stirred with a jet of 95% O2-5% CO2, maintaining submerged slices viable for many hours. The system has exceptional advantages for investigating synaptic pharmacology of scarce and expensive drugs.

The antagonism of amino acid-induced excitations of rat hippocampal CA1 neurones in vitro

1. The effects of the ionophoretic application of a number of excitatory amino acids and antagonists to the dendrites of CA1 neurones of rat hippocampal slices maintained in vitro were examined. Cells were excited by N-methyl-DL-aspartate (NMA), kainate, quisqualate, L-aspartate and L-glutamate; NMA was unique in causing cells to fire in bursts of repetitive discharges in contrast to the sustained firing seen with the other compounds. 2. D-(-)-alpha-aminoadipate (DAA) and (+/-)-2-amino-5-phosphonovalerate (APV) were selective NMA antagonists, the latter appearing to be the more potent; in addition both compounds potentiated the responses to kainate and quisqualate. L-glutamate excitations were affected less by APV than were those of L-aspartate. The antagonist properties of APV appeared to reside with the D-(-)-isomer. 3. gamma-D-glutamylglycine (DGG) in low ionophoretic doses inhibited NMA-, kainate- and aspartate-induced cell firing but at higher doses the quisqualate and glutamate responses were also decreased. 4. Kainate and NMA responses were blocked by D-(-)-2-amino-4-phosphonobutyrate (D-APB) which also had some action against the excitatory effects of L-aspartate. L-APB had no antagonistic effects, but often produced potentiation of amino acid excitations or was itself an excitant. 5. The effects of NMA and those of kainate and quisqualate were blocked by (+/-)-cis-2,3-piperidine dicarboxylate (PDA), but this compound itself had a direct excitatory effect. L-glutamate diethylester (GDEE) did not show specific antagonism of any amino acid excitations. 6. DGG and APV did not affect ACh excitations and these selective antagonists should be of value in studying the involvement of the excitatory amino acids in synaptic transmission in the hippocampus. Because they are less potent and/or have complicating direct effects PDA, GDEE, D- and L-APB may be less useful in this regard.

Excitatory amino acids in synaptic transmission in the Schaffer collateral-commissural pathway of the rat hippocampus

1. The effects of excitatory amino acids and some antagonists applied by ionophoresis to stratum radiatum in the CA1 region of rat hippocampal slices were examined on the locally recorded field e.p.s.p. evoked by stimulation of the Schaffer collateral-commissural projection. 2. L-glutamate, L-aspartate and the more potent and selective excitatory amino acids quisqualate, kainate and N-methyl-DL-aspartate (NMA) depressed the e.p.s.p., presumably through depolarization and/or a change in membrane conductance. 3. The depression induced by kainate considerably outlasted the period of ejection whereas NMA depressions were rapidly reversible and were often followed by a potentiation of the e.p.s.p. In higher doses NMA also depressed the presynaptic fibre volley. The possible involvement of these effects in neurotoxicity and synaptic plasticity is raised. 4. The selective NMA antagonist, DL-2-amino-5-phosphonovalerate (APV) applied in doses which abolished responses to NMA, had no effect on the e.p.s.p. but prevented long term potentiation (l.t.p.) of synaptic transmission evoked by high frequency stimulation of the Schaffer collateral-commissural pathway. Other antagonists which had little or no effect on normal synaptic transmission included D-alpha-aminoadipate (DAA), the optical isomers of 2-amino-4-phosphonobutyrate (APB) and L-glutamate diethylester (GDEE). 5. In contrast, gamma-D-glutamylglycine (DGG), applied in amounts which affected quisqualate and kainate actions as well as those of NMA, was an effective synaptic antagonist whilst having no effect on the presynaptic fibre volley. 6. These results indicate that the synaptic receptor in the Schaffer collateral-commissural pathway may be of the kainate or quisqualate type. Although NMA receptors do not appear to be involved in normal synaptic transmission in this pathway they may play a role in synaptic plasticity. The interaction of L-glutamate and L-aspartate with these receptors is discussed.

Acidic amino acid antagonists of lateral perforant path synaptic transmission: agonist-antagonist interactions in the dentate gyrus

Acidic amino acid antagonists were tested for their ability to block depolarizations produced by excitatory amino acids in the outer molecular layer of the dentate gyrus in hippocampal slices. 2-Amino-4-phosphonobutyrate (APB) and serine-O-phosphate (SOP), potent synaptic blockers of the lateral perforant path input to this system, are moderately selective antagonists of kainate and N-methyl-DL-aspartate depolarizations, but not depolarizations produced by L-glutamate, quisqualate, or serine-O-sulfate. N-Methyl-DL-aspartate responses were potently blocked by 2-amino-5-phosphonovalerate, but this antagonist is less potent than APB or SOP against the synaptic response.

Response of Schaffer collateral-CA 1 pyramidal cell synapses of the hippocampus to analogues of acidic amino acids

Analogues of the putative excitatory transmitters aspartic acid and glutamic acid were tested for antagonism against stimulus-evoked activation of Schaffer collateral-CA 1 pyramidal cell synapses in slices of rat hippocampus. Responses to the analogues, applied via the superfusing medium, were extracellularly recorded. The compounds examined included D- and L-alpha-aminodicarboxylic acids, diaminodicarboxylic acids, phosphonate analogues of acidic amino acids, D- and L-gamma-glutamyl glycine, and the cis- and trans-isomers of piperidine 2,3-, and 2,4-dicarboxylic acid. Many of these compounds are known to be potent and selective antagonists for excitatory amino acids and a few excitatory pathways. In this hippocampal pathway most of these analogues showed relatively low and similar potency. The most potent antagonist uncontaminated with agonist activity was D-alpha-aminosuberate, with an apparent antagonist dissociation constant (Kd) of 3 mM. Only 5 of the analogues, 3 of the piperidine dicarboxylates, kainic acid, and L-alpha-aminopimelic acid, reduced the amplitude of the extracellularly recorded field potentials more than 30% at 0.5 mM. However, all of the others reduced the potential by more than 30% at 5 mM. Most analogues also evoked extracellular responses which can be attributed to depolarization of the pyramidal neurons. Agonist activity was particularly strong among the most potent analogues. These results contrast with the responses documented by others for the N-methyl-D-aspartate receptor of the dorsal-ventral root excitatory pathway of the spinal cord in which the higher homologues tested here were the most potent antagonists, and the D-isomers were more potent than the L-isomers. It also contrasts with the response of the perforant path synapses to granule cells of the dentate gyrus in which the portion derived from the lateral entorhinal cortex is sensitive to L-2-amino-4-phosphonobutyric acid. Thus the Schaffer-CA 1 pyramidal cell synaptic field utilizes a novel excitatory transmitter receptor which interacts detectably but only weakly with a variety of acidic amino acids with potent specific inhibitory action for receptors elsewhere in the central nervous system.