Excitatory Amino Acid-Evoked Release of γ-[3H]Aminobutyric Acid from Striatal Neurons in Primary Culture

The actions of excitatory amino acids on the release of previously incorporated gamma-[3H]aminobutyric acid ([3H]GABA) were examined in purified (greater than 93%) striatal neurons derived from the fetal mouse brain and differentiated in primary culture. Glutamate, KCl, and veratrine evoked a dose-dependent, saturable, and reversible release of [3H]GABA from striatal neurons. Glutamate actions were not reduced in the absence of calcium, and were insensitive to tetrodotoxin. The dose-response relationships of excitatory amino acids demonstrated the following rank order of potency: glutamate greater than aspartate = N-methyl-D-aspartate greater than kainate much greater than quisqualate. Kainate, however, was the most effective agonist, evoking an eightfold increase over baseline levels of [3H]GABA release. Aspartate- and N-methyl-D-aspartate-evoked release was abolished in the presence of either 2-aminophosphonovaleric acid or gamma-D-glutamylglycine. Release due to glutamate and kainate was partially or ineffectively attenuated by these agents. Glutamate-, aspartate-, and N-methyl-D-aspartate-evoked GABA releases were augmented when calcium was omitted from the bathing medium and reduced when sodium was replaced with choline or lithium. Kainate-evoked release was unaffected when calcium was omitted, virtually unchanged when choline replaced sodium, and markedly potentiated when lithium was substituted for sodium. These findings suggest that at least two distinct receptor systems for excitatory amino acids mediate the evoked release of [3H]GABA from striatal neurons in primary culture. These two systems, aspartate/N-methyl-D-aspartate- and kainate-preferring, are distinguishable on the basis of their pharmacological and ionic properties.     

Maitotoxin-Induced Release of γ-[3H]Aminobutyric Acid from Cultures of Striatal Neurons

The potent marine toxin, maitotoxin, induced the release of gamma-[3H]aminobutyric acid (GABA) from reaggregate cultures of striatal neurons in a dose-dependent manner. Maitotoxin-induced release occurred following a lag period of several minutes and was persistent. Release induced by 70 mM K+ on the other hand was immediate and transient in nature. Co2+ (3 mM) and Cd2+ (1 mM) inhibited maitotoxin-induced release of GABA as did removal of extracellular Ca2+. However, the organic calcium antagonists nisoldipine, nitrendipine, and D-600 at concentrations of 10(-6) M did not block maitotoxin-induced or 70 mM K+-induced release. High concentrations of D-600 (10(-4) M) partially blocked both maitotoxin- and 70 mM K+-induced release. The dihydropyridine calcium agonist BAY K8644 (10(-6) M) did not enhance maitotoxin-induced or 70 mM K+-induced release. Replacement of Na+ in the incubation medium with choline led to an increased basal output of GABA and an apparent inhibition of the effect of maitotoxin. These data are discussed with reference to the hypothesis that maitotoxin can directly activate voltage-sensitive calcium channels.     

Stimulation of Dopamine Release from Cultured Rat Mesencephalic Cells by Naturally Occurring Excitatory Amino Acids: Involvement of Both N-Methyl-D-Aspartate (NMDA) and Non-NMDA Receptor Subtypes

In rat mesencephalic cell cultures, L-glutamate at concentrations ranging from 100 microM to 1 mM stimulated release of [3H]dopamine that was attenuated by the non-N-methyl-D-aspartate (non-NMDA) receptor antagonist 6,7-dinitroquinoxalinedione, but not by the selective NMDA receptor antagonists (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801; 10 microM) and 3-(2-carboxypiperazine-4-yl)propyl-1-phosphonate (300 microM). Even at 1 mM glutamate, this release was Ca2+ dependent. These observations suggest that the release was mediated by a non-NMDA receptor. Only release stimulated by a lower concentration (10 microM) of glutamate was inhibited by MK-801 (10 microM), indicating that glutamate at this concentration activates the NMDA receptor. By contrast, L-aspartate at concentrations of 10 microM to 1 mM evoked [3H]dopamine release that was completely inhibited by MK-801 (10 microM) and was also Ca2+ dependent (tested at 1 and 10 mM aspartate). Thus, effects of aspartate involved activation of the NMDA receptor. Sulfur-containing amino acids (L-homocysteate, L-homocysteine sulfinate, L-cysteate, L-cysteine sulfinate) also evoked [3H]dopamine release. Release evoked by submillimolar concentrations of these amino acids was attenuated by MK-801 (10 microM), indicating involvement of the NMDA receptor.(ABSTRACT TRUNCATED AT 250 WORDS)     

Release of Previously Incorporated γ-[3H]Aminobutyric Acid in Rabbit Caudate Nucleus Slices

The release of gamma-aminobutyric acid (GABA) was studied in slices of the head of the rabbit caudate nucleus. The slices were preincubated with [3H]GABA and then superfused. Aminooxyacetic acid was present throughout. Both the tritium in the slices and that in the superfusate consisted practically entirely of [3H]GABA. Stimulation for 2 min by electrical field pulses of 3 ms width and 9 V/cm voltage drop (36 mA current strength) at 5 or 20 Hz elicited an overflow of [3H]GABA that amounted to 0.23 or 0.47% of the tritium content of the tissue, respectively, and was diminished by 85% in the presence of tetrodotoxin. At higher current strength, less of the stimulation-evoked overflow was tetrodotoxin-sensitive. cis-1,3-Aminocyclohexane carboxylic acid diminished the uptake of [3H]GABA into the tissue but did not change the percentage released by electrical stimulation. Ca2+ withdrawal greatly accelerated basal [3H]GABA efflux and almost abolished the response to stimulation. Nipecotic acid 10-1,000 microM enhanced both the basal and (up to eightfold) the stimulation-evoked overflow. The method described allows us to elicit electrically a quasiphysiological, i.e., Ca2+-dependent and tetrodotoxin-sensitive, neuronal release of [3H]GABA. Nipecotic acid diverts released [3H]GABA from reuptake to overflow.     

[3H]Norepinephrine Release from Hippocampal Slices Is an In Vitro Biochemical Tool for Investigating the Pharmacological Properties of Excitatory Amino Acid Receptors

[3H]Norepinephrine ([3H]NE) efflux from preloaded rat hippocampal slices was increased in a dose-dependent manner by excitatory amino acids, with the following order of potencies: N-methyl-D-aspartate (NMDA) greater than kainic acid (KA) greater than L-glutamate greater than or equal to D,L-homocysteate greater than L-aspartate greater than quinolinic acid greater than quisqualic acid. The effect of the excitatory amino acids was blocked by physiological concentrations of Mg2+, with the exception of KA. D,L-2-Amino-7-phosphonoheptanoic acid dose-dependently inhibited the NMDA effect (ID50 = 69 microM), whereas at 1 mM it was ineffective versus KA. The release of [3H]-NE induced by quinolinic acid was blocked by 0.1 mM D,L-2-amino-7-phosphonohepatanoic acid. gamma-D-Glutamylglycine dose-dependently inhibited the KA effect with an ID50 of 1.15 mM. Tetrodotoxin (2 microM) reduced by 40 and 20% the NMDA and KA effects, respectively. The data indicate that [3H]NE release from hippocampal slices can be used as a biochemical marker for pharmacological investigations of excitatory amino acid receptors and their putative agonists and antagonists.     

Glutamate Receptor Subtypes in Cultured Cerebellar Neurons: Modulation of Glutamate and γ-Aminobutyric Acid Release

Using cerebellar, neuron-enriched primary cultures, we have studied the glutamate receptor subtypes coupled to neurotransmitter amino acid release. Acute exposure of the cultures to micromolar concentrations of kainate and quisqualate stimulated D-[3H]aspartate release, whereas N-methyl-D-aspartate, as well as dihydrokainic acid, were ineffective. The effect of kainic acid was concentration dependent in the concentration range of 20-100 microM. Quisqualic acid was effective at lower concentrations, with maximal releasing activity at about 50 microM. Kainate and dihydrokainate (20-100 microM) inhibited the initial rate of D-[3H]aspartate uptake into cultured granule cells, whereas quisqualate and N-methyl-DL-aspartate were ineffective. D-[3H]Aspartate uptake into confluent cerebellar astrocyte cultures was not affected by kainic acid. The stimulatory effect of kainic acid on D-[3H]aspartate release was Na+ independent, and partly Ca2+ dependent; the effect of quisqualate was Na+ and Ca2+ independent. Kynurenic acid (50-200 microM) and, to a lesser extent, 2,3-cis-piperidine dicarboxylic acid (100-200 microM) antagonized the stimulatory effect of kainate but not that of quisqualate. Kainic and quisqualic acid (20-100 microM) also stimulated gamma-[3H]-aminobutyric acid release from cerebellar cultures, and kynurenic acid antagonized the effect of kainate but not that of quisqualate. In conclusion, kainic acid and quisqualic acid appear to activate two different excitatory amino acid receptor subtypes, both coupled to neurotransmitter amino acid release. Moreover, kainate inhibits D-[3H]aspartate neuronal uptake by interfering with the acidic amino acid high-affinity transport system.     

Effects of Creatine on Synthesis and Release of γ-[3H]Aminobutyric Acid

Creatine has been used previously to alter the energy balance of neurons in brain slices. In the present experiments, it was found to reduce the accumulation of gamma-[3H]aminobutyric acid ([3H]GABA) as synthesized from [3H]glutamine or [3H]glutamic acid in slices of rat neostriatum. The lowest effective concentration was 5 mM. Creatine (25 mM) was also effective when the degrading enzyme of GABA, i.e., GABA-alpha-oxoglutarate transaminase, was blocked by gabaculine. Creatine (25 mM) did not inhibit the uptake and subsequent accumulation of [3H]GABA. Thus, indirect evidence was obtained that creatine decreased the activity of the synthesizing enzyme of GABA, i.e., glutamate decarboxylase. When the direct effect of creatine (25 mM) on glutamate decarboxylase was studied in vitro, the agent indeed decreased the activity of the enzyme. Creatine (25 mM) also diminished the release of [3H]GABA (expressed as dpm/mg wet weight) from rat neostriatal slices, probably by reducing its synthesis and thus its readily releasable pool. These data are of importance for studies with creatine in complex neuronal systems, because they show that the agent changes not only neuronal energy balance, but also synthesis and release of the ubiquitous transmitter GABA.     

Release of γ-[3H]Aminobutyric Acid from Rat Olfactory Bulb and Substantia Nigra: Differential Modulation by Glutamic Acid

We have studied the glutamate modulation of gamma-[3H]aminobutyric acid ([3H]GABA) release from GABAergic dendrites of the external plexiform layer of the olfactory bulb and from GABAergic axons of the substantia nigra. In the olfactory bulb, [3H]GABA release was induced by high K+ and kainate, and not by aspartate and glutamate alone. However, when the tissue was conditioned by a previous K+ depolarization, glutamate and aspartate caused [3H]GABA release. The effect of glutamate was significantly enhanced when the GABA uptake mechanism was blocked by nipecotic acid. N-Methyl-D-aspartate and quisqualate did not cause [3H]GABA release under the same conditions. The acidic amino acid receptor antagonist 2-amino-4-phosphonobutyric acid and the N-methyl-D-aspartate receptor antagonist 2-amino-5-phosphonovaleric acid significantly inhibited the K+-glutamate- and the kainate-induced [3H]GABA release. Mg2+ (5 mM), which blocks the N-methyl-D-aspartate receptors, significantly inhibited the K+-glutamate-induced but not the kainic acid-induced [3H]GABA release. The K+-glutamate-stimulated release, but not the K+-stimulated [3H]GABA release, was strongly inhibited by Na+-free solutions or by 300 nM tetrodotoxin. Apparently the glutamate-induced release of [3H]GABA occurs through an interneuron because it is dependent on the presence of nerve conduction. In the substantia nigra no [3H]GABA release was elicited by any of the glutamate agonists tested. The present results clearly differentiate between the effects of glutamate on the release of [3H]GABA from the substantia nigra and from the olfactory bulb.(ABSTRACT TRUNCATED AT 250 WORDS)     

Regional Selectivity of a γ-Aminobutyric Acid-Induced [3H]Acetylcholine Release Sensitive to Inhibitors of γ-Aminobutyric Acid Uptake

The effects of gamma-aminobutyric acid (GABA) on the release of [3H]acetylcholine ([3H]ACh) were studied in synaptosomes prepared from rat hippocampus, cerebral cortex, hypothalamus, and striatum and prelabelled with [3H]choline. When synaptosomes were exposed in superfusion to exogenous GABA (0.01-0.3 mM) the basal release of newly synthesized [3H]ACh was increased in a concentration-dependent way in hippocampus, cortex, and hypothalamus nerve endings. In contrast, the release of [3H]ACh was not significantly affected by GABA in striatal synaptosomes. The effect of GABA was not antagonized significantly by bicuculline or picrotoxin. Muscimol caused only a slight not significant increase of [3H]ACh release when tested at 0.3 mM whereas, at this concentration, (-)-baclofen was totally inactive. The GABA-induced release of [3H]ACh was counteracted by SKF 89976A, SKF 100561, and SKF 100330A, three strong and selective GABA uptake inhibitors. The data suggest that, in selective areas of the rat brain, GABA causes release of [3H]ACh following penetration into cholinergic nerve terminals through a GABA transport system.     

Release of γ-Amino[3H]butyric Acid from Cultured Amacrine-Like Neurons Mediated by Different Excitatory Amino Acid Receptors

The release of preaccumulated gamma-amino[3H]butyric acid ([3H]GABA) from putative GABAergic amacrine cells was studied in neuronal monolayer cultures made from embryonic chick retina. Release was specifically stimulated by excitatory amino acid agonists. N-Methyl-D-aspartate (NMDA; EC50, 19.1 +/- 5.0 microM), kainic acid (EC50, 15.6 +/- 2.3 microM), and the presumptive endogenous ligand glutamate (EC50, 3.6 +/- 0.5 microM) showed the same efficacy. Quisqualic acid, although the most potent agonist (EC50, 0.56 +/- 0.12 microM), was only half as efficacious. The time course of [3H]GABA release and autoradiographic visualization of responsive GABA-accumulating cells suggest that approximately 50% of the [3H]GABA-accumulating cells possess no or very low responsiveness to quisqualic acid. Depolarization (56 mM KCl)-induced release was fivefold lower than the maximal effect elicited by excitatory amino acids. Release of [3H]GABA and of endogenous GABA was entirely independent of extracellular Ca2+ but was completely abolished after replacement of Na+ by choline or Li+. The effects of NMDA and low concentrations of glutamate (up to 10 microM) were blocked by 2-amino-5-phosphonovaleric acid, by MK 801, and (in a voltage-dependent manner) by Mg2+. The reduction of NMDA responses by kynurenic acid was reversed by D-serine, and quisqualic acid competitively inhibited kainic acid-evoked release. Our results show that the cultured [3H]GABA-accumulating neurons, which probably represent the in vitro counterparts of GABAergic amacrine cells, express at least two types of excitatory amino acid receptors (of the NMDA and non-NMDA type), both of which can mediate a Ca2(+)-independent but Na2(+)-dependent release of GABA.     

Presynaptic Mechanisms Underlying the γ-Aminobutyric Acid-Evoked Receptor-Independent Release of [3H]Norepinephrine in Rat Hippocampus

The effects of gamma-aminobutyric acid (GABA) on the spontaneous efflux of [3H]norepinephrine ([3H]NE) were studied in synaptosomes prepared from rat hippocampus and prelabelled with [3H]NE. It had been observed previously that, when synaptosomes were exposed in superfusion to GABA, the basal release of the tritiated catecholamine was enhanced, apparently with no involvement of the known GABA receptors. The mechanisms underlying this effect have now been investigated. The potency of GABA as a releaser of [3H]NE was decreased by lowering the Na+ content of the superfusion medium, and its effect disappeared at 23 mM Na+. The GABA-induced [3H]NE release was counteracted by the GABA uptake inhibitor N-(4,4-diphenyl-3-butenyl)nipecotic acid (SKF 89976A), but it was unaffected by the NE uptake blockers desmethylimipramine and nisoxetine. The GABA-induced release of [3H]NE was Ca2+-dependent and tetrodotoxin-sensitive. The data support the hypothesis that GABA provoked [3H]NE release by a novel mechanism which involves penetration into the noradrenergic nerve terminals through a GABA carrier located on the NE terminals themselves. This uptake process might be electrogenic and provoke depolarization of the nerve terminals, causing an exocytotic release of [3H]NE.     

A Search for Receptors Modulating the Release of γ-[3H]Aminobutyric Acid in Rabbit Caudate Nucleus Slices

Various putative striatal transmitters and related compounds were studied for their effects on the release of gamma-aminobutyric acid (GABA) from slices of the head of the rabbit caudate nucleus. The slices were preincubated with [3H]GABA and then superfused and stimulated electrically at 5 or 20 Hz. Aminooxyacetic acid was present throughout. The main changes observed were the following. The basal and, less consistently, the electrically evoked overflow of [3H]GABA were enhanced by 3,4-dihydroxyphenylethylamine (dopamine), an effect not blocked by cis-flupentixol or domperidone and not mimicked by apomorphine and D1-selective agonists. The electrically evoked overflow was diminished by 5-hydroxytryptamine (serotonin); the inhibition was prevented by methiothepin. The basal but not the electrically evoked overflow was enhanced by carbachol; acetylcholine and nicotine also accelerated the basal outflow whereas oxotremorine caused no consistent change; the effect of carbachol and acetylcholine were blocked by hexamethonium but not by atropine or by tetrodotoxin. These findings indicate that the GABA neurons in the caudate nucleus may be stimulated by dopamine, although the receptor type involved remains unclear; inhibited by serotonin; and stimulated by acetylcholine acting via a nicotine receptor. However, all drug effects observed were relatively small. No evidence was obtained for autoreceptors, alpha 2-adrenoceptors or receptors for opioids, adenosine or substance P at the GABA neurons.     

γ-Aminobutyric AcidB Receptor Activation Modifies Agonist Binding to β-Adrenergic Receptors in Rat Brain Cerebral Cortex

The interaction of isoproterenol with beta-adrenergic receptor (beta AR) binding sites was measured in membranes prepared from rat brain cerebral cortical slices previously incubated in the presence or absence of gamma-aminobutyric acid (GABA) receptor agonists. Both GABA and baclofen, but not isoguvacine, altered beta AR agonist binding by increasing the affinity of both the low- and high-affinity binding sites and by increasing the proportion of low-affinity receptors. The response to baclofen was stereoselective, and the effect of GABA was not inhibited by bicuculline. The results suggest that GABAB, but not GABAA, receptor activation modifies the coupling between beta AR and stimulatory guanine nucleotide-binding protein, which may in part explain the ability of baclofen to augment isoproterenol-stimulated cyclic AMP accumulation in brain slices.     

Inositol Hexakisphosphate (Phytic Acid) Enhances Ca2+Influx and D-[3H]Aspartate Release in Cultured Cerebellar Neurons

Inositol hexakisphosphate (InsP6) increased 45Ca2+ uptake in cultured cerebellar granule cells. This increase was concentration dependent (EC50 = 20 microM), exhibited slow kinetics, and was present after 5 days of cell maturation in culture. InsP6 also enhanced D-[3H]aspartate release in cerebellar granule cells at 11-12 days in vitro. Stimulation of 45Ca2+ uptake was also produced by inositol pentakisphosphate but not by inositol 1,3,4,5-tetrakisphosphate. The increase in 45Ca2+ influx induced by InsP6 was independent of extracellular Na+ and was only partially reduced by the organic calcium channel blocker nifedipine. The intrinsic action of InsP6 was not affected by competitive or noncompetitive glutamate receptor antagonists. In addition, stimulations of 45Ca2+ uptake by InsP6 and glutamate were additive. These data provide evidence that InsP6 directly activates a specific population of neurons in the CNS.     

Dopamine Inhibits Calcium-Independent γ-[3H]Aminobutyric Acid Release Induced by Kainate and High K+ in the Fish Retina

Kainic acid (KA) at micromolar concentrations stimulated the release of gamma-[3H]aminobutyric acid [( 3H]GABA) from a particulate fraction of the carp (Cyprinus carpio) retina. The KA action was dose-dependent but Ca2+-independent. A similar response was elicited by another glutamate receptor agonist, quisqualic acid, and high K+, but not by an aspartate agonist, N-methyl-D-aspartic acid. The stimulatory action of KA on the [3H]GABA release was selectively blocked by the KA blockers gamma-D-glutamylglycine and cis-2,3-piperidine dicarboxylic acid. Dopamine (DA), which is contained in DA interplexiform cells in the carp retina, inhibited the [3H]GABA release induced by KA and high K+ in a dose-dependent manner. 5-Hydroxytryptamine and two well-known GABA antagonists, bicuculline (Bic) and picrotoxin (Pic), also mimicked the DA effect on the GABA release at a comparable concentration. This inhibitory effect of DA as well as Bic and Pic on the [3H]GABA release evoked by KA was clearly antagonized by a DA blocker, haloperidol. The action of these agents (KA, DA, GABA antagonist) belonging to three different receptor categories on the GABAergic neurons (possibly external horizontal cells; H1 cells) is discussed in relation to other electrophysiological studies on the lateral spread of S-potentials between H1 cells.     

α-[3H]Amino-3-Hydroxy-5-Methylisoxazole-4-Propionic Acid Binding to Human Cerebral Cortical Membranes: Minimal Changes in Postmortem Brains of Chronic Schizophrenics

The binding of alpha-[3H]amino-3-hydroxy-5-methylisoxazole-4-propionic acid ([3H]AMPA), a selective ligand for the ion channel-linked quisqualate receptor, was evaluated in Triton X-100-treated membranes of human cerebral cortex. The presence of chaotropic ions produced divergent effects on specific [3H]AMPA binding: A twofold increase in the binding was observed with thiocyanide at 100 mM, although iodide (100 mM) and perchlorate (100 mM) reduced the binding. Chemical modifications of the sulfhydryl group with p-chloromercuriphenylsulfonic acid (PCMBS) produced threefold increases in specific [3H]-AMPA binding in the absence of KSCN as well as in the presence of KSCN. Treatment with dithiothreitol restored the enhanced specific [3H]AMPA binding by PCMBS to the basal level. Although specific [3H]AMPA binding in the absence of KSCN showed a single site (KD = 220 nM, Bmax = 235 fmol/mg of protein), curvilinear Scatchard plots of specific [3H]AMPA binding in the presence of 100 mM KSCN can be resolved into two binding sites with the following parameters: KD1 = 5.82 nM, Bmax1 = 247 fmol/mg of protein; KD2 = 214 nM, Bmax2 = 424 fmol/mg of protein. Quisqualate and AMPA were the most potent inhibitors of the [3H]AMPA binding in the presence of KSCN. Potent inhibitors of the binding included beta-N-oxalylamino-L-alanine (L-BOAA), cysteine-S-sulfate, L-glutamate, 6-cyano-7-nitroquinoxaline-2,3-dione, and 6,7-dinitroquinoxaline-2,3-dione. Kainate, L-homocysteine sulfinic acid, and L-homocysteic acid were active with an IC50 value of a micromolar concentration, whereas L-cysteic acid and L-cysteine sulfinic acid were weakly active.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of Two Convulsant β-Carboline Derivatives, DMCM and β-CCM, on Regional Neurotransmitter Amino Acid Levels and on In Vitro D-[3H]Aspartate Release in Rodents

Clonic seizures were induced in Swiss or DBA/2 mice by methyl-6-7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), 0.048 mmol/kg i.p., or by methyl-beta-carboline-3-carboxylate (beta-CCM), 0.044 mmol/kg i.p. Measurement of regional brain (cortex, hippocampus, striatum, and cerebellum) amino acid levels after 15 min of seizure activity showed increases in gamma-aminobutyric acid (GABA) (in all regions after beta-CCM, and in cortex and hippocampus after DMCM), and an increase in glycine in the striatum after beta-CCM. Aspartate levels fell (in cortex and hippocampus) after DMCM, but were unchanged in all regions after beta-CCM. Glutamate levels fell in cortex after beta-CCM and in striatum after DMCM. Pretreatment with the excitatory amino acid receptor antagonist, 2-amino-7-phosphonoheptanoic acid, 0.5 mmol/kg i.p., 45 min prior to the beta-carboline, significantly increased the ED50 for DMCM-induced clonic seizures (4.68 mumol/kg vs. 9.39 mumol/kg). Similar pretreatment did not significantly alter the ED50 for beta-CCM (4.22 mumol/kg vs. 6.6 mumol/kg). Pretreatment with 2-amino-7-phosphonoheptanoic acid, 1.0 mmol/kg, blocked the increase in GABA content produced by DMCM but not the fall in cortical aspartate content. Potassium-induced release of preloaded D-[3H]aspartate from rat cortical or hippocampal minislices was enhanced in the presence of DMCM (100 microM). In contrast, stimulated release of D-[3H]aspartate (from cortex or hippocampus) was not altered in the presence of beta-CCM (100 microM). Although DMCM and beta-CCM are both considered to induce convulsion by acting at the GABA--benzodiazepine receptor complex, the convulsions differ in several pharmacological and biochemical respects. It is suggested that enhanced release of excitatory amino acid neurotransmitters plays a more important role in seizures induced by DMCM.