GABA and benzodiazepine-induced modification of [14C]L-glutamic acid release from rat spinal cord slices

Flumazenil prevents diazepam-elicited anticonvulsant action and concomitant attenuation of glutamate overflow

Systemic administration of diazepam (5 mg/kg, i.p.) produced a prompt anticonvulsant effect in pilocarpine-induced seizures in freely moving rats. The anticonvulsant effect was associated with significant attenuation of pilocarpine-evoked increases in extracellular hippocampal glutamate levels to below the baseline levels. The purpose of the present microdialysis study, therefore, was to investigate if the effect of diazepam on glutamate release was mediated at the level of the benzodiazepine gamma-aminobutyric acid(A) (GABA(A)) receptor complex to preclude any non-GABAergic mechanisms. Systemic administration of the specific benzodiazepine-receptor antagonist flumazenil (10 mg/kg, i.p. )-elicited complete reversal of diazepam-evoked anticonvulsant action and concomitant attenuation of extracellular glutamate efflux below the baseline levels. This provides evidence that under the given experimental conditions, diazepam-evoked alterations in glutamate overflow associated with the anticonvulsant action were indeed mediated at the level of benzodiazepine-GABA(A) receptor complex, possibly involving the modulation of both pre- and post-synaptic sites of the receptor complex.

Efflux of putative transmitters from superfused rat brain slices induced by low chloride ion concentrations

Slices of rat cerebral cortex, preloaded with [14C]gamma-aminobutyric acid (GABA) and either [3H]5-hydroxytryptamine (5-HT) or [3H]noradrenaline, were superfused with media in which varying concentrations of Cl- had been replaced with other monovalent anions. Rapid reduction of [Cl-], by superfusion with media containing instead the impermeant anions propionate, isethionate, gluconate, or methyl sulphate, caused increases in the efflux of tritiated biogenic amines, but the increase in that of [14C]-GABA was not significant. The increased efflux of [3H]5-HT evoked by superfusion with low Cl- levels when propionate was the replacement anion, was transient and was linearly related to the log[Cl-]-1. It was not affected by removal of Ca2+ or by addition of 10 mM Mg2+ and was delayed but not abolished by tetrodotoxin. The low Cl(-)-evoked efflux of [3H]5-HT was not affected by pretreatment with neuronal reuptake blockers but was inhibited by picrotoxin, strychnine, and 4-acetamido-4-isothiocyanostilbene-2,2-disulphonic acid and was enhanced by glycine. Muscimol and GABA were without effect. These observations are taken to indicate that the efflux of biogenic amines is brought about by terminal depolarisation due to outward movement of Cl- in low chloride-containing media. They are of relevance to other physiological and pharmacological studies in which anion concentrations are manipulated and suggest that the anion-evoked release phenomenon may provide a model for the analysis of Cl(-)-dependent mechanisms in nerve terminals.