Using a new in vitro superfusion device, the release of preloaded [H-3]GABA was examined in microdiscs of tissues taken from sagittal slices in matrix-enriched areas of the rat striatum. Potassium (9 mM, 15 mM) stimulated the release of [H-3]GABA in a concentration- and calcium-dependent manner and the veratridine (1 muM)-evoked release of [H-3]GABA was completely abolished in the presence of tetrodotoxin (1 muM). The selective glutamatergic agonist alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (1 mM) enhanced the potassium-evoked release of [H-3]GABA as well as the basal outflow of [H-3]GABA. This latter effect was found to be calcium-dependent, partially diminished by tetrodotoxin (1 muM), completely blocked by 6,7-dinitro-quinoxaline-2,3-dione (0.1 mM), which is generally used as an antagonist of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors. but not affected by (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK801, 10 muM), a specific antagonist of N-methyl-D-aspartate receptors. Similarly, N-methyl-D-aspartate (1 mM) enhanced both the potassium (9 mM) and the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (1 mM)-evoked release of [H-3]GABA but when used alone, due to the presence of magnesium in the superfusion medium, was ineffective on the basal efflux of [H-3]GABA. A stimulatory effect of N-methyl-D-aspartate (1 mM) on the basal outflow of [H-3]GABA was observed, however, when magnesium was omitted from the superfusion medium. The stimulatory effect of N-methyl-D-aspartate (1 mM) observed in the presence of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate was not potentiated by glycine (1 muM, in the presence of strychnine 1 muM) and the N-methyl-D-aspartate-evoked response seen in the absence of magnesium was not enhanced by D-serine (1 mM), suggesting that endogenous glycine is already actin on N-methyl-D-aspartate receptors. In fact, in the absence of magnesium, 7-chloro-kynurenate (1 mM) completely abolished the stimulatory effect of N-methyl-D-aspartate on the release of [H-3]GABA confirming that under our conditions, the glycine site of the N-methyl-D-aspartate receptor is saturated. N-methyl-D-aspartate-evoked responses were all blocked by MK801 (10 muM). Finally, the N-methyl-D-aspartate-evoked response seen in the absence of magnesium was markedly reduced in the presence of tetrodotoxin (1 muM). While potassium (9 mM) markedly stimulated the release of [H-3]GABA from purified synaptosomes from the rat striatum, neither alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate alone nor N-methyl-D-aspartate in the presence of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate or in the absence of magnesium enhanced the release of [H-3]GABA. A slight but not significant stimulatory effect was observed with N-methyl-D-aspartate and D-serine using a magnesium-free superfusion medium. Altogether, these results strongly suggest that alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate and N-methyl-D-aspartate receptors are present on dendrites and/or cell bodies of efferent GABAergic neurons but not on their nerve terminals in matrix-enriched areas of the rat striatum.
