RELEASE OF GAMMA-AMINO[H-3]BUTYRIC ACID FROM CULTURED AMACRINE-LIKE NEURONS MEDIATED BY DIFFERENT EXCITATORY AMINO-ACID RECEPTORS

The release of preaccumulated gamma-amino[H-3]butyric acid ([H-3]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-mu-M), kainic acid (EC50, 15.6 +/- 2.3-mu-M), and the presumptive endogenous ligand glutamate (EC50, 3.6 +/- 0.5-mu-M) showed the same efficacy. Quisqualic acid, although the most potent agonist (EC50, 0.56 +/- 0.12-mu-M), was only half as efficacious. The time course of [H-3]GABA release and autoradiographic visualization of responsive GABA-accumulating cells suggest that approximately 50% of the [H-3]GABA-accumulating cells possess no or very low responsiveness to quisqualic acid. Depolarization (56 mM KCI)-induced release was fivefold lower than the maximal effect elicited by excitatory amino acids. Release of [H-3]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-mu-M) 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 [H-3]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 Na+-dependent release of GABA.