l-Glutamate evoked release of GABA from cultured avian retina cells does not require glutamate receptor activation

γ-Aminobutyric acid (GABA) and l-glutamate are the major inhibitory and excitatory transmitters in the central nervous system. Recent evidence has indicated that l-glutamate may stimulate GABA release by a novel exchange mechanism (Nascimento and De Mello, J. Neurochem., 1985, 45: 1820-1827). Here we provide strong support for this hypothesis by showing that the l-glutamate-evoked release of [³H]GABA from cultured avian retina cells is not dependent on the activation of excitatory amino acid receptors. Retina cells were found to incorporate [³H]GABA into a pool that was released when cultures were treated with l-glutamate (100 μM). This release was unaffected when calcium ions were removed, but was prevented when NaCl was replaced by LiCl. d-Aspartate, which in tracer experiments was shown to be taken into cells by the same carrier as l-glutamate, was also able to evoke release of [³H]GABA, with the same requirement for NaCl. In addition, l-glutamate and d-aspartate uptake by retina cells was inhibited in more then 80% when the uptake was measured in the presence of LiCl. As opposed to GABA, the release of acetylcholine (ACh) promoted by l-glutamate showed characteristics of classical mechanisms of neurotransmitter release. Glutamate-induced efflux of ACh was Ca²⁺-dependent and was not affected when NaCl was replaced by LiCl. Also, d-aspartate was ineffective in eliciting the release of ACh. Even at high concentrations, antagonists of excitatory amino acid receptors were unable to diminish the glutamate-evoked release of [³H]GABA. The antagonists tested were: 2-amino-5-phosphonovalerate and d-α-aminoadipate, which are relatively selective for N-methyl-d-aspartate receptors; l-glutamate diethyl ester, which selectively blocks quisqualate sites, and cis-2,3-piperidine dicarboxylate, w which non-selectively antagonizes all types of excitatory amino acid receptors. The data show that excitatory amino acid receptors are not involved in the Na-dependent l-glutamate-evoked release of [³H]GABA and support the concept that a glutamate-GABA exchange mechanism operates in the central nervous system. Since glutamate and GABA exert antagonistic effects on the electrophysiology of nerve cells, this mechanism might be important in regulating neuronal excitability.