Study of the Mechanism of Release of [3H]GABA from a Teleost Retina In Vitro

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Abstract
GABA is thought to be a neurotransmitter in the vertebrate retina. The voltage and Ca2+ dependency of the process of release of [3H]GABA was studied in the retina of the teleost Eugerres plumieri, using a microsuperfusion technique. Two depolarizing agents, veratridine and high K+, produced a concentration-dependent release of [3H]GABA. The veratridine effect was inhibited in Na+-free solution, but was not affected by 1 .mu.M tetrodotoxin. A substantial inhibition (.apprx. 75%) of the veratridine- and K+-stimulated release of [3H]GABA occurred in Ca2+-free medium. Inhibitors of the Ca2+ channel, such as Mg2+ (20 mM), La3+ (0.1 mM), and methoxy-verapamil (4 .mu.M-0.4 mM), inhibited the veratridine- and K+-stimulated release. However, Co2+ and Cd2+ caused a potentiation and no change of the K+- and veratridine-stimulated release, respectively. This release process is apparently specific, since both depolarizing agents were unable to release [3H]methionine, a nontransmitter amino acid, under the same experimental conditions. Auotradiographic studies with [3H]GABA, using the same incubation conditions as for the release experiments, showed a high density of silver grains over the horizontal cells with almost no accumulation by amacrine cells and Mueller cells. .beta.-Alanine and nipecotic acid were used as 2 relative specific inhibitors of the glial and neuronal GABA uptake mechanisms, respectively. Only a small heteroexchange with [3H]GABA was found with .beta.-alanine, and no inhibition of the subsequent veratridine-stimulated release. Nipecotic acid produced a strong heteroexchange with [3H]GABA and lacked the capacity to induce the veratridine-stimulated release of [3H]GABA. There is a voltage- and Ca2+-dependent neuronal release of [3H]GABA from retina.