Study of the mechanism of release of [3H]GABA from a teleost retina in vitro.
gamma-Aminobutyric acid (GABA) is thought to be a neurotransmitter in the vetebrate retina. We studied the voltage and Ca2+ dependency of the process of release of [3H]GABA from the retina of the teleost Eugerres plumieri, using a microsuperfusion technique. Two depolarizing agents, veratridine and high potassium, produced a concentration-dependent release of [3H]GABA. The veratridine effect was inhibited in Na+-free solution, but was not affected by 1 microM tetrodotoxin. A substantial inhibition (about 75%) of the veratridine- and potassium-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 microM-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. Autoradiographic 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 Müller cells. beta-Alanine and nipecotic acid were used as two 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. On the other hand, nipecotic acid produced a strong heteroexchange with [3H]GABA and lacked the capacity to induce the veratridine-stimulated release of [3H]GABA. These results suggest a voltage- and Ca2+-dependent neuronal release of [3H]GABA from retina.
D000409:Alanine / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000494:pharmacology; D002121:Calcium Channel Blockers / Q000494:pharmacology; D005399:Fishes / Q000378:metabolism*; D009557:Nipecotic Acids / Q000494:pharmacology; D011188:Potassium / Q000494:pharmacology; D011392:Proline / Q000031:analogs & derivatives*; D012160:Retina / Q000187:drug effects / Q000378:metabolism*; D012964:Sodium / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology; D014701:Veratridine / Q000494:pharmacology; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
D002121:Calcium Channel Blockers; D009557:Nipecotic Acids; C030278:nipecotic acid; D013779:Tetrodotoxin; D005680:gamma-Aminobutyric Acid; D014701:Veratridine; D011392:Proline; D012964:Sodium; C010983:homoproline; D000409:Alanine; D011188:Potassium; D002118:Calcium