Stimulation of GABA release from retinal horizontal cells by potassium and acidic amino acid agonists

Abstract

The release of [ 3H]GABA from horizontal cells of goldfish retina was studied by biochemical analysis of perfused isolated retina. Retinas were incubated for 15 min in 0.72 μM[ 3H]GABA, rinsed for 30 min and then perfused with 1 min pulses of increasing concentrations of K + and acidic amino acid agonists under a variety of conditions. Radioactivity in the perfusate was determined by liquid scintillation spectroscopy. The main findings are: (1) virtually all of the [ 3H]GABA released by l-glutamate ( l-Glu) and l-aspartate ( l-Asp) and 50% of the K +-evoked release, is calcium independent; (2) K +-evoked [ 3H]GABA release is only 10% of that released by l-Glu; (3) threshold [ 3H]GABA release occurs with 320 μM l-Glu, 1175 μM l-Asp, 4μM quisqualic acid (QA), 4μM kainic acid (KA) and 53 μM N-methyl- dl-aspartate (NMDLA); (4) the quisqualate antagonist glutamic acid diethyl ester (GDEE), has no specific inhibitory action on any of the agonists, whereas d-α-aminoadipic acid (DαAA), an NMDA antagonist, potently inhibits the action of NMDLA and l-Asp; (5) the presence of Mg 2+, even at 1 mM, totally inhibits NMDLA and also inhibits the action of l-Glu and l-Asp below 1 mM; (6) d-Asp potentiates the action of l-Glu by 0.6–0.8 log units and completely inhibits the action of l-Asp; (7) l-Asp at a ratio of 3:1 potentiates the effect of l-Glu. From these and other results one concludes that: (a) [ 3H]GABA release from H1 cells is calcium independent and depends on factors other than passive depolarization, probably sodium; (2) the likely transmitter of red cones is l-Glu acting on quisqualate or kainate receptors, and (3) l-Asp acts predominantly on NMDA receptors and may provide a modulatory role in the outer retina by potentiating the action of l-Glu.