Dopamine Inhibits Calcium‐Independent γ‐[3H]Aminobutyric Acid Release Induced by Kainate and High K+ in the Fish Retina

Abstract

Kainic acid (KA) at micromolar concentrations stimulated the release of γ‐[³H]aminobutyric acid ([³H]GABA) from a participate fraction of the carp (Cyprinus carpio) retina. The KA action was dose‐dependent but Ca²⁺‐independent. A similar response was elicited by another glutamate receptor agonist, quisqualic acid, and high K⁺, but not by an aspartate agonist, N‐methyl‐d‐aspartic acid. The stimulatory action of KA on the [³H]GABA release was selectively blocked by the KA blockers γ‐d‐glutamylglycine and cis‐2,3‐piperidine dicarboxylic acid. Dopamine (DA), which is contained in DA interplexiform cells in the carp retina, inhibited the [³H]GABA release induced by KA and high K⁺ in a dose‐dependent manner. 5‐Hydroxytryptamine and two well‐known GABA antagonists, bicuculline (Bic) and picrotoxin (Pic), also mimicked the DA effect on the GABA release at a comparable concentration. This inhibitory effect of DA as well as Bic and Pic on the [³H]GABA release evoked by KA was clearly antagonized by a DA blocker, haloperidol. The action of these agents (KA, DA, GABA antagonist) belonging to three different receptor categories on the GABAergic neurons (possibly external horizontal cells; H1 cells) is discussed in relation to other electrophysiological studies on the lateral spread of S‐potentials between H1 cells.