Factors affecting release of 3H-dopamine from perfused carp retina

The effects of putative retinal neurotransmitters and several neuropeptides on 3H-dopamine release from isolated perfused carp (Cyprinus carpio) retinas were studied. Of the transmitter candidates tested, only serotonin (5-HT) and the 5-HT agonist tryptamine released 3H-dopamine. However, the release evoked by these agents was calcium (Ca2+) independent and not blockable by the 5-HT antagonist methysergide. We also investigated the antagonism of inhibitory inputs as a potential regulatory mechanism for dopamine release and found that the GABA antagonists, bicuculline and picrotoxin, stimulated a dose-dependent release of 3H-dopamine. The effects of the GABA antagonists were dependent on extracellular Ca2+ and could be inhibited by perfusion of the retina with GABA. Bicuculline and picrotoxin also stimulated an increase in cAMP accumulation, which was inhibited by the dopamine antagonist haloperidol. Our results support the hypothesis that the dopaminergic interplexiform cells of the teleost retina are under GABAergic inhibitory control.