5-hydroxytryptamine: its facilitative action on [3H]dopamine release from the retina

5-Hydroxytryptamine (5-HT) at 5 X 10(-4) M caused a two-fold increase in [3H]dopamine (DA) release from retinal particulate fractions of the carp (Cyprinus carpio). The 5-HT action was dose- and Ca2+-dependent. The three 5-HT agonists examined (5-methoxytryptamine, 5-methoxy-N,N-dimethyltryptamine and tryptamine) were more effective than 5-HT on [3H]DA release. 5,6-Dihydroxytryptamine was the strongest competitor among drugs tested for [3H ]DA uptake, but did not evoke any significant release of [3H]DA. Noradrenaline (or DA) at 5 X 10(-4) M produced a large increase in [3H]DA release from the particulate fractions, but its action was Ca2+-independent. The 5-HT-induced DA release could be seen in the frog retina but not in the rat retina, which does not contain indoleamine-accumulating cells. The results obtained strongly suggest that 5-HT stimulates [3H ]DA release through a receptor mechanism on DAergic terminals and modifies the DA-cell's function in the retina.