REGULATION OF ENDOGENOUS DOPAMINE RELEASE IN AMPHIBIAN RETINA BY MELATONIN - THE ROLE OF GABA

In the retina of the African clawed frog (Xenopus laevis), endogenous dopamine release increases in light and decreases in darkness. Exogeno us melatonin and several chemical analogs of melatonin suppressed ligh t-evoked dopamine release from frog retina in a concentration-dependen t manner. The rank order of potency for inhibition of light-evoked dop amine release was melatonin >> 5-methoxytryptamine greater than or equ al to N-acetylserotonin > 5-methoxytryptophol >>> serotonin. Melatonin did not suppress dopamine release below levels seen in darkness. The putative melatonin receptor antagonist luzindole inhibited the effect of melatonin. Luzindole enhanced dopamine release in darkness but had little effect in light. These data suggest a role for endogenous melat onin in dark-induced suppression of retinal dopamine. Picrotoxin and b icuculline, GABA-A receptor antagonists, blocked melatonin-induced sup pression of dopamine release. In the presence of melatonin, bicucullin e was significantly less potent in stimulating dopamine release. These results suggest that melatonin enhances GABAergic inhibition of light -evoked dopamine release. This mechanism may underlie the light/dark d ifference in dopamine release in vertebrate retina.