Dopamine decreases melatonin content in golden hamster retina

Dopamine significantly decreased melatonin levels in Golden hamster retinas excised at noon and incubated under light. The effect of dopamine was reve rsed by spiperone and clozapine (selective antagonists for D-2 and for D-4/ D-2 dopaminergic receptors, respectively) but not by SCH 23390 (a selective D-1 dopamine receptor antagonist). Both clozapine and spiperone per se sig nificantly increased melatonin levels, whereas SCH 23390 was ineffective. Q uinpirole (an agonist for D-2-subfamily dopaminergic receptor) decreased me latonin content in retinas excised at midday. Dopamine increased, whereas q uinpirole decreased, cAMP accumulation in retinas excised at noon. Retinal dopaminergic turnover rate (assessed as the ratio of 3,4-dihydroxyphenylace tic acid to dopamine) was significantly higher at midday than at midnight. In retinas excised at midnight, melatonin content in vitro was unaffected b y dopamine or quinpirole. At midnight, dopamine increased cAMP accumulation , whereas quinpirole was ineffective. When hamsters were kept under constan t darkness for 48 h and sacrificed at subjective midday or midnight, dopami ne increased cAMP accumulation at both times, whereas quinpirole decreased this parameter only at subjective midday. Dopaminergic turnover rate was si gnificantly higher at subjective midday than at subjective midnight. These results show that dopamine regulates melatonin biosynthesis in the Golden h amster retina.