Vol. 293, Issue 1, 91-95, April 2000

Dopamine Decreases Melatonin Content in Golden Hamster Retina¹

Carolina O. Jaliffa, Francisco Firpo Lacoste, Diego Weinberg Llomovatte, María I. Keller Sarmiento and Ruth E. Rosenstein

Laboratorio de Neuroquímica Retiniana y Oftalmología Experimental, Departamento de Bioquímica Humana, Facultad de Medicina, Universidad de Buenos Aires, Buenos Aires, Argentina

Dopamine significantly decreased melatonin levels in Golden hamster retinas excised at noon and incubated under light. The effect of dopamine was reversed by spiperone and clozapine (selective antagonists for D₂ and for D₄/D₂ dopaminergic receptors, respectively) but not by SCH 23390 (a selective D₁ dopamine receptor antagonist). Both clozapine and spiperone per se significantly increased melatonin levels, whereas SCH 23390 was ineffective. Quinpirole (an agonist for D₂-subfamily dopaminergic receptor) decreased melatonin content in retinas excised at midday. Dopamine increased, whereas quinpirole decreased, cAMP accumulation in retinas excised at noon. Retinal dopaminergic turnover rate (assessed as the ratio of 3,4-dihydroxyphenylacetic acid to dopamine) was significantly higher at midday than at midnight. In retinas excised at midnight, melatonin content in vitro was unaffected by dopamine or quinpirole. At midnight, dopamine increased cAMP accumulation, whereas quinpirole was ineffective. When hamsters were kept under constant darkness for 48 h and sacrificed at subjective midday or midnight, dopamine increased cAMP accumulation at both times, whereas quinpirole decreased this parameter only at subjective midday. Dopaminergic turnover rate was significantly higher at subjective midday than at subjective midnight. These results show that dopamine regulates melatonin biosynthesis in the Golden hamster retina.