DOES D-4 DOPAMINE-RECEPTOR MEDIATE THE INHIBITORY EFFECT OF LIGHT ON MELATONIN BIOSYNTHESIS IN CHICK RETINA

The dopamine (DA) receptor mediating the inhibitory effect of light on melatonin formation in the chick retina was characterized pharmacolog ically. Nighttime serotonin N-acetyltransferase (NAT) activity was sig nificantly decreased by either light exposure or by intraocular (i.oc. ) administration of DA and quinpirole (a predominant D-3/D-4 DA recept or agonist). Several D-2-like DA receptor antagonists, i.e. clozapine, haloperidol, spiroperidol, sulpiride, (+)-UH-232 and YM-09151-2, give n i.oc. to light-adapted chicks markedly elevated retinal NAT activity . In contrast, raclopride (a D-2/D-3 DA receptor antagonist) and remox ipride (a D-2-selective DA receptor antagonist) were ineffective. Spir operidol, clozapine, haloperidol and sulpiride significantly increased melatonin content in the light-exposed retina, but had no effect on t he activity of hydroxyindole-O-methyltransferase. None of D-2-like DA receptor blockers tested modified the nighttime NAT activity in the ch ick retina. Our results indicate that the light-evoked inhibition of t he nocturnal increase in melatonin biosynthesis in chick retina may in volve stimulation of the D-4 subtype DA receptor.