Does D4 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 pharmacologically. Nighttime serotonin N-acetyltransferase (NAT) activity was significantly decreased by either light exposure or by intraocular (i.oc.) administration of DA and quinpirole (a predominant D3/D4 DA receptor agonist). Several D2-like DA receptor antagonists, i.e. clozapine, haloperidol, spiroperidol, sulpiride, (+)-UH-232 and YM-09151-2, given i.oc. to light-adapted chicks markedly elevated retinal NAT activity. In contrast, raclopride (a D2/D3 DA receptor antagonist) and remoxipride (a D2-selective DA receptor antagonist) were ineffective. Spiroperidol, clozapine, haloperidol and sulpiride significantly increased melatonin content in the light-exposed retina, but had no effect on the activity of hydroxyindole-O-methyltransferase. None of D2-like DA receptor blockers tested modified the nighttime NAT activity in the chick retina. Our results indicate that the light-evoked inhibition of the nocturnal increase in melatonin biosynthesis in chick retina may involve stimulation of the D4 subtype DA receptor.