Catecholamine receptors regulating serotonin N-acetyltransferase activity and melatonin content of chicken retina and pineal gland: D2-dopamine receptors in retina and alpha-2 adrenergic receptors in pineal gland.

The type of catecholamine receptor involved in the regulation of serotonin N-acetyltransferase (NAT) activity and melatonin (N-acetyl-5-methoxytryptamine) level in chicken retina was compared to that regulating these parameters in the pineal gland. Systemic administration of apomorphine, a dopamine receptor agonist, resulted in marked inhibition of the nocturnal increase of retinal NAT activity and melatonin content. Apomorphine did not affect NAT activity or melatonin content of the pineal gland. In contrast, clonidine, an alpha-2 adrenergic receptor agonist, inhibited the nocturnal rise in pineal NAT activity and melatonin content although being without effect on these parameters in retina. Apomorphine-induced inhibition of retinal NAT activity was blocked by spiperone, a D2-dopamine receptor antagonist, but not by antagonists of D1-dopamine, alpha-1, alpha-2 and beta adrenergic receptors. Systemic or intraocular injection of quinpirole, a D2-dopamine receptor agonist, in the middle of the dark phase of the light-dark cycle markedly reduced retinal NAT activity and melatonin level, whereas injections of SKF 38393-A, a D1-dopaminergic agonist, had no effect. The inhibitory effect of clonidine on pineal NAT activity was blocked by yohimbine, an alpha-2 adrenergic receptor antagonist. The results presented in this paper demonstrate that NAT activity and melatonin content in chicken retina and pineal gland are differentially modulated in vivo by D2-dopamine and alpha-2 adrenergic receptors, respectively. Despite the different types of receptors involved, both tissues may share a common pathway for catecholamine-mediated inhibition of melatonin biosynthesis, i.e., inhibition of adenylate cyclase activity.