Possible involvement of D4-like dopamine receptor in regulation of serotonin N-acetyltransferase activity in duck retina.

The dopamine (DA) receptor regulating serotonin N-acetyltransferase (NAT) activity in duck retina was characterized pharmacologically. Systemic administration to ducks of quinpirole (a D3/D4-DA receptor agonist) and &-OH-DPAT (a D3-DA receptor agonist) suppressed the nighttime NAT activity of the retina in a dose-dependent manner, with quinpirole being two orders of magnitude more potent than 7-OH-DPAT. Bromocriptine (a D2/D3-DA receptor agonist) and SKF 38393 (an agonist of the D1 family DA receptors) were ineffective. The suppressive effect of quinpirole on NAT activity was blocked by spiroperidol and clozapine (antagonists of the D2 family of DA receptors), and not affected by SCH 23390 (an antagonist of the D1 family). The results extend our data on chick retina (which demonstrate an important role of D4-like DA receptor in the control of melatonin biosynthesis) and suggest that DA receptor regulating NAT activity in the duck retina belongs to the D2 family and may represent a D4-like subtype.