DOPAMINE-RECEPTOR REGULATING SEROTONIN N-ACETYLTRANSFERASE ACTIVITY IN CHICK RETINA REPRESENTS A D-4-LIKE SUBTYPE - PHARMACOLOGICAL CHARACTERIZATION

The dopamine (DA) receptor regulating serotonin N-acetyltransferase (N AT) activity in chick retina was characterized pharmacologically. Intr aocular (i.o.) administration of DA significantly decreased the nightt ime NAT activity of chick retina. The effect of DA was antagonized by blockers of the D2 family of DA receptors, spiroperidol, YM-09151-2 an d clozapine, and it was not affected by SCH 23390, a selective antagon ist of D1 DA receptor. Several agonists of D2 family of DA receptors g iven i.o. suppressed the nighttime NAT activity of the chick retina wi th quinpirole (D3/D4 receptor selective) and bromocriptine (D2/D3 rece ptor selective) being the most and the least potent drugs, respectivel y. The rank-order potency of antagonists of D2 family of DA receptors to block the inhibitory effect of quinpirole on the enzyme activity (w ith clozapine being relatively very potent, and (+)-butaclamol, raclop ride and remoxipride-ineffective) match the characteristics of the D4 DA receptor. Moreover, although sulpiride effectively prevented the qu inpirole-induced decline in the nighttime NAT activity of the chick re tina, there was no marked stereoselectivity in its action. It is sugge sted that DA receptor regulating NAT activity in chick retina represen ts a D4-like subtype.