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.