STIMULATION OF D-4-LIKE DOPAMINE-RECEPTOR SUPPRESSES SEROTONIN N-ACETYLTRANSFERASE ACTIVITY BUT DOES NOT PHASE-SHIFT THE CIRCADIAN OSCILLATOR IN CHICK RETINA

We have recently reported that dopamine (DA) receptor-regulating serot onin N-acetyltransferase (NAT) activity in chick retina represents a D -4-like subtype. In this work, effects of stimulation of DA-D-4 recept ors upon the circadian rhythm of retinal NAT activity were investigate d in chicks. Systemic administration of quinpirole (QNP), a DA-D-4 rec eptor agonist, resulted in a potent suppression of the nighttime NAT a ctivity of chick retina but did not significantly affect the phase of subsequent cycles of the rhythmic oscillation of the enzyme activity i n constant darkness. 6-h pulses of white light caused an acute suppres sion of NAT activity in chick retina and, in contrast to QNP, resetted the phase of the free-running rhythm. It is suggested that, although D-4-like DA receptors in chick retina are involved in the regulation o f NAT activity by the circadian clock, they do not appear to be implic ated in the regulation of the clock by light.