Chick retina and pineal gland differentially respond to constant light and darkness: in vivo studies on serotonin N-acetyltransferase (NAT) activity and melatonin content.

Oscillations in serotonin N-acetyltransferase (NAT) activity and melatonin content were investigated in retina and pineal gland of chicks kept for 5 days in constant darkness (DD) or continuous light (LL). Under DD the rhythm of the pineal melatonin biosynthesis resembled that found under diurnal illumination (LD), whereas in the retina DD resulted in significant elevations of NAT activity and melatonin level during subjective light. A low-amplitude rhythm of retinal NAT activity continued under LL with a period close to 24 h. In the pineal gland, light exposure suppressed the level of NAT activity and melatonin content (an effect being substantially weaker than that observed in retina), delayed the first peak of NAT activity by 3 h, and prolonged the rhythm's period to about 26 h. Our data suggest the existence of some differences in the activity of circadian oscillators that regulate the melatonin generating system in the retina and pineal gland of chick.