Circadian rhythm in pineal N-acetyltransferase activity: phase shifting by light pulses (I)

N-Acetyltransferase activity (NAT) in the pineal gland has a circadian rhythm which is responsive to environmental light-dark cycles. The rhythm entrains to (synchronizes with or is phased by) light-dark cycles. To assess the means by which phase resetting is accomplished, the phase response of the pineal NAT rhythm to light pulses was examined using chicks (Gallus domesticus). When 4-h light pulses were imposed on chicks at intervals over a 24-h period of darkness and the time of the next rise in NAT was determined in darkness, it was found that: (1) pulses early in the subjective dark time delayed the rise in NAT, (2) pulses late in the subjective dark time advanced the rise in NAT, and (3) pulses in the subjective light time were relatively ineffective. These results are typical of a circadian phase response curve and show that an enzyme with a circadian rhythm is similar to other circadian rhythms studied.