PHOTIC ENTRAINMENT OF THE MAMMALIAN RHYTHM IN MELATONIN PRODUCTION

This review summarizes studies on the photic entrainment of the circad ian rhythm in the rat pineal melatonin production, namely of the rhyth m in N-acetyltransferase (NAT) activity, and compares the NAT rhythm r esetting with preliminary results on the resetting of an intrinsic rhy thmicity in the suprachiasmatic nucleus (SCN) of the hypothalamus, nam ely with the entrainment of the rhythm in the light-induced c-fos gene expression. Phase delaying of the NAT rhythm after various Light stim uli proceeds within 1 day with almost no transients, whereas during ph ase advancing of the rhythm only the morning NAT decline is phase adva nced within 1 day and the evening rise phase shifts through transients . A Light stimulus encompassing the middle of the night may phase dela y the evening NAT rise, phase advance the morning decline, compress th e rhythm waveform, and eventually lower its amplitude. Similarly, a lo ng photoperiod compresses the NAT rhythm waveform. The magnitude of ph ase shifts of the NAT rhythm, as well as their direction, depends on a previous photoperiod. Phase shifts of the evening rise in c-fos gene photoinduction in the SCN and of the morning decline are similar to th ose of the pineal NAT rhythm after all light stimuli studied so far. T he data indicate that the resetting of the rhythm in melatonin product ion in the rat pineal gland reflects changes in the SCN functional sta te and suggest that the underlying SCN pacemaking system is complex.