PHOTIC RESETTING OF INTRINSIC RHYTHMICITY OF THE RAT SUPRACHIASMATIC NUCLEUS UNDER VARIOUS PHOTOPERIODS

To date, photic entrainment of the mammalian circadian system has been studied by following phase shifts of overt rhythms in the periphery g overned by a circadian pacemaker located in the suprachiasmatic nucleu s (SCN). The present study follows for the first time photic resetting of intrinsic rhythmicity of the SCN itself. Rats maintained under eit her a shorter photoperiod, with 12 h of light and 12 h of darkness per day, or under a long, 18:6-h light-dark photoperiod were exposed to a light stimulus during the dark period and then released into darkness , and the next day the SCN rhythm in the light-stimulated c-Fos protei n immunoreactivity was followed as a marker of the SCN endogenous rhyt hmicity. After a light stimulus in the early night, the evening rise i n the photic elevation of Fos protein photoinduction as well as the mo rning decline were phase delayed within one cycle. After a light stimu lus in the late night, only the morning decline in the photic elevatio n of Fos was phase advanced the next night, not the evening rise; cons equently, the interval enabling high photic elevation of Fos was reduc ed. After a light stimulus was administered around the middle of the n ight, the next night the evening rise in the light-stimulated Fos was eventually phase delayed, the morning decline was phase advanced, and the rhythm amplitude was reduced significantly; under 18:6-h light-dar k, a mere 5-min light exposure exhibited such effects. The data indica te that resetting of the SCN rhythmicity in the light-elevated c-Fos 1 day after a resetting stimulus administration, i.e., during transient cycles, may proceed via nonparallel phase shifts of the evening rise and of the morning decline of the light-stimulated Fos, and via amplit ude lowering and suggest a complex circadian pacemaking system in the rat SCN.