Entrainment of the circadian clock as a function of time when a light stimulus is presented has been studied in detail while little attention has been paid to a role photoperiod may play in the resetting. To find out whether and how photoperiod affects the entrainment, resetting of the rat circadian pacemaker by delays in the evening light offset and by advances in the morning light onset, respectively, was studied in rats maintained either under a short photoperiod, with 8 h of light and 16 h of darkness per day (LD 8:16) or under a long, LD 18:6 photoperiod. To assess phase shifts of the clock, the suprachiasmatic nucleus controlled rhythm in the pineal N-acetyltransferase (NAT), namely the time of the evening NAT rise and the time of the morning decline, were followed. One day after a delay in lights off, on LD 8:16 the NAT rhythm with a normal amplitude was retained following longer delays of the light offset and the maximum phase delay of the NAT rise was 3 times larger than on LD 18:6. One day after an advance in lights on, the NAT decline was phase advanced under both photoperiods; on LD 8:16 the maximum shift was 3 times as large as on LD 18:6. On LD 8:16, the NAT rise was not shifted after shorter advances in lights on and became phase delayed only when the light onset was brought forward to before midnight while on LD 18:6 the NAT rise was phase delayed after any, even a mere 1 h, advance in lights on. The data show that magnitude and direction of phase shifts of the NAT rhythm depend not only on the time of light presentation but on photoperiod as well. Difference in resetting of the rhythm under various photoperiods may reflect photoperiod-dependent changes of an underlying pacemaker.

Institute of Physiology Czechoslovak Academy of Sciences Prague

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The rat suprachiasmatic nucleus is a clock for all seasons