Entrainment of the circadian rhythm in rat pineal N-acetyltransferase activity under extremely long and short photoperiods.

Entrainment of a pacemaker driving the circadian rhythm in rat pineal N-acetyltransferase activity was studied under extremely long and short photoperiods. Adult male rats maintained under the light-dark regime (LD) 18:6 or under the regime LD 6:18 were exposed to a 1-min light pulse at different times at night, then they were released into darkness, and the next night phase-shifts of the evening N-acetyltransferase rise and of the morning N-acetyltransferase decline caused by light pulses were determined. The evening rise was phase-delayed by at most 0.5 h under LD 18:6, but by as much as 2.8 h under LD 6:18. The morning decline was phase-advanced by at most 1.9 h under LD 18:6, but by as much as 3.5 h under LD 6:18. Hence, the magnitude of phase-shifts and consequently patterns of phase-response curves, which show possibilities of discrete entrainment, depend on the photoperiods under which animals are maintained. A 1-min light pulse applied within 1 h before the end of the dark period phase-advanced the morning N-acetyltransferase decline under LD 18:6 as well as under LD 6:18, while a pulse applied within 1 h after the beginning of the dark period phase-delayed the evening N-acetyltransferase rise only in rats maintained under LD 18:6, but not in those kept under LD 6:18. It seems that under very long photoperiods, the N-acetyltransferase rhythm may be entrained by evening as well as by the morning light, while under very short photoperiods the rhythm may be synchronized by morning light only.