Entrainment of the circadian rhythm in the rat pineal N-acetyltransferase activity by prolonged periods of light

Summary Entrainment of the circadian rhythm in the pineal N-acetyltranferase activity by prolonged periods of light was studied in rats synchronized with a light:dark regime of 1212 h by observing phase-shifts in rhythm after delays in switching off the light in the evening or after bringing forward of the morning onset of light. When rats were subjected to delays in switching off the light of up to 10 h and then were released into darkness, phase-delays of the evening N-acetyltransferase rise during the same night corresponded roughly to delays in the light switch off. However, phasedelays of the morning decline were much smaller. After a delay in the evening switch off of 11 h, no N-acetyltransferase rhythm was found in the subsequent darkness. The evening N-acetyltransferase rise was phase-delayed by 6.2 h at most 1 day after delays. Phase-delays of the morning Nacetyltransferase decline were shorter than phasedelays of the N-acetyltransferase rise by only 0.7 h to 0.9 h at most. Hence, 1 day after delays in the evening switch off, the period of the high night N-acetyltransferase activity may be shortened only slightly. The N-acetyltransferase rhythm was abolished only after a 12 h delay in switching off the light.Rats were subjected to a bringing forward of the morning light onset and then were released into darkness 4 h before the usual switch off of light. In the following night, the morning N-acetyltransferase decline, but not the evening rise, was phase advanced considerably. Moreover, when the onset of light was brought forward to before midnight, the N-acetyltransferase rise was even phase-delayed. Hence, 1 day after bringing forward the morning onset of light, the period of the high night N-acetyltransferase activity may be drastically reduced. When rats were subjected to a 4 h light pulse around midnight and then released into darkness, the N-acetyltransferase rhythm in the next night was abolished.The data are discussed in terms of a two-component pacemaker controlling the N-acetyltransferase rhythm. It is suggested that delays in the evening switch off of light may disturb the N-acetyltransferase rhythm the next day only a little, as the morning component may adjust to phasedelays of the evening component almost within one cycle. On the other hand, bringing forward the morning onset of light may disturb the N-acetyltransferase rhythm heavily the next day, as the evening component not only does not adjust to phase-advances of the morning component, but it may even be phase-delayed when the light onset occurs before midnight.Abbreviations NAT N-acetyltransferase - PRC phase response curve - E evening component of the N-acetyltransferase rhythm or of its pacemaker - M morning component of the N-acetyltransferase rhythm or of its pacemaker - LD xy light dark cycle comprising x h of light and y h of darkness