Physiology & behavior
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Physiology & behavior · Aug 1991
Rapid reentrainment of the circadian clock itself, but not the measurable activity rhythms to a new light-dark cycle in the rat.
Experiments were performed to determine if the circadian clock reentrains more quickly to an 8-hour phase shift in light-dark (LD) cycles than does the overt rhythm of activity. To investigate the reentrainment of the clock itself to an 8-hour advance or delay in the LD cycle, the rats were released into constant darkness only two or three days after a shift in LD cycle, and the amount of the phase shift of the clock itself was estimated from where free-running rhythm started by backward extrapolation. If the circadian clock could rapidly reset itself to the new LD cycle, it was predicted that the free-running rhythm of activity would start from near the dark period of the new LD cycle rather than the preceding one. ⋯ When rats (n = 24) were released into constant darkness two days after the LD cycle was advanced by 8 hours, 12 rats started the activity near time of dark period of the new LD cycle, while 9 rats started the activity near time of dark period of the preceding LD cycle. The remaining 3 rats showed the activity of the free-running rhythm near intermediate phase (transient phase). On the other hand, when the rats were not released into constant darkness after LD cycle was advanced by 8 hours, it took 6.4 days for activity rhythm to reentrain to the advanced LD cycle.(ABSTRACT TRUNCATED AT 250 WORDS)