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 were released into constant darkness three days after the LD cycle was advanced by 8 hours, the activity of the free-running rhythm started near time of dark period of the new LD cycle in all rats (n = 16). 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. In the delayed LD cycle, when rats were released into constant darkness two days after the LD cycle was delayed by 8 hours, the free-running activity rhythm started near the dark period of the new LD cycle in all rats (n = 16). These results suggest that the circadian clock itself reentrains very rapidly (i.e., with at least 3 days) following either 8-hour phase advance or delay in the LD cycle, while the overt rhythms of activity take many cycles.