Phase-shifting the light-dark cycle resets the food-entrainable circadian pacemaker.

This experiment examined coupling between two circadian pacemakers, one entrained by light and the other by food. Rats were housed in running wheels: one-half on restricted feeding from 0900 to 1100 h whereas the others were free fed. After timed-fed rats developed bursts of running before 0900 h, all rats were given free access to food. A month later, the light-dark (LD) cycles for one-half the rats in each group were delayed by 6 h. After entrainment to the new LD cycles, all rats were probed with 96-h periods of food deprivation. During these probes, unshifted timed-fed rats ran more than their free-fed controls at 0600-1200 h, but delayed timed-fed rats ran more than their controls at 1200-1500 h, i.e., 6 h later than unshifted rats. Next, the unshifted rats were subjected to a 6-h advance in their LD cycle. One month later, all rats were again probed with food deprivation. Delayed timed-fed rats continued to run more at 1200-1500 h than their free-fed controls, but timed-fed and free-fed phase-advanced rats showed similar activity patterns after the phase advance. However, when activity patterns before these probes were subtracted, timed-fed advanced rats showed greater "responses" to food deprivation at 0001-0300 h than their free-fed controls, i.e., 6 h earlier than before the phase advance. Thus LD shifts delayed and advanced the running that was previously associated with food availability and reset the food-entrainable circadian pacemaker.