Resetting of a feeding-entrainable circadian clock in the rat.

Reentrainment of anticipatory activity (AA) after phase shifts of food access was studied in rats with lesions of the suprachiasmatic nuclei. Eight- or 10-h phase delays of feeding time resulted in delaying transients of AA. Twelve-h phase shifts and some 10-h phase advances resulted in rapid reentrainment (2-3 days) without visible transients. Most phase advances of 10 h resulted in delaying transients while 8-h advances induced transients with advancing and delaying components in a number of rats. Split transients were not prevented by advancing mealtime in 1-h steps. Phase shifts of food in multiple steps failed to accelerate delay shifts but retarded advance shifts. After the first 8-h phase delay shift, increased activity reappeared at the preshift phase of AA, simultaneously with anticipation of the phase-shifted meal time. The observation of split transients indicates that two or more circadian oscillators mediate entrainment to mealtime, and the reappearance of AA at a previously established phase suggest the possibility that this system has a memory of phase displacement.