Forced dissociation of activity entrained to T cycles of food access in rats with suprachiasmatic lesions.

The ability of rats with suprachiasmatic lesions to entrain anticipatory wheel running to two food access times per day was investigated. In the presence of meal schedules with periods of 23.75 hr and 24 hr, two of seven rats entrained activity to both for many consecutive days, while other rats repeatedly shifted activity from one schedule to the other. A second group of rats was maintained on 25-hr and 26-hr meal schedules. One of nine rats showed prolonged entrainment to both schedules (i.e., forced dissociation). In the other rats, anticipatory activity (AA) waxed and waned repeatedly on each schedule. In both experiments, AA to the leading schedule increased when the interval between meals was about 5 hr or less, and AA to the trailing schedule diminished or ceased. Changes in AA were also common when the interval between meals was between 11 and 16 hr. The results are consistent with the hypothesis that entrainment of AA to periodic food access is mediated by at least two mutually coupled circadian pacemakers. Interactions between these putative pacemakers appear to be strongest when the pacemakers are nearly in phase or in antiphase.