Entrainment of aged, dysrhythmic rats to a restricted feeding schedule.

Aged rats often display abnormal circadian activity rhythms; the rhythm amplitude is low and entrainment to light-dark cycles is irregular. The activity rhythm of young rats can be entrained by both light and nonphotic cues, specifically food availability. In young rats, entrainment to restricted feeding cycles does not depend on intact suprachiasmatic nuclei, the presumed anatomical location of the light-entrainable oscillator. In this study, aged rats that displayed disrupted entrainment to light were assessed for their ability to entrain to restricted feeding schedules. Aged rats, young controls, and young suprachiasmatic nuclei-lesioned (SCN) rats were placed on a food restriction schedule (FR) for 14 days. Food was available for 2 h during the light phase of a 12-h light-dark cycle. Despite the absence of entrainment to light/dark cycles, both SCN-lesioned and aged groups showed entrainment to FR, with clear bouts of anticipatory activity during a period of complete food deprivation following 2 weeks of FR. The results suggest that the dysrhythmia of aged rats is a result of natural deterioration of a central circadian light-entrainable pacemaker, but that a secondary oscillator entrainable to food cycles is spared.