Evidence for a separate meal-associated oscillator in the rat

Three experiments were conducted to explore the relationship between food availability and the wheel running activity rhythms of intact rats. In two experiments re-entrainment when meal times were changed was studied. The rats showed an increase in activity immediately prior to a regular feeding time and this increase was more rapid when rats had been fed at that time in an earlier condition. Some evidence of transients at a former meal time was observed when the meal time was shifted to later in the day, but not when the meal time was shifted to earlier in the day. This led to the hypothesis that ad lib feeding masks rather than abolishes meal-entrained activity. In a third experiment, therefore, rats were entrained to a daily meal under a light-dark (LD) cycle, then placed in constant dark (DD) on ad lib food with occasional periods of deprivation. A burst of activity associated with the former meal time occurred during the deprivation period, but not during ad lib periods; it returned during subsequent deprivation. It is concluded that meal feeding entrains a separate oscillator with a period different from the oscillator entrained by the LD cycle.