2 POSSIBLE DETERMINANTS OF THE TIMING OF DAILY EPISODES OF BEHAVIOR IN RATS

The present experiment examined endogenous ultradian rhythms and regul atory metabolic processes as two potential determinants of daily out-o f-nest episodes (ONEs) and feeding episodes (FEs) in rats living in a 24-h environment. Two types of photoperiod were used: a Standard 12:12 h L/D schedule and a Skeleton 12:12 h schedule. During each type of p hotoperiod rats were exposed to 4 feeding conditions of 7 to 12 days e ach: (a) a baseline of ad lib food and water; (b) food restricted to t he diurnal (inactive) portion of the cycle; (c) both food and water re stricted to the inactive period; and (d) return to baseline. Time seri es analyses of nocturnal ONEs in baseline revealed a strong circadian rhythm and weaker ultradian rhythms with periods between 2 and 6 h. An alyses of FEs, though, revealed a general absence of circadian rhythms but strong ultradian rhythms with periods similar to those observed i n ONEs. When food and water were restricted to the inactive part of th e cycle, ONEs showed no change in frequency, but a decrease in average duration and changes in ultradian periodicities. The results indicate d control of daily episodes of behavior in rats by ultradian oscillato rs that are weaker and more variable in effect than those found in vol es, but similar in period.