Effects of daily schedules of forced activity on free-running rhythms in the rat.

Circadian rhythms of hamsters can be phase-shifted or entrained by single or daily sessions of induced wheel running. In contrast, observations of rats under restricted-feeding schedules suggest that their free-running rhythms are not readily entrainable by a daily bout of intense activity. A formal test of this idea was made by subjecting rats to daily 2-hr or 3-hr sessions of forced treadmill activity. None of 18 rats entrained to a daily treadmill schedule when tested in constant dim light, but 1 of 16 did entrain when tested after blinding, when the period of its free-running activity rhythm was very close to the period of the treadmill schedule and when the onset of its daily active phase overlapped with the treadmill sessions. These conditions were recreated in a final group of eight rats; the rats were trained in a light-dark cycle, blinded, and subjected to a treadmill schedule with a period of 23.91 hr that was initiated at the onset of the rats' active phase on day 1. Six of these rats entrained. The mechanism for entrainment by activity schedules clearly exists in rats, but the conditions under which this occurs are highly constrained, suggesting that activity is a very weak zeitgeber in this species. It is argued that the evolution of functionally separable food- and light-entrainable oscillators in the rat demands a very low sensitivity to feedback effects of activity.