Development of circadian cycle of core temperature in juvenile rats.

To find out whether or not juvenile rats can sustain circadian rhythms of core temperature (Tc) in the absence of diurnal cues, we continuously recorded Tc for 12 days in isolated pups artificially reared in continuous dim light. Possible effects of uncontrolled environmental 24-h cycles were excluded by simultaneously studying pups born on the same day to two mothers entrained to 12:12 h light-dark cycles 180 degrees out of phase. Age-dependent changes were discriminated from the effects of sustained isolation by comparing Tc records between pups isolated from 10 or from 4 days of age (late- or early-isolated pups). Both groups of pups showed Tc cycles in phase with the light cycle in the colony of origin and with highest amplitudes from 10 to 13 days of age. Tc cycles with a period length close to 24 h could be seen through the 12th day of artificial rearing for early-isolated pups but only until the 6th day for late-isolated pups. In the third postnatal week the rhythm faded gradually. Self-sustained circadian Tc rhythms observed in isolated 1- to 2-wk-old pups demonstrate a functional endogenous clock in the juvenile rat.