Temperature rhythm reentrains faster than locomotor rhythm after a light phase shift

Abstract

Mammalian endogenous circadian rhythms are entrained to the environmental light–dark (LD) cycle. Although the circadian rhythms of core body temperature (Tb) and spontaneous locomotor activity (LA) are well synchronized under stable LD conditions, it is thought that these two parameters are regulated by distinct mechanisms. The purpose of the present study was to examine the adaptability of these two rhythms to an abrupt change in the environmental light phase. Tb and LA were simultaneously recorded in individual mice kept under 12:12-h LD cycle conditions before and after an 8-h photic phase advance. The onset of LA required 8 days to reentrain to the new LD cycle, whereas 6 days were required for reentrainment of the acrophase of Tb. Resting Tb, i.e., the Tb level independent of LA, was extracted from the same data source. The resting Tb level exhibited a robust daily rhythm with a difference of 1.0 °C between LD phases. After the photic phase advance, the resting Tb rapidly reached a stable level within 4 days, whereas the uncorrected Tb required 6 days for reentrainment. Based on these findings, we revealed that, independent of LA, the adaptability of the Tb rhythm to a new light cycle is half as rapid as that of LA. These results therefore suggest that the circadian rhythms of Tb and LA are intrinsically regulated by different pacemaker or effector mechanisms.