Circadian rhythm of core body temperature in two laboratory mouse lines

The circadian rhythm of core body temperature (Tb) was examined in two mouse lines bidirectionally selected for nest-building behavior (small (SNB) and big nest-builders (BNB)). This selection also resulted in more robust circadian organization of wheel-running activity in the SNB compared to the BNB mice. Tb was measured by an e-mitter implanted in the abdominal cavity. The circadian Tb rhythm of the SNB was more robust compared to the BNB regardless of whether the animals had access to a running wheel or not and regardless of the lighting conditions, i.e., 12 h:12 h light:dark (LD) cycle or constant dark (DD). Wheel-running activity rhythms of SNB were more robust in LD and DD compared to BNB. The amplitude of the circadian Tb rhythm increased significantly in response to wheel access in both mouse lines, but was not significantly different between the BNB and SNB. However, BNB tended to have lower amplitudes of circadian Tb rhythm in the absence of running wheels and a larger increase in the amplitude upon access to a running wheel compared to SNB. No differences were found in LD and DD between the lines in mean Tb and wheel-running activity levels. In addition, no differences between the two mouse lines were found in the free-running period of the Tb or wheel-running activity rhythms in DD. Overall, our findings reveal a more robust circadian phenotype of the SNB compared to the BNB.