Interactions between body temperature and wheel running over the estrous cycle in rats

Abstract

Female rats were housed in Wahmann wheels under a 12:12 light-dark (LD) photoperiod for 36 days and then were switched to LD 10:14 for 36–64 days. Running was 95–100% nocturnal. Overall amounts of running were higher in LD 12:12, but changes over the estrous cycle were similar. On the night of proestrus running increased by 64% in LD 12:12 and 123% in LD 10:14 ( p<0.0001) compared to the first night of diestrus. Nighttime Tb's were correlated with running levels. In LD 12:12 Tb on proestrus was higher by 0.3 ± 0.0°C ( p<0.0001); in LD 10:14 it was higher by 0.2 ± 0.0°C ( p<0.001) compared to diestrus-1. During L, Tb was lowest on proestrus and highest on estrus. Half of the rats exhibited a consistent phase advance of at least 30 min in both Tb and activity on proestrus. The advanced Tb acrophase was correlated with both the activity acrophase (r= .91; p<0.0001) and the amount of running (r= .60; p<0.01). The wheels of 6 rats were then locked at night in LD 10:14. There were no significant Tb changes from diestrus-1 across the cycle. However, Tb acrophase was delayed a mean of 76 ± 16 min ( p<0.01) in these rats. Also, the Tb acrophase across all days of the estrous cycle was delayed by 94 ± 18 min ( p<0.001) compared to when the wheels were open. Six rats that were housed in basin cages under LD 12:12 and then 10:14 showed changes in Tb that were similar to the rats in the locked wheels, and Tb acrophases on proestrus were even more delayed (mean 128 ± 20 min). These data suggest that most of the Tb changes over the estrous cycle are related to changes in activity, and that the hormonal influence on Tb is minor.