Contribution of spontaneous motor activity to the 24 hour control of body temperature in male and female rats

The metabolic heat produced from motor activity contributes to the control of core temperature (T-c); however, little is known regarding the role of motor activity in thermoregulation in warm and cold environments and the influence of sex and the estrous cycle. To address these issues, motor activity and T-c were monitored by radiotelemetry in male and female rats maintained at ambient temperatures (T-a) ranging from 12 to 33.5 degrees C for 24 h under a 12:12 L:D cycle. The contribution of the mean level of motor activity to the elevation in T-c, termed Delta T-ma, was determined using linear regression analysis of T-c vs motor activity recorded at 5 min intervals during the light and dark phases. Delta T-ma was significantly affected by T-a, sex, and stage of the estrous cycle, whereas the light and dark cycle had marginal effects. Delta T-ma was highest in female rats at T(a)s of 22 to 33.5 degrees C. Both sexes exhibited a reduction in Delta T-ma at T(a)s greater than or equal to 26 degrees C. There was no difference in Delta T-ma in rats housed at a T-a of 12 degrees C as compared to 22 degrees C, suggesting that the contribution of motor activity to T-c does not change with cold exposure. There was a 50% reduction in Delta T-ma during proestrus concomitant with a 160% elevation in motor activity compared to metestrus and diestrus but an increase in Delta T-ma during estrus. Overall, the contribution of motor activity on T-c is reduced in the male rat when exposed to warm T(a)s and also in the female rat during proestrus, a period when T-c is elevated. (C) 1997 Published by Elsevier Science Ltd.