Selected ambient temperatures of rats acclimated to heat given on various schedules.

We investigated the behavioral thermoregulation of heat-acclimated rats by measuring their selected ambient temperatures (Ts). Rats kept in a light:dark cycle of 12:12 h were subjected to one of four different heat exposure regimes for 10 consecutive days; a constant ambient temperature (Ta) of 32 degrees C (CH), a Ta of 32 degrees C for 5 h daily in the latter half of the dark phase (IHF), a Ta of 32 degrees C for 5 h daily at a random time of day (IHR), or a constant Ta of 24 degrees C (control). After the heat exposure schedule, the rats were placed in a thermal gradient and their intra-abdominal temperature (Tab), Ts and spontaneous activity were measured for 3 days. There were clear day-night variations of Tab and Ts in all groups. The levels of Tab and Ts of the CH rats were significantly higher than those of the IHF, IHR and control rats. The Tab and activity levels of the IHF rats were significantly lower than those of the IHR and control rats only in the latter half of the dark phase. The Ts values of the IHF rats did not differ from those of the IHR and control rats. These results suggest that, after rats were constantly subjected to heat, heat-seeking behavior was induced so that the core temperature was maintained at a high level. However, when rats were acclimated to heat given for several hours at a fixed time daily, core temperature was lowered during the same time period of previous heat exposure in association with a depression of thermogenic behavior.