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 (T ^sub s^). 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 (T ^sub a^) of 32°C (CH), a T ^sub a^ of 32°C for 5 h daily in the latter half of the dark phase (IHF), a T ^sub a^ of 32°C for 5 h daily at a random time of day (IHR), or a constant T ^sub a^ of 24°C (control). After the heat exposure schedule, the rats were placed in a thermal gradient and their intra-abdominal temperature (T ^sub ab^), T ^sub s^ and spontaneous activity were measured for 3 days. There were clear day-night variations of T ^sub ab^ and T ^sub s^ in all groups. The levels of T ^sub ab^ and T ^sub s^ of the CH rats were significantly higher than those of the IHF, IHR and control rats. The T ^sub ab^ 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 T ^sub s^ 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.[PUBLICATION ABSTRACT]