Effect of heat acclimation on diurnal changes in body temperature and locomotor activity in rats

1. The present study was performed to examine the effects of heat exposure hours hours at a fixed time once a day on diurnal variations of deep body temperature, heat balance and locomotor activity in rats. 2. The heat-exposed group (HE) was subjected to an ambient temperature of 33.5 degrees C for about 5 h in the last half of the dark phase daily for at least 10 consecutive days, while the control rats were constantly kept at 24 degrees C. 3. After the completion of the heat exposure schedule, hypothalamic temperature (Thy), heat loss, heat production and locomotor activity of HE rats significantly decreased for 3-4 h during the period of previous heat exposure time and formed a characteristic trough in the dark phase, which was never observed in the control rats or in the HE rats before the start of the heat exposure schedule. 4. The troughs of Thy, heat loss and heat production in the dark phase were persistent for at least 2 days after the end of the heat exposure schedule. The reductions of locomotor activity in this period were, however, observed only on the first day after stopping the heat exposure schedule. 5. These results suggest that the time memory for heat exposure was formed in heat-acclimated rats and was persistent for at least 2 days after the removal of heat exposure. It is also concluded that the reduction in metabolic heat production contributed to the fall of Thy during the period of heat exposure.