DAY-NIGHT CHANGES OF BODY-TEMPERATURE AND FEEDING-ACTIVITY IN HEAT-ACCLIMATED RATS

Male Wistar rats were divided into two groups. The controls (CN) were kept at a constant ambient temperature (T-a) of 24 degrees C throughou t the experiment. Heat-acclimated rats (HA) were subjected to T-a of 3 3 degrees C in the last half of the dark phase for 16 consecutive days . After the schedule was terminated, hypothalamic temperature (T-hy), oxygen consumption (VO2), heat loss, and feeding activity were measure d for the following 2 days at T-a of 24 degrees C with a direct calori meter in constant darkness. Hypothalamic temperature, VO2, and heat lo ss of the HA were significantly lower than those of the CN for 3-4 h d uring the period when the rats had been previously exposed to heat. Fe eding activity during the specific period was significantly less in th e HA than in the CN only on the first day. Under starved conditions, t he decreases of T-hy and VO2 during the previous heat exposure time we re consistent in the HA. The results suggest that body core temperatur e and feeding behavior decrease during the previous heat exposure time in rats after subjection to repeated heat exposure at a fixed time da ily. The characteristic fall in body core temperature is mainly attrib uted to the reduction of VO2. It also seems that a decrease in postpra ndial thermogenesis is not a predominant contributor to the depression of VO2 during the previous heat exposure time in heat-acclimated rats .