24-HOUR CONTROL OF BODY-TEMPERATURE IN RATS .I. INTEGRATION OF BEHAVIORAL AND AUTONOMIC EFFECTORS

Some studies suggest that the nocturnal elevation in core temperature (T-c) of the rat is mediated by an elevation in the set point. The role of set point can be assessed if behavioral effecters are measured simultaneously with other thermoregulatory effecters and T-c over a 24-h period. Selected ambient temperature (STa) and motor activity (MA) were measured in rats housed in a temperature gradient system with a 12:12-h photoperiod (lights on 0600 h). T-c and heart rate (HR) were monitored by telemetry. During the light phase, STa, T-c, HR, and MA were relatively stable with values 29.0 degrees C, 37.1 degrees C, 310 beats/min, and 1-2 m/h, respectively. During the light-to-dark transition there were abrupt elevations in T-c, HR, and MA but no change in STa. STa decreased during the dark phase and reached a nadir of 23 degrees C at 0500 h. All variables recovered to basal levels within 3-4 h after the onset of the Light phase. Overall, autonomic effecters control the elevation in T-c during the onset of the dark phase while behavioral effecters have little if any role. Behavioral thermoregulation is important in two ways: 1) the selection of cooler T-a values at night to prevent an excess elevation in T-c and 2) a preference for cooler T-a values before the light phase to facilitate the recovery of T-c.