Day-night variations of behavioral and autonomic thermoregulatory responses to lipopolysaccharide in rats

This study investigated the day-night differences in behavioral and autonomic thermoregulatory responses to bacterial lipopolysaccharide (LPS) in rats. Male rats were housed individually in cages with a 12: 12 h light dark cycle at an ambient temperature of 24 degrees C. The rats were placed in a box with a temperature gradient and intraperitoneally injected with LPS (10 micrograms/kg). The preferred ambient temperature (Tpr) was estimated by the location of the rats in the box, and intraperitoneal temperature (Tb) was measured by a biotelemetry system. Measurements were taken during the light and dark phases of the day. LPS produced fever in both phases. The magnitude of rise in Tb did not differ between the two periods. In the dark phase, Tpr significantly increased during the development of fever and decreased during the defervescence, while it did not change throughout the febrile course during the light phase. In a separate experiment, rats were loosely restrained and placed in a direct calorimeter. Their colonic temperature (Tcol), evaporative and nonevaporative heat loss and heat production were measured before and after intraperitoneal injections of LPS (10 micrograms/kg). Measurements were taken during the light and dark phases of the day. LPS induced fever in both phases. The magnitude of change in T col, heat loss, and heat production due to LPS did not differ between the two periods. These results suggest that the fertile response of rats to intraperitoneal LPS is not affected by the time of day. However, it seems that during LPS-induced fever, thermoregulatory behavior is not fully activated during the light phase of the day.