Body temperature during and following 10-day subcutaneous infusion of clonidine in the rat.

Body temperature in the rat was measured during and after cessation of the continuous subcutaneous infusion of clonidine (10 micrograms/kg per hr) for 10 days. The body temperature of control animals displayed a distinct circadian rhythm. On each day the mean body temperature over the dark phase (2000-0800 hr) was consistently higher (0.6-0.9 degrees C) than the following light phase. The infusion of clonidine was essentially devoid of initial effects on body temperature. However, during the light phases of day 2 onwards the mean body temperature of the animals treated with clonidine was consistently higher (0.4-0.6 degrees C) than that of controls. No such differences were observed during the dark phases. It appeared that the infusion of clonidine limited the fall in body temperature which normally occurred at the onset of the light phases and this resulted in the treated rats displaying a relative hyperthermia. On cessation of the infusion of clonidine (at 2400 hr on day 11) a distinct hyperthermia was observed within 2 hr and was sustained for the remainder of the dark phase and subsequent light phase. This post-infusion hyperthermia was more pronounced than that observed during the period of infusion of clonidine. These results demonstrate that the circadian control of body temperature is disturbed both during and after continuous infusion of clonidine.