Effect of total body core cooling during poly I:C-induced fever in rabbits

At ambient temperature (Ta) 20 and 10 degrees C, metabolic heat production and hypothalamic temperature (Thypo) were measured to determine the fever response in six rabbits injected with polyriboinosinic-polyribocytidylic acid (poly I:C; 5 mg/kg iv). Similar measurements were made in afebrile and febrile animals subjected to 3 h of body cooling, in which heat was extracted with a chronically implanted intravascular heat exchanger in a ramplike manner. The fever time course showed a biphasic pattern. During cooling in the febrile experiments, Thypo remained constant or even slightly increased during the time corresponding to the first phase of fever but rapidly fell during the second phase because of a depressed shivering response. The net effect at the end of the cooling period was that Thypo decreased by 0.4 and 0.6 degree C more than in the afebrile cooling experiments at Ta 20 and 10 degrees C, respectively. The results indicate normal shivering responses during phase I of poly I:C-induced fever and depressed shivering in phase I, possibly because of a reduced thermosensitivity.