The Effect of Low Ambient Temperature on the Febrile Responses of Rats to Semi-Purified Human

The febrile responses of Sprague-Dawley rats to semi-purified human endogenous pyrogen were studied at a thermoneutral ambient temperature (260C) and in the cold (30C). It was found that while rats developed typical monophasic febrile responses at thermoneutrality, febrile responses were absent in the cold-exposed rats. Experiments were conducted to determine whether this lack of febrile responses in cold-exposed rats was due to an inability of these animals to generate or retain heat in the cold. Thermogenesis and vasoconstriction were stimulated in cold-exposed rats by selectively cooling the hypothalamus, using chronically implanted ther-modes. It was shown that, using this stimulus, metabolic rate could be increased by more than 50 percent and body temperature could be driven up at a rate of 5°C/hour in rats exposed to the cold. Therefore, it was concluded that the lack of febrile responses of cold-exposed rats to pyrogen is in no way due to a physical or physiological inability to retain heat. Instead, it appears that in some manner cold exposure suppresses the sensitivity or responsiveness of the rat to pyrogenic stimuli