Behavioral responses to pyrogen in cold-stressed and starved newborn rabbits.

Starvation for 2 days and a chronic mild cold stress prevented the rises in body temperature that normally occur in newborn rabbits that have been injected with pyrogen (Piromen, 5 mg/kg). Nevertheless, the stressed pups selected significantly warmer positions in a thermal gradient than did saline-injected littermates and thereby raised their body temperatures. Enhanced heat seeking and subsequent fever were also observed in normally fed pups that were incubated at 24 degrees C and had become hypothermic after pyrogen injection. The responses of the pups before they were allowed to thermoregulate behaviorally resemble the types of thermal responses to infection seen in human newborns. The temperature selection of these pups, and others, indicates that pyrogen elevates the set points of newborn rabbits when endothermic fever is attenuated or even absent.