Fever and motor activity in rats following day and night injections ofStaphylococcus aureuscell walls

Abstract

Body temperature and physical activity are affected by both circadian cycles and pyrogens. We injected intraperitoneally 2.5 × 109cell walls of the gram-positive organism Staphylococcus aureus or sterile saline at three different times in the circadian temperature and activity rhythm of Sprague-Dawley rats. Irrespective of whether pyrogen injections were made when the rats were inactive (injection at 0900), just before the nighttime rise in activity and body temperature (1630), or during high activity (2100), the peak body temperature attained and the time to reach peak temperature were indistinguishable. The fever response, as measured by the thermal-response index, was greatest, however, when body temperature and activity were in the lowest phase. Physical activity was inhibited by night but not day injection of S. aureus. Our results provide the first description of experimental fever resulting from a gram-positive pyrogen in rats and the first time an aspect of sickness behavior (suppressed motor activity) has been associated with fever resulting from simulated gram-positive bacterial infection.