Diurnal variations in lipopolysaccharide-induced sleep, sickness behavior and changes in corticosterone levels in the rat

Inoculation of rats with microorganisms or microbial constituents that acti vate host defense promotes nonrapid eye movement sleep (non-REMS) and suppr esses REMS. In this study, we evaluated circadian influences on the effects of lipopolysaccharide (LPS) on sleep, sickness behavior and plasma cortico sterone levels in the rat. Three sets of experiments were performed. In eac h, the animals were intraperitoneally injected with vehicle for LPS (30 mu g/kg) during 2 consecutive days, at the beginning of either the circadian r est or the activity phase. In experiment 1, sleep-wake behavior and brain t emperature were recorded, and in experiment 2, core body temperature, locom otor activity as well as food a nd water intake. in experiment 3, corticost erone blood levels were measured. The results show that LPS-evoked changes in temperature, sleep and other behavioral parameters depend markedly on th e time of day LPS is administered. However, a direct comparison of the LPS data demonstrates that, except for sleep parameters, the absolute time cour se of the assessed parameters was rather similar between the rest and activ ity phases. These findings suggest that LPS evokes a state characterized by high temperature and low vigilance, which is reached independently of the circadian phase. Copyright (C) 2000 S. Karger AG, Basel.