Differential behavioral, physiological and hormonal sensitivity to LPS challenge in rats.

In animals, systemic administration of lipopolysaccharide (LPS) induces a characteristic set of responses that has been termed sickness behavior. In the present study, the effects of a wide dose range of LPS injections on behavior and physiology were examined in rats. Male rats were injected IP with either saline or LPS (1, 5, 15, 50, 125, or 250 µg/kg). Body temperature, body weight, and home-cage activity were monitored over 24 h after injection. Behaviorally the social interaction and the saccharin preference tests were assessed at 2 and 24 h post-injection, respectively. Further animals were treated with LPS to assess hypothalamic–pituitary adrenal (HPA) axis and immune system responses 2 h post-injection. LPS signifi cantly reduced body weight (1 µg/kg and above), increased ACTH (15 µg/kg and above) and serum corticosterone levels (5 µg/kg and above). In addition LPS raised serum interleukins (5 g/kg and above) and tumor necrosis factor-alpha (1 µg/kg and above). Social behavior and preference for saccharin were signifi cantly decreased at all doses of LPS tested (1–250 µg/kg) while a robust reduction in home-cage activity was observed starting at 15 µg/kg. The present finding suggests that LPS caused profound behavioral alterations and stimulation of the immune system even at very low doses.