Effects of prior stress on LPS-induced cytokine and sickness responses.

It has recently been reported that exposure to inescapable tailshock (IS) enhances the release of proinflammatory cytokines following bacterial challenge. However, it is not known whether the level of potentiation of proinflammatory cytokines is sufficient to exaggerate any of the physiological processes that are regulated by these cytokines. Thus, LPS was administered and fever, activity, hypothalamic-pituitary-adrenal (HPA) responses, and proinflammatory cytokine release were assessed during both the light and dark phases of the light cycle following IS. Exposure to IS resulted in elevated basal core body temperature during the light phase but not the dark phase and decreased activity during the dark phase but not the light phase. IS animals had significantly greater fever, corticosterone, and ACTH responses following LPS during both the light and dark phases, whereas enhanced proinflammatory cytokine responses were only observed during the light phase. These data suggest that enhanced proinflammatory cytokine responses are not necessary to observe enhanced HPA or fever responses.