Immune challenge induces differential corticosterone and interleukin-6 responsiveness in rats bred for extremes in anxiety-related behavior.

Disturbances in mood such as anxiety and depression are often associated with altered hypothalamo-pituitary-adrenal (HPA) axis reactivity, but also with changes in cytokine production, such as interleukin-6 (IL-6), an essential immune factor produced by macrophages and lymphocytes during inflammatory processes. The reciprocal relationship between the HPA axis and the immune system is now well established. In order to understand better the endocrine reactivity of anxious individuals faced with an immune challenge, a model of innate anxiety-related behavior, HAB and LAB rats (HABs, high and LABs, low anxiety-related behavior) was used in this study. We sought to determine whether injection of lipopolysaccharide (LPS) induced a differential HPA axis reactivity and plasma IL-6 release in HABs and LABs. After LPS injection, the plasma adrenal corticotrophic hormone increase did not differ between HABs and LABs, whereas a larger increase in plasma corticosterone levels occurred in HABs than in LABs at 2 h after injection. Moreover, basal IL-6 levels were lower in HABs than in LABs, leading to a higher IL-6 2 h/basal ratio in HABs. In conclusion, we propose for the first time a link between the endocrine and immune systems of HABs and LABs and suggest that IL-6 could be a neuroendocrine correlate of trait anxiety in HABs.