Abstract

The present study tested the hypothesis that the cytokine tumor necrosis factor-α (TNF-α) is an important CNS mediator of the hypothalamo-pituitary-adrenal (HPA) axis response to local inflammation in the rat. Recombinant murine TNF-α administered directly into the cerebroventricles of normal rats produced a dose-dependent increase in plasma adrenocorticotropin (ACTH) concentration. Local inflammation induced by the intramuscular injection of turpentine (50 μl/100 gm body weight) also produced an increase in plasma ACTH, peaking at 160–200 pg/ml at 7.5 hr after injection (compared with 10–30 pg/ml in controls). Intracerebroventricular pretreatment with either 5 μl of anti-TNF-α antiserum or 1–50 μg of soluble TNF receptor construct (rhTNFR:Fc) reduced the peak of the ACTH response to local inflammation by 62–72%. In contrast, intravenous treatment with the same doses of anti-TNF-α or rhTNFR:Fc had no significant effect on the ACTH response to local inflammation. Although these data indicated an action of TNF-α specifically within the brain, no increase in brain TNF-α protein (measured by bioassay) or mRNA (assessed using either in situ hybridization histochemical or semi-quantitative RT-PCR procedures) was demonstrable during the onset or peak of HPA activation produced by local inflammation. Furthermore, increased passage of TNF-α from blood to brain seems unlikely, because inflammation did not affect plasma TNF-α biological activity. Collectively these data demonstrate that TNF-α action within the brain is critical to the elaboration of the HPA axis response to local inflammation in the rat, but they indicate that increases in cerebral TNF-α synthesis are not a necessary accompaniment.