In the Mouse, the Activation of the Hypothalamic Pituitary-Adrenal Axis by a Lipopolysaccharide (Endotoxin) Is Mediated through Interleukin-1*

The mechanisms through which endotoxin stimulates the hypothalamic-pituitary-adrenal axis are not well understood. In the studies reported here we tested the hypothesis that endotoxin increases plasma ACTH levels by releasing interleukin-1 (Il-1). Two experimental tools reported to interfere with the biological activity of IL-1 were used: antibodies directed against IL-1 receptors and alpha MSH. In a first series of experiments, adult male mice were injected with a lipopolysaccharide (LPS; 25 micrograms), antibodies against IL-1 receptor, alpha MSH (1-30 micrograms), or LPS and either IL-1 antibodies or alpha MSH. All treatments were administered ip. The endotoxin LPS caused a marked increase in plasma ACTH levels, measured 6 h later. Both alpha MSH and the Il-1 receptor antibodies, while having no effect by themselves, significantly (P less than or equal to 0.01) blocked LPS-induced ACTH release. In a second series of experiments, mice were injected ip with 500 ng recombinant human (rh) Il-1 alpha or 100 ng rhIl-1 beta in the presence or absence of alpha MSH (1-30 micrograms, ip). While not altering ACTH secretion induced by rhIl-1 alpha, 10-30 micrograms alpha MSH significantly (P less than or equal to 0.01) interfered with the effect of rhIl-1 beta. These results suggest 1) that endotoxin activates the hypothalamic-pituitary-adrenal axis through a mechanism involving the activation of interleukin-1 receptors; and 2) that the effect of rhIl-1 beta, but not -alpha, on ACTH secretion by the mouse can be partially blocked by alpha MSH.