Interactions between Tumor Necrosis Factor-α, Hypothalamic Corticotropin-Releasing Hormone, and Adrenocorticotropin Secretion in the Rat*

We studied the effects of tumor necrosis factor-alpha (TNF alpha), a macrophage-derived pleiotropic cytokine produced during the inflammatory/immune response, on the function of the hypothalamic-pituitary-adrenal (HPA) axis of the rat. Intravenous injections of TNF alpha stimulated plasma ACTH and corticosterone secretion in a dose-dependent fashion. This effect was inhibited by a rat CRH antiserum that was administered to the rats 1 h before the TNF alpha injections. This suggested that CRH is a major mediator of the HPA axis response to TNF alpha. We subsequently evaluated the ability of TNF alpha to influence CRH and ACTH secretion in vitro by explanted rat hypothalami in organ culture and by dispersed rat anterior pituicytes in primary culture respectively. Hypothalami were incubated for 40 min with graded concentrations of TNF alpha (10 pM to 1 microM). This cytokine stimulated CRH secretion in a dose-dependent fashion, with an EC50 of 6.7 x 10 pM (P less than 0.05). Preincubation of hypothalamic explants with dexamethasone, indomethacin (1 microM), eicosatetraynoic acid (10 microM), or nordihydroguaiaretic acid (30 microM) resulted in inhibition of TNF alpha-stimulated CRH secretion (P less than 0.05). Interestingly, 4-h incubation with TNF alpha had no effect on ACTH secretion from rat anterior pituicytes at a concentration of 10 nM. Higher concentrations of TNF alpha (100 nM and 1 microM), however, elicited a dose-dependent increase in the ACTH concentration in the medium. Our results suggest that TNF alpha represents one of the immune response mediators that directly or via stimulation of other cytokines act as activators of the HPA axis during immune/inflammatory reactions. This effect appears to be glucocorticoid suppressible and eicosanoid mediated. The primary site of action of TNF alpha appears to by the hypothalamic CRH-secreting neuron. Some pituitary and adrenal effects of TNF alpha, however, cannot be excluded.