Interleukin (IL)-18 is a proinflammatory cytokine and a stimulator of cell-mediated immune responses. We have previously reported that acute stress stimulates the production of IL-18 mRNA in the glucocorticoid (GC)-producing cells of the adrenal cortex. In order to investigate the mechanisms governing the expression of IL-18 in the adrenal cortex, the effects of acute ACTH or chronic corticosterone treatment on the levels of IL-18 mRNA and protein were examined by in situ hybridization and Northern and Western blot assays. Adult male Sprague-Dawley rats received a subcutaneous injection of ACTH or subcutaneous implantation of slow-release corticosterone pellets followed by an injection of saline or ACTH. After 4 h, ACTH induced a 4-fold increase in IL-18 mRNA levels and elevated the content of pro-IL-18 peptide. Six days of chronic corticosterone treatment did not alter the basal levels of IL-18 mRNA and reduced those of pro-IL-18. Finally, ACTH treatment of animals under the corticosterone regimen induced a 2-fold increase in IL-18 mRNA and elevated the levels of the pro-IL-18 protein. The levels of the precursor, p45, and the active subunit p10 peptides of the IL-18-processing enzyme, IL-1β-converting enzyme (ICE), showed no substantial differences in all the conditions tested. IL-1β was not detected under these experimental conditions. These data demonstrate that the production of IL-18 in the adrenal cortex is stimulated by ACTH treatment and is not inhibited by the direct action of corticosterone. In contrast to the anti-inflammatory action of GCs, IL-18 may have an immunostimulatory role during acute stress.

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