Glucocorticoids (GCs) are released in response to immune activation by the bacterial endotoxin, lipopolysaccharide (LPS). However, GC secretion in response to immune activation and other stressors is attenuated at term of pregnancy. GCs are important modulators of the immune response, and both pro- and anti-inflammatory effects are described. Here, we examined whether GC secretion in response to LPS is maintained in earlier pregnancy before term, and investigated the role of endogenous GCs in

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... dulating LPS-induced circulating cytokines, interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α), in pregnant compared to non-pregnant female rats. Plasma corticosterone (Cort) and ACTH responses to LPS were well maintained in pregnant rats at embryonic days 15/16 (E15/16) and E18/19 compared to non-pregnant rats. At E19, maternal LPS administration increased fetal plasma Cort and decreased testosterone in male fetuses. In non-pregnant animals, pretreatment with the GC synthesis inhibitor, metyrapone, inhibited the LPS-induced increase in IL-6, and the IL-6 response was restored by Cort replacement, indicating that LPS induction of IL-6 is Cort-dependent. In E15 pregnant animals, metyrapone had no effect on LPS-induced IL-6 levels, indicating that LPS-induction of IL-6 is not dependent on Cort. These contrasting patterns of IL-6 induction in non-pregnant and pregnant animals were reflected in levels of hypothalamic Socs3 mRNA, an indicator of IL-6 signaling pathway activation. In both non-pregnant and pregnant rats, LPS-induced plasma TNF-α responses were inhibited by metyrapone but not re-instated by Cort replacement. It is suggested that altered GC regulation of IL-6 may be required to sustain specialized functions of IL-6 during pregnancy.