Estradiol, but not testosterone, heightens cortisol-mediated negative feedback on pulsatile ACTH secretion and ACTH approximate entropy in unstressed older men and women.

How sex steroids modulate glucocorticoid feedback on the hypothalamic-pituitary-corticotrope (HPC) unit is controversial in humans. We postulated that testosterone (T) in men and estradiol (E2) in women govern unstressed cortisol-mediated negative feedback on ACTH secretion. To test this hypothesis, 24 men and 24 women age 58 ± 2.4 yr were pretreated with leuprolide and either sex steroid (E2 in women, T in men) or placebo addback. Placebo or ketoconazole (KTCZ) was administered overnight to inhibit adrenal steroidogenesis during overnight 14-h intravenous infusions of saline or cortisol in a continuous versus pulsatile manner to test for feedback differences. ACTH was measured every 10 min during the last 8 h of the infusions. The main outcome measures were mean ACTH concentrations, pulsatile ACTH secretion, and ACTH approximate entropy (ApEn). ACTH concentrations were lower in women than men (P < 0.01), and in women in the E2+ compared with E2- group under both continuous (P = 0.01) and pulsatile (P = 0.006) cortisol feedback, despite higher cortisol binding globulin and lower free cortisol levels in women than men (P < 0.01). In the combined groups, under both modes of cortisol addback, ACTH concentrations, pulsatile ACTH secretion, and ACTH secretory-burst mass correlated negatively and univariately with E2 levels (each P < 0.005). E2 also suppressed ACTH ApEn (process randomness) during continuous cortisol feedback (P = 0.004). T had no univariate effect but was a positive correlate of ACTH when assessed jointly with E2 (negative) under cortisol pulses. In conclusion, sex steroids modulate selective gender-related hypothalamic-pituitary adrenal-axis adaptations to cortisol feedback in unstressed humans.