Late feedback effects of hypothalamic-pituitary-adrenal axis hormones in healthy subjects

We tested the hypothesis that hypothalamic-pituitary-adrenal (HPA) axis hormones exert prolonged negative feedback on corticotropin (ACTH) secretion. Ten healthy subjects underwent a standard protocol 4 x and each time received i.v., under double blind conditions and in random order, corticotropin-releasing hormone (CRH) 1 microgram/kg, co-syntropin (ACTH1-24) 0.25 mg, cortisol (hydrocortisone) 15 mg, or placebo. Subjects had a venipuncture for cortisol and ACTH levels at 0900h on Day 1, then had i.v. insertion and cortisol and ACTH levels measured at 1600 and 1855h. The test substance was given at 1900h and cortisol and ACTH levels were monitored until 2300h, when the i.v.'s were discontinued. Subjects then had venipunctures for cortisol and ACTH levels at 0900 and 1600h on Day 2 and 3. Hormones had the expected acute effects. Hormones did not differ from placebo in effects on cortisol levels measured over Days 2 and 3. There were significant differences between test substances in effects on afternoon ACTH levels on Days 2 and 3, with ACTH levels increasing significantly less from baseline to Day 2 and 3 after CRH administration than after placebo, and tending to increase less from baseline to Day 3 after ACTH administration than after placebo. Examination of Day 2 and 3 morning ACTH levels showed a significant interaction between the test substances and time (Day 2 vs. 3), and interpretation of this interaction is not straightforward. We conclude that CRH and possibly ACTH exert late inhibitory effects on ACTH secretion measured in the afternoon of the 2 days following hormone administration.