Exogenous opioid regulation of the hypothalamic-pituitary-adrenal axis in fetal sheep

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Abstract
To test the hypothesis that endogenous opioids participate in the regulation of the ontogenic development of the hypothalamic-pituitary-adrenal axis in fetal sheep, we measured changes in immunoreactive (ir) ACTH and cortisol concentrations in response to bolus injections of either the [Met]-enkephalin analogue, [d-Ala2,N-Phe4,Met(0)ol5]-enkephalin (FK 33-824; 25 μg), the opioid antagonist naloxone (1 mg), a combination of both, or saline vehicle, administered to chronically catheterized fetal sheep through late gestation. There were no effects of either FK 33-824, naloxone or saline on the release of ir-ACTH and cortisol at the earliest stage of gestation studied (days 110–115). By days 125–130, FK 33-824 caused a rapid but short-lived (30 min) increase in plasma ir-ACTH (P J. Endocr. (1988) 119, 389–395