Opioid receptors are present in the hypothalamus but not detectable in the anterior pituitary of the developing ovine fetus.
Exogenous opioids stimulate adrenocorticotropic hormone (ACTH) release in fetal sheep after day 125 of pregnancy (term 145 days), but not at day 110. To determine if the different response is due to an alteration in opioid-binding sites and to examine sites of opioid action on ACTH release, we established an in vitro binding assay to study changes and characteristics of opioid receptor binding sites in ovine fetal hypothalamus and pituitary during development. [3H]-naloxone was used as the radiolabelled ligand. The binding of [3H]-naloxone to hypothalamic membrane preparations was specific, saturable with respect to [3H]-naloxone concentration, and linear with the hypothalamic membrane protein content. Binding assays were conducted on tissues collected from fetuses at three gestational ages: days 110-115 and 125-130 and term. In all cases, Scatchard analysis revealed a single class of binding sites with high binding affinity (0.9-1.2 nM) which did not differ significantly with gestational ages. The binding capacity increased significantly from 45.4 +/- 2.5 fmol/mg protein at days 110-115 to 76.8 +/- 2.3 fmol/mg at days 125-130, but did not change further in term fetuses. When anterior pituitaries from the three groups of fetuses were processed and analyzed in a similar manner, no detectable binding was found. These results indicate (1) that endogenous opioid peptides may act at the hypothalamus rather than pituitary to regulate hypothalamic-pituitary-adrenal activity in the ovine fetus, and (2) the developmental changes in the binding capacity may contribute, in part, to the altered ACTH response reported in previous in vivo studies.
Yang
K P
KP
Lawson Research Institute, St. Joseph's Hospital, Department of Obstetrics and Gynaecology, London, Ont., Canada.
Challis
J R
JR
D000324:Adrenocorticotropic Hormone / Q000378:metabolism; D000818:Animals; D002462:Cell Membrane / Q000378:metabolism; D004723:Endorphins / Q000494:pharmacology; D005865:Gestational Age; D007031:Hypothalamus / Q000196:embryology* / Q000378:metabolism; D009270:Naloxone / Q000378:metabolism*; D010903:Pituitary Gland, Anterior / Q000196:embryology* / Q000378:metabolism; D011957:Receptors, Opioid / Q000378:metabolism*; D012756:Sheep
D004723:Endorphins; D011957:Receptors, Opioid; D009270:Naloxone; D000324:Adrenocorticotropic Hormone