MULTIPLE OPIATE RECEPTOR SUBTYPES AVE INVOLVED IN THE STIMULATION OF GROWTH-HORMONE RELEASE BY BETA-ENDORPHIN IN FEMALE RATS

The growth hormone (GH) secretory response to beta-endorphin and the involvement of opiate receptor subtypes in this response were determined in diestrous female rats. The involvement of the mu (mu), delta (delta) and/or kappa (kappa) site was determined by administering specific antagonists for each of these sites prior to beta-endorphin. beta-Funaltrexamine (1 or 5 mu g) was administered to block mu sites, ICI 154,129 (5 or 25 mu g) blocked delta sites and nor-binaltorphimine (8 mu g) blocked kappa sites. The ability of these antagonists to block GH secretion following intravenous morphine administration was also determined. The opiate antagonists and beta-endorphin were administered into the lateral ventricle. A dose-response study for beta-endorphin indicated that 0.5 mu g beta-endorphin was the minimum stimulatory dose for GH release, producing an approximately 4-fold increase in circulating levels of GH; lower doses of beta-endorphin did not stimulate secretion. Ah three antagonists were capable of blocking the stimulatory effects of beta-endorphin. These results provide evidence that all three opiate receptor subtypes are involved in the stimulatory effect of beta-endorphin on GH release.