OPIATE RECEPTOR SUBTYPE INVOLVEMENT IN THE STIMULATION OF PROLACTIN-RELEASE BY BETA-ENDORPHIN IN FEMALE RATS

The prolactin secretory response to beta-endorphin and the involvement of opiate receptor subtypes in this response was determined in both diestrous and postpartum, lactating female rats. The involvement of the mu-, delta- and/or kappa-site was determined by administering specific antagonists for each of these sites prior to beta-endorphin. Beta-Funaltrexamine (beta-FNA, 1 or 5 mug) was administered to block mu-sites, ICI 154,129 (5, 10 or 25 mug) blocked delta-sites and nor-binaltorphimine (norBNI, 8 mug) blocked kappa-sites. The ability of beta-FNA and ICI 154, 129 to block prolactin secretion following morphine administration was also determined. A dose response study for beta-endorphin indicated that beta-endorphin, at doses as low as 25 ng, was a potent stimulus for prolactin release producing an increase in prolactin that mimicked the suckling-induced prolactin increase. In addition, all three antagonists were capable of antagonizing the stimulatory effect of beta-endorphin in both diestrous and postpartum female rats. These results indicate that beta-endorphin is a potent stimulus for prolactin secretion and that these three opiate receptor subtypes interact to produce its stimulatory effect on prolactin release.