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 micrograms) was administered to block mu-sites, ICI 154,129 (5, 10 or 25 micrograms) blocked delta-sites and nor-binaltorphimine (norBNI, 8 micrograms) 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.