Ovarian influence on gonadotropin and prolactin release in mated rabbits

In 17beta-estradiol (E)-treated ovariectomized (OVX) rabbits, the coitus-induced luteinizing hormone (LH) surge is only one fourth that in ovarian-intact rabbits. In this study, we determined the pattern of the coitus-induced gonadotropin release, i.e., LH and follicle-stimulating hormone (FSH), in OVX + E animals without or with continuous 3-wk treatment of 20-alphahydroxypregn-4-en-3-one (20alphaP). For positive and negative experimental controls, ovarian-intact rabbits were either mated or sham mated, respectively. The pituitary hormones prolactin (PRL) and growth hormone (GH) were measured to serve as collateral controls for gonadotropins. The addition of continuous 20alphaP in OVX + E does fail to stimulate a coitus-induced LH surge equal in magnitude and duration to the LH surge in ovarian-intact rabbits. Postcoital levels of FSH were greater in OVX + E + 20alphaP animals than those in OVX + E rabbits. Coitus induced a PRL surge in ovarian-intact and OVX + steroid-treated females, but not in mated males, thereby suggesting a gender difference in this neuroendocrine circuit. Neither coitus nor steroids altered plasma GH values in female or male animals. We conclude that chronic administration of neither E nor E + 20alphaP can restore full-scale gonadotropin surges in OVX rabbits, whereas replacement of one or both of these steroids is sufficient for a coitus-induced PRL surge. Moreover, the presented observation that activin stimulates hypothalamic gonadotropin-releasing hormone (GnRH) release suggests a possible involvement of ovarian proteins in the production of a full-scale coitus-induced GnRH/LH surge.