Hyperprolactinemia-Induced Precocious Puberty: Studies on the Mechanism(s) by which Prolactin Enhances Ovarian Progesterone Responsiveness to Gonadotropins in Prepubertal Rats*

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Abstract
Hyperprolactinemia induced in immature female rats by treatment with sulpiride, a dopaminergic receptor blocker, increased the in vitro release of ovarian progesterone (P) in response to different doses of both highly purified hCG and human FSH. The increased P response to gonadotropins was also observed in ovaries of animals injected with ovine PRL or in rats in which the hyperprolactinemic condition was induced by pimozide, a more typical dopaminergic receptor blocker. In addition, the pimozide treatment advanced the time of puberty in a manner similar to that previously observed with sulpiride. In other experiments in which only the ovarian response to hCG, but not that to FSH, was evaluated, it was found that the in vivo treatment of hypophysectomized immature rats with sulpiride did not modify the almost undetectable serum PRL levels of these animals and failed to increase the in vitro ovarian P response to hCG. By contrast, sc injection of PRL to hypophysectomized rats clearly en anced the in vitro release of P in response to the gonadotropin. Adrenalectomy of otherwise intact rats significantly decreased the in vitro ovarian P response to hCG and blunted the increase in the P response induced by hyperprolactinemia. These effects, however, were almost completely reversed by concomitant corticosterone replacement therapy. The ovarian content of hCG receptor in normal rats was found to increase during juvenile development (days 22–31). Hyperprolactinemic animals showed a greater ovarian hCG receptor content than age-matched 31-day-old controls. In contrast, the FSH receptor contents were similar in both groups. The increase in hCG receptor content induced by hyperprolactinemia was even more clearly manifested in isolated granulosa cells, but, as in the case of the whole ovaries, the FSH receptor content in these cells remained essentially the same in hyperprolactinemic and control rats. The results indicate that the enhanced ovarian P response to hCG induced by PRL in prepubertal rats is, at least in part, mediated by an increase in the LH receptor content of the granulosa cells of the developing follicle. It also appears that the sensitizing effect of PRL on the prepubertal ovarian P response to gonadotropins is modulated by the adrenal gland through an effect exerted by corticosterone. The mechanisms by which PRL enhances the ovarian P response to FSH do not appear to involve changes in FSH receptors. However, the occurrence of enlarged uteri in hyperprolactinemic rats suggests that the PRL-induced increase in the P response to FSH may be related to the presence of more mature, estrogensecreting follicles resulting from the hyperprolactinemic condition.