Prolactin-Releasing Action of a Low Dose of Exogenous Gonadotropin-Releasing Hormone Throughout the Human Menstrual Cycle

Abstract

Pharmacological doses of gonadotropin-releasing hormone (GnRH) are known to induce prolactin (PRL) release in different pathological states. The same effect can be observed in postmenopausal women and during the phases of menstrual cycle characterized by high estrogen levels. With the aim to evaluate whether nonpharmacological doses of GnRH are also able to induce PRL release, gonadotropin and PRL response to a low dose of GnRH (10 µg, i.v. bolus) was evaluated in 70 normal women during different phases of their menstrual cycle. A significant PRL increase was observed in 33% of subjects during the first days of the cycle (menstrual phase; days 1–3 from the beginning of menstrual bleeding; n = 6), in 24% of subjects during early follicular phase (days – 10 to -8 from LH peak; n= 17); in 38% of subjects during midfollicular phase (days – 6 to – 4 from LH peak; n = 8); in 78% of subjects during preovulatory phase (days – 2 to – 1 from LH peak; n = 9); in 67% of subjects during postovulatory phase (days + 1 to +2 from LH peak; n = 6) and in 42% of subjects during midluteal phase (days +5 to +8 from LH peak; n = 24). In brief, the increase of mean PRL levels after GnRH administration was only significant (p < 0.05) during pre- and postovulatory phases. The percentage of patients who showed a PRL response during the different phases of menstrual cycle was significantly correlated to the mean maximal net increase of LH (r = 0.927; p < 0.01) and to the mean maximal net increase of FSH (r = 0.926; p < 0.01). In conclusion, the present data show that in normal cycling women the administration of exogenous GnRH induces PRL release even at low doses. The PRL-releasing effect of GnRH is variable throughout the menstrual cycle and it is maximal during the periovulatory period. The significant correlation between PRL release and gonadotrophs responsiveness to GnRH suggests that a paracrine interaction between gonadotrophs and lactotrophs might exist also in humans.