Regulation of gonadotropin secretion: implications for polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is a disorder characterized by hyperandrogenism and chronic anovulation. Although the etiology of PCOS is unknown, perturbations of gonadotropin secretion are one of the hallmarks of this disorder. In normal menstrual physiology, the monotropic rise of plasma follicle-stimulating hormone (FSH) during the luteal-follicular transition is critical for follicular development and subsequent ovulation. One of the mechanisms by which FSH is differentially synthesized involves the luteal slowing of gonadotropin-releasing hormone (GnRH) pulse frequency by ovarian steroids. In PCOS, plasma leutinizing hormone (LH) is commonly increased, FSH is typically in the lower follicular range, and LH (and by inference GnRH) pulse frequency is persistently rapid at approximately one LH pulse per hour. The etiology of the neuroendocrine abnormalities in PCOS remain unclear; however, recent studies have revealed decreased sensitivity of the GnRH pulse generator to inhibition by ovarian steroids, particularly progesterone. This abnormality is reversed by the androgen receptor antagonist flutamide, suggesting that elevated androgen levels may alter the sensitivity of the hypothalamic GnRH pulse generator to steroid inhibition and lead to enhanced LH secretion. As such, women with PCOS require higher levels of progesterone to slow the frequency of GnRH pulse secretion, resulting in inadequate FSH synthesis and persistent LH stimulation of ovarian androgens. The decreased sensitivity of the GnRH pulse generator may help to explain the genesis of PCOS during puberty. In normal early puberty, sleep-entrained increases in LH stimulate ovarian steroids, which subsequently suppress LH frequency and amplitude during the subsequent day. In hyperandrogenemic girls destined to develop PCOS, this nocturnal increase in ovarian steroids may not be adequate to suppress the GnRH pulse generator, leading to a persistently rapid LH pulse frequency, impaired FSH production, and inadequate follicular development.