Attenuating activity of the ovary on LH response to GnRH during the follicular phase of the cycle

Gonadotropin surge-inhibiting/attenuating factors: a review of current evidence, potential applications, and future directions for research

Animal studies in the 1980s suggested the existence of an ovarian hormone, termed gonadotropin surge-inhibiting/attenuating factor (GnSIF/AF), that modulates pituitary secretion of luteinizing hormone (LH). Given the importance of identifying regulatory factors of the hypothalamic-pituitary-ovarian axis and the accumulating data suggesting its existence, we conducted a comprehensive literature search using PubMed, Web of Science, Scopus, and Embase to identify articles related to GnSIF/AF. The search generated 161 publications, of which 97 were included in this study. Several attempts have been made to identify and characterize this hormone and several candidates have been identified, but the protein sequences of these putative GnSIF/AF factors differ widely from one study to another. In addition, while the RF-amide RFRP-3 is known foremost as a neuropeptide, some research supports an ovarian origin for this non-steroidal hormone, thereby suggesting a role for RFRP-3 either as a co-modulator of GnSIF/AF or as a gonadotropin-inhibiting factor in the hypothalamus (GnIH). Discovery of the KNDy neurons that modulate GnRH secretion, on the other hand, further encourages the search for substance(s) that modulate their activity and that indirectly affect LH secretion and the hypothalamic-pituitary-ovarian axis. While it has remained an elusive hormone, GnSIF/AF holds many potential applications for contraception, in vitro fertilization, and/or cancer as well as for understanding polycystic ovary syndrome, metabolic diseases, and/or pubertal development. In this review, we rigorously examine the available evidence regarding the existence of GnSIF/AF, previous attempts at its identification, limitations to its discovery, future directions of research, and potential clinical applications.

Novel aspects of the endocrinology of the menstrual cycle

Ovarian control of gonadotrophin secretion is normally achieved via the feedback mechanisms mediated by oestradiol and progesterone. Evidence has been provided that nonsteroidal substances, such as inhibin A and B, participate in the negative feedback control of FSH secretion. Another nonsteroidal ovarian substance is gonadotrophin surge-attenuating factor (GnSAF), the activity of which is particularly evident in women undergoing ovulation induction. Accumulating evidence has suggested that GnSAF plays a physiological role during the menstrual cycle. In particular, this factor antagonizes the sensitizing effect of oestradiol on the pituitary response to gonadotrophin-releasing hormone during the follicular phase of the cycle. A hypothesis has been developed that, in the late follicular phase, the activity of GnSAF is reduced and this facilitates the sensitizing effect of oestradiol on the pituitary, thus enforcing the massive discharge of gonadotrophins at the midcycle LH surge. The interaction of oestradiol, progesterone and GnSAF on the hypothalamic-pituitary system provides a novel approach to explain the mechanisms which control LH secretion during the normal menstrual cycle. The ovarian control of gonadotrophin secretion during the normal menstrual cycle is achieved via negative and positive feedback mechanisms. The steroids oestradiol and progesterone are the main regulators; however, nonsteroidal substances, such as inhibin A and inhibn B, also participate. Accumulating evidence has demonstrated that another nonsteroidal ovarian substance, gonadotrophin surge-attenuating factor (GnSAF), plays a key role in the control of LH secretion during the follicular phase and at midcycle, providing thus a novel aspect in the ovarian control of gonadotrophin secretion during the human menstrual cycle. The ovarian control of gonadotrophin secretion during the normal menstrual cycle is achieved via negative and positive feedback mechanisms. The steroids oestradiol and progesterone are the main regulators; however, nonsteroidal substances, such as inhibin A and inhibn B, also participate. Accumulating evidence has demonstrated that another nonsteroidal ovarian substance, gonadotrophin surge-attenuating factor (GnSAF), plays a key role in the control of LH secretion during the follicular phase and at midcycle, providing thus a novel aspect in the ovarian control of gonadotrophin secretion during the human menstrual cycle.