The following are the functions of GnIH:[3][9][10][11]

• Inhibits gonadotropin (FSH and LH) secretion from the anterior pituitary gland
• Inhibits reproduction and reproductive behavior
• Inhibition of gonadal development
• Regulator of steroidogenesis and gametogenesis
• Decreases LH pulse amplitude
• Involved in pubertal delay due to imbalance in the hypothalamic-pituitary-gonadal axis (HPG) and hypothalamic-pituitary-thyroid (HPT) axes
• Mediates stress-induced reproductive dysfunction

GnIH binds and stimulates the GPR147 receptor to suppress adenylyl cyclase formation which ultimately suppresses gonadotrope function. The inhibitory effects of GnIH on reproduction is mainly accomplished at the hypothalamic-pituitary level. Gonadotropin-releasing hormone (GnRH) neurons, gonadotropes, and the gonads are major targets of GnIH action based on the interaction and distribution of the GnIH receptor.[6][12] Apart from being a negative regulator of reproductive behavior and inhibiting gonadotropins, GnIH also acts on kisspeptins (a group of peptide fragments encoded by the KISS1 gene in humans).[13] Kisspeptins are present in the lateral preoptic area and arcuate nucleus of the hypothalamus and play a role in initiating the preovulatory GnRH/luteinizing hormone (LH) surge, which is crucial for ovulation.[14] Kisspeptins and GnIH are two neuropeptides of the hypothalamus. They have an essential role in the regulation of the reproductive axis. Kisspeptins are stimulators of the reproductive axis, while GnIH is the inhibitory opponent.

Numerous studies have shown the presence of GnIH in several species including non-human primates, vertebrates, mammals, and humans.[15][8] Since its discovery, GnIH has advanced our knowledge of hypothalamic control and regulation of gonadotropes, reproductive physiology, and behavior, by acting on the brain and pituitary gland. Recent evidence indicates that GnIH may be useful in the treatment of endometriosis, uterine fibroids, precocious puberty, breast cancer, benign prostatic hyperplasia, and prostate cancer, and may function as a contraceptive.[7]

GnIH (aka RFamide-related peptide, RFRP) is a recently discovered hypothalamic neuropeptide that regulates reproduction by working on the gonadotropes, GnRH-I and II neurons, and the gonads via the GnIH receptor to exert its functions.[6][16] One main function is decreasing the synthesis and release of LH and FSH to regulate steroidogenesis and gametogenesis.[16]

GnIH is believed to be involved in pubertal delay due to imbalances in the hypothalamic axes (HPT and HPG). The thyroid hormones are regulators of development and growth with an important role in pubertal onset.[10] Mice studies suggest GnIH is involved in keeping the balance of the thyroid-hormone mediated HPG regulation that is vital for proper pubertal development, as GnIH may have a role in cross-talk between HPT and HPG axes.[10]

LH, FSH, and Gonads

GnIH induces a significant decrease in the expression of LH and FSH mRNA, as seen in in-vitro studies of quail birds and chickens.[16] Both in vivo and in vitro studies in birds suggest that GnIH inhibits the synthesis and release of gonadotropins.[16] Studies on sheep also have shown RFRP-3 (GnIH homolog found in humans) is secreted into portal blood to act on pituitary gonadotropes to inhibit GnRH and LH secretion.[6] Since LH normally stimulates synthesis and release of testosterone in Leydig cells, studies on male quails demonstrated GnIH decreased plasma testosterone concentration during development, thus suppressing testicular growth in male quails and inducing apoptosis in spermatocytes, spermatogonia, and Sertoli cells to ultimately decrease the diameter of the seminiferous tubules.[16]

Immunochemistry done on the avian population detected GnIH peptide in the ovary (theca and granulosa cells) and testicle (interstitial, germ, and pseudostratified columnar cells of the epididymis) suggesting the possibility of autocrine/paracrine regulation of gonadal steroid production, germ cell differentiation, and maturation.[6] The hypothesis that GnIH may be an autocrine regulator of gonadal steroid production was supported by a study that demonstrated RFRP-1/3 and GPR147 were expressed in granulosa cells of normal human ovaries. In this study, RFRP-3 was shown to bind to GPR147 and inhibit gonadotropin signaling to ultimately decrease progesterone synthesis.[17]

Human RFRP-3 (human GnIH) has potential as a therapeutic agent for the treatment of the reproductive cycle and hormone-dependent diseases due to its ability to decrease levels of gonadotropins and steroid hormones.[1]

Reproductive Behavior

GnIH may also act in the brain to regulate reproductive behaviors (sexual and aggressive) by controlling the synthesis of neurosteroids. Recent research done on quail suggests that GnIH may affect neurosteroid biosynthesis. GnIH increased neuroestrogen production by stimulation of the cytochrome P450 aromatase through dephosphorylation which subsequently increases aggressive behavior.[4] Neuroestrogen is thought to typically precipitate aggressive behavior unless over a certain threshold. Pass the threshold, neuroestrogen decreases aggressive behavior. It is believed that GnIH raises the neuroestrogen over this threshold to decrease aggression.

In addition, intracerebroventricular (ICV) injection of GnIH in mammals showed reduced sexual behavior in male rats, reduced sexual motivation and vaginal scent marking in hamsters, and altered fos expression in different parts of the hamster brain, which also suggests GnIH has implications in reproductive behavior.[4] This may be due to GnIH’s inhibitory effect on GnRH-II neurons that are responsible for certain sexual behaviors.[6]

Thyroid Hormone-Mediated GnIH Regulation

Thyroid hormones are essential for body growth, brain development, metabolism, and proper function of the reproductive system. Therefore, children with thyroid disorders can experience a delay in pubertal development.

Researchers have conducted molecular studies in an attempt to understand the effect of thyroid hormone on puberty.[4] GnIH neurons in the hypothalamus express thyroid-receptor alpha (TR-alpha) and thyroid-receptor beta (TR-beta). Furthermore, putative thyroid hormone-response elements (TREs) are present in the promoter region of the GnIH gene in mice. Studies have found that thyroid dysfunction alters GnIH expression in the hypothalamus by inducing chromatin modification in the GnIH promoter region in female mice through H3-acetylation and H3K9-trimethylation, which induces and represses expression of GnIH respectively.[4] It is considered that an elevated level of thyroid hormone decreases GnIH expression, whereas a lower level of thyroid hormone increases GnIH expression.[4] It also has been shown that elevated levels of thyrotropin-releasing hormone, a hormone released from the hypothalamus that stimulates the anterior pituitary to synthesize thyroid-stimulating hormone, in hypothyroidism induces hyperprolactinemia and alters GnRH pulsatile secretion, which leads to delayed puberty.[4] In conclusion, the effects of abnormal thyroid hormone levels on puberty may be mediated by GnIH.

Glucocorticoid and Melatonin-Mediated GnIH Modulation

Stress is known to inhibit reproductive function mediated via the hypothalamic GnIH system. Studies on birds and mammals have shown that the inhibitory effect of stress on reproductive function becomes mediated by high concentrations of circulating glucocorticoids acting via the GC receptor (GR) and GC response element (GRE) in the GnIH promoter region.[11][12] The higher the level of stress, the more GC bound GR is recruited to GRE, thus upregulating GnIH expression. This data implies that GnIH may act as a gating system for the effects of stress on the reproductive axis at different times of the year.[12]

Avian and hamster studies suggest that melatonin also modulates both GnIH expression and release.[4][18][19] The exact activity within humans is not known, but quail studies suggest melatonin induces GnIH expression and release, whereas Syrian and Siberian hamsters studies suggest melatonin decreases GnIH expression. These studies demonstrate the photoperiodic regulation of GnIH by a melatonin dependent process that has species-specific differences.[4]