Reproductive neuroendocrinology of mammalian gonadotropin‐inhibitory hormone

BACKGROUND: Gonadotropin‐inhibitory hormone (GnIH) was discovered in the Japanese quail brain in 2000 as a hypothalamic neuropeptide that suppresses luteinizing hormone release from cultured quail anterior pituitary. METHODS: The authors investigated the existence of mammalian orthologous peptides to GnIH and their physiological functions in the following 19 years of research. MAIN FINDINGS: Mammals have orthologous peptide to GnIH, often described RFamide‐related peptide, expressed in the hypothalamus and gonads. Mammalian GnIH may also suppress gonadotropin synthesis and release by suppressing gonadotropin‐releasing hormone (GnRH) synthesis and release in addition to directly suppressing gonadotropin synthesis and release from the pituitary. Mammalian GnIH may also suppress kisspeptin, a stimulator of GnRH, release. Mammalian GnIH is also expressed in the testis and ovary and suppresses gametogenesis and sex steroid production acting in an autocrine/paracrine manner. Thus, mammalian GnIH may act at all levels of the hypothalamic‐pituitary‐gonadal axis to suppress reproduction. GnIH may be involved in the regulation of puberty, estrous or menstrual cycle, seasonal reproduction, and stress responses. CONCLUSION: Studies suggest that mammalian GnIH is an important neuroendocrine suppressor of reproduction in mammals.