The Involvement of Gonadotropin-Inhibitory Hormone (GnIH) in Fish Reproduction

Gonadotropin-inhibitory hormone (GnIH) is a hypothalamic neuropeptide that was discovered from the brain of Japanese quail. GnIH belongs to the RFamide peptide family having LPXRFamide (X = L or Q) sequence at C-terminal. GnIH has an inhibitory effect on the reproductive axis in birds and most of the mammals, but in fishes, it showed both inhibitory and stimulatory effects depending on species. Even in single fish species, it can have different impacts during different breeding seasons. GnIH also showed involvement in other functions in fishes affecting growth, stress, and behavior. All these functions are mediated via G protein-coupled GnIH receptors, GPR147. Fishes even with more than half of the known vertebrate species, the study about GnIH, its physiological effect are very limited, and even results obtained in some studies are conflicting. So in this chapter, we summarize the available information about the GnIH and its distribution, evolutionary origin, ontogeny, interaction with other molecules of the HPG axis, and physiological effects in fishes. With GnIH, as a fascinating molecule of the HPG axis having different types of impacts, further study is required in different groups of fishes.