Gonadotropin-releasing hormone (GnRH) in fishes: A review on basic and practical findings

Similar to mammals, neurohormones regulate reproductive process in fish. Gonadotropin-releasing hormone (GnRH) is one of the most important factors controlling reproduction, which causes gametogenesis by regulating the amount and time of LH and FSH secretion. The highest variations in GnRH isoforms in vertebrates is related to teleost. Phylogenetic study classifies GnRH into three forms (GnRH1, GnRH2, GnRH3), and all fishes have at least two forms. In fish that lack GnRH1, GnRH3 plays an alternative role. Unlike mammals, it seems that GnRH is not the main gate of reproduction, at least in zebrafish and medaka. However, farming fishes in captivity leads to a decrease in the level of GnRH and LH, which causes reproductive impairment. Therefore, the synthesis of GnRH analogs that have prolonged stability is important for the aquaculture industry. Over the last decade molecular techniques have been well developed and the role of other neuropeptides on the production of LH hormone have confirmed which raised new challenges of the relationship between novel peptides and GnRH. The present study is centered on the most important applied and fundamental findings on GnRH in fishes.