Gonadotropin-releasing hormone (GnRH) in fishes: structural and functional diversity

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 amount of GnRH isoforms in vertebrates is related to teleost. According to the phylogeny tree, GnRH is classified 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 ornamental fish (zebrafish and medaka). However, farming fishes in closed environments such as farms 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.