Gonadotropin-releasing Hormone (GnRH) Genes in Fish

Abstract

The decapeptide gonadotropin-releasing hormone (GnRH) is a neuropeptide which is synthesized in the brain. The GnRH stimulates the synthesis and secretion of gonadotropic hormone (GtH) which triggers steroidogenesis of gonads. Among vertebrates, the highest number of GnRH type (8 types) were identified in fish. GnRH1, cfGnRH, hgGnRH, mGnRH, mdGnRH, sbGnRH and wfGnRH were found in the preoptic area of a fish brain, while GnRH2 (cGnRH-II) and GnRH3 (sGnRH) were found in the midbrain and telencephalon. Generally two to three types of GnRH were detected in a fish species. A function of GnRH1 involves regulation of the reproductive system, while GnRH2 acts as a neurotransmitter. In a species bearing two types of GnRH, GnRH3 also plays a role in controlling the reproductive system. The bioactive and processing sites of GnRH mRNA showed the highly conserved region. The expression of the GnRH gene was site- and time-specific. It can be concluded that GnRH is involved in the regulation of the hypothalamic-pituitary-gonadal axis of fish. The knowledge on various types and expressions of GnRH are essential for the development of a more efficient GnRHa-induced breeding.