Molecular characterization and expression profiles of three GnRH forms in the brain and pituitary of adult chub mackerel (Scomber japonicus) maintained in captivity

Gonadotropin-releasing hormone (GnRH) is a key neuroendocrine peptide involved in the reproduction of fish and other vertebrates. However, characterizing the involvement of GnRH in fish reproduction has been complicated by the discovery of multiple GnRH forms. In the present study, we isolated full-length cDNAs encoding three GnRH forms and analyzed seasonal changes in the concentrations of mRNA in the brain and corresponding peptides in the brain and pituitary, in relation to seasonal gonadal development of chub mackerel (Scomber japonicus). Chub mackerel sbGnRH, cGnRH-II, and sGnRH cDNAs encode 98, 85, and 90 deduced amino acids, respectively. In females, brain sbGnRH mRNA and peptide concentrations were significantly higher only during the post-spawning season (August); however, pituitary peptide concentrations were higher during late vitellogenesis (April) and the post-spawning season, in comparison to immature stage (November). In males, brain sbGnRH mRNA and pituitary peptide concentrations were higher during spermiation (April). No significant differences in cGnRH-II mRNA or peptide concentrations were found in either sex. Furthermore, in females, brain sGnRH mRNA concentrations did not vary significantly; however, corresponding peptide concentrations in the brain and pituitary were higher during late vitellogenesis and the post-spawning season, respectively. In males, only brain sGnRH mRNA concentrations were higher during the post-spawning season, with no significant change in peptide concentrations. This study quantified the seasonal expression changes of three GnRH mRNAs and peptides in both sexes of chub mackerel, and the present results combined with our previous immunocytochemical report indicates that sbGnRH form plays a dominant role in seasonal gonadal development.