Gonadotropin-releasing hormone and gonadotropin in goldfish and masu salmon

Reproductive activities in vertebrates are regulated by an endocrine system, consisting of the brain-pituitary-gonad axis. In teleosts, gonadotropin-releasing hormone (GnRH) in the brain stimulates gonadotropin (GTH) release in the pituitary gland, but because of lack of the portal vessel, it is not known when and how much GnRH is released for the regulation of GTH release. There are multiple molecular types of GnRH in teleosts and several distinct populations of GnRH neurons in the brain. However, we do not know which types and populations of GnRH neurons regulate reproductive activities. Here we summarize our recent studies on GnRH and GTH in masu salmon Oncorhynchus masou and goldfish Carassius auratus. Immunocytochemistry showed the location and molecular types of GnRH neurons. Salmon (sGnRH) and chicken-II GnRH (cGnRH-II) neuronal fibers were widely distributed in the brain of both masu salmon and goldfish. Only sGnRH fibers were observed in the pituitary of masu salmon, whereas both sGnRH and cGnRH-II fibers were observed in the goldfish pituitary, indicating that species specific GnRH profiles are involved in the regulation of pituitary function in teleosts. A series of experiments in masu salmon and goldfish suggest that among GnRH neuron populations GnRH neurons in the ventral telencephalon and the hypothalamus regulate GTH release, and that GnRH of the terminal nerve origin is not essential to gonadal maturation and ovulation. The biological function of other GnRH neurons remains unkown. Two GTHs appear to be characteristic of teleost; however, regulation of reproduction by these GTHs is a question that remains to be elucidated. In salmonid species, it is proposed that GTH I stimulates early gonadal development, whereas GTH II acts in later stages. When GTH expression was examined in goldfish, both GTH I and II mRNA levels in the pituitary gland showed increases in accordance with gonadal development, unlike the sequential expression of GTH subunits in salmonids. The expression of these GTH subunit mRNAs were affected by water temperature, starvation, and steroid hormones in goldfish, but in what manner these two GTHs regulate gonadal development remains to be clarified.