Molecular Neuroanatomical Bases of Control of Salmonid Spawning Migration

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Gonadotropin-releasing hormone (GnRH) regulates synthesis and release of gonadotropins (GTHs) and other pituitary hormones, such as growth hormone (GH), prolactin (PRL), and somatolactin (SL) in a seasonally different manner. In masu salmon (Oncorhynchus masou), expression of two genes encoding salmon GnRH (sGnRH) precursors increases at prepubertal stages and changes with seasons during sexual maturation. Recently, we demonstrated in masu salmon that five types of GnRH receptor (GnRH-R) genes are expressed in the brain, pituitary and other peripheral tissues in different patterns. In the present study, we examined seasonal expression patterns of sGnRH and GnRH-R genes, in the brain and pituitary during growth and sexual maturation, to elucidate roles of the multiple GnRH-Rs in GnRH action. Moreover, seasonal changes in response to GnRH of GnRH-R genes _were examined by a GnRH analog (GnRHa) implantation experiment. The fish were treated with GnRHa for one week and the brains and pituitaries were taken out. This implantation experiment was repeated every month from immature to spawning periods. The absolute amounts of mRNAs for two sGnRHs and five GnRH-Rs were determined by a quantitative real-time PCR method. In the brain, the amounts of two sGnRH mRNAs tended to be high in winter and spring, and decreased toward summer, and again increased from late summer to spawning. The changes in the amounts of five types of GnRH-R mRNA were similar to that of sGnRHs. In the pituitary, the amounts of five GnRH-R mRNAs showed different changes during immature and maturing stages but increased in pre-spawning periods, in which GnRHa treatment increased the amounts in the females, whereas decreased in the males. The coincidental elevation of expression of sGnRH, GnRH-R and GTH subunit genes in the pre-spawning stage may be important in the regulation of GTH subunit gene expression by GnRH, and further control of upstream migration.