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Our previous studies suggested that salmon gonadotropin-releasing hormone (sGnRH) neurons regulate both final maturation and migratory behavior in homing salmonids. Activation of sGnRH neurons can occur during upstream migration. We therefore examined expression of genes encoding the precursors of sGnRH, sGnRH-I, and sGnRH-II, in discrete forebrain loci of prespawning chum salmon, Oncorhynchus keta. Fish were captured from 1997 through 1999 along their homing pathway: coastal areas, a midway of the river, 4 km downstream of the natal hatchery, and the hatchery. Amounts of sGnRH mRNAs in fresh frozen sections including the olfactory bulb (OB), terminal nerve (TN), ventral telencephalon (VT), nucleus preopticus parvocellularis anterioris (PPa), and nucleus preopticus magnocellularis (PM) were determined by quantitative real-time polymerase chain reactions. The amounts of sGnRH-II mRNA were higher than those of sGnRH-I mRNA, while they showed similar changes during upstream migration. In the OB and TN, the amounts of sGnRH mRNAs elevated from the coast to the natal hatchery. In the VT and PPa, they elevated along with the progress of final maturation. Such elevation was also observed in the rostroventral, middle, and dorsocaudal parts of the PM. The amounts of gonadotropin IIbeta and somatolactin mRNAs in the pituitary also increased consistently with the elevation of gene expression for sGnRH. These results, in combination with lines of previous evidence, indicate that sGnRH neurons are activated in almost all the forebrain loci during the last phases of spawning migration, resulting in coordination of final gonadal maturation and migratory behavior to the spawning ground.

Pacific salmon migrate from ocean through the natal river for spawning. Information on expression of genes encoding osmoregulatory hormones and migratory behavior is important for understanding of molecular events that underlie osmoregulation of homing salmon. In the present article, regulation of gene expression for osmoregulatory hormones in pre-spawning salmon was briefly reviewed with special reference to neurohypophysial hormone, vasotocin (VT), and pituitary hormones, growth hormone (GH) and prolactin (PRL).Thereafter, we introduced recent data on migratory behavior from SW to FW environment.In pre-spawning chum salmon, the hypothalamic VT mRNA levels increased in the males, while decreased in the females with loss of salinity tolerance when they were kept in SW.The amounts of GH mRNA in the pituitary decreased during ocean migration prior to entrance into FW. Hypo-osmotic stimulation by SW-to-FW transfer did not significantly affect the amount of PRL mRNA, but it was elevated in both SW and FW environments along with progress in final maturation. Behaviorally, homing chum salmon continued vertical movement between SW and FW layers in the mouth of the natal river for about 12 hours prior to upstream migration. Pre-spawning chum salmon in an aquarium, which allowed fish free access to SW and FW, showed that individuals with the lower plasma testosterone (T) and higher estradiol-17β (E2) levels spent longer time in FW when compared with the SW fish.Taken together, neuroendocrine mechanisms that underlie salt and water homeostasis and migratory behavior from SW to FW may be under the control of the hypothalamus-pituitary -gonadal axis in pre-spawning salmon.