Molecular Basis for Neurohormonal Control of Salmonid Spawning Migration

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Spawning migration of salmon is a typical stereotyped or instinctive behavior which is genetically programmed. However, molecular approaches are rare to reveal what is a genetic program of salmon migration. Aiming clarification of such genetic program as the final goal, we examined molecular bases of neuroendocrine mechanisms for salmon spawning migration. Since spawning migration is reproductive behavior, we hypothesized that preoptic neurons producing gonadotropin-releasing hormone (GnRH) and vasotocin (VT) have a crucial role in coordination of the sensory, motor and motivational function of the central nervous system and the neuroendocrine function of the brain-pituitary-gonadal axis. The coordinated activity of the central nervous system and the neuroendocrine system should be crucial for the success of reproduction, that is, the neural controls of migratory and spawning behavior must be temporally and spacially coincident. Herewith. we determined absolute changes in the amounts of mRNAs encoding neurohormones and pituitary hormones along migratory pathways to elucidate changes in expression of hormonal genes during migration. We further analyzed regulatory mechanisms of expression of hormonal genes in salmonids. The results obtained through the present project actually support our hypothesis mentioned above.