Meeting Abstract

76.6  Wednesday, Jan. 6  Neuroendocrine bases of spawning migration in chum salmon URANO, Akihisa; Hokkaido Uiversity, Sapporo, Japan aurano@sci.hokudai.ac.jp

Chum salmon of Japanese stock spend several years to grow and mature in the summer Bering Sea and the winter Gulf of Alaska, and then home their natal rivers over thousands of kilometers for spawning. Since the control of such migratory behavior includes neural and endocrine, or neuroendocrine mechanisms, we tried to accumulate confirmable evidence along two working hypothesis: 1) dual controls by neuroendocrine neurons of the nervous and endocrine systems are crucial, and 2) transition from regulation of growth to that of reproduction is involved in the onset of homing. Information on the latter is particularly important to realize “when, where and how salmon initiate homing or spawning migration”. Since transcription of genes was considered to precede physiological events, we focused to determine the exact amounts of hormonal mRNAs and assessed the changes in magnitudes of gene expression. Liquid nitrogen-frozen brain and pituitary samples of oceanic fish were repeatedly collected through several research cruises. In terms of the first working hypothesis, anatomical and physiological evidence showed importance of the GnRH neuronal system. GnRH neurons multiply project to various brain loci and the neurohypophysis, indicating dual controls of the nervous and endocrine systems. In terms of the second working hypothesis, IGF-I is considered to initiate gonadal development in the winter Gulf of Alaska. Since expression of GnRH gene was also high in fish which started sexual maturation, interaction of the brain GnRH system and the somatic IGF-I system may be important for the control of homing migration.