| Research Abstract |
Some salmonid fish reproduce once, and die soon after spawning, while others spawn several times. We used land-locked coho salmon Oncorhynchus kisutch and land-locked steelhead trout Oncorhynchus mykis for the model of these two types, respectively, and investigated the plasma cortisol levels, total reucocyte numbers, and plasma immunoglobulin M (IgM) concentration during sexual maturation. In the coho salmon, plasma cortisol concentration was less than 80 ng/mL in immature fish, but it drastically increased to 400-500 ng/mL after ovulation. In these ovulated fish, plasma leucocyte number, as well as plasma IgM concentration, significantly decreased compared with those of immature fish. In contrast, steelhead trout represented no significant increase in cortisol level, reucocyte number, or IgM concentration during sexual maturation. These findings suggest that in coho salmon, disorders in corticosteroid feedback mechanisms after ovulation lead to hyper corticalism, which cause immunodeficiency, then, death of the salmon . We also obtained the evidence that some β-amyloid proteins, which are thought to be concemed in brain aging in mammals, are responsible for this feedback disorder.
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