Cortisol and the immune system of juvenile Pacific salmon during stress and development Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/9w032599j

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  • Plasma concentrations of cortisol, a glucocorticoid hormone that is immunosuppressive in mammals, increase during stress and smoltification of salmonids. There was a reduction in the ability of lymphocytes to generate antibody producing cells (APC) in vivo concurrent with increased plasma cortisol titers during smoltification of coho salmon (Oncorhvnchus kisutch). Cortisol implants increased circulating cortisol concentrations, reduced immune response and increased fish mortality to vibriosis. Acute stress of chinook salmon (0. tshawytscha) in laboratory and field trials caused transient increases in plasma cortisol that were concurrent with decreased ability to generate APC in vitro 4 h after stress. However, 24 h after stress cortisol had returned to resting levels and immune response was enhanced. Disease challenges validated the APC assay as an accurate measure of immunocompetence as there was decreased survival at 4 h, but enhanced survival at 24 h after stress. Fish that were acutely stressed or treated with cortisol had transient reductions in number of cells in spleen and peripheral blood and increases in thymus and anterior kidney. Chronically stressed fish responded in a similar manner, except that the number of cells in the anterior kidney was reduced throughout the experiment. High affinity, low capacity glucocorticoid receptors (GR) were identified in gill and whole leukocytes from spleen and anterior kidney. Leukocytes from the anterior kidney had 2- to 3-fold more GR than did those from the spleen. The dissociation constants (Kd) and numbers of GR in leukocytes increased as a result of stress; the Kd of GR in gill of stressed fish also increased, but the number decreased. During smoltification, number and Kd of GR in anterior kidney leukocytes increased and Kd of GR in splenic leukocytes decreased with no change in number. Anterior kidney leukocytes were initially insensitive to cortisol added to cell cultures, but developed sensitivity concurrent with changes in GR. Similar to a model proposed for mammals, I speculate that the functional significance of the immunosuppressive effects of cortisol in salmon may be to downregulate the immune system to avoid autoimmunity during recovery from stress, or as a result of tissue changes associated with smoltification.
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