The involvement of cortisol and corticosteroid receptor types in mediating the effects of a high salt diet on rainbow trout (Oncorhynchus mykiss)

Abstract

Rainbow trout (Oncorhynchus mykiss) are able to migrate between freshwater and seawater environments. These transitions induce remodeling of the gill, which aids in maintaining proper ionic balance in the face of changing salt and water requirements. Specific gill phenotypes for both freshwater and seawater rainbow trout have been observed, and are identifiable based on the locations and quantities of ion transporters and on the specific cell types present. Previous research has shown that internal salt loading is sufficient to induce a seawater gill phenotype in freshwater rainbow trout. The present study hypothesized that the developing phenotype was due to increases in circulating cortisol levels or corticosteroid receptor expression; this correlation was attempted by examining the time course of development of the seawater phenotype, and by examining concomitant changes in two other key osmoregulatory tissues, gut and kidney. A novel finding of the present study is that cortisol may play a role in promoting the development of a seawater phenotype in salt-fed freshwater fish, probably through regulation of corticosteroid receptor abundance rather than through modulation of cortisol levels.