Expression of corticoid‐regulatory genes in the gills of Atlantic salmon (<scp><i>Salmo salar</i></scp>) parr and smolt and during salinity acclimation

In teleost fishes, cortisol is the major corticoid and has both glucocorticoid and mineralocorticoid actions. However, how fish tissues discriminate between these distinct corticosteroid actions is unclear. In mammals, the major factors responsible for intracellular corticosteroid regulation are glucocorticoid receptors (grs) and the mineralocorticoid receptor (mr), but their role in osmoregulation of fish is unclear. 11β-hydroxysteroid dehydrogenases (hsd11bs) control the levels of intracellular corticosteroids by converting from bioactive forms to inert forms. To investigate how Atlantic salmon (Salmo salar) respond to cortisol in different physiological or environmental conditions, we performed comparisons of parr and smolt, and osmotic challenge experiments to examine the physiological responses and gill transcript levels of genes underlying cortisol-signalling, including gr1, gr2, mr, hsd11b2 and hsd11b3. Because cortisol may interact with growth hormone and prolactin during salinity changes, transcript levels encoding growth hormone receptors (ghr1, ghr2) and the prolactin receptor (prlr) were also examined. Hsd11b2 transcript levels in seawater-acclimated fish were consistently lower compared to fish acclimated to fresh water. After transfer to seawater, prlr transcript levels in fish significantly decreased and transcript levels of ghr1, ghr2 and hsd11b3 showed no change or were slightly higher than those of freshwater control groups. Gr1, gr2 and mr transcript levels were slightly but consistently higher in fish acclimated to fresh water relative to seawater. Our results indicate that changes in corticosteroid receptor and hsd11b2 transcript levels in the gills may be important mechanisms that regulate corticoid signals to achieve ion homeostasis in Atlantic salmon.