Dynamics of cortisol receptor activity in the gills of the brook trout, Salvelinus fontinalis, during seawater adaptation.

Injection of cortisol into freshwater-adapted brook trout caused a rapid increase in plasma cortisol and a significant drop in plasma osmolality, thyroxine (T4), and chloride concentrations, but a nonsignificant drop in plasma triiodothyronine (T3). Concomitantly, cortisol cytosolic receptor activity decreased and cortisol nuclear extract receptor activity increased significantly. Transfer of brook trout into 30% seawater evoked a significant rise in plasma cortisol, osmotic, and chloride concentrations and a significant drop in plasma T4 and T3. Cortisol cytosol receptor activity rapidly dropped following entry into seawater, and cortisol nuclear extract receptor activity rose significantly in a manner similar to the effects of the injection of cortisol. Although there was a lack of correlation between the cytosol cortisol concentration and the cytosol receptor concentration, a significant and positive correlation occurred between the nuclear extract cortisol concentration and the nuclear extract cortisol receptor activity. This positive correlation may reflect an important role for the putative nuclear receptor. These results support the hypothesis that cortisol has an important role in the adaptation of brook trout to the marine environment.