In vitro evidence that cortisol directly modulates stress-related responses in the skin epidermis of the rainbow trout (Oncorhynchus mykiss Walbaum)

Exposure of fish to stressors leads to multiple changes in the skin epithelium. We investigated the role of the stress hormone cortisol in the control of these changes by exposure of pieces of skin from the rainbow trout Oncorhynchus mykiss with an in vitro tissue culture incubation procedure. The effects of 24 h exposure to 4 cortisol concentrations (0, 50, 500 and 1000 ng/ml) were determined. Numbers of mucous, mitotic and apoptotic cells were quantitatively assessed using immunohistochemical techniques, in situ DNA nick end labelling (TUNEL), as well as conventional light and scanning electron microscopy (SEM). The cortisol receptor blocker mifepristone was used to investigate whether the effects could be attributed to the direct action of the hormone via glucocorticoid receptors. Overall, cortisol had no effect on the mucous cell population at 24 h. Incubation with the receptor blocker reduced the number of mucous cells. Cell proliferation was stimulated by the addition of 50 and 500 ng/ml cortisol, but not at 1000 ng/ml. Incubation with the receptor blocker increased proliferation in the control group (0 ng/ml) only. An increase in apoptosis occurred at 500 and 1000 ng/ml cortisol. This increase was blocked by incubation with the receptor blocker, which resulted in lower numbers of apoptotic cells in all except the 0 ng/ml controls. SEM observations corroborated the quantitative data. The results indicated that the effects of stressors on the fish epidermis mentioned above are mediated by cortisol, with the exception of mucous cell release.