Stress gives less?: Cortisol effects on E2 induced vitellogenin expression in salmon hepatocytes

Abstract

The Atlantic salmon (Salmo salar) is an important species in the fish farming industry in several countries. As high production volume is wanted, a corresponding survival rate of the fry is required. To ensure the development of the embryo and survival of the fry during the first post-embryo stages, the yolk sac content derived from the mother needs to be sufficient.Reproduction is an energy demanding process securing the future continuation of the genes. This investment is not compatible with the physiological changes caused by stress, as these processes also require a lot of energy. When the internal equilibrium is shifted due to stress, the individual has to choose between reproduction or barren survival. Routines in the breeding process may expose the fish for stress, increasing the plasma cortisol levels, and may therefore interfere with the quality of the eggs stripped from the female breeding stock. In order to explore the direct effect of cortisol on the hepatocytes Vtg mRNA expression, an in vitro model of male salmon hepatocytes were exposed to 1 nM 17-β-estradiol singly, cortisol (10-1000 nM) singly and in combination with 1 nM 17-β-estradiol for 12 and 24 hours, respectively (the exposure of male hepatocytes to E2 induces vitellogenesis giving a condition similar to female hepatocytes).The results showed a decrease in the expression of vitellogenin mRNA at all exposure concentrations to cortisol singly or in combination with 1 nM E2 after 12 hours. After 24 hours the negative effects of the lower exposure concentrations were gone, however, a decline in vitellogenin mRNA was still observed of the higher exposure concentrations. An induction in the expression of Vtg mRNA was observed for the 24 hours incubation with E2 singly.In conclusion, this study indicate that moderate cortisol levels may reduce the expression of Vtg mRNA for more than 12 hours, while high levels keeps the expression low longer (24 hours). Long lasting exposures to stress may thus affect the salmon egg yolk production, and consequently further stress exposure studies of maturing salmon are warranted.