Cortisol-induced hepatic vitellogenin mRNA in Oreochromis aureus (Steindachner)

Cortisol triggers rapid but transient transcription of the silent vitellogenin (Vg) gene in male Oreochromis aureus, producing short-lived Vg mRNA. Implantation of cocoa butter containing cortisol at 0.8, 8, 20, and 40 micrograms/g fish body wt induced hepatic Vg mRNA within 1 or 2 hr. Implantation of cortisol at the lowest of these dosages (0.8 micrograms/g body wt) produced a transient surge in serum cortisol within 1 hr. This is equivalent to a 30-fold increase over that of the resting male fish in which the endogenous cortisol levels measured 10.5 +/- 2.39 and 11.4 +/- 3.1 ng/ml, respectively, for fish kept communally or singly. Although sham implantation with cocoa butter also resulted in the elevation of serum cortisol, the rise was much more gradual, only reaching a peak after 6 hr. Thus, parameters like the treatment regime, sampling time intervals after implantation, and the interaction between these two factors can significantly affect the serum cortisol concentrations (P = 0.001). Although implantation of cocoa butter with or without cortisol increased serum cortisol, albeit unsynchronized, Vg mRNA was detected only in the hormone-treated group. The profile of Vg mRNA accumulation appeared to coincide with that of serum cortisol, further suggesting the activating effect of exogenous cortisol on Vg gene. Compared with an earlier study on estradiol-induced Vg gene expression, the action of cortisol was more rapid, but transient. This is reflected in the rapid clearance of the cortisol-induced Vg mRNA and, hence, the lack of plasma Vg protein during cortisol treatment.