CORTISOL-INDUCED HEPATIC VITELLOGENIN MESSENGER-RNA IN OREOCHROMIS-AUREUS (STEINDACHNER)

Cortisol triggers rapid but transient transcription of the silent vite llogenin (Vg) gene in male Oreochromis aut eus, producing short-lived Vg mRNA. Implantation of cocoa butter containing cortisol at 0.8, 8, 2 0, and 40 mu g/g fish body wt induced hepatic Vg mRNA within 1 or 2 hr . Implantation of cortisol at the lowest of these dosages (0.8 mu g/g body wt) produced a transient surge in serum cortisol within 1 hr. Thi s is equivalent to a 30-fold increase over that of the resting male fi sh 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 elev ation of serum cortisol, the rise was much more gradual, only reaching a peak after 6 hr. Thus, parameters like the treatment regime, sampli ng time intervals after implantation, and the interaction between thes e two factors can significantly affect the serum cortisol concentratio ns (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 acc umulation appeared to coincide with that of serum cortisol, further su ggesting the activating effect of exogenous cortisol on Vg gene. Compa red with an earlier study on estradiol-induced Vg gene expression, the action of cortisol was more rapid, but transient. This is reflected i n the rapid clearance of the cortisol-induced Vg mRNA and, hence, the lack of plasma Vg protein during cortisol treatment. (C) 1994 Academic Press, Inc.