Biosynthesis in vivo and excretion of cortisol by fish larvae

There is a posthatching rise in levels of endogenous cortisol during the ontogeny of those teleosts studied to date. This is thought to be the result of de novo synthesis of cortisol by the larvae, although there is no direct evidence for this. The present study aimed to demonstrate this process in Asian seabass (Lates calcarifer). Larvae (4 days posthatching) were maintained for up to 12 hours in seawater containing [3H]17 alpha-hydroxyprogesterone. High performance liquid chromatography analysis of extracts of the medium, before and after treatment with glucuronidase, indicates conversion of the precursor to several metabolites. One of these was identified as cortisol on the basis of its isopolarity with authentic standard in thin-layer chromatography, and confirmed by recrystallisation to constant specific activity. Immunohistochemistry on siblings shows that the interrenals are immunoreactive for adrenodoxin (adrenal ferredoxin) and cytochrome P-450(21) (steroid 21-monooxygenase [steroid, hydrogen-donor:oxygen oxidoreductase, 21-hydroxylating]; EC 1.14.99.10), and the pituitary for adrenocorticotrophic hormone. These findings suggest that the pituitary-interrenal axis is functional even at this early stage, and are consistent with the hypothesis that the posthatching rise in endogenous cortisol levels is the result of de novo steroidogenesis.