SERRANO, X.; GARCIA, M.M.; VEILLETTE, P.A.; SPECKER, J.L.; University of Rhode Island: Cortisol regulation in larval summer flounder (Paralichthys dentatus)

In teleosts, cortisol is primarily produced in the diffuse interrenal tissue and circulating levels are regulated through feedback inhibition on the hypothalamus and pituitary. However, it is not clear when auto-regulation of the hypothalamic-pituitary-interrenal (HPI) axis becomes functional in early development. To examine this, we measured whole-body cortisol concentrations in summer flounder (a marine teleost) at 1, 7, and 21 days after hatching (dah). At each time-point, larvae were immersed in 0.12 µM RU486 (cortisol receptor antagonist), 20 µM dexamethasone (cortisol agonist), or both, 4 hours prior to obtaining tissues for ether-extraction and cortisol measurements by radioimmunoassay. One day after hatching, cortisol concentrations were significantly depressed in larvae treated with RU486, dexamethasone, or both, compared to controls, perhaps due to displacement of endogenous cortisol by exogenous steroid in the yolk-sac. There were no changes in tissue-cortisol among groups at 7 dah. At 21 dah though, RU486 significantly increased tissue cortisol concentrations above controls, while dexamethasone (with or without RU486) decreased tissue cortisol below controls. Thus RU486 likely blocked, and dexamethasone stimulated, feedback inhibition of the HPI axis. Taken together, these results indicate that auto-regulation of the HPI axis by cortisol occurs after the first week after hatching. Development of a feedback mechanism for the HPI axis within 3 weeks after hatching coincides with, and is likely critical for, the increase in whole-body cortisol concentrations that we observe in metamorphosing summer flounder at 4 weeks of age. [Supported by NSF�s Summer Undergraduate Research Fellowships in Oceanography and IBN-0220196.]