ONTOGENY OF THE CORTISOL STRESS-RESPONSE IN LARVAL RAINBOW-TROUT

The ontogeny of the interrenal stress response in rainbow trout was ch aracterized by measuring resting and acute-stress-induced changes in w hole-body cortisol levels in embryos and larvae at different early dev elopmental stages. In Experiment 1, resting cortisol levels averaged 6 .0 ng/g in newly fertilized eggs, fell to less than 0.3 ng/g by the ti me of hatching at Week 4 (incubation at 10 degrees), and increased to 1.4 ng/g by Week 5. Cortisol levels did not change in response to acut e stress in 3-, 4-, or 5-week-old fish. In Experiment 2, resting corti sol averaged 1.4 ng/g in newly fertilized eggs, fell to less than 0.03 ng/g by Week 2, and then steadily increased between Weeks 3 and 6 to a peak of 4.8 ng/g before falling to 1.2 ng/g by Week 7. Cortisol leve ls did not change in response to acute stress in 3-, 4-, or 5-week-old fish. Six-week-old fish showed a 2.3-fold increase in cortisol levels at 1 hr poststress, indicating that the hypothalamic-pituitary-interr enal axis first develops responsiveness to stress 2 weeks after hatchi ng and 1 week before the onset of exogenous feeding. The stress hypore sponsive period after hatching in rainbow trout may be homologous to t he 2-week stress hyporesponsive period after birth in rodents, the fun ction of which may be to maintain low, constant corticosteroid levels during a critical developmental period when these steroids can have pe rmanent effects on neural organization. As suggested for mammals, this period may be a time when rainbow trout are particularly vulnerable t o environmental effects on their subsequent development. (C) 1995 Acad emic Press, Inc.