Ontogeny of the corticotropin-releasing factor system in rainbow trout and differential effects of hypoxia on the endocrine and cellular stress responses during development

Abstract

To further our understanding of the development of the stress axis and the responsiveness of embryonic and larval fish to environmental stressors, this study examined the ontogeny of whole-body cortisol levels and of the corticotropin-releasing factor (CRF) system in rainbow trout, as well as the endocrine and cellular stress responses to hypoxia. After depletion of a maternal deposit, de novo synthesis of cortisol increased exponentially between the ‘eyed’ stage and first feeding. Whole body CRF mRNA levels dominated over those of the related peptide urotensin I (UI) from hatch through complete yolk sac absorption. The mRNA levels of CRF-binding protein (CRF-BP) closely paralleled those of CRF and UI throughout ontogeny except at first feeding when an increase in CRF gene expression was not matched by change in CRF-BP transcript abundance. In the hypoxia challenge, fish were exposed to 15% O2 saturation for either 90min or 24h at three key developmental stages: hatch, swim up and first feeding. While the embryos were unaffected, chronic hypoxia elicited a transient 2-fold increase in whole-body cortisol levels in the larval stages. The hypoxia challenge also generally suppressed the mRNA levels of CRF and CRF-BP, had no effect on the expression of UI, but had a marked stimulatory effect on heat shock protein 70 (Hsp70) gene expression. Taken together, these results suggest a role for the CRF system in the ontogenic regulation of corticosteroidogenesis and show that hypoxia has developmental stage-specific effects on the endocrine and cellular stress responses in rainbow trout.