Inhibition of cortisol signaling and development of the hypothalamus-pituitary-interrenal axis (HPI-axis) during early ontogeny of Atlantic salmon (Salmo salar L.)

Abstract

During early development much of the fish’s physiological capacity and fitness is established. Still, our understanding of how the early development (endogenous and external) communicate with and influence embryonic development in fish is limited. This study investigated how signaling of maternal cortisol may influence early embryogenesis, with particular focus on the development of the hypothalamus-pituitary-interrenal (HPI) axis in Atlantic salmon (Salmo salar L.). In this study maternal cortisol signaling was investigated through two approaches: 1. knockdown (KD) of glucocorticoid receptors (GR) using antisense oligonucleotides (GAPmers) and 2. inhibitors to block GRs and mineralocorticoid receptors (MR), both alone and in combination. Expression of genes critical to the HPI-axis: gr1a, gr1b, gr2, pomca and pomcb were measured using quantitative polymerase chain reactions (qPCR) and analyzed for possible effects on the timing of the HPI-axis. Embryo morphology was recorded from photos during important life stages: cell cleavage, mid-blastula, 50 % epiboly, 95 % epiboly, “eyed” stage and at hatching. GAPmer treatment resulted in significant reduction in gr1b expression during cell cleavage indicating successful KD of the GR1b receptor. From 50 % epiboly until hatch GAPmer treated embryos had generally lower gene expression than controls, significantly for gr1a. GAPmer injected embryos showed a delay or stop in development during gastrulation and appeared to be slightly larger in size, but with rounder and smaller yolk sacs at hatching. Embryos treated with inhibitors were smaller but had larger yolk sacs. Overall, gene expression matched the ontology based on other species. Use of GR and MR inhibitors resulted in few consistent effects except for the MR inhibitor which significantly elevated pomca expression from ca. 95 % epiboly until hatch. Results obtained in this study indicate that some important regulators of the HPI-axis may be affected by inhibited cortisol signaling during early development. Cortisol signaling may also influence early embryo morphology through effects of yolk allocation. To clarify if the observed changes result in altered HPI-axis functionality, further research is needed.