Osmoregulatory contributions of the corticotropin-releasing factor system in the intestine of Atlantic salmon

The ability of euryhaline fishes to tolerate different environmental salinities depends upon the flexibility of their osmoregulatory organs, including the intestine. Several endocrine pathways contribute to the coordination of osmoregulatory processes in the teleost intestine; however, while the corticotropin-releasing factor (CRF) system has established osmoregulatory actions in the mammalian intestine, it is unclear whether the intestinal CRF system serves similar functions in teleosts. Therefore, we sought to determine whether the CRF system contributes to osmoregulatory processes in the intestine of Atlantic salmon (Salmo salar). We first showed using in vitro sac preparations that activation of CRF receptor type 2 (CRFR2) in the middle and posterior regions of the intestine reduces water, Na⁺, and Cl⁻ absorption. However, co-activation of CRFR1 and CRFR2 inhibited water and Na⁺ absorption without affecting net Cl⁻ absorption. We then assessed how the CRF system in the middle and posterior regions of the intestine was transcriptionally regulated during the seasonal acquisition of seawater tolerance (i.e., smoltification) and following changes in environmental salinity. Smolts had higher transcript levels of CRF ligands than parr and this difference persisted following seawater transfer. Additionally, seawater transfer caused transient increases in transcription of urocortin 2 (UCN2) and CRFR2 (posterior intestine only). Similar increases in UCN2 and CRFR2 were observed following seawater to freshwater transfer of post-smolts. Our results indicate that the intestinal CRF system of Atlantic salmon contributes to osmoregulation during the initial days following changes in environmental salinity and that osmoregulatory actions of the intestinal CRF system are conserved across vertebrates.