Stressor‐ and Tissue‐Specific Regulation of the Corticotropin‐Releasing Factor System Across Epithelial Tissues in Rainbow Trout

The corticotropin-releasing factor (CRF) system is a major neural regulator of stress responses in vertebrates. However, stress-related roles for the CRF system in other tissues-and whether these roles vary between stressor types-remain unclear. To address this gap, we first characterized the CRF system in the gills and intestine of rainbow trout (Oncorhynchus mykiss) and then evaluated how it is transcriptionally regulated following either an immune (vaccination) or osmotic (seawater transfer) stressor. Additionally, since the CRF system is involved in food intake regulation, we also evaluated whether feeding state affects the intestinal CRF system. Vaccination against Vibrio anguillarum caused increases in intestinal CRF binding protein transcripts paired with reductions in ligand (crfa2) and receptor (crfr1b) transcripts but did not affect transcript levels of CRF system components in the gills. In contrast, seawater transfer caused the abundance of most CRF system transcripts to increase in the middle (but not posterior) portion of the intestine, while transcript levels of CRF binding proteins and receptors in the gills declined. Finally, levels of CRF system transcripts in the intestine varied with feeding state in a region-specific manner. In the middle intestine, transcript levels of most components declined with fasting and increased when feeding was resumed, whereas the opposite pattern occurred in the posterior intestine. Overall, our results implicate the peripheral CRF system as a stressor- and epithelial tissue-specific modulator of immune and osmoregulatory functions in teleosts.