Response of rainbow trout mucosal tissues under repeated hypoxia episodes

Hypoxic events are common in both natural and aquaculture environments due to external factors or anthropogenic activities. To cope with this fluctuating environment, fish activate complex systemic stress responses, but less is known about their mucosal responses. To investigate this, five experimental groups were established: absolute control (AC), manipulated control (MC), and three groups exposed to a single hypoxic shock (H1) or repeated hypoxic shocks (H2 and H3), sampled at 1, 6, and 24 h after hypoxia. Mucus cortisol levels increased 1 h after manipulation and hypoxic shocks, confirming that the stress response was activated. However, cortisol levels decreased as the fish underwent more shocks. A significant up-regulation of the crh gene by treatment was observed in the gills, while no expression was detected in the intestine. Additionally, an up-regulation of gr1 and mr gene expression was found in the gills, whereas the skin and intestine showed an opposite response, with gene expression returning to control levels after the third hypoxic shock (H3). Hif-1α and myoglobin gene expression showed clear tissue-specific regulation, likely related to their primary functions. In the skin and intestine, as hif-1α levels increased, myoglobin expression decreased, while in the gills, the opposite pattern was observed, with higher myoglobin expression during the reoxygenation phase. Overall, the results suggest that repeated hypoxia promotes tolerance and a certain degree of habituation in rainbow trout, without compromising their survival.