Cross-regulatory interaction between the HPI axis and appetite regulation in Atlantic salmon (Salmo salar) parr under chronic and acute stress

Reduced feed intake is a common stress response in Atlantic salmon (<i>Salmo salar</i>), yet the interaction between the stress-related hypothalamic-pituitary-interrenal (HPI) axis and the appetite-regulating hypothalamic melanocortin system remains poorly understood. This study explored the potential cross-regulatory interaction between these two systems by assessing feed intake, gastrointestinal content, and hypothalamic mRNA expression of <i>crf1</i> paralogs, a key factor in stress handling, and appetite-related genes <i>npy</i>, <i>agrp1</i>, <i>pomc</i>, and <i>cart</i> in Atlantic salmon parr exposed to 21 days of unpredictable chronic stress, followed by a novel acute stressor. Our results indicated that regulation of the HPI axis and appetite-regulatory mechanisms appeared to depend on the type and duration of stress. Chronic stress reduced feed intake and gut content and increased feed conversion ratio, without changes in hypothalamic <i>crf1</i> paralog expression but with reduced orexigenic <i>npya1</i> transcript levels. Simultaneously, levels of the anorexigenic <i>cart2b</i> transcript variant were also reduced, possibly reflecting a compensatory response to prolonged appetite suppression. In contrast, exposure to the novel acute stressor induced a transient increase in <i>crf1</i> paralogs and upregulated orexigenic neuropeptides <i>npya1</i> and <i>npya2</i>, suggesting compensatory regulation to counteract stress-induced anorexia. In contrast, <i>cart2b</i> expression normalized to control levels, possibly reflecting an acute stress-induced compensatory response restoring appetite regulation. These results highlight a dynamic, stress-duration-dependent interaction between stress and appetite-regulatory systems in Atlantic salmon.