Background: The communication between the brain and the immune system is a cornerstone in animal physiology. This interaction is mediated by immune factors acting in both health and pathogenesis, but it is unclear how these systems molecularly and mechanistically communicate under changing environmental conditions. Behavioural fever is a well-conserved immune response that promotes dramatic changes in gene expression patterns during ectotherms’ thermoregulatory adaptation, including those orchestrating inflammation. However, the molecular regulators activating the inflammatory reflex in ectotherms remain unidentified. Methods: We revisited behavioural fever by providing groups of fish a thermal gradient environment during infection. Our novel experimental setup created temperature ranges in which fish freely moved between different thermal gradients: (1) wide thermoregulatory range; T° = 6.4 °C; and (2) restricted thermoregulatory range; T° = 1.4 °C. The fish behaviour was investigated during 5-days post-viral infection. Blood, spleen, and brain samples were collected to determine plasmatic pro- and anti-inflammatory cytokine levels. To characterize genes’ functioning during behavioural fever, we performed a transcriptomic profiling of the fish spleen. We also measured the activity of neurotransmitters such as norepinephrine and acetylcholine in brain and peripheral tissues. Results: We describe the first set of the neural components that control inflammatory modulation during behavioural fever. We identified a neuro-immune crosstalk as a potential mechanism promoting the fine regulation of inflammation. The development of behavioural fever upon viral infection triggers a robust inflammatory response in vivo, establishing an activation threshold after infection in several organs, including the brain. Thus, temperature shifts strongly impact on neural tissue, specifically on the inflammatory reflex network activation. At the molecular level, behavioural fever causes a significant increase in cholinergic neurotransmitters and their receptors’ activity and key anti-inflammatory factors such as cytokine Il10 and Tgfβ in target tissues. Conclusion: These results reveal a cholinergic neuronal-based mechanism underlying anti-inflammatory responses under induced fever. We performed the first molecular characterization of the behavioural fever response and inflammatory reflex activation in mobile ectotherms, identifying the role of key regulators of these processes. These findings provide genetic entry points for functional studies of the neural–immune adaptation to infection and its protective relevance in ectotherm organisms.

The feeding behavior in fish is a complex activity that relies on the ability of the brain to integrate multiple signals to produce appropriate responses in terms of food intake, energy expenditure, and metabolic activity. Upon stress cues including viral infection or mediators such as the proinflammatory cytokines, prostaglandins, and cortisol, both Pomc and Npy/Agrp neurons from the hypothalamus are stimulated, thus triggering a response that controls both energy storage and expenditure. However, how appetite modulators or neuro-immune cues link pathogenesis and energy homeostasis in fish remains poorly understood. Here, we provide the first evidence of a molecular linkage between inflammation and food intake in Salmon salar. We show that in vivo viral challenge with infectious pancreatic necrosis virus (IPNV) impacts food consumption by activating anorexic genes such as mc4r, crf, and pomcb and 5-HT in the brain of S. salar. At the molecular level, viral infection induces an overall reduction in lipid content in the liver, favoring the production of AA and EPA associated with the increment of elovl2 gene. In addition, infection upregulates leptin signaling and inhibits insulin signaling. These changes are accompanied by a robust inflammatory response represented by the increment of Il-1b, Il-6, Tnfa, and Pge2 as well as an increased cortisol level in vivo. Thus, we propose a model in which hypothalamic neurons respond to inflammatory cytokines and stress-related molecules and interact with appetite induction/inhibition. These findings provide evidence of crosstalk between pathogenesis-driven inflammation and hypothalamic–pituitary–adrenocortical axes in stress-induced food intake behavior in fish.

Cumulative welfare is defined as the balance of all experiences shaping an animal’s quality of life over its lifespan. For cultured fish, optimizing cumulative welfare requires identifying the factors that modulate biological processes linked to negative and positive experiences. This study investigated whether behavioral thermoregulation represents one such factor in Atlantic salmon ( Salmo salar ) during the freshwater phase. The fish were reared for 5 months in thermally enriched (allowing thermoregulation) or constant‐temperature (non‐thermoregulatory) environments, and their thermal behaviors were documented. Welfare was assessed through mortality, growth, and parameters linked to oxidative damage in brain and muscle tissues. These parameters included free radicals, superoxide dismutase 1 ( sod1 ), glutathione peroxidase 1a ( gpx1a ), 8‐hydroxy‐2 ^′ ‐deoxyguanosine (8‐OHdG), telomere‐to‐single‐copy gene ( T / S ) ratio, and 5‐methylcytosine (5‐mC). The ability to behaviorally thermoregulate was associated with lower mortality, greater growth, and elevated antioxidant enzyme expression compared to the non‐thermoregulatory group. Thermoregulatory fish also showed lower 8‐OHdG levels in both tissues, a stable brain T / S ratio over time, and 5‐mC changes indicative of brain hypomethylation and muscle hypermethylation. Despite these complex time, tissue, and group‐dependent patterns, the principal component analysis (PCA)‐derived index revealed consistently higher welfare scores for the thermoregulatory group. Among all indicators evaluated, PCA‐based coefficient values identified 8‐OHdG and mortality as the variables with the largest contribution to the cumulative welfare index. These findings provide experimental evidence of a link between behavioral thermoregulation and the modulation of biological processes related to an optimized cumulative welfare state. This study contributes to the integration of retrospective and prospective dimensions into animal welfare management in aquaculture.