Stress and immune responses in rainbow trout (Oncorhynchus mykiss) treated by a low dose vaccine injection

The animal stress response is initiated by the locus coeruleus-noradrenaline (LC-NE) system and followed by an activation of hypothalamic-pituitary-interrenal (HPI) axis. Pathogens represent a common physiological stressor, and vaccination is the most used method to prevent bursts of infectious disease in aquaculture. Moreover, vaccination is an alternative way to study the physiological changes corresponding to an infectious process. In the present study, with the aim of investigating the interaction between neuroendocrine and immune responses during the initial stage of natural pathogen infection, rainbow trout (Oncorhynchus mykiss) were injected with inactivated Vibrio anguillarum bacterin (100 μL). Fish were sampled at different time-points: 1, 6, 24, and 72 h post-infection (hpi), and blood, brain, pituitary, head kidney, spleen, intestine, and liver were sampled. Biochemical parameters were analyzed in plasma (e.g., concentrations of corticotropin-releasing hormone (CRH) and cortisol), and the mRNA levels of genes involved in catecholamine synthesis and metabolism, hypothalamic-pituitary-interrenal (HPI) axis, and immune responses were assessed in the sampled tissues. Results showed that stress parameters, including plasma glucose, protein level, cortisol, and expression of pomcα in the pituitary were significantly changed at 1hpi (p < 0.05), while plasma IL1β levels and expression of immune related genes, including il1β, il8, il6, were mainly significantly changed at the latter stages (24 hpi, 72 hpi) in the spleen and brain (p < 0.05). This indicates a first stress response with a latter immune response. Significant changes in the expression of mao, pnmt and dbh in the brain were also observed, implying firstly the effective local and systemic role of catecholamines and secondly interactivities between the LC-NE system and HPI in the maintenance of stress response.