Sammendrag
Fish exposed to stressful conditions are prone to decreased immunocompetence. Using a cold-water fish, Atlantic cod, Gadus morhua as a model for the stress response in gadoid fish, the changes in primary (plasma cortisol), secondary responses (plasma glucose and total antioxidant capacity), humoral immune parameters, expression of selected stress-, antioxidant defense- and immune-related genes, as well as in vitro proliferation of bacterial pathogens were examined. Adult fish were kept at a density of ca. 100 kg m-3 by lowering the water level in the rearing tank for 1 h and this short-term crowding stress was repeated thrice over a 12 h interval period. Blood and plasma samples were collected prior to exposure and at 2, 24 and 72 h post-crowding. Plasma cortisol level significantly increased at 2 h post-crowding but returned to pre-crowding levels 24 h after exposure. Plasma glucose and total antioxidant capacity significantly increased at 2 h post-crowding; however the former returned to its pre-stressed levels at 24 h post-crowding while the total antioxidant capacity remained elevated until 72 h. Among the humoral immune parameters, there was transient upregulation of plasma protein, while plasma globulin significantly increased at 24 h post-crowding. Both myeloperoxidase activity and lysozyme activity increased at 2 h post-crowding, but the former returned to its pre-stress levels at 24 h, while the latter continued to be upregulated until 72 h. Alkaline phosphatase activity increased, while haemolytic activity was significantly depressed at 72 h post-crowding. At the transcription level, the relative expression of the stress response genes: glucose transporter 3 (GLUT-3), GLUT-4 and glyceraldehyde-3-phosphate dehydrogenase significantly increased at 2 h after crowding, while the putative heat shock protein 70 significantly increased at 24 h post-crowding. Gene transcripts of the antioxidant enzymes, Cu/Zn superoxide dismutase (Cu/Zn SOD), catalase and phospholipid hydroperoxide glutathione peroxidase also significantly increased at 2 h post-crowding, but thereafter they returned to their pre-crowding levels with the exception of Cu/Zn SOD that remained significantly higher than the initial group until 72 h. Significant upregulation of the pro-inflammatory cytokines, IL-1? and IL-8, as well as antibacterial genes, g-type lysozyme and bactericidal permeability-increasing protein/lipopolysaccharide-binding protein (BPI/LBP) was also observed at 2 h and the levels were maintained until 72 h post-exposure, except for BPI/LBP that had maximum upregulation at 24 h. There was increased proliferation of the bacterial pathogens, Vibrio anguillarum and Aeromonas salmonicida in the plasma of fish at 24 h post-crowding. The present results indicate that short-term crowding stress is capable of modulating the innate immune responses in Atlantic cod showing transient enhancement of several immune parameters and subsequent susceptibility to proliferation of bacterial pathogens in vitro.
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