Acute low temperature and lipopolysaccharide differentially modulated the innate immune and antioxidant responses in a subtropical fish, the pacu (Piaractus mesopotamicus).

Exogenous factors such as low water temperature can be stressful and elicit negative immune system effects, especially for fish, which are ectothermic. Stress and immune responses require energy overload, which can affect the cellular redox balance, causing oxidative damage. These overall responses impair the animal's health and negatively affect fish farming. To evaluate indicators of stress, immune and antioxidant systems, and oxidative stress responses in fish during thermal challenge, the present study reduced the water temperature from 29.5 °C to 16 °C and then inoculated pacu (Piaractus mesopotamicus) with lipopolysaccharide (LPS) from Escherichia coli. Our results revealed that acute exposure to low water temperature itself increased blood glucose, impaired the serum lysozyme concentration and increased GSH-Px activity. There was an interaction effect between low temperature and LPS inoculation. After LPS inoculation, leukocytes were initially activated (3 h); glucose levels increased (3 h); GST activity initially decreased (3 h) but then increased (6 h); SOD, CAT and GSH-Px activities decreased; and lysozyme activity remained depressed in fish subjected to cold shock. The results showed that thermal and immunological challenges impaired the maintenance of leucocyte activation and compromised the pacu oxidant response. The overall response of pacu to thermal challenge indicates that the species proved to be acutely sensitive to a drop in water temperature, reducing its ability to maintain homeostasis, especially when subjected to immunological challenge.