Inhibitory effect of cortisol on the defense activities of tilapia neutrophils in vitro

On the premise that fish neutrophil activities are depressed under stressed conditions, this study investigated the direct effect of cortisol on the in vitro activity of neutrophiles collected from tilapia Oreochromis niloticus. Neutrophils were obtained from swim bladders and exposed to different levels of cortisol (0-1000 ng/mL), and their chemotactic, phagocytic and respiratory burst activities were assessed. This treatment suppressed neutrophil activities as seen in the reduction of chemotactic, phagocytic and respiratory burst activities. Neutrophil activities appeared to be suppressed in a dose-dependent manner leading to a considerable reduction in neutrophil activities with high concentrations of cortisol. Nevertheless, we showed that the respiratory burst activity was able to recover from the observed effect after cortisol was removed. The results in this study strongly suggest that cortisol has a directly adverse effect on fish neutrophil defense activities, but that the effect is reversible.