Social Stress Suppresses Defense Activities of Neutrophils in Tilapia

Abstract

Defense activities of neutrophils in tilapia, Oreochromis niloticus were investigated under the conditions of social stress. Two fish, one from the large fish group (mean weight; 143 g) and the other from the small fish group (mean weight; 91 g) were transferred into an aquarium. As a control, a large fish or a small fish was held individually in aquaria of the same size. Five such pairs were observed in this study. Within a few hours post-pairing, the large fish became dominant in all pairs tested, and charged, chased and rammed the subordinate small fish. The cortisol and glucose concentrations in the plasma of the subordinates were significantly higher than those of the dominants and the controls, indicating that the subordinates were stressed. The swim bladders of both fish were injected with formalin-killed Escherichia coli cells just before pairing, and after 24 h, the neutrophils exuded into the swim bladder were collected and their defense activities measured. The number of neutrophils that migrated to the swim bladders of the subordinates was significantly decreased, and their phagocytic ability and respiratory. burst activity were both reduced. These results indicate that social stress suppresses the defense activities of neutrophils. Interestingly, the dominant fish showed high respiratory burst activity compared with the controls, indicating that slight stress may stimulate the defense activities of neutrophils.