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.
ABSTRACT-We examined the effect of cortisol on the degranulation and number of eosinophilic granular cells (EGCs) in tilapia, Oreochromis niloticus. One hour after injection of stimulants (killed Escherichia coli or substance P, a neurotransmitter) into the swim bladder, the percent of the degranulated EGCs in the swim bladder membrane was determined. Peritoneal implantation of coconut oil containing cortisol (50 mg/kg body weight) 24 h before injection of stimulants signifi cantly reduced the percent of the degranulated EGCs, compared with the control fish administered with coconut oil without Cortisol. Cortisol also suppressed the in vitro degranulation of EGCs stimulated with substance P or with tilapia normal sera containing zymosan.To investigate the effect of cortisol on the abundance of EGCs, the number of EGCs in the swim bladder membrane was counted. Density of EGCs was decreased after implantation of cortisol, compared with that of the control group. These results suggest that cortisol suppresses the degranulating activity of EGCs and decreases the density of EGCs in the tissue.
ABSTRACT--In order to reveal the effect of cortisol on tilapia neutrophil defense activities in vivo, fish were given intraperitoneal injections of cortisol mixed with coconut oil as an "implant" (5 mg/0.5 mL/fish). Fish were injected with Escherichia coli into their swim bladders before 24 h prior to col lecting neutrophils, which were then assessed for their chemotactic, phagocytic and respiratory burst activities. In addition, concentrations of plasma cortisol and glucose were measured. Cortisol-implanted fish showed considerably higher levels of cortisol and their neutrophil activities were significantly suppressed compared to the control group at 1 day post-implantation. Both groups of fish were challenged with Edwardsiella tarda (1 mg/fish). Cortisol-implanted fish started to die 6 days after the challenge, whereas no deaths were observed in the controls. After the chal lenge experiment, E. tarda was recovered from more than half of the cortisol-treated fish group and from the only one of the controls. Results from this study have demonstrated that elevated levels of cortisol in fish bodies, similar to that observed during stressful experiences, decrease neutrophil activities and increase susceptibility to bacterial pathogens.
ABSTRACT--On the premise that fish neutrophil activities are depressed under stressed condi tions, this study investigated the direct effect of cortisol on the in vitro activity of neutrophiles col lected 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 respi ratory 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 respira tory 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.
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