Stress Leukogram Induced by Acute and Chronic Stress in Zebrafish (Danio rerio)

The use of zebrafish (Danio rerio) as an animal model for experimental studies of stress has increased rapidly over the years. Although many physiologic and behavioral characteristics associated with stress have been defined in zebrafish, the effects of stress on hematologic parameters have not been described. The purpose of our study was to induce a rise in endogenous cortisol through various acute and chronic stressors and compare the effects of these stressors on peripheral WBC populations. Acutely stressed fish underwent dorsal or full-body exposure to air for 3 min, repeated every 30 min over the course of 90 min. Chronically stressed fish underwent exposure to stressors twice daily over a period of 5 d. After the last stressful event, fish were euthanized, and whole blood and plasma were obtained. A drop of whole blood was used to create a blood smear, which was subsequently stained with a modified Wright-Giemsa stain and a 50-WBC differential count determined. Plasma cortisol levels were determined by using a commercially available ELISA. Endogenous cortisol concentrations were significantly higher in both stressed groups as compared with control fish. Acutely stressed fish demonstrated significant lymphopenia, monocytosis, and neutrophilia, compared with unstressed, control fish. Chronic stress induced lymphopenia and monocytosis but no significant changes in relative neutrophil populations in zebrafish. The changes in both stressed groups most likely are due to increases in endogenous cortisol concentrations and represent the first description of a stress leukogram in zebrafish.