Swim performance as an effective, environmentally relevant measure of sublethal toxicity in zebrafish (Danio rerio)
Examination of the swimming capabilities of fish is increasingly being considered as an effective method for determining sublethal toxicity. Acute toxicant exposure is known to cause decreases in swim performance in fish but less is known about how developmental exposure can cause persistent effects that hinder swimming. In addition, little is known about how triglyceride levels fluctuate during fish swimming upon both acute and developmental exposure to toxicant. In this thesis, two studies, one acute and one developmental, were carried out using two different toxicants in order to address these issues. In order to examine acute effects, adult zebrafish (Danio rerio) were exposed to ethanol vehicle or increasing concentrations of 2,4-dinitrophenol (DNP), a mitochondrial electron transport chain uncoupler, for a 24 h period. Following exposure, fish were placed in a swim tunnel for critical swimming speed (Ucrit) determination and swim motion analysis. Whole body triglyceride levels were then determined. Ucrit was decreased in a concentration dependent manner in both the 6 mg/L and 12 mg/L DNP exposure groups, with 6 mg/L DNP being considered sublethal and 12 mg/L approaching the LC50. A decrease in tail beat frequency was observed and is likely the main cause for the decrease in Ucrit in the DNP exposure groups. Triglyceride levels were elevated in a concentration dependent manner in the DNP exposure groups. This increase in triglyceride stores may be due to a behavioral adaption limiting swimming capabilities or due to a direct toxic action of DNP on lipid catabolism. The second study examined whether developmental 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure would cause persistent toxic effects. Zebrafish embryos were exposed to dimethyl sulfoxide control or increasing concentrations of TCDD between 2-4 days post fertilization (dpf). At 5 dpf, cytochrome P450 1A (CYP1A) activity was determined. Fish were raised to 90 dpf with mortalities and deformities being recorded at 5 dpf, 10 dpf, and 90 dpf. At 90 dpf, fish were placed in swim tunnel and Ucrit , swimming motion, and aerobic scope (oxygen consumption rate during exercise minus oxygen consumption rate during rest) were determined. Following swimming, some fish were used for whole body triglyceride analysis while others were used for histological examination. Ucrit was shown to be decreased in the two highest sublethal TCDD exposure groups (0.1 and 1 ng/L) but not in the lowest TCDD exposure group (0.01 ng/L). The exact cause of the decrease in Ucrit is not known, but may be linked to the observed decrease in dorsal aorta diameter, an inability to mobilize triglyceride stores, behavioral adaptations limiting swimming, decreased body length, or a combination of these factors. This TCDD related defect in swimming ability is not due to any increases in gross deformity or mortality rates, nor does it appear that CYP1A induction is required to mediate the toxic effects. Thus, it appears that examination of swim performance may serve as an effective measure of both sublethal acute and developmental toxicities.
zebrafish, dioxin, triglycerides, dinitrophenol, sublethal toxicity, critical swimming speed
Master of Science (M.Sc.)
Veterinary Biomedical Sciences
Veterinary Biomedical Sciences