Euthanasia methods, corticosterone and haematocrit levels in Xenopus laevis: evidence for differences in stress?

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AbstractAmphibians, like other vertebrates, respond to acute stressors by releasing glucocorticoid steroid hormones that mediate physiological and behavioural responses to stress. Measurement of stress hormones provides a potential means to improve the welfare of laboratory animals. For example, manipulations of laboratory housing and procedures combined with measurement of glucocorticoids may identify which conditions are more stressful to animals. This is important because there is very little experimental evidence to guide best practice for welfare in amphibians and other lower vertebrates. We investigated the effect of different methods of euthanasia on the circulating plasma corticosterone levels in the African clawed frog (Xenopus laevis), a model amphibian organism that is frequently used in laboratories. In particular, we investigated the effect of different concentrations and pH of the anaesthetic tricaine methanesulphonate (MS-222). Low concentration and unbuffered (low pH) solutions of MS-222 caused elevated corticosterone levels, but only after the effect of MS-222 treatment on blood fluid volume had been taken into account. The level of disturbance that animals experienced also affected corticosterone levels. Thus, our data suggest that to minimise stress to X. laevis, animals should be euthanised after minimal disturbance and in a 3 g L−1 MS-222 solution, buffered to pH 7. The potential for the improvement of amphibian welfare using corticosterone measures as a tool is discussed.