Refining anaesthesia and euthanasia for zebrafish and mice

Sedation is necessary for zebrafish during procedures such as imaging, biopsy, and surgery to ensure animal welfare and high-quality science. But the effects of sedation can last beyond the administration period and should also be carefully considered. A paper by Gressler et al. explores the use of eugenol and propofol during a 3-hour sedation and their subsequent effects after a 1 hour washout period. Both drugs had effects on behavior and physiology even after the washout period. For behavior, in a novel tank test, eugenol was found to amplify diving response while propofol induced anti-anxiety responses. For physiology, both drugs caused alterations in gill structure. Clearly, as with other species, sedation of zebrafish can significantly affect behavior and physiology beyond the administration period. Therefore, sedation procedures must be carefully designed and reported to refine experiments. Read more: https://doi.org/10.1016/j.applanim.2021.105482

Next, let’s look at euthanasia, an important consideration for any animal study, and those involving zebrafish are no exception. Despite zebrafish being widely used in research, there is no consensus on which method to use when euthanizing them. The most commonly-used method – an overdose of tricaine, or MS-222 – is versatile, readily available and, in fact, is the only legal option in some areas, but it is now known to be aversive. A new paper by von Krogh et al. describes work to address this discrepancy. The team screened overdoses of 7 common alternative anaesthetics dissolved in water, to determine whether they led adult zebrafish to lose reflexes in a rapid, reliable, and non-aversive manner. Other substances were used to buffer the anaesthetics where needed, adjusting the water pH to reduce irritation. The authors found adding 1g/L lidocaine hydrochloride buffered with 2g/L sodium bicarbonate was particularly effective, reliably inducing loss of all reflexes within 2 mins and provoking little aversive behavior. Adding 50mL/L ethanol further reduced these issues. While the authors recommend this method, they also stress the importance of further investigation, including for zebrafish at different developmental stages. Read the paper: https://doi.org/10.3390/biology10111133

Finally, let’s focus on mice and how researchers are working to ensure they are euthanized humanely. Gradual exposure to carbon dioxide remains the most common method for euthanising laboratory rodents. Although CO2 is generally considered an acceptable option when properly administered, there are concerns that CO2 is aversive to rodents and might cause distress and pain above certain concentrations. In a new study, Rodriguez-Sanchez and colleagues investigated using a voluntarily ingested sedative to reduce CO2 aversiveness. Cream cheese mixed with different doses of a rapid-acting anesthetic was provided to C57BL/6 mice before exposing the animals to CO2. Using a broad range of behavioural parameters, the team showed 20 mg/kg of sedative resulted in a mild sedation and likely reduced the aversiveness of CO2. While the investigators acknowledge more work is needed to determine the experience of the mice during sedation, they suggest voluntary oral administration of a sedative is potentially an effective, affordable, and easy way to minimise the stress of mice during CO2 euthanasia. Find out more: https://doi.org/10.3390/ani11102879

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