Summary: Makowska et al. (2009)

Laboratory mice are the most commonly used species in biomedical research and are most commonly euthensased with carbon dioxide, which they may find aversive, as has been shown in rats. In many countries, Animal care policies generally require death to be painless and there should be minimal fear and anxiety. Makowska et al. (2009) assessed male CD-1 mouse aversion to CO2 usuing approach-avoidance testing in response to CO2 and 4 alternatives (argon, CO, halothane and isoflurane). Mouse motivation to avoid gas exposure was measured against motiavtion to assess a highly palatable sweet food reward. During preliminary testing, they recorded time to recumbency and concentration of gas at recumbency. Mice were trained to enter the test chamber through provision of a piece of sweetened shredded coconut as a reward presented in response to gentle tapping of the cage. When exposed to CO2, argon and CO, mice always left the test cage before losing consciousness, showing that mice forego access to a preferred food reward in the presence of the gas. In most cases, when exposed to halothane and isoflurane, mice also left the test cage before losing consciousness, however motivation to avoid isoflurance appeared weaker. Makowska et al. (2009) suggest that dyspnea is the cause of the aversion to CO2. Furthermore, they suggest that all substances tested were aversive to mice, therefore the test for a non-aversive agent should continue. They also suggest that, as isoflurane appears to be the least aversive, it should be used (at least in the interim) as the preferred alternative to CO2 or other gases.