The most common method of euthanasia of laboratory rodents is exposure to carbon dioxide (CO2), but recent studies have shown that rodents find this gas aversive. Inhalant anaesthetics such as halothane and isoflurane are commonly used to induce unconsciousness in small animals and may be a suitable alternative. We conducted two experiments to record rat responses to induction with halothane and isoflurane using approach-avoidance testing. In Experiment 1, anaesthetics were each delivered through a vaporizer at four concentrations shown to induce recumbency within 155 s, 136 s, 113 s and 83 s. Rats were not allowed to re-enter the test cage once they left. In Experiment 2, halothane and isoflurane were delivered from soaked cotton wool balls in amounts that induced recumbency within 205 s and 168 s, respectively. Rats were allowed to re-enter the test cage during a trial. On the first day of exposure to anaesthetics in Experiment 1, rats remained in the test cage for an average of 64 s and six of the eight rats were ataxic before leaving the test cage. During subsequent exposures rats remained for just 8 s on average, and ataxia was only observed in 19 out of 120 trials. Rats remained in the test cage longer with isoflurane and with lower concentrations of both anaesthetics, but remained closer to the time of expected recumbency with higher concentrations. In Experiment 2, rats remained in the test cage for an average of 129 s on the first day of exposure and eight of the nine rats remained until ataxic. Upon re-exposure to an anaesthetic many rats again left the test cage within seconds. However, all rats promptly returned to the cage and remained until ataxic, indicating that the learned aversion is transient. Rats were likely in a state of conscious sedation by the time they chose to leave, suggesting that forced exposure from the onset of aversion until loss of consciousness may be more humane than forced exposure to other agents. We conclude that inhalant halothane and isoflurane are aversive to rats, but may nonetheless be a more humane method for inducing unconsciousness prior to euthanasia than the commonly used CO2.
|Publication Title||Applied Animal Behaviour Science|
|Author Address||Animal Welfare Program, Faculty of Land and Food Systems, University of British Columbia, 2357 Main Mall, Vancouver, BC V6T 1Z4, Canada. email@example.com|
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