Rat aversion to carbon monoxide

The most common method of killing laboratory rodents is exposure to carbon dioxide, but recent studies have shown that rodents find this gas aversive. Carbon monoxide (CO) is a colourless and odourless gas that reduces oxygen supply to the tissues and leads to hypoxia. The aim of this study was to use approach–avoidance testing to assess whether rats show aversion to CO-induced hypoxia when CO is delivered at 3%, 6% or 7% of the test cage volume per min. Although most animals chose to avoid CO, one rat remained in the test cage until recumbency when exposed to CO delivered at the highest flow rate, and another rat became recumbent immediately after leaving the test cage with the intermediate flow rate. A follow-up study revealed that all rats exposed to CO without the possibility of escape exhibited behaviours indicative of aversion. In conclusion, CO is the first agent that was tolerated until recumbency by a rat, but this agent was still aversive to most rats. The search for non-aversive euthanasia agents should continue.