Rats show aversion to argon-induced hypoxia

Laboratory rats and mice are commonly killed using carbon dioxide gas. However, recent studies have shown that rodents find this gas aversive. Argon is a tasteless and odourless gas that causes hypoxia by displacing air. The aim of the present study was to use approach-avoidance testing to evaluate rats' responses to argon-induced hypoxia when argon was introduced over a range of flow rates. The experiment was run in two phases: Phase 1 tested low flow rates (40-120% of the test cage volume per min) and Phase 2 tested higher flow rates (120-239%). Rats were trained to enter the bottom cage of a two-cage apparatus for a reward of 20 Cheerios™. Argon (or air as a control) entered the cage at the assigned flow rate as soon as the rats started eating. During control trials with air, rats ate for an average (±standard deviation) of 252 ± 22 s in Phase 1 and 232 ± 51 s in Phase 2, with no effect of flow rate. When tested with argon, rats never remained in the test cage long enough to lose consciousness. They consumed fewer Cheerios™, stopped eating sooner, and left the test cage quicker than when tested with air. Rats ate for only 104 ± 31 s when tested at even the lowest argon flow rate, and this time decreased with increasing flow rates in both Phase 1 (P < 0.0001) and Phase 2 (P < 0.0001). The oxygen (O2) concentration at which rats stopped eating (P < 0.0001) and left the test cage (P < 0.0009) decreased over the lower range of flow rates tested in Phase 1, but stabilized at about 7.7 and 6.8%, respectively, with the higher flow rates tested in Phase 2; these O2 concentrations are too high to cause unconsciousness or death. Although humans exposed to hypoxia report only subtle symptoms, rats are more sensitive than humans to changes in O2. We conclude that rats are averse to argon-induced hypoxia and that alternative methods of euthanasia are still required.