Evidence for consistent individual differences in rat sensitivity to carbon dioxide

Carbon dioxide (CO(2)) gradual-fill is commonly used to kill laboratory rats, but this use remains controversial due to a lack of agreement between studies. Inconsistencies may arise from differences in behaviors measured (e.g. active versus passive behaviors), in how rats cope with threats, or in rat sensitivity to CO(2). The aims of the current study were to 1) describe active and passive responses during CO(2) forced exposure, 2) determine if these responses are consistent within individuals and across aversive stimuli, 3) assess individual differences in aversion to CO(2) in aversion-avoidance and approach-avoidance tests and 4) determine how responses in aversion tests relate to individual differences in behavior during forced exposure. Twelve Sprague Dawley female rats were exposed twice to three treatments: CO(2), oxygen (O(2)), and fox scent, and were exposed to CO(2) twice in each aversion test. The change in behavior from baseline was higher for rearing and locomotion when rats were exposed to CO(2) than when exposed to O(2) and fox scent. Responses varied among rats but were consistent across multiple tests within rats. For example, rearing was consistent within individuals between two exposures to CO(2). Similarly, the strength of aversion was consistent within individuals across multiple exposures to CO(2) in aversion-avoidance and approach-avoidance testing. Latency to avoid CO(2) in aversion-avoidance tests was negatively correlated with rearing during CO(2) forced exposure. Collectively, these results indicate that rat responses to CO(2) vary between (but are consistent within) individuals, suggesting that rats vary in CO(2) sensitivity. However, even the less responsive rats avoided CO(2) concentrations far below those necessary to achieve unconsciousness, indicating that all rats likely experience negative states when euthanized with CO(2).