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 .