Abstract
Carbon dioxide (CO2) is one of the most commonly used gas euthanasia agents in mice, despite reports of aversion and nociception. Inert gases such as nitrogen (N2) may be a viable alternative to carbon dioxide. Here we compared behavioural and electrophysiological reactions to CO2 or N2 at either slow fill or rapid fill in C57Bl/6 mice undergoing gas euthanasia. We found that mice euthanised with CO2 increased locomotor activity compared to baseline, whereas mice exposed to N2 decreased locomotion. Furthermore, mice exposed to CO2 showed significantly more vertical jumps and freezing episodes than mice exposed to N2. We further found that CO2 exposure resulted in increased theta:delta of the EEG, a measure of excitation, whereas the N2 decreased theta:delta. Differences in responses were not oxygen-concentration dependent. Taken together, these results demonstrate that CO2 increases both behavioural and electrophysiological excitation as well as producing a fear response, whereas N2 reduces behavioural activity and central neurological depression and may be less aversive although still produces a fear response. Further studies are required to evaluate N2 as a suitable euthanasia agent for mice.
Highlights
Carbon dioxide (CO2) is the most commonly used gas euthanasia agent in mice due to rapid action, low cost and easy availability [1, 2]
To determine whether nitrogen is less aversive than carbon dioxide as a euthanasia agent for mice, we first assessed the behavioural parameters of groups of non-instrumented C57Bl/6 mice being euthanised in four different treatment groups (CO2R; n = 10 cages; female: n = 5; male: n = 5; N2 rapid (N2R); n = 10 cages; female: n = 5; male: n = 5; CO2 slow fill (CO2S); n = 10 cages; female: n = 5; male: n = 5; and N2 slow (N2S); n = 10 cages; female: n = 5; male: n = 5)
Both compared to N2 groups (CO2R) and CO2S resulted in significantly more jumps compared to N2S ( P < 0.05) (H) Cumulative curves indicating the relative number of jumps in relation with oxygen concentration of the four treatment groups. (I) Freezing shown as mean (± s.e.m) number of freezing episodes per minute per animal
Summary
Carbon dioxide (CO2) is the most commonly used gas euthanasia agent in mice due to rapid action, low cost and easy availability [1, 2]. Multiple studies have demonstrated that carbon dioxide exposure may cause pain in humans as well as fear responses and aversion in rodents [3,4,5,6] its use for euthanasia is controversial [7,8,9,10,11,12]. The formation of carbonic acid in the mucous membranes following CO2 absorption is considered to cause pain and discomfort as reported in humans [3, 16], inert gas absorption does not result in carbonic acid formation.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
