Is xenon a suitable euthanasia agent for mice?

OBJECTIVE: To compare behavioural and electrophysiological variables of mice undergoing gas euthanasia with either xenon (Xe) or carbon dioxide (CO2). STUDY DESIGN: Single animals chronically instrumented for electroencephalography (EEG) recording were randomized to undergo euthanasia with either CO2 or Xe (n = 6 animals per group). ANIMALS: Twelve adult (>6 weeks old) male C57Bl6/n mice. METHODS: Mice were surgically instrumented with EEG and electromyogram electrodes. Following a 7-day recovery period, animals were placed individually in a sealed chamber and a 5-minute baseline recorded in 21% O2. Gas [100% Xe (n = 6) or 100% CO2 (n = 6)] was then added to the chamber at 30% chamber volume minute-1 (2.8 L minute-1) until cessation of breathing. EEG, behaviour (jumping and freezing) and locomotion speed were recorded throughout. RESULTS: Mice undergoing single gas euthanasia with Xe did not show jumping or freezing behaviours and had reduced locomotion speed compared to baseline, in contrast to CO2, which resulted in increases in these variables. EEG recordings revealed sedative effects from Xe but heightened arousal from CO2. CONCLUSIONS: Our data suggest that Xe may be less aversive than CO2 when using a 30% chamber volume minute-1 fill rate and could improve the welfare of mice undergoing gas euthanasia.

Nitrogen gas produces less behavioural and neurophysiological excitation than carbon dioxide in mice undergoing euthanasia.

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.