you cannot ask rhodents about how dfferent gases feel.

But you can ask humans that almost died or got knocked unconscious by different gases. And all mammals share certain similarities.

So my rejection of CO2 and recommendation of alternatives is entirely based on human witness accounts. since nitrous oxide has a long history of being used to initiate anaesthesia in humans without complaints by patients and also because ive experienced it myself, i would use this to knock out the rhodents, then switch to something else when they are unconscious or jsut finish them with more nitrous oxide.

You cannot deny that CO2 got some really bad reputation from events that happened on some submarines or apollo 13 or divers or people that almost died from lung failure, all these accounts seem to agree: a rise in CO2-concentration in the blood really sucks.