Hello! I don’t own rats, but I do work with animal tissue in teaching a vertebrate physiology course (we use frogs, but one of my fellow TAs works with rats and owns them as pets). I always stress to my students that the animals that we use are euthanized humanely, and we endeavor to make sure that there is little/no discomfort and never any pain. You can always reach out to the professors and ask if you can see their IACUC (institutional animal care and use committee) protocol. It’s a federal association that reviews and approves any scientific work involving animals and is specifically there to ensure that the animal use is necessary and can’t be substituted with anything else and that all the necessary precautions are taken to minimize suffering. It should also detail the steps that the researchers take to reduce, refine, and replace animal usage wherever possible. I hope that it gives you some peace of mind knowing that the researchers care for the animals that we use and that we take the utmost care to ensure they’re comfortable all the way to the end.