If you can articulate the significance of that research experience to your desires to pursue medicine and maybe include some clinical implications, I think that would make for a very interesting PS. Best wishes! 🙂
I did neuro research for 3 years in undergrad. Part of it was testing drugs on rats, killing them (in a humane way), and make brain section slides for study. Didn't write about it, but it did come up in an interview and it was fine.