Yes, I think it would represent a new kind of slavery, and should be extremely illegal.
Have you ever heard of dairy farms, or horses? If not, I'd advise against searching.
Cows and horses are not people. The use of human brains or human brain matter brings up slavery in a way that rat brains don't because owning animals is not considered morally equivalent to owning people.
Ah, sorry, I skipped over the 'human brain in...' part of the post, and thought GP was responding to 'rat brains'. Please ignore.