If you had a friend that wasn't able to suffer through some quirk of atypical DNA, would that person still have rights? Would it be unethical to enslave them?
(Hint: yes)
There are such people (depending on your definition of suffering) who cannot experience pain. The condition is called congenital analgesia and, to my knowledge, they retain their rights.
Well, if it is as simple as you say, I agree. But I bet that "the structures that are responsible for suffering" is not clearly defined. And who is to say that these structures are not the very ones we need to be researching to find treatments for various mental illnesses?