First, how realistic is it to have a hypothetical situation where one would need gene therapy in order to correct for the genetic coding of a neurotransmitter? (This is a science-fiction setting.) Assuming that the organism could survive without the neurotransmitter being produced, is it techinically (or hypothetically technically) possible to use gene therapy to activate its production during its adult life?
Also on the gene therapy line of thinking, how complicated (roughly, here) would it be to code for something like a neurotransmitter? Would multiple genes be involved? I would think that multiple genes would, by extension, make gene therapy more difficult to enact.