"In the central nervous system, ACh has a variety of effects as a neuromodulator, e.g. for plasticity and excitability. Other effects are arousal and reward."
Do you remember Andro (androstenedione, used to be available in stores, used by Mark McGwire)? It's a testosterone precursor. Your liver converts it to testosterone (and also some amount converts to estrogens). So a neurotransmitter precursor would seem to be a bit like steroids for the brain.
Not sure how external supplementation affects endogenous neurotransmitter production, but with hormone precursors the biofeedback loop tells your endocrine system to stop its own production. E.g. if you take testosterone, your body shuts down its own production (when your testosterone level hits a certain point, your body thinks "oh, that's enough for now" and hits the off-switch on your testicles. This turns off ALL their functions, so it's actually also a form of birth control. And since they aren't doing anything, they atrophy).
Fun note: Your body tries to keep your testosterone:estrogen ratio at about 50:1 (for young men). So if you take androgenic steroids, your body increases its own estrogen production. Once you hit a certain absolute amount of estrogen, you're growing breast tissue.
Then you are a man. With titties.
(Bodybuilders combat this by using certain steroids that don't aromatize into estrogen as much, and also using anti-estrogens that either bind to the aromatase enzyme to stop conversion into estrogen, or bind to the actual estrogen receptor sites like Clomid. One interesting effect of Clomid -- that makes it attractive to male porn stars -- is it causes big pop shots.)