Great analogy. I love it. And many, MANY things are not neurotransmitters. Soup, for example. Not a neurotransmitter. Bourbon, K-Mart, the SR-71... all not classified as neurotransmitters.

(on a side note: I haven't done any particle physics work in a few years, but still can't type anything that starts with "neu" without automatically typing a 't' afterwards. NEUTRINOS!!! There. Ahh. I feel better. Neutrinos. :D)