Brain
There are dozens of other chemical neurotransmitters that are used in more limited areas of the brain, often areas dedicated to a particular function. Serotonin, for example—the primary target of antidepressant drugs and many dietary aids—comes exclusively from a small brainstem area called the raphe nuclei. Norepinephrine, which is involved in arousal, comes exclusively from a nearby small area called the locus coeruleus. Other neurotransmitters such as acetylcholine and dopamine have multiple sources in the brain, but are not as ubiquitously distributed as glutamate and GABA.
Henry Hallett Dale
Dale also originated the scheme used to differentiate neurons according to the neurotransmitters they release. Thus, neurons releasing noradrenaline (known in the United States as norepinephrine) are called noradrenergic, neurons releasing GABA are GABAergic, and so on. This is called "Dale's principle" (sometimes erroneously referred to as "Dale's Law"), one interpretation of which holds that each neuron releases only one type of neurotransmitter. This particular interpretation of Dale's principle has been shown to be false, as many neurons release neuropeptides and amino acids in addition to classical neurotransmitters such as acetylcholine or biogenic amines (see cotransmission) (Bear, et al. 2001). This finding, that numerous neurotransmitters can be released by the same neuron, is referred to as the "coexistence principle." This phenomenon was most popularized by the Swedish neuroanatomist and neuropharmacologist Tomas Hökfelt, who is considered to be the "Father of the Coexistence Principle."