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.
Dysosmia
There are approximately 1000 olfactory receptors coded in the human genome. Less than 500 receptors are functional in the nasal epithelium. Each receptor neuron is a single type of olfactory receptor and is not specific to any one odorant. An odorant is recognized by more than one type of receptor and thus odorants are recognized by a combination of receptors. The olfactory system relies on different excitation patterns to obtain different codes for different odorants. Nobel prize recipient Linda B. Buck compared this system to combining different letters of the alphabet to produce different words. In this case, each word represents an odor. This coding explains why we can detect more odors than there are receptors in the nasal epithelium.