By the 1950s, the list of neurotransmitters (defined by the criteria described in Box A) had expanded to include four amines—epinephrine, norepinephrine, dopamine, and serotonin—in addition to ACh. Over the following decade, three amino acids—glutamate, γ-aminobutyric acid (GABA), and glycine—were also shown to be neurotransmitters. Subsequently, other small molecules were added to the list, and considerable evidence now suggests that histamine, aspartate, and ATP should be included (Figure 6.3). The most recent class of molecules discovered to be transmitters are a large number of polypeptides; since the 1970s, more than 100 such molecules have been shown to meet at least some of the criteria outlined in Box A.

Figure 6.3

Examples of small-molecule and peptide neurotransmitters. Small-molecule transmitters can be subdivided into acetylcholine, the amino acids, purines, and biogenic amines. The catcholamines, so named because they all share the catechol moiety (i.e., a (more...)

For purposes of discussion, it is useful to separate this variety of agents into two broad categories based simply on their size (Figure 6.3). Neuropeptides are relatively large transmitter molecules composed of 3 to 36 amino acids (Figure 6.4). Individual amino acids, such as glutamate and GABA, as well as the transmitters acetylcholine, serotonin, and histamine, are much smaller than neuropeptides and have therefore come to be called small-molecule neurotransmitters. Within the category of small-molecule neurotransmitters, the biogenic amines (dopamine, norepinephrine, epinephrine, serotonin, and histamine) are often discussed separately because of their similar chemical properties and postsynaptic actions.

Figure 6.4

Neuropeptides vary in length, but usually contain between 3 and 36 amino acids. Note that one peptide can include the sequence of other neuroactive peptides. For example, β-endorphin contains both α-endorphin and methionine enkephalin (more...)