CHEMISTRY OF THE BRAIN: MOLECULES, MOOD, AND MENTAL ILLNESS - Chemistry: Notable Research and Discoveries - page 78

neurotransmitters

The list of identifi ed neurotransmitters contains more than
50 substances, although only a dozen or so are common in
the brain. To be included, a chemical must be found in the
presynaptic neuron and have a defi nite effect on the postsyn-
aptic target. Neurotransmitter molecules are usually pack-
aged in a container known as a synaptic vesicle, as shown
in the fi gure at right. The vesicle is bound by a membrane,
similar to the cell's membrane. Electrical activity initiated by
an action potential causes a fl ow of calcium ions, which in
turn cause the vesicle to bind and join the cell's membrane,
releasing the contents into the synaptic gap. In the most
common situation, neurotransmitters bind to a specifi c pro-
tein embedded in the postsynaptic cell's membrane. This
protein is called a receptor for the specifi c neurotransmitter
or ligand it binds. ( Ligand is a general term for a chemical
that binds to a specifi c target—the word comes from a Latin
term, ligare, to bind.) The binding of a ligand activates the
receptor, which has some sort of effect on the postsynap-
tic cell—some receptors impact ion channels, and others
generate molecules known as second messengers, which in
turn activate certain enzymes or other proteins in the cell.
Neurotransmitters include the following substances:

• amino acids: glutamate, gamma-aminobutyric acid

(GABA), and glycine
small molecules, including acetylcholine, dopamine,
norepinephrine (also known as noradrenalin), and se-
rotonin (also known as 5-hydroxytryptamine, or 5-HT)
peptides (short chains of amino acids) such as en-
kephalin and vasopressin
soluble gases such as nitric oxide

•

•

•

Many of these substances play additional roles in the
body. For example, besides acting as neurotransmitters,