throughout the cerebral cortex, cerebellum, and the entire brain.
These neurotransmitters are involved in global modulation of alert-
ness and arousal, wakefulness and sleep, attention and memory,
and selection and initiation of behavioral actions—many important
things.
Another category of neurotransmitter is composed of polypep-
tides, ranging in length from five to thirty-one amino acids.
These peptide neurotransmitters include the opioids, such as the
enkephalins, dynorphins, and beta-endorphin—collectively called
the endorphins—and a variety of other neuropeptides, such as va-
sopressin, substance P, somatostatin, oxytocin, orexin, neurotensin,
neuropeptide Y, galanin, and cholecystokinin. Interestingly, some of
these neuropeptides are coreleased from neurons together with other
neurotransmitters. Mostly we do not yet know very much about what
they do.
The known neurotransmitters in the human brain may be grouped
into several categories. There are amino acid neurotransmitters:
glutamate and glycine (alpha-amino acids) and GABA (a gamma-
amino acid) constitute the known members of this category. There
are neurotransmitters made directly from amino acids by small
molecular modifications—serotonin, dopamine, norepinephrine,
epinephrine, and histamine represent these. There is acetylcholine.
There are polypeptide neurotransmitters, the neuropeptides. And
there are several more: adenosine, the endocannabinoids, nitric oxide,
and a few others that are known, and no doubt a few others are yet to
be identified. The diversity of neurotransmitter molecules allows for
nuanced regulation of the networks of interconnected neurons in the
brain.
The two major categories of neurotransmitter receptor are the