11
NEUROTRANSMITTER
FUNCTIONAL CLASSES
SITES WHERE SECRETED
COMMENTS
Amino Acids, continued
Glutamate
Generally excitatory
Direct action
CNS: spinal cord; widespread
in brain where it represents
the major excitatory
neurotransmitter
Important in learning and memory.
The “stroke neurotransmitter”: exces-
sive release produces excitotoxicity—
neurons literally stimulated to death;
most commonly caused by ischemia
(oxygen deprivation, usually due to a
blocked blood vessel).
Glycine
Generally inhibitory
Direct action
CNS: spinal cord and brain stem,
retina
Principal inhibitory neurotransmitter of
the spinal cord. Strychnine blocks gly-
cine receptors, resulting in uncontrolled
convulsions and respiratory arrest.
Peptides
Endorphins, e.g., beta
endorphin, dynorphin,
enkephalins
Generally inhibitory
Indirect action via second
messengers
CNS: widely distributed in brain;
hypothalamus; limbic system;
pituitary; spinal cord
Natural opiates; inhibit pain by inhib-
iting substance P. Effects mimicked by
morphine, heroin, and methadone.
Tachykinins: substance P,
neurokinin A (NKA)
Excitatory
Indirect action via second
messengers
CNS: basal nuclei, midbrain,
hypothalamus, cerebral cortex
PNS: certain sensory neurons
of dorsal root ganglia (pain
afferents), enteric neurons
Substance P mediates pain
transmission in the PNS. In the
CNS, tachykinins are involved in
respiratory and cardiovascular
controls and in mood.
Somatostatin
Generally inhibitory
Indirect action via second
messengers
CNS: hypothalamus, septum,
basal nuclei, hippocampus,
cerebral cortex
Pancreas
Often released with GABA. A gut-
brain peptide. Inhibits growth
hormone release.
Cholecystokinin (CCK)
Generally excitatory
Indirect action via second
messengers
Throughout CNS
Small intestine
Involved in anxiety, pain, memory. A
gut-brain peptide hormone. Inhibits
appetite.
Purines
ATP
Excitatory or inhibitory
depending on receptor type
bound
Direct and indirect actions via
second messengers
CNS: basal nuclei, induces Ca
2
1
wave propagation in astrocytes
PNS: dorsal root ganglion
neurons
ATP released by sensory neurons (as
well as that released by injured cells)
provokes pain sensation.
Adenosine
Generally inhibitory
Indirect action via second
messengers
Throughout CNS and PNS
Caffeine (coffee), theophylline (tea),
and theobromine (chocolate) stimulate
by blocking brain adenosine receptors.
May be involved in sleep-wake cycle
and terminating seizures. Dilates arte-
rioles, increasing blood flow to heart
and other tissues as needed.
Gases and Lipids
Nitric oxide (NO)
Excitatory or inhibitory
Indirect action via second
messengers
CNS: brain, spinal cord
PNS: adrenal gland; nerves to
penis
Its release potentiates stroke
damage. Some types of male
impotence treated by enhancing NO
action [e.g., with sildenafil (Viagra)].
Carbon monoxide (CO)
Excitatory or inhibitory
Indirect action via second
messengers
Brain and some neuromuscular
and neuroglandular synapses
 
Endocannabinoids, e.g.,
2-arachidonoylglycerol,
anandamide
Inhibitory
Indirect action via second
messengers
Throughout CNS
Involved in memory (as a retrograde
messenger), appetite control, nausea
and vomiting, neuronal development.
Receptors activated by THC, the prin-
cipal active ingredient of cannabis.
Receptors also found on immune cells.
Table 11.3
Neurotransmitters and Neuromodulators
(continued)
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