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in portions of the ANS. Norepinephrine is also called Chapter 12 Neural Tissue 443
noradrenaline, and synapses that release NE are known as
adrenergic synapses. Norepinephrine typically has an in several regions of the brain. Carbon monoxide (CO), best
excitatory, depolarizing effect on the postsynaptic mem- known as a component of automobile exhaust, is also gener-
brane, but the mechanism is quite distinct from that of ated by specialized axon terminals in the brain, where it func-
ACh, as we will see in Chapter 16. tions as a neurotransmitter.
Dopamine (DO. -puh-me. n) is a CNS neurotransmitter Neuromodulators
released in many areas of the brain. It may have either It is convenient to discuss each synapse as if it were releasing
inhibitory or excitatory effects. Inhibitory effects play an only one chemical, but axon terminals may release a mix-
important role in our precise control of movements. For ture of active compounds, either through diffusion across the
example, dopamine release in one portion of the brain membrane or by exocytosis, along with neurotransmitter mol-
prevents the overstimulation of neurons that control skel- ecules. These compounds may have a variety of functions.
etal muscle tone. If the neurons that produce dopamine Those that alter the rate of neurotransmitter release by the
are damaged or destroyed, the result can be the character- presynaptic neuron or change the postsynaptic cell’s response
istic rigidity and stiffness of Parkinson’s disease, a condi- to neurotransmitters are called neuromodulators (noo-ro. -
tion we describe in Chapter 14. At other sites, dopamine MOD-u. -la. -torz). These substances are typically neuropep-
release has excitatory effects. Cocaine inhibits the removal tides, small peptide chains synthesized and released by the
of dopamine from synapses in specific areas of the brain. axon terminal. Most neuromodulators act by binding to recep-
The resulting rise in dopamine concentrations at these tors in the presynaptic or postsynaptic membranes and activat-
synapses is responsible for the “high” experienced by co-
caine users. 12ing cytoplasmic enzymes.
Neuromodulators called opioids (O. -pe. -oydz) have effects
Serotonin (ser-o. -TO. -nin) is another important CNS neu- similar to those of the drugs opium and morphine, because they
bind to the same group of postsynaptic receptors. Four classes
rotransmitter. Inadequate serotonin production can have of opioids in the CNS are (1) endorphins (en-DOR-finz),
widespread effects on a person’s attention and emotional (2) enkephalins (en-KEF-a-linz), (3) endomorphins, and
states and may be responsible for many cases of severe (4) dynorphins (DI.-nor-finz). The primary function of opi-
chronic depression. Fluoxetine (Prozac), paroxetine (Paxil), oids is probably to relieve pain. They inhibit the release of the
sertraline (Zoloft), and related antidepressant drugs inhibit neurotransmitter substance P at synapses that relay pain sensa-
the reabsorption of serotonin by axon terminals (hence tions. Dynorphins have far more powerful pain-relieving effects
their classification as selective serotonin reuptake inhibi- than morphine or the other opioids.
tors, or SSRIs). This inhibition leads to increased serotonin
concentrations at synapses, and over time, the increase &T i p s T r i c k s
may relieve the symptoms of depression. Interactions Endorphins are so named because they act like endogenous
among serotonin, norepinephrine, and other neurotrans- (coming from within the body) morphine.
mitters are thought to be involved in the regulation of
sleep and wake cycles. In general, neuromodulators (1) have long-term effects
that are relatively slow to appear; (2) trigger responses that
Gamma-aminobutyric (a-ME. -no. -bu. -TE. R-ik) acid, or involve a number of steps and intermediary compounds;
(3) may affect the presynaptic membrane, the postsynaptic
GABA, generally has an inhibitory effect. Roughly 20 per- membrane, or both; and (4) can be released alone or along
cent of the synapses in the brain release GABA, but its func- with a neurotransmitter. Table 12–4 lists major neurotrans-
tions remain incompletely understood. In the CNS, GABA mitters and neuromodulators of the brain and spinal cord,
release appears to reduce anxiety, and some antianxiety and their primary effects (if known). In practice, it can be
drugs work by enhancing this effect. very difficult to distinguish neurotransmitters from neuro-
modulators on either biochemical or functional grounds: A
The functions of many neurotransmitters are not well neuropeptide may function in one site as a neuromodula-
understood. In a clear demonstration of the principle “the tor and in another as a neurotransmitter. For this reason,
more you look, the more you see,” over 100 neurotransmitters Table 12–4 does not distinguish between neurotransmitters
have been identified, including certain amino acids, peptides, and neuromodulators.
polypeptides, prostaglandins, and ATP.
In addition, two gases, nitric oxide and carbon monoxide,
are known to be important neurotransmitters. Nitric oxide
(NO) is generated by axon terminals that innervate smooth
muscle in the walls of blood vessels in the PNS, and at synapses
