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One of the best-understood neurotransmitters, acetylcholine (ACh), plays a role in
learning and memory. ACh also enables muscle action, by acting as the messenger at
every junction between motor neurons (which carry information from the brain and spi-
nal cord to the body’s tissues) and skeletal muscles. When ACh is released to our muscle
cell receptors, the targeted muscle contracts. If ACh transmission is blocked, as happens
during some kinds of anesthesia, with some poisons, and with the neuromuscular disease
myasthenia gravis, the muscles cannot contract. The result is weakness, difficulties with
muscle control, or paralysis.
Candace Pert and Solomon Snyder (1973) made an exciting discovery about neuro-
transmitters when they attached a harmless radioactive tracer to morphine, an opioid drug
that elevates mood and eases pain. As the researchers tracked the morphine in an animal’s
Distributed by Bedford, Freeman & Worth Publishers. Not for redistribution.
®
AP Science Practice brain, they noticed it was binding to receptors in areas linked with mood and pain sensa-
tions. But why would the brain have these “opioid receptors”? Why would it have a chemi-
Research
cal lock, unless it also had a key — a natural painkiller — to open it?
To conclude that vigorous exercise Researchers soon confirmed that the brain does, indeed, produce its own naturally
causes the release of endorphins,
researchers would need to manip- occurring opioids. Our body releases several types of neurotransmitter molecules similar to
ulate (or randomly assign) some morphine in response to pain and vigorous exercise. These endorphins (short for endog-
participants to exercise and others enous [produced within] morphine) help explain good feelings such as the “runner’s high,”
not to exercise, then measure all the painkilling effects of acupuncture, and the indifference to pain in some severely injured
participants’ endorphin levels and
compare the two groups. You can people (Boecker et al., 2008; Fuss et al., 2015). Physician Lewis Thomas (1983) called the
draw cause-effect conclusions endorphins “a biologically universal act of mercy. I cannot explain it, except to say that
from studies that use an experi- I would have put it in had I been around at the very beginning, sitting as a member of a
mental research design.
planning committee.”
How Drugs and Other Chemicals Alter Neurotransmission

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AP Exam Tip Copyright © Bedford, Freeman & Worth Publishers.
If natural endorphins lessen pain and boost mood, why shouldn’t we increase this effect by
flooding the brain with artificial opioids, thereby intensifying the brain’s own “feel-good”
Be clear on this: Neurotransmitters chemistry? The answer: Because that would disrupt the brain’s chemical balancing act.
are produced inside the body; When flooded with opioid drugs such as heroin, morphine, and fentanyl, the brain — to
they can excite and inhibit neural
communication. Drugs and other maintain its chemical balance — may stop producing its own natural opioids. When the
chemicals come from outside the drug is withdrawn, the brain may then be deprived of any form of opioid, causing intense
body; they can have an agonistic discomfort. For suppressing the body’s own neurotransmitter production, nature charges
effect or an antagonistic effect on a price.
neurotransmission. Students have
confused these concepts on the Drugs and other chemicals affect brain chemistry, often by either exciting or inhib-
AP exam. iting neurons’ firing. Agonist molecules increase a neurotransmitter’s action. Some
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agonists increase the production or release of neurotransmitters, or block synaptic
reuptake. Other agonists may be similar enough to a neurotransmitter to bind to its
receptor and mimic its excitatory or inhibitory effects. Some opioid drugs are agonists
endorphins [en-DOR-fins] and produce a temporary “high” by amplifying normal sensations of arousal or plea-
“morphine within”; natural, sure (Figure 1.3-6).
opioid-like neurotransmitters Antagonists decrease a neurotransmitter’s action by blocking production or release.
linked to pain control and to Botulin, a poison that can grow in improperly canned food, causes paralysis by blocking
pleasure.
ACh release. (Small injections of botulin — known by the brand name Botox — smooth
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agonist a molecule that wrinkles by paralyzing the underlying facial muscles.) These antagonists are enough like the
increases a neurotransmitter’s natural neurotransmitter to occupy its receptor site and block its effect, but are not similar
action.
enough to stimulate the receptor (rather like foreign coins that fit into, but won’t operate,
antagonist a molecule
that inhibits or blocks a a vending machine). Curare, a poison that some South American Indigenous people have
neurotransmitter’s action. applied to hunting-dart tips, occupies and blocks ACh receptor sites on muscles, producing
paralysis in their prey.

34 Unit 1 Biological Bases of Behavior

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