Download LESSON 5.2 WORKBOOK How do drugs alter synaptic transmis-

LESSON 5.2 WORKBOOK
How do drugs alter synaptic transmission?
DEFINITIONS OF TERMS
Reuptake – process that involves
transport of neurotransmitter out of
the synaptic cleft by the same cell
that released the neurotransmitter.
For a complete list of defined
terms, see the Glossary.
Wo r k b o o k
Lesson 5.2
Now that we know which neural pathway is activated in
response to rewarding stimuli, let’s take a closer look
at the synapses in the pathway and see how different
drugs of abuse change their synaptic signaling.
Dopamine signaling
All the natural reinforcers that have been studied
so far (such as food for a hungry animal, water for
a thirsty one, or sexual contact) have one physiological effect in common – they cause the release
of dopamine in the nucleus accumbens. The release of dopamine appears to be a necessary
condition for positive reinforcement to take place.
What do all natural reinforcers have in common?
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How does the dopamine synapse clear dopamine from the synaptic cleft?
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Addictive drugs – including amphetamine, co___________________________________
caine, opiates, nicotine, alcohol, PCP, and canna___________________________________
Figure
3:
Dopamine
synapse.
The
dopamine
bis – trigger the VTA to release dopamine in the
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synaptic cleft does not contain an inactivating
nucleus accumbens. Different drugs stimulate the
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enzyme, therefore in order to turn off doparelease of dopamine in different ways. Before we
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mine signaling, dopamine must be recaptured
get into the specifics of how different drugs stimu___________________________________
and transported back into the presynaptic
terminal by transporters in a process known as
late dopamine signaling, we need to mention one
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reuptake.
important feature of the dopamine synapse.
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Once dopamine has been released, moved across the synaptic cleft and interacted with its receptors, ___________________________________
the dopamine synapse clears dopamine from the synaptic cleft by pumping it back into the presynaptic ___________________________________
terminal using specific transporters. This process is known as reuptake (Figure 3). This means that any ___________________________________
drug inhibiting the reuptake of dopamine from the synaptic cleft will cause dopamine levels to stay high,
and its effects to persist, because it can’t be taken back up by the presynaptic terminal. Keep this in mind
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as we discuss how specific drugs change dopamine signaling.
LESSON READING
What are the psychological and behavioral
effects of taking cocaine?
Stimulant drugs: Cocaine and Amphetamine
Cocaine and amphetamine have similar behavioral effects: both are stimulants and both act as potent dopamine agonists. However, their sites of action are different. Cocaine binds with and inactivates the dopamine transporter proteins, thus blocking dopamine reuptake and keeping dopamine levels in the synaptic
cleft high. Amphetamine has two effects: It also inhibits the reuptake of dopamine, but its most important
effect is to directly stimulate the release of dopamine from axon terminals.
DEFINITIONS OF TERMS
Agonist – drug that facilitates the
effects of a neurotransmitter on the
postsynaptic cell .
Stimulants – class of drugs that
increase the activity of the central
nervous system.
For a complete list of defined
terms, see the Glossary.
Cocaine
When people take cocaine (Figure 4), they become euphoric, active and talkative. They say that they feel powerful and alert. Some of them become addicted to the drug,
and obtaining it becomes an obsession to which they devote more and more time and money.
Laboratory animals, which quickly learn to self-administer
cocaine, also act excited and show intense exploratory activity. If animals are given continuous access to self-inject
cocaine, they often self-inject so much that they die from
an overdose.
Figure 4: Cocaine. When people take
cocaine they become euphoric, active
and talkative. Once addicted to cocaine,
obtaining it becomes an obsession.
One of the alarming effects seen in people who abuse
either cocaine or amphetamine is psychotic behavior: hallucinations, delusions of persecution, mood disturbances, and repetitive behaviors. These behaviors so closely resemble those of paranoid schizophrenia
that even trained mental health professionals cannot distinguish between them unless he or she knows
about the person’s history of drug abuse. These effects disappear once people stop taking the drug.
As mentioned previously, cocaine use increases alertness, energy, motor activity and feelings of well-being. But feelings of anxiety, paranoia and restlessness are also common effects of cocaine. With excessive
use, cocaine can cause tremors, convulsions, stroke and even death.
Cocaine effects dopamine synaptic transmission by altering the way in which dopamine is removed from
the synaptic cleft. Under normal conditions, the VTA releases dopamine onto the nucleus accumbens.
Dopamine binds to receptors within the nucleus accumbens and this initiates downstream signaling in the
nucleus accumbens neurons. As we saw before, dopamine signaling is normally stopped when the dopamine transporters (also known as pumps) in the dopamine reuptake system pump dopamine back into the
presynaptic neurons, thus decreasing the levels of dopamine in the synaptic cleft.
Wo r k b o o k
Lesson 5.2
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Where does cocaine act within the reward
pathway?
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LESSON READING
What is the effect of cocaine on dopamine
levels in the nucleus accumbens?
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Cocaine ___________________________________
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Figure 5: Mechanism of cocaine action. Cocaine
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increases synaptic dopamine by binding to the dopamine
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transporter and inhibiting dopamine reuptake.
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Compare the healthy brain at the top with the
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brain of a cocaine abuser at the bottom of Fig___________________________________
ure 6. Didn’t we say cocaine is a stimulant?
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Why then does cocaine reduce brain activity
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if it increases the activity of dopamine neuro___________________________________
transmission at the nucleus accumbens syn___________________________________
apse? This is because the output neurons from
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the nucleus accumbens are actually inhibitory.
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That is to say when they are activated they shut
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down signaling in the regions they synapse
Figure 6: PET scans showing decreased activity
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with. When they are stimulated by cocaine the
in cocaine addict. Cocaine reduces brain activity
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because output from the nucleus accumbens is
inhibition is even more intense.
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largely inhibitory.
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You can watch an animation describing how cocaine affects the brain online — see this unit on the student ___________________________________
website or click below:
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■■ Video: How Does Cocaine Affect the Brain?
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Cocaine blocks the activity of dopamine
transporters. Since dopamine transporters are responsible for removing excess
dopamine from the synaptic cleft, when
they are blocked, dopamine cannot be
removed from the synaptic cleft, and dopamine levels increase which increases
dopamine signaling at the nucleus accumbens (Figure 5).
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Lesson 5.2
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LESSON READING
Amphetamine
Amphetamines include methamphetamine, crystal meth, and crack. Amphetamines are also central nervous system stimulants, increasing alertness and focus, while decreasing fatigue and appetite. They also
produce hyperactivity and anxiety. With chronic use and/or high doses, amphetamines can also cause
seizures, stroke, coma, and death.
Amphetamine (Figure 7) has two effects on the VTA-nucleus
accumbens synapse: The first effect is similar to cocaine – it
also prevents dopamine reuptake. The most important effect
of amphetamine however is to stimulate release of dopamine
into the synaptic cleft (Figure 8).
Figure 7: Amphetamines include methamphetamine, crystal meth and crank.
Amphetamines are CNS stimulants.
Under normal conditions, the VTA releases moderate amounts
of dopamine onto the nucleus accumbens. Dopamine then
binds to receptors within the nucleus accumbens and effects
downstream signaling, as we have seen before. Then, to stop
dopamine signaling, dopamine transporters (also called dopamine reuptake pumps) pump dopamine back into the presynaptic neurons, which decreases the levels of dopamine in the
synaptic cleft.
Amphetamines can alter dopamine synaptic transmission by blocking dopamine transporters like cocaine
does. When the dopamine transporters are blocked, dopamine cannot be removed from the synaptic cleft,
and dopamine levels remain high, which increases dopamine signaling in the nucleus accumbens, just as
we saw for cocaine.
Wo r k b o o k
Lesson 5.2
The second way amphetamines can alter synaptic transmission is by entering
the presynaptic terminals of the VTA’s
neurons and causing dopamine to be released even in the absence of action potentials. This release also increases the
level of dopamine signaling to the nucleus accumbens. Scientists are still trying
to figure out exactly how amphetamines
cause the release of dopamine in the absence of action potentials, but somehow
this mechanism gives amphetamines another way to alter dopamine signaling in
the reward pathway.
Amphetamine Figure 8: Mechanism of amphetamine action. Amphetamine (AMPH) increases synaptic dopamine in two ways.
First, it binds to the dopamine transporter and inhibits
dopamine reuptake. Second, it enters the presynaptic
terminal and stimulates dopamine release. The combined
effect is a massive increase in synaptic dopamine levels.
What are the psychological and behavioral
effects of taking amphetamine?
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Where does amphetamine act within the reward pathway?
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How does amphetamine alter synaptic signaling of the reward pathway? What is the
drug target?
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What is the effect of amphetamine on dopamine levels in the nucleus accumbens?
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LESSON READING
What are the psychological and behavioral
effects of taking heroin?
Opiates: Heroin and Morphine
Opium, derived from a sticky resin produced by the opium poppy, has been eaten and smoked for centuries, and in 1847 scientists figured out to make it themselves, producing heroin, the most commonly abused
opiate. Addiction to opiates, like heroin, has several high personal and societal costs.
• First, because heroin (Figure 9) is an illegal drug is
most countries, an addict becomes by definition, a
criminal.
DEFINITIONS OF TERMS
• Second, because of tolerance, a person must take
Opiates – class of drugs with pain
reducing qualities.
increasing amount of the drug to achieve a “high”.
The habit thus becomes more and more expensive, and the person often turns to crime to obtain
enough money to support his or her habit.
• Third, addicts that inject opiates often don’t have
For a complete list of defined
terms, see the Glossary.
Where does heroin act within the reward
pathway?
Figure 9: Heroin is the most commonly
abused opiate.
access to sterile one-use needles, so a substantial
percentage of people who inject illicit drugs have been exposed to hepatitis or HIV.
• Fourth, if the addict is a pregnant woman, her infant will also become dependent on the drug, which
easily crosses the placenta barrier. The infant must be given opiates right after birth, and then weaned
off the drug with gradually decreasing doses.
• Fifth, the uncertainty about the strength of a given batch of heroin means that a user could unwittingly
take an overdose, with possibly fatal consequences.
Heroin and morphine are both opiates. They are analgesics, meaning they reduce pain without producing
unconsciousness. Heroin and morphine produce a sense of relaxation and sleep, and at high doses can
cause coma and death.
Heroin and morphine increase the activity at the nucleus accumbens in two ways, both are different from
how cocaine and amphetamines act. Both heroin and morphine work on a group of inhibitory neurons that
normally inhibit the VTA. Under normal circumstances these neurons decrease the activity of the VTA.
However when heroin and morphine bind to their receptors, they can no longer decrease the activity of the
VTA. When the inhibition to the VTA is blocked, the VTA’s activity is increased, which results in an increase
in dopamine signaling to the nucleus accumbens. Thus when either heroin or morphine are present, more
dopamine is released by the VTA to the nucleus accumbens because the VTA is no longer inhibited.
Wo r k b o o k
Lesson 5.2
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Heroin and morphine can also affect the nucleus accumbens directly by binding to receptors for opiates
found on the nucleus accumbens itself. The body itself produces natural opiates, which play an important
role in reducing pain sensations and many neurons have receptors for these natural opiates. The nucleus
accumbens is no exception. Both heroin and morphine can bind to these receptors and in this way can
directly affect the activity of the nucleus accumbens.
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How does heroin alter synaptic signaling of
the reward pathway? What is the drug target?
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What is the effect of heroin on dopamine levels in the nucleus accumbens?
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LESSON READING
Nicotine
While nicotine (Figure 10) might seem rather tame in comparison to cocaine, amphetamine and opiates, it
is still an addictive drug, and it accounts for more deaths than all the other so called “hard” drugs combined.
The combination of nicotine and other substances in tobacco smoke is carcinogenic and leads to cancer
in the lungs, mouth, throat and esophagus. The World Health Organization (WHO) estimates that 50% of
people who begin to smoke as adolescents and continue to smoke will die from smoking-related diseases.
Nicotine is extremely addictive; many people continue to smoke even when their health is seriously affected. Although tobacco companies and others with vested interests have tried to argue that smoking is a
“habit” rather than an “addiction”, it is clear that people who regularly use tobacco behave like compulsive
drug users. Smokers tend to smoke regularly or not at all; few smoke just a little. Male smokers smoke an
average of 17 cigarettes per day, while female smokers smoke an average of 14. Nineteen out of twenty
smokers smoke every day, and only 60 out of 3500 smokers surveyed smoke fewer than 5 cigarettes a day. Of those who attempt
to quit smoking by enrolling in a special program, only 20% manage to abstain for one year. The record is much poorer for those
who try to quit on their own: one-third manage to stop for one day,
Figure 10: Nicotine is the active
one-fourth manage for one week, but only 4% manage to stop for
ingredient in cigarettes and chewing tobacco.
six months.
What are the psychological and behavioral
effects of taking nicotine?
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Where does nicotine act within the reward
pathway?
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How does nicotine alter synaptic signaling of
Ours is not the only species that will willingly self-administers nicotine, laboratory animals will too. Nicotine the reward pathway? What is the drug target?
binds to and stimulates nicotinic acetylcholine receptors in the VTA. Once nicotine binds to these receptors,
the neurons in the VTA fire action potentials. These action potentials cause the VTA to release dopamine
onto the nucleus accumbens, thus increasing dopamine levels in the synaptic cleft.
Scientists have also learned that nicotine can increase the actual amount of dopamine that the VTA releases, although they do not understand how. They do know that when nicotine binds to its receptors, the
VTA releases more dopamine than normal. Thus nicotine binding to its receptor not only causes dopamine
to be released in response to an action potential, but also the amount of dopamine released is greater.
Wo r k b o o k
Lesson 5.2
Through binding to nicotinic receptors, nicotine stimulates the central nervous system, causing increased
alertness, and decreased appetite. Like other drugs nicotine also has unpleasant effects such as nausea,
vomiting, diarrhea, and confusion.
When people stop smoking, they often start overeating and gain weight. One lab discovered why: As you
will see in the Metabolic Diseases module, eating and changes in metabolism are regulated by the activity
of two different types of neurons whose cell bodies are located in the hypothalamus. One of these sets of
neurons secretes the peptide called melanocyte-stimulating hormone (MSH). MSH affects neurons that
normally increase appetite. When nicotine is present, it inhibits these neurons, suppressing appetite. However when nicotine is removed, such as when people try to stop smoking, this inhibition is removed and the
MSH neurons can stimulate eating again — so people overeat and gain weight.
You can watch a video about nicotine’s effect on the brain online — see the student website or click below:
■■ Video: Visualization award winner in Science - Nicotine addiction and molecule diffusion
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What is the effect of nicotine on dopamine
levels in the nucleus accumbens?
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LESSON READING
Alcohol
DEFINITIONS OF TERMS
Apoptosis – type of cell death in
which the cell uses specialized
cellular machinery to kill itself.
Depressant – class of drugs that
decrease the activity of the central
nervous system.
For a complete list of defined
terms, see the Glossary.
Wo r k b o o k
Lesson 5.2
What are the psychological and behavioral
effects of taking alcohol?
Alcohol (Figure 11) also has enormous
costs to society. A large percentage of
deaths and injuries caused by car accidents are related to alcohol use. Additionally, alcohol contributes to violence and
aggression. Chronic alcoholics often lose
their jobs, homes, and families; and many
Figure 11: Alcohol is the most commonly used drug in
die of cirrhosis of the liver, exposure, or
the United States.
diseases caused by poor living conditions
and abuse of their bodies. Alcohol consumption by pregnant women leads to fetal alcohol syndrome, one
of the leading causes of mental retardation in the Western world today. Therefore, understanding the physiological and behavioral effects of alcohol is an imporant issue.
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Alcohol acts as a depressant in the central nervous system. Alcohol consumption produces a decrease in
anxiety, disinhibition, intoxication, memory impairment and sleep. At high doses, alcohol can cause vomiting, unconsciousness and even death due to the inhibition of the brain’s breathing center.
Alcohol is considered a sedative because it reduces the excitability of neurons. Alcohol has this effect by
increasing the activity of the brain’s main inhibitory neurotransmitter, GABA. Alcohol can bind to the GABA
receptor and increasing its activity. When alcohol is present, GABA signaling is increased, thus increasing
inhibition.
How does alcohol alter synaptic signaling of
the reward pathway? What is the drug target?
Where does alcohol act within the reward
pathway?
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Alcohol’s serious effects on fetal development occur during the brain growth spurt period, which occurs
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during the last trimester of pregnancy and for several years after birth. One study found that exposure of an
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immature rat brain to alcohol triggered widespread death of neurons called apoptosis. This resulted in the
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death of various areas of the immature brain, and abnormal brain development.
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Alcohol can also affects the brain’s reward system. It does this by inhibiting the same inhibitory neurons that ___________________________________
project to the VTA that opiates work on. Recall that when they are active these inhibitory neurons decrease ___________________________________
the activity of the VTA, which decreases the levels of dopamine the VTA releases onto the nucleus accumbens. However, when alcohol is present, the activity of these inhibitory neurons is blocked which removes What is the effect of alcohol on dopamine
their inhibition of the VTA. This removal of inhibition increases the activity of the VTA. The VTA is then able levels in the nucleus accumbens?
to increase dopamine signaling to the nucleus accumbens. Thus when alcohol is present, more dopamine ___________________________________
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is released by the VTA onto the nucleus accumbens because inhibition is removed.
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LESSON READING
What are the psychological and behavioral
effects of taking marijuana?
Marijuana
Another drug that people regularly self-administer is THC, the active ingredient in marijuana (Figure 12).
THC comes from the flowering hemp plant, Cannabis sativa, and is a class of chemical called cannabinoids. THC is found in several different forms such as marijuana and hashish, both of which may be
smoked or eaten. The consumption of cannabis for its intoxicating effects is thought to date back thousands
of years to Eastern cultures. The practice of marijuana smoking was introduced in the United States in the
early 1900s by Mexican and West Indian immigrants.
Marijuana is the most heavily used illicit drug in the United
States. Marijuana use causes euphoria, disinhibition,
relaxation, altered sensations and increased appetite.
Long-term use of marijuana has been associated with
deficits in cognitive function, respiratory problems and
impaired immune function.
Studies with laboratory animals show that in rodents THC
produces changes in motor activity, catalepsy (inability to
move), hypothermia, and analgesia. Cannabinoids disrupt
memory in several kinds of learning tasks, an effect that
is thought to be related to activation of the hippocampus.
Where does marijuana act within the reward
pathway?
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Figure 12: Marijuana. The active
When marijuana is smoked, THC rapidly passes from
ingredient in marijuana, THC is regularly
self-administered, resulting in euphoria,
the lungs into the bloodstream which carries THC into the
disinhibition, relaxation, altered sensabrain. Once in the brain, THC binds to receptors, called
tions and increased appetite.
cannabinoid receptors. These receptors are particularly
concentrated on the VTA. The binding of THC to cannabinoid receptors on the VTA stimulates them to fire an action potential. These action potentials cause the
VTA to release dopamine, thus increasing dopamine signaling at the nucleus accumbens.
There are many concerns about the adverse effects of chronic cannabis use. In young people the amount
of cannabis use directly correlates with poor educational performance, although whether this is a causal
effect is not clear. Long-term use has been associated with at least temporary decrements in cognitive
function, although whether these effects persist has been disputed. Likewise, it has been suggested that
heavy cannabis use produces persistent cognitive deficits and/or apathy, loss of achievement motivation
and decreased productivity, but the evidence supporting these suggestions is not strong. Because of this,
researchers currently favor the hypothesis that early cannabis use is linked to lifestyles that devalue educational achievement.
Wo r k b o o k
Lesson 5.2
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On the other hand, there are real health risks associated with marijuana smoking that involve respiratory
problems.
How does marijuana alter synaptic signaling
of the reward pathway? What is the drug target?
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What is the effect of marijuana on dopamine
levels in the nucleus accumbens?
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LESSON READING
Drugs of abuse have similar effects
All drugs of abuse increase dopamine activity at the synapse between the VTA and the NAc, but they do
it in slightly different ways:
•
Cocaine and amphetamine both inhibit dopamine reuptake transporters, thus increasing the
amount of dopamine remaining in the synaptic cleft.
•
Heroin and other synthetic opiates, like morphine, effect dopamine signaling in two ways. First,
they inhibit inhibitory neurons projecting to the VTA. By removing this inhibition, the drugs increase
VTA activity and the release of dopamine. Second, the drugs can also directly affect the nucleus
accumbens by binding to opiate receptors.
•
Alcohol, like the opiates, inhibits inhibitory projections to the VTA, thereby increasing VTA activity
and its release of dopamine.
•
Nicotine stimulates the VTA, where the dopamine neurons originate, directly, increasing dopamine release.
•
THC the active ingredient in marijuana binds to cannabinoid receptors on the VTA which stimulates it to release dopamine.
Drugs of abuse can increase the amount of dopamine in the synaptic cleft between 2 - 10 fold more than
normal. In some cases, this occurs almost immediately (as when drugs are smoked or injected), and the
effects can last much longer than those produced by natural rewards.
Wo r k b o o k
Lesson 5.2
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STUDENT RESPONSES
On the diagram of the reward circuit below, draw in where each drug of abuse affects the pathway. Make sure to include: cocaine,
amphetamine, heroin & morphine, nicotine, alcohol, and marijuana.
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Inhibitory _____________________________________________________________________________________________________
Neuron _____________________________________________________________________________________________________
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Prefrontal _____________________________________________________________________________________________________
Cortex VTA _____________________________________________________________________________________________________
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NAc _____________________________________________________________________________________________________
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What do all drugs of abuse have in common? In terms of their activity on the reward pathway? In terms of their abuse potential?
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