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Chapter 17

PSY240H5 Chapter Notes - Chapter 17: Dopamine Receptor, Nmda Receptor, Twin Study

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Ayesha Khan

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Theories of Substance Use, Abuse and Dependence
The brain appears to have its own “pleasure pathway,” which affects
our experience of reward
This pathway begins in the midbrain ventral tegmental area and then
goes forward through the nucleus accumbens and on to the frontal
This pathway is rich in neurons sensitive to the neurotransmitter
Some drugs, such as amphetamines and cocaine, act directly to
increase the availability of dopamine in this pathway, leading to the
strong sense of reward or “high” that these drugs produce
Other drugs increase the availability of dopamine in more indirect ways
The neurons in the ventral tegmental area are kept from continuous
firing by GABA neurons, so the firing of GABA neurons reduces the
“high” caused by activity in the dopamine neurons
The opiate drugs inhibit GABA, which in turn stops the GABA neurons
from inhibiting dopamine, which makes dopamine available in the
reward centre
The chronic use of psychoactive substances may produce permanent
changes in the reward centres, causing a craving for these substances
even after withdrawal symptoms pass.
The repeated use of such substances as cocaine, heroin, and
amphetamines causes dopamine neurons to become hyperactive or
This sensitization can be permanent, so that these neurons will be
activated more highly by subsequent exposure to the psychoactive
substance or by stimuli that are associated with the substance (such as
the pipe that a cocaine user formerly used to smoke crack)
This sensitization creates a chronic, strong craving for the substance,
which is made worse every time a former user comes into contact with
stimuli that remind him or her of the substance
This craving can create a powerful physiological motivation for
relapsing back into substance abuse and dependence
Substances that have especially rapid and powerful effects on the brain
but also wear off very quickly, such as cocaine, create great risk for
Psychoactive drugs affect a number of other biochemical and brain
Alcohol has its sedative and antianxiety effects largely by enhancing
the activity of the neurotransmitter GABA in the septal-hippocampal
Alcohol also affects serotonin systems, which in turn are associated
with changes in mood
Alcoholics and other drug addicts were simply perceived as weak, bad
people who would not exert control over their impulses.

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The disease model of alcoholism and other drug addictions views
these disorders as incurable physical diseases, like epilepsy or
Biological Theories:
People who become substance dependent or abusive may react
differently physiologically to substances than do those who do not
become dependent or abusive
Alcoholism, and perhaps other forms of substance dependence, really
represents an underlying biological depression
Genetic Factors
Family history, adoption, and twin studies conducted in Canada and the
United States all suggest that genetics may play a substantial role in
determining who is at risk for substance use disorders
Family studies show that the relatives of people with substance-related
disorders are eight times as likely to also have a substance disorder as
are the relatives of people with no substance-related disorder
There seems to be a common underlying genetic vulnerability to
substance abuse and dependence in general, perhaps accounting for
the fact that individuals who use one substance are likely to use
multiple substances.
Twin studies have clearly shown that a substantial portion of the family
transmission of substance abuse and dependence is due to genetics
Some studies suggest that genetics play a stronger role in alcohol use
disorders among men than women
Some twin studies find no evidence for a genetic contribution to alcohol
dependence in women, or they find that the genetic contribution for
women is less than that for men
Genetic effects were found only in males, while in females substance
use problems were entirely caused by environmental factors
One large twin study found similar heritability for alcohol dependence in
women and men, whereas another study found modestly higher
heritability for women than for men
Environmental circumstances, such as sexual abuse, are stronger
predictors of alcoholism in women than in men, however
The first reports suggesting that genes play a role in smoking were
published half a century ago by Fisher (1958), who found that the
concordance rate for smoking is significantly higher in monozygotic
twins than in dizygotic twins
The genes that confer vulnerability to the initiation of smoking are
distinct from the genes that pertain to the persistence of smoking
There is less than 40% overlap in the genes implicated in smoking
persistence versus smoking initiation
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