NIDA BRAIN SLIDE SHOW (continued) PSY273 - QCC
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Dopamine
neurotransmission and modulation by endogenous opiates
The
dopamine is synthesized in the nerve terminal and packaged in
vesicles. Note how the vesicle fuses with the membrane and releases
dopamine. The dopamine molecules can then bind to a dopamine receptor
(in pink). After the dopamine binds, it comes off the receptor and is
removed from the synaptic cleft by uptake pumps (also proteins) that
reside on the terminal (arrows show the direction of movement).
This process is important because it ensures that not too much
dopamine remains in the synaptic cleft at any one time.
Also point out that there are neighboring neurons that release
another compound called a neuromodulator.
Neuromodulators help to enhance
or inhibit neurotransmission that is controlled by neurotransmitters
such as dopamine. In this
case, the neuromodulator is
an "endorphin" (in red).
Endorphins bind to opiate receptors (in yellow) which can
reside on the post-synaptic cell (shown here) or, in some cases, on
the terminals of other neurons (this is not shown so it must be
pointed out). The
endorphins are destroyed by enzymes rather than removed by uptake
pumps.
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The reward pathway and addiction
Now
we examine the concept of reward.
Humans, as well as other organisms engage in behaviors that are
rewarding; the pleasurable
feelings provide positive reinforcement so that the behavior is
repeated.
There are natural rewards as well as artificial rewards, such
as drugs.
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Natural rewards
Natural
rewards such as food, water, sex and nurturing allow the organism to
feel pleasure when eating, drinking, procreating
and being nurtured.
Such pleasurable feelings reinforce the behavior so that it
will be repeated.
Each of these behaviors is required for the
survival of the species.
Remember there is a pathway in the brain that is responsible
for rewarding behaviors.
This can be viewed in more detail in the next slide.
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The
reward pathway
This
is a view of the brain cut down the middle.
An important part of the reward pathway is shown and the major structures
are highlighted:
the ventral tegmental area (VTA), the nucleus accumbens and the
prefrontal cortex.
The VTA is connected to both the nucleus accumbens and the
prefrontal cortex via this pathway and it sends information to these
structures via its neurons.
The neurons of the VTA contain the neurotransmitter dopamine
which is released in the nucleus accumbens and in the prefrontal
cortex. Remember that this pathway is activated by a rewarding
stimulus.
[Note: the pathway shown here is not the only pathway activated
by rewards, other structures are involved too, but only this part of
the pathway is shown for simplicity.]
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Activation
of the reward pathway by an electrical stimulus
The
discovery of the reward pathway was achieved with the help of animals
such as rats. Rats were
trained to press a lever for a tiny electrical jolt to certain parts
of the brain. Show that
when an electrode is placed in the nucleus accumbens, the rat keeps
pressing the lever to receive the small electrical stimulus because it
feels pleasurable. This
rewarding feeling is also called positive reinforcement.
See the area of the brain close to the nucleus accumbens. When
the electrode is placed there, the rat will not press the lever for
the electrical stimulus because stimulating neurons in a nearby area
that does not connect with the nucleus accumbens does not activate the
reward pathway. The
importance of the neurotransmitter dopamine has been determined in
these experiments because scientists can measure an increased release
of dopamine in the reward pathway after the rat receives the reward.
And, if the dopamine release is prevented (either with a drug
or by destroying the pathway), the rat won't press the bar for the
electrical jolt. So with
the help of the rats, scientists figured out the specific brain areas
as well as the neurochemicals involved in the reward pathway.
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