NIDA BRAIN SLIDE SHOW (continued) PSY273 - QCC
Addiction
Now
that we have defined the concept of reward, let’s define addiction.
Addiction is a state in which an organism engages in a
compulsive behavior, even when faced with negative consequences.
This behavior is reinforcing, or rewarding.
A major feature of addiction is the loss of control in limiting
intake of the addictive substance.
The most recent research indicates that the reward pathway may
be even more important in the craving associated with addiction,
compared to the reward itself.
Scientists have learned a great deal about the biochemical,
cellular and molecular bases of addiction; it is clear that addiction
is a disease of the brain.
|
|
 |
Localization
of opiate binding sites within the brain and spinal cord
When
a person injects heroin (or morphine), the drug
travels quickly to the brain through the bloodstream.
Actually, heroin can reach the brain just as quickly if it is
smoked. Abusers also
snort heroin to avoid problems with needles.
In this case, the heroin doesn't reach the brain as quickly as
if it were injected or smoked, but its effects can last longer.
Once in the brain, the heroin is converted to morphine by
enzymes; the morphine binds to opiate receptors in certain areas of
the brain. Note the areas
where opiates bind (green dots). Part of the cerebral cortex, the VTA,
nucleus accumbens, thalamus, brainstem and spinal cord are
highlighted. The morphine
binds to opiate receptors that are concentrated in areas within the
reward pathway (including the VTA, nucleus accumbens and cortex).
Morphine also binds to areas involved in the pain pathway
(including the thalamus, brainstem and spinal cord). Binding of
morphine to areas in the pain pathway leads to analgesia.
|
Opiates
binding to opiate receptors in the nucleus accumbens: increased
dopamine release
This
is a close-up view of a synapse in
the nucleus accumbens. Three
types of neurons participate in opiate action; one that releases
dopamine (on the left), a
neighboring terminal (on the right) containing a different
neurotransmitter (probably GABA for those who would like to know), and
the post-synaptic cell containing dopamine receptors (in pink).
Opiates bind to opiate receptors (yellow) on the neighboring
terminal and this sends a signal to the dopamine terminal to release
more dopamine. [how--one theory is that opiate receptor activation
decreases GABA release, which normally inhibits dopamine release--so
dopamine release is increased.]
|
 |
 |
Rats
self-administer heroin
Just
as a rat will stimulate itself with a small electrical jolt (into the
reward pathway), it will also press a bar to receive heroin.
In this slide, the rat is self-administering heroin through a
small needle placed directly into the nuclues accumbens.
The rat keeps pressing the bar to get more heroin because the
drug makes the rat feel good. The
heroin is positively reinforcing and serves as a reward.
If the injection needle is placed in an area nearby the nucleus
accumbens, the rat won't self-administer the heroin.
Scientists have found that dopamine release is increased within
the reward pathway of rats self-administering heroin.
So, since more dopamine is present in the synaptic space, it
binds to more dopamine receptors and activates the reward pathway.
|
