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Source: University of Michigan Health System
http://www.eurekalert.org/pub_releas...-pap101806.php


Pleasure and pain:
Study shows brain's 'pleasure chemical'
is involved in response to pain too


Research may help explain why some people are
more vulnerable to drug abuse when they're under stress or suffer chronic pain

ANN ARBOR, Mich. -- For years, the brain chemical
dopamine has been thought of as the brain's
"pleasure chemical," sending signals between
brain cells in a way that rewards a person or
animal for one activity or another. More
recently, research has shown that certain drugs
like cocaine and heroin amplify this effect – an
action that may lie at the heart of drug addiction.

Now, a new study from the University of Michigan
adds a new twist to dopamine's fun-loving reputation: pain.

Using sophisticated brain-scanning and a
carefully controlled way of inducing muscle pain,
the researchers show that the brain's dopamine
system is highly active while someone experiences
pain – and that this response varies between
individuals in a way that relates directly to how
the pain makes them feel. It's the first time
that dopamine has been linked to pain response in humans.

The finding, published in the October 18 issue of
the Journal of Neuroscience, may help explain why
people are more likely to acquire a drug
addiction during times of intense stress in their
lives. It may also yield clues to why some, but
not other chronic pain patients may be prone to
developing addictions to certain pain
medications. And, it gives further evidence that
vulnerability to drug addiction is a very
individual phenomenon – and one that can't be
predicted by current knowledge of genetics and physiology.

"It appears from our study that dopamine acts as
an interface between stress, pain and emotions,
or between physical and emotional events, and
that it's activated with both positive and
negative stimuli," says senior author Jon-Kar
Zubieta, M.D., Ph.D., professor of psychiatry and
radiology at the U-M Medical School and a member
of the U-M Molecular and Behavioral Neuroscience
Institute and U-M Depression Center. "It appears
to act as a mechanism that responds to the
salience of a stimuli – the importance of it to
the individual – and makes it relevant for them to respond to."

The study, which involved 25 healthy men and
women, showed that dopamine was active in areas
of the brain region known as the basal ganglia,
the same region where it has been observed to
respond to positive stimuli, such as food or sex.

But when the researchers induced pain in the
volunteers' jaw muscle, and asked them to rate
different aspects of how they were feeling,
differences emerged in specific sub-areas of the
basal ganglia. For example, the more a person
rated the pain as causing emotional distress and
fear, the more dopamine was released in the area
known as the nucleus accumbens – the same region implicated in drug addiction.

That effect persisted even after the researchers
controlled for the negative emotional effects
caused by the actual research setup, which
included a needle inserted into a large jaw
muscle, and the expectation of pain and repeated questioning.

Similarly, dopamine release in two other areas of
the basal ganglia – the putamen and caudate
nucleus – was strongly correlated with the rating
of how intense and unpleasant the pain itself was
on a scale of 0 to 100. The authors concluded
that in some areas of the basal ganglia, dopamine
was involved in the assessment of pain itself,
while in the ventral area, or nucleus accumbens,
it was related to the emotional experience of pain.

The study used positron emission tomography, or
PET, scanning that allowed the researchers to
calculate the level of dopamine activity by
measuring the percentage of dopamine receptors on
the surface of brain cells that were active. To
do this, they used the drug raclopride, to which
had been attached a short-lived radioactive form
of carbon. The drug binds to the same receptors
that dopamine does, so the more of it that could
be seen in a specific brain area, the less
dopamine was present and vice versa.

The researchers also scanned each volunteer's
brain using magnetic resonance imaging (MRI) in
order to create a precise map of the brain's
structure, and combined that with their PET scans
to find the exact areas of dopamine activity.

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