Oct. 23, 2006
 
HEALTH: Dopamine Plays Role in Pain, Researchers Find
 
By Lee Bowman
Scripps Howard News Service
 
Just as a tickle can turn to torture, researchers have determined that the brain's main messenger of pleasurable experience is also directly involved in feeling pain.
 
The finding by researchers at the University of Michigan may help explain why some people are more susceptible to drug addiction when they're under stress or experiencing chronic pain.
 
Using sophisticated brain-scanning techniques to measure dopamine activity levels in various parts of the brain and a carefully controlled system for inducing muscle pain in 25 healthy male and female volunteers, the researchers showed that the neurotransmitter dopamine is highly active while someone experiences pain.
 
They also found that the dopamine response relates directly to how the pain makes the individual feel.
 
"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 by both positive and negative stimuli," said Dr. Jon-Kar Zubieta, a professor of psychiatry and radiology at the university's medical school and senior author of the study. The report appeared last week in the Journal of Neuroscience.
 
The dopamine system "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," Zubieta added.
 
The researchers, led by graduate student David Scott, used positron emission tomography and magnetic resonance imaging of each subject's brain to pinpoint the exact locations responding to the pain. Each patient was treated with a short-lived radioactive tracer combined with a drug that binds to the same receptors in brain cells that dopamine does. The more that images showed the tracer, the less dopamine was present in a brain region, and vice versa.
 
Imaging showed that dopamine was active in the region of the brain called the basal ganglia, the same region where it normally responds to positive stimuli like food or sex.
 
But when the scientists caused pain in the volunteers' jaw muscles, and asked them to rate different aspects of how they were feeling, responses differed within that region. For instance, the more a volunteer rated the pain as causing emotional distress and fear, the more dopamine was released in an area called the nucleus accumbens, the same region implicated in drug addiction.
 
Dopamine release in two other areas of the basal ganglia matched up closely with how the subjects rated the intensity and unpleasantness of the pain on a scale of 0 to 100.
 
Researchers concluded that in some areas of the basal ganglia, dopamine was involved in the assessment of pain intensity, while in the nucleus accumbens, it was related to the emotional experience of pain.
 
Brain scans also showed that while certain areas of the brain were activated by pain for all participants, there were substantial differences in the degree of dopamine response and the self-rated pain and emotional response to it.
 
Such variation may help explain differences among individuals who are exposed to addictive drugs, with some becoming addicted to the pleasure of the high while others do not.
 
Susceptibility to drug abuse and addiction "could be mediated by individual differences in the response (of the dopamine system) to various forms of stress, with pain being itself a physical and emotional stressor," the researchers said.
 
Now the team is working to find hormonal and genetic factors that may explain why dopamine systems respond differently to pain in different people.
 
On the Net: www.jneurosci.org
 
Contact Lee Bowman at BowmanL@SHNS.com. Distributed by Scripps Howard News Service, http://www.shns.com