The brain chemical dopamine has always been considered as a 'pleasure chemical,' in charge of sending signals between brain cells in a manner which extols a person or animal for the continuous activity that go on in succession. Novel research has shown that certain drugs like cocaine and heroin can simply intensify this effect .

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.