Pain and pleasure may be antonyms but a research has shown that dopamine, also known as the pleasure chemical, is involved in response to pain too.

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.

In addition to the differences in dopamine receptor activation in certain areas of the brain across all the participants, the scans also revealed differences between individuals in the level of their dopamine response and their self-rated pain and emotional response. This kind of variation may help explain the major variation between individuals who are exposed to addictive drugs - some become addicted to the pleasures of the "high" the drugs cause, while others do not.