For the first time, researchers have examined in real time how different people feel pain in the brain. By monitoring healthy humans experiencing sustained pain, scientists at the University of Michigan got to watch the brain's painkiller system in action and determined that not all brains handle pain equally well. Their results appear in today's issue of the journal Science.

Thirteen men and seven women volunteered to undergo 20 minutes of constant pain, caused by an injection of highly-concentrated salt water into their jaw muscles. While they suffered, the scientists took brain scans using positron emission tomography (PET). The subjects also received a 20-minute injection of a placebo solution in the randomized, double-blind study. Patients recorded the level of pain they felt every 15 seconds during the injections and completed a pain questionnaire about their experience at the end of the experiment.

Using a small amount of a radioactive substance as a tracer, the scientists focused on the brain's mu-opioid system in which chemicals called endogenous opioids bind to receptors and hinder the spread of pain messages in the brain. The researchers saw the greatest change in brain regions involved in emotional responses and those responsible for processing sensations. They controlled the experimental conditions so that the subjects experienced similar levels of pain, but found that individuals showed different patterns of mu-opioid activity. There were differences in both the amount of chemicals released and the timing of the release. As it turned out, subjects who experienced the largest change in the mu-opioid system between the placebo injection and the painful one tended to report the least pain.

"This may help explain why some people are more sensitive, or less sensitive, than others when it comes to painful sensations," lead author Jon-Kar Zubieta says. "We show that people vary both in the number of receptors that they have for these anti-pain brain chemicals, and in their ability to release the anti-pain chemicals themselves." The scientists hope the study's results will be helpful in understanding and perhaps treating persistent pain syndromes. "It may also be quite relevant," Zubieta says, "to why some people, but not others, develop chronic pain conditions."