From Scientific American:
Sometimes, just thinking you are receiving treatment is enough to make you feel better, a phenomenon known as the placebo effect. Scientists have long wondered what causes this outcome, the magnitude of which is not the same for all people. A new brain imaging study suggests that the body’s natural painkillers, endorphins, play a significant role.
Previous studies had shown general changes in brain activity associated with the placebo effect by using functional magnetic resonance imaging, and scientists had hypothesized that the brain’s opioid system was involved. This time, by utilizing positron emission tomography (PET) brain scans for the new work, the researchers were able to focus on a specific type of brain receptor and track its response to a placebo. The PET scans employed by Jon-Kar Zubieta of the University of Michigan and his colleagues measured the activity of mu-opioid receptors, which are an integral part of the body’s natural painkilling system and help transmit pain signals from one nerve cell to the next.
More here.