A natural system acts to slow transmission of pain signals among nerve cells.
Researchers have demonstrated for the first time that just thinking that medicine will make pain go away prompts the brain to release its own natural painkillers.
Although this "placebo effect" against pain has long been dismissed as a psychological rather than physical phenomena, scientists have recently spotted specific activity in brain centers related to pain after sham treatments were administered.
A new study nails down the actual brain chemistry invoked by a pain-related placebo. Researchers at the University of Michigan Health System are reporting their work today in the Journal of Neuroscience.
The effort involved brain scans on 14 healthy young men who agreed to let the scientists inject their jaw muscles with a concentrated saltwater solution to induce pain. The injections were done as the men's brains were being scanned through tomography. During one session, the volunteers were told they would receive a medicine (actually a placebo) that might relieve the pain.
All participants showed an increase in activation of their endogenous opioid systems after they were told the medicine was coming and the placebo was given. This system naturally acts to stop or slow the transmission of pain signals from one nerve cell to the next, and also reacts to drugs such as heroin, morphine, methadone and various anesthetics.
"This deals another serious blow to the idea that the placebo effect is purely psychological," said Dr. Jon-Kar Zubieta, an associate professor of psychiatry and radiology at the health system and a research scientist at the university's Molecular and Behavioral Neurosciences Institute.
"We were able to see that the endorphin system was activated in pain-related areas of the brain and that activity increased when someone was told they were receiving a medicine to ease their pain. They then reported feeling less pain. The mind-body connection is quite clear."
Every 15 seconds during scans, the patients were asked to rate the intensity of pain sensations on a scale of 0 to 100. The men gave more detailed ratings in interviews after the experiment. The researchers also asked the participants about mood, emotions and other details of how they felt during each scan.
There were significant differences in ratings given by participants during the scan in which they got the placebo and when they got the injection alone.
Moreover, the researchers actually secretly upped the pain ante for participants after telling them they were getting the medicine. The scientists gradually increased the amount of saltwater injected into the jaw muscles as the scans continued, yet the tolerance for injections stayed about the same. This indicated a reduction in pain sensitivity that the volunteers weren't aware of.
Nine participants were classified as "high placebo responders" because they had a difference of more than 20 percent in their average pain ratings between the pain alone and placebo scans, as the volume of saltwater injected increased. The placebo effect was less pronounced in the other five volunteers.
Dr. Zubieta said the results might help explain why so many people say they get relief from therapies and remedies that have no physical action against an ailment, and may promote better use of psychological therapy in treating people suffering from chronic pain.
But because the research was done only in healthy men between the ages of 20 and 30, Dr. Zubieta said more study would be needed to determine if the same chemical pathways are activated in women and people suffering from various illnesses.