Nine of the participants were classified as "high placebo responders" because they had more than a 20 percent difference between pain and placebo scans in their average pain ratings per volume of salt water infused -- in other words, the placebo effect was strong. The other five were classified as "low placebo responders."
These subjective ratings are consistent with previous findings, Zubieta notes. But the simultaneous imaging of the participants' endogenous pain-reducing opioid systems sheds new light on why the placebo effect occurs.
The imaging method used in the study involves tiny doses of a medicine called carfentanil that is attached to a short-lived radioactive form of carbon, which releases subatomic particles known as positrons. These positrons are detected with the PET scanner, which acts like a photographic camera to capture those particles. It then determines exactly which part of the brain they originated from, and how many of them are coming from each brain region. The researchers also made MRI scans of the participants' brains, which they cross-registered with the PET scans to give accurate information on exactly which brain regions were active.
Because carfentanil competes with the brain's natural endogenous opioid painkillers for space on nerve cell receptors, the PET scans can be used to see how active the opioid system and mu-opioid receptors are. The stronger the positron signal from a particular brain region, the less active the mu opioid system, and vice versa.
All of the participants showed an increase in the activation of their mu opioid endorphin system after they were told that the "medicine" was coming and the placebo was giv
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Source:University of Michigan Health System