Why do sugar pills and other sham medications often ease pain even though they contain no painkilling properties? Scientists think they may have the answer. New research indicates that when people expect a treatment to numb their hurt, their brains release endorphins, the body's own opioid painkillers. Experts say the findings are the first to nail down the molecular basis of the placebo effect.
Scientists have long suspected that placebos tap into opioid circuitry within the brain. Chemical blockades of these circuits abolish the placebo effect, and placebos increase blood flow to areas of the brain rich in opioid receptors (ScienceNOW, 7 February 2002). To obtain direct evidence that opioid receptors mediate the placebo response, neuroscientist Jon-Kar Zubieta of the University of Michigan in Ann Arbor and colleagues used a radioactive tracer molecule that sticks to vacant opioid receptors.
First, the team induced a painful, sustained ache in 14 men by injecting a salt solution into their jaw muscles. The researchers then scanned the subjects' brains using positron emission tomography, which locates the radioactive tracer. During one scan, the researchers offered no relief from the pain. During another, they told the subjects they were administering an experimental drug thought to ease pain, but instead gave only saline. When subjects got the placebo, they experienced a decreased sensitivity to pain, and their brain scans revealed occupied opiate receptors, an indication that opioids had bound to them. "This is not just a psychological phenomenon," says Zubieta, whose team reports its findings 24 August in the Journal of Neuroscience. "It is a real chemical mechanism."
"This is the first direct demonstration of opioid release in the brain after placebo administration," says neuroscientist Fabrizio Benedetti of the University of Turin, Italy. The study is also the first to pinpoint specific brain areas where opiates are released under placebo, notes Predrag Petrovic of the Karolinska Institute in Stockholm, Sweden. He believes this approach, which has so far only been used by Zubieta's group, will be widely adopted to study other aspects of the placebo effect, such as the tempered emotional responses to pain.
Related Sites
More
on placebos from the FDA
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Zubieta's
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