September 23, 2005

Placebo Effect Tied to Brain Receptor Activity

NEW YORK -- The activation in the brain of chemical receptors, called mu-opioid receptors, appears to be involved in producing what is known as the "placebo effect," according to a report in The Journal of Neuroscience.

The placebo effect occurs when people are given an inactive drug or therapy, but experience an improvement in their symptoms anyway. Researchers often compare new drugs to placebo to gauge the true benefit of a therapy.

"Placebo effects probably form part of brain mechanisms involved in the resiliency of the organism to respond to environmental events (e.g., injury, etc.) and not biases in reporting," Dr. Jon-Kar Zubieta from The University of Michigan, Ann Arbor, Michigan told Reuters Health. "Further understanding of these mechanisms would help harness them for innovative treatment strategies."

Zubieta and colleagues examined the effect of deep sustained pain -- produced by a prolonged infusion of 5 percent hypertonic saline into the masseter muscle --with or without a placebo in young male volunteers.

The researchers used positron emission tomography (PET) and molecular imaging to examine whether introducing a placebo with the expectation of pain relief activates the brain to produce opiate-like substances.

Introducing the placebo was effective in reducing the sensation of pain, the team reports, as evidenced by higher ratings of pain relief and by an increase in the rate of painful stimulus required to maintain the same level of pain.

The changes in the sensation of pain intensity correlated with mu-opioid system activation in the several different regions of the brain, the researchers note.

The study provides the "first direct evidence" that the administration of a placebo with presumed analgesic properties activates pain and stress inhibitory systems in the brain "through direct effects on the mu-opioid receptors," the investigators write.

"The results...are consistent with the notion that placebo-responding regions and neurochemical systems are an intrinsic part of neuronal processes that mediate the interaction between positive environmental conditions (in the present case the suggestion of analgesia) and the corresponding physical and emotional responses of the individual," the researchers conclude.

"We are now looking at individual differences in the placebo response -- why some individuals have a more profound response than others -- for example, the effects of gender, hormones and genes," Zubieta said.

SOURCE: Journal of Neuroscience, August 24, 2005.