Dopamine (red) approaching one of the receptors (blue)
Juan Gartner/Science Photo Library/Alamy
Dopamine, a hormone and neurotransmitter, is commonly believed to be the driving force behind the placebo effect pain relief experienced, but in fact may play little or no role in this phenomenon.
The placebo effect occurs when a person’s medical symptoms are alleviated through the power of suggestion or expectation, such as taking a sugar pill. Dopamine, along with opioids and cannabinoids that are naturally produced in the body, was thought to be specifically involved in pain relief.
To get a clearer picture, Ulrike Bingel of the University Hospital Essen in Germany and her colleagues teamed up with the Center for Therapeutic Expectations, also in Germany. The scientists asked 168 people aged 18 to 40 with no pre-existing medical conditions to apply two different creams to different parts of their arms and then touch them with a heated wand, causing mild discomfort.
The creams were identical, but participants were told that one contained the active pain-relieving ingredient and the other acted as a placebo.
Just before that, the researchers asked participants to take a drug that either suppressed dopamine, stimulated the release of dopamine, or left dopamine levels unchanged.
Although participants’ dopamine levels changed in the expected way, this did not appear to affect the amount of pain they experienced, which was rated on a scale of 0 to 10, or how much pain they expected to feel.
This suggests that dopamine isn’t directly involved in the placebo effect on pain relief, Bingel said. Opioids and cannabinoids probably play a stronger role, he said. Hormones such as oxytocin and noradrenaline (norepinephrine) may also have a role and could be explored in future studies, Bingel said.
But she says dopamine can also come into play when people are more motivated to relieve pain, such as when the discomfort is greater than what was seen in the current study.
Understanding the placebo effect could lead to treatments that harness its power to better manage pain, Bingel said.
Lauren Atlas of the National Institutes of Health in Bethesda, Maryland, says the placebo effect “may involve verbal cues or social factors that depend on the context surrounding the treatment, and these factors are unlikely to be mediated by dopamine.”
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