Dopamine (red) approaching one of its receptors (blue)
JUAN GAERTNER/SCIENCE PHOTO LIBRARY/ALAMY
The hormone and neurotransmitter dopamine is generally believed to be a driving force behind experiencing pain relief with the placebo effect, but it may actually play little or no part in the phenomenon.
The placebo effect occurs when someone’s medical symptoms are lessened through the power of suggestion and expectation, such as by taking a sugar pill. Dopamine, along with opioids and cannabinoids that are produced naturally in our bodies, was thought to be involved in this for pain relief specifically.
To get a clearer picture, Ulrike Bingel at University Hospital Essen in Germany and her colleagues teamed up with the Treatment Expectation research centre, also in Germany. The scientists asked 168 people, aged 18 to 40 and with no known medical conditions, to apply two creams to different parts of their arms, before being touched with a heated rod, which caused mild discomfort.
The creams were identical, but participants were told that one contained an active pain-relieving ingredient and the other was acting as a placebo.
Shortly before, the researchers asked the participants to take medications that suppressed dopamine, encouraged its release or didn’t alter its level.
The participants’ dopamine levels changed as expected, but this didn’t seem to affect how much pain they experienced or how much they anticipated that they would feel, which were both rated on a scale of 0 to 10.
This suggests that dopamine isn’t directly linked to the placebo effect for pain relief, says Bingel. Opioids and cannabinoids probably play a stronger role, she says. Hormones such as oxytocin and noradrenaline (norepinephrine) may also have an effect, which future studies could investigate, says Bingel.
However, it is also possible that dopamine comes into play when people are more motivated to feel pain relief, such as when the discomfort is more intense than it was in this study, she says.
Understanding the placebo effect could lead to therapies that harness its action for better pain management, says Bingel.
Lauren Atlas at the National Institutes of Health in Bethesda, Maryland, says that the placebo effect probably involves “verbal instructions and social factors that depend on the context surrounding treatment, and these factors are unlikely to be mediated by dopamine”.
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