New research has challenged previous findings about the role of dopamine in the placebo effect.
Dopamine is involved in the brain’s reward and learning mechanisms and has previously been shown to contribute to the placebo effect – the phenomenon where a person receiving a “non-intervention,” such as a sugar pill, experiences improvement in their condition.
The placebo effect can significantly affect medication and treatment effectiveness, but why this is the case remains largely a mystery to scientists.
To investigate the role of dopamine, Ulrike Bingel from University Hospital Essen in Germany and colleagues conducted a study published in PLOS Biology. It showed that changes in dopamine levels did not affect positive treatment expectations or placebo analgesia (placebo-induced pain relief) in 168 healthy volunteers.
The participants had two creams applied to different parts of their arms before being exposed to moderate pain induced by a heated rod. Both creams were identical but participants were told that one contained an active pain-relieving ingredient while the other was an inactive “control cream”.
Shortly before testing, the participants were given either L-dopa (dopamine precursor), sulpiride (dopamine antagonist) or an inactive pill (control).
Dopamine levels changed in those who took L-dopa and sulpiride, but neither medication seemed to affect how much pain was expected or experienced when compared to the control.
Although the results suggest that dopamine is not directly linked to the placebo effect, it is possible that dopamine may play a role when “active behaviour” or “motivational aspects” are involved.
Neither were explored in this study – the placebo cream was administered passively by the experimenter, and the researchers say the participants may not have developed “sufficient motivation” for pain relief due to the design of the experiment.
Understanding the placebo effect “remains paramount in the quest to exploit [its] effects for optimal treatment outcomes [such as better pain management],” say the authors.
“Our research is driven by the motivation to target the underlying mechanisms of placebo effects to make active medical treatments more effective. The results of our study help to redirect the search for novel treatment targets to achieve this goal.”
