New findings argue against a direct causal role for dopamine during the experience of a treatment effect in the establishment of positive treatment expectations and placebo analgesia in healthy volunteers, according to a study published September 24 th in the open-access journal PLOS Biology by Ulrike Bingel from University Hospital Essen, Germany, and colleagues. Dopamine-based reward and learning mechanisms have been suggested to contribute to placebo effects. However, the exact role of the brain messenger molecule dopamine in their generation and maintenance is still unclear.
To fill this knowledge gap, Bingel and colleagues examined the causal role of dopamine in expecting positive treatment effects, as well as the magnitude and duration of their effects on pain. To this end, they used an established placebo pain relief paradigm in combination with two opposing medications to change dopamine levels in the brain, i.e.
, the dopamine antagonist sulpiride, the dopamine precursor L-dopa, and an inactive pill with no medication as control, which were applied in an experimental, double-blind, randomized, placebo-controlled trial involving 168 healthy volunteers. The study medication successfully altered dopaminergic tone during the conditioning procedure. Contrary to the hypothesis, the medication did not modulate the formation of positive treatment expectation and placebo analgesia tested one day later.
Placebo analgesia was no longer detectable on day eight after conditioning..