Placebo and Nocebo Effects Are Defined by Opposite Opioid and Dopaminergic Responses

To examine the contribution of 2 different neurotransmitters, the endogenous opioid and the dopaminergic (DA) systems, to the development of placebo and nocebo effects. DESIGN AND SETTING: Using a within-subject design, subjects twice underwent a 20-minute standardized pain challenge, in the absence and presence of a placebo with expected analgesic properties. Studies were conducted in a university hospital setting.

Twenty healthy men and women aged 20 to 30 years recruited by advertisement. MAIN OUTCOME MEASURES: Activation of DA and opioid neurotransmission by a pain stressor with and without placebo (changes in the binding potential of carbon 11 [11C]-labeled raclopride and [11C] carfentanil with positron emission tomography) and ratings of pain, affective state, and anticipation and perception of analgesia.