Expectations play a central role in the mechanism of the placebo effect. In Parkinson disease (PD), the placebo effect is associated with release of endogenous dopamine in both nigrostriatal and mesoaccumbens projections, yet the factors that control this dopamine release are undetermined.
To determine how the strength of expectation of clinical improvement influences the degree of striatal dopamine release in response to placebo in patients with moderate PD.
Randomized, repeated-measures study with perceived expectation as the independent between-subjects variable.
University of British Columbia Hospital, Vancouver, British Columbia, Canada.
Thirty-five patients with mild to moderate PD undergoing levodopa treatment.
Verbal manipulation was used to modulate the expectations of patients, who were told that they had a particular probability (25%, 50%, 75%, or 100%) of receiving active medication when they in fact received placebo.
Main Outcome Measures
The dopaminergic response to placebo was measured using [11C]raclopride positron emission tomography. The clinical response was also measured (Unified Parkinson Disease Rating Scale) and subjective responses were ascertained using patient self-report.
Significant dopamine release occurred when the declared probability of receiving active medication was 75%, but not at other probabilities. Placebo-induced dopamine release in all regions of the striatum was also highly correlated with the dopaminergic response to open administration of active medication. Whereas response to prior medication was the major determinant of placebo-induced dopamine release in the motor striatum, expectation of clinical improvement was additionally required to drive dopamine release in the ventral striatum.
The strength of belief of improvement can directly modulate dopamine release in patients with PD. Our findings demonstrate the importance of uncertainty and/or salience over and above a patient's prior treatment response in regulating the placebo effect and have important implications for the interpretation and design of clinical trials.