Placebo responses in Parkinson's disease.

Parkinson's disease (PD) patients exhibit strong placebo responses in clinical trials. Patient characteristics that affect placebo include patients' expectations of good outcomes, genetic variants, and personality. The presence of motor fluctuation and high baseline UPDRS motor scores predicted placebo response. However, gender, age, duration of PD, religion, or level of education do not correlate with placebo response. PD patients who are preconditioned with active treatment such as apomorphine have more robust placebo effects. Studies that focused on patients with motor fluctuations, surgical intervention, or higher probability of placebo assignment had higher rates of placebo response. Patients view participating in placebo-controlled trials positively. Placebo effect can be measured objectively using neuroimaging and neurophysiological techniques. PET studies show that placebo-induced improvement is associated with dopamine release in the dorsal striatum and that the expectation of receiving the reward, not the reward itself, increased dopamine release in the ventral striatum. Expectations of benefitting from repetitive transcranial magnetic stimulation also induced dopamine release. Expectations of receiving a dopaminergic drug induced changes in fMRI in a reward-learning task. Single cell recordings demonstrate that placebo response is associated with changes of single neuronal activities in the basal ganglia circuit. These studies demonstrate that placebo effects are genuine biological responses to the administration of placebo. In clinical trials, we can use several approaches to minimize placebo responses. In clinical practice, we can use approaches to harness the power of placebo and minimize nocebo effects to improve patients' outcome.