40 巻 (2009) 4 号 p. 145-150
The placebo effect can be observed in various medical conditions, and has been documented by randomized placebo-controlled drug studies. One of the best examples is the placebo effect in Parkinson's disease (PD). The placebo effect is observable not only in drug trials but also with surgical therapies, such as deep brain stimulation, fetal tissue transplantation, and infusion therapy of a neurotrophic factor. A recent study using positron emission tomography (PET) with raclopride demonstrated that the release of endogeneous dopamine in the dorsal striatum mediates the placebo effect in PD, suggesting that the placebo effectiveness is achieved by endogenous dopamine supplementation. Although the detailed pathophysiological mechanism underlying the placebo effect is unclear, the placebo effect has been explained by two mechanisms: the conditioning theory (pavlovian conditioning) and the cognitive or expectancy theory (expectation of clinical improvement). Speculatively, it may be that the placebo effect in PD is related more to cognitive mechanisms. Numerous studies have suggested a functional relationship between dopamine and the reward system. Understanding the placebo effect is important for designing clinical trials and interpreting their results.