Serotonin and Amyloidopathy

Parkinson's disease (PD) is a common problem of older people often causing significant problems with thinking and memory. Abnormal accumulation within in the brain of a protein called amyloid is felt to contribute to impaired thinking and memory in PD. Excessive accumulation of this protein is also involved in Alzheimer disease (AD) and may influence thinking and memory in otherwise healthy older people. Recent evidence suggests that changes in the amount of another brain chemical, serotonin, influences the amount of amyloid accumulating in the brains of people with PD and otherwise healthy older people. The purpose of this study is to determine if changes in brain serotonin affects the accumulation of amyloid in the brain. The investigators use brain imaging methods to measure the amount of serotonin and amyloid in the brain of individuals with PD and otherwise healthy older people. PD participants will undergo repeat brain imaging to assess amyloid accumulation 2 years after their first brain imaging session. All participants will undergo examinations to assess their motor function, and asked questions to assess their mood and thinking.