The underlying goal of this study is to assess [123I] 5-IA and SPECT imaging as a tool to detect nicotinic receptor activity in the brain of PD patients. All study procedures will be conducted at the Institute for Neurodegenerative Disorders (IND) and Molecular NeuroImaging (MNI) in New Haven, CT. Approximately 10 patients with a diagnosis of PD without cognitive changes will be recruited to participate in this study. Patients will be eligible to participate if they have a diagnosis of PD of more than 2 years duration and have no significant cognitive changes.

All subjects will undergo written informed consent and a screening evaluation including baseline clinical laboratory testing, a baseline physical and neurological evaluation and baseline cognitive evaluations using the MMSE, the ANAM computerized cognitive battery, and other tests of executive function. All subjects will be evaluated with United Parkinson Disease Rating Scales (UPDRS) following an overnight withdrawal of anti-parkinson medication. Subjects will be asked to undergo an injection of [123I] 5-IA followed by SPECT imaging as described below. A second [123I] 5-IA and SPECT imaging study will be obtained for reliability testing between 2 weeks and 2 months following the initial [123I] 5IA imaging session.

The primary imaging outcome measure will be VT', the equilibrium distribution volume in brain regions is determined from an MRI-directed region-of-interest (ROI) analysis. The baseline imaging VT' will be compared to the follow-up imaging study to the reliability of the nicotinic [123I] 5-IA imaging.

Inclusion Criteria:

• The presence of idiopathic Parkinson disease
• A clear clinical response to dopaminergic therapy treatment
• Hoehn and Yahr Stages < 3;
• 2-7 year Duration from time of diagnosis.
• Mini-mental status exam score of >24,
• For females, non-child bearing potential or negative urine pregnancy test on day of [123I] 5-IA injection.

Exclusion Criteria:

• Secondary Parkinsonism;
• Nicotine dependence or use within the previous 12 months prior to enrollment;
• Treatment with Aricept (donepezil), Exelon (rivastigmine), Cognex (tacrine) within the past 30 days; treatment with medications that bind to the nicotinic receptor.
• Clinically significant clinical laboratory value and/or medical or psychiatric illness;
• Mini-mental status exam score of ≤24.
• The subject has evidence of clinically significant thyroid disease, gastrointestinal, cardiovascular, hepatic, renal, hematologic, neoplastic, endocrine, neurologic, immunodeficiency, pulmonary, or other medical or psychiatric disorder;
• The subject has received an investigational drug within 30 days of the screening visit;
• Pregnancy