Terazosin and Parkinson's Disease Extension Study

Primary procedures in this study are MIBG scan, DAT scan, NM-MRI, and terazosin titration. Subjects will return for research visits and imaging every six months for three years. The investigators hypothesize that the rate of decline in DAT scan123I-Ioflupane uptake will be slower in subjects who have received the a1- Adrenergic Receptor Antagonist Terazosin, resulting in a decreased clinical conversion rate to parkinsonism.