The purpose of this study is to investigate the long-term effects of treatment with the selective post-synaptic a1-adrenergic blocker terazosin on serial 123 Ioflupane Dopamine Transporter single-photon emission-computed tomography (123I-FP DAT-SPECT) in a population of subjects with defined pre-motor PD risks (i.e., RBD and at least one among hyposmia, constipation, depression and color vision abnormality) and abnormal Iodine-123 meta-iodobenzylguanidine (123I-MIBG) uptake. Imaging changes will be correlated to the presence and severity of motor and non-motor symptoms of PD, measured by validated clinical scales and cardiac autonomic function tests. The rate of RBD clinical conversion to PD will be estimated and compared to available data in the literature.