Can a common drug reveal Parkinson's before symptoms start?

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

Inclusion Criteria: * Male or female of age between 50 and 85 years at time of enrollment. * Diagnosis of idiopathic REM sleep behavior disorder (iRBD), established either as 'definite RBD' according to the criteria proposed by the International Classification of Sleep Disorders (ICSD)-2 \[AASM, 2005\] or 'probable RBD' following a score of 6 or higher in the RBD questionnaire (RBDSQ) \[Nomura et al, 2011\], with a score of at least 1 in subitems 6.1 to 6.4 of question 6 \[Halsband et al, 2018\]. * At least one of the following: 1. Diagnosis of hyposmia, established as a University of Pennsylvania Smell Identification Test (UPSIT) score \< 20th percentile for the individual's age group and sex. 2. Functional constipation assessed by a scores \> 4 on a questionnaire based on modified ROME IV diagnostic criteria. 3. Color vision abnormality, as assessed using HRR Pseudoisochromatic Plates, in the absence of congenital dyschromatopsia 4. Symptoms of depression, as assessed by a Beck Depression Inventory (BDI) fast screen score \>3 or concurrent use of antidepressant medications. * Abnormal 123I-MIBG myocardial scintigraphy, as defined by a Late H/M ratio \< 2.2 and/or a WR \>20%, with normal cardiac ejection fraction (LVEF \>55%). * Capacity to give informed consent Exclusion Criteria: * Secondary Parkinsonism, including tardive * Concurrent dementia defined by a score lower than 22 on the MOCA * Concurrent severe depression defined by a BDI fast screen score greater than 13 * Comorbidites related to SNS hyperactivity * Heart failure (LVEF\< 45%) * Recent myocardial revascularization (\< 12 weeks) * Hypertension (SBP \>150 mmHg or DBP\> 100mmHg) * Chronic Atrial fibrillation * Concurrent use of Alpha- adrenergic antagonist * Diabetes mellitus * COPD * Untreated Severe Sleep Apnea; Apnea-Hypopnea Index (AHI) \> 30/h. * Contraindication to the use of Terazosin * Recent myocardial infarction (\< 48 h) * Ongoing angina pectoris * Cardiogenic shock or prolonged * Breast feeding * Current use of Phosphodiesterase type 5 inhibitors: sildenafil (Viagra TM), tadalafil (Cialis TM), or vardenafil (Levitra TM) * History of Priapism * Neurogenic orthostatic hypotensiondefiened as symptomatic decrease in BP\> 20 mmHg systolic or \> 10mmHg diastolic and HR increase \< 20bpm on supine to sitting or standing * Blood pressure less than 110 mmHG systolic at screening or baseline visit * Use of investigational drugs whitin 30 days before screening * For female participant, Pregnacy, or plans for child-bearing during study period * Allergy/hypersenstivity to iodine or study medication