Brain Blood Vessel Responses to Changes in Blood Flow

Inclusion Criteria:

* Between the ages of 55-69 years
* Have a BMI less than or equal to 34.5 kg/m2
* Nonsmoker
* are less than 72 in (182.88 cm) height
* Females must be postmenopausal

Exclusion Criteria:

* Diagnosis of Mild Cognitive Impairment (MCI) or Alzheimer's disease (AD)
* History or evidence of hepatic disease, hematological disease, or peripheral vascular disease; severe kidney injury requiring hemodialysis
* History of cardiovascular disease including: severe congestive heart failure, coronary artery disease, ischemic heart disease (stents, coronary artery bypass grafts) and tachycardia
* Uncontrolled hypertension
* History of clinically significant ischemic or hemorrhagic stroke, or significant cerebrovascular disease
* Severe untreated obstructive sleep apnea
* History of diabetes with HbA1c greater than 9.5%
* Major neurologic disorders other than dementia (e.g., MS, ALS, brain surgery, etc.)
* Current or recent (\<1 year) major psychiatric condition (Axis I) or addictive disorders
* Significant surgical history
* Other significant medical conditions at investigators' discretion
* Contraindications to MRI
* Prescribed medications that interfere with prazosin
* Lactose allergy
* lower body will not fit in lower body negative pressure (LBNP) chamber (regardless of overall height and BMI).