 - Subjects taking alpha-adrenergic antagonists, e.g. prazosin for hypertension; and beta-adrenergic antagonists, e.g. propranolol. Because they may attenuate effects of yohimbine. Subjects will be eligible for the study if they are willing to titrate off potentially confounding agents prior to yohimbine administration for a period of five half-lives, given that such titration is also clinically appropriate and deemed to be in the patients best interests. 