Neurogenic orthostatic hypotension (NOH) is a debilitating condition associated with reduced quality of life, impaired function and is also an independent predictor of mortality(Bendini et al., 2007; Cordeiro et al., 2009; Rose et al., 2006). NOH is clinically defined as a sustained reduction in systolic blood pressure (SBP) ≥30mmHg within 3 minutes of standing or head-up tilt to at least 60 degrees on a tilt table(Freeman et al., 2011). Specifically, neurogenic OH can be differentiated from other causes of orthostatic hypotension, such as hypotension due to endocrine issues, generalized low blood pressure, low blood volume, etc., in that NOH is associated with autonomic dysfunction. Specifically, dysfunction of the reflexive regulation mediated by the sympathetic nervous system(Goldstein and Sharabi, 2009; Low et al., 2008).

Studies have implicated specific dysfunction of the peripheral sympathetic nerves in disorders that have accompanying NOH such as Multiple System Atrophy (MSA), Pure Autonomic Failure (PAF) and Parkinson Disease (PD+NOH)(Imrich et al., 2009; Senard et al., 1993; Sharabi et al., 2006). In clinical NOH populations with known diagnoses such as MSA, PAF and PD+NOH, infusions of yohimbine have been used to detect whether post-ganglionic sympathetic nerves are intact or denervated. Yohimbine is an alpha-adrenoceptor antagonist that, in healthy/intact sympathetic nerves, causes an increase in the release of norepinephrine (NE) from sympathetic nerves via increased sympathetic neuronal outflow. NE is a natural neurotransmitter that is released when the sympathetic nervous system is required to increase its activity. In persons with intact post-ganglionic sympathetic nerves an infusion of yohimbine results in an increase in blood pressure, arterial NE levels, and heart rate levels, with a decrease in forearm blood flow indicative of vasoconstriction. In contrast, patients with sympathetic denervation these responses are attenuated(Senard et al., 1993; Shannon et al., 2000; Sharabi et al., 2006). However, in these studies, the clinical population consisted of MSA, PAF and PD+NOH. Little research has been done in NOH populations without an underlying diagnosis, and in fact, 1/3 of patients with NOH have no identifiable underlying cause (Robertson and Robertson, 1994).

Furthermore, it has been hypothesized that supine hypertension in this select patient population is due to residual sympathetic tone in patients with intact post-ganglionic sympathetic nerves. Approximately 50% of NOH patients have associated supine hypertension(Shannon et al., 2000), which if left untreated, comes with its very own unique set of cardiovascular complications, such as significantly higher left-ventricular mass indices, specific end organ damage(Vagaonescu et al., 2000), heart attack and stroke. Therefore, clinicians are left with the challenging dilemma of finding a near impossible balance between the risks associated with supine hypertension versus the risks of sudden hypotension upon standing and the associated consequences of falls, fractures and head injuries resulting in more immediately morbid events. Medications such as nitrates and other antihypertensives can be prescribed, however their use is strongly cautioned as it is quite frequent that NOH patients are often older and have nocturia, and as a result are up frequently throughout the night. Other options such as raising the head of the bed 4 inches from the ground in order to reduced renal hyper-perfusion pose as an additional conservative measure, however, this does not act as a treatment for the supine hypertension.

In contrast, melatonin is a natural hormone secreted by the pineal gland in response to low light and is involved in maintaining proper circadian rhythms and sleep patterns. However, more recently, there has been a growing source of literature supporting melatonin as having an important role in blood pressure control: i) In rats, following pinealectomy, there is evidence of vasoconstriction (Cunnane et al., 1980) and hypertension (Zanoboni et al., 1978; Zanoboni and Zanoboni-Muciaccia, 1967). ii) Experimental hypertension elicited via pinealectomy can be reversed through exogenous administration of melatonin(Holmes and Sugden, 1976). iii) Continuous light exposure, results in a melatonin deficiency, peripheral vasoconstriction and hypertension(Briaud et al., 2004; Brown et al., 1991).

Therefore, melatonin is now being looked at as a non-traditional anti-hypertensive medication in patients with essential and nocturnal hypertension. In a study of 34 patients with nocturnal hypertension, administration of melatonin proved to have a slight, yet significant, reduction in nighttime blood pressure measurements(Grossman et al., 2006). In these studies, melatonin was taken for 3 or 4 weeks via an oral prescription 1 hour before bed. The dose was formulated as a controlled- or slow-release throughout the night. In these studies, there was an average systolic BP drop of 6.5mmHg and 4mmHg diastolic in supine/nighttime blood pressures. While this reduction may not seem significant, clinical it is. In a study of 2156 hypertensive patients, following a median follow-up period of 5.6 years it was found that the cardiovascular risk adjustment per 5mmHg reduction of nocturnal blood pressure in patients aged 55 years and above, was 0.92 (95%CI0.88-0.96) and per 5mmHg reduction in nocturnal diastolic blood pressure was 0.82 (95%CI0.77-0.88). The decrease in mean asleep BP during follow-up was most significantly associated with event-free survival (Hermida et al., 2010). In women, a mean decrease of 6mmHg in diastolic pressure significantly reduced overall mortality from vascular disease by 21%, fatal and nonfatal stroke by 42%, and fatal and nonfatal coronary heart disease by 14% (Rich-Ewards et al., 1995). Currently, the posed mechanisms of melatonin to reduced blood pressure consist of both central and peripheral mechanisms (Capsoni et al., 1994; Pogan et al., 2002; Ray, 2003; Satake et al., 1991; Stankov et al., 1993; Weekley, 1993). Therefore, the objectives of the current study are: 1. Identify NOH patients as having either peripherally intact vs denervated post-ganglionic sympathetic innervation to help identify a group of patients potentially more susceptible to supine hypertension. 2. Administer melatonin and monitor its effects on supine/nocturnal blood pressures in patients with supine hypertension, and 3. Investigate the proposed mechanisms of melatonin by comparing its effects in patients with peripherally intact vs denervated sympathetic nerves.