Intranasal orexin A modulates sympathetic vascular tone: a pilot study in healthy male humans.

Previous research suggests that the neuropeptide orexin A contributes to sympathetic blood pressure (BP) control inasmuch as hypothalamic injection of orexin A increases sympathetic vasomotor tone and arterial BP in rodents. In humans with narcolepsy, a disorder associated with loss of orexin-producing neurons, vasoconstrictive muscle sympathetic nerve activity (MSNA) is reduced. Since intranasally administered oligopeptides like orexin are known to modulate brain function, we investigated the effect of intranasal orexin A on vascular sympathetic baroreflex function in healthy humans. In a balanced, double-blind crossover study, orexin A (500 nmol) and placebo, respectively, were intranasally administered to 10 lean healthy males (age 25.8 ± 4.6 yr). MSNA was assessed microneurographically before and 30-45 min after either substance administration. Additionally, baroreflex was challenged via graded infusions of vasoactive drugs before and after substance administration. Baroreflex function was defined as the correlation of BP with MSNA and heart rate. Intranasal orexin A compared with placebo induced a significant increase in resting MSNA from pre-to postadministration [Δburst rate, orexin A vs. placebo: +5.8 ± 0.8 vs. +2.1 ± 0.6 bursts/min, P = 0.007; total activity 169 ± 11.5% vs. 115 ± 5.0%; P = 0.002]. BP, heart rate, and sympathovagal balance to the heart, as represented by heart rate variability (HRV), as well as baroreflex sensitivity during the vasoactive challenge were not altered. Intranasally administered orexin A acutely induced vasoconstrictory sympathoactivation in healthy male humans. This result suggests that orexin A mediates upward resetting of the vascular baroreflex set point at centers superordinate to the mere baroreflex feedback loop. NEW & NOTEWORTHY Our pilot study adds another important part to the complex network of neuroendocrine-sympathetic interaction. Our results demonstrate that intranasal orexin A elicits an excitatory effect on sympathetic vascular tone superordinate to mere baroreflex feedback regulation. This resetting of the baroreflex set point suggests an activation of hypothalamic core centers such as the paraventricular nucleus (PVN). The role of the orexinergic system in the development of neurogenic arterial hypertension warrants further investigations.