Involvement of Selective Alpha-2 Adrenoreceptor in Sympathetically Maintained Pain

ABSTRACT

OBJECTIVE: Peripheral nerve injury often leads to neuropathic pain, which is characterized by burning pain, allodynia, and hyperalgesia. The role of the sympathetic nervous system in neuropathic pain is a complex and controversial issue. It is generally accepted that the alpha adrenoreceptor (AR) in sympathetic nerve system plays a significant role in the maintenance of pain. Among alpha adrenoreceptor, alpha-1 receptors play a major role in the sympathetic mediated pain. The primary goal of this study is to test the hypothesis that sympathetically maintained pain involves peripheral alpha-2 receptors in human. METHODS: The study was a randomized, prospective, double-blinded, crossover study involving twenty patients. The treatments were Yohimbine (30 mg mixed in 500 mL normal saline), and Phentolamine (1 mg/kg in 500 mL normal saline) in 500 mL normal saline at 70 mL/hr initially then titrated. The patients underwent infusions on three different appointments, at least one month apart. Thus, all patients received all 2 treatments. Pain measurement was by visual analogue scale, neuropathic pain questionnaire, and McGill pain questionnaire. RESULTS: There were significant decreases in the visual analogue scale, neuropathic score, McGill pain score of yohimnine, and phentolamine. CONCLUSION: We conclude that alpha-2 adrenoreceptor, along with alpha-2 adrenoreceptor, may be play role in sympathetically maintained pain in human.