Sympathectomy and Sympathetic Blockade Reduce Pain Behavior Via Alpha-2 Adrenoceptor of the Dorsal Root Ganglion Neurons in a Lumbar Radiculopathy Model

Study Design. Animal experimental study with intervention. Objective. We investigated whether sympathectomy and pharmacological sympathetic blockade reduced pain behavior and reversed adrenoceptor mRNA expression of the dorsal root ganglion (DRG) in a lumbar radiculopathy model. Summary of Background Data. The abnormal sympathetic-somatosensory interaction may underlie some forms of neuropathic pain. There are several reports that sympathectomy and pharmacological sympathetic blockades are often effective to treat neuropathic pain. However, its pathophysiological mechanisms remain obscure. Methods. We used 91 male Sprague-Dawley rats. Just after root constriction (RC), the rats underwent sympathectomy or received 3 local injections of subtype-specific alpha-adrenergic receptor antagonists around the DRG. We evaluated the analgesic effects of sympathectomy and sympathetic blockade using behaviors indicative mechanical allodynia and thermal hyperalgesia. We estimated the mRNA expression levels of the DRG adrenoceptor subtypes using real time reverse transcription polymerase chain reaction. Results. Sympathectomy and alpha(2)-antagonist significantly reduced the mechanical allodynia and thermal hyperalgesia after RC. Real time reverse transcription polymerase chain reaction analysis indicated that sympathectomy possibly reversed alpha(2A)- and alpha(2B)-adrenoceptors mRNA overexpression in the DRG after RC. Conclusion. We considered that pain behaviors of neuropathic pain are due, at least in part, to enhanced sympathetic noradrenergic transmission within the DRG. Suppression of sympathetic activity by reducing adrenergic release, alpha(2)-adrenoceptor stimulation, and/or alpha(2)-adrenoceptor upregulation in the DRG may relieve neuropathic pain.