Adrenergic sensitivity of uninjured C-fiber nociceptors in neuropathic rats

We investigated the adrenergic sensitivity of afferent fibers in the L4 dorsal roots of rats with a unilateral ligation of the L5-L6 spinal nerves. About 12% of nociceptive fibers on the affected side were excited by sympathetic stimulation or by intra-arterial injection of norepinephrine which did not affect A β-fiber activity. Sympathetic excitation of nociceptive fibers was suppressed by α₁-antagonist prazosin, while it was unaffected by α₂-antagonist yohimbine. Most of these fibers were excited by intra-arterial injection of α₁-agonist phenylephrine, without being affected by an injection of α₂-agonist clonidine. Sympathetic excitation was blocked by lidocaine applied near the receptive fields of recorded fibers. The results suggested that some nociceptors remaining intact after partial nerve injury become sensitive to sympathetic activity by the mediation of α₁-adrenoceptors in the peripheral endings.