Adrenergic sensitivity of uninjured C-fiber nociceptors in neuropathic rats

We investigated the adrenergic sensitivity of afferent fibers in the L4 dor sal roots of rats with a unilateral ligation of the L5-L6 spinal nerves. Ab out 12% of nociceptive fibers on the affected side were excited by sympathe tic stimulation or by intra-arterial injection of norepinephrine which did nor affect A beta-fiber activity. Sympathetic excitation of nociceptive fib ers was suppressed by alpha(1)-antagonist prazosin, while it was unaffected by alpha(2)-antagonist yohimbine. Most of these fibers were excited by int ra-arterial injection of alpha(1)-agonist phenylephrine, without being affe cted by an injection of alpha(2)-agonist clonidine. Sympathetic excitation was blocked by lidocaine applied near the receptive fields of recorded fibe rs. The results suggested that some nociceptors remaining intact after part ial nerve injury become sensitive to sympathetic activity by the mediation of alpha(1)-adrenoceptors in the peripheral endings.