Effects of capsaicin applied locally to adult peripheral nerve. I. Physiology of peripheral nerve and spinal cord

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Abstract
Systemic capsaicin treatment of neonatal and adult rats is known to affect unmyelinated afferents. The systemic route of administration presents several disadvantages and in order to overcome these a method was explored where a single nerve in adult rats was locally treated. Sciatic nerves were exposed and a 10-mm length was soaked for 15 min in 1.5% capsaicin in vehicle or in the vehicle alone (10% Tween 80, 10% ethyl alcohol in saline). Both the capsaicin solution and the vehicle caused acute block of the C compound action potential while in contact with the nerve. Removal of the solutions resulted in substantial recovery of C fiber conduction. The A fiber volley was totally unaffected. After 13-21 days the size of the myelinated and unmyelinated volleys evoked by maximal stimulation of the capsaicin treated nerve were unchanged but there was a 20% decrease of conduction velocity in the C fibers. The ability of the maximal C volley from the treated nerve to excite cells in the spinal cord was substantially decreased (by 50%) 13-21 days after local capsaicin.