Chronically sensitized C nociceptors in skin. Patient with hyperalgesia and ongoing pain: [formula omitted], J. Ochoa and E. Torebjörk (Departments of Neurology, Good Samaritan Hospital and Medical Center, Portland, OR, U.S.A., Neurophysiology, University of Wisconsin, Madison, WI, U.S.A., Clinical Neurophysiology, Uppsala, Sweden)

Abstract

Potassium ion-containing solutions have been shown to initially excite then depress intradental nerve activity (INA) when applied into deep dentinal cavities. The INA reflects activity originating in intradental A fibers. Application of KCl to deep dentinal cavities also induces an increase in pulpal blood flow (PBF). Capsaicin is known to exert a highly selective desensitizing effect on polymodal C-fiber nerve endings. These C fibers are generally believed to release vasoactive substances in response to stimulation. In order to determine if KCl exerts its vascular effect via activation of capsaicin-sensitive nerve fibers, we examined blood flow and sensory nerve responses to KCl obtained before and after capsaicin desensitization. The A-fiber nerve activity was determined by using INA recording technique. Blood flow was measured simultaneously from the same tooth using laser Doppler flowmetry. Local application of 0.25 M KCl to a deep dentinal cavity induced a brief spike burst and an increase in PBF of 76.0 ± 14.6% (n = 8). Repeated applications of 0.25 M KCl caused a consistent increase in the peak PBF amplitude (n = 8). Local application of 100 μM capsaicin to a deep dentinal cavity caused an increase in PBF of 116.8 ± 26.3% (n = 8) lasting 12–18 min, but capsaicin application did not appear to evoke any INA response. The amplitude of PBF in response to capsaicin application declined during repeated applications. Following repeated applications of capsaicin the PBF response to KCl was significantly reduced (9.9 ± 4.3%, n = 8). Capsaicin treatment, however, had no effect on the vasodilatory response to papaverine, a known smooth-muscle relaxant, which was used as a control. The brief excitation of KCl on the INA after repeated applications of capsaicin did not seem to be affected by comparing the oscillographic spike activity. These results suggest that the inhibition of KCl-induced vasodilation by capsaicin may be caused by desensitization of capsaicin-sensitive nerve fibers and that local application of KCl into a deep dentinal cavity can stimulate both A fibers and capsaicin-sensitive nerve fibers.