Capsaicin-sensitive Aδ Fibers in Cat Tooth Pulp

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Abstract
How close a correlation there is between the conduction velocity and receptive properties of pulpal nerve fibers is still unclear. Our specific aims were to confirm whether: (1) capsaicin affects not only polymodal C fibers but A8 fibers as well, and (2) Aδ polymodal nociceptors exist in the tooth pulp. A total of 139 functional single cat tooth pulp nerve fibers was isolated for analysis, of which 21 were Ap, 37 C, and 81 AS fibers. The A8 fibers were divided into two groups: One (n = 38) consisted of those fibers whose conduction velocities were more than 2.0 m/s both inside and outside the tooth pulp, and the other (n = 43) consisted of those fibers whose intrapulpal conduction velocities were less than 2.0 m/s, with extrapulpal conduction velocities greater than 2.0 m/s. We used 82 fibers to record the neural response following the topical application of capsaicin for 60 min at increasing concentrations (1 nM, 100 nM, 10 μM) through thin dentin. Six of 25 slow Aδ, 10/20 C, and no Ap (0/11) or fast AS (0/26) fibers responded to 1 nM or 100 nM of capsaicin. When the three concentrations of capsaicin solution were applied in turn, the electrical threshold and latency of Aβ and fast AS fibers did not change, whereas those of slow AS and C fibers gradually increased. In 0/11 Ap, 0/26 fast AS, 13/25 slow AS, and 18/20 C fibers, the conduction was blocked reversibly or irreversibly following the application of 10 uM of capsaicin. The amplitude of the late component of antidromic action potential of fast AS fibers decreased after the capsaicin application. No neural discharge could be recorded from 19 (3 Aβ, 5 fast AS, 6 slow AS, and 5C) fibers following the application of a single high concentration of capsaicin (10 μM). A single low concentration of capsaicin (100 nM) activated only some slow-conducting fibers (0/4 Ap, 0/4 FAS, 3/6 SAS, and 4/6 C). Response properties recorded from the remaining 18 fibers (3 Ap, 3 fast AS, 6 slow Aδ, and 6 C) were not changed following the application of the control vehicle. These results confirm that a low concentration of capsaicin has an excitatory effect on the response of slow pulpal AS as well as C fibers, and that a high concentration of capsaicin blocks the conduction of slow Aδ and C fibers as well as the terminals of fast AS fibers in the pulp.