Figure 2. Effect of eugenol on voltage-gated sodium channel currents (I_Na) in dental primary afferent neurons. (Aa) Time-course of the effects of eugenol (1 mM) and capsaicin (1 µM) on I_Na in a capsaicin-insensitive neuron. Eugenol (1 mM) inhibited I_Na in capsaicin-insensitive neurons (n = 20). (Ab) Superimposed I_Na evoked by a test pulse at the points indicated in Aa. (Ba) The I_Na inhibition by eugenol was also observed in capsaicin-sensitive neurons (n = 30). (Bb) Superimposed I_Na evoked by a test pulse at the points indicated in Ba. A and B are representative traces illustrating the effect of eugenol on I_Na from capsaicin-insensitive and capsaicin-sensitive neurons, respectively. (C) Dose-response relationship of eugenol-induced I_Na inhibition. The inhibition of the peak I_Na by eugenol was dose-dependent with IC₅₀ of 600 µM. (Da) The summary of I_Na inhibition in dental primary afferent neurons. Eugenol (1 mM)-induced I_Na inhibition in capsaicin-sensitive neurons (Cap-S) was similar to that obtained in capsaicin-insensitive neurons (Cap-Ins). (Db) The effect of capsazepine (10 µM), a competitive TRPV1 antagonist, on eugenol-induced I_Na inhibition. The I_Na inhibition by combined application of eugenol and capsazepine was not significantly different from that of eugenol (mean ± SEM, p > 0.05), indicating that eugenol-induced I_Na inhibition was TRPV1-independent. The numbers in parentheses represent the number of cells studied.