Figure 1. Effect of eugenol on action potentials in dental primary afferent neurons. At –65 mV, action potentials were evoked with 5-ms depolarizing current pulses with increasing amplitude (0.2 ~ 1 nA in 50.0-pA steps). Dental primary afferent neurons were classified into two types based on action potential shape: (A) The chemical structures of eugenol and capsaicin. Eugenol shares the vanilloid-moiety with capsaicin. (B) Type I neurons that had a prominent shoulder in the falling phase of the action potential were inhibited by capsaicin (n = 15). (C) Type II neurons that had no hump in the falling phase of the action potential were not affected by capsaicin (n = 15). Eugenol inhibited the generation of action potentials in both types of dental primary afferent neurons. B and C are representative responses of type I and type II neurons, respectively.