Pre-treatment with capsaicin reduces the excitability of small DRG neurons of rats. (A) Whole-cell patch-clamp recording in small neurons acutely dissociated from the rat DRGs showed that the firing frequency of action potentials induced by a current ramp (1 s duration; peak ranging from 100–500 pA) was firstly increased, and was then decreased after 5 min washout of capsaicin. (B) The action potential frequency during capsaicin treatment was increased to 3.12 (3.12 ± 0.65)-folds of that recorded before treatment, and such a change was attenuated in the same neuron after 5 min washout. **p < 0.01, ***p < 0.001, versus the same group of neurons before capsaicin treatment (n = 15 neurons). (C) The capsaicin-induced suppression of action potential was dose-dependent. The action potential frequency after 5 min washout reduced to 0.25-fold and 0.18-fold of that before 0.25 μM (n = 26) and 0.5 μM (n = 9) capsaicin treatment, respectively, and was almost totally suppressed with 2 μM capsaicin (n = 11).