Changes in the firing frequency of evoked action potentials recorded from small-sized DRG neurons in rats with intra-tibial injections of MRMT-1 cells. (A) - (C): Summary of the action potential numbers (spikes/s) evoked by a 1-s, 300-pA depolarizing current pulse in LT, MT, and HT neurons in naïve, PBS-, HK-, and MRMT-1-rats. *P < 0.05, ***P < 0.001, as compared with HK groups, two-tailed Student's t-test. (D): Representative of evoked action potentials recorded from LT (a), MT (b), and HT (c) neurons. LT, MT, and HT mean low-, medium-, and high-firing threshold neurons, respectively. Note that implantation of MRMT-1 tumor cells into the tibial canal in rats increased the firing frequency of evoked action potentials in LT and MT, but not HT neurons.