Figure 5.

Tonic neurons express a persistent sodium current (I_Na,P). A, Responses from a typical tonic neuron illustrate that, under control conditions, R_in calculated from small hyperpolarizing steps from prestimulus V_m of -60 mV was 1190 MΩ, whereas from prestimulus V_m of -75 mV it was 780 MΩ. Voltage sensitivity of R_in was significantly less after TTX. TTX itself reduced R_in to 560 and 585 MΩ for V_m of -60 and -75 mV, respectively. This is consistent with a TTX-sensitive inward current that is persistently active at -60 mV and less so at -75 mV (a simple calculation suggests 23 and 7 pA, respectively) being deactivated by hyperpolarization. B, Cumulative data showing voltage sensitivity of R_in calculated from 20 pA hyperpolarizing steps under control conditions. R_in calculated from prestimulus V_m of -75 mV was 24 ± 5% less than R_in calculated from prestimulus V_m of -60 mV (n = 9; ^**p < 0.01; Wilcoxon signed-rank test). C, Effect of TTX on R_in expressed as difference in R_in before and after TTX for different prestimulus V_m. The effect at -60 mV was significantly greater than at -75 (n = 5; ^*p < 0.05; Wilcoxon signed-rank test), although both changes were significant relative to 0 (^*p < 0.05; Wilcoxon signed-rank tests). D, I-V curves were measured in voltage clamp by ramping command voltage at 10 mV/s between -110 and -10 mV. Insets show response before and after TTX (black and gray, respectively), and main graphs show the difference. The representative tonic cell clearly exhibited a TTX-sensitive inward current that peaked at approximately -40 mV but was still active at subthreshold potentials, consistent with amplitudes calculated in A. A representative single-spike cell showed no such inward current. E, I-V curve averaged across three tonic neurons tested and smoothed with a running average.