Fig. 2. The role of sodium in spontaneous tonic firing.A, Spontaneous firing was observed in the majority of cholinergic interneurons. B, Application of TTX (1 μm) prevented action potential generation and established a stable subthreshold membrane potential (approximately −60 mV). C, Stepping the membrane from a holding potential of −60 to −52.5 mV elicited a slowly developing inward current that persisted throughout the pulse. The dashed line indicates the zero current point. D, After TTX (1 μm) treatment, the inward current produced by the depolarizing voltage step was absent, and a small outward current was observed. Additionally, the zero current point was shifted to −60 mV. E, The I–V plot for a range of voltage steps (Vcmd) shows that, under control conditions, there is a region of negative slope conductance at potentials positive to −60 mV and no zero current point in the subthreshold voltage range. The inward current generated by depolarizing steps was blocked by TTX. F, Examination of the TTX-sensitive current that was obtained by subtraction for nine neurons and pooled (mean ± SD). G–J, Spiking rate was reduced by injection of constant negative current and failed to reveal any subthreshold oscillation in the absence of action potential generation. The firing pattern was related to firing rate, and spike trains became increasingly irregular at lower rates. K, For seven neurons exhibiting tonic, regular spiking (2.45–4.03 Hz; CV, 0.10–0.19), the firing rate and pattern was measured for control (0 pA) and during steady injection of −10 and −20 pA (open circles). Pooled data (filled circles; mean ± SD) confirmed that firing pattern was a function of spiking rate.