Figure 6. Persistent sodium current is prominent at pacemaker voltages after muscarinic stimulation, but Ih and low-threshold calcium current are minimal. A–E, Illustration of strategy used to measure INaP, Ih, and low-threshold calcium current in a spontaneously firing neuron. A, Firing behavior recorded in current clamp before and after application of 5 μm ACh. B–D, Currents evoked by slow voltage ramps (20 mV/s) from −98 to −28 mV were then recorded from the cell while serially adding 1 μm TTX (B, red) to define INaP, 10 μm ZD7288 (ZD; C, blue) to define Ih, and 100 μm NiCl2 (D, green) to define low-threshold calcium current. Each trace in B–D was signal averaged from two sweeps. E, TTX-, ZD7288-, and NiCl2-sensitive current during ACh stimulation obtained by subtracting the traces in B–D. F, Collected results from the same experimental protocol in multiple CA1 neurons, showing average INaP (circles; n = 7), Ih (squares; n = 7), and ICaT/L (triangles; n = 6). INaP was defined as TTX-sensitive current. Ih was defined as ZD7288-sensitive current in the presence of TTX. ICaT/L was defined as current sensitive to 100 μm nickel and 10 μm nimodipine coapplied in the presence of TTX. The pacemaking voltage region is bounded by the lowest trough voltage observed in a spontaneously active cell (−70 mV) and 4 mV below the mean spike threshold during spontaneous activity (−60 mV).