Fig. 5. Both the sADP and PP were dependent on Na+ influx, independent of TTX-sensitive channels and Ca2+ influx via HVA channels. A, In 20 μm carbachol, depolarizing current injection evoked both an sADP and a PP. Lowering external Na+ from 152 to 26 mm reversibly depressed the sADP and abolished the PP, suggesting that Na+ influx was required for these afterpotentials. The resting potentials before and after cholinergic stimulation were −68 and −59 mV, respectively. The calibration bars in the inset represent 40 msec and 20 mV. B, In a different pyramidal neuron, both the sADP and PP were evoked by intracellular current injection. Lowering external Ca2+ from 2 to 0.1 mmreversibly abolished both the sADP and PP. The concentration of Mg2+ was simultaneously increased from 2 to 10 mm to maintain divalent cation charge screening. This finding suggests that Ca2+ influx was also necessary for these potentials. The resting potentials of this neuron before and after cholinergic stimulation were −67 and −60 mV, respectively. The calibration bars in the inset represent 40 msec and 20 mV. C, In 20 μmcarbachol, an sADP and a PP were observed after evoked action potentials. Coapplication of 1.2 μm TTX, a concentration sufficient to block action potentials, failed to depress either the sADP or the PP. Notice that the PP followed a slow regenerative potential, presumably Ca2+-dependent, whereas the sADP did not. Activation of voltage-dependent Na+ channels, therefore, was not required for either of these afterpotentials. The resting membrane potentials before and after carbachol were −65 and −62mV, respectively. D, In carbachol, an sADP and a PP were elicited by intracellular current injection. Coapplication of 100 μm Cd2+ abolished both the sADP and PP. Activation of HVA Ca2+ channels, therefore, was necessary to evoke these potentials. The resting membrane potential in control was −69 mV, whereas 20 μmcarbachol depolarized the membrane potential to −62 mV.