To test when iPSC-derived neurons have physiological activity, we performed whole cell recordings on RFP-expressing neurons at D4, D7, D24 and D38 (Fig. 5A). As shown in Fig. 5B, the sodium currents can be evoked by depolarizing steps. From D4 to D38, the threshold of evoking sodium currents shifted towards a more negative potential (Fig. 5C, D4 -23.4 ± 1.4 mV, n = 16; D7 -29.3 ± 1.3 mV, n = 28; D24 -30.2 ± 1.1 mV, n = 25; D38 -32.7 ± 1.4 mV, n = 24). In addition, the evoked sodium currents gradually increased during maturation (Fig. 5D, D4 -546 ± 91.6 pA, n = 16; D71048 ± 159.9 pA, n = 28; D24 1135.3 ± 148.2 pA, n = 25; D38 1862.2 ± 312.9 pA, n = 25). The resting membrane potentials shifted towards a more negative potential (RMP, Fig. 5E, D4 -30 ± 2.3 mV, n = 25; D7 -31.8 ± 2 mV, n = 29; D24 -34.7 ± 2.5 mV, n = 21; D38 -36.8 ± 1.6 mV, n = 26) and membrane resistances showed
