To compare the electrical excitability of iPSC-derived neurons from probands with those from familial controls, we made whole-cell patch-clamp recordings from neurons in dissociated cultures or at the edge of organoids. All neurons examined (n=99) expressed voltage-activated sodium and potassium currents that were of similar amplitude in control and proband neurons from two different families (Figure S3E–G). Because we observed that iPSC differentiated using dissociated monolayers were not able to robustly generate ventral telencephalic neurons and express virtually no GAD1+ cells, we limited our experiments to organoid preparations. In recordings from neurons in the organoids, the voltage-activated currents supported action potential firing in 18/18 control neurons and 12/14 proband neurons, with thresholds of −42.5 ± 0.8 mV and −37.9 ± 2.1 mV (controls, probands; p = 0.06) and action potential overshoots of 30 to 65 mV. Most neurons fired only a single action potential, but some fired multiple spikes (Figure 1C). In total, the electrophysiological data indicate that the cells studied here display voltage-gated channels similar to those in central neurons. In addition to these signatures of electrical excitability, we
