To characterize the functional properties of AD-iN cells, we performed patch–clamp recordings from cells at 5 and 7 WPI. We found that AD-iN cells, at both timepoints, elicited spontaneous and current-induced action potentials and expressed voltage-gated ion channels (Fig. 3a,b and Supplementary Fig. 6). Their intrinsic membrane properties, including capacitance and input resistance, were similar between the two timepoints; but upon coculturing with glutamatergic Ngn2-iN cells, AD-iN cells showed improved maturation in terms of larger cell sizes and more ion channels in their membranes (Fig. 3a,b). Resting membrane potential was not affected by coculture. We then analyzed the synaptic properties of AD-iN cells. Immunoblotting showed that a large panel of synaptic markers was robustly expressed at 5 WPI (Supplementary Fig. 6). Moreover, MAP2-positive dendrites were decorated with the presynaptic proteins SYN1 and vGAT in a punctate pattern, which suggested the formation of GABAergic synapses (Supplementary Fig. 6). Spontaneous and evoked IPSCs could be readily recorded at 5 and 7 WPI, but both frequency and amplitude of synaptic events were increased at least two-fold by 7 WPI, showing a progressive maturation
