We next determined whether GFP+ cells from day 10-18 treated cultures (enriched for GABA neurons), undergo GABAergic synaptic transmission. Whole-cell patch-clamp electrophysiological studies (Figure 4F–K) of identified GFP+ cells showed spontaneous firing in both day 10-18 and day 6-18 treated cultures (Figure 4H,I). However, only the day 10-18 group showed modulation of the firing rate of NKX2.1::GFP+ cells in response to the GABA-A receptor antagonist bicuculline (Figure 4J). Since the cortical feeders contained very few murine interneurons (in both day 10-18 and day 6-18 co-culture conditions), these results indicate that hPSC-derived GABAergic neurons mediate the inhibitory synaptic output. Accordingly, in the day 6-18 cultures in which GABAergic neurons are more rare (Figure 4C), the firing rates following bicuculline exposure remained unchanged (Figure 4K).
