Next we asked whether iN cells were capable of forming synapses with each other. To address this question we plated FACS-sorted TauEGFP-positive, MEF-derived 5F-iN cells eight days after infection onto a monolayer culture of primary astrocytes, which are thought to play an essential role in synaptogenesis29,30. Importantly, we confirmed that these cultures were free of preexisting Tuj1 or MAP2-positive neurons (data not shown). Patch clamp recordings at 12–17 days after sorting indicated the presence of spontaneous post synaptic currents in (5/11 cells) (Fig. 4g). Upon extracellular stimulation, evoked EPSCs could be elicited in a majority of the cells (9/11 cells, Fig 4h). Similar to iN cells cultured with primary cortical neurons, we were able to record both NMDA receptor mediated (9/11 cells) and AMPA receptor-mediated EPSCs (8/11 cells; Fig. 4h–i). Interestingly, we were unable to detect obvious IPSCs in a total of fifteen recorded 5F-iN cells. These data indicate that iN cells are capable of forming functional synapses with each other, and that the majority of iN cells exhibit an excitatory phenotype.
