One rationale for designing this strategy of culturing different neuronal subtypes in the microdevice is to have the ability to specifically manipulate individual subtypes. As a proof of principle, we expressed ChR221 in the excitatory neurons before seeding them into the outer chambers of the microdevices (Fig. 6a,b). We then performed whole-cell recordings combined with optogenetic stimulation. Photostimulation of the presynaptic neuronal inputs expressing ChR2 to the neurons located in the central chamber can reliably evoke synaptic responses (Fig. 6c) as well as short-term synaptic plasticity induced by a train of photostimulation (Fig. 6d). Interestingly, sometimes the train stimulation can induce some asynchronous synaptic activity between and shortly after the train stimulations (Fig. 6e). These evoked responses were abolished by the addition of CNQX, indicating excitatory synaptic transmission in a network (Fig. 6f). This experiment demonstrates that the device promotes formation of an active circuit of iNs, and that we can accurately measure physiological properties of trans-synaptic signaling and synaptic plasticity.
