To test whether synapses between two distinct neuronal populations can form within microgrooves, neurons were plated in the two somatic compartments, cultured for 14 days and then infected with either a GFP or RFP Sindbis virus in separate compartments in order to visualize potential connections (Figure 2A). The compartment-specific expression of either GFP or RFP demonstrates the ability to genetically manipulate each population independently. In these chambers, axons can grow the entire 900 μm extent of the microgrooves, whereas dendrites extend less than 450 μm into the microgrooves (Taylor et al., 2005); therefore, only axons will reach the distal end of the microgrooves. As described above, dendrites are easily identified based on morphological features, and when examined carefully, individual dendrites can be followed back to their somata of origin in the cell compartment. Together, these properties allow us to identify presynaptic processes originating from cell bodies in one compartment and postsynaptic processes originating from cell bodies in the other compartment (Figure 2B).
