Two-Photon Optogenetic Mapping of Excitatory Synaptic Connectivity and Strength

The development of optical methods to activate neurons with single-cell resolution has enabled systematic mapping of inhibitory connections. In contrast, optical mapping of excitatory connections between pyramidal neurons (PCs) has been a major challenge due to their high densities in cortical tissue and their weak and stochastic connectivity. Here we present an optogenetic two-photon mapping method in mouse neocortical slices by activating PCs with the red-shifted opsin C1V1 while recording postsynaptic responses in whole-cell configuration. Comparison of delays from triggered action potentials (APs) with those from synaptic inputs allowed us to predict connected PCs in three dimensions. We confirmed these predictions with paired recordings, and used this method to map strong connections among large populations of layer 2/3 PCs. Our method can be used for fast, systematic mapping of synaptic connectivity and weights.