Tracing inputs to inhibitory or excitatory neurons of mouse and cat visual cortex with a targeted rabies virus

Background Cortical inhibition plays a critical role in controlling and modulating cortical excitation and a more detailed understanding of the neuronal circuits contributing to each will provide more insight into their roles in complex cortical computations. Traditional neuronal tracers lack a means for easily distinguishing between circuits of inhibitory and excitatory neurons. To overcome this limitation, we developed a technique for retrogradely labeling inputs to local clusters of inhibitory or excitatory neurons, but not both, using neurotropic adeno-associated and lentiviral vectors, cell-type specific promoters and a modified rabies virus. Results Applied to primary visual cortex (V1) in mouse, the cell-type specific tracing technique labeled thousands of presynaptically connected neurons, and revealed that the dominant source of input to inhibitory and excitatory neurons is local in origin. Neurons in other visual areas are also labeled; the percentage of these inter-cortical inputs to excitatory neurons is somewhat higher (~20%) than to inhibitory neurons (