Wickersham, I.R. et al. Nat. Commun. 4, 2332 (2013).

To study neural circuits, one needs to identify the neurons that make synaptic contacts with a cell of interest (input connections) as well as where the axon terminal of that cell projects to (output connection). For the former, one can use viral tracers that are transported retrogradely, from the synapse of a target cell to the soma of the presynaptic partner. Rabies virus shows this type of tropism and has been engineered into a popular neuronal tracer. In new work, Wickersham et al. engineer rabies viruses that are transported in the opposite direction: from the soma of the target cell to its axon terminal, or anterogradely. The authors accomplish this by coating the rabies virus with the envelope glycoprotein of vesicular stomatitis virus. They show several new capabilities for neuronal tracing that are enabled by the combination of retrograde and anterograde rabies vectors, such as labeling a brain region's inputs and outputs simultaneously.