Axonal and subcellular labelling using modified rabies viral vectors.

An important aspect of any neural circuit is the placement of its output synapses, at levels ranging from macroscopic to subcellular. The many new molecular tools for locating and manipulating synapses are limited by the viral vectors available for delivering them. Adeno-associated viruses are the best current means of labelling and manipulating axons and synapses, but they have never expressed more than one transgene highly enough to label fine axonal structure while also labelling or perturbing synapses. Their slow expression also makes them incompatible with retrograde and transsynaptic vectors, preventing powerful combinatorial experiments. Here we show that deletion-mutant rabies virus can be specifically targeted to cells local to an injection site, brightly labelling axons even when coexpressing two other transgenes. We demonstrate several novel capabilities: simultaneously labelling axons and presynaptic terminals, labelling both dendrites and postsynaptic densities, and simultaneously labelling a region's inputs and outputs using co-injected vectors.