Reardon, T.R. et al. Neuron 89, 711–724 (2016).

Rabies-virus-based tools have become invaluable for analyses of neural connectivity, as they retrogradely label input neurons. However, the commonly used rabies virus strain infects synaptically connected neurons inefficiently and also exerts neurotoxic effects. Reardon et al. revisited other laboratory strains of rabies virus and selected a strain called CVS-N2c, which has high neuronal affinity and reduced neurotoxicity. After introducing the same modifications that converted the commonly used strain into a retrograde-labeling tool, the researchers found that the CVS-N2c–based tool was more efficiently transmitted to connected neurons and strongly reduced neurotoxicity, allowing experiments over longer periods of time than possible with the traditional strain. The researchers used the improved retrograde-tracing tool to express fluorescent proteins, channelrhodopsin-2 or the calcium sensor GCaMP6f in neurons upstream of the neurons of interest.