[Transneuronal tracing using neurotropic viruses in the mammals]

For fifteen years, the knowledge of the organization of neuronal networks inside the central nervous system of mammals has been considerably enhanced with the use of neurotropic viruses as tracers. Those viruses, pseudorabies, herpes simplex type 1 and rabies propagate in the brain through chains of connected neurons. This means that the transfer from a neuron to another one occurs only at the synaptic level. Each strain of virus has its own specificities which are used to study neuronal networks of different nature. Thus, the Bartha strain of pseudorabies, mostly used for tracing studies in rodents, has been very useful to characterize the networks belonging to the autonomic nervous system following peripheral inoculation. Less employed, rabies virus has been mainly used for the characterization of motor networks and, recently, to map neuronal networks in the cortex of primates after stereotaxic injections. On the other hand, the injection at distinct sites of recombinant pseudorabies viruses expressing different exogenous genes allows the study of interactions between two networks. This revue presents the milestones of the development of the transneuronal tracing using neurotropic viruses and emphasizes what virology and neurosciences bring to each other thanks to this technique.