Rabies Virus as a Transneuronal Tracer of Neuronal Connections

Powerful transneuronal tracing technologies exploit the ability of some neurotropic viruses to travel across neuronal pathways and to function as self-amplifying markers. Rabies virus is the only viral tracer that is entirely specific, as it propagates exclusively between connected neurons by strictly unidirectional (retrograde) transneuronal transfer, allowing for the stepwise identification of neuronal connections of progressively higher order. Transneuronal tracing studies in primates and rodent models prior to the development of clinical disease have provided valuable information on rabies pathogenesis. We have shown that rabies virus propagation occurs at chemical synapses but not via gap junctions or cell-to-cell spread. Infected neurons remain viable, as they can express their neurotransmitters and cotransport other tracers. Axonal transport occurs at high speed, and all populations of the same synaptic order are infected simultaneously regardless of their neurotransmitters, synaptic strength, and distance, showing that rabies virus receptors are ubiquitously distributed within the CNS. Conversely, in the peripheral nervous system, rabies virus receptors are present only on motor endplates and motor axons, since uptake and transneuronal transmission to the CNS occur exclusively via the motor route, while sensory and autonomic endings are not infected. Infection of sensory and autonomic ganglia requires longer incubation times, as it reflects centrifugal propagation from the CNS to the periphery, via polysynaptic connections from sensory and autonomic neurons to the initially infected motoneurons. Virus is recovered from end organs only after the development of rabies because anterograde spread to end organs is likely mediated by passive diffusion, rather than active transport mechanisms.