Viruses as transneuronal tracers for defining neural circuits.

Live viruses can be used as tools to label chains of neurons and thus to define functionally connected CNS circuits. This review summarizes the background and general principles involved in using the viral tracing technology. An attenuated form of a pig herpes virus, known as the Bartha's K strain of pseudorabies virus, has proven to be a useful type of virus for the analysis of CNS systems in the rat. The properties of this virus and the evidence for its specificity in causing trans-synaptic infections is discussed.