Use of pseudorabies virus to delineate multisynaptic circuits in brain: opportunities and limitations

Transsynaptic tracing with live virus is a powerful tool that has been used extensively to analyze central efferents that regulate peripheral targets. More recently, investigators have begun to use this new methodology with c entral injections to identify circuit anatomy within the brain. Although tr anssynaptic tracing with peripheral injection of pseudorabies virus has bee n extensively characterized, several methodological issues related to centr al application of this tracer have not been addressed. Here, we review the following issues relevant to the use of pseudorabies virus (PRV; Bartha str ain) in experiments involving injection of virus into rat brain: (i) factor s that determine the zone of viral uptake; (ii) uptake of pseudorabies viru s by fibers of passage; (iii) viral invasion of the brain after leakage of virus into the brain ventricles; (iv) considerations for double labeling fo r PRV with peptides and neurotransmitters; (v) use of PRV with conventional retrograde tracers to anatomically identify relays in a multisynaptic path way; and (vi) transport of PRV throughout the dendritic tree as a means of identifying inputs to distal dendrites. Collectively, the data demonstrate that PRV provides a powerful means of dissecting the synaptology of CNS cir cuitry when appropriate controls are incorporated into the experimental des ign. A set of recipes for various procedures are included at the end of thi s article. (C) 2000 Elsevier Science B.V. All rights reserved.