Retrograde tracing

The pseudorabies virus (PRV; Bartha strain), for example, may be used as a suitable tracer due to the propensity of the infection to spread upstream through a pathway of synaptically linked neurons, thus revealing the nature of their circuitry.

[3] These factors allow for highly specific traces that can reveal individual neuronal connections in a circuit without inflicting physical damage on the cells.

This virus was capable of infecting a single cell and jumping across one synapse; this allowed the researchers to investigate the local connectivity of neurons.

[3] Deletion of the gene encoding glycoprotein (G protein) in rabies limits the spread of the virus strictly to cells that were initially infected.

Transsynaptic spread of the virus can be limited to monosynaptic transmission to a neuron of origin by pseudotyping the G protein and putting the gene under Cre-control.