Herpes simplex virus
Certain viruses can replicate in brain cells and cross synapses. So, viruses modified to express markers (such as fluorescent proteins) can be used to trace connectivity between brain regions across multiple synapses. Two tracer viruses which replicate and spread transneuronal/transsynaptic are the Herpes simplex virus type1 (HSV) and the Rhabdoviruses. Herpes simplex virus was used to trace the connections between the brain and the stomach, in order to examine the brain areas involved in viscero-sensory processing. Another study injected herpes simplex virus into the eye, thus allowing the visualization of the optical pathway from the retina into the visual system. An example of a tracer virus which replicates from the synapse to the soma is the pseudorabies virus. By using pseudorabies viruses with different fluorescent reporters, dual infection models can parse complex synaptic architecture.