Viruses have the advantage of self-replication over molecular tracers but can also spread too quickly and cause degradation of neural tissue.
[1][2][3] The use of viruses to label functionally connected neurons stems from the work and bioassay developed by Albert Sabin.
[4] Subsequent research allowed for the incorporation of immunohistochemical techniques to systematically label neuronal connections.
[5] Neuronal tracing methods offer an unprecedented view into the morphology and connectivity of neural networks.
Typically, viruses can infect only a small number of organisms, and even then, only a specific cell type within the body.
Researchers are able to incorporate numerous genes into the infected neurons, including fluorescent proteins used for visualization.
[10] Further advances in neuronal tracing allow for the targeted expression of fluorescent proteins to specific cell types.
[9][11] It is important to note that different tracers show characteristic affinities for dynein and kinesin, and so will spread at different rates.
At times, it is desirable to trace neurons upstream and downstream to determine both the inputs and the outputs of neural circuitry.
Few molecular tracers are able to do this, and those that can usually have a decreased signal in secondary neurons, which leads to the other benefit of viral tracing - viruses can self-replicate.
As such, the timing of tracer studies must be precise to allow adequate propagation before neural death occurs, causing large-scale harm to the body.
A virus used for tracing should ideally be just mildly infectious to give good results, but not deadly as to destroy neural tissue too quickly or pose unnecessary risks to those exposed.
Another drawback is that viral neuronal tracing currently requires the additional step of attaching fluorescent antibodies to the viruses to visualize the path.
In contrast, most molecular tracers are brightly colored and can be viewed with the naked eye, without additional modification.
Researchers use one of the previously mentioned viruses to study how neurons in the brain are connected to each other with a very fine level of detail.