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= Viral neuronal tracing = Viral neuronal tracing is the use of a virus to trace neural pathways, providing a self-replicating tracer. Neurotropic viruses infect the nervous system through spatially close assemblies of neurons through synapses, allowing for their use in studying functionally connected neural networks. The use of viruses to label functionally connected neurons stems from work done by Albert Sabin who developed a bioassay which could assess the infection of viruses across neurons. Subsequent research allowed for incorporation of Immunohistochemical techniques to systematically label neuronal connections. To date, viruses have been used to study retinal ganglion circuits, cortical circuits, and spinal circuits, among others.

Infection
The viral tracer may be introduced in peripheral organs, such as a muscle or gland. Certain viruses, such as adeno-associated virus can be injected into the blood stream and cross the blood–brain barrier to infect the brain. Viruses may also be introduced into a ganglion or injected directly into the brain using a stereotactic device. These methods offer unique insight into how the brain and its periphery are connected. There are two major methods to introduce tracer into the target tissues. Pressure injection requires the tracer, in liquid form, to be injected directly into the cell. This is the most common method. Iontophoresis involves the application of current to the tracer solution within an electrode. The tracer molecules pick up a charge and are driven into the cell via the electric field. This is a useful method if you wish to label a cell after performing the patch clamp technique. Once the tracer is introduced into the cell, the aforementioned transport mechanisms take over.

Once introduced into the brain, the virus will begin to infect cells in the local area. The viruses function by incorporating their own genetic material into the genome of the infected cells. The host cell will then produce the proteins encoded by the gene. Researchers are able to incorporate numerous genes into the infected neurons, including fluorescent proteins used for visualization. Further advances in neuronal tracing allow for targeted expression of fluorescent proteins to specific cell types, defined using cre-recombinase expression in the cell type of interest.

Histology and imaging
Once the virus has spread to the desired extent, the brain is sliced and mounted on slides. Then, fluorescent antibodies specific for the virus or fluorescent complementary DNA probes for viral DNA are washed over the slices and imaged under a fluorescence microscope.

Direction of transmission
Once a virus has successfully infected a neuron, the spread of infection to adjacent neurons is largely determined by properties of the cellular cytoskeleton, however the exact mechanisms which govern direction of transport are largely unknown. Transport can proceed in one of two directions: anterograde (from soma to synapse), or retrograde (from synapse to soma). The direction of transport and the ability of viruses to infect cells across a synapse can be engineered genetically by researchers. Neurons naturally transport proteins, neurotransmitters, and other macromolecules via these cellular pathways. Neuronal tracers, including viruses, take advantage of these transport mechanisms to distribute a tracer throughout a cell. Researchers can use this to study synaptic circuitry.

Benefits
One of the benefits of using viral tracers is the ability of the virus to jump across synapses. This allows for tracing of microcircuitry as well as projection studies. Few molecular tracers are able to do this, and those that can usually have a decreased signal in secondary neurons. Therefore, another benefit of viral tracing is the ability of viruses to self-replicate. As soon as the secondary neuron is infected, the virus begins multiplying and replicating. There is no loss of signal as the tracer propagates through the brain.

Drawbacks
Although some characteristics of viruses present a number of advantages in tracing, others present potential problems. As they propagate through the nervous system, the viral tracers infect neurons and ultimately destroy them. Therefore, the timing of tracer studies must be precise to allow adequate propagation before neural death occurs. The viruses can be not only harmful to neural tissue, but also harmful to the body at large. A virus used for tracing should ideally be just infectious enough to give good results, but not so much as to destroy neural tissue too quickly or pose unnecessary risks to those exposed.

Viruses in use
The following is a list of viruses currently in use for the purpose of neuronal tracing.