Current uses




Viral tracing is primarily used to trace neuronal circuits. 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. Connectivity largely determines how the brain functions. Viruses have been used to study retinal ganglion circuits, cortical circuits, and spinal circuits, among others.

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Direction of transmission

Viruses can be transmitted in one of two directions. First, one must understand the underlying mechanism of axoplasmic transport. Within the axon are long slender protein complexes called microtubules. They act as a cytoskeleton to help the cell maintain its shape. These can also act as highways within the axon and facilitate transport of neurotransmitter-filled vesicles and enzymes back and forth between the cell body, or soma and the axon terminal, or synapse. Transport can proceed in either direction: anterograde (from soma to synapse), or retrograde (from synapse to soma). 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. Anterograde transport edit Anterograde tracing is the use of a tracer that moves from soma to synapse. Anterograde transport us
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Benefits and drawbacks

The use of viruses as tracers has its benefits and its drawbacks. As such, there are some applications in which viruses are an excellent tracer, and other applications in which there are better methods to use. Benefits edit 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 edit 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
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Methods

Infection edit 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. It 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. Viruses are introduced into neuronal tissue in many different ways. 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
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