Viral neuronal tracing

Viral neuronal tracing is the use of a virus to trace neural pathways, providing a self-replicating tracer. Viruses have the advantage of self-replication over molecular tracers but can also spread too quickly and cause degradation of neural tissue. Viruses that can infect the nervous system, called neurotropic viruses, spread 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 the work and bioassay developed by Albert Sabin. Subsequent research allowed for the incorporation of immunohistochemical techniques to systematically label neuronal connections. To date, viruses have been used to study multiple circuits in the nervous system. The individual connections of neurons have long evaded neuroanatomists. Neuronal tracing methods offer an unprecedented view into the morphology and connectivity of neural networks. Depending on the tracer used, this can be limited to a single neuron or can progress trans-synoptically to adjacent neurons. After the tracer has spread sufficiently, the extent may be measured either by fluorescence (for dyes) or by immunohistochemistry (for biological tracers). An important innovation in this field is the use of neurotropic viruses as tracers. These not only spread throughout the initial site of infection but can jump across synapses. The life cycle of viruses, such as those used in neuronal tracing, is different from cellular organisms. Viruses are parasitic in nature and cannot proliferate on their own. Therefore, they must infect another organism and effectively hijack cellular machinery to complete their life cycle. The first stage of the viral life cycle is called viral entry. This defines the manner in which a virus infects a new host cell. In nature, neurotropic viruses are usually transmitted through bites or scratches, as in the case of the rabies virus or certain strains of herpes viruses.