Neuronal tracing

Received 2 January 2003; accepted 29 January 2003 ABSTRACT The tracing of neural pathways continues to be an important concern for neuroanatomy. During the second half of the nineteenth century, at a time when the foundations of neuroanatomy were being established, tracing pathways often required laborious dissections by hand. Then, neuroanatomy experienced a methodological revolution in the beginning of the 1970s with the development of powerful techniques based on the axonal transport of tracers. Axonal tracing of neuronal pathways using anterograde and retrograde transport is available. Axonal transport can be combined with immunohistochemistry for the neurochemical characterization of specific neuronal pathways. After tracing, tissue is collected following appropriate survival time for the tracer to be transported. Immunohistochemical methods, virus immunolabeling or autoradiography at the axon terminals are used for expressing the tracers. Retrograde axonal transport is used by certain viruses (e.g., herpes and pseudorabies virus) to spread from one neuron to the next in a chain of neurons (transneuronal transfer). It is also the method whereby toxins (e.g., tetanus) are transported from the periphery into the central nervous system. This has led to the idea that viral tracing may be an important way as a possible gene delivery method for the gene therapy in nervous system diseases.