Retrograde tracing

Retrograde tracing is a research method which is used in neuroscience to trace neural connections from their point of termination (the synapse) to their source (the cell body). The opposite technique is anterograde tracing, which is used to trace neural connections from their source to their point of termination (i.e. from cell body to synapse). Both the anterograde and retrograde tracing techniques are based on the visualization of the biological process of axonal transport.

The anterograde and retrograde tracing techniques allow the detailed assessment of neuronal connections from a single population of neurons to their various targets throughout the nervous system. These techniques allow the "mapping" of connections between neurons in a particular structure (e.g. the eye) and the target neurons in the brain. Much of what is currently known about connectional neuroanatomy was discovered through the use of the anterograde and retrograde tracing techniques.

Techniques

Retrograde tracing can be achieved through various means, including the use of viral strains as markers of a cell’s connectivity to the injection site. The pseudorabies virus (PRV; Bartha strain), for example, may be used as a suitable tracer due to the propensity of the infection to spread upstream through a pathway of synaptically linked neurons, thus revealing the nature of their circuitry.

Another technique involves injecting special "beads" into the brain nuclei of anaesthetized animals. The animals are allowed to survive for a few days and then euthanized. The cells in the origin of projection are visualized through an inverted fluorescence microscope.

A specialist technique was developed by Wickersham and colleagues, which employed a modified rabies virus. This virus was only capable of infecting a single cell, and only capable of jumping across 1 synapse; this allowed the researchers to investigate the local connectivity of neurons.