Answer: The rabies virus infects neurons in a retrograde manner, meaning they move presynaptically backwards across the synapse towards the previous neuron.
Because of the efficiency with which viruses can deliver genetic material into cells, research scientists have used genetically modified viruses to deliver DNA into target neurons. Many viruses used, such as an adeno-associated virus (AAV) are used to deliver DNA is an anterograde direction, away from the soma towards the axons. The virus is first injected into the brain using a stereotaxic device and identified brain coordinates. Then, the soma takes up the viral content, and the proteins are then synthesized by ribosomes and sent down microtubules towards the axon.
Rabies virus, on the other hand, is unique because it has retrograde activity. This means that when a virus infects a neuron, it travels across the synapse backwards, infecting the cells that sent connections onto it.
It has become very useful in the field of connectomics, the study of how different populations of neurons are connected to one another. In doing these experiments, a fluorescent tag such as GFP is added to the genetic content of the virus, which can then be imaged using fluorescence microscopy.
A monosynaptic rabies virus is used to trace one set of projecting neurons that send synapses towards a cell. (Rabies as a transneuronal tracer of circuits in the central nervous system)