Rabies-mediated monosynaptic circuit tracing is a highly specialized and powerful tool in neuroscience, used for mapping neural circuits in the brain. This technique leverages the unique properties of modified rabies virus to trace neural connections. The modified rabies virus, engineered to be non-toxic and to lack certain native viral components, is used to infect specific "starter" neurons. Once these neurons are infected, the virus replicates and then travels retrogradely (backwards) across synapses to infect neurons that are directly connected to the starter neuron.

The method employs a modified rabies virus that lacks the glycoprotein (G) necessary for infecting other cells. The G protein is only provided to the starter cells, ensuring that the virus can spread from these cells to the first-order presynaptic neurons, but not beyond. This selective spread is critical for identifying the direct inputs to specific neurons and understanding their connections within the brain. One of the key strengths of this technique is its ability to reveal the direct, monosynaptic connections to specific neurons, providing a detailed map of neural circuitry.