Development and application of a barcoded rabies viral tracing method for mapping brain-wide inputs to single neurons

Mapping the input connections of a single neuron, or the "inputome," is crucial for constructing mesoscopic connectomes at the cellular resolution of the brain. By combining retrograde viral tracing with single-cell RNA sequencing, we developed a barcoded rabies viral tracing (BRT) method that enables mapping both local and long-range input connections to transcriptome-defined neurons at the single-cell level. When applied to the mouse medial prefrontal cortex (mPFC), BRT revealed that certain starter cells were innervated by a large number of input cells while others received fewer than expected inputs. Interestingly, for each inputome, the number of local input neurons was positively correlated with the number of distant input regions, suggesting a dependence of local circuit complexity on distant input diversity. Thus, the BRT method provides a valuable foundation for constructing comprehensive mesoscopic connectomes of the brain.