Mapping of projection of sprouting neuron in neonate and adult mice after pyramidotomy

Although severed axons of the corticospinal tract rarely regenerate, neural function is partially recovered by axonal sprouting from intact neurons. The sprouting capability is higher in neonates than in adults, making neonates an attractive model for finding novel therapeutic targets for central nervous system injury. The axonal projection pattern of the motor cortex neurons is highly heterogeneous, and high-throughput analysis at the single-cell level is required to correctly understand the projection pattern of sprouting neurons. The recently developed multiplexed analysis of projections by sequencing (MAPseq) has revealed axonal projection complexity in many brain areas; however, axonal sprouting of the corticospinal tract in neonates and adults has not yet been analyzed. Here, we evaluated the sprouting pattern of individual neurons using MAPseq in neonate and adult mice after pyramidotomy, using the intact and denervated (sprouting) sides of the cervical cord. The ratio of neurons projecting solely to the intact or denervated side or projecting to both sides was not significantly different. The cumulative projection strength on the denervated side was significantly higher in the neonatal pyramidotomy group than in the other groups. The ratio of the projection strength on the denervated side to the total projection strength (sprouting index) correlated with the projection strength on the denervated side. The ratio of the neuron with a sprouting index of 0.6-0.7 was significantly higher in the neonatal pyramidotomy group. These results show the usefulness of MAPseq in sprouting studies and provide important information regarding the sprouting patterns of individual neurons in neonates.