Formation of somatosensory detour circuits mediates functional recovery following dorsal column injury

Anatomically incomplete spinal cord injuries can be followed by functional recovery mediated, in part, by the formation of intraspinal detour circuits. Here, we show that adult mice recover tactile and proprioceptive function following a unilateral dorsal column lesion. We therefore investigated the basis of this recovery and focused on the plasticity of the dorsal column-medial lemniscus pathway. We show that ascending dorsal root ganglion (DRG) axons branch in the spinal grey matter and substantially increase the number of these collaterals following injury. These sensory fibers exhibit synapsin-positive varicosities, indicating their integration into spinal networks. Using a monosynaptic circuit tracing with rabies viruses injected into the cuneate nucleus, we show the presence of spinal cord neurons that provide a detour pathway to the original target area of DRG axons. Notably the number of contacts between DRG collaterals and those spinal neurons increases by more than 300% after injury. We then characterized these interneurons and showed that the lesion triggers a remodeling of the connectivity pattern. Finally, using re-lesion experiments after initial remodeling of connections, we show that these detour circuits are responsible for the recovery of tactile and proprioceptive function. Taken together our study reveals that detour circuits represent a common blueprint for axonal rewiring after injury.