Rapid synaptic remodeling in the adult somatosensory cortex following peripheral nerve injury and its association with neuropathic pain.

Structural and functional plastic changes in the primary somatosensory cortex (S1) have been observed following peripheral nerve injury that often leads to neuropathic pain, which is characterized by tactile allodynia. However, remodeling of cortical connections following injury has been believed to take months or years; this is not temporally correlated with the rapid development of allodynia and S1 hyperexcitability. Here we first report, by using long-term two-photon imaging of postsynaptic dendritic spines in living adult mice, that synaptic connections in the S1 are rewired within days following sciatic nerve ligation through phase-specific and size-dependent spine survival/growth. Spine turnover in the S1 area corresponding to the injured paw markedly increased during an early phase of neuropathic pain and was restored in a late phase of neuropathic pain, which was prevented by immediate local blockade of the injured nerve throughout the early phase. New spines that generated before nerve injury showed volume decrease after injury, whereas more new spines that formed in the early phase of neuropathic pain became persistent and substantially increased their volume during the late phase. Further, preexisting stable spines survived less following injury than controls, and such lost persistent spines were smaller in size than the surviving ones, which displayed long-term potentiation-like enlargement over weeks. These results suggest that peripheral nerve injury induces rapid and selective remodeling of cortical synapses, which is associated with neuropathic pain development, probably underlying, at least partially, long-lasting sensory changes in neuropathic subjects.