Peripheral nerve injury triggers maladaptive plastic changes along the somatosensory nervous system so that altered nociceptive signal processing, represented by neuropathic pain hypersensitivity (e.g. tactile allodynia (painful response to innocuous mechanical stimuli), occurs. Previous studies have suggested that structural and functional plastic changes in the primary somatosensory cortex (S1) following peripheral nerve injury contribute to neuropathic pain. However, remodeling of cortical connections following injury has been believed to take months or years, based on macroscopic imaging or static measurement; this is not temporally correlated with the rapid development of allodynia and S1 hyperexcitability. Recently, we first reported, 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 injury 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 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, pre-existing stable spines survived less following injury than controls and such lost persistent spines were smaller in size than the survived ones that displayed long-term potentiation (LTP)-like enlargement over weeks. These results suggest that peripheral nerve injury induce 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.

However, it is still unknown whether and how the other members of ‘tripartite synapse’, presynaptic axonal boutons and astrocytes, in the S1 change after nerve injury. Here, we found that formation and elimination of axonal boutons slightly increased and decreased, respectively, only in the early phase of neuropathic pain. Furthermore, in vivo two-photon Ca2+ imaging experiments showed that the frequency and amplitude of astrocytic Ca2+ activity significantly increased in the early phase and subsequently, but not completely, decreased in the late phase of neuropathic pain. These results suggest that the correlated, phase-specific changes in the S1 astrocytes and synaptic structures may play a role in neuropathic pain development.