Endoneurial pathology of the needlestick-nerve-injury model of Complex Regional Pain Syndrome, including rats with and without pain behaviors

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Current rodent models of neuropathic pain produce pain hypersensitivity in almost all lesioned animals and not all identified experimental effects are pain specific. 18G needlestick-nerve-injury (NNI) to one tibial nerve of outbred Sprague–Dawley rats models the phenotype of Complex Regional Pain Syndrome (CRPS), a post-traumatic neuropathic pain syndrome, leaving roughly half of NNI rats with hyperalgesia. We compared endoneurial data from these divergent endophenotypes searching for pathological changes specifically associated with pain-behaviors. Tibial, sural, and common sciatic nerves from 12 NNI rats plus 10 nerves from sham-operated controls were removed 14 days post-surgery for morphometric analysis. PGP9.5+ unmyelinated-fibers were quantitated in plantar hindpaw skin. Distal tibial nerves of NNI rats had endoneurial edema, 30% fewer axons, twice as many mast cells, and thicker blood-vessel walls than uninjured tibial nerves. However the only significant difference between nerves from hyperalgesic versus non-hyperalgesic NNI rats was greater endoneurial edema in hyperalgesic rats (p < 0.01). We also discovered significant axonal losses in uninjured ipsilateral sural nerves of NNI rats, demonstrating spread of neuropathy to nearby nerves formerly thought spared. Tibial and sural nerves contralateral to NNI had significant changes in endoneurial blood-vessels. Similar pathological changes have been identified in CRPS-I patients. The current findings suggest that severity of endoneurial vasculopathy and inflammation may correlate better with neuropathic pain behaviors than degree of axonal loss. Spread of pathological changes to nearby ipsilateral and contralateral nerves might potentially contribute to extraterritorial pain in CRPS.

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