Behavioral characterization of neuropathic pain on the glabrous skin areas reinnervated solely by axotomy-regenerative axons after adult rat sciatic nerve crush.

In cranial and spinal nerve ganglia, both axotomized primary sensory neurons without regeneration (axotomy-nonregenerative neurons) and spared intact primary sensory neurons adjacent to axotomized neurons (axotomy-spared neurons) have been definitely shown to participate in pain transmission in peripheral neuropathic pain states. However, whether axotomized primary sensory neurons with regeneration (axotomy-regenerative neurons) would be integral components of neural circuits underlying peripheral neuropathic pain states remains controversial. In the present study, we utilized an adult rat sciatic nerve crush model to systematically analyze pain behaviors on the glabrous plantar surface of the hindpaw sural nerve skin territories. To the best of our knowledge, our results for the first time showed that heat hyperalgesia, cold allodynia, mechanical allodynia, and mechanical hyperalgesia emerged and persisted on the glabrous sural nerve skin areas after adult rat sciatic nerve crush. Interestingly, mechanical hyperalgesia was sexually dimorphic. Moreover, with our optimized immunofluorescence staining protocol of free-floating thick skin sections for wide-field epifluorescence microscopic imaging, changes in purely regenerative reinnervation on the same skin areas by axotomized primary sensory afferents were shown to be paralleled by those pathological pain behaviors. To our surprise, Protein Gene Product 9.5-immunoreactive nerve fibers with regular and large varicosities ectopically emigrated into the upper dermis of the glabrous sural nerve skin territories after adult rat sciatic nerve crush. Our results indicated that axotomy-regenerative primary sensory neurons could be critical elements in neural circuits underlying peripheral neuropathic pain states. Besides, our results implied that peripheral neuropathic pain transmitted by axotomy-regenerative primary sensory neurons alone might be a new dimension in the clinical therapy of peripheral nerve trauma beyond regeneration.