Gracile Neurons Contribute to the Maintenance of Neuropathic Pain in Peripheral and Central Neuropathic Models

In the present study, we compared the roles of gracile neurons in mechanically-induced neuropathic pain caused by spinal injury and L5 spinal nerve ligation in rats. Behavioral and electrophysiological methods were used to measure mechanical allodynia in the hindpaws, and excitability of the gracile neurons in the medulla, respectively. In the spinal hemisection and spinal contusion models, mechanical allodynia developed in both hindpaws and lasted over a month. Three weeks following the hemisection, gracile neurons identified as wide-dynamic-range (WDR) and low-threshold (LT) neurons, showed increased neuronal activity to non-noxious mechanical stimuli compared to control groups, whereas the spinal contusion groups did not show evoked activity (*p < 0.05). A lesion of the gracile nucleus partially reversed the existing mechanical allodynia in both hindpaws compared to prior to the injury in the hemisection group, whereas the spinal contusion groups did not show significant changes (*p < 0.05). In the spinal nerve ligation model, mechanical allodynia developed at the ipsilateral (injured) side of the hindpaw. In addition, WDR neuronal activity at the ipsilateral gracile neurons showed a significant increase with non-noxious mechanical stimuli, whereas the LT neurons did not show significant changes (*p < 0.05). Similarly to the hemisection model, a lesion of the gracile nucleus attenuated the mechanical allodynia in spinal nerve ligation models. The present data suggest that gracile neurons contribute to the maintenance of non-noxious mechanically-induced neuropathic pain in both hemisection-and ligation-induced neuropathic pain in rats.