Signs of neuropathic pain depend on signals from injured nerve fibers in a rat model.

The present study was undertaken to determine the role of injured fibers in the development of neuropathic pain using our earlier established rat model. Our model was produced by placing tight ligatures to the L5 or both the L5 and L6 spinal nerves on one side in the rat. These rats showed long-lasting behavioral signs of mechanical allodynia and heat hyperalgesia. Using the uniqueness of our model, 3 specific questions are being asked concerning the initiation and maintenance of behavioral signs for neuropathic pain. The results of behavioral tests performed after various surgical manipulations suggest that: (1) peripheral nerve injury itself is the critical factor for the development of behavioral signs of neuropathic pain; (2) signs of neuropathic pain appear only when injury occurs at a part of the peripheral nerve distal to the dorsal root ganglion; and (3) signals (either electrical or chemical) entering the spinal cord from the injured fibers or the dorsal root ganglion cells play a critical role for both initiation and maintenance of the neuropathic pain state.