Both motor and sensory abnormalities contribute to changes in foot posture in an experimental rat neuropathic model

This study was undertaken to find out whether pain contributes to abnormal posture of the foot in rats displaying neuropathic pain behaviors, and if so, whether there is a specific abnormality which reflects the level of pain. We used a rat model of neuropathic pain which is produced by tightly ligating the L5 and L6 spinal nerves just distal to the dorsal root ganglion (DRG). The operation produced neuropathic pain behaviors along with abnormal foot posture as has been shown previously. On the other hand, injury to the same somatic sensory and motor fibers at a more proximal site by sectioning dorsal and ventral roots of the same segments did not produce neuropathic pain behaviors. Rats with dorsal and ventral rhizotomies showed an abnormality of foot posture despite the absence of neuropathic pain behaviors. However, the abnormality in these rats was less severe and lasted for a shorter period of time as compared to rats with spinal nerve ligation. This suggests that peripheral nerve injury in our rat neuropathic pain model produces a foot postural abnormality partially due to motor deficit. However, there is an additional component of the abnormality that is related to sensory function, possibly pain. Close examination of the data indicate that there is no single obvious abnormal foot posture which exclusively represents motor deficit or sensory abnormality. Rather, neuropathic rats are likely to display abnormal foot posture as a result of a complex mixture of motor and sensory abnormalities.