Swing time ratio, a new parameter of gait disturbance, for the evaluation of the severity of neuropathic pain in a rat model of partial sciatic nerve ligation.

INTRODUCTION: Dynamic weight bearing tests are used to evaluate the chronic pain severity in animal models of nociceptive pain (such as osteoarthritis); however, common tests frequently fail to collect the characteristics of neuropathic pain such as allodynia, because surgical intervention which is sometimes required to establish the models causes both nociceptive and neuropathic pain. METHODS: In this study, we used rats with partial sciatic nerve ligation (PSL) as the neuropathic and chronic pain model. To assess the severity of pain by gait disturbance, we applied automatic analysis on walking function using the GAIT® system. The system employs a novel index of abnormal step cycles, the swing time ratio (STR), of laboratory animals. Data were compared to those obtained with conventional tests, including a von Frey test and a hot plate test. Finally, we analyzed recovery of walking function after single or repeated administration of pregabalin. RESULTS: By using rats with PSL, we confirmed that results obtained by the GAIT® system were comparable to those obtained by both von Frey tests and hot plate tests. Single administration of pregabalin transiently improved STR, on the other hand, repeated pregabalin treatment showed lasting STR recovery. DISCUSSION: STR is sensitive to claudication of rats with PSL, providing a new scale to evaluate neuropathic pain in addition to conventional tests. Moreover, STR analysis enables us to evaluate walking function of animal models after neuropathic injury, which is quite important to judge the effectiveness of new treatments and analgesics.