Analgesic Effect of Intrathecal Gabapentin in a Rat Model of Persistent Muscle Pain

OBJECTIVE To evaluate the analgesic effect of intrathecal gabapentin therapy on secondary hyperalgesia in a rat model of persistent muscle pain. METHODS Intrathecal catheters were implanted into rats. Mechanical secondary hyperalgesia was induced by repeated intramuscular injections of acidic solution into the gastrocnemius muscle. Gabapentin was administrated intrathecally. Rats were allocated to control and experimental (gabapentin 30, 100, 300, and 1,000 µg) group. After gabapentin administration, mechanical withdrawal threshold was measured every 15 minutes and the motor function was measured 30 minutes later. RESULTS Mechanical hyperalgesia was evoked after the second acidic buffer injection. There was a significant improvement on the mechanical threshold after administration of 100, 300, and 1,000 µg gabapentin compared to pre-injection and the control group. The analgesic effect continued for 105, 135, and 210 minutes, respectively. To discern side effects, motor function was measured. Motor function was preserved in both groups after gabapentin administration, except for rats who received 1,000 µg gabapentin. CONCLUSION Intrathecal gabapentin administration produces dose-dependent improvements in mechanical hyperalgesia in a persistent muscle pain rat model. This implicates the central nervous system as having a strong influence on the development of persistent mechanical hyperalgesia. These results are helpful in understanding the pathophysiology of secondary hyperalgesia and in the treatment of patients with chronic muscle pain.