Gabapentin increases analgesic effect of morphine in male rats

The clinical usefulness of morphine for treatment of chronic pain is limited by development of tolerance. So many authors have proposed the co-administration of other drugs. This study was performed to evaluate the role of gabapentin on analgesic effect of morphine. Four groups of male rats (220-250 g) were examined for time latency using tail flick test: control, morphine (M), gabapentin (GB), and gabapentin-morphine (GB-M)-treated groups. Rats received morphine (10 mg/kg, s.c.) or gabapentin (75 mg/kg, i.p.) or both of them twice a day for 10 days. Control rats received normal saline as scheduled time. Tail flick latency time was recorded 3 times (5 minute interval) before drug injection and at 60, 65 and 70 minutes after drug injection. In this respect, %MPE was calculated for all groups. Analysis of variance showed no significant difference of %MPE in control and GB groups, while in M and GB-M groups, %MPE changed significantly during the study. GB had no analgesic effect, while M and GB-M had significant analgesic effect as compared to control. %MPE of GB-M was also significantly higher than M at days 5, 7 and 9. It can be concluded that co-administration of gabapentin enhances the analgesic effect of chronic use of morphine. Precise mechanism(s) should be investigated in future.