Antinociceptive drug interaction between intrathecal vitamin E and gabapentin in the rat formalin test

BACKGROUND: Gabapentin is thought to exert an effect through the voltage-dependent calcium channel. Vitamin E is a widely known antioxidant which neutralizes the harmful effect of ROS which is considered to play a prominent role in various painful conditions. This study was therefore conducted to assess the antinociceptive effects of gabapentin and vitamin E and the interaction of these drugs in the modulation of pain in rats subjected to a formalin test. METHODS: Sprague-Dawley rats with a lumbar intrathecal catheter were tested for their paw flinches by 5% formalin injection after intrathecal injection of gabapentin or vitamin E. After obtaining dose-response curves for each drug, the effect of the combination was tested by the total dose fraction value and isobolographic analysis. RESULTS: When a single drug was injected intrathecally, significant dose-dependent decreases in flinches were shown only in the late phase. ED(50) values of intrathecal gabapentin and vitamin E in the late phase were 75.3 ± 9.58 µg, and 17.56 ± 1.65 mg/kg respectively. The combination of gabapentin and vitamin E produced dose-dependent decreases in the number of flinches in both phases induced by the formalin test. The ED(50) value of the combination was lower than the theoretical additive values in the late phase, but did not show a significant difference with the theoretical additive value. CONCLUSIONS: Gabapentin and vitamin E (by itself) have no antinociceptive effect in the early phase; however their combination has shown an antinociceptive effect. In addition, they show additive effects in the late phase of the formalin test.