Anti-nociceptive synergism of morphine and gabapentin in neuropathic pain induced by chronic constriction injury.

In order to detect an anti-nociceptive interaction between morphine and gabapentin, the anti-allodynic and anti-hyperalgesic effects of these drugs, administered either separately or in combination, were determined using the von Frey and acetone tests in a rat model of neuropathic pain (Bennett model). Morphine and gabapentin individually induced moderate attenuation of mechanical hyperalgesia, whereas the morphine and gabapentin combination completely decreased hyperalgesia. Morphine showed its maximal effect at 30 min post-injection in the acetone test; however, this effect gradually returned to the baseline value. Gabapentin did not produce an anti-allodynic effect, whereas the morphine and gabapentin combination completely decreased allodynia behavior at 30 min post-injection, an effect that persisted until 120 min. The area under the curve (AUC) of the anti-allodynic or anti-hyperalgesic effects produced by the combinations were significantly greater than the theoretical sum of effects produced by each drug alone or similar to the theoretical sum. The analysis of the effect, expressed as the AUC of the time course, supports the hypothesis that the combination of these drugs is useful in neuropathic pain therapy.