Lack of effect of naltrexone on the spinal synergism between morphine and non steroidal anti-inflammatory drugs.

To enhance analgesia, the combinatorial use of analgesic drugs with proven efficacies is a widely-used strategy to reduce adverse side effects. The present study characterizes the antinociceptive interaction of intrathecal morphine co-administered with different NSAIDs using isobolographic analysis.Antinoceptive activity was evaluated using a model for acute visceral pain, the writhing test of mice. The possible involvement of opioid receptors in the mechanism of action of the intrathecal co-administration of morphine and NSAIDs was investigated using the non-selective receptor antagonist naltrexone. The study demonstrated a synergistic antinociception of intrathecal administered combinations of morphine with the following NSAIDs: diclofenac, ketoprofen, meloxicam, metamizol, naproxen, nimesulide, parecoxib and piroxicam. The supra additive effect was obtained with very low doses of each drug and it appeared to be independent of the COX-1 or COX-2 inhibition selectivity of each NSAID and was not significantly modified by intrathecal naltrexone. The findings of the present work suggest that the combination of opioids and NSAIDs has a direct action on spinal nociceptive processing, which may be achieved via mechanisms that are independent of the activation of opioid receptors. The ineffectiveness of naltrexone to reverse the analgesic activity of opioids + NSAIDs combinations indicates that other complex pain regulatory systems are involved in this effect.