Interactions of NMDA antagonists and an alpha 2 agonist with mu, delta and kappa opioids in an acute nociception assay.

Animal and clinical studies have reported potentiation of opioid antinociception by co-administration of alpha-2 adrenoceptor agonists such as clonidine and NMDA receptor antagonists such as ketamine and dextromethorphan. The aim of this study was to compare these clinically available compounds in combination with classical morphine, fentanyl-like opioids, the delta opioid agonist SNC80 and the kappa opioid agonist U50,488H. Using a mouse hot-plate test, dose-response relationships were first determined for all compounds individually and then for opioids co-administered with fixed doses of clonidine, ketamine or dextromethorphan. Clonidine was also evaluated in combination with ketamine and dextromethorphan. ED50 values were calculated from the proportion of animals reaching a fixed cut-off criterion of 30 s. To varying degrees, all compounds produced increases in response latencies over time. Dextromethorphan produced lower ED50 values for morphine, fentanyl and sufentanil but exerted no effect on SNC80 or U50,488H. Similarly, ketamine potentiated the antinociceptive efficacy of morphine and sufentanil but not SNC80 or U50,488H. By contrast, clonidine potentiated all opioids tested. In addition, the potency of clonidine was found to increase with co-administration of ketamine but not dextromethorphan. The strongest opioid sparing interactions occurred between clonidine and the lipophilic mu opioids fentanyl and sufentanil and the delta opioid SNC80. In summary, these results suggest an important role for lipophilic opioids in combination therapies particularly with clonidine as well as possible advantages of specific delta or kappa opioid combinations with alpha-2 agonists.