Abstract
This study evaluated the interaction of the analgesic effects of a selective Îș- (U50,488H) and a selective ÎŽ- ([d-Pen2,5]enkephalin, DPDPE) opioid agonist, co-injected intrathecally, using the Randall-Selitto paw-withdrawal test, in the rat. Intrathecal administration of both U50,488H and DPDPE, as single agents, produced dose-dependent increases in mechanical nociceptive threshold. However, when the dose-response curves for both U50,488H and DPDPE in the presence of a low-analgesic dose of the other agent were compared with the dose-response curves for the respective agonist administered alone, the curves for the combination regimes were shifted to the left. A statistically significant deviation from parallelism between the dose-response curves of the single versus the combined agents, as well as isobolographic analysis, demonstrates that the simultaneous administration of opioid agonists that act at Îș- and ÎŽ-opioid receptor sites, can produce analgesic synergy.
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