Antinociceptive synergy in a peripheral hyperalgesia model: interplay of cannabinoidergic, adrenergic, and opioidergic systems and their antagonism

There is growing interest in co-administering know analgesics for pain management, to reduce side effects and maximize therapeutic effects by pharmacological synergism, defined as supra-additive effects to biological stimuli. This work aimed to evaluate, using isobolgraphic analysis, synergistic effects of three antinociceptive substances-- anandamide (AEA), a cannabinoid CB1 receptor agonist; xylazine (XYL), an adrenergic 2-receptor agonist; and DAMGO, an {micro}-opioid receptor agonist--administered in binary doses in a prostaglandin E2 (PGE2)-induced peripheral pain model. Hyperalgesia was induced in Swiss male mice, and subsequently, animals were treated with binary agonist combinations administered to the hind paw. Mechanical nociceptive thresholds were measured using an algesimetric task, and the results obtained were compared with additive predicted effects. For AEA+XYL and AEA+DAMGO combinations, the observed effects were significantly greater than those predicted by Loewes additivity principles at all tested effect levels (10%, 30%, and 50% maximum possible effect, MPE). DAMGO+XYL combination showed significant synergistic effects at 10% and 30% MPE but not at 50% MPE. Confirming these findings, combination indexes (CI) for AEA+XYL and AEA+DAMGO were less than 1, indicating synergism, while CI for DAMGO+XYL was near 1, indicating additivity. Notably, single-system antagonism with either AM251, a CB1 antagonist, yohimbine, an 2C-receptor antagonist or naloxone, pan-opioid receptor antagonist, could prevent synergy or any analgesia at all for AEA+XYL and AEA+DAMGO. Furthermore, the binary agonist combinations did not produce systemic effects, sedation, or motor impairments. The results suggest synergistic antinociceptive effects for AEA+XYL and AEA+DAMGO, which are dependent on concomitant agonism upon known metabotropic receptors.