BACKGROUND: Synergy between drugs manifests with increased potency and/or efficacy of the combination relative to either agonist given alone. Synergy is typically observed between drugs of different classes, as is the case with the α-adrenergic-opioid receptor synergy often observed in preclinical studies. However, rare studies report synergy between agonists of the same class. The current study examined the analgesic interaction between two intrathecally injected α2-adrenergic receptor (AR) agonists previously thought to act at the same receptor subtype when given spinally. METHODS: Mice were given clonidine, dexmedetomidine, or the combination spinally to evaluate the interaction between these two agonists. The ED50 values were calculated, and the interactions were tested by isobolographic analysis. The rotarod test was performed in the same mice after the completion of analgesic assessment to assess motor or sedative effects. These experiments were performed in outbred mice as well as in mice with mutant α2AARs, α2CAR knockout mice, or wild-type controls. Finally, analgesic cross-tolerance between clonidine and dexmedetomidine was evaluated. RESULTS: Clonidine and dexmedetomidine interacted synergistically in all lines except the α2CAR knockout line, implicating α2CARs in the interaction. In addition, clonidine and dexmedetomidine did not show analgesic cross-tolerance in the outbred strain, suggesting that the two drugs have distinct mechanisms of action. CONCLUSIONS: The current study introduces a new synergistic agonist pair, clonidine-dexmedetomidine. These two drugs seem to require the α2AAR for spinal analgesia when given separately; when delivered as a combination, the resultant synergistic interaction requires the α2CAR as well.
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