α2C-adrenergic receptors mediate spinal analgesia and adrenergic-opioid synergy

The alpha(2A)-adrenergic receptor (AR) subtype mediates antinociception induced by the alpha(2)AR agonists clonidine, dexmedetomidine, norepinephrine, and 5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine (UK-14,304) as well as antinociceptive synergy of UK-14,304 with opioid agonists [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin and deltorphin II. Differential localization of alpha(2)-adrenergic (alpha(2A)-, alpha(2B)-, alpha(2C)-) and opioid (mu-, delta-, kappa-) subtypes suggests differential involvement of subtype pairs in opioid-adrenergic analgesic synergy. The present study applies a novel imidazoline(1)/alpha(2)-adrenergic receptor analgesic, moxonidine, to test for involvement of alpha(2B)- and alpha(2C)ARs in antinociception and antinociceptive synergy, because spinal antinociceptive activity of moxonidine shows minimal dependence on alpha(2A)AR. Intrathecal administration of moxonidine produced similar (2-3-fold) decreases in both mutant mice with a functional knockout of alpha(2A)AR (D79N-alpha(2A)AR) and alpha(2C)AR knockout (KO) mice. The potency of moxonidine was not altered in alpha(2B)KO mice, indicating that this subtype does not participate in moxonidine-induced spinal antinociception. Moxonidine-mediated antinociception was dose dependently inhibited by the selective alpha(2)-receptor antagonist SK&F 86466 in both D79N-alpha(2A) mice and a,,KO mice, indicating that a2AR activation is required in the absence of either alpha(2A)- or alpha(2C)AR. Spinal administration of antisense oligodeoxynucleotides directed against the alpha(2C)AR decreased both alpha(2C)AR immunoreactivity and the antinociceptive potency of moxonidine. Isobolographic analysis demonstrates that moxonidine-deltorphin antinociceptive synergy is present in the D79N-alpha(2A) mice but not in the alpha(2C)AR-KO mice. These results confirm that the a,,AR subtype contributes to spinal antinociception and synergy with opioids.