• Atsushi Nakamura, Masahide Fujita, Hiroko Ono, Yoshie Hongo, Tomoe Kanbara, Koichi Ogawa, Yasuhide Morioka, Atsushi Nishiyori, Masahiro Shibasaki, Tomohisa Mori, Tsutomu Suzuki, Gaku Sakaguchi, Akira Kato, and Minoru Hasegawa.
• Pain & Neurology, Medicinal Research Laboratories, Shionogi Co., Ltd., Osaka, Japan; Department of Toxicology, Hoshi University School of Pharmacy and Pharmaceutical Sciences, Tokyo, Japan.
• Br. J. Pharmacol. 2014 Jan 1;171(1):253-64.

Background And PurposeOxycodone and morphine are μ-opioid receptor agonists prescribed to control moderate-to-severe pain. Previous studies suggested that these opioids exhibit different analgesic profiles. We hypothesized that distinct mechanisms mediate the differential effects of these two opioids and investigated the role of G protein-gated inwardly rectifying potassium (K(IR)3 also known as GIRK) channels in their antinociceptive effects.Experimental ApproachOpioid-induced antinociceptive effects were assessed in mice, using the tail-flick test, by i.c.v. and intrathecal (i.t.) administration of morphine and oxycodone, alone and following inhibition of K(IR)3.1 channels with tertiapin-Q (30 pmol per mouse, i.c.v. and i.t.) and K(IR)3.1-specific siRNA. The antinociceptive effects of oxycodone and morphine were also examined after tertiapin-Q administration in the mouse femur bone cancer and neuropathic pain models.Key ResultsThe antinociceptive effects of oxycodone, after both i.c.v. and i.t. administrations, were markedly attenuated by K(IR)3.1 channel inhibition. In contrast, the antinociceptive effects of i.c.v. morphine were unaffected, whereas those induced by i.t. morphine were attenuated, by K(IR)3.1 channel inhibition. In the two chronic pain models, the antinociceptive effects of s.c. oxycodone, but not morphine, were inhibited by supraspinal administration of tertiapin-Q.Conclusion And ImplicationsThese results demonstrate that K(IR)3.1 channels play a primary role in the antinociceptive effects of oxycodone, but not those of morphine, at supraspinal sites and suggest that supraspinal K(IR)3.1 channels are responsible for the unique analgesic profile of oxycodone.© 2013 The British Pharmacological Society.

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