G protein-gated inwardly rectifying potassium (KIR3) channels play a primary role in the antinociceptive effect of oxycodone, but not morphine, at supraspinal sites
- Atsushi Nakamura and Masahide Fujita contributed equally to this work.
Background and Purpose
Oxycodone 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 (KIR3 also known as GIRK) channels in their antinociceptive effects.
Opioid-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 KIR3.1 channels with tertiapin-Q (30 pmol per mouse, i.c.v. and i.t.) and KIR3.1-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.
The antinociceptive effects of oxycodone, after both i.c.v. and i.t. administrations, were markedly attenuated by KIR3.1 channel inhibition. In contrast, the antinociceptive effects of i.c.v. morphine were unaffected, whereas those induced by i.t. morphine were attenuated, by KIR3.1 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 Implications
These results demonstrate that KIR3.1 channels play a primary role in the antinociceptive effects of oxycodone, but not those of morphine, at supraspinal sites and suggest that supraspinal KIR3.1 channels are responsible for the unique analgesic profile of oxycodone.