G-protein-gated Inwardly Rectifying Potassium Channels Modulate Respiratory Depression by Opioids

Background: Drugs acting on mu-opioid receptors (MORs) are widely used as analgesics but present side effects including life-threatening respiratory depression. MORs are G-protein-coupled receptors inhibiting neuronal activity through calcium channels, adenylyl cyclase, and/or G-protein-gated inwardly rectifying potassium (GIRK) channels. The pathways underlying MOR-dependent inhibition of rhythmic breathing are unknown. Methods: By using a combination of genetic, pharmacological, and physiological tools in rodents in vivo, the authors aimed to identify the role of GIRK channels in MOR-mediated inhibition of respiratory circuits. Results: GIRK channels were expressed in the ventrolateral medulla, a neuronal population regulating rhythmic breathing, and GIRK channel activation with flupirtine reduced respiratory rate in rats (percentage of baseline rate in mean +/- SD: 79.4 +/- 7.4%, n = 7), wild-type mice (82.6 +/- 3.8%, n = 3), but not in mice lacking the GIRK2 subunit, an integral subunit of neuronal GIRK channels (GIRK2(-/-), 101.0 +/- 1.9%, n = 3). Application of the MOR agonist [d-Ala(2), N-MePhe(4), Gly-ol]-enkephalin (DAMGO) to the ventrolateral medulla depressed respiratory rate, an effect partially reversed by the GIRK channel blocker Tertiapin-Q (baseline: 42.1 +/- 7.4 breath/min, DAMGO: 26.1 +/- 13.4 breath/min, Tertiapin-Q + DAMGO: 33.9 +/- 9.8 breath/min, n = 4). Importantly, DAMGO applied to the ventrolateral medulla failed to reduce rhythmic breathing in GIRK2(-/-) mice (percentage of baseline rate: 103.2 +/- 12.1%, n = 4), whereas it considerably reduced rate in wild-type mice (62.5 +/- 17.7% of baseline, n = 4). Respiratory rate depression by systemic injection of the opioid analgesic fentanyl was markedly reduced in GIRK2(-/-) (percentage of baseline: 12.8 +/- 15.8%, n = 5) compared with wild-type mice (72.9 +/- 27.3%). Conclusions: Overall, these results identify that GIRK channels contribute to respiratory inhibition by MOR, an essential step toward understanding respiratory depression by opioids.