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

Abstract

Background Drugs acting on μ-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-Ala2, N-MePhe4, 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.

Contribution of the respiratory network to rhythm and motor output revealed by modulation of GIRK channels, somatostatin and neurokinin-1 receptors

“…To further test this concept we manipulated GIRK channels in the preBötC region of adult rats and measured the subsequent changes in respiratory activity. Our results showed that GIRK channel activation at the preBötC decreased rhythmic breathing, a result that was previously identified17. Importantly, we further identified that GIRK channels decreased genioglossus muscle activity.…”

“…NK-1R-expressing preBötC neurons are selectively inhibited by MOR in vitro 14 and respiratory inhibition by MOR depends on GIRK channels17. Since NK-1R excitation inhibits GIRK channels in the nucleus basalis18, excitation of preBötC neurons by NK-1R may also involve GIRK channels.…”

“…Importantly, a subpopulation of GIRK2-expressing neurons also expressed NK-1Rs. In mice, GIRK2 subunits were also found in the preBötC region17. Small fusiform preBötC interneurons express NK-1R and SST8 and are thought to be the site of respiratory rhythm generation in rodents7.…”

“…We recorded diaphragm and genioglossus muscle activities using a similar approach to the rats and as previously described17. The mice were also kept warm with a heating pad.…”

“…In the preBötC, MORs inhibit rhythmic breathing through potassium channels16, and we recently identified that GIRK channels contribute to MOR respiratory slowing17. In nucleus basalis neurons, GIRK channels contribute to neuronal excitation by the endogenous NK-1R agonist substance P18.…”