GIRK Channel Activation Contributes to the Muscarinic Modulation of XII Motoneuron Excitability in Neonatal Mice in Vitro

Sleep specific decreases in airway muscle tone, and particularly the genioglossus muscle of the tongue (primary tongue protruder), have been causally implicated in obstructive sleep apnea. During REM (rapid eye movement) sleep, decreased excitability of the XII motoneurons that innervate the genioglossus muscle may be due to a loss of excitatory noradrenergic modulation, or due to activation of muscarinic acetylcholine receptors that may act via G‐protein coupled inward rectifying potassium (GIRK) channels. GIRK channel activation results in membrane hyperpolarization and thus could mediate a decrease in excitability of XII motoneurons during REM sleep. Here we test the hypothesis that muscarine acts via GIRK channels to inhibit inspiratory burst amplitude of XII motoneurons in vitro. Using immunohistochemical techniques, our preliminary data indicate that XII motoneurons express GIRK1, GIRK2, and GIRK3 isoforms. Next, using rhythmically medullary slice preparations from neonatal CD1 mice (postnatal day, P0–5), we investigated the effects of GIRK channel and muscarinic acetycholine receptor activation on XII inspiratory burst characteristics including burst amplitude and burst period. Local application of muscarine (30 s, 100 mM) to the XII motor nucleus had the net effect of increasing XII inspiratory burst amplitude, although local pre‐application of the GIRK channel antagonist Tertiapin Q (TQ, 60 s, 100 nM) led to two different responses. In P0–2 mice (n = 4), muscarine increased burst amplitude to 120% of control (range 105–133%) while pre‐application of TQ followed by local application of muscarine led to a similar 115% (range 104–123%) increase in amplitude, suggesting that muscarine does not activate GIRK channels in P0–2 mice. In P3–5 mice (n = 4), however, local application of muscarine increased burst amplitude to 135% (range 122–156%) of baseline, and pre‐application of TQ to block GIRK channel activation further increased the response to muscarine to 160% (range 130–170%) of baseline amplitude. These results indicate that muscarine acts at GIRK channels to inhibit XII motoneuron excitability in P3–5 mice, but also acts at other channels to enhance XII motoneuron excitability. Preliminary data suggests this effect is specific to muscarinic activation of GIRK channels because bath application of TQ (10 nM) in P3–5 mice resulted in no change in inspiratory burst amplitude (mean: 102% of baseline, range 90–109%, after 30 min in TQ, n = 3). In contrast, bath application of the GIRK channel agonist ML297 (10 mM) resulted burst amplitude decrease to 80–88% of baseline in 2 out of 3 P3–5 neonatal mice and an increase in burst amplitude to 120% of baseline in a third prep. Burst period increased to 179% of baseline (range: 128–263%) indicating that GIRK channels are likely also present in preBötzinger Complex where inspiratory rhythm is generated. These data support the hypothesis that activation of muscarinic receptors in XII motoneurons leads to GIRK channel activation in P3–5 neonatal mice, which may be one mechanism through which muscarine mediates a decrease in XII motoneuron excitability in vivo. This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .