Muscarinic modulation increases excitability inspiratory and non-inspiratory hypoglossal motoneurons in mice postnatal days 0-5
Authors: Fernandez Sarena , Revill Ann 
Published: May 2023

Hypoglossal motoneurons (XII MNs) innervate the tongue and their activity normally contributes to stiffening of the airway to maintain airway patency. During sleep, there is normal reduction in airway tone. The specific mechanisms that contribute to decreased airway tone during sleep remain to be completely resolved, although research from adult rodents suggests activation of muscarinic acetylcholine receptors (mAChRs) is an important contributor by inhibiting XII MNs. Conversely, data from neonatal rodents, have shown that muscarinic modulation leads to an overall excitatory effect in XII MNs. Therefore, the overarching objective of this research project is to uncover the mechanisms through which muscarinic modulation can influence the excitability of XII MNs across postnatal maturation. Rhythmic medullary slices from neonatal mice (postnatal day 0-5) were used for whole cell electrophysiology experiments to test the effects of activating mAChRs at XII MNs in both non-inspiratory and inspiratory XII MNs. Data are reported as mean±SD. We hypothesized that mAChR activation at XII MNs would increase excitability of non-inspiratory and inspiratory XII MNs, and potentiate inspiratory bursting in inspiratory XII MNs. Starting at -60 mV, muscarinic modulation depolarized non-inspiratory and inspiratory XII MNs similarly (3.6±2.6 vs 4.6±2.1 mV, n=12, p>0.05), and had no effect on input resistance (CTL vs Musc; non-insp: 120±23 vs 128±33 MΩ; inspiratory: 140±28 vs 137±32 MΩ; n =12; p>0.05). Muscarinic modulation can increase the hyperpolarization-activated cation current (Ih), but we observed no difference in the sag potential in response to -150 (CTL vs Musc; non-insp: 2.3±2.3 vs 3.8±3.0 mV; inspiratory: 2.7±2.8 vs 4.9±2.8 mV; n =12; p > 0.05), -300 (CTL vs Musc; non-insp: 9.0±7.8 vs 11.7±8.0 mV; inspiratory: 9.9±6.4 vs 12.9±7.5 mV; n=12; p>0.05), and -500 pA (CTL vs Musc; non-insp: 21.9±15.0 vs 23.6±14.2 mV; inspiratory: 22.0±9.5 vs 23.5±9.5 mV; n=12; p>0.05) current steps from -45 mV in non-inspiratory compared to inspiratory XII MNs. Muscarinic modulation significantly increased the firing frequency after muscarine application for inspiratory cells with the current step injections 100 pA (CTL vs Musc; 14.7±6.8 vs 21.1±6.4 Hz, n=9 and 10; p<0.05) and 175 pA (CTL vs Musc; 19.5±6.4 vs 25.0±9.4 Hz, n=11 and 8; p<0.05). However, muscarinic modulation did not affect the slope of firing rate/injected current relationship for non-inspiratory and inspiratory XII MNs (CTL vs Musc; non-insp: 0.16±0.12 vs 0.15±0.04 Hz/pA, n=9; inspiratory: 0.12±0.02 vs 0.16±0.07 Hz/pA, n=11 and 12; p>0.05). Finally, muscarinic modulation significantly increased inspiratory burst amplitude (CTL vs Musc; 15.2±3.1 vs 18.3±4.5 mV; n=3; p<0.05), and a trend to increase inspiratory burst area (CTL vs Musc; 4260±779 vs 5354±3063 mV ms, n=3; p=0.08). These data indicated that muscarinic modulation has similar effects on non-inspiratory and inspiratory XII MNs early in postnatal growth. Funding: NIH This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.