Contribution of Ih to Postnatal Maturation of Muscarinic Modulation of Inspiratory Bursting at Hypoglossal Motoneurons in Neonatal Mice

Activation of hypoglossal motoneurons (XII MNs) contributes to stiffening of the upper airway during inspiration. The magnitude of this excitation is decreased during sleep, likely due to a combination of withdrawal of noradrenaline and acetylcholine release at XII MNs. Acetylcholine acts at muscarinic acetylcholine receptors (mAChRs) and surprisingly mAChR activation has an excitatory effect on inspiratory bursting in neonatal preparations, which becomes inhibitory in adult preparations. mAChRs modulate several ion channels and characterizing muscarinic effect changes at the XII nucleus will help to explain the mechanism of the inhibitory shift over postnatal maturation. The hyperpolarization activated cyclic nucleotide gated (HCN) channel is positively modulated by mAChRs. HCN channels give rise to I h , a mixed cation current activated at hyperpolarized membrane potentials that contribute to membrane depolarization. I h increases dramatically with postnatal maturation in XII MNs. Thus, we hypothesize that 1) the effects of muscarinic modulation of I h increases with postnatal maturation, and 2) that muscarinic activation of I h contributes to the net inhibitory component of muscarinic modulation of inspiratory bursting at XII motoneurons. To test our hypotheses, we used the rhythmic medullary slice preparation to test the functional effects of muscarinic modulation of I h on inspiratory bursting in neonatal CD1 mice (postnatal day, P0-P4) in combination with pharmacological block of I h with ZD7288 (25 μM, 150s). Local application of muscarine (100μM, 30s) in the XII motor nucleus increased inspiratory burst amplitude (189 ± 58% of baseline, n = 9). Contrary to our hypothesis, there was no statistically significant difference between inspiratory burst amplitude elicited by muscarine in the presence of ZD7288 (192 ± 47% of baseline, n = 9) versus with the aCSF vehicle control (174 ± 43% of baseline, p = 0.098). We next tested a higher ZD7288 concentration (100 μM) and longer application times (150s, 330s) in neonatal preparations. Preliminary data (n=2) indicate that ZD7288 may increase muscarinic potentiation of inspiratory burst amplitude at XII MNs (muscarinic potentiation (100μM, 30s): control — 112 ±65 % of baseline, aCSF vehicle control - 131 ± 38% of baseline; ZD7288 100μM, 150s - 148 ± 44% of baseline; ZD7288 100μM, 330s - 107 ± 72% of baseline). Future research will elucidate whether the I h contribution to muscarinic modulation of inspiratory bursting increases with postnatal maturation. This is the full abstract presented at the American Physiology Summit 2024 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.