Characterization of HCN channel subtypes at hypoglossal motoneurons throughout postnatal maturation in mice

Abstract

The inspiratory phase of breathing includes a stiffening of the upper airway to decrease airway resistance and is mediated by excitatory motor output from hypoglossal motoneurons (XII MNs). This excitation decreases from wakefulness to sleep. This decrease in excitation is in large part due to increasing acetylcholine levels, which activate muscarinic acetylcholine receptors (mAChRs) that have a net inhibitory effect in adult preparations. By contrast, activation of mAChRs in neonatal rodent preparations has a net effect of potentiating inspiratory burst amplitude. Therefore, characterizing changes in muscarinic effects at XII MNs across postnatal maturation will help elucidate the mechanism underlying the shift from muscarinic excitation to inhibition. mAChRs modulate several effector ion channels including the hyperpolarization activated cyclic nucleotide gated (HCN) channel. HCN channels give rise to I h, a mixed cation current activated at hyperpolarized membrane potentials. I h additionally shows upregulation at XII MNs with postnatal maturation. Since there are four HCN subtypes (HCN1-4), the reported changes in I h may be caused by changes in HCN channel subtype levels. HCN1 and HCN2 are most prominent in adult XII MNs, whereas the expression of HCN1-4 in neonates is unknown. Thus, the purpose of this research is to characterize changes in HCN gene and channel expression levels at XII MNs across postnatal maturation. We hypothesis that both HCN1 and HCN2 subtypes will increase with postnatal maturation, and that HCN2 has higher levels than HCN1. To test this hypothesis, we used neuroanatomical techniques to evaluate changes in expression patterns of the four HCN proteins at the XII nuclei across postnatal maturation in CD-1 mice. We performed double-labeled immunofluorescence experiments against HCN1 and HCN2 and co-labeled with choline acetyltransferase (ChAT) on 20μm transverse brainstem slices across six postnatal age groups under identical conditions: P0-P1, P4-P6, P9-P10, P12-P14, P17-P19, and adult. Images of the XII nuclei were collected using a confocal microscope. ImageJ was used to determine the average XII MN HCN subtype intensity across postnatal maturation. Preliminary data suggest that HCN1 (n=2) channels undergo a decrease in expression at P17-19 before a final increase in adulthood (P0-1= 97-100%, P4-6= 92-100%, P9-10= 85-95%, P12-14= 77-97%, P17-19= 61-67%, adult= 71-82%). By contrast, preliminary data suggest that HCN2 (n=2) expression intensity remained relatively consistent with a decrease in labeling intensity at P4-6 (P0-1= 97-100%, P4-6=62-84%, P9-10= 45-95%, P12-14= 67-90%, P17-19= 50-93%, adult= 75-100%). These modest changes in labeling intensity of HCN channel subtypes may not contribute to the observed shift of mAChR modulation at XII MNs from excitation to inhibition with postnatal maturation. Biomedical Sciences, College of Graduate Studies; Department of Physiology, College of Graduate Studies, Midwestern University; NIH/NHLBI Funding: R15HL148870. 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.