Muscarinic Inhibition of Hypoglossal Motoneurons: Possible Implications for Upper Airway Muscle Hypotonia during REM Sleep

Chronic intermittent hypoxia attenuates noradrenergic innervation of hypoglossal motor nucleus

The state-dependent noradrenergic activation of hypoglossal motoneurons plays an important role in the maintenance of upper airway patency and pathophysiology of obstructive sleep apnea (OSA). Chronic intermittent hypoxia (CIH), a major pathogenic factor of OSA, contributes to the risk for developing neurodegenerative disorders in OSA patients. Using anterograde tracer, channelrhodopsin-2, we mapped axonal projections from noradrenergic A7 and SubCoeruleus neurons to hypoglossal nucleus in DBH-cre mice and assessed the effect of CIH on these projections. We found that CIH significantly reduced the number of axonal projections from SubCoeruleus neurons to both dorsal (by 68%) and to ventral (by73%) subregions of the hypoglossal motor nucleus compared to sham-treated animals. The animals' body weight was also negatively affected by CIH. Both effects, the decrease in axonal projections and body weight, were more pronounced in male than female mice, which was likely caused by less sensitivity of female mice to CIH as compared to males. The A7 neurons appeared to have limited projections to the hypoglossal nucleus. Our findings suggest that CIH-induced reduction of noradrenergic innervation of hypoglossal motoneurons may exacerbate progression of OSA, especially in men.

A muscarinic, GIRK channel-mediated inhibition of inspiratory-related XII nerve motor output emerges in early postnatal development in mice

In neonatal rhythmic medullary slices, muscarinic acetylcholine receptor (mAChR) activation of hypoglossal (XII) motoneurons that innervate the tongue has a net excitatory effect on XII inspiratory motor output. Conversely, during rapid eye movement sleep in adult rodents, XII motoneurons experience a loss of excitability partly due to activation of mAChRs. This may be mediated by activation of G-protein-coupled inwardly rectifying potassium (GIRK) channels. Therefore, this study was designed to evaluate whether muscarinic modulation of XII inspiratory motor output in mouse rhythmic medullary slices includes GIRK channel-mediated inhibition and, if so, when this inhibitory mechanism emerges. Local pressure injection of the mAChR agonist muscarine potentiated inspiratory bursting by 150 ± 28% in postnatal day (P)0-P5 rhythmic medullary slice preparations. In the absence of muscarine, pharmacological GIRK channel block by Tertiapin-Q did not affect inspiratory burst parameters, whereas activation with ML297 decreased inspiratory burst area. Blocking GIRK channels by local preapplication of Tertiapin-Q revealed a developmental change in muscarinic modulation of inspiratory bursting. In P0-P2 rhythmic medullary slices, Tertiapin-Q preapplication had no significant effect on muscarinic potentiation of inspiratory bursting (a negligible 6% decrease). However, preapplication of Tertiapin-Q to P3-P5 rhythmic medullary slices caused a 19% increase in muscarinic potentiation of XII inspiratory burst amplitude. Immunofluorescence experiments revealed expression of GIRK 1 and 2 subunits and M1, M2, M3, and M5 mAChRs from P0 to P5. Overall, these data support that mechanisms underlying muscarinic modulation of inspiratory burst activity change postnatally and that potent GIRK-mediated inhibition described in adults emerges early in postnatal life.NEW & NOTEWORTHY Muscarinic modulation of inspiratory bursting at hypoglossal motoneurons has a net excitatory effect in neonatal rhythmic medullary slice preparations and a net inhibitory effect in adult animals. We demonstrate that muscarinic modulation of inspiratory bursting undergoes maturational changes from postnatal days 0 to 5 that include emergence of an inhibitory component mediated by G-protein-coupled inwardly rectifying potassium channels after postnatal day 3 in neonatal mouse rhythmic medullary slice preparations.

Is exposure to chemical pollutants associated with sleep outcomes? A systematic review

Environmental exposures may influence sleep; however, the contributions of environmental chemical pollutants to sleep health have not been systematically investigated. We conducted a systematic review to identify, evaluate, summarize, and synthesize the existing evidence between chemical pollutants (air pollution, exposures related to the Gulf War and other conflicts, endocrine disruptors, metals, pesticides, solvents) and dimensions of sleep health (architecture, duration, quality, timing) and disorders (sleeping pill use, insomnia, sleep-disordered breathing)). Of the 204 included studies, results were mixed; however, the synthesized evidence suggested associations between particulate matter, exposures related to the Gulf War, dioxin and dioxin-like compounds, and pesticide exposure with worse sleep quality; exposures related to the Gulf War, aluminum, and mercury with insomnia and impaired sleep maintenance; and associations between tobacco smoke exposure with insomnia and sleep-disordered breathing, particularly in pediatric populations. Possible mechanisms relate to cholinergic signaling, neurotransmission, and inflammation. Chemical pollutants are likely key determinants of sleep health and disorders. Future studies should aim to evaluate environmental exposures on sleep across the lifespan, with a particular focus on developmental windows and biological mechanisms, as well as in historically marginalized or excluded populations.

Targets for obstructive sleep apnea pharmacotherapy: principles, approaches, and emerging strategies

INTRODUCTION

Obstructive sleep apnea (OSA) is a common and serious breathing disorder. Several pathophysiological factors predispose individuals to OSA. These factors are quantifiable, and modifiable pharmacologically.

AREAS COVERED

Four key pharmacotherapeutic targets are identified and mapped to the major determinants of OSA pathophysiology. PubMed and Clinicaltrials.gov were searched through April 2023.

EXPERT OPINION

Target #1: Pharyngeal Motor Effectors. Increasing pharyngeal muscle activity and responsivity with noradrenergic-antimuscarinic combination is central to recent breakthrough OSA pharmacotherapy. Assumptions, knowledge gaps, future directions, and other targets are identified. #2: Upper Airway Sensory Afferents. There is translational potential of sensitizing and amplifying reflex pharyngeal dilator muscle responses to negative airway pressure via intranasal delivery of new potassium channel blockers. Rationales, advantages, findings, and potential strategies to enhance effectiveness are identified. #3: Chemosensory Afferents and Ventilatory Control. Strategies to manipulate ventilatory control system sensitivity by carbonic anhydrase inhibitors are supported in theory and initial studies. Intranasal delivery of agents to stimulate central respiratory activity are also introduced. #4: Sleep-Wake Mechanisms. Arousability is the fourth therapeutic target rationalized. Evolving automated tools to measure key pathophysiological factors predisposing to OSA will accelerate pharmacotherapy. Although not currently ready for general clinical settings, the identified targets are of future promise.

Advances in Molecular Pathology of Obstructive Sleep Apnea

Obstructive sleep apnea (OSA) is a common syndrome that features a complex etiology and set of mechanisms. Here we summarized the molecular pathogenesis of OSA, especially the prospective mechanism of upper? airway dilator fatigue and the current breakthroughs. Additionally, we also introduced the molecular mechanism of OSA in terms of related studies on the main signaling pathways and epigenetics alterations, such as microRNA, long non-coding RNA, and DNA methylation. We also reviewed small molecular compounds, which are potential targets for gene regulations in the future, that are involved in the regulation of OSA. This review will be beneficial to point the way for OSA research within the next decade.

Differential pharmacological and sex-specific effects of antimuscarinic agents at the hypoglossal motor nucleus in vivo in rats

Successful cholinergic-noradrenergic pharmacotherapy for obstructive sleep apnea (OSA) is thought to be due to effects at the hypoglossal motor nucleus (HMN). Clinical efficacy varies with muscarinic-receptor (MR) subtype affinities. We hypothesized that oxybutynin (cholinergic agent in successful OSA pharmacotherapy) is an effective MR antagonist at the HMN and characterized its efficacy with other antagonists. We recorded tongue muscle activity of isoflurane anesthetized rats (121 males and 60 females, 7–13 per group across 13 protocols) in response to HMN microperfusion with MR antagonists with and without: (i) eserine-induced increased endogenous acetylcholine at the HMN and (ii) muscarine. Eserine-induced increased acetylcholine decreased tongue motor activity (p < 0.001) with lesser cholinergic suppression in females versus males (p = 0.017). Motor suppression was significantly attenuated by the MR antagonists atropine, oxybutynin, and omadacycline (MR2 antagonist), each p < 0.001, with similar residual activity between agents (p ≥ 0.089) suggesting similar efficacy at the HMN. Sex differences remained with atropine and oxybutynin (p < 0.001 to 0.05) but not omadacycline (p = 0.722). Muscarine at the HMN also decreased motor activity (p < 0.001) but this was not sex-specific (p = 0.849). These findings have translational relevance to antimuscarinic agents in OSA pharmacotherapy and understanding potential sex differences in HMN suppression with increased endogenous acetylcholine related to sparing nicotinic excitation.

Screening of plasma exosomal lncRNAs to identify potential biomarkers for obstructive sleep apnea

Background

Obstructive sleep apnea (OSA) is highly prevalent, but frequently undiagnosed. The existing biomarkers of OSA are relatively insensitive and inaccurate. Long non-coding RNAs (lncRNAs) have no protein-coding ability but have a role in regulating gene expression. They are stably expressed in exosomes, easily and rapidly measurable. Changes in expression of exosomal lncRNAs can be useful for disease diagnoses. However, there are few reports on the association of exosomal lncRNAs with OSA. We aimed to investigate the exosomal lncRNA profiles to establish the differences between non-OSA, OSA with or without hypertension (HTN) and serve as a potential diagnostic biomarker.

Methods

This diagnostic test included 63 participants: [normal control (NC) =25], (OSA =23), and (HTN-OSA =15). Expression profiling of lncRNAs in isolated exosomes was performed through high-throughput sequencing in 9 participants. Subsequently, OSA/HTN-OSA related lncRNAs were selected for validation by droplet digital polymerase chain reaction (ddPCR), receiver operating characteristic (ROC) curves were used to determine the diagnostic value. The reliabilities of the screened gene were further validated in another independent cohort: (NC =10), (OSA mild =10), (OSA moderate =11), and (OSA severe =10), the correlation between clinical features and its expression was analyzed. The MiRanda software was used to predict the binding sites of interaction between microRNA (miRNA) and target genes regulated by screened lncRNA.

Results

We identified the differentially expressed lncRNAs and mRNAs in plasma exosomes of the NC, OSA, HTN-OSA groups. Most pathways enriched in differentially expressed lncRNAs and mRNAs had previously been linked to OSA. Among them, ENST00000592016 enables discrimination between NC and OSA individuals [area under curve (AUC) =0.846, 95% confidence interval (CI): 0.72–0.97]. The severity of OSA was associated with changes in the ENST00000592016 expression. Furthermore, ENST00000592016 affected the PI3K-Akt, MAPK, and TNF pathways by regulating miRNA expressions.

Conclusions

This is the first report about differential expression of lncRNA in OSA and HTN-OSA exosomes. ENST00000592016 enables discrimination between NC and OSA individuals. This work enabled characterization of OSA and provided the preliminary work for the study of biomarker of OSA.

Activity of Pontine A7 Noradrenergic Neurons is suppressed during REM sleep

The activity of hypoglossal motoneurons plays a key role in the maintenance of upper airway patency. The withdrawal of noradrenergic excitatory drive and increase of cholinergic inhibition markedly decreases excitability of hypoglossal motoneurons during sleep and especially during rapid-eye-movement (REM) stage. This leads to increased collapsibility of upper airway during sleep, which is the major neurological factor of obstructive sleep apnea (OSA) pathophysiology. Anatomical and functional data suggests that noradrenergic A7 neurons are the main source of noradrenergic drive to hypoglossal motoneurons. However, it is unknown whether the behavior of A7 neurons during sleep-wake cycle is in accord with their proposed involvement in sleep-related depression of hypoglossal motoneuron activity. Therefore, we sought to assess the behavior of A7 neurons during sleep and wakefulness in naturally sleeping head-restrained rats. We have found that, similar to other pontine noradrenergic neurons, the putative A7 noradrenergic neurons fired with relatively long-lasting action potentials with a low frequency regular discharge. Importantly, all noradrenergic A7 neurons were predominantly silent during REM sleep. The REM-off activity of the A7 neurons supports our hypothesis that these neurons may significantly contribute to the withdrawal of excitatory noradrenergic drive from upper airway motoneurons during REM sleep and, consequently, play a critical role in maintaining upper airway patency and pathophysiology of OSA. Therefore, noradrenergic A7 neurons may serve as an additional target for designing pharmacological approaches to treat OSA.

Obstructive sleep apnea

Obstructive sleep apnea (OSA) is a disease that results from loss of upper airway muscle tone leading to upper airway collapse during sleep in anatomically susceptible persons, leading to recurrent periods of hypoventilation, hypoxia, and arousals from sleep. Significant clinical consequences of the disorder cover a wide spectrum and include daytime hypersomnolence, neurocognitive dysfunction, cardiovascular disease, metabolic dysfunction, respiratory failure, and pulmonary hypertension. With escalating rates of obesity a major risk factor for OSA, the public health burden from OSA and its sequalae are expected to increase, as well. In this chapter, we review the mechanisms responsible for the development of OSA and associated neurocognitive and cardiometabolic comorbidities. Emphasis is placed on the neural control of the striated muscles that control the pharyngeal passages, especially regulation of hypoglossal motoneuron activity throughout the sleep/wake cycle, the neurocognitive complications of OSA, and the therapeutic options available to treat OSA including recent pharmacotherapeutic developments.

Gene delivery to the hypoglossal motor system: preclinical studies and translational potential

Dysfunction and/or reduced activity in the tongue muscles contributes to conditions such as dysphagia, dysarthria, and sleep disordered breathing. Current treatments are often inadequate, and the tongue is a readily accessible target for therapeutic gene delivery. In this regard, gene therapy specifically targeting the tongue motor system offers two general strategies for treating lingual disorders. First, correcting tongue myofiber and/or hypoglossal (XII) motoneuron pathology in genetic neuromuscular disorders may be readily achieved by intralingual delivery of viral vectors. The retrograde movement of viral vectors such as adeno-associated virus (AAV) enables targeted distribution to XII motoneurons via intralingual viral delivery. Second, conditions with impaired or reduced tongue muscle activation can potentially be treated using viral-driven chemo- or optogenetic approaches to activate or inhibit XII motoneurons and/or tongue myofibers. Further considerations that are highly relevant to lingual gene therapy include (1) the diversity of the motoneurons which control the tongue, (2) the patterns of XII nerve branching, and (3) the complexity of tongue muscle anatomy and biomechanics. Preclinical studies show considerable promise for lingual directed gene therapy in neuromuscular disease, but the potential of such approaches is largely untapped.

Measurement and State-Dependent Modulation of Hypoglossal Motor Excitability and Responsivity In-Vivo

Motoneurons are the final output pathway for the brain’s influence on behavior. Here we identify properties of hypoglossal motor output to the tongue musculature. Tongue motor control is critical to the pathogenesis of obstructive sleep apnea, a common and serious sleep-related breathing disorder. Studies were performed on mice expressing a light sensitive cation channel exclusively on cholinergic neurons (ChAT-ChR2(H134R)-EYFP). Discrete photostimulations under isoflurane-induced anesthesia from an optical probe positioned above the medullary surface and hypoglossal motor nucleus elicited discrete increases in tongue motor output, with the magnitude of responses dependent on stimulation power (P < 0.001, n = 7) and frequency (P = 0.002, n = 8, with responses to 10 Hz stimulation greater than for 15–25 Hz, P < 0.022). Stimulations during REM sleep elicited significantly reduced responses at powers 3–20 mW compared to non-rapid eye movement (non-REM) sleep and wakefulness (each P < 0.05, n = 7). Response thresholds were also greater in REM sleep (10 mW) compared to non-REM and waking (3 to 5 mW, P < 0.05), and the slopes of the regressions between input photostimulation powers and output motor responses were specifically reduced in REM sleep (P < 0.001). This study identifies that variations in photostimulation input produce tunable changes in hypoglossal motor output in-vivo and identifies REM sleep specific suppression of net motor excitability and responsivity.