Modulation of genioglossus muscle activity across sleep-wake states by histamine at the hypoglossal motor pool

The medullary serotonergic centres involved in cardiorespiratory control are disrupted by fetal growth restriction

Fetal growth restriction (FGR) is associated with cardiovascular and respiratory complications after birth and beyond. Despite research showing a range of neurological changes following FGR, little is known about how FGR affects the brainstem cardiorespiratory control centres. The primary neurons that release serotonin reside in the brainstem cardiorespiratory control centres and may be affected by FGR. At two time points in the last trimester of sheep brain development, 110 and 127 days of gestation (0.74 and 0.86 of gestation), we assessed histopathological alterations in the brainstem cardiorespiratory control centres of the pons and medulla in early-onset FGR versus control fetal sheep. The FGR cohort were hypoxaemic and asymmetrically growth restricted. Compared to the controls, the brainstem of FGR fetuses exhibited signs of neuropathology, including elevated cell death and reduced cell proliferation, grey and white matter deficits, and evidence of oxidative stress and neuroinflammation. FGR brainstem pathology was predominantly observed in the medullary raphé nuclei, hypoglossal nucleus, nucleus ambiguous, solitary tract and nucleus of the solitary tract. The FGR groups showed imbalanced brainstem serotonin and serotonin 1A receptor abundance in the medullary raphé nuclei, despite evidence of increased serotonin staining within vascular regions of placentomes collected from FGR fetuses. Our findings demonstrate both early and adaptive brainstem neuropathology in response to placental insufficiency. KEY POINTS: Early-onset fetal growth restriction (FGR) was induced in fetal sheep, resulting in chronic fetal hypoxaemia. Growth-restricted fetuses exhibit persistent neuropathology in brainstem nuclei, characterised by disrupted cell proliferation and reduced neuronal cell number within critical centres responsible for the regulation of cardiovascular and respiratory functions. Elevated brainstem inflammation and oxidative stress suggest potential mechanisms contributing to the observed neuropathological changes. Both placental and brainstem levels of 5-HT were found to be impaired following FGR.

Histamine/H1 receptor signaling in the parafacial region increases activity of chemosensitive neurons and respiratory activity in rats

Histaminergic neurons of the tuberomammillary nucleus (TMN) are pH-sensitive and contribute to CO₂/H⁺-dependent behaviors including arousal and respiratory activity. TMN neurons project to several respiratory centers including the ventral parafacial region (pF) where chemosensitive retrotrapezoid (RTN) neurons are located, and since RTN neurons are an important source of CO₂/H⁺-dependent respiratory drive, we wondered whether histamine contributes to RTN chemoreception. To test this, we characterized effects of histamine on mean arterial pressure (MAP) and diaphragm muscle activity (DIA_EMG) in urethane-anaesthetized, vagotomized and artificially ventilated male Wistar rats. Unilateral injection of histamine (25 mM) in the pF increased DIA_EMG amplitude without changing DIA_EMG frequency and MAP. Bilateral pF injections of the H1 receptor antagonist diphenhydramine hydrochloride (DPH; 0.5 mM) decreased baseline DIA_EMG amplitude and frequency and MAP. Despite the strong inhibitory effect of DPH on baseline breathing, the hypercapnic ventilatory response was preserved under these experimental conditions. At the cellular level, chemosensitive RTN neurons showed a dose-dependent excitatory response to histamine that was blunted by DPH and mimicked by the H1 receptor agonist 2-pyridylethylamine dihydrochloride (2PYEA) under both control conditions and when fast neurotransmitter receptors are blocked. We also tested effects of 2PYEA in the presence of serotonin, another wake-on neurotransmitter that activates RTN chemoreceptors partly by activation of Gq-coupled receptors. We found the response to 2PYEA was diminished in serotonin, suggesting RTN neurons have a limited capacity to respond to multiple Gq-coupled modulators. These results suggest histamine can modulate breathing at the pF level by a mechanism involving H1 receptors.

Breathing during sleep

The depth, rate, and regularity of breathing change following transition from wakefulness to sleep. Interactions between sleep and breathing involve direct effects of the central mechanisms that generate sleep states exerted at multiple respiratory regulatory sites, such as the central respiratory pattern generator, respiratory premotor pathways, and motoneurons that innervate the respiratory pump and upper airway muscles, as well as effects secondary to sleep-related changes in metabolism. This chapter discusses respiratory effects of sleep as they occur under physiologic conditions. Breathing and central respiratory neuronal activities during nonrapid eye movement (NREM) sleep and REM sleep are characterized in relation to activity of central wake-active and sleep-active neurons. Consideration is given to the obstructive sleep apnea syndrome because in this common disorder, state-dependent control of upper airway patency by upper airway muscles attains high significance and recurrent arousals from sleep are triggered by hypercapnic and hypoxic episodes. Selected clinical trials are discussed in which pharmacological interventions targeted transmission in noradrenergic, serotonergic, cholinergic, and other state-dependent pathways identified as mediators of ventilatory changes during sleep. Central pathways for arousals elicited by chemical stimulation of breathing are given special attention for their important role in sleep loss and fragmentation in sleep-related respiratory disorders.

The Combination of Betahistine and Oxybutynin Increases Respiratory Control Sensitivity (Loop Gain) in People with Obstructive Sleep Apnea: A Randomized, Placebo-Controlled Trial

RATIONALE

There are widespread histaminergic projections throughout the brain, including hypoglossal nuclei, that modulate pharyngeal muscle tone and respiratory control. Hence, histaminergic stimulation pharmacologically may increase pharyngeal muscle tone and stabilize respiratory control (loop gain) to reduce obstructive sleep apnea (OSA) severity. Antimuscarinics also increase REM pharyngeal muscle tone in rats. Thus, a combination of histaminergic and anti-muscarinic drugs may be a novel target for OSA pharmacotherapy. However, this has not been investigated. Accordingly, we aimed to test the effects of betahistine (Beta), an H3-autoreceptor antagonist which thereby increases histamine levels, in combination with the antimuscarinic oxybutynin (Oxy), on OSA severity, OSA endotypes, polysomnography parameters and next-day sleepiness and alertness.

METHODS

Thirteen adults with OSA received either Beta-Oxy (96-5mg) or placebo according to a randomized, crossover, double-blind design, prior to polysomnography. Participants completed the Karolinska Sleep Scale and Leeds Sleep Evaluation Questionnaire and a driving simulation task to quantify next-day sleepiness and alertness. OSA endotypes were estimated through validated algorithms using polysomnography.

RESULTS

Compared to placebo, Beta-Oxy increased respiratory control sensitivity (loop gain) (0.52[0.24] vs 0.60[0.34], median [IQR], P = 0.021) without systematically changing OSA severity (34.4±17.2 vs 40.3±27.3 events/h, mean±SD, P = 0.124), sleep efficiency, arousal index or markers of hypoxemia. Beta-Oxy was well tolerated and did not worsen next-day sleepiness/alertness.

CONCLUSION

Rather than stabilize breathing during sleep, Beta-Oxy increases loop gain, which is likely to be deleterious for most people with OSA. However, in certain conditions characterized by blunted respiratory control (eg, obesity hypoventilation syndrome), interventions to increase loop gain may be beneficial.

Impact of cold and flu medication on obstructive sleep apnoea and its underlying traits: A pilot randomized controlled trial

BACKGROUND AND OBJECTIVE

Animal studies indicate that alpha-1 adrenergic receptor agonists and antimuscarinic agents improve genioglossus muscle activity during sleep and may be candidates for the pharmacological treatment of OSA. On the other hand, noradrenergic stimulants may be wake-promoting or cause insomnia symptoms if taken before bedtime, and the addition of a medication with sedative properties, such as an antihistaminic, may reduce these side effects. In this study, we aimed to determine the effects of the combination of an alpha-1 adrenergic agonist (pseudoephedrine) and an antihistaminic-antimuscarinic (diphenhydramine) on OSA severity (AHI), genioglossus responsiveness and other endotypic traits (V_passive , muscle compensation, LG and arousal threshold).

METHODS

Ten OSA patients performed a randomized, placebo-controlled, double-blind, crossover trial comparing one night of pseudoephedrine 120 mg plus diphenhydramine 50 mg (DAW1033D) to placebo administered prior to sleep. The AHI, genioglossus muscle responsiveness to negative oesophageal pressure and the endotypic traits were measured via PSG.

RESULTS

The participants' median (interquartile range) age was 50 (46-53) years and body mass index (BMI) was 34.3 (30.6-39.2) kg/m² . The drug combination had no effect on AHI (21.6 (9.1-49.8) on placebo vs 37.9 (5.1-55.4) events/h on DAW1033D, P > 0.5) or genioglossus responsiveness (6.0 (2.6-9.2) on placebo vs 4.0 (3.5-7.3) %/cm H₂ O). Amongst the phenotypic traits, only V_passive was improved by 29 (3-55) % eupnoea, P = 0.03 (mean (95% CI)).

CONCLUSION

The combination of pseudoephedrine and diphenhydramine did not improve OSA severity or genioglossus responsiveness but induced a small improvement in upper airway collapsibility, possibly due to the decongestant effect of the medications. The results of this study do not support the use of these medications for OSA treatment.

Impact of histaminergic H3 receptor antagonist on hypoglossal nucleus in chronic intermittent hypoxia conditions

RATIONALE

The hypoglossal nucleus (HN) controls the movement of the genioglossus (GG) muscle whose dysfunction leads to airway occlusion and occurrence of obstructive sleep apnea (OSA). Histamine produced by the tuberomammillary nucleus (TMN) has a potent excitatory action on GG muscle activity.

OBJECTIVES

The aim of the study was to investigate the role histaminergic neurons play in the regulation of the genioglossus.

METHODS

C57BL/6 mice were exposed to chronic intermittent hypoxia (CIH) for 3 weeks to resemble OSA. The histamine H3 receptor (H3R) antagonist ciproxifan was applied to increase histamine in the brain. Histamine levels and GG activity were measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS) and electromyogram (EMG) separately. Neuronal activity and repair ability of the HN and TMN and key proteins of histamine were analyzed by immunohistochemistry and western blots.

RESULTS

Significant decline of histamine level and GG activity of the HN and TMN induced by CIH exposure could be ameliorated by ciproxifan. Application of ciproxifan could also partly reverse the decline of the histidine decarboxylase (HDC) by CIH.

CONCLUSIONS

This investigation studied the impacts of ciproxifan on the HN and TMN in CIH conditions and revealed that the negative effects on the HN and TMN caused by CIH could be partly ameliorated by ciproxifan, which might open new perspectives for the development of pharmacological treatment for OSA.

Standard Correction of Vision Worsens EMG Activity of Pericranial Muscles in Chronic TMD Subjects

Recent studies showed an evident correlation between the stomatognathic system and the visual system. These results suggest that subjects who are affected by both temporomandibular (TMD) disorders and refractive disorders present with altered control of pericranial musculature tone and higher open-eye electromyographic (EMG) values. The objective of this work was to evaluate the effects of standard vision correction on EMG in subjects suffering from TMD compared with application of the same vision treatments to non-TMD subjects. 40 subjects were enrolled in this study. The test group included 20 myopic subjects and also included patients with TMD. The control group included 20 healthy myopic subjects. All of the participants underwent a complete ocular examination and a sEMG analysis. The results showed that TMD subjects with vision disorders that are corrected with standard glasses present EMG values that are significantly higher than those presented by non-TMD subjects with vision disorders and standard glasses. Infact, in TMD subjects, eye correction did not have a positive effect on the stomatognathic or pericranial musculature.

Muscles of Breathing: Development, Function, and Patterns of Activation

This review is a comprehensive description of all muscles that assist lung inflation or deflation in any way. The developmental origin, anatomical orientation, mechanical action, innervation, and pattern of activation are described for each respiratory muscle fulfilling this broad definition. In addition, the circumstances in which each muscle is called upon to assist ventilation are discussed. The number of "respiratory" muscles is large, and the coordination of respiratory muscles with "nonrespiratory" muscles and in nonrespiratory activities is complex-commensurate with the diversity of activities that humans pursue, including sleep (8.27). The capacity for speech and adoption of the bipedal posture in human evolution has resulted in patterns of respiratory muscle activation that differ significantly from most other animals. A disproportionate number of respiratory muscles affect the nose, mouth, pharynx, and larynx, reflecting the vital importance of coordinated muscle activity to control upper airway patency during both wakefulness and sleep. The upright posture has freed the hands from locomotor functions, but the evolutionary history and ontogeny of forelimb muscles pervades the patterns of activation and the forces generated by these muscles during breathing. The distinction between respiratory and nonrespiratory muscles is artificial, as many "nonrespiratory" muscles can augment breathing under conditions of high ventilator demand. Understanding the ontogeny, innervation, activation patterns, and functions of respiratory muscles is clinically useful, particularly in sleep medicine. Detailed explorations of how the nervous system controls the multiple muscles required for successful completion of respiratory behaviors will continue to be a fruitful area of investigation. © 2019 American Physiological Society. Compr Physiol 9:1025-1080, 2019.

REM obstructive sleep apnea: risk for adverse health outcomes and novel treatments

Rapid eye movement (REM) sleep was discovered nearly 60 years ago. This stage of sleep accounts for approximately a quarter of total sleep time in healthy adults, and it is mostly concentrated in the second half of the sleep period. The majority of research on REM sleep has focused on neurocognition. More recently, however, there has been a growing interest in understanding whether obstructive sleep apnea (OSA) during the two main stages of sleep (REM and non-REM sleep) leads to different cardiometabolic and neurocognitive risk. In this review, we discuss the growing evidence indicating that OSA during REM sleep is a prevalent disorder that is independently associated with adverse cardiovascular, metabolic, and neurocognitive outcomes. From a therapeutic standpoint, we discuss limitations of continuous positive airway pressure (CPAP) therapy given that 3 or 4 h of CPAP use from the beginning of the sleep period would leave 75% or 60% of obstructive events during REM sleep untreated. We also review potential pharmacologic approaches to treating OSA during REM sleep. Undoubtedly, further research is needed to establish best treatment strategies in order to effectively treat REM OSA. Moreover, it is critical to understand whether treatment of REM OSA will translate into better patient outcomes.

Neuroanatomical Basis of State-Dependent Activity of Upper Airway Muscles

Obstructive Sleep Apnea (OSA) is a common sleep-related respiratory disorder that is associated with cognitive, cardiovascular, and metabolic morbidities. The major cause of OSA is the sleep-related reduction of upper airway muscle tone that leads to airway obstructions in individuals with anatomically narrow upper airway. This reduction is mainly due to the suppressant effect of sleep on hypoglossal motoneurons that innervate upper airway muscles. The hypoglossal motoneurons have state-dependent activity, which is decreased during the transition from wakefulness to non-rapid eye movement sleep and is further suppressed during rapid eye movement sleep. Multiple neurotransmitters and their receptors have been implicated in the control of hypoglossal motoneuron activity across the sleep-wake states. However, to date, the results of the rigorous testing show that withdrawal of noradrenergic excitation and cholinergic inhibition essentially contribute to the depression of hypoglossal motoneuron activity during sleep. The present review will focus on origins of noradrenergic and cholinergic innervation of hypoglossal motoneurons and the functional role of these neurons in the state-dependent activity of hypoglossal motoneurons.

Modulation of Motoneuronal Activity With Sleep-Wake States and Motoneuronal Gene Expression Vary With Circadian Rest-Activity Cycle

In both nocturnal and diurnal mammals, sleep and wake states differentially aggregate during the rest and active phases of circadian cycle. Closely associated with this rhythm are prominent changes in motor activity. Here, we quantified the magnitudes of electromyographic activity (EMG) measured separately during different sleep-wake states across the rest-activity cycle, thereby separating amplitude measurements from the known dependance of the timing of wake and sleep on the phase of circadian rest-activity cycle. In seven rats chronically instrumented for electroencephalogram and EMG monitoring, nuchal and lingual muscle EMGs were measured as a commonly used postural output in behavioral sleep studies and as a cranial motor output with potential clinical relevance in obstructive sleep apnea (OSA) syndrome, respectively. We found that, for both motor outputs, EMG measured during wake episodes was significantly higher during the active phase, than during the rest phase, of circadian cycle. The corresponding patterns observed during slow-wave sleep (SWS) and rapid eye movement sleep (REMS) were different. During SWS, lingual EMG was very low and did not differ between the rest and active phase, whereas nuchal EMG had pattern similar to that during wakefulness. During REMS, lingual EMG was, paradoxically, higher during the rest phase due to increased twitching activity, whereas nuchal EMG was very low throughout the rest and active periods (postural atonia). In the follow-up comparison of differences in transcript levels in tissue samples obtained from the medullary hypoglossal motor nucleus and inferior olive (IO) at rest onset and active period onset conducted using microarrays, we identified significant differences for multiple transcripts representing the core members of the molecular circadian clock and other genes important for the regulation of cell metabolism and activity (up to n = 130 at p < 0.001). Collectively, our data indicate that activity of motoneurons is regulated to optimally align it with the rest-activity cycle, with the process possibly involving transcriptional mechanisms at the motoneuronal level. Our data also suggest that OSA patients may be relatively better protected against sleep-related upper airway obstructions during REMS episodes generated during the rest phase, than during active phase, of the circadian cycle.

Signaling mechanism underlying the histamine-modulated action of hypoglossal motoneurons

Histamine, an important modulator of the arousal states of the central nervous system, has been reported to contribute an excitatory drive at the hypoglossal motor nucleus to the genioglossus (GG) muscle, which is involved in the pathogenesis of obstructive sleep apnea. However, the effect of histamine on hypoglossal motoneurons (HMNs) and the underlying signaling mechanisms have remained elusive. Here, whole-cell patch-clamp recordings were conducted using neonatal rat brain sections, which showed that histamine excited HMNs with an inward current under voltage-clamp and a depolarization membrane potential under current-clamp via histamine H1 receptors (H1Rs). The phospholipase C inhibitor U-73122 blocked H1Rs-mediated excitatory effects, but protein kinase A inhibitor and protein kinase C inhibitor did not, indicating that the signal transduction cascades underlying the excitatory action of histamine on HMNs were H1R/Gq/11 /phospholipase C/inositol-1,4,5-trisphosphate (IP3). The effects of histamine were also dependent on extracellular Na(+) and intracellular Ca(2+), which took place via activation of Na(+)-Ca(2+) exchangers. These results identify the signaling molecules associated with the regulatory effect of histamine on HMNs. The findings of this study may provide new insights into therapeutic approaches in obstructive sleep apnea. We proposed the post-synaptic mechanisms underlying the modulation effect of histamine on hypoglossal motoneuron. Histamine activates the H1Rs via PLC and IP3, increases Ca(2+) releases from intracellular stores, promotes Na(+) influx and Ca(2+) efflux via the NCXs, and then produces an inward current and depolarizes the neurons. Histamine modulates the excitability of HMNs with other neuromodulators, such as noradrenaline, serotonin and orexin. We think that these findings should provide an important new direction for drug development for the treatment of obstructive sleep apnea.

Orexin A activates hypoglossal motoneurons and enhances genioglossus muscle activity in rats

BACKGROUND AND PURPOSE

Orexins have been demonstrated to play important roles in many physiological processes. However, it is not known how orexin A affects the activity of the hypoglossal motoneuron (HMN) and genioglossus (GG) muscle.

EXPERIMENTAL APPROACH

GG muscle electromyograms (GG-EMG) were recorded in anaesthetized adult rats after orexin A or orexin receptor antagonists were applied to the hypoglossal nucleus, and in adult rats in which orexin neurons were lesioned with the neurotoxin orexin-saporin (orexin-SAP). HMN membrane potential and firing were recorded from neonatal rat brain slices using whole-cell patch clamp after an infusion of orexin A or orexin receptor antagonists.

KEY RESULTS

Unilateral micro-injection of orexin A (50, 100 or 200 μM) into the hypoglossal nucleus significantly enhanced ipsilateral GG activity in adult rats. Orexin A (4, 20, 100 or 500 nM) depolarized the resting membrane potential and increased the firing rate of HMNs in a dose-dependent manner in the medullary slices of neonatal rats. Both SB 334867, a specific OX1 receptor antagonist and TCS OX2 29, a specific OX2 receptor antagonist not only blocked the depolarized membrane potential and the increased firing rate of HMNs by orexin A in the neonatal model but also attenuated GG-EMG in the adult model. A significant decrease in GG-EMG was observed in adult orexin neuron-lesioned rats compared with sham animals.

CONCLUSION AND IMPLICATIONS

Orexin A activates OX1 and OX2 receptors within the hypoglossal motor pool and promotes GG activity, indicating that orexin A is involved in controlling respiratory motor activity.

Neural control of the upper airway: integrative physiological mechanisms and relevance for sleep disordered breathing

The various neural mechanisms affecting the control of the upper airway muscles are discussed in this review, with particular emphasis on structure-function relationships and integrative physiological motor-control processes. Particular foci of attention include the respiratory function of the upper airway muscles, and the various reflex mechanisms underlying their control, specifically the reflex responses to changes in airway pressure, reflexes from pulmonary receptors, chemoreceptor and baroreceptor reflexes, and postural effects on upper airway motor control. This article also addresses the determinants of upper airway collapsibility and the influence of neural drive to the upper airway muscles, and the influence of common drugs such as ethanol, sedative hypnotics, and opioids on upper airway motor control. In addition to an examination of these basic physiological mechanisms, consideration is given throughout this review as to how these mechanisms relate to integrative function in the intact normal upper airway in wakefulness and sleep, and how they may be involved in the pathogenesis of clinical problems such obstructive sleep apnea hypopnea.

The effect of tongue exercise on serotonergic input to the hypoglossal nucleus in young and old rats

PURPOSE

Breathing and swallowing problems affect elderly people and may be related to age-associated tongue dysfunction. Hypoglossal motoneurons that innervate the tongue receive a robust, excitatory serotonergic (5HT) input and may be affected by aging. We used a rat model of aging and progressive resistance tongue exercise to determine whether age-related alterations in 5HT inputs to the hypoglossal nucleus can be modified. We hypothesized that tongue forces would increase with exercise, 5HT input to the tongue would decrease with age, and tongue exercise would augment 5HT input to the hypoglossal nucleus.

METHOD

Young (9-10 months), middle-aged (24-25 months), and old (32-33 months) male F344/BN rats received tongue exercise for 8 weeks. Immunoreactivity for 5HT was measured in digital images of sections through the hypoglossal nucleus using ImageJ software.

RESULTS

Tongue exercise resulted in increased maximum tongue forces at all ages. There was a statistically significant increase in 5HT immunoreactivity in the hypoglossal nucleus in exercised, young rats but only in the caudal third of the nucleus and primarily in the ventral half.

CONCLUSION

Specificity found in serotonergic input following exercise may reflect the topographic organization of motoneurons in the hypoglossal nucleus and the tongue muscles engaged in the exercise paradigm.