The central motor conductivity of genioglossus in obstructive sleep apnoea

Tongue Strength Training Increases Daytime Upper Airway Stability in Rats

PURPOSE

Tongue strength training (TST) has been shown to decrease the apnea-hypopnea index in some patients with obstructive sleep apnea (OSA). However, whether TST modulates the central regulation of genioglossus and influences the stability of the upper airway remains unknown. The purpose of this study was to dynamically assess the effect of TST on the upper airway.

METHODS

Sixteen adult male Sprague-Dawley rats were studied to explore the mechanism of TST improving the upper airway function. The rats were randomly assigned to the normal control (NC) and TST groups. The TST group underwent 8-week progressive resistance tongue exercise training. Transcranial magnetic stimulation (TMS) responses and EMG activities were consistently recorded for 2 h on days 0, 14, 28, and 56 of the experiments in both groups. Theoretical critical pressure (Pcrit) value was measured on days 0, 14, 28, and 56.

RESULTS

The TST group showed shorter TMS latency and higher genioglossus EMG activity, which lasted from 5 min to 80 min after training on day 56 of training, than the NC group. The TST group showed significantly lower theoretical Pcrit values on days 28 and 56 of training than the NC group (-4.07±0.92 vs -3.12±0.77 cmH₂O, P< 0.05, -4.66±0.74 vs -3.07±0.38 cmH₂O, P< 0.01).

CONCLUSION

This study revealed that an 8-week TST could gradually and transiently increase corticomotor excitability of genioglossus, elevate the genioglossus EMG activity, and ultimately enhance the stability of the upper airway during daytime. Moreover, improved neuromuscular excitability occurred prior to the enhanced upper airway stability. These findings provide a theoretical foundation for TST as a promising treatment for OSA patients.

ERS statement on respiratory muscle testing at rest and during exercise

Assessing respiratory mechanics and muscle function is critical for both clinical practice and research purposes. Several methodological developments over the past two decades have enhanced our understanding of respiratory muscle function and responses to interventions across the spectrum of health and disease. They are especially useful in diagnosing, phenotyping and assessing treatment efficacy in patients with respiratory symptoms and neuromuscular diseases. Considerable research has been undertaken over the past 17 years, since the publication of the previous American Thoracic Society (ATS)/European Respiratory Society (ERS) statement on respiratory muscle testing in 2002. Key advances have been made in the field of mechanics of breathing, respiratory muscle neurophysiology (electromyography, electroencephalography and transcranial magnetic stimulation) and on respiratory muscle imaging (ultrasound, optoelectronic plethysmography and structured light plethysmography). Accordingly, this ERS task force reviewed the field of respiratory muscle testing in health and disease, with particular reference to data obtained since the previous ATS/ERS statement. It summarises the most recent scientific and methodological developments regarding respiratory mechanics and respiratory muscle assessment by addressing the validity, precision, reproducibility, prognostic value and responsiveness to interventions of various methods. A particular emphasis is placed on assessment during exercise, which is a useful condition to stress the respiratory system.

Transient upregulation of TASK-1 expression in the hypoglossal nucleus during chronic intermittent hypoxia is reduced by serotonin 2A receptor antagonist

Hypoglossal motoneurons innervate genioglossus muscle, the contraction of which is critical in the maintenance of upper airway patency in patients with obstructive sleep apnea. As a potassium channel distributed in hypoglossal motoneurons, TWIK-related acid-sensitive K+ channel-1 (TASK-1) could be inhibited by 5-HT. This study aimed to investigate if TASK-1 expression in hypoglossal nucleus could be influenced by chronic intermittent hypoxia (CIH) and 5-HT_2A receptors antagonist. Two hundred twenty-eight rats were exposed to CIH or normoxia (NO) in the presence and absence of 5-HT _2A receptor antagonist (MDL-100907) microinjected into the hypoglossal nucleus. The expression of 5-HT and TASK-1 in the hypoglossal nucleus were detected by immunohistochemistry and reverse transcription quantitative polymerase chain reaction on the 1st, 3rd, 7th, 14th and 21st day of CIH exposure. The mean optical density (MOD) of 5-HT in the XII nucleus was significantly increased in the CIH and CIH + MDL group than the NO group on the 7th and 21st day ( p < 0.05). Compared with the NO group, the MOD and gene expression of TASK-1 in the CIH group was significantly increased on the 7th and 14th day ( p < 0.05), then normalized on the 21st day. The TASK-1 expression in the CIH + MDL group was significantly lower than the CIH + PBS and CIH group on the 7th and 14th day ( p < 0.05). The CIH-induced transiently upregulation of the TASK-1 expression in the hypoglossal nucleus could be reversed by 5-HT _2A receptor antagonist, indicating that the modulation of the TASK-1 expression in response to CIH involves 5-HT and 5-HT _2A receptors, and this CIH effect might be 5-HT _2A receptor-dependent.

Genistein Protects Genioglossus Myoblast Against Hypoxia-induced Injury through PI3K-Akt and ERK MAPK Pathways

Obstructive sleep apnea and hypopnea syndrome (OSAHS) is a clinical syndrome characterized by recurrent episodes of obstruction of the upper airway during sleep that leads to a hypoxic condition. Genioglossus, an important pharyngeal muscle, plays an important role in maintaining an open upper airway for effective breathing. Our previous study found that genistein (a kind of phytoestrogen) protects genioglossus muscle from hypoxia-induced oxidative injury. However, the underlying mechanism is still unknown. In the present study, we examined the effects of hypoxia on genioglossus myoblast proliferation, viability and apoptosis, and the protective effect of genistein and its relationship with the PI3K/Akt and ERK MAPK pathways. Cell viability and Bcl-2 were reduced under hypoxic condition, while ROS generation, caspase-3, MDA, and DNA damage were increased following a hypoxia exposure. However, the effects of hypoxia were partially reversed by genistein in an Akt- and ERK- (but not estrogen receptor) dependent manner. In conclusion, genistein protects genioglossus myoblasts against hypoxia-induced oxidative injury and apoptosis independent of estrogen receptor. The PI3K-Akt and ERK1/2 MAPK signaling pathways are involved in the antioxidant and anti-apoptosis effect of genistein on genioglossus myoblasts.

Noradrenergic Activation of Hypoglossal Nucleus Modulates the Central Regulation of Genioglossus in Chronic Intermittent Hypoxic Rats

Neuromuscular compensation of the genioglossus muscle can be induced by chronic intermittent hypoxia (CIH) in obstructive sleep apnea to maintain upper airway stability. Noradrenergic activation of hypoglossal nucleus plays a critical role in the central control of the genioglossus. However, it remains unknown whether norepinephrine takes part in the central regulation of the genioglossus during CIH. Adult male Wistar rats (n = 32) were studied to explore the influence of noradrenergic activation of hypoglossal nucleus on the central control of the genioglossus at different stages of CIH. The rats were divided into four groups: normal control or normoxic (NO) group, CIH group, CIH + normal saline (NS) group, and CIH + prazosin (PZ, α1-adrenergic antagonist) group. PZ (0.2 mM, 60 nl) and NS (0.9%, 60 nl) were microinjected into the hypoglossal nucleus. The responses of the genioglossus corticomotor area to transcranial magnetic stimulation (TMS) were recorded on the 1st, 7th, 14th, and 21st day of CIH. The CIH group showed significantly shorter TMS latencies on days 1, 7, and 14 (3.85 ± 0.37 vs. 4.58 ± 0.42, 3.93 ± 0.17 vs. 4.49 ± 0.55, 3.79 ± 0.38 vs. 4.39 ± 0.30 ms, P < 0.05), and higher TMS amplitudes on day 1 (2.74 ± 0.87 vs. 1.60 ± 0.52 mV, P < 0.05) of CIH than the NO group. Compared to the CIH + NS group, the CIH + PZ group showed decreased TMS responses (longer latencies and lower amplitudes) only on the 14th day of CIH (3.99 ± 0.28 vs. 4.61 ± 0.48 ms, 2.51 ± 0.67 vs. 1.18 ± 0.62 mV, P < 0.05). These results indicated that noradrenergic activation of the hypoglossal nucleus played a role in the central compensation of genioglossus through α1-adrenoceptor on the 14th day of CIH.

Assessment of tongue mechanical properties using different contraction tasks

Inadequate upper airway (UA) dilator muscle function may play an important role in the pathophysiology of obstructive sleep apnea (OSA). To date, tongue mechanical properties have been assessed mainly using protrusion protocol with conflicting results. Performance during elevation tasks among patients with OSA remains unknown. This study aimed at assessing tongue muscle strength, strength stability, endurance time, fatigue indices, and total muscle work, using elevation and protrusion tasks with repetitive isometric fatiguing contractions in 12 normal plus mild, 17 moderate, and 11 severe patients with OSA, and to assess the influence of body mass index (BMI) and age. Endurance time was longer in protrusion than elevation task (P = 0.01). In both tasks, endurance time was negatively correlated with baseline value of strength coefficient of variation (P < 0.01). Compared with other groups, patients with moderate OSA had the lowest total muscle work for protrusion (P = 0.01) and shortest endurance time (P = 0.04), regardless of the type of task. Additionally, in patients with moderate-severe OSA, the total muscle work for both tasks was lower in nonobese compared with obese (P < 0.05). Total muscle work for protrusion was positively correlated with apnea hypopnea index (AHI) in obese subjects (P < 0.01). Endurance time was shorter (P < 0.01) and recovery time longer (P = 0.02) in the old compared with young subjects. In conclusion, the tongue is more prone to fatigue during the elevation task and in patients with moderate OSA. Obesity appeared to prevent alteration of tongue mechanical properties in patients with OSA. Baseline strength stability and endurance were related, illustrating the role of central neuromuscular output in tongue resistance to fatigue.NEW & NOTEWORTHY To our knowledge, this is the first study to assess and compare tongue function using both elevation and protrusion tasks with repetitive isometric fatiguing contractions in subjects with different OSA status. Tongue mechanical performance seemed to differ between protrusion and elevation tasks and depend on the severity of OSA.

Distinctive patterns of cortical excitability to transcranial magnetic stimulation in obstructive sleep apnea syndrome, restless legs syndrome, insomnia, and sleep deprivation

Altered responses to transcranial magnetic stimulation (TMS) in obstructive sleep apnea syndrome (OSAS), restless legs syndrome (RLS), insomnia, and sleep-deprived healthy subjects have been reported. We have reviewed the relevant literature in order to identify eventual distinctive electrocortical profiles based on single and paired-pulse TMS, sensorimotor modulation, plasticity-related and repetitive TMS measures. Although obtained from heterogeneous studies, the detected changes might be the result of the different pathophysiological substrates underlying OSAS, RLS, insomnia and sleep deprivation rather than reflect the general effect of non-specific sleep loss and instability. OSAS tends to exhibit an increased motor cortex inhibition, which is reduced in RLS; intracortical excitability seems to be in favor of an "activating" profile in chronic insomnia and in sleep-deprived healthy individuals. Abnormal plasticity-related TMS phenomena have been demonstrated in OSAS and RLS. This review provides a perspective of TMS techniques by further understanding the role of neurotransmission pathways and plastic remodeling of neuronal networks involved in common sleep disorders. TMS might be considered a valuable tool in the assessment of sleep disorders, the evaluation of the effect of therapy and the design of non-pharmacological approaches.

Raphe serotonergic neurons modulate genioglossus corticomotor activity in intermittent hypoxic rats

Background

Genioglossus activity is greater during wakefulness but decreases to a weaker state during sleep in obstructive sleep apnea syndrome (OSAS) patients, compared to healthy subjects. Previous studies suggested that the corticomotor control of the genioglossus was modified in OSAS patients. Intermittent hypoxia (IH), the typical pathophysiological change in OSAS, can induce genioglossus facilitation. The serotonergic neurons of the raphe dorsal (DRN) and magnus nuclei (RMg) are responsive to hypoxia and play important roles in the control of the genioglossus. However, it remains unknown whether DRN and RMg serotonergic neurons are responsible for the facilitated corticomotor activity of the genioglossus during IH. This study explored the influence of IH on the corticomotor activity of the genioglossus by transcranial magnetic stimulation (TMS), and the role of DRN and RMg serotonergic neurons in this effect.

Methods

Rats were exposed to IH and divided into two groups. In one group, anti-SERT-SAP was microinjected into the DRN and RMg respectively to kill serotonergic neurons. In the other group, artificial cerebrospinal fluid (ACSF) was injected. Comparisons were conducted between the two groups during four weeks of IH and four weeks after IH.

Results

Compared to the corresponding ACSF-injected group, the DRN lesion group and RMg lesion group showed longer TMS latencies and lower amplitudes during IH from the 1^st to the 28^th day. After 28 days of IH, longer latencies and lower amplitudes were seen only in the DRN lesion group.

Conclusion

These results indicate that DRN and RMg serotonergic neurons play different roles in the facilitation of genioglossus corticomotor activity induced by IH.

Transcranial magnetic stimulation and sleep disorders: pathophysiologic insights

The neural mechanisms underlying the development of the most common intrinsic sleep disorders are not completely known. Therefore, there is a great need for noninvasive tools which can be used to better understand the pathophysiology of these diseases. Transcranial magnetic stimulation (TMS) offers a method to noninvasively investigate the functional integrity of the motor cortex and its corticospinal projections in neurologic and psychiatric diseases. To date, TMS studies have revealed cortical and corticospinal dysfunction in several sleep disorders, with cortical hyperexcitability being a characteristic feature in some disorders (i.e., the restless legs syndrome) and cortical hypoexcitability being a well-established finding in others (i.e., obstructive sleep apnea syndrome narcolepsy). Several research groups also have applied TMS to evaluate the effects of pharmacologic agents, such as dopaminergic agent or wake-promoting substances. Our review will focus on the mechanisms underlying the generation of abnormal TMS measures in the different types of sleep disorders, the contribution of TMS in enhancing the understanding of their pathophysiology, and the potential diagnostic utility of TMS techniques. We also briefly discussed the possible future implications for improving therapeutic approaches.

Intermittent hypoxia, respiratory plasticity and sleep apnea in humans: present knowledge and future investigations

This review examines the role that respiratory plasticity has in the maintenance of breathing stability during sleep in individuals with sleep apnea. The initial portion of the review considers the manner in which repetitive breathing events may be initiated in individuals with sleep apnea. Thereafter, the role that two forms of respiratory plasticity, progressive augmentation of the hypoxic ventilatory response and long-term facilitation of upper airway and respiratory muscle activity, might have in modifying breathing events in humans is examined. In this context, present knowledge regarding the initiation of respiratory plasticity in humans during wakefulness and sleep is addressed. Also, published findings which reveal that exposure to intermittent hypoxia promotes breathing instability, at least in part, because of progressive augmentation of the hypoxic ventilatory response and the absence of long-term facilitation, are considered. Next, future directions are presented and are focused on the manner in which forms of plasticity that stabilize breathing might be promoted while diminishing destabilizing forms, concurrently. These future directions will consider the potential role of circadian rhythms in the promotion of respiratory plasticity and the role of respiratory plasticity in enhancing established treatments for sleep apnea.

Central motor conduction time

Central motor conduction time (CMCT) is the time taken for neural impulses to travel through the central nervous system on their way to the target muscles. When the motor cortex is stimulated with transcranial magnetic stimulation (TMS), CMCT is calculated by subtracting the peripheral conduction time from the motor evoked potential latency elicited by motor cortical TMS. CMCT in infants and children reaches adult level at about age of 6 years for the lower limbs. The alterations of CMCT in various neurological conditions are reviewed in this chapter. Prolongation of CMCT occurs due to slowing of conduction through rapidly conducting corticospinal fibers, as seen in various disorders such as demyelinating diseases (multiple sclerosis, MS), amyotrophic lateral sclerosis, structural lesions in the corticospinal tract such as stroke and compressive myelopathy, and neurodegenerative disorders including multiple system atrophy and progressive supranuclear palsy. As CMCT is prolonged in certain clinical conditions, it is of diagnostic value in some neurological disorders such as myelopathy, amyotrophic lateral sclerosis, and MS when used together with other clinical and electrophysiological measures. It could also be used as a prognostic marker in some of neurological conditions, such as myelopathy and MS.

Functional role of neural injury in obstructive sleep apnea

The causes of obstructive sleep apnea (OSA) are multifactorial. Neural injury affecting the upper airway muscles due to repetitive exposure to intermittent hypoxia and/or mechanical strain resulting from snoring and recurrent upper airway closure have been proposed to contribute to OSA disease progression. Multiple studies have demonstrated altered sensory and motor function in patients with OSA using a variety of neurophysiological and histological approaches. However, the extent to which the alterations contribute to impairments in upper airway muscle function, and thus OSA disease progression, remains uncertain. This brief review, primarily focused on data in humans, summarizes: (1) the evidence for upper airway sensorimotor injury in OSA and (2) current understanding of how these changes affect upper airway function and their potential to change OSA progression. Some unresolved questions including possible treatment targets are noted.