Electromyographic activity at the base and tip of the tongue across sleep-wake states in rats

Noradrenergic terminal density varies among different groups of hypoglossal premotor neurons

In obstructive sleep apnea (OSA) patients, contraction of the muscles of the tongue is needed to protect the upper airway from collapse. During wakefulness, norepinephrine directly excites motoneurons that innervate the tongue and other upper airway muscles but its excitatory effects decline during sleep, thus contributing to OSA. In addition to motoneurons, NE may regulate activity in premotor pathways but little is known about these upstream effects. To start filling this void, we injected a retrograde tracer (beta-subunit of cholera toxin-CTb; 5-10 nl, 1%) into the hypoglossal (XII) motor nucleus in 7 rats. We then used dual immunohistochemistry and brightfield microscopy to count dopamine beta-hydroxylase (DBH)-positive axon terminals closely apposed to CTb cells located in five anatomically distinct XII premotor regions. In different premotor groups, we found on the average 2.2-4.3 closely apposed DBH terminals per cell, with ˜60% more terminals on XII premotor neurons located in the ventrolateral pontine parabrachial region and ventral medullary gigantocellular region than on XII premotor cells of the rostral or caudal intermediate medullary reticular regions. This difference suggests stronger control by norepinephrine of the interneurons that mediate complex behavioral effects than of those mediating reflexes or respiratory drive to XII 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.

Hypoglossal motoneurons are endogenously activated by serotonin during the active period of circadian cycle

In obstructive sleep apnea patients, contraction of lingual muscles protects the pharyngeal airway from collapse. Hypoglossal (XII) motoneurons innervate the muscles of the tongue and are themselves under wake-related excitatory drives, including that mediated by serotonin (5-HT). Estimates of endogenous 5-HT activation vary among different studies. We tested whether endogenous drive mediated by 5-HT is present in rat XII motoneurons when measured during the active period of the circadian cycle. We monitored sleep-wake states and lingual and nuchal electromyograms (EMGs) while perfusing the XII nucleus with a vehicle or a 5-HT₂ receptor antagonist (mianserin, 0.2mM) at the active period onset. EMG levels were measured during each behavioral state and normalized by the mean EMG activity during wakefulness at 4-7am. Wake-related lingual EMG was significantly lower during mianserin perfusion than with the vehicle (53.0±9.7% vs. 84.5±8.7%; p=0.002). Mianserin had no effect on nuchal EMG or sleep-wake behavior. Thus, rat XII motoneurons receive endogenous serotonergic activation during wakefulness when measured during the dark period. This indicates that XII motoneuronal activity is enhanced by 5-HT output during the active period of the circadian cycle.

Tongue strength and endurance: Comparison in active and non-active young and older adults

PURPOSE

Tongue strength and endurance are important for swallowing and upper airway patency. Physical activity positively affects targeted and non-targeted skeletal muscles; however, little is known about the indirect effect of physical activity on tongue muscles. This study sought to determine if tongue muscle performance differs between highly active and non-active individuals and if such an effect varies with age.

METHOD

Forty-eight healthy adults were divided into two age groups (24 young, 20.96 ± 3.22 years; 24 older, 65 ± 3.72 years) and further divided into highly active and non-active based on The General Practice Physical Activity Questionnaire. Tongue strength (TS) and tongue endurance (TE) were obtained using the Iowa Oral Performance Instrument.

RESULT

A significant main effect of activity level on TS and TE was found. Although the main effect of age on TS and TE and age × activity level interactions were not significant, the effect of activity level on TS and TE was more pronounced in older adults than younger adults.

CONCLUSION

Findings suggest physical activity may affect TS and TE, particularly in older adults. Future research is warranted to understand the underlying mechanisms contributing to these group differences. Clinical implications of these findings are discussed.

Neural Control of the Upper Airway: Respiratory and State-Dependent Mechanisms

Upper airway muscles subserve many essential for survival orofacial behaviors, including their important role as accessory respiratory muscles. In the face of certain predisposition of craniofacial anatomy, both tonic and phasic inspiratory activation of upper airway muscles is necessary to protect the upper airway against collapse. This protective action is adequate during wakefulness, but fails during sleep which results in recurrent episodes of hypopneas and apneas, a condition known as the obstructive sleep apnea syndrome (OSA). Although OSA is almost exclusively a human disorder, animal models help unveil the basic principles governing the impact of sleep on breathing and upper airway muscle activity. This article discusses the neuroanatomy, neurochemistry, and neurophysiology of the different neuronal systems whose activity changes with sleep-wake states, such as the noradrenergic, serotonergic, cholinergic, orexinergic, histaminergic, GABAergic and glycinergic, and their impact on central respiratory neurons and upper airway motoneurons. Observations of the interactions between sleep-wake states and upper airway muscles in healthy humans and OSA patients are related to findings from animal models with normal upper airway, and various animal models of OSA, including the chronic-intermittent hypoxia model. Using a framework of upper airway motoneurons being under concurrent influence of central respiratory, reflex and state-dependent inputs, different neurotransmitters, and neuropeptides are considered as either causing a sleep-dependent withdrawal of excitation from motoneurons or mediating an active, sleep-related inhibition of motoneurons. Information about the neurochemistry of state-dependent control of upper airway muscles accumulated to date reveals fundamental principles and may help understand and treat OSA. © 2016 American Physiological Society. Compr Physiol 6:1801-1850, 2016.

Lingual muscle activity across sleep-wake States in rats with surgically altered upper airway

Obstructive sleep apnea (OSA) patients have increased upper airway muscle activity, including such lingual muscles as the genioglossus (GG), geniohyoid (GH), and hyoglossus (HG). This adaptation partially protects their upper airway against obstructions. Rodents are used to study the central neural control of sleep and breathing but they do not naturally exhibit OSA. We investigated whether, in chronically instrumented, behaving rats, disconnecting the GH and HG muscles from the hyoid (H) apparatus would result in a compensatory increase of other upper airway muscle activity (electromyogram, EMG) and/or other signs of upper airway instability. We first determined that, in intact rats, lingual (GG and intrinsic) muscles maintained stable activity levels when quantified based on 2 h-long recordings conducted on days 6 through 22 after instrumentation. We then studied five rats in which the tendons connecting the GH and HG muscles to the H apparatus were experimentally severed. When quantified across all recording days, lingual EMG during slow-wave sleep (SWS) was modestly but significantly increased in rats with surgically altered upper airway [8.6 ± 0.7% (SE) vs. 6.1 ± 0.7% of the mean during wakefulness; p = 0.012]. Respiratory modulation of lingual EMG occurred mainly during SWS and was similarly infrequent in both groups, and the incidence of sighs and central apneas also was similar. Thus, a weakened action of selected lingual muscles did not produce sleep-disordered breathing but resulted in a relatively elevated activity in other lingual muscles during SWS. These results encourage more extensive surgical manipulations with the aim to obtain a rodent model with collapsible upper airway.

Time- and behavioral state-dependent changes in posterior hypothalamic GABAA receptors contribute to the regulation of sleep

Sleep-wake behavior is regulated by a circadian rhythm, homeostatically and by additional mechanisms that determine the timing of slow-wave sleep and rapid eye movement sleep (REMS) episodes. The posterior hypothalamus coordinates the neural and humoral signals with the rest-activity cycle. It contains wake-active neurons, and is a site where stimulation of inhibitory GABA_A receptors promotes sleep, whereas their antagonism enhances wakefulness. We explored whether GABAergic mechanisms present in the posterior hypothalamus contribute to the homeostatic and other aspects of sleep-wake regulation. Using micropunches of tissue extracted from either the perifornical (PF) or dorsomedial (DM) regions of the posterior hypothalamus of rats, we determined that mRNA levels for selected subunits of GABA_A receptors (β1, β3 and ε) were higher at the end of the active period or following sleep deprivation, when the need for sleep is high, than after several hours of sleep, when sleep need is partially fulfilled. Such a pattern was present in the PF region only, and was consistent with changes in β1 subunit and GABA synthesizing enzyme (GAD) protein levels. In contrast, in the DM region, the levels of GABA_A receptor subunit mRNAs and proteins (α1, α2, β1) and GAD varied with circadian time, but were not responsive to sleep deprivation. Separate experiments with sleep-wake monitoring and local perfusion of the PF region with the GABA_A receptor antagonist bicuculline revealed that the antagonist had a weaker sleep-reducing effect when sleep need was enhanced by sleep deprivation and that the increased amount of REMS characteristic of the late sleep period was dependent on endogenous GABAergic inhibition. These results support the concept that a varying magnitude of GABAergic inhibition exerted within the PF region contributes to the homeostatic regulation of sleep and shapes its temporal pattern, whereas GABAergic mechanisms in the DM region contribute to circadian regulation.

Intranodose ganglion injections of dronabinol attenuate serotonin-induced apnea in Sprague-Dawley rat

Obstructive sleep apnea represents a significant public health concern. Afferent vagal activation is implicated in increased apnea susceptibility by reducing upper airway muscle tone via activation of serotonin receptors in the nodose ganglia. Previous investigations demonstrated that systemically administered cannabinoids can be used therapeutically to decrease the apnea/hypopnea index in rats and in humans. However, cannabinoids have effects on both the central and peripheral nervous systems, and the exact mechanism of decreased apnea/hypopnea index with cannabinoids is unknown. Here, we hypothesized that intranodose ganglion injections of a cannabinoid will attenuate 5-HT-induced reflex apnea and increase upper airway muscle tone. We show that dronabinol injected locally into the nodose ganglia suppresses 5-HT-induced reflex apnea, and increases phasic, but not tonic, activation of the genioglossus. These data support the view that dronabinol stabilizes respiratory pattern and augments upper airway muscles by acting at the nodose ganglia. These findings underscore a therapeutic potential of dronabinol for the treatment of obstructive sleep apnea.

Spontaneous sleep-like brain state alternations and breathing characteristics in urethane anesthetized mice

Brain state alternations resembling those of sleep spontaneously occur in rats under urethane anesthesia and they are closely linked with sleep-like respiratory changes. Although rats are a common model for both sleep and respiratory physiology, we sought to determine if similar brain state and respiratory changes occur in mice under urethane. We made local field potential recordings from the hippocampus and measured respiratory activity by means of EMG recordings in intercostal, genioglossus, and abdominal muscles. Similar to results in adult rats, urethane anesthetized mice displayed quasi-periodic spontaneous forebrain state alternations between deactivated patterns resembling slow wave sleep (SWS) and activated patterns resembling rapid eye movement (REM) sleep. These alternations were associated with an increase in breathing rate, respiratory variability, a depression of inspiratory related activity in genioglossus muscle and an increase in expiratory-related abdominal muscle activity when comparing deactivated (SWS-like) to activated (REM-like) states. These results demonstrate that urethane anesthesia consistently induces sleep-like brain state alternations and correlated changes in respiratory activity across different rodent species. They open up the powerful possibility of utilizing transgenic mouse technology for the advancement and translation of knowledge regarding sleep cycle alternations and their impact on respiration.

Respiratory modulation of lingual muscle activity across sleep-wake states in rats

In obstructive sleep apnea (OSA) patients, inspiratory activation (IA) of lingual muscles protects the upper airway from collapse. We aimed to determine when rats' lingual muscles exhibit IA. In 5 Sprague-Dawley and 3 Wistar rats, we monitored cortical EEG and lingual, diaphragmatic and nuchal electromyograms (EMGs), and identified segments of records when lingual EMG exhibited IA. Individual segments lasted 2.4-269 s (median: 14.5 s), most (89%) occurred during slow-wave sleep (SWS), and they collectively occupied 0.3-6.1% of the total recording time. IA usually started to increase with a delay after SWS onset and ended with an arousal, or declined prior to rapid eye movement sleep. IA of lingual EMG was not accompanied by increased diaphragmatic activity or respiratory rate changes, but occurred when cortical EEG power was particularly low in a low beta-1 frequency range (12.5-16.4 Hz). A deep SWS-related activation of upper airway muscles may be an endogenous phenomenon designed to protect the upper airway against collapse.

Circadian dependence of receptors that mediate wake-related excitatory drive to hypoglossal motoneurons

Serotonin (5-HT), norepinephrine and orexins (ORX) are the three best established mediators of wake-related activation of hypoglossal (XII) motoneurons that innervate the muscles of the tongue. Since the tongue's use is temporarily closely aligned with the rest-activity cycle, we tested whether expression of mRNA for relevant 5-HT, norepinephrine and ORX receptors varies in the XII nucleus with the rest-activity cycle. Adult rats (n=7-9/group) were decapitated at 8-9 am (near rest period onset) or at 6-7 pm (near active period onset). Tissue micropunches were extracted from medullary slices containing the XII motor and sensory external cuneate (ECN) nuclei. 5-HT2A, α1-adrenergic and ORX type 2 receptor mRNAs were quantified using RT-PCR. Only 5-HT2A receptor mRNA levels differed between the two time points and were higher at the active period onset; no differences were detected in the ECN. Consistent with the mRNA results, 5-HT2A protein levels were also higher in the XII nucleus at the active period onset than at rest onset. Thus, the endogenous serotonergic excitatory drive to XII motoneurons may be enhanced through circadian- or activity-dependent mechanisms that increase the availability of 5-HT2A receptors prior to the active period. Conversely, reduced levels of 5-HT2A receptors during the rest-sleep period may exacerbate the propensity for sleep-disordered breathing in subjects with anatomically compromised upper airway.

State-dependent modulation of breathing in urethane-anesthetized rats

Respiratory activity is most fragile during sleep, in particular during paradoxical [or rapid eye movement (REM)] sleep and sleep state transitions. Rats are commonly used to study respiratory neuromodulation, but rodent sleep is characterized by a highly fragmented sleep pattern, thus making it very challenging to examine different sleep states and potential pharmacological manipulations within them. Sleep-like brain-state alternations occur in rats under urethane anesthesia and may be an effective and efficient model for sleep itself. The present study assessed state-dependent changes in breathing and respiratory muscle modulation under urethane anesthesia to determine their similarity to those occurring during natural sleep. Rats were anesthetized with urethane and respiratory airflow, as well as electromyographic activity in respiratory muscles were recorded in combination with local field potentials in neocortex and hippocampus to determine how breathing pattern and muscle activity are modulated with brain state. Measurements were made in normoxic, hypoxic, and hypercapnic conditions. Results were compared with recordings made from rats during natural sleep. Brain-state alternations under urethane anesthesia were closely correlated with changes in breathing rate and variability and with modulation of respiratory muscle tone. These changes closely mimicked those observed in natural sleep. Of great interest was that, during both REM and REM-like states, genioglossus muscle activity was strongly depressed and abdominal muscle activity showed potent expiratory modulation. We demonstrate that, in urethane-anesthetized rats, respiratory airflow and muscle activity are closely correlated with brain-state transitions and parallel those shown in natural sleep, providing a useful model to systematically study sleep-related changes in respiratory control.

Reduced sleep and impaired sleep initiation in adult male rats exposed to alcohol during early postnatal period

Prenatal alcohol exposure (AE) is associated with cognitive and neurobehavioral abnormalities, such as increased motor activity and elevated anxiety, that may last a lifetime. Persistent sleep disruption may underlie these problems. Using a rat model, we investigated long-term alterations of sleep-wake behavior following AE during a critical early developmental period. Male rats received 2.6 g/kg of alcohol intragastrically twice daily on postnatal days (PD) 4-9, a developmental period equivalent to the third trimester of human pregnancy (AE group), or were sham-intubated (S group). On PD52-80, they were instrumented for tethered electroencephalogram and nuchal electromyogram recording and habituated to the recording procedures. Sleep-wake behavior was then recorded during one 24 h-long session. Wake, slow-wave sleep (SWS) and rapid eye movement sleep (REMS) were scored in 10 s epochs during 6h of the lights-on (rest) and 6h of the lights-off (active) periods. During the active period, REMS percentage was significantly lower (4.7 ± 0.9 (SE) vs. 8.2 ± 0.9; p < 0.02) and the percentage of SWS tended to be lower (p = 0.07) in AE than S rats (N = 6/group). During the rest period, sleep and wake amounts did not differ between the groups, but AE rats had longer latency to both SWS and REMS onset (p = 0.02 and 0.003, respectively). Our data demonstrate that, in a rat model of prenatal AE, impaired sleep-wake behavior persists into the adulthood. Disordered sleep may exacerbate cognitive and behavioral disorders seen in human victims of prenatal AE.

Ketamine activates breathing and abolishes the coupling between loss of consciousness and upper airway dilator muscle dysfunction

BACKGROUND

Procedural sedation is frequently performed in spontaneously breathing patients, but hypnotics and opioids decrease respiratory drive and place the upper airway at risk for collapse.

METHODS

In a randomized, controlled, cross-over, pharmaco-physiologic study in 12 rats, we conducted acute experiments to compare breathing and genioglossus electromyogram activity at equianesthetic concentrations of ketamine, a noncompetitive N-methyl-D-aspartate receptor antagonist that combines potent analgesic with hypnotic action effects, versus propofol. In 10 chronically instrumented rats resting in a plethysmograph, we measured these variables as well as electroencephalography during five conditions: quiet wakefulness, nonrapid-eye-movement sleep, rapid eye movement sleep, and low-dose (60 mg/kg intraperitoneally) and high-dose ketamine anesthesia (125 mg/kg intraperitoneally).

RESULTS

Ketamine anesthesia was associated with markedly increased genioglossus activity (1.5 to fivefold higher values of genioglossus electromyogram) compared with sleep- and propofol-induced unconsciousness. Plethysmography revealed a respiratory stimulating effect: higher values of flow rate, respiratory rate, and duty-cycle (effective inspiratory time, 1.5-to-2-fold higher values). During wakefulness and normal sleep, the δ (f = 6.51, P = 0.04) electroencephalogram power spectrum was an independent predictor of genioglossus activity, indicating an association between electroencephalographic determinants of consciousness and genioglossus activity. Following ketamine administration, electroencephalogram power spectrum and genioglossus electroencephalogram was dissociated (P = 0.9 for the relationship between δ/θ power spectrum and genioglossus electromyogram).

CONCLUSIONS

Ketamine is a respiratory stimulant that abolishes the coupling between loss-of-consciousness and upper airway dilator muscle dysfunction in a wide dose-range. Ketamine compared with propofol might help stabilize airway patency during sedation and anesthesia.

Respiratory related control of hypoglossal motoneurons--knowing what we do not know

Because tongue position and stiffness help insure that the pharyngeal airspace is sufficiently open during breathing, the respiration-related behavior of the tongue muscles has been studied in detail, particularly during the last two decades. Although eight different muscles act upon the mammal tongue, we know very little about the respiration-related control of the majority of these, and almost nothing about how they work together as a complex electro-mechanical system. Other significant gaps include how hypoglossal motoneuron axons find their appropriate muscle target during development, whether the biophysical properties of hypoglossal motoneurons driving different muscles are the same, and how afferent information from cardiorespiratory reflex systems is transmitted from major brainstem integrating centers to the hypoglossal motoneuron pool. This brief review outlines some of these issues, with the hope that this will spur research in the field, ultimately leading to an improved understanding of the respiration-related control of the mammalian tongue musculature.

Single motor unit recordings in human geniohyoid reveal minimal respiratory activity during quiet breathing

Maintenance of airway patency during breathing involves complex interactions between pharyngeal dilator muscles. The few previous studies of geniohyoid activity using multiunit electromyography (EMG) have suggested that geniohyoid shows predominantly inspiratory phasic activity. This study aimed to quantify geniohyoid respiration-related activity with single motor unit (SMU) EMG recordings. Six healthy subjects of normal body mass index were studied. Intramuscular EMG recordings of geniohyoid activity were made with a monopolar needle with subjects in supine and seated positions. The depth of the geniohyoid was identified by ultrasound, and the electrode position was confirmed with maneuvers to isolate activity in geniohyoid and genioglossus. Activity was recorded at 85 sites in the geniohyoid during quiet breathing (45 supine and 40 seated). When subjects were supine, 33 sites (73%) showed no activity during breathing and 10 (22%) showed tonic activity. In addition, one site showed a tonic SMU with increased expiratory discharge, and one site in another subject had one unit with expiratory phasic activity. When subjects were seated, 27 sites (68%) in the geniohyoid showed no activity, 12 sites (30%) showed tonic activity that was not respiration related, and one unit at one site showed phasic expiratory activity. The average peak discharge frequency of geniohyoid motor units was 16.2 ± 3.1 impulses/s during the “geniohyoid maneuver,” which was the first part of a swallow. In contrast to previous findings, the geniohyoid shows some tonic activity but minimal respiration-related activity in healthy subjects in quiet breathing. The geniohyoid has little active role in airway stability under these conditions.