Cyclic nucleotides modulate genioglossus and hypoglossal responses to excitatory inputs in rats

Pathophysiological mechanisms and therapeutic approaches in obstructive sleep apnea syndrome

Obstructive sleep apnea syndrome (OSAS) is a common breathing disorder in sleep in which the airways narrow or collapse during sleep, causing obstructive sleep apnea. The prevalence of OSAS continues to rise worldwide, particularly in middle-aged and elderly individuals. The mechanism of upper airway collapse is incompletely understood but is associated with several factors, including obesity, craniofacial changes, altered muscle function in the upper airway, pharyngeal neuropathy, and fluid shifts to the neck. The main characteristics of OSAS are recurrent pauses in respiration, which lead to intermittent hypoxia (IH) and hypercapnia, accompanied by blood oxygen desaturation and arousal during sleep, which sharply increases the risk of several diseases. This paper first briefly describes the epidemiology, incidence, and pathophysiological mechanisms of OSAS. Next, the alterations in relevant signaling pathways induced by IH are systematically reviewed and discussed. For example, IH can induce gut microbiota (GM) dysbiosis, impair the intestinal barrier, and alter intestinal metabolites. These mechanisms ultimately lead to secondary oxidative stress, systemic inflammation, and sympathetic activation. We then summarize the effects of IH on disease pathogenesis, including cardiocerebrovascular disorders, neurological disorders, metabolic diseases, cancer, reproductive disorders, and COVID-19. Finally, different therapeutic strategies for OSAS caused by different causes are proposed. Multidisciplinary approaches and shared decision-making are necessary for the successful treatment of OSAS in the future, but more randomized controlled trials are needed for further evaluation to define what treatments are best for specific OSAS patients.

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.

Protein kinase G-dependent mechanisms modulate hypoglossal motoneuronal excitability and long-term facilitation

Since protein kinase-dependent modulation of motoneuronal excitability contributes to adaptive changes in breathing, we hypothesized that cGMP-dependent pathways activating protein kinase G (PKG) modulate motoneuronal inspiratory drive currents and long-term plasticity. In a medullary slice preparation from neonatal rat (postnatal days 0-4) generating spontaneous respiratory-related rhythm, hypoglossal (XII) motoneuronal inspiratory drive currents and respiratory-related XII nerve activity were recorded. Focal application of a PKG activator, 8-bromoguanosine-3',5'-cyclomonophosphate (8-Br-cGMP), to voltage-clamped XII motoneurones decreased inspiratory drive currents. In the presence of tetrodotoxin (TTX), 8-Br-cGMP decreased the exogenous postsynaptic inward currents induced by focal application of AMPA. Intracellular dialysis of XII motoneurones with an inhibitory peptide to PKG (PKGI) increased endogenous inspiratory-drive currents and exogenous AMPA-induced currents. Application of 8-Br-cGMP with PKGI had no further effect on spontaneous or evoked currents, confirming that the observed effects were induced by PKG. However, PKG differentially increased longer-term plasticity. Three 3 min applications (separated by 5 min) of the α(1)-adrenergic agonist phenylephrine (PE) in combination with 8-Br-cGMP yielded greater in vitro long-term facilitation than PE alone. These data indicate the presence of a cGMP/PKG-dependent signalling pathway in XII motoneurones that modulates inspiratory drive currents and plasticity of XII motoneurones, possibly contributing to their adaptation during physiological challenges, such as sleep and exercise.

Protein kinase A activators produce a short-term, but not long-term, increase in respiratory-drive transmission at the hypoglossal motor nucleus in vivo

Synaptic plasticity is an intrinsic and conserved feature of neuronal activity that has been most extensively studied in the context of learning and memory in Aplysia and the mammalian hippocampus. However, the intracellular mechanisms underlying plasticity at motor nuclei, influencing motor behaviour, are less well studied. In vitro experiments in neonatal rodents indicate that protein kinase A (PKA) modulates respiratory-drive transmission at the hypoglossal motor nucleus (HMN), which innervates the genioglossus muscle of the tongue. We hypothesised that PKA activators at the HMN would increase genioglossus activity in vivo, whereas a PKA inhibitor would suppress activity indicative of constitutive PKA activation. Since PKA activators are importantly involved in models of long-term augmentation of neuronal activity following massed stimulation [16], we also hypothesised that application of PKA activators to the HMN would produce long-term facilitation of genioglossus activity. Experiments were performed in 25 isoflurane-anaesthetised, tracheotomised, spontaneously breathing adult rats. Microdialysis perfusion of 8-Br-cAMP (direct PKA activator) into the HMN increased genioglossus activity compared to baseline levels with artificial cerebrospinal fluid (P<0.001). Application of forskolin (indirect PKA activator) had a similar effect (P<0.002). Genioglossus activity progressively decreased back to baseline during a 90-min washout with artificial cerebrospinal fluid, demonstrating a lack of long-term facilitation of genioglossus activity. Similar to massed application of 8-Br-cAMP to the HMN, intermittent application produced a short-term (P<0.001), but not long-term, increase in genioglossus activity in vivo. Application of Rp-8-Cl-cAMPS (PKA inhibitor) did not decrease genioglossus activity, indicating a lack of constitutive PKA activation.

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

STUDY OBJECTIVES

Histamine neurons comprise a major component of the aminergic arousal system and significantly influence sleep-wake states, with antihistamines widely used as sedative hypnotics. Unlike the serotonergic and noradrenergic components of this arousal system, however, the role of histamine in the central control of respiratory motor activity has not been determined. The aims of this study were to characterize the effects of histamine receptor agonists and antagonists at the hypoglossal motor pool on genioglossus muscle activity across sleep and awake states, and also determine if histamine contributes an endogenous excitatory drive to modulate hypoglossal motor outflow to genioglossus muscle.

DESIGN, PARTICIPANTS, AND INTERVENTIONS

Thirty-three rats were implanted with electroencephalogram and neck electrodes to record sleep-wake states, and genioglossus and diaphragm electrodes for respiratory muscle recordings. Microdialysis probes were inserted into the hypoglossal motor nucleus.

MEASUREMENTS AND RESULTS

Histamine at the hypoglossal motor nucleus significantly increased tonic genioglossus muscle activity in wakefulness, non-REM sleep and REM sleep. The activating effects of histamine on genioglossus muscle activity also occurred with a histamine type-1 (H1) but not H2 receptor agonist. However, H1 receptor antagonism at the hypoglossal motor nucleus did not decrease genioglossus muscle activity in wakefulness or sleep.

CONCLUSIONS

The results suggest that histamine at the hypoglossal motor pool increases genioglossus muscle activity in freely behaving rats in wakefulness, non-REM, and REM sleep via an H1 receptor mechanism.

Glycinergic and GABA(A)-mediated inhibition of somatic motoneurons does not mediate rapid eye movement sleep motor atonia

A hallmark of rapid eye movement (REM) sleep is a potent suppression of postural muscle tone. Motor control in REM sleep is unique because it is characterized by flurries of intermittent muscle twitches that punctuate muscle atonia. Because somatic motoneurons are bombarded by strychnine-sensitive IPSPs during REM sleep, it is assumed that glycinergic inhibition underlies REM atonia. However, it has never been determined whether glycinergic inhibition of motoneurons is indeed responsible for triggering the loss of postural muscle tone during REM sleep. Therefore, we used reverse microdialysis, electrophysiology, and pharmacological and histological methods to determine whether glycinergic and/or GABA(A)-mediated neurotransmission at the trigeminal motor pool mediates masseter muscle atonia during REM sleep in rats. By antagonizing glycine and GABA(A) receptors on trigeminal motoneurons, we unmasked a tonic glycinergic/GABAergic drive at the trigeminal motor pool during waking and non-rapid eye movement (NREM) sleep. Blockade of this drive potently increased masseter muscle tone during both waking and NREM sleep. This glycinergic/GABAergic drive was immediately switched-off and converted into a phasic glycinergic drive during REM sleep. Blockade of this phasic drive potently provoked muscle twitch activity in REM sleep; however, it did not prevent or reverse REM atonia. Muscle atonia in REM even persisted when glycine and GABA(A) receptors were simultaneously antagonized and trigeminal motoneurons were directly activated by glutamatergic excitation, indicating that a powerful, yet unidentified, inhibitory mechanism overrides motoneuron excitation during REM sleep. Our data refute the prevailing hypothesis that REM atonia is caused by glycinergic inhibition. The inhibitory mechanism mediating REM atonia therefore requires reevaluation.

Control of genioglossus muscle by sleep state-dependent neuromodulators

Pharyngeal muscle tone decreases in sleep and this predisposes some individuals to obstructive sleep apnea. This review summarizes the control of the genioglossus muscle by sleep-state dependent neuromodulators at the hypoglossal motor nucleus, the source of motor output to the genioglossus muscle of the tongue. Knowledge of such mechanisms is relevant to identifying and developing new strategies to augment pharyngeal muscle activity in sleep, potentially as treatments for obstructive sleep apnea.

Ca++ influx is essential for the hypotensive response to arginine vasopressin-induced neuron activation of the area postrema in the rat

We attempted to relate the signal pathway to the hypotension induced by arginine vasopressin (AVP) injection into the area postrema (AP) in urethane-anesthetized and ventilated rats with vagotomy. A femoral artery and vein were catheterized to measure the blood pressure (BP) and administer drugs, respectively. The rat was placed on a stereotaxic apparatus to expose the calamus sriptorius (CS) by craniostomy and maintained at normocapnia in hyperoxia. In protocol 1, hypotension evoked by AVP (3.0 x 10(-5) IU) microinjected into the AP 0.2 mm rostral to the CS of the midline was abolished by V(1A) antagonist, U73122 (phospholipase C blocker), and BAPTA-AM (Ca(++) chelator), suggesting that an increasing intracellular Ca(++) is essential for AVP-induced hypotension. In protocol 2, AVP-induced hypotension was abolished by EGTA (extracellular Ca(++) chelator) and Ca(++) blockers such as nifedipine, nimodipine (L-types), and omega-conotoxin MVIIC (P/Q-type), but not by omega-conotoxin GVIA (N-type). In protocol 3, AVP-induced hypotension was blocked by calphostin C (protein kinase C inhibitor) and mimicked by an increase in intracellular K(+) ions that was reversed by EGTA. Vehicle injections produced no changes in BP. In protocol 4, glutamate-induced hypotension was reversed by BAPTA-AM but not by EGTA or V(1A) antagonist. Our data suggest that AVP-induced hypotension depends on Ca(++) influx through a signal pathway from phospholipase C to protein kinase C which inactivates K(+) channels that may depolarize AP neurons to activate L- and P/Q-type Ca(++) channels. This may provide new insights into establishing a relationship between the signal pathway and physiological functions.

Respiratory motor activity: influence of neuromodulators and implications for sleep disordered breathing

Sleep, especially rapid-eye-movement sleep, causes fundamental modifications of respiratory muscle activity and control mechanisms, modifications that can predispose individuals to sleep-related breathing disorders. One of the most common of these disorders is obstructive sleep apnea (OSA) that affects approximately 4% of adults. OSA is caused by repeated episodes of pharyngeal airway obstruction that can occur hundreds of times per night, leading to recurrent asphyxia, arousals from sleep, daytime sleepiness, and adverse cardiovascular and cerebrovascular consequences. OSA is caused by the effects of sleep on pharyngeal muscle tone in individuals with already narrow upper airways. Moreover, since OSA occurs only in sleep, this disorder by definition is a state-dependent process ultimately caused by the influence of sleep neural mechanisms on the activity of pharyngeal motoneurons. This review synthesizes recent findings relating to control of pharyngeal muscle activity across sleep-wake states, with special emphasis on the influence of neuromodulators acting at the hypoglossal motor nucleus that inervates the genioglossus muscle of the tongue. The results of such basic physiological studies may be relevant to identifying and developing new pharmacological strategies to augment pharyngeal muscle activity in sleep, especially rapid-eye-movement sleep, as potential treatments for OSA.

Episodic stimulation of alpha1-adrenoreceptors induces protein kinase C-dependent persistent changes in motoneuronal excitability

In vitro long-term facilitation (ivLTF) is a novel form of activity-independent postsynaptic enhancement of AMPA receptor function in hypoglossal (XII) motoneurons that can be induced by intermittent activation of 5-HT2 receptors. In vivo respiratory long-term facilitation (LTF) is characterized by a persistent 5-HT2 receptor-dependent increase in respiratory motor output or ventilation after episodic exposures to hypoxia in adult rats. Here, we demonstrate that ivLTF can also be induced by episodic but not continuous stimulation of alpha1-adrenergic receptors that requires protein kinase C (PKC), but not PKA (protein kinase A), activation. Additionally, we show that in vivo respiratory LTF is also alpha1-adrenergic receptor dependent. We suggest that, in vivo, concurrent episodic activation of 5-HT2 and alpha1-adrenergic receptors is necessary to produce long-lasting changes in the excitability of respiratory motoneurons, possibly involving PKC activation via the G alpha(q)-PLC (phospholipase C) signaling pathway common to both receptor subtypes. Such plasticity of XII motor output may increase upper airway muscle (innervated by XII nerve) tone and improve the likelihood that airway patency will be maintained. Elucidating the mechanism underlying LTF can be of clinical importance to the patients suffering from sleep-disordered breathing.

Genioglossus muscle activity and serotonergic modulation of hypoglossal motor output in obese Zucker rats

Obese Zucker rats have a narrower and more collapsible upper airway compared with lean controls, similar to obstructive sleep apnea (OSA) patients. Genioglossus (GG) muscle activity is augmented in awake OSA patients to compensate for airway narrowing, but the neural control of GG activity in obese Zucker rats has not been investigated to determine whether such neuromuscular compensation also occurs. This study tests the hypotheses that GG activity is augmented in obese Zucker rats compared with lean controls and that endogenous 5-hydroxytryptamine (5-HT) contributes to GG activation. Seven obese and seven lean Zucker rats were implanted with electroencephalogram and neck muscle electrodes to record sleep-wake states, and they were implanted with GG and diaphragm wires for respiratory muscle recordings. Microdialysis probes were implanted into the hypoglossal motor nucleus for perfusion of artificial cerebrospinal fluid and the 5-HT receptor antagonist mianserin (100 microM). Compared with lean controls, respiratory rates were increased in obese rats across sleep-wake states (P=0.048) because of reduced expiratory durations (P=0.007); diaphragm activation was similar between lean and obese animals (P=0.632). Respiratory-related, tonic, and peak GG activities were also similar between obese and lean rats (P>0.139). There was no reduction in GG activity with mianserin at the hypoglossal motor nucleus, consistent with recent observations of a minimal contribution of endogenous 5-HT to GG activity. These results suggest that despite the upper airway narrowing in obese Zucker rats, these animals have a sufficiently stable airway such that pharyngeal muscle activity is normal across sleep-wake states.

Endogenous Excitatory Drive Modulating Respiratory Muscle Activity across Sleep–Wake States

RATIONALE

The concept of a tonic drive activating respiratory muscle in wakefulness but not sleep has been an important and enduring notion in respiratory medicine, not least because it is useful in modeling sleep effects on breathing and understanding the pathogenesis of sleep-related breathing disorders such as obstructive sleep apnea. However, a neurotransmitter substrate mediating respiratory muscle activation across sleep-wake states has not been identified.

OBJECTIVES

We determined if alpha1 receptor antagonism at the hypoglossal motor nucleus (HMN) decreases genioglossus (GG) activity consistent with a role for an endogenous noradrenergic drive contributing to GG activation across sleep-wake states. We also determined if alpha1 receptor stimulation could counteract reduced endogenous noradrenergic drive and increase sleeping GG activity.

METHODS

Thirty-five rats were implanted with electroencephalogram and neck electrodes to record sleep-wake states and GG and diaphragm electrodes for respiratory muscle recordings. Microdialysis probes were inserted into the HMN.

MEASUREMENTS AND MAIN RESULTS

Microdialysis perfusion of the alpha1 receptor antagonist terazosin into the HMN significantly decreased GG activity in wakefulness and nonrapid eye movement (non-REM) sleep but not REM sleep. The alpha1 receptor agonist phenylephrine increased GG activity in wakefulness and sleep, but periods of motor inactivity persisted in REM sleep; there was no potentiating effect of combined alpha1 and 5-HT2 receptor stimulation.

CONCLUSIONS

Identification of an endogenous noradrenergic drive contributing to GG activation in wakefulness and non-REM sleep, but not REM sleep, is important given the prevalence and clinical significance of sleep-induced hypoventilation and obstructive sleep apnea in humans and the potential for pharmacologic treatment.