5-HT at hypoglossal motor nucleus and respiratory control of genioglossus muscle in anesthetized rats

Optical and pharmacological manipulation of hypoglossal motor nucleus identifies differential effects of taltirelin on sleeping tonic motor activity and responsiveness

Pharyngeal muscle activity and responsiveness are key pathophysiological traits in human obstructive sleep apnea (OSA) and strong contributors to improvements with pharmacotherapy. The thyrotropin-releasing hormone (TRH) analog taltirelin is of high pre-clinical interest given its neuronal-stimulant properties, minimal endocrine activity, tongue muscle activation following microperfusion into the hypoglossal motor nucleus (HMN) or systemic delivery, and high TRH receptor expression at the HMN compared to rest of the brain. Here we test the hypothesis that taltirelin increases HMN activity and/or responsivity to excitatory stimuli applied across sleep-wake states in-vivo. To target hypoglossal motoneurons with simultaneous pharmacological and optical stimuli we used customized "opto-dialysis" probes and chronically implanted them in mice expressing a light sensitive cation channel exclusively on cholinergic neurons (ChAT-ChR2, n = 12) and wild-type mice lacking the opsin (n = 10). Both optical stimuli applied across a range of powers (P < 0.001) and microperfusion of taltirelin into the HMN (P < 0.020) increased tongue motor activity in sleeping ChAT-ChR2 mice. Notably, taltirelin increased tonic background tongue motor activity (P < 0.001) but not responsivity to excitatory optical stimuli across sleep-wake states (P > 0.098). This differential effect on tonic motor activity versus responsivity informs human studies of the potential beneficial effects of taltirelin on pharyngeal motor control and OSA pharmacotherapy.

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

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

Obstructive sleep apnea

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

Modulation of TASK-1/3 channels at the hypoglossal motoneuron pool and effects on tongue motor output and responses to excitatory inputs in vivo: implications for strategies for obstructive sleep apnea pharmacotherapy

Obstructive sleep apnea (OSA) occurs exclusively during sleep due to reduced tongue motor activity. Withdrawal of excitatory inputs to the hypoglossal motor nucleus (HMN) from wake to sleep contributes to this reduced activity. Several awake-active neurotransmitters with inputs to the HMN (e.g. serotonin [5-HT]) inhibit K+ leak mediated by TASK-1/3 channels on hypoglossal motoneurons, leading to increased neuronal activity in vitro. We hypothesize that TASK channel inhibition at the HMN will increase tongue muscle activity in vivo and modulate responses to 5-HT. We first microperfused the HMN of anesthetized rats with TASK channel inhibitors: doxapram (75 μM, n = 9), A1899 (25 μM, n = 9), ML365 (25 μM, n = 9), acidified artificial cerebrospinal fluid (ACSF, pH = 6.25, n = 9); and a TASK channel activator terbinafine (50 μM, n = 9); all with and without co-applied 5-HT (10 mM). 5-HT alone at the HMN increased tongue motor activity (202.8% ± 45.9%, p < 0.001). However, neither the TASK channel inhibitors, nor activator, at the HMN changed baseline tongue activity (p > 0.716) or responses to 5-HT (p > 0.127). Tonic tongue motor responses to 5-HT at the HMN were also not different (p > 0.05) between ChAT-Cre:TASKf/f mice (n = 8) lacking TASK-1/3 channels on cholinergic neurons versus controls (n = 10). In freely behaving rats (n = 9), microperfusion of A1899 into the HMN increased within-breath phasic tongue motor activity in wakefulness only (p = 0.005) but not sleep, with no effects on tonic activity across all sleep-wake states. Together, the findings suggest robust maintenance of tongue motor activity despite various strategies for TASK channel manipulation targeting the HMN in vivo, and thus currently do not support this target and direction for potential OSA pharmacotherapy.

Differential activating effects of thyrotropin-releasing hormone and its analog taltirelin on motor output to the tongue musculature in vivo

Thyrotropin-releasing hormone (TRH) is produced by the hypothalamus but most brain TRH is located elsewhere where it acts as a neuromodulator. TRH-positive neurons project to the hypoglossal motoneuron pool where TRH receptor RNA shows a high degree of differential expression compared with the rest of the brain. Strategies to modulate hypoglossal motor activity are of physiological and clinical interest given the potential for pharmacotherapy for obstructive sleep apnea (OSA), a common and serious respiratory disorder. Here, we identified the effects on tongue motor activity of TRH and a specific analog (taltirelin) applied locally to the hypoglossal motoneuron pool and systemically in vivo. Studies were performed under isoflurane anesthesia and across sleep–wake states in rats. In anesthetized rats, microperfusion of TRH (n = 8) or taltirelin (n = 9) into the hypoglossal motoneuron pool caused dose-dependent increases in tonic and phasic tongue motor activity (both p < 0.001). However, the motor responses to TRH were biphasic, being significantly larger “early” in the response versus at the end of the intervention (p ≤ 0.022). In contrast, responses to taltirelin were similar “early” versus “late” (p ≥ 0.107); i.e. once elicited, the motor responses to taltirelin were sustained and maintained. In freely behaving conscious rats (n = 10), microperfusion of 10 μM taltirelin into the hypoglossal motoneuron pool increased tonic and phasic tongue motor activity in non-rapid-eye-movement (REM) sleep (p ≤ 0.038). Intraperitoneal injection of taltirelin (1 mg/kg, n = 16 rats) also increased tonic tongue motor activity across sleep–wake states (p = 0.010). These findings inform the studies in humans to identify the potential beneficial effects of taltirelin for breathing during sleep and OSA.

Effect of Venlafaxine on Apnea-Hypopnea Index in Patients With Sleep Apnea: A Randomized, Double-Blind Crossover Study

BACKGROUND

One of the key mechanisms underlying OSA is reduced pharyngeal muscle tone during sleep. Data suggest that pharmacologic augmentation of central serotonergic/adrenergic tone increases pharyngeal muscle tone.

RESEARCH QUESTION

We hypothesized that venlafaxine, a serotonin-norepinephrine reuptake inhibitor, would improve OSA severity.

STUDY DESIGN AND METHODS

In this mechanistic, randomized, double-blind, placebo-controlled crossover trial, 20 patients with OSA underwent two overnight polysomnograms ≥ 4 days apart, receiving either 50 mg of immediate-release venlafaxine or placebo before bedtime. Primary outcomes were the apnea-hypopnea index (AHI) and peripheral oxygen saturation (Spo₂) nadir, and secondary outcomes included sleep parameters and pathophysiologic traits with a view toward understanding the impact of venlafaxine on mechanisms underlying OSA.

RESULTS

Overall, there was no significant difference between venlafaxine and placebo regarding AHI (mean reduction, -5.6 events/h [95% CI, -12.0 to 0.9]; P = .09) or Spo₂ nadir (median increase, +1.0% [-0.5 to 5]; P = .11). Venlafaxine reduced total sleep time, sleep efficiency, and rapid eye movement (REM) sleep, while increasing non-REM stage 1 sleep (P_all < .05). On the basis of exploratory post hoc analyses venlafaxine decreased ("improved") the ventilatory response to arousal (-30%; P = .049) and lowered ("worsened") the predicted arousal threshold (-13%; [P = .02]; ie, more arousable), with no effects on other pathophysiologic traits (P_all ≥ .3). Post hoc analyses further suggested effect modification by arousal threshold (P = .002): AHI improved by 19% in patients with a high arousal threshold (-10.9 events/h [-3.9 to -17.9]) but tended to increase in patients with a low arousal threshold (+7 events/h [-2.0 to 16]). Other predictors of response were elevated AHI and less collapsible upper airway anatomy at baseline (|r| > 0.5, P ≤ .02).

INTERPRETATION

In unselected patients, venlafaxine simultaneously worsened and improved various pathophysiologic traits, resulting in a zero net effect. Careful patient selection based on pathophysiologic traits, or combination therapy with drugs countering its alerting effects, may produce a more robust response.

TRIAL REGISTRY

ClinicalTrials.gov; No.: NCT02714400; URL: www.clinicaltrials.gov.

The Combination of Atomoxetine and Oxybutynin Greatly Reduces Obstructive Sleep Apnea Severity. A Randomized, Placebo-controlled, Double-Blind Crossover Trial

Rationale: There is currently no effective pharmacological treatment for obstructive sleep apnea (OSA). Recent investigations indicate that drugs with noradrenergic and antimuscarinic effects improve genioglossus muscle activity and upper airway patency during sleep. Objectives: We aimed to determine the effects of the combination of a norepinephrine reuptake inhibitor (atomoxetine) and an antimuscarinic (oxybutynin) on OSA severity (apnea-hypopnea index [AHI]; primary outcome) and genioglossus responsiveness (secondary outcome) in people with OSA. Methods: A total of 20 people completed a randomized, placebo-controlled, double-blind, crossover trial comparing 1 night of 80 mg atomoxetine plus 5 mg oxybutynin (ato-oxy) to placebo administered before sleep. The AHI and genioglossus muscle responsiveness to negative esophageal pressure swings were measured via in-laboratory polysomnography. In a subgroup of nine patients, the AHI was also measured when the drugs were administered separately. Measurements and Main Results: The participants' median (interquartile range) age was 53 (46-58) years and body mass index was 34.8 (30.0-40.2) kg/m2. ato-oxy lowered AHI by 63% (34-86%), from 28.5 (10.9-51.6) events/h to 7.5 (2.4-18.6) events/h (P < 0.001). Of the 15/20 patients with OSA on placebo (AHI > 10 events/hr), AHI was lowered by 74% (62-88%) (P < 0.001) and all 15 patients exhibited a ≥50% reduction. Genioglossus responsiveness increased approximately threefold, from 2.2 (1.1-4.7)%/cm H2O on placebo to 6.3 (3.0 to 18.3)%/cm H2O on ato-oxy (P < 0.001). Neither atomoxetine nor oxybutynin reduced the AHI when administered separately. Conclusions: A combination of noradrenergic and antimuscarinic agents administered orally before bedtime on 1 night greatly reduced OSA severity. These findings open new possibilities for the pharmacologic treatment of OSA. Clinical trial registered with www.clinicaltrials.gov (NCT02908529).

Mice Lacking Brain-Derived Serotonin Have Altered Swallowing Function

The intricate sensorimotor neural circuits that control swallowing are heavily reliant on serotonin (5-hydroxytryptamine [5-HT]); however, the impact of 5-HT deficiency on swallow function remains largely unexplored. We investigated this using mice deficient in tryptophan-hydroxylase-2 (TPH2), the enzyme catalyzing the rate-limiting step in 5-HT synthesis. Videofluoroscopy was utilized to characterize the swallowing function of TPH2 knockout (TPH2^-/-) mice as compared with littermate controls (TPH2^+/+). Results showed that 5-HT deficiency altered all 3 stages of swallowing. As compared with controls, TPH2^-/- mice had significantly slower lick and swallow rates and faster esophageal transit times. Future studies with this model are necessary to determine if 5-HT replacement may rescue abnormal swallowing function. If so, supplemental 5-HT therapy may have vast applications for a large population of patients with a variety of neurologic disorders resulting in life-diminishing dysphagia, particularly amyotrophic lateral sclerosis and Parkinson's disease, for which 5-HT deficiency is implicated in the disease pathogenesis.

Effect of Gender on Chronic Intermittent Hypoxic Fosb Expression in Cardiorespiratory-Related Brain Structures in Mice

We aimed to delineate sex-based differences in neuroplasticity that may be associated with previously reported sex-based differences in physiological alterations caused by repetitive succession of hypoxemia-reoxygenation encountered during obstructive sleep apnea (OSA). We examined long-term changes in the activity of brainstem and diencephalic cardiorespiratory neuronal populations induced by chronic intermittent hypoxia (CIH) in male and female mice by analyzing Fosb expression. Whereas the overall baseline and CIH-induced Fosb expression in females was higher than in males, possibly reflecting different neuroplastic dynamics, in contrast, structures responded to CIH by Fosb upregulation in males only. There was a sex-based difference at the level of the rostral ventrolateral reticular nucleus of the medulla, with an increase in the number of FOSB/ΔFOSB-positive cells induced by CIH in males but not females. This structure contains neurons that generate the sympathetic tone and which are involved in CIH-induced sustained hypertension during waking hours. We suggest that the sex-based difference in neuroplasticity of this structure contributes to the reported sex-based difference in CIH-induced hypertension. Moreover, we highlighted a sex-based dimorphic phenomenon in serotoninergic systems induced by CIH, with increased serotoninergic immunoreactivity in the hypoglossal nucleus and a decreased number of serotoninergic cells in the dorsal raphe nucleus in male but not female mice. We suggest that this dimorphism in the neuroplasticity of serotoninergic systems predisposes males to a greater alteration of neuronal control of the upper respiratory tract associated with the greater collapsibility of upper airways described in male OSA subjects.

Desipramine Increases Genioglossus Activity and Reduces Upper Airway Collapsibility during Non-REM Sleep in Healthy Subjects

RATIONALE

Obstructive sleep apnea is a state-dependent disease. One of the key factors that triggers upper airway collapse is decreased pharyngeal dilator muscle activity during sleep. To date, there have not been effective methods to reverse pharyngeal hypotonia pharmacologically in sleeping humans.

OBJECTIVES

We tested the hypothesis that administration of desipramine 200 mg prevents the state-related reduction in genioglossus activity that occurs during sleep and thereby decreases pharyngeal collapsibility.

METHODS

We conducted a placebo-controlled, double-blind, crossover trial with 10 healthy participants. Participants received active treatment or placebo in randomized order 2 hours before sleep in the physiology laboratory.

MEASUREMENTS AND MAIN RESULTS

Genioglossus activity during wakefulness and sleep, genioglossus muscle responsiveness to negative epiglottic pressure, and upper airway collapsibility during passive and active conditions were compared between on- and off-drug states. Desipramine abolished the normal reduction of genioglossus activity from wakefulness to non-REM sleep that occurred on the placebo night. Specifically, tonic (median, 96% [86-120] vs. 75% [50-92] wakefulness; P = 0.01) but not phasic genioglossus activity was higher with desipramine compared with placebo. Upper airway collapsibility was also reduced with desipramine compared with placebo (-10.0 cm H₂O [-15.2 to -5.8] vs. -8.1 cm H₂O [-10.4 to -6.3]; P = 0.037).

CONCLUSIONS

Desipramine reduces the state-related drop in tonic genioglossus muscle activity that occurs from wakefulness to non-REM sleep and reduces airway collapsibility. These data provide a rationale for a new pharmacologic therapy for obstructive sleep apnea. Clinical trial registered with www.clinicaltrials.gov (NCT02428478).

The influence of 5-HT1A receptors in the dorsal raphé nucleus on genioglossus activity

Genioglossus activity maintains the patency of the upper airway. 5-HT neurons in the raphe nucleus regulate genioglossus activity. In order to study the influence of 5-HT_1A receptors in dorsal raphé nucleus (DRN) on genioglossus EMG during normoxia, adult male Wistar rats were randomly divided into four groups: the artificial cerebrospinal fluid group (ACSF group), the low-concentration of 5-HT_1A receptors agonist 8-OH-DPAT group (0.1 mM group), the mid-concentration 8-OH-DPAT group (0.4 mM group) and the high-concentration 8-OH-DPAT group (1.0 mM group). Rats received 0.1 μl ACSF/8-OH-DPAT microinjections into the DRN. EMG activity of the genioglossus was recorded at 5 min, 15 min and 30 min after microinjection. In three 8-OH-DPAT groups, genioglossus EMG activity significantly decreased at 5 min after microinjection and persisted for 30 min. The significantly decreased EMG activity was more pronounced in the mid- and high-concentration groups than in the low-concentration group, indicating that 5-HT_1A receptors in the DRN could rapidly and continuously inhibit genioglossus EMG activity during normoxia.

Ventilation and neurochemical changes during µ-opioid receptor activation or blockade of excitatory receptors in the hypoglossal motor nucleus of goats

Neuromodulator interdependence posits that changes in one or more neuromodulators are compensated by changes in other modulators to maintain stability in the respiratory control network. Herein, we studied compensatory neuromodulation in the hypoglossal motor nucleus (HMN) after chronic implantation of microtubules unilaterally ( n = 5) or bilaterally ( n = 5) into the HMN. After recovery, receptor agonists or antagonists in mock cerebrospinal fluid (mCSF) were dialyzed during the awake and non-rapid eye movement (NREM) sleep states. During day studies, dialysis of the µ-opioid inhibitory receptor agonist [d-Ala², N-MePhe⁴, Gly-ol]enkephalin (DAMGO; 100 µM) decreased pulmonary ventilation (V̇i), breathing frequency ( f), and genioglossus (GG) muscle activity but did not alter neuromodulators measured in the effluent mCSF. However, neither unilateral dialysis of a broad spectrum muscarinic receptor antagonist (atropine; 50 mM) nor unilateral or bilateral dialysis of a mixture of excitatory receptor antagonists altered V̇i or GG activity, but all of these did increase HMN serotonin (5-HT) levels. Finally, during night studies, DAMGO and excitatory receptor antagonist decreased ventilatory variables during NREM sleep but not during wakefulness. These findings contrast with previous dialysis studies in the ventral respiratory column (VRC) where unilateral DAMGO or atropine dialysis had no effects on breathing and bilateral DAMGO or unilateral atropine increased V̇i and f and decreased GABA or increased 5-HT, respectively. Thus we conclude that the mechanisms of compensatory neuromodulation are less robust in the HMN than in the VRC under physiological conditions in adult goats, possibly because of site differences in the underlying mechanisms governing neuromodulator release and consequently neuronal activity, and/or responsiveness of receptors to compensatory neuromodulators. NEW & NOTEWORTHY Activation of inhibitory µ-opioid receptors in the hypoglossal motor nucleus decreased ventilation under physiological conditions and did not affect neurochemicals in effluent dialyzed mock cerebral spinal fluid. These findings contrast with studies in the ventral respiratory column where unilateral [d-Ala², N-MePhe⁴, Gly-ol]enkephalin (DAMGO) had no effects on ventilation and bilateral DAMGO or unilateral atropine increased ventilation and decreased GABA or increased serotonin, respectively. Our data support the hypothesis that mechanisms that govern local compensatory neuromodulation within the brain stem are site specific under physiological conditions.

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.

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.

TASK Channels on Basal Forebrain Cholinergic Neurons Modulate Electrocortical Signatures of Arousal by Histamine

Basal forebrain cholinergic neurons are the main source of cortical acetylcholine, and their activation by histamine elicits cortical arousal. TWIK-like acid-sensitive K(+) (TASK) channels modulate neuronal excitability and are expressed on basal forebrain cholinergic neurons, but the role of TASK channels in the histamine-basal forebrain cholinergic arousal circuit is unknown. We first expressed TASK channel subunits and histamine Type 1 receptors in HEK cells. Application of histamine in vitro inhibited the acid-sensitive K(+) current, indicating a functionally coupled signaling mechanism. We then studied the role of TASK channels in modulating electrocortical activity in vivo using freely behaving wild-type (n = 12) and ChAT-Cre:TASK(f/f) mice (n = 12), the latter lacking TASK-1/3 channels on cholinergic neurons. TASK channel deletion on cholinergic neurons significantly altered endogenous electroencephalogram oscillations in multiple frequency bands. We then identified the effect of TASK channel deletion during microperfusion of histamine into the basal forebrain. In non-rapid eye movement sleep, TASK channel deletion on cholinergic neurons significantly attenuated the histamine-induced increase in 30-50 Hz activity, consistent with TASK channels contributing to histamine action on basal forebrain cholinergic neurons. In contrast, during active wakefulness, histamine significantly increased 30-50 Hz activity in ChAT-Cre:TASK(f/f) mice but not wild-type mice, showing that the histamine response depended upon the prevailing cortical arousal state. In summary, we identify TASK channel modulation in response to histamine receptor activation in vitro, as well as a role of TASK channels on cholinergic neurons in modulating endogenous oscillations in the electroencephalogram and the electrocortical response to histamine at the basal forebrain in vivo.

SIGNIFICANCE STATEMENT

Attentive states and cognitive function are associated with the generation of γ EEG activity. Basal forebrain cholinergic neurons are important modulators of cortical arousal and γ activity, and in this study we investigated the mechanism by which these neurons are activated by the wake-active neurotransmitter histamine. We found that histamine inhibited a class of K(+) leak channels called TASK channels and that deletion of TASK channels selectively on cholinergic neurons modulated baseline EEG activity as well as histamine-induced changes in γ activity. By identifying a discrete brain circuit where TASK channels can influence γ activity, these results represent new knowledge that enhances our understanding of how subcortical arousal systems may contribute to the generation of attentive states.

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.

Physiology in medicine: obstructive sleep apnea pathogenesis and treatment--considerations beyond airway anatomy

We review evidence in support of significant contributions to the pathogenesis of obstructive sleep apnea (OSA) from pathophysiological factors beyond the well-accepted importance of airway anatomy. Emphasis is placed on contributions from neurochemical control of central respiratory motor output through its effects on output stability, upper airway dilator muscle activation, and arousability. In turn, we consider the evidence demonstrating effective treatment of OSA via approaches that address each of these pathophysiologic risk factors. Finally, a case is made for combining treatments aimed at both anatomical and ventilatory control system deficiencies and for individualizing treatment to address a patient's own specific risk factors.

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.

Recruitment and rate-coding strategies of the human genioglossus muscle

Single motor unit (SMU) analysis provides a means to examine the motor control of a muscle. SMUs in the genioglossus show considerable complexity, with several different firing patterns. Two of the primary stimuli that contribute to genioglossal activation are carbon dioxide (CO(2)) and negative pressure, which act through chemoreceptor and mechanoreceptor activation, respectively. We sought to determine how these stimuli affect the behavior of genioglossus SMUs. We quantified genioglossus SMU discharge activity during periods of quiet breathing, elevated CO(2) (facilitation), and continuous positive airway pressure (CPAP) administration (inhibition). CPAP was applied in 2-cmH(2)O increments until 10 cmH(2)O during hypercapnia. Five hundred ninety-one periods (each ∼ 3 breaths) of genioglossus SMU data were recorded using wire electrodes(n = 96 units) from 15 awake, supine subjects. Overall hypercapnic stimulation increased the discharge rate of genioglossus units (20.9 ± 1.0 vs. 22.7 ± 0.9 Hz). Inspiratory units were activated ∼ 13% earlier in the inspiratory cycle, and the units fired for a longer duration (80.6 ± 5.1 vs. 105.3 ± 4.2% inspiratory time; P < 0.05). Compared with baseline, an additional 32% of distinguishable SMUs within the selective electrode recording area were recruited with hypercapnia. CPAP led to progressive SMU inhibition; at ∼ 6 cmH(2)O, there were similar numbers of SMUs active compared with baseline, with peak frequencies of inspiratory units close to baseline, despite elevated CO(2) levels. At 10 cmH(2)O, the number of units was 36% less than baseline. Genioglossus inspiratory phasic SMUs respond to hypercapnic stimulation with changes in recruitment and rate coding. The SMUs respond to CPAP with derecruitment as a homogeneous population, and inspiratory phasic units show slower discharge rates. Understanding upper airway muscle recruitment/derecruitment may yield therapeutic targets for maintenance of pharyngeal patency.

Pathophysiology of sleep apnea

Sleep-induced apnea and disordered breathing refers to intermittent, cyclical cessations or reductions of airflow, with or without obstructions of the upper airway (OSA). In the presence of an anatomically compromised, collapsible airway, the sleep-induced loss of compensatory tonic input to the upper airway dilator muscle motor neurons leads to collapse of the pharyngeal airway. In turn, the ability of the sleeping subject to compensate for this airway obstruction will determine the degree of cycling of these events. Several of the classic neurotransmitters and a growing list of neuromodulators have now been identified that contribute to neurochemical regulation of pharyngeal motor neuron activity and airway patency. Limited progress has been made in developing pharmacotherapies with acceptable specificity for the treatment of sleep-induced airway obstruction. We review three types of major long-term sequelae to severe OSA that have been assessed in humans through use of continuous positive airway pressure (CPAP) treatment and in animal models via long-term intermittent hypoxemia (IH): 1) cardiovascular. The evidence is strongest to support daytime systemic hypertension as a consequence of severe OSA, with less conclusive effects on pulmonary hypertension, stroke, coronary artery disease, and cardiac arrhythmias. The underlying mechanisms mediating hypertension include enhanced chemoreceptor sensitivity causing excessive daytime sympathetic vasoconstrictor activity, combined with overproduction of superoxide ion and inflammatory effects on resistance vessels. 2) Insulin sensitivity and homeostasis of glucose regulation are negatively impacted by both intermittent hypoxemia and sleep disruption, but whether these influences of OSA are sufficient, independent of obesity, to contribute significantly to the "metabolic syndrome" remains unsettled. 3) Neurocognitive effects include daytime sleepiness and impaired memory and concentration. These effects reflect hypoxic-induced "neural injury." We discuss future research into understanding the pathophysiology of sleep apnea as a basis for uncovering newer forms of treatment of both the ventilatory disorder and its multiple sequelae.

Emerging principles and neural substrates underlying tonic sleep-state-dependent influences on respiratory motor activity

Respiratory muscles with dual respiratory and non-respiratory functions (e.g. the pharyngeal and intercostal muscles) show greater suppression of activity in sleep than the diaphragm, a muscle almost entirely devoted to respiratory function. This sleep-related suppression of activity is most apparent in the tonic component of motor activity, which has functional implications of a more collapsible upper airspace in the case of pharyngeal muscles, and decreased functional residual capacity in the case of intercostal muscles. A major source of tonic drive to respiratory motoneurons originates from neurons intimately involved in states of brain arousal, i.e. neurons not classically involved in generating respiratory rhythm and pattern per se. The tonic drive to hypoglossal motoneurons, a respiratory motor pool with both respiratory and non-respiratory functions, is mediated principally by noradrenergic and glutamatergic inputs, these constituting the essential components of the wakefulness stimulus. These tonic excitatory drives are opposed by tonic inhibitory glycinergic and gamma-amino butyric acid (GABA) inputs that constrain the level of respiratory-related motor activity, with the balance determining net motor tone. In sleep, the excitatory inputs are withdrawn and GABA release into the brainstem is increased, thus decreasing respiratory motor tone and predisposing susceptible individuals to hypoventilation and obstructive sleep apnoea.

SEROTONERGIC MECHANISMS IN AMYOTROPHIC LATERAL SCLEROSIS

Serotonin (5-HT) has been intimately linked with global regulation of motor behavior, local control of motoneuron excitability, functional recovery of spinal motoneurons as well as neuronal maturation and aging. Selective degeneration of motoneurons is the pathological hallmark of amyotrophic lateral sclerosis (ALS). Motoneurons that are preferentially affected in ALS are also densely innervated by 5-HT neurons (e.g., trigeminal, facial, ambiguus, and hypoglossal brainstem nuclei as well as ventral horn and motor cortex). Conversely, motoneuron groups that appear more resistant to the process of neurodegeneration in ALS (e.g., oculomotor, trochlear, and abducens nuclei) as well as the cerebellum receive only sparse 5-HT input. The glutamate excitotoxicity theory maintains that in ALS degeneration of motoneurons is caused by excessive glutamate neurotransmission, which is neurotoxic. Because of its facilitatory effects on glutaminergic motoneuron excitation, 5-HT may be pivotal to the pathogenesis and therapy of ALS. 5-HT levels as well as the concentrations 5-hydroxyindole acetic acid (5-HIAA), the major metabolite of 5-HT, are reduced in postmortem spinal cord tissue of ALS patients indicating decreased 5-HT release. Furthermore, cerebrospinal fluid levels of tryptophan, a precursor of 5-HT, are decreased in patients with ALS and plasma concentrations of tryptophan are also decreased with the lowest levels found in the most severely affected patients. In ALS progressive degeneration of 5-HT neurons would result in a compensatory increase in glutamate excitation of motoneurons. Additionally, because 5-HT, acting through presynaptic 5-HT1B receptors, inhibits glutamatergic synaptic transmission, lowered 5-HT activity would lead to increased synaptic glutamate release. Furthermore, 5-HT is a precursor of melatonin, which inhibits glutamate release and glutamate-induced neurotoxicity. Thus, progressive degeneration of 5-HT neurons affecting motoneuron activity constitutes the prime mover of the disease and its progression and treatment of ALS needs to be focused primarily on boosting 5-HT functions (e.g., pharmacologically via its precursors, reuptake inhibitors, selective 5-HT1A receptor agonists/5-HT2 receptor antagonists, and electrically through transcranial administration of AC pulsed picotesla electromagnetic fields) to prevent excessive glutamate activity in the motoneurons. In fact, 5HT1A and 5HT2 receptor agonists have been shown to prevent glutamate-induced neurotoxicity in primary cortical cell cultures and the 5-HT precursor 5-hydroxytryptophan (5-HTP) improved locomotor function and survival of transgenic SOD1 G93A mice, an animal model of ALS.

Discharge of the hypoglossal nerve cannot distinguish eupnea from gasping, as defined by phrenic discharge, in the in situ mouse

If normal, eupneic breathing fails, gasping is recruited. Serotonin was proposed as essential for gasping, based on findings using an in vitro mouse preparation. This preparation generates rhythmic activities of the hypoglossal nerve that are considered to be akin to both eupnea and gasping. In previous studies, gasping of in situ rat and mouse preparations continued unabated following blockers of receptors for serotonin. However, hypoglossal activity was not recorded in the mouse, and we hypothesized that its discharge during gasping might be dependent on serotonin. In the in situ mouse preparation, hypoglossal discharge had varying and inconsistent patterns during eupnea, discharging concomitant with the phrenic burst, at varying intervals between phrenic bursts, or was silent in some respiratory cycles. In eupnea, phrenic discharge was incrementing, whereas hypoglossal discharge was decrementing in 15 of 20 preparations. During ischemia-induced gasping, peak phrenic height was reached at 205 +/- 17 ms, compared with 282 +/- 27.9 ms after the start of the eupneic burst (P < 0.002). In contrast, rates of rise of hypoglossal discharge in gasping (peak at 233 +/- 25 ms) and eupnea (peak at 199 +/- 19.2 ms) were the same. The uncoupling of hypoglossal from phrenic discharge in eupnea was exacerbated by methysergide, an antagonist of serotonin receptors. These findings demonstrate that hypoglossal discharge alone cannot distinguish eupnea from gasping nor, in eupnea, can hypoglossal activity be used to differentiate neural inspiration from expiration. These findings have significant negative implications for conclusions drawn from the in vitro medullary slice of mouse.

Serotonergic projections from the caudal raphe nuclei to the hypoglossal nucleus in male and female rats

The respiratory control system is sexually dimorphic. In many brain regions, including respiratory motor nuclei, serotonin (5HT) levels are higher in females than in males. We hypothesized that there could be sex differences in 5HT input to the hypoglossal nucleus, a region of the brainstem involved in upper airway control. Adult Fischer 344 rats were anesthetized and a retrograde transsynaptic neuroanatomical tracer, Bartha pseudorabies virus (PRV), was injected into the tongue. Sections through the medulla were reacted immunocytochemically for the presence of (i) PRV, (ii) tryptophan hydroxylase (TPH; marker of 5HT neurons), (iii) PRV combined with TPH, and (iv) 5HT. Sex hormone levels were measured in female rats and correlated with TPH immunoreactivity, as hypoglossal 5HT levels vary with the estrous cycle. The number of PRV neurons was comparable in male and female rats. The number and distribution of TPH immunoreactive neurons in the caudal raphe nuclei were similar in male and female rats. The subset of 5HT neurons that innervate hypoglossal motoneurons was also similar in male and female rats. With the exception of the ventrolateral region of the hypoglossal nucleus, 5HT immunoreactivity was similar in male and female rats. These data suggest that sex differences in 5HT modulation of hypoglossal motoneurons in male and female rats are not the result of sex differences in TPH or 5HT, but may result from differences in neurotransmitter release and reuptake, location of 5HT synaptic terminals on hypoglossal motoneurons, pre- and postsynaptic 5HT receptor expression, or the distribution of sex hormone receptors on hypoglossal or caudal raphe neurons.

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.

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.

Persistence of eupnea and gasping following blockade of both serotonin type 1 and 2 receptors in the in situ juvenile rat preparation

In severe hypoxia or ischemia, normal eupneic breathing is replaced by gasping, which can serve as a powerful mechanism for "autoresuscitation." We have proposed that gasping is generated by medullary neurons having intrinsic pacemaker bursting properties dependent on a persistent sodium current. A number of neuromodulators, including serotonin, influence persistent sodium currents. Thus we hypothesized that endogenous serotonin is essential for gasping to be generated. To assess such a critical role for serotonin, a preparation of the perfused, juvenile in situ rat was used. Activities of the phrenic, hypoglossal, and vagal nerves were recorded. We added blockers of type 1 and/or type 2 classes of serotonergic receptors to the perfusate delivered to the preparation. Eupnea continued following additions of any of the blockers. Changes were limited to an increase in the frequency of phrenic bursts and a decline in peak heights of all neural activities. In ischemia, gasping was induced following any of the blockers. Few statistically significant changes in parameters of gasping were found. We thus did not find a differential suppression of gasping, compared with eupnea, following blockers of serotonin receptors. Such a differential suppression had been proposed based on findings using an in vitro preparation. We hypothesize that multiple neurotransmitters/neuromodulators influence medullary mechanisms underlying the neurogenesis of gasping. In greatly reduced in vitro preparations, the importance of any individual neuromodulator, such as serotonin, may be exaggerated compared with its role in more intact preparations.

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.

The development of the medullary serotonergic system in early human life

The serotonergic (5-HT) neurons of the medulla oblongata are postulated to comprise a system that modulates homeostatic function in response to metabolic imbalances in the internal milieu in a state-dependent manner. In this study, we define the baseline development of the topography of the human medullary 5-HT system in 30 cases ranging from the embryonic period through infancy. We used immunocytochemical techniques with the PH8 antibody which recognizes the key 5-HT synthetic enzyme, tryptophan hydroxylase, and computer-based methods of cell quantitation. In the infant medulla, 5-HT neurons were distributed in raphé, extra-raphé, and ventral positions that place these neurons adjacent to, or intermingled with, the neurons in the lower cranial nerve nuclei and reticular formation that directly mediate respiration, upper airway reflexes, and autonomic function. Along the ventral and ventrolateral surface, 5-HT neurons formed two lateral and one midline "columns" in the rostrocaudal axis that are homologous in position to chemosensitive 5-HT neurons in rats, and that correspond in part to the classic respiratory chemosensitive fields. Serotonergic neurons comprised a subpopulation of the arcuate nucleus along the ventral surface; their short processes directly abutted the surface, suggesting a role for them in monitoring carbon dioxide levels in the cerebrospinal fluid. The medullary 5-HT system began to form in the embryo, with the raphé primordia appearing as early as 7 weeks (the earliest time-point available). By 10-12 weeks, the lateral tegmental 5-HT neurons clustered into the early primordia of extra-raphé subnuclei. By 20 weeks, the "adult-like" topography of the medullary 5-HT system was in place, with subtle (quantitative) changes occurring thereafter. Thus, protracted changes occur from the prenatal period through infancy. These data provide a foundation for 5-HT neuronal analysis in pediatric brainstem disorders, as proposed in the sudden infant death syndrome.

Sex-specific densities of estrogen receptors alpha and beta in the subnuclei of the nucleus tractus solitarius, hypoglossal nucleus and dorsal vagal motor nucleus weanling rats

In rats ventilatory responses to N-methyl-d-aspartate (NMDA) receptor modulation are sexually dimorphic and may be altered by manipulating brain levels of estrogen receptors. Here we used image analysis and immunohistochemistry in weanling male and female rats to quantitate areas and densities of ER alpha and ER beta-positive neurons within medullary regions associated with cardiopulmonary regulation including the hypoglossal nucleus, subnuclei of the nucleus of the solitary tract (NTS), and the dorsal motor nucleus of the vagus. Weanling rats were selected because ventilation, metabolic rate, and body and brain weights are comparable at this age and there are no large fluctuations in plasma hormone levels. Females, relative to males, had smaller areas in the A2 region and parts of the NTS. Counts and densities for ER alpha were greater in females than males in almost all regions studied. In contrast sex differences in ER beta were found in fewer nuclei, but in those higher counts and densities were noted in females. In general, ER beta-positive neurons in the brainstem regions examined were less prevalent than ER alpha neurons. Thus, in weanling rats sex affected ER alpha and ER beta neuronal densities in brainstem regions associated with cardiopulmonary regulation that may be responsible for sex differences in control of breathing.

Inhibition of serotonergic medullary raphe obscurus neurons suppresses genioglossus and diaphragm activities in anesthetized but not conscious rats

Although exogenous serotonin at the hypoglossal motor nucleus (HMN) activates the genioglossus muscle, endogenous serotonin plays a minimal role in modulating genioglossus activity in awake and sleeping rats (Sood S, Morrison JL, Liu H, and Horner RL. Am J Respir Crit Care Med 172: 1338–1347, 2005). This result therefore implies that medullary raphe neurons also play a minimal role in the normal physiological control of the HMN, but this has not yet been established because raphe neurons release other excitatory neurotransmitters onto respiratory motoneurons in addition to serotonin. This study tests the hypothesis that inhibition of medullary raphe serotonergic neurons with 8-hydroxy-2-(di- n-propylamino)tetralin (8-OH-DPAT) suppresses genioglossus and diaphragm activities in awake and sleeping rats. Ten rats were implanted with electrodes to record sleep-wake states and genioglossus and diaphragm activities. Microdialysis probes were also implanted into the nucleus raphe obscurus (NRO). Experiments in 10 anesthetized and vagotomized rats were also performed using the same methodology. In anesthetized rats, microdialysis perfusion of 0.1 mM 8-OH-DPAT into the NRO decreased genioglossus activity by 60.7 ± 9.0% and diaphragm activity by 13.3 ± 3.4%. Diaphragm responses to 7.5% CO2 were also significantly reduced by 8-OH-DPAT. However, despite the robust effects observed in anesthetized and vagotomized rats, there was no effect of 0.1 mM 8-OH-DPAT on genioglossus or diaphragm activities in conscious rats awake or asleep. The results support the concept that endogenously active serotonergic medullary raphe neurons play a minimal role in modulating respiratory motor activity across natural sleep-wake states in freely behaving rodents. This result has implications for pharmacological strategies aiming to manipulate raphe neurons and endogenous serotonin in obstructive sleep apnea.

Serotonergic modulation of inspiratory hypoglossal motoneurons in decerebrate dogs

Inspiratory hypoglossal motoneurons (IHMNs) maintain upper airway patency. However, this may be compromised during sleep and by sedatives, potent analgesics, and volatile anesthetics by either depression of excitatory or enhancement of inhibitory inputs. In vitro data suggest that serotonin (5-HT), through the 5-HT2A receptor subtype, plays a key role in controlling the excitability of IHMNs. We hypothesized that in vivo 5-HT modulates IHMNs activity through the 5-HT2A receptor subtype. To test this hypothesis, we used multibarrel micropipettes for extracellular single neuron recording and pressure picoejection of 5-HT or ketanserin, a selective 5-HT2A receptor subtype antagonist, onto single IHMNs in decerebrate, vagotomized, paralyzed, and mechanically ventilated dogs. Drug-induced changes in neuronal discharge frequency (F(n)) and neuronal discharge pattern were analyzed using cycle-triggered histograms. 5-HT increased the control peak F(n) to 256% and the time-averaged F(n) to 340%. 5-HT increased the gain of the discharge pattern by 61% and the offset by 34 Hz. Ketanserin reduced the control peak F(n) by 68%, the time-averaged F(n) by 80%, and the gain by 63%. These results confirm our hypothesis that in vivo 5-HT is a potent modulator of IHMN activity through the 5-HT2A receptor subtype. Application of exogenous 5-HT shows that this mechanism is not saturated during hypercapnic hyperoxia. The two different mechanisms, gain modulation and offset change, indicate that 5-HT affects the excitability as well as the excitation of IHMNs in vivo.

Effects of electrical stimulation of the medullary raphe nuclei on respiratory movement in rats

The present study was undertaken to examine the effects of electrical stimulation of the medullary raphe nuclei on respiration in rats anesthetized with ketamine and xylazine. Train pulse stimuli (100 Hz, 10-30 microA) were applied in the regions of the caudal raphe nuclei: the raphe magnus (RM), raphe pallidus (RP) and raphe obscurus (RO). Stimulation of the RM depressed inspiratory movements measured by means of an abdominal pneumograph, whereas stimulation of the RP augmented inspiratory movements. It was revealed that stimulation of the RO induced either inhibitory or facilitatory effects on respiratory movements depending on the stimulation sites. These findings confirm and extend previous studies concerning the effects of raphe stimulation on respiratory activity in cats. The present results demonstrate that in rats the caudal raphe nuclei are involved in respiratory control.

Respiratory activation of the genioglossus muscle involves both non-NMDA and NMDA glutamate receptors at the hypoglossal motor nucleus in vivo

Brainstem respiratory neurons innervate the hypoglossal motor nucleus which in turn transmits this respiratory drive signal to the genioglossus muscle of the tongue. The mechanism of this transmission is important to help maintain an open airspace for effective breathing, and is thought to rely almost exclusively on non-N-methyl-d-aspartate (non-NMDA) glutamate receptor activation during respiration. However those studies were performed in slices of medulla from neonatal animals in vitro which may have led to an underestimation of the contribution of NMDA glutamate receptors that may normally operate in intact preparations. The current study tests the hypothesis that both NMDA and non-NMDA receptors contribute to respiratory drive transmission at the hypoglossal motor nucleus in vivo. Experiments were performed in urethane-anesthetized and tracheotomized adult Wistar rats in which vagus nerves were either intact or sectioned. In the presence of augmented genioglossus activity produced by vagotomy, microdialysis perfusion of either an NMDA receptor antagonist (D-2-amino-5-phosphonovaleric acid, 0.001-10 mM) or a non-NMDA receptor antagonist (6-cyano-7-nitroquinoxaline-2, 3-dione disodium salt, 0.001-1 mM) to the hypoglossal motor nucleus reduced respiratory-related genioglossus activity in a dose-dependent manner (P < 0.001) indicating that both NMDA and non-NMDA glutamate receptors are necessary for transmission of the respiratory drive signal to genioglossus muscle in vivo. Similar effects were observed in the vagus nerve intact rats. Further experiments demonstrated that each delivered antagonist had effects that were specific to its respective receptor. Regression analysis also revealed that the activity of both NMDA and non-NMDA receptors at the hypoglossal motor nucleus is related to levels of the prevailing respiratory drive. These results show that both NMDA and non-NMDA glutamate receptors at the hypoglossal motor nucleus are involved in transmission of the respiratory drive signal to genioglossus muscle in vivo.

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

RATIONALE

Previous studies modulating pharyngeal muscle activity with pharmacologic approaches have targeted membrane receptors on pharyngeal motoneurons. Whether modulation of intracellular pathways can increase pharyngeal muscle activity, however, has not been investigated but is relevant to pharmacologic treatments of obstructive sleep apnea.

OBJECTIVES

To determine if modulating the second messenger cyclic adenosine-3'-5'-monophosphate (cAMP) at the hypoglossal motor nucleus (HMN) will increase genioglossus activity across sleep- wake states.

METHODS

Forty-eight 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 HMN to perfuse artificial cerebrospinal fluid and (1) forskolin (500 microM, adenylyl cyclase activator to increase cAMP), (2) a cAMP analog (500 microM), (3) iso-butyl-methylxanthine (IBMX; 300 microM, phosphodiesterase inhibitor), or (4) a cyclic guanosine-3'-5'-monophosphate (cGMP) analog (500 microM, 8-Br-cGMP).

MEASUREMENTS AND MAIN RESULTS

Forskolin and the cAMP analog at the HMN increased respiratory-related and tonic genioglossus activities in wakefulness and non-REM sleep but not REM sleep. IBMX did not affect genioglossus activity in awake or sleeping rats. However, IBMX abolished the robust excitatory responses to serotonin and phenylephrine at the HMN, but responses to non-N-methyl-D-aspartate receptor activation remained. These effects of IBMX were mimicked by 8-Br-cGMP.

CONCLUSIONS

Genioglossus responses to manipulation of cAMP at the HMN are differentially modulated by sleep-wake state. Selective abolition of serotonin and phenylephrine responses after IBMX suggests that under conditions of nonspecific phosphodiesterase inhibition the HMN is unresponsive to certain, otherwise potent, excitatory inputs. Similar responses with 8-Br-cGMP suggest this effect is likely mediated by cGMP pathways.

Role of Endogenous Serotonin in Modulating Genioglossus Muscle Activity in Awake and Sleeping Rats

RATIONALE

Exogenous serotonin at the hypoglossal motor nucleus (HMN) stimulates genioglossus (GG) muscle activity. However, whether endogenous serotonin contributes to GG activation across natural sleep-wake states has not been determined, but is relevant given that serotonergic neurons have decreased activity in sleep and project to pharyngeal motoneurons.

OBJECTIVES

To determine the role of endogenous serotonin at the HMN in modulating GG activity across natural sleep-wake states.

METHODS

Ten rats were implanted with electroencephalogram and neck muscle electrodes to record sleep-wake states, and GG and diaphragm wires for respiratory muscle recordings. Microdialysis probes were implanted into the HMN for perfusion of artificial cerebrospinal fluid and the serotonin receptor antagonist mianserin (100 microM).

MEASUREMENTS AND MAIN RESULTS

In room air, there was no effect of mianserin on respiratory-related or tonic GG activities across sleep-wake states (p > 0.300). In hypercapnia, however, the normal declines in GG activity from non-REM to REM sleep, and wakefulness to REM sleep, were reduced with mianserin (p < 0.005). These data demonstrate a normally low endogenous serotonergic drive modulating GG activity unless augmented by reflex inputs. We also demonstrated a significant serotonergic drive modulating GG activity in vagotomized rats, but not in vagi-intact rats, under anesthesia, suggesting that previous results in reduced preparations may have been influenced by vagotomy.

CONCLUSIONS

The results show a minimal endogenous serotonergic drive at the HMN modulating GG activity across sleep-wake states, unless augmented by reflex inputs. This result has implications for pharmacologic strategies aiming to increase GG activity by manipulating endogenous serotonin in patients with obstructive sleep apnea.

Shared genetic risk factors for obstructive sleep apnea and obesity

Both obesity and obstructive sleep apnea (OSA) are complex disorders with multiple risk factors, which interact in a complicated fashion to determine the overall phenotype. In addition to environmental risk factors, each disorder has a strong genetic basis that is likely due to the summation of small to moderate effects from a large number of genetic loci. Obesity is a strong risk factor for sleep apnea, and there are some data to suggest sleep apnea may influence obesity. It is therefore not surprising that many susceptibility genes for obesity and OSA should be shared. Current research suggests that approximately half of the genetic variance in the apnea hypopnea index is shared with obesity phenotypes. Genetic polymorphisms that increase weight will also be risk factors for apnea. In addition, given the interrelated pathways regulating both weight and other intermediate phenotypes for sleep apnea such as ventilatory control, upper airway muscle function, and sleep characteristics, it is likely that there are genes with pleiotropic effects independently impacting obesity and OSA traits. Other genetic loci likely interact with obesity to influence development of OSA in a gene-by-environment type of effect. Conversely, environmental stressors such as intermittent hypoxia and sleep fragmentation produced by OSA may interact with obesity susceptibility genes to modulate the importance that these loci have on defining obesity-related traits.

Alpha-7 and alpha-4 nicotinic receptor subunit immunoreactivity in genioglossus muscle motoneurons

In the present study, immunohistochemistry combined with retrograde labeling techniques were used to determine if hypoglossal motoneurons (HMNs), retrogradely labeled after cholera toxin B subunit (CTB) injection to the genioglossus muscle in rats, show immunoreactivity for alpha-7 and alpha-4 subunits of nicotinic acetylcholine receptors (nAChRs). CTB-positive HMNs projecting to the genioglossus muscle were consistently labeled throughout the rostrocaudal extent of the hypoglossal nuclei with the greatest labeling at and caudal to area postrema. Alpha-7 subunit immunoreactivity was found in 39.44+/-5.10% of 870 CTB-labeled motoneurons and the alpha-4 subunit in 51.01+/-3.71% of 983 CTB-positive neurons. Rostrally, the number of genioglossal motoneurons demonstrating immunoreactivity for the alpha-7 subunit was 45.85+/-10.04% compared to 34.96+/-5.11% at and caudal to area postrema (P>0.1). The number of genioglossal motoneurons that showed immunoreactivity for the alpha-4 subunit was 55.03+/-4.83% at and caudal to area postrema compared to 42.98+/-3.90% in rostral areas (P=0.074). These results demonstrate that nAChR immunoreactivity is present in genioglossal motoneurons and suggest a role for alpha-7 and alpha-4 subunits containing nAChRs in the regulation of upper airway patency.

Opposing muscarinic and nicotinic modulation of hypoglossal motor output to genioglossus muscle in rats in vivo

The genioglossus (GG) muscle of the tongue, innervated by the hypoglossal motor nucleus (HMN), helps maintain an open airway for effective breathing. In vitro studies in neonatal rodents have separately characterized muscarinic and nicotinic receptor influences at the HMN but the net effects of combined nicotinic and muscarinic receptor activation and increased endogenous acetylcholine have not been determined in adult animals in vivo. Urethane-anaesthetized, tracheotomized and vagotomised rats were studied. Microdialysis perfusion of acetylcholine into the HMN significantly decreased respiratory-related GG activity (28.5 +/- 11.0% at a threshold dose of 0.1 mm). Application of the cholinergic agonists carbachol and muscarine have similar suppression effects (GG activity was decreased 11.8 +/- 4.3 and 20.5 +/- 5.8%, respectively, at 0.01 microm). Eserine, an acetylcholinesterase inhibitor, also decreased the amplitude of respiratory-related GG activity (36.4 +/- 11.3% at 1.0 microm) indicating that endogenous acetylcholine modulates GG activity. Although these results showed that suppression of GG activity predominates during cholinergic stimulation at the HMN, application of the nicotinic receptor agonist dimethyl-4-phenylpiperazinium iodide significantly increased tonic and respiratory-related GG activity (156 +/- 33% for respiratory activity at 1.0 mm) showing that excitatory responses are also present. Consistent with this, 100 microm carbachol decreased GG activity by 44.2 +/- 7.5% of control, with atropine (10 microm) reducing this suppression to 13.8 +/- 4.0% (P < 0.001). However, the nicotinic receptor antagonist dihydro-beta-erythroidine (100 microm) increased the carbachol-mediated suppression to 69.5 +/- 5.9% (P = 0.011), consistent with a role for nicotinic receptors in limiting the overall suppression of GG activity during cholinergic stimulation. Application of eserine to increase endogenous acetylcholine also showed that inhibitory muscarinic and excitatory nicotinic receptors together determine the net level of GG activity during cholinergic stimulation at the HMN. The results suggest that acetylcholine has mixed effects at the HMN with muscarinic-mediated GG suppression masking nicotinic excitation.

Long-Term Intermittent Hypoxia

Humans with long-standing sleep apnea show mixed responses to serotonergic therapies for obstructive sleep apnea. We hypothesize that long-term intermittent hypoxia may result in oxidative injury to upper airway motoneurons, thereby diminishing serotonergic motoneuronal excitation. Unilateral serotonin and glutamate agonist and antagonist microinjections into the hypoglossal motor nuclei in adult rats exposed to 3 weeks of intermittent hypoxia showed reduced hypoglossal nerve responsiveness (logEC50) for serotonin and N-methyl-D-aspartate. However, long-term intermittent hypoxia did not appear to alter hypoglossal response to alpha-amino-3-hydroxy-methylisoxazole-4-propionic acid injections. There was no reduction in hypoglossal motoneuron soma number or in serotonergic postsynaptic receptor mRNA copy numbers within single-cells; in contrast, there was an increase in isoprostanes in the dorsal medulla. Systemic 4-hydroxyl-2,2,6,6-tetramethylpiperidin-1-oxyl (tempol) throughout exposure to intermittent hypoxia improved the EC50 for serotonin to a larger extent than glutamate and normalized medullary isoprostanes. Protein kinase C activity within the hypoglossal nucleus was increased after long-term intermittent hypoxia. These results suggest that long-term intermittent hypoxia reduces serotonergic and N-methyl-D-aspartate excitatory output of hypoglossal nerves, and that reduced excitatory responsiveness and lipid peroxidation are largely prevented with superoxide dismutase treatment throughout hypoxia/reoxygenation. Similar alterations in neurochemical responsiveness may occur in select persons with obstructive sleep apnea.