Serotonin receptor mRNA expression in the hypoglossal motor nucleus

Systemic 8-OH-DPAT challenge causes hyperventilation largely via activating pre-botzinger complex 5-HT1A receptors

Systemic 8-OH-DPAT (a 5-HT_1A receptor agonist) challenge evokes hyperventilation independent of peripheral 5-HT_1A receptors. Though the pre-Botzinger Complex (PBC) is critical in generating respiratory rhythm and activation of local 5-HT_1A receptors induces tachypnea via disinhibition of local GABA_A neurons, its role in the respiratory response to systemic 8-OH-DPAT challenge is still unclear. In anesthetized rats, 8-OH-DPAT (100 μg/kg, iv) was injected twice to confirm the reproducibility of the evoked responses. The same challenges were performed after bilateral microinjections of (S)-WAY-100135 (a 5-HT_1A receptor antagonist) or gabazine (a GABA_A receptor antagonist) into the PBC. Our results showed that: 1) 8-OH-DPAT caused reproducible hyperventilation associated with hypotension and bradycardia; 2) microinjections of (S)-WAY-100135 into the PBC attenuated the hyperventilation by ˜60 % without effect on the evoked hypotension and bradycardia; and 3) the same hyperventilatory attenuation was also observed after microinjections of gabazine into the PBC. Our data suggest that PBC 5-HT_1A receptors play a key role in the respiratory response to systemic 8-OH-DPAT challenge likely via disinhibiting local GABAergic neurons.

Quipazine Elicits Swallowing in the Arterially Perfused Rat Preparation: A Role for Medullary Raphe Nuclei?

Pharmacological neuromodulation of swallowing may represent a promising therapeutic option to treat dysphagia. Previous studies suggested a serotonergic control of swallowing, but mechanisms remain poorly understood. Here, we investigated the effects of the serotonergic agonist quipazine on swallowing, using the arterially perfused working heart-brainstem (in situ) preparation in rats. Systemic injection of quipazine produced single swallows with motor patterns and swallow-breathing coordination similar to spontaneous swallows, and increased swallow rate with moderate changes in cardiorespiratory functions. Methysergide, a 5-HT2 receptor antagonist, blocked the excitatory effect of quipazine on swallowing, but had no effect on spontaneous swallow rate. Microinjections of quipazine in the nucleus of the solitary tract were without effect. In contrast, similar injections in caudal medullary raphe nuclei increased swallow rate without changes in cardiorespiratory parameters. Thus, quipazine may exert an excitatory effect on raphe neurons via stimulation of 5-HT2A receptors, leading to increased excitability of the swallowing network. In conclusion, we suggest that pharmacological stimulation of swallowing by quipazine in situ represents a valuable model for experimental studies. This work paves the way for future investigations on brainstem serotonergic modulation, and further identification of neural populations and mechanisms involved in swallowing and/or swallow-breathing interaction.

Targeting Endotypic Traits with Medications for the Pharmacological Treatment of Obstructive Sleep Apnea. A Review of the Current Literature

Obstructive sleep apnea (OSA) is a highly prevalent condition with few therapeutic options. To date there is no approved pharmacotherapy for this disorder, but several attempts have been made in the past and are currently ongoing to find one. The recent identification of multiple endotypes underlying this disorder has oriented the pharmacological research towards tailored therapies targeting specific pathophysiological traits that contribute differently to cause OSA in each patient. In this review we retrospectively analyze the literature on OSA pharmacotherapy dividing the medications tested on the basis of the four main endotypes: anatomy, upper airway muscle activity, arousal threshold and ventilatory instability (loop gain). We show how recently introduced drugs for weight loss that modify upper airway anatomy may play an important role in the management of OSA in the near future, and promising results have been obtained with drugs that increase upper airway muscle activity during sleep and reduce loop gain. The lack of a medication that can effectively increase the arousal threshold makes this strategy less encouraging, although recent studies have shown that the use of certain sedatives do not worsen OSA severity and could actually improve patients' sleep quality.

Increased tongue use enhances 5-HT2C receptor immunostaining in hypoglossal motor nucleus

Hypoglossal (XII) motoneurons are activated by type 2 receptors for serotonin (5-HT). This activation is especially strong during wakefulness which facilitates diverse motor functions of the tongue, including the maintenance of upper airway patency in obstructive sleep apnea (OSA) patients. We tested whether 5-HT₂ receptor levels in the XII nucleus vary with intensity of tongue use. Three groups of rats were housed overnight under conditions of increasing oromotor activity: W-water available ad lib; S-sweetened water to stimulate drinking; S + O-sweetened water + oil applied on fur to increase grooming. After the exposures, immunostaining for 5-HT_2C, but not 5-HT_2A, receptors was higher in the XII nucleus in S + O than in W rats (65 ± 1.8 (SE) vs. 60 ± 2.0 arbitrary units; p = 0.008). In the medullary raphé obscurus region, the percentage of c-Fos-positive 5-HT cells was 13% higher (p = 0.03) in S + O than in W rats. The positive feedback between tongue use and 5-HT_2C receptor immunostaining reveals a novel mechanism potentially relevant for OSA and neuromuscular disorders.

Neuroanatomical Basis of State-Dependent Activity of Upper Airway Muscles

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

Intraspinal Grafting of Serotonergic Neurons Modifies Expression of Genes Important for Functional Recovery in Paraplegic Rats

Serotonin (5-hydroxytryptamine; 5-HT) plays an important role in control of locomotion, partly through direct effects on motoneurons. Spinal cord complete transection (SCI) results in changes in 5-HT receptors on motoneurons that influence functional recovery. Activation of 5-HT_2A and 5-HT₇ receptors improves locomotor hindlimb movements in paraplegic rats. Here, we analyzed the mRNA of 5-HT_2A and 5-HT₇ receptors (encoded by Htr2a and Htr7 genes, resp.) in motoneurons innervating tibialis anterior (TA) and gastrocnemius lateralis (GM) hindlimb muscles and the tail extensor caudae medialis (ECM) muscle in intact as well as spinal rats. Moreover, the effect of intraspinal grafting of serotonergic neurons on Htr2a and Htr7 gene expression was examined to test the possibility that the graft origin 5-HT innervation in the spinal cord of paraplegic rats could reverse changes in gene expression induced by SCI. Our results indicate that SCI at the thoracic level leads to changes in Htr2a and Htr7 gene expression, whereas transplantation of embryonic serotonergic neurons modifies these changes in motoneurons innervating hindlimb muscles but not those innervating tail muscles. This suggests that the upregulation of genes critical for locomotor recovery, resulting in limb motoneuron plasticity, might account for the improved locomotion in grafted animals.

Hypoxia-induced increases in serotonin-immunoreactive nerve fibers in the medulla oblongata of the rat

Hypoxia induces respiratory responses in mammals and serotonergic neurons in the medulla oblongata participate in respiratory control. However, the morphological changes in serotonergic neurons induced by hypoxia have not yet been examined and respiratory controls of serotonergic neurons have not been clarified. We herein investigated the distribution of immunoreactivity for serotonin (5-hydroxytryptamine; 5-HT) in the medulla oblongata of control rats and rats exposed to 1-6h of hypoxia (10% O₂). We also examined the medulla oblongata by multiple immunofluorescence labeling for 5-HT, neurokinin 1 receptors (NK1R), a marker for some respiratory neurons in the pre-Bötzinger complex (PBC), and dopamine β-hydroxylase (DBH), a marker for catecholaminergic neurons. The number of 5-HT-immunoreactive nerve cell bodies in the raphe nuclei was higher in rats exposed to hypoxia than in control rats. The number of 5-HT-immunoreactive nerve fibers significantly increased in the rostral ventrolateral medulla of rats exposed to 1-6h of hypoxia, caudal ventrolateral medulla of rats exposed to 2-6h of hypoxia, and lateral part of the nucleus of the solitary tract and dorsal motor nucleus of the vagus nerve of rats exposed to 1-2h of hypoxia. Multiple immunofluorescence labeling showed that 5-HT-immunoreactive nerve fibers were close to NK1R-immunoreactive neurons in ventrolateral medulla and to DBH-immunoreactive neurons in the medulla. These results suggest that serotonergic neurons partly regulate respiratory control under hypoxic conditions by modulating the activity of NK1R-expressing and catecholaminergic neurons.

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.

Dual effects of 5-HT(1a) receptor activation on breathing in neonatal mice

Inhibitory 5-HT(1a) receptors are located on serotonin (5-HT) neurons (autoreceptors) as well as neurons of the respiratory network (heteroreceptors). Thus, effects on breathing of 5-HT(1a) agonists, such as (R)-(+)-8-hydroxy-2-(di-N-propylamino) tetralin (8-OH-DPAT), could either be due to decreased firing of 5-HT neurons or direct effects on the respiratory network. Mice in which the transcription factor LMX1B is genetically deleted selectively in Pet1-1-expressing cells (Lmx1b(f/f/p)) essentially have complete absence of central 5-HT neurons, providing a unique opportunity to separate the effect of activation of downstream 5-HT(1a) heteroreceptors from that of autoreceptors. We used rhythmically active medullary slices from wild-type (WT) and Lmx1b(f/f/p) neonatal mice to differentiate autoreceptor versus heteroreceptor effects of 8-OH-DPAT on hypoglossal nerve respiratory output. 8-OH-DPAT transiently increased respiratory burst frequency in Lmx1b(f/f/p) preparations, but not in WT slices. This excitation was abolished when synaptic inhibition was blocked by GABAergic/glycinergic receptor antagonists. Conversely, after 10 min of application, frequency in Lmx1b(f/f/p) slices was not different from baseline, whereas it was significantly depressed in WT slices. In WT mice in vivo, subcutaneous injection of 8-OH-DPAT produced similar biphasic respiratory effects as in Lmx1b(f/f/p) mice. We conclude that 5-HT1a receptor agonists have two competing effects: rapid stimulation of breathing due to excitation of the respiratory network, and delayed inhibition of breathing due to autoreceptor inhibition of 5-HT neurons. The former effect is presumably due to inhibition of inhibitory interneurons embedded in the respiratory network.

Sleep-wake control of the upper airway by noradrenergic neurons, with and without intermittent hypoxia

Hypoglossal (XII) motoneurons innervate muscles of the tongue whose tonic and inspiratory modulated activity protects the upper airway from collapse in patients affected by the obstructive sleep apnea (OSA) syndrome. Both norepinephrine and serotonin provide wakefulness-related excitatory drives that maintain activity in XII motoneurons, with the noradrenergic system playing a particularly prominent role in rats. When noradrenergic and serotonergic drives are antagonized, no further decline of XII nerve activity occurs during pharmacologically induced rapid eye movement (REM) sleep-like state. This is the best evidence to date that, at least in this model, the entire REM sleep-related decline of upper airway muscle tone results from withdrawal of these two excitatory inputs. A major component of noradrenergic input to XII motoneurons originates from pontine noradrenergic neurons that have state-dependent patterns of activity, maximal during wakefulness, and minimal, or absent during REM sleep. Our data suggest that not all ventrolateral medullary catecholaminergic neurons follow this pattern, with adrenergic C1 neurons probably increasing their activity during REM sleep. When rats are subjected to chronic-intermittent hypoxia, noradrenergic drive to XII motoneurons is increased by mechanisms that include sprouting of noradrenergic terminals in the XII nucleus, and increased expression of α1-adrenoceptors; an outcome that may underlie the elevated baseline activity of upper airway muscles during wakefulness in OSA patients.

Loss of motoneurons in the ventral compartment of the rat hypoglossal nucleus following early postnatal exposure to alcohol

Perinatal alcohol exposure (AE) has multiple detrimental effects on cognitive and various behavioral outcomes, but little is known about its impact on the autonomic functions. In a rat model of fetal alcohol spectrum disorders (FASD), we investigated neurochemical and neuroanatomical alterations in two brainstem nuclei, the hypoglossal nucleus (XIIn) and the dorsal nucleus of the vagus nerve (Xdn). One group of male Sprague-Dawley rats (n=6) received 2.625 g/kg ethanol intragastrically twice daily on postnatal days (PD) 4-9, a period equivalent to the third trimester of human pregnancy, and another group (n=6) was sham-intubated. On PD 18-19, the rats were perfused and medullary sections were immunohistochemically processed for choline acetyltransferase (ChAT) or two aminergic receptors that mediate excitatory drive to motoneurons, α₁-adrenergic (α₁-R) and serotonin 2A (5-HT(2A)-R), and c-Fos. Based on ChAT labeling, AE rats had reduced numbers of motoneurons in the ventral XIIn (XIIn-v; 35.4±1.3 motoneurons per side and section vs. 40.0±1.2, p=0.022), but not in the dorsal XIIn or Xdn. Consistent with ChAT data, both the numbers of α₁-R-labeled motoneurons in the XIIn-v and the area of the XIIn-v measured using 5-HT(2A)-R staining were significantly smaller in AE rats (19.7±1.5 vs. 25.0±1.4, p=0.031 and 0.063 mm² ±0.002 vs. 0.074±0.002, p=0.002, respectively). Concurrently, both 5-HT(2A)-R and c-Fos staining tended to be higher in AE rats, suggesting an increased activation. Thus, postnatal AE causes motoneuronal loss in the XIIn-v. This may compromise upper airway control and contribute to increased risk of upper airway obstructions and sudden infant death in FASD victims.

Serotonergic mechanisms are necessary for central respiratory chemoresponsiveness in situ

Evidence from in vivo and in vitro experiments conclude that serotonin (5-HT) neurons are involved in and play an important role in central respiratory CO2/H(+) chemosensitivity. This study was designed to assess the importance of 5-HT neurons and 5-HT receptor activation in the frequency and amplitude components of the hypercapnic response of the respiratory network in the unanesthetized perfused in situ juvenile rat brainstem preparation that exhibits patterns of phrenic nerve discharge similar to breathing in vivo. Exposure to a hypercapnic perfusate increased phrenic burst frequency and/or amplitude, the neural correlates of breathing frequency and tidal volume in vivo. Hypercapnic responses were also assessed during exposure to ketanserin (5-HT2 receptor antagonist), and 8-OH-DPAT (inhibiting 5-HT neurons via 5-HT1A autoreceptors). Neither of these drugs substantially altered baseline activity, however, both abolished hypercapnic responses of the respiratory network. These data illustrate that 5-HT neurons and 5-HT receptor activation are not required for respiratory rhythm generation per se, but are critical for CO2 responses in situ, supporting the hypothesis that 5-HT neurons play an important role in central ventilatory chemosensitivity in vivo.

5-Hydroxytryptamine 5HT2C receptors form a protein complex with N-methyl-D-aspartate GluN2A subunits and activate phosphorylation of Src protein to modulate motoneuronal depolarization

N-Methyl-D-aspartate (NMDA)-gated ion channels are known to play a critical role in motoneuron depolarization, but the molecular mechanisms modulating NMDA activation in the spinal cord are not well understood. This study demonstrates that activated 5HT2C receptors enhance NMDA depolarizations recorded electrophysiologically from motoneurons. Pharmacological studies indicate involvement of Src tyrosine kinase mediates 5HT2C facilitation of NMDA. RT-PCR analysis revealed edited forms of 5HT2C were present in mammalian spinal cord, indicating the availability of G-protein-independent isoforms. Spinal cord neurons treated with the 5HT2C agonist MK 212 showed increased Src(Tyr-416) phosphorylation in a dose-dependent manner thus verifying that Src is activated after treatment. In addition, 5HT2C antagonists and tyrosine kinase inhibitors blocked 5HT2C-mediated Src(Tyr-416) phosphorylation and also enhanced NMDA-induced motoneuron depolarization. Co-immunoprecipitation of synaptosomal fractions showed that GluN2A, 5HT2C receptors, and Src tyrosine kinase form protein associations in synaptosomes. Moreover, immunohistochemical analysis demonstrated GluN2A and 5HT2C receptors co-localize on the processes of spinal neurons. These findings reveal that a distinct multiprotein complex links 5-hydroxytryptamine-activated intracellular signaling events with NMDA-mediated functional activity.

Chronic intermittent hypoxia alters density of aminergic terminals and receptors in the hypoglossal motor nucleus

RATIONALE

Patients with obstructive sleep apnea (OSA) adapt to the anatomical vulnerability of their upper airway by generating increased activity in upper airway-dilating muscles during wakefulness. Norepinephrine (NE) and serotonin (5-HT) mediate, through α₁-adrenergic and 5-HT₂A receptors, a wake-related excitatory drive to upper airway motoneurons. In patients with OSA, this drive is necessary to maintain their upper airway open. We tested whether chronic intermittent hypoxia (CIH), a major pathogenic factor of OSA, affects aminergic innervation of XII motoneurons that innervate tongue-protruding muscles in a manner that could alter their airway-dilatory action.

OBJECTIVES

To determine the impact of CIH on neurochemical markers of NE and 5-HT innervation of the XII nucleus.

METHODS

NE and 5-HT terminal varicosities and α₁-adrenergic and 5-HT₂A receptors were immunohistochemically visualized and quantified in the XII nucleus in adult rats exposed to CIH or room air exchanges for 10 h/d for 34 to 40 days.

MEASUREMENTS AND MAIN RESULTS

CIH-exposed rats had approximately 40% higher density of NE terminals and approximately 20% higher density of 5-HT terminals in the ventromedial quadrant of the XII nucleus, the region that controls tongue protruder muscles, than sham-treated rats. XII motoneurons expressing α₁-adrenoceptors were also approximately 10% more numerous in CIH rats, whereas 5-HT₂A receptor density tended to be lower in CIH rats.

CONCLUSIONS

CIH-elicited increase of NE and 5-HT terminal density and increased expression of α₁-adrenoceptors in the XII nucleus may lead to augmentation of endogenous aminergic excitatory drives to XII motoneurons, thereby contributing to the increased upper airway motor tone in patients with OSA.

Serotoninergic modulation of cortical and respiratory responses to episodic hypoxia

Biphasic respiratory response to hypoxia in anesthetized animals is accompanied by changes in the EEG mostly in the low EEG frequency bands. Serotonin is a potent modulator of cortical and respiratory activity through 5-HT(2) receptors. Present study investigated whether 5-HT(2) receptors might be involved in the EEG and respiratory relationship during normoxic and hypoxic respiration assessed from integrated phrenic (Phr) and hypoglossal (HG) nerve activities. EEG signal recorded from the frontal cortex was subjected to power spectral analysis in delta, theta, alpha, and beta frequency bands. Systemic administration of 5-HT(2) agonist DOI (1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane) enhanced tonic and lowered peak phasic respiratory activity, and increased frequency of bursts of Phr and HG activity. At the same time, EEG activity became desynchronized and arterial blood pressure (ABP) increased. Following DOI pretreatment, 11% hypoxia induced an augmented respiratory response in comparison with the response in the baseline condition. ABP fell less then in the control hypoxia. EEG pattern changed less than in the baseline state. Subsequent administration of ketanserin, a 5-HT(2) antagonist increased respiratory activity, elicited a synchronization of EEG activity and hypotension. The respiratory response to hypoxia was attenuated and cortical response was more potent in comparison with that after DOI injection. Arterial blood pressure decreased more then during baseline hypoxic response. The results suggest that modulation of cortical synchronization and desynchronization through 5-HT(2) receptor active agents may impact to hypoxic respiratory response.

Contributions of 5-HT neurons to respiratory control: neuromodulatory and trophic effects

Serotonin (5-hydroxytryptamine; 5-HT) is a neurotransmitter produced by a small number of neurons in the midbrain, pons and medulla. These neurons project widely throughout the neuraxis, where they release 5-HT and co-localized neuropeptides such as substance P (SP) and thyrotropin-releasing hormone (TRH). Each of these chemicals produce effects largely through G protein-coupled receptors, second messenger systems and subsequent neuromodulatory effects on target neurons. Emerging evidence suggests that 5-HT has additional modes of action during development and in adult mammals, including trophic effects (neurogenesis, cell differentiation, proliferation, migration and maturation) and influences on synaptic plasticity. Here, we discuss some of the neuromodulatory and trophic roles of 5-HT in general and in the context of respiratory control, as well as the regulation of release of modulatory neurotransmitters from 5-HT neurons. Future directions of study are also discussed.

5-HT2A receptors are concentrated in regions of the human infant medulla involved in respiratory and autonomic control

The serotonergic (5-HT) system in the human medulla oblongata is well-recognized to play an important role in the regulation of respiratory and autonomic function. In this study, using both immunocytochemistry (n=5) and tissue section autoradiography with the radioligand (125)I-1-(2,5-dimethoxy-4-iodo-phenyl)2-aminopropane (n=7), we examine the normative development and distribution of the 5-HT(2A) receptor in the human medulla during the last part of gestation and first postnatal year when dramatic changes are known to occur in respiratory and autonomic control, in part mediated by the 5-HT(2A) receptor. High 5-HT(2A) receptor binding was observed in the dorsal motor nucleus of the vagus (preganglionic parasympathetic output) and hypoglossal nucleus (airway patency); intermediate binding was present in the nucleus of the solitary tract (visceral sensory input), gigantocellularis, intermediate reticular zone, and paragigantocellularis lateralis. Negligible binding was present in the raphé obscurus and arcuate nucleus. The pattern of 5-HT(2A) immunoreactivity paralleled that of binding density. By 15 gestational weeks, the relative distribution of the 5-HT(2A) receptor was similar to that in infancy. In all nuclei sampled, 5-HT(2A) receptor binding increased with age, with significant increases in the hypoglossal nucleus (p=0.027), principal inferior olive (p=0.044), and medial accessory olive (0.038). Thus, 5-HT(2A) receptors are concentrated in regions involved in autonomic and respiratory control in the human infant medulla, and their developmental profile changes over the first year of life in the hypoglossal nucleus critical to airway patency and the inferior olivary complex essential to cerebellar function.

Time- and dose-related effects of three 5-HT receptor ligands on the genioglossus activity in anesthetized and conscious rats

Clinical trials in obstructive sleep apnea syndrome patients reported moderate effects of serotoninergic drugs on oropharyngeal apneas, although numerous specific 5-HT ligands highly modulate the genioglossus muscle (GG) activity in experiments performed in anesthetized animals. The purpose of this study was to investigate time- and dose-related effects of central and systemic injections of 8-OHDPAT (5-HT1A agonist), SB224289 (5-HT1B antagonist), and DOI (5-HT2A/2C agonist) on the GG activity in anesthetized and conscious rats. Electromyographic recordings of the GG activity (GGemg) were analyzed after central and systemic injections of each drug in ketamine-xylazine anesthetized rats. Electroencephalograms (EEG), as well as neck and GG muscle activities (Nemg and GGemg), were recorded in 15 additional rats to analyze changes in sleep-wake states before and after systemic injection of the drugs. Central injections of 8-OHDPAT and DOI in anesthetized rats induced clear dose-related increases in phasic and tonic GGemg activities, respectively. The time-responses were inferior to 30 min with 8-OHDPAT and over 50 min with DOI. Moderate increases in phasic GGemg activity were also observed after central, but not peripheral injection of SB and DOI. The total sleep time measured in conscious rats significantly decreased after systemic injections of DOI and 8-OHDPAT, although no change was observed in phasic or tonic GGemg activity. The dose- and time-responses of the DOI in anesthetized rat partly explain the lack of GGemg tonic change in conscious rat. The moderate effect on the GGemg phasic activity of peripheral 5-HT1A ligand injection easily explains the lack of change in conscious rat. The serotonergic modulation of the respiratory component of the GGemg remains complex, but is highly sensitive to 5-HT1A receptors after central injection in rats under anesthesia. Forthcoming therapy in OSAS should be made of mixed profiled neurotransmitters and different routes of administration.

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.

Gasping activity in vitro: a rhythm dependent on 5-HT2A receptors

Many rhythmic behaviors are continuously modulated by endogenous peptides and amines, but whether neuromodulation is critical to the expression of a rhythmic behavior often remains unknown, particularly in mammals. Here, we address this issue in the respiratory network that was isolated in spontaneously rhythmic medullary slice preparations from mice. Under control conditions, the respiratory network generates fictive eupneic activity. We hypothesized previously that this activity depends on two types of pacemaker neurons. The bursting properties of one pacemaker rely on the persistent sodium current (INa(p)) and are insensitive to blockade of calcium channels with cadmium (CI-pacemakers), whereas bursting mechanisms of a second pacemaker are sensitive to cadmium (CS-pacemakers) and the calcium-dependent nonspecific cation current blocker flufenamic acid. During hypoxia, fictive eupneic activity is supplanted by the neural correlate of gasping, which is proposed to depend only on CI-pacemakers. Because CI-pacemakers require endogenous activation of 5-HT2A receptors, we tested the hypothesis that 5-HT2A receptor activation is critical for gasping. Here, we demonstrate that fictive gasping and CI-pacemaker bursting were selectively eliminated by the 5-HT2A receptor antagonist piperidine or ketanserin. Neither 5-HT2A antagonist eliminated bursting by CS-pacemakers and ventral respiratory group (VRG) population activity. However, this VRG activity was very different from eupneic activity. In the presence of 5-HT2A antagonists, VRG activity was eliminated by flufenamic acid and could not be reliably restored by adding substance P. These data support the hypothesis that two types of pacemaker bursting mechanisms underlie fictive eupnea, whereas only one burst mechanism is critical for gasping.

Noradrenergic, serotonergic and GABAergic antagonists injected together into the XII nucleus abolish the REM sleep-like depression of hypoglossal motoneuronal activity

Recently, we reported that the suppression of hypoglossal (XII) motoneuronal activity that occurs during the carbachol-induced, rapid eye movement (REM) sleep-like state is abolished by the microinjection into the XII nucleus of a drug mix that antagonizes aminergic excitation and amino acid-mediated inhibition (prazosin, methysergide, bicuculline and strychnine). We now assess the role of glycinergic inhibition in the depression of XII motoneuronal activity and estimate the distribution of the antagonists around the XII nucleus at the time when they are effective. Towards the first goal, REM sleep-like episodes were elicited in urethane-anesthetized rats by 10 nl carbachol microinjections into the dorsomedial pons prior to, and at different times after, combined microinjections into the XII nucleus of only three antagonists (strychnine omitted). As in our previous study, the carbachol-induced depression of XII activity was abolished during tests performed 42-88 min after the antagonists, whereas other characteristic effects of carbachol (appearance of hippocampal theta, cortical activation, decreased respiratory rate) remained intact. The depressant effect of carbachol on XII motoneurons partially recovered after 2.5 h. Towards the second goal, using a drug diffusion model, we determined that the tissue concentrations of the antagonists at the time when they were effective were within the range of their selective actions, and the drugs acted within 0.9-1.4 mm from the injection sites, thus within a space containing XII motoneurons and their dendrites. We conclude that antagonism of alpha-adrenergic, serotonergic, and GABA(A) receptors are sufficient to abolish the REM sleep-like atonia of XII motoneurons.

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.

REM sleep-like atonia of hypoglossal (XII) motoneurons is caused by loss of noradrenergic and serotonergic inputs

RATIONALE

Studies of hypoglossal (XII) motoneurons that innervate the genioglossus muscle, an upper airway dilator, suggested that the suppression of upper airway motor tone during REM sleep is caused by withdrawal of excitation mediated by norepinephrine and serotonin.

OBJECTIVES

Our objectives were to determine whether antagonism of aminergic receptors located in the XII nucleus region can abolish the REM sleep-like atonia of XII motoneurons, and whether both serotonergic and noradrenergic antagonists are required to achieve this effect.

METHODS

REM sleep-like episodes were elicited in anesthetized rats by pontine carbachol injections before and at various times after microinjection of prazosin and methysergide combined, or of only one of the drugs, into the XII nucleus.

MEASUREMENTS AND MAIN RESULTS

Spontaneous XII nerve activity was significantly reduced, by 35 to 81%, by each antagonist alone and in combination, indicating that XII motoneurons were under both noradrenergic and serotonergic endogenous excitatory drives. During the 32 to 81 min after microinjections of both antagonists, pontine carbachol caused no depression of XII nerve activity, whereas other characteristic effects (activation of the hippocampal and cortical EEG, and slowing of the respiratory rate) remained intact. A partial recovery of the depressant effect of carbachol then occurred parallel to the recovery of spontaneous XII nerve activity from the depressant effect of the antagonists. Microinjections of either antagonist alone did not eliminate the depressant effect of carbachol.

CONCLUSIONS

The REM sleep-like depression of XII motoneuronal activity induced by pontine carbachol can be fully accounted for by the combined withdrawal of noradrenergic and serotonergic effects on XII motoneurons.

Maturational changes in neuromodulation of central pathways underlying hypoxic ventilatory response

The neuromodulator systems mediating the central component of the hypoxic ventilatory response (HVR) during development are complex and diverse. The early component of the HVR is mediated through N-methyl-D-aspartate (NMDA) glutamate receptors in the caudal brainstem. The intracellular downstream signal transductions of the NMDA receptors involve protein kinase C (PKC), neuronal nitric oxide synthase (nNOS) and tyrosine kinase (TK). Activation of NMDA receptors will also lead to activation of the early gene transcription factors including AP-1 (c-fos, c-jun) and NF-kappaB which may play a role in modulation of the subsequent response to hypoxia. NMDA receptors in the caudal brainstem play a critical role in the development of the HVR and increasing dependency on NMDA receptors emerges over time. Similarly, hypoxia-induced PKC, NOS and c-Fos activation in the caudal brainstem is relatively weak in the immature animals, but this activation increases with age and the strength of the response appears to increase concomitantly with the appearance of NMDA expression. Several neurotransmitters including adenosine, gamma-aminobutyric acid (GABA), serotonin and opioids are involved in the late component of the HVR. In addition, the late phase of the HVR is mediated in part through platelet-derived growth factor (PDGF)-beta receptors. PDGF-beta receptor activation is an important contributor of the hypoxic ventilatory depression at all postnatal ages, but its role is more critical in the developing animals. Maturation of these neuromodulators, especially the NMDA and PDGF-beta receptors-mediated pathways, occurs primarily during the early postnatal period. Perturbation of these developmental processes may result in short-term or sustained alterations to the HVR and may also affect neuronal survival during hypoxia.

Serotonin agonists and antagonists in obstructive sleep apnea: therapeutic potential

Obstructive sleep apnea hypopnea syndrome (OSAHS) is a prevalent disorder associated with substantial cardiovascular and neurobehavioral morbidity. Yet this is a disorder for which there are no widely effective pharmacotherapies. The pathophysiology of obstructive sleep apnea namely, normal respiration in waking with disordered breathing only in sleep, suggests that this disorder should be readily amenable to drug therapy. Over the past 10 years, we have gained tremendous insight into the neurochemical mechanisms involved in state-dependent control of respiration. It is apparent from this work that there are many potential avenues for pharmacotherapies, including several seemingly conflicting directions for serotonergic therapies. Serotonin delivery is reduced to upper airway dilator motor neurons in sleep, and this contributes, at least in part, to sleep-related reductions in dilator muscle activity and upper airway obstruction. The dilator motor neuron post-synaptic serotonin receptors are 5-HT(2A) and 5-HT(2C) subtypes, and in adults the presynaptic 5-HT receptor in motor nuclei is 5-HT(1B), an inhibitory receptor. Serotonin receptors are also found within central respiratory neuronal groups, and these receptor subtypes include 5-HT(1A) (inhibitory) and 5-HT(2) receptors. Peripherally, stimulation of 5-HT(2A), 5-HT(2C) and 5-HT(3) receptor subtypes have an inhibitory effect on respiration via action at the nodose ganglion. Many of these receptor subtypes and their signal transduction pathways may be affected by oxidative stress in obstructive sleep apnea. These alterations will make finding drug therapies for sleep apnea more challenging, but not insurmountable. Future directions are suggested for elucidating safe, well-tolerated serotonergic drugs for this disorder. Tryptophan was one of the first serotonergic drugs tested for OSAHS. This drug was withdrawn from the market as a result of reports linking tryptophan use with eosinophilic myalgia syndrome and life-threatening pulmonary hypertension. Newer drugs with serotonergic activity tested in persons with sleep-disordered breathing include buspirone, fluoxetine and paroxetine. Trials are presently being conducted to evaluate the effects of 5-HT(2A) and 5-HT(3) antagonists on OSAHS. Many of the drugs tested have not shown significant improvement in sleep apnea. However, with continued effort to elucidate the pharmacology of neurochemical control of state-dependent changes in respiratory control, the availability of pharmacological therapy for this disorder is not too far away.

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

Serotonin (5-HT) from medullary raphe neurons excites hypoglossal motoneurons innervating genioglossus (GG) muscle. Since some raphe neurons also show increased activity in hypercapnia, we tested the hypothesis that serotonergic mechanisms at the hypoglossal motor nucleus (HMN) modulate GG activity and responses to CO2. Seventeen urethane-anesthetized, tracheotomized and vagotomized rats were studied. Microdialysis probes were used to deliver mianserin (5-HT receptor antagonist, 0 and 0.1 mM) or 5-HT (eight doses, 0-50 mM) to the HMN during room air or CO2-stimulated breathing. Mianserin decreased respiratory-related GG activity during room air and CO2-stimulated breathing (P<0.001), and also suppressed GG responses to CO2 (P=0.05). In contrast, GG activity was increased by 5-HT at the HMN, and was further increased in hypercapnia (P<0.02). However, 5-HT increased respiratory-related GG activity at levels lower (1 mM) than those eliciting tonic GG activity (10-30 mM 5-HT). The results show that 5-HT at the HMN contributes to the respiratory control of GG muscle.

The modulation by 5-HT of glutamatergic inputs from the raphe pallidus to rat hypoglossal motoneurones, in vitro

Decreases in the activity of 5-HT-containing caudal raphe neurones during sleep are thought to be partially responsible for the resultant disfacilitation of hypoglossal motoneurones. Whilst 5-HT has a direct excitatory action on hypoglossal motoneurones as a result of activation of 5-HT2 receptors, microinjection of 5-HT2 antagonists into the hypoglossal nucleus reduces motor activity to a much lesser extent compared to the suppression observed during sleep suggesting other transmitters co-localised in caudal raphe neurones may also be involved. The aim of the present study was therefore to characterise raphe pallidus inputs to hypoglossal motoneurones. Whole cell recordings were made from hypoglossal motoneurones in vitro. 5-HT evoked a direct membrane depolarisation (8.45 +/- 3.8 mV, P < 0.001) and increase in cell input resistance (53 +/- 40 %, P < 0.001) which was blocked by the 5-HT2 antagonist, ritanserin (2.40 +/- 2.7 vs. 7.04 +/- 4.6 mV). Stimulation within the raphe pallidus evoked a monosynaptic EPSC that was significantly reduced by the AMPA/kainate antagonist, NBQX (22.8 +/- 16 % of control, P < 0.001). In contrast, the 5-HT2 antagonist, ritanserin, had no effect on the amplitude of these EPSCs (106 +/- 31 % of control, P = n.s.). 5-HT reduced these EPSCs to 50.0 +/- 13 % of control (P < 0.001), as did the 5-HT1A agonist, 8-OH-DPAT (52.5 +/- 17 %, P < 0.001) and the 5-HT1B agonist, CP 93129 (40.6 +/- 29 %, P < 0.01). 8-OH-DPAT and CP 93129 increased the paired pulse ratio (1.38 +/- 0.27 to 1.91 +/- 0.54, P < 0.05 & 1.27 +/- 0.08 to 1.44 +/- 0.13, P < 0.01 respectively) but had no effect on the postsynaptic glutamate response (99 +/- 4.4 % and 100 +/- 2.5 %, P = n.s.). They also increased the frequency (P < 0.001), but not the amplitude, of miniature glutamatergic EPSCs in hypoglossal motoneurones. These data demonstrate that raphe pallidus inputs to hypoglossal motoneurones are predominantly glutamatergic in nature, with 5-HT decreasing the release of glutamate from these projections as a result of activation of 5-HT1A and/or 5-HT1B receptors located on presynaptic terminals.

Postnatal development of serotonin 1B, 2 A and 2C receptors in brainstem motoneurons

The effects of serotonin (5-HT) on motoneurons are mediated via multiple receptor subtypes. In hypoglossal (XII) motoneurons, the prototypic brainstem motoneurons whose functions change during the postnatal period, 5-HT effects evolve from inhibitory to excitatory, probably in association with changes in receptor expression. We studied 5-HT1B, 5-HT2A and 5-HT2C receptor mRNA in 414 dissociated XII motoneurons and 5-HT2A protein in the XII, facial and spinal cervical (C2-3) motor nuclei. The percentage of motoneurons expressing distinct mRNAs varied with the postnatal age (P3-33 days) and receptor subtype. Initially, 5-HT1B mRNA was present in 50-85% of cells, but on P14 its expression transiently decreased below 35%. 5-HT2A mRNA was present in nearly all cells after P6, but in less than 65% on P3-5. Normal and/or short splice variants of the 5-HT2C mRNA were expressed in less than 20% of motoneurons on P3-9, and in approximately 35% thereafter. 5-HT1B and 5-HT2A mRNAs often were expressed in different cells during early and intermediate postnatal periods, whereas 5-HT2C mRNA never occurred alone. The 5-HT2A receptor protein level gradually increased through P15 in the XII and facial nuclei, with dendritic labelling appearing in XII motoneurons only after P12. In spinal motoneurons, both somatic and dendritic labelling was strongest on P5 and then decreased. The development of 5-HT receptors in XII motoneurons may be related to changes in feeding behaviour, whereas different cues regulate 5-HT receptor expression in upper spinal motoneurons.

Activation of 5HT1B receptors inhibits glycinergic synaptic inputs to mammalian motoneurons during postnatal development

We investigated whether 5-HT(1B) receptor-mediated inhibition of evoked glycinergic inhibitory postsynaptic currents (eIPSCs) in hypoglossal motoneurons (HMs) changed postnatally. In HMs from postnatal days 2-3 (P2-3, neonate) and P10-11 (juvenile) rats bath application of 5-HT (10 microM) caused a not significantly different large reduction in eIPSC amplitude to 35.0+/-22.5% (mean+/-S.D.) and 35.4+/-10.6% of control; respectively. The dose-response relationship for the 5-HT(1B) receptor agonist, CP-93,129, revealed that the mean agonist concentration at half-maximal inhibition (IC(50)) was similar, 1.6 and 2.0 nM, respectively. Additionally, strong antibody labeling of 5-HT(1B) receptors in the hypoglossal motor nucleus was observed in neonates, juveniles and adults. These results demonstrate that over the postnatal period studied, 5-HT(1B) receptor-mediated inhibition of glycinergic eIPSCs is not age dependent.

Age and gender effects on serotonin-dependent plasticity in respiratory motor control

We review recent evidence indicating that serotonin-dependent plasticity in respiratory control is influenced by age and gender. Specifically, respiratory long-term-facilitation following intermittent hypoxia decreases with age in male rats, but increases in female rats. We speculate about a possible relationship between age and gender effects on serotonin-dependent plasticity in upper airway motoneuron pools and the prevalence of obstructive sleep apnea.

Hyperpolarization-activated current I(h) in nucleus of solitary tract neurons: regional difference in serotonergic modulation

The nucleus of solitary tract (NTS) contains diverse neural circuits responsible for basic vital functions. We examined the effect of serotonin (5-HT) on hyperpolarization-activated current (I(h)) in neurons acutely isolated from caudal, medial and rostral parts of the NTS. Caudal and medial NTS neurons showed a large amplitude of I(h) compared with rostral neurons. In these neurons, perfusion with 5-HT potentiated Ih amplitude in a concentration-dependent manner. The effect of 5-HT was blocked by NAN-190, a 5-HT1A receptor antagonist. Thus, 5-HT1A receptors may regulate I(h) channel activity in caudal and medial NTS neurons.

The neuropharmacology of upper airway motor control in the awake and asleep states: implications for obstructive sleep apnoea

Obstructive sleep apnoea is a common and serious breathing problem that is caused by effects of sleep on pharyngeal muscle tone in individuals with narrow upper airways. There has been increasing focus on delineating the brain mechanisms that modulate pharyngeal muscle activity in the awake and asleep states in order to understand the pathogenesis of obstructive apnoeas and to develop novel neurochemical treatments. Although initial clinical studies have met with only limited success, it is proposed that more rational and realistic approaches may be devised for neurochemical modulation of pharyngeal muscle tone as the relevant neurotransmitters and receptors that are involved in sleep-dependent modulation are identified following basic experiments.

Neural control of tongue movement with respect to respiration and swallowing

The tongue must move with remarkable speed and precision between multiple orofacial motor behaviors that are executed virtually simultaneously. Our present understanding of these highly integrated relationships has been limited by their complexity. Recent research indicates that the tongue s contribution to complex orofacial movements is much greater than previously thought. The purpose of this paper is to review the neural control of tongue movement and relate it to complex orofacial behaviors. Particular attention will be given to the interaction of tongue movement with respiration and swallowing, because the morbidity and mortality associated with these relationships make this a primary focus of many current investigations. This review will begin with a discussion of peripheral tongue muscle and nerve physiology that will include new data on tongue contractile properties. Other relevant peripheral oral cavity and oropharyngeal neurophysiology will also be discussed. Much of the review will focus on brainstem control of tongue movement and modulation by neurons that control swallowing and respiration, because it is in the brainstem that orofacial motor behaviors sort themselves out from their common peripheral structures. There is abundant evidence indicating that the neural control of protrusive tongue movement by motoneurons in the ventral hypoglossal nucleus is modulated by respiratory neurons that control inspiratory drive. Yet, little is known of hypoglossal motoneuron modulation by neurons controlling swallowing or other complex movements. There is evidence, however, suggesting that functional segregation of respiration and swallowing within the brainstem is reflected in somatotopy within the hypoglossal nucleus. Also, subtle changes in the neural control of tongue movement may signal the transition between respiration and swallowing. The final section of this review will focus on the cortical integration of tongue movement with complex orofacial movements. This section will conclude with a discussion of the functional and clinical significance of cortical control with respect to recent advances in our understanding of the peripheral and brainstem physiology of tongue movement.

Alpha(1B) receptors are the main postsynaptic mediators of adrenergic excitation in brainstem motoneurons, a single-cell RT-PCR study

Norepinephrine (NE) is an important modulator of brainstem motoneurons. It is released at high levels during wakefulness, whereas its reduced release during sleep may contribute to motor suppression, including upper airway hypotonia. To identify the receptors that mediate postsynaptic effects of NE in brainstem motoneurons of juvenile and adult rats, we determined the pattern of adrenoceptor mRNA expression and co-expression in retrogradely labeled and acutely dissociated hypoglossal (XII) motoneurons (n=121) using single-cell, real-time reverse transcription-polymerase chain reaction (RT-PCR). The alpha(1B) receptor mRNA was present in most motoneurons (33/39 or 85%). The remaining six adrenoceptor mRNA species investigated were consistently present in micropunches of tissue extracted from the XII nucleus, but were either rarely expressed in individual motoneurons (alpha(1A) mRNA in 15%, alpha(1D) in 14%, alpha(2B/C) in 2% of cells) or absent (alpha(2A), beta(1) and beta(2)). When present, the alpha(1A) and alpha(1D) mRNAs were co-expressed with alpha(1B) mRNA. The adrenoceptor mRNA expression profiles in dissociated locus coeruleus and inferior olive neurons were significantly different. We conclude that postsynaptic effects of NE in XII motoneurons are primarily mediated by alpha(1B) receptors; the effects ascribed to alpha(2) and/or beta adrenoceptors may be exerted presynaptically.

Microdialysis perfusion of 5-HT into hypoglossal motor nucleus differentially modulates genioglossus activity across natural sleep-wake states in rats

1. Serotonin (5-hydroxytryptamine, 5-HT) excites hypoglossal (XII) motoneurons in reduced preparations, and it has been suggested that withdrawal of 5-HT may underlie reduced genioglossus (GG) muscle activity in sleep. However, systemic administration of 5-HT agents in humans has limited effects on GG activity. Whether 5-HT applied directly to the XII motor nucleus increases GG activity in an intact preparation either awake or asleep has not been tested. 2. The aim of this study was to develop a novel freely behaving animal model for in vivo microdialysis of the XII motor nucleus across sleep-wake states, and test the hypothesis that 5-HT application will increase GG activity. 3. Eighteen rats were implanted with electroencephalogram and neck muscle electrodes to record sleep-wake states, and GG and diaphragm electrodes for respiratory muscle recording. Microdialysis probes were implanted into the XII motor nucleus and perfused with artificial cerebrospinal fluid (ACSF) or 10 mM 5-HT. 4. Normal decreases in GG activity occurred from wakefulness to non-rapid eye movement (non-REM) and REM sleep with ACSF (P < 0.01). Compared to ACSF, 5-HT caused marked GG activation across all sleep-wake states (increases of 91-251 %, P < 0.015). Importantly, 5-HT increased sleeping GG activity to normal waking levels for as long as 5-HT was applied (3-5 h). Despite tonic stimulation by 5-HT, periods of phasic GG suppression and excitation occurred in REM sleep compared with non-REM. 5. The results show that sleep-wake states differentially modulate GG responses to 5-HT at the XII motor nucleus. This animal model using in vivo microdialysis of the caudal medulla will enable the determination of neural mechanisms underlying pharyngeal motor control in natural sleep.

Nucleus raphé obscurus modulates hypoglossal output of neonatal rat in vitro transverse brain stem slices

Nucleus raphé obscurus (NRo) modulates hypoglossal (XII) nerve motor output in the in vitro transverse brain stem slice of neonatal rats (1-5 days old); chemical ablation of NRo and its focal CO(2) acidification modulated the bursting rhythm of XII nerves. We microinjected a 4.5 mM solution of kainic acid into the NRo to disrupt cellular activity and observed that XII nerve activity was temporarily abolished (n = 10). We also microinjected CO(2)-acidified (pH = 6.00 +/- 0.01) artificial cerebrospinal fluid (aCSF) into the NRo (n = 6), the pre-Bötzinger complex (PBC) (n = 6), as well as a control region in the lateral tegmental field equidistant to NRo, PBC, and the XII motor nuclei (n = 12). CO(2) acidification of the control region had no effect on XII motor output. CO(2) acidification of the NRo significantly (P < 0.05) increased the burst discharge frequency of XII nerves by 77%; integrated burst amplitude and burst duration increased by 64% and 52%, respectively. CO(2) acidification of the PBC significantly (P < 0.05) increased the burst discharge frequency of XII nerves by 65%, but neither integrated burst amplitude nor burst duration changed. These results demonstrate that chemical ablation of the NRo can abolish XII nerve bursting rhythm and that stimulation of the NRo with CO(2)-acidified aCSF can excite XII nerve bursting activity. From these observations, we conclude that, in transverse brain stem slices, the NRo contains pH/CO(2)-sensitive cells that modulate XII motor output.

Single-cell RT-PCR as a tool to study gene expression in central and peripheral autonomic neurones

In studies of the central and peripheral autonomic nervous system, it has become increasingly important to be able to investigate mRNA expression patterns within specific neuronal populations. Traditionally, the identification of mRNA species in discrete populations of cells has relied upon in situ hybridization. An alternative, relatively simple procedure is 'multiplex' reverse transcription-polymerase chain reaction (RT-PCR), conducted on single neurons after their in vitro isolation. Multiplex single-cell RT-PCR can be used to examine the expression of multiple genes within individual cells, and can be combined with electrophysiological, pharmacological and anatomical (retrograde labelling) studies. This review focuses on a number of key aspects of this approach, methodology, and both the advantages and the limitations of the technique. We also provide specific examples of work performed in our laboratory, examining the expression of alpha 2-adrenergic receptors in catecholaminergic cells of the rat brainstem and adrenal medulla. The application of single-cell RT-PCR to future studies of the autonomic nervous system will hopefully provide information on how physiological and pathological conditions affect gene expression in autonomic neurones.

Serotonergic stimulation of the genioglossus and the response to nasal continuous positive airway pressure

In obstructive sleep apnea (OSA), abnormal pharyngeal collapsibility may be offset by increased mechanoreflex-mediated activity of dilator muscles while awake, but this reflex is inhibited during sleep and during application of nasal continuous positive airway pressure (CPAP). Direct activation of upper airway (UA) motor neurons in the hypoglossal nucleus by a selective serotonin reuptake inhibitor (SSRI), paroxetine hydrochloride, may increase genioglossal electromyographic (EMG) activity (EMGgg) in a manner resistant to mechanoreflex inhibition. We studied the effects of paroxetine on EMGgg using an intraoral surface electrode during eupnea or room air breathing (RA), hypercapnia (HYP), and CPAP application in the presence of hypercapnia (CPAP + HYP) in 11 normal volunteers, using a double-blind, placebo-controlled crossover design. After 5 d of paroxetine, EMGgg activity increased significantly within each condition (p = 0.02). EMGgg during the conditions of HYP and HYP + CPAP were significantly greater than during RA for both placebo and paroxetine treatments (p = 0.006). EMGgg activity in HYP persisted during HYP + CPAP on paroxetine (183% versus 182% of placebo, respectively). We conclude that paroxetine produces an augmentation in EMGgg in normal subjects during wakefulness and that this effect persists during mechanoreflex inhibition. This is consistent with a central serotonergic effect.

Postnatal development of 5-HT(1A) receptor expression in rat somatic motoneurons

Prior work has established that hypoglossal motoneurons (HMs) change postnatally in their response to serotonin (5-HT), in part as a result of a decline in expression of 5-HT(1A) receptors. In the current study, two issues were addressed. First, using in situ hybridization we found that transient expression of 5-HT(1A) receptors occurs in other populations of brainstem (facial and trigeminal) and spinal (cervical and lumbar) motoneurons. Second, the participation of motoneuronal afferent (serotonergic) and efferent (neuromuscular) innervation in inducing and maintaining this decline in expression was investigated. Serotonergic innervation of the hypoglossal nucleus (nXII) was disrupted in neonatal rats by intra-cisternal injection of the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT), and 5-HT(1A) receptor mRNA levels in nXII from these rats were assayed at postnatal day 21. In spite of an almost complete loss of serotonergic fibers in the region, the postnatal decrease in 5-HT(1A) receptor expression by HMs still occurred. To test for potential regulation by target-derived factors or by nerve injury, receptor mRNA levels were assayed after unilateral transection of the hypoglossal nerve in adult rats. Though this treatment resulted in re-induction of developmentally transient expression of the p75 neurotrophin receptor, 5-HT(1A) receptor expression remained low. Thus, neonatal expression of 5-HT(1A) receptors appears to be common to somatic motoneurons, but we found no evidence for changes in serotonergic innervation in influencing this expression, nor did we find evidence for its regulation by peripheral factors.

Synaptic control of motoneuronal excitability

Movement, the fundamental component of behavior and the principal extrinsic action of the brain, is produced when skeletal muscles contract and relax in response to patterns of action potentials generated by motoneurons. The processes that determine the firing behavior of motoneurons are therefore important in understanding the transformation of neural activity to motor behavior. Here, we review recent studies on the control of motoneuronal excitability, focusing on synaptic and cellular properties. We first present a background description of motoneurons: their development, anatomical organization, and membrane properties, both passive and active. We then describe the general anatomical organization of synaptic input to motoneurons, followed by a description of the major transmitter systems that affect motoneuronal excitability, including ligands, receptor distribution, pre- and postsynaptic actions, signal transduction, and functional role. Glutamate is the main excitatory, and GABA and glycine are the main inhibitory transmitters acting through ionotropic receptors. These amino acids signal the principal motor commands from peripheral, spinal, and supraspinal structures. Amines, such as serotonin and norepinephrine, and neuropeptides, as well as the glutamate and GABA acting at metabotropic receptors, modulate motoneuronal excitability through pre- and postsynaptic actions. Acting principally via second messenger systems, their actions converge on common effectors, e.g., leak K(+) current, cationic inward current, hyperpolarization-activated inward current, Ca(2+) channels, or presynaptic release processes. Together, these numerous inputs mediate and modify incoming motor commands, ultimately generating the coordinated firing patterns that underlie muscle contractions during motor behavior.

The effects of trazodone with L-tryptophan on sleep-disordered breathing in the English bulldog

Obstructive sleep apnea hypopnea syndrome (OSAHS) is a prevalent disorder, for which there are no universally effective pharmacotherapeutics. We hypothesized that in OSAHS, excitatory serotoninergic influences are important for maintaining patency of the upper airway in waking, and that in sleep, reduced serotoninergic drive plays a significant role in upper airway collapse and OSAHS. The previously reported small responses in humans with OSAHS to serotoninergics may relate, in part, to study design and the drugs/doses selected. We therefore performed multitrials/dose, multidose, randomized sleep studies testing the effectiveness of a combination of serotoninergics, trazodone, and L-tryptophan, in our animal model of OSAHS, the English bulldog. Trazodone/L-tryptophan caused dose-dependent reductions in respiratory events in non-rapid-eye-movement sleep (NREMS) and rapid-eye-movement sleep (REMS). During NREMS, the respiratory disturbance index (RDI) +/- standard error was 6.3 +/- 1.4 events/h (placebo) and 0.9 +/- 0.3 (highest dose), p < 0.01. During REMS, the RDI was 31.4 +/- 6.1 events/h (placebo) and 11.5 +/- 4.3 (highest dose), p = 0.002. Trazodone/ L-tryptophan dose-dependently reduced sleep fragmentation, p = 0.03, increased sleep efficiency, p = 0.005, enhanced slow-wave sleep, p = 0.0004, and minimized sleep-related suppression of upper airway dilator activity, p < 0.02. Trazodone with L-tryptophan can treat sleep-disordered breathing (SDB) in an animal model of OSAHS; the effectiveness of this therapy may be related to increased upper airway dilator activity in sleep and/or enhanced slow-wave sleep.

Age-related changes in serotonin in the hypoglossal nucleus of rat: implications for sleep-disordered breathing

We are attempting to determine the neuronal factors that influence upper airway patency during sleep in the elderly. Serotonin has a facilitatory effect on hypoglossal motoneurons that innervate the tongue, and manipulations of the serotonergic system alter airway patency. We hypothesized that age-associated changes in serotonergic input to the hypoglossal nucleus might be a factor in the increased susceptibility to sleep-disordered breathing in the elderly. We used light microscopic immunocytochemistry to study the distribution of serotonin in the hypoglossal nucleus in young and old rats. Rats > 18 months had fewer serotonin immunoreactive axons and boutons in the hypoglossal nucleus than rats < 6 months. These data suggest that normal aging may result in a change in the availability of serotonin that results in decreased facilitation of hypoglossal motoneurons.

Maturation of the mammalian respiratory system

In this review, the maturational changes occurring in the mammalian respiratory network from fetal to adult ages are analyzed. Most of the data presented were obtained on rodents using in vitro approaches. In gestational day 18 (E18) fetuses, this network functions but is not yet able to sustain a stable respiratory activity, and most of the neonatal modulatory processes are not yet efficient. Respiratory motoneurons undergo relatively little cell death, and even if not yet fully mature at E18, they are capable of firing sustained bursts of potentials. Endogenous serotonin exerts a potent facilitation on the network and appears to be necessary for the respiratory rhythm to be expressed. In E20 fetuses and neonates, the respiratory activity has become quite stable. Inhibitory processes are not yet necessary for respiratory rhythmogenesis, and the rostral ventrolateral medulla (RVLM) contains inspiratory bursting pacemaker neurons that seem to constitute the kernel of the network. The activity of the network depends on CO2 and pH levels, via cholinergic relays, as well as being modulated at both the RVLM and motoneuronal levels by endogenous serotonin, substance P, and catecholamine mechanisms. In adults, the inhibitory processes become more important, but the RVLM is still a crucial area. The neonatal modulatory processes are likely to continue during adulthood, but they are difficult to investigate in vivo. In conclusion, 1) serotonin, which greatly facilitates the activity of the respiratory network at all developmental ages, may at least partly define its maturation; 2) the RVLM bursting pacemaker neurons may be the kernel of the network from E20 to adulthood, but their existence and their role in vivo need to be further confirmed in both neonatal and adult mammals.

Serotonin induces an increase in D-myo-inositol (1,4,5)-trisphosphate 3-kinase activity in rat brainstem slices

Serotonin robustly potentiated the activity of the InsP3 3-kinase in rat brainstem slices. This potentiation was mediated through activation of 5-HT2 receptors since it was only retrieved with the selective 5-HT2 agonist DOI but not with the 5-HT1A agonist 8OHDPAT. The enhancement of the InsP3 3-kinase activity by serotonin is positively modulated by pretreatment of the slices with the phosphatase inhibitor okadaic acid. Moreover, the specific CaMKII antagonists KN-62 and KN-93 dramatically reduced the serotonin-evoked increase in the InsP3 3-kinase activity. It is thus concluded that InsP3 3-kinase up-regulation occurs through activation of PLC-coupled serotoninergic receptors and requires the phosphorylation of the enzyme by the ubiquitous multimeric protein kinase CaMKII.