Serotonin receptor subtypes required for ventilatory long-term facilitation and its enhancement after chronic intermittent hypoxia in awake rats

Sleep state dependence of ventilatory long-term facilitation following acute intermittent hypoxia in Lewis rats

Ventilatory long-term facilitation (vLTF) is a form of respiratory plasticity induced by acute intermittent hypoxia (AIH). Although vLTF has been reported in unanesthetized animals, little is known concerning the effects of vigilance state on vLTF expression. We hypothesized that AIH-induced vLTF is preferentially expressed in sleeping vs. awake male Lewis rats. Vigilance state was assessed in unanesthetized rats with chronically implanted EEG and nuchal EMG electrodes, while tidal volume, frequency, minute ventilation (Ve), and CO(2) production were measured via plethysmography, before, during, and after AIH (five 5-min episodes of 10.5% O(2) separated by 5-min normoxic intervals), acute sustained hypoxia (25 min of 10.5% O(2)), or a sham protocol without hypoxia. Vigilance state was classified as quiet wakefulness (QW), light and deep non-rapid eye movement (NREM) sleep (l-NREM and d-NREM sleep, respectively), or rapid eye movement sleep. Ventilatory variables were normalized to pretreatment baseline values in the same vigilance state. During d-NREM sleep, vLTF was observed as a progressive increase in Ve post-AIH (27 + or - 5% average, 30-60 min post-AIH). In association, Ve/Vco(2) (36 + or - 2%), tidal volume (14 + or - 2%), and frequency (7 + or - 2%) were increased 30-60 min post-AIH during d-NREM sleep. vLTF was significant but less robust during l-NREM sleep, was minimal during QW, and was not observed following acute sustained hypoxia or sham protocols in any vigilance state. Thus, vLTF is state-dependent and pattern-sensitive in unanesthetized Lewis rats, with the greatest effects during d-NREM sleep. Although the physiological significance of vLTF is not clear, its greatest significance to ventilatory control is most likely during sleep.

The Ventilatory Response to Hypoxia in Mammals: Mechanisms, Measurement, and Analysis

The respiratory response to hypoxia in mammals develops from an inhibition of breathing movements in utero into a sustained increase in ventilation in the adult. This ventilatory response to hypoxia (HVR) in mammals is the subject of this review. The period immediately after birth contains a critical time window in which environmental factors can cause long-term changes in the structural and functional properties of the respiratory system, resulting in an altered HVR phenotype. Both neonatal chronic and chronic intermittent hypoxia, but also chronic hyperoxia, can induce such plastic changes, the nature of which depends on the time pattern and duration of the exposure (acute or chronic, episodic or not, etc.). At adult age, exposure to chronic hypoxic paradigms induces adjustments in the HVR that seem reversible when the respiratory system is fully matured. These changes are orchestrated by transcription factors of which hypoxia-inducible factor 1 has been identified as the master regulator. We discuss the mechanisms underlying the HVR and its adaptations to chronic changes in ambient oxygen concentration, with emphasis on the carotid bodies that contain oxygen sensors and initiate the response, and on the contribution of central neurotransmitters and brain stem regions. We also briefly summarize the techniques used in small animals and in humans to measure the HVR and discuss the specific difficulties encountered in its measurement and analysis.

Differential expression of respiratory long-term facilitation among inbred rat strains

We tested the hypotheses that: (1) long-term facilitation (LTF) following acute intermittent hypoxia (AIH) varies among three inbred rat strains: Fischer 344 (F344), Brown Norway (BN) and Lewis rats and (2) ventral cervical spinal levels of genes important for phrenic LTF (pLTF) vary in association with pLTF magnitude. Lewis and F344, but not BN rats exhibited significant increases in phrenic and hypoglossal burst amplitude 60min post-AIH that were significantly greater than control experiments without AIH, indicating strain differences in phrenic (98%, 56% and 20%, respectively) and hypoglossal LTF (66%, 77% and 5%, respectively). Ventral spinal 5-HT(2A) receptor mRNA and protein levels were higher in F344 and Lewis versus BN, suggesting that higher 5-HT(2A) receptor levels are associated with greater pLTF. More complex relationships were found for 5-HT(7), BDNF and TrkB mRNA. BN had higher 5-HT(7) and TrkB mRNA versus F344; BN and Lewis had higher BDNF mRNA levels versus F344. Genetic variations in serotonergic function may underlie strain differences in AIH-induced pLTF.

Glossopharyngeal long-term facilitation requires serotonin 5-HT2 and NMDA receptors in rats

Although the glossopharyngeal nerve (IX) is mainly a sensory nerve, it innervates stylopharyngeus and some other pharyngeal muscles, whose excitations would likely improve upper airway patency since electrical IX stimulation increases pharyngeal airway size. As acute intermittent hypoxia (AIH) induces hypoglossal and genioglossal long-term facilitation (LTF), we hypothesized that AIH induces glossopharyngeal LTF, which requires serotonin 5-HT(2) and NMDA receptors. Integrated IX activity was recorded in anesthetized, vagotomized, paralyzed and ventilated rats before, during and after 5 episodes of 3-min isocapnic 12% O(2) with 3-min intervals of 50% O(2). Either saline, ketanserin (5-HT(2) antagonist, 2mg/kg) or MK-801 (NMDA antagonist, 0.2mg/kg) was (i.v.) injected 30-60 min before AIH. Both phasic and tonic IX activities were persistently increased (both P<0.05) after AIH in vehicle, but not ketanserin or MK-801, rats. Hypoxic glossopharyngeal responses were minimally changed after either drug. These data suggest that AIH induces both phasic and tonic glossopharyngeal LTF, which requires activation of 5-HT(2) and NMDA receptors.

Progressive augmentation and ventilatory long-term facilitation are enhanced in sleep apnoea patients and are mitigated by antioxidant administration

Progressive augmentation (PA) and ventilatory long-term facilitation (vLTF) of respiratory motor output are forms of respiratory plasticity that are initiated during exposure to intermittent hypoxia. The present study was designed to determine whether PA and vLTF are enhanced in obstructive sleep apnoea (OSA) participants compared to matched healthy controls. The study was also designed to determine whether administration of an antioxidant cocktail mitigates PA and vLTF. Thirteen participants with sleep apnoea and 13 controls completed two trials. During both trials participants were exposed to intermittent hypoxia which included twelve 4-min episodes of hypoxia (P(ETCO(2)), 50 mmHg; P(ETCO(2)), 4 mmHg above baseline) followed by 30 min of recovery. Prior to exposure to intermittent hypoxia, participants were administered, in a randomized fashion, either an antioxidant or a placebo cocktail. Baseline measures of minute ventilation during the placebo and antioxidant trials were not different between or within groups. During the placebo trial, PA was evident in both groups; however it was enhanced in the OSA group compared to control (last hypoxic episode 36.9 +/- 2.8 vs. 27.7 +/- 2.2 l min(-1); P Collapse

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MESH Headings

• Adult
• Antioxidants/administration & dosage
• Humans
• Hypoxia/blood
• Hypoxia/drug therapy
• Hypoxia/physiopathology
• Male
• Oxidative Stress/drug effects
• Oxidative Stress/physiology
• Polysomnography/methods
• Pulmonary Ventilation/drug effects
• Pulmonary Ventilation/physiology
• Sleep Apnea, Obstructive/blood
• Sleep Apnea, Obstructive/drug therapy
• Sleep Apnea, Obstructive/physiopathology
• Superoxide Dismutase/administration & dosage
• Time Factors
• Ubiquinone/administration & dosage
• Ubiquinone/analogs & derivatives
• Vitamin E/administration & dosage

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Grants

• R01 HL085537 NHLBI NIH HHS
• R01HL085537 NHLBI NIH HHS

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Affiliation(s)

Dorothy S Lee John D. Dingell Veterans Administration Medical Center, Wayne State University, Detroit, MI 48201, USA
M Safwan Badr
Jason H Mateika

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5-HT2 receptors modulate excitatory neurotransmission to cardiac vagal neurons within the nucleus ambiguus evoked during and after hypoxia

To examine the role of 5-HT2 receptors in the central cardiorespiratory network, and in particular the respiratory modulation of parasympathetic activity to the heart, we used an in vitro medullary slice that allowed simultaneous examination of rhythmic inspiratory-related activity recorded from hypoglossal rootlet and excitatory inspiratory-related neurotransmission to cardioinhibitory vagal neurons (CVNs) within the nucleus ambiguus (NA). Focal application of ketanserin, a 5-HT2 receptor antagonist, did not significantly alter the frequency of spontaneous excitatory postsynaptic excitatory currents (EPSCs) in CVNs in control conditions. However, ketanserin diminished spontaneous excitatory neurotransmission to CVNs during hypoxia. The inhibitory action of ketanserin was on 5-HT3 mediated EPSCs during hypoxia since these responses were blocked by the 5-HT3 receptor antagonist ondansetron. In addition, a robust inspiratory-related excitatory neurotransmission was recruited during recovery from hypoxia. Focal application of ketanserin during this posthypoxia period evoked a significant augmentation of the frequency of inspiratory-related, but not spontaneous EPSCs in CVNs. This excitatory effect of ketanserin was prevented by application of the purinergic receptor blocker pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS). These results demonstrate 5-HT2 receptors differentially modulate excitatory neurotransmission to CVNs during and after hypoxia. Activation of 5-HT2 receptors acts to maintain excitatory neurotransmission to CVNs during hypoxia, likely via presynaptic facilitation of 5-HT3 receptor-mediated neurotransmission to CVNs. However, activation of 5HT2 receptors diminishes the subsequent inspiratory-related excitatory neurotransmission to CVNs that is recruited during the recovery from hypoxia likely exerting an inhibitory action on inspiratory-related purinergic signaling.

Transgenic mice lacking serotonin neurons have severe apnea and high mortality during development

Central serotonin (5-HT) neurons modulate many vital brain functions, including respiratory control. Whether breathing depends critically on 5-HT neurons, or whether their influence is excitatory or inhibitory, remains controversial. Here we show that neonatal Lmx1b(flox/flox;ePet-Cre/+) mice (also called Lmx1b(f/f/p) mice), which selectively lack serotonin neurons, display frequent and severe apnea lasting as long as 55 s. This was associated with a marked decrease in ventilation to less than one-half of normal. These respiratory abnormalities were most severe during the postnatal period, markedly improving by the time the pups were 2-4 weeks old. Despite the severe breathing dysfunction, many of these mice survived, but there was a high perinatal mortality, and those that survived had a decrease in growth rate until the age at which the respiratory defects resolved. Consistent with these in vivo observations, respiratory output was markedly reduced in isolated brainstem-spinal cord preparations from neonatal Lmx1b(f/f/p) mice and completely blocked in perfused brain preparations from neonatal rats treated with selective antagonists of 5-HT(2A) and neurokinin 1 (NK-1) receptors. The ventilatory deficits in neonatal Lmx1b(f/f/p) mice were reversed in vitro and in vivo with agonists of 5-HT(2A) and/or NK-1 receptors. These results demonstrate that ventilatory output in the neonatal period is critically dependent on serotonin neurons, which provide excitatory drive to the respiratory network via 5-HT(2A) and NK-1 receptor activation. These findings provide insight into the mechanisms of sudden infant death syndrome, which has been associated with abnormalities of 5-HT neurons and of cardiorespiratory control.

Intermittent hypoxia induces functional recovery following cervical spinal injury

Respiratory-related complications are the leading cause of death in spinal cord injury (SCI) patients. Few effective SCI treatments are available after therapeutic interventions are performed in the period shortly after injury (e.g. spine stabilization and prevention of further spinal damage). In this review we explore the capacity to harness endogenous spinal plasticity induced by intermittent hypoxia to optimize function of surviving (spared) neural pathways associated with breathing. Two primary questions are addressed: (1) does intermittent hypoxia induce plasticity in spinal synaptic pathways to respiratory motor neurons following experimental SCI? and (2) can this plasticity improve respiratory function? In normal rats, intermittent hypoxia induces serotonin-dependent plasticity in spinal pathways to respiratory motor neurons. Early experiments suggest that intermittent hypoxia also enhances respiratory motor output in experimental models of cervical SCI (cervical hemisection) and that the capacity to induce functional recovery is greater with longer durations post-injury. Available evidence suggests that intermittent hypoxia-induced spinal plasticity has considerable therapeutic potential to treat respiratory insufficiency following chronic cervical spinal injury.

The role of 5-HT3 and other excitatory receptors in central cardiorespiratory responses to hypoxia: implications for sudden infant death syndrome

Although brainstem serotonergic (5-HT) systems are involved in the protective responses to hypoxia, abnormalities of 5-HT function are strongly implicated in SIDS, and the neurochemical mechanisms by which 5-HT receptors influence brainstem cardiorespiratory responses to hypoxia remains unclear. This study focuses on the role of excitatory neurotransmission, including 5-HT3 signaling, to cardiac vagal neurons (CVNs) that dominate the control of heart rate. Excitatory synaptic inputs to CVNs, located in the nucleus ambiguus (NA), were recorded simultaneously with respiratory activity in in vitro brainstem slices. During control conditions excitatory inputs to CVNs were blocked by application of NMDA and AMPA/kainate glutamatergic receptor antagonists, whereas the 5-HT3 and purinergic receptor antagonists ondansetron and pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), respectively, had no effect. However, during hypoxia ondansetron inhibited excitatory neurotransmission to CVNs. In recovery from hypoxia, spontaneous and respiratory-related excitatory events were blocked by glutamatergic and purinergic receptor blockers, respectively, whereas ondancetron had no effect. These results demonstrate that hypoxia recruits a 5-HT pathway to CVNs that activates 5-HT3 receptors on CVNs to maintain parasympathetic cardiac activity during hypoxia. Exaggeration of this 5-HT neurotransmission could increase the incidence of bradycardia and risk of sudden infant death during hypoxia.

Dorsomedial medullary 5-HT2 receptors mediate immediate onset of initial hyperventilation, airway dilation, and ventilatory decline during hypoxia in mice

The dorsomedial medulla oblongata (DMM) includes the solitary tract nucleus and the hypoglossal nucleus, to which 5-HT neurons project. Effects of 5-HT in the DMM on ventilatory augmentation and airway dilation are mediated via 5-HT2 receptors, which interact with the CO(2) drive. The interaction may elicit cycles between hyperventilation with airway dilation and hypoventilation with airway narrowing. In the present study, effects of 5-HT2 receptors in the DMM on hypoxic ventilatory and airway responses were investigated, while 5-HT release in the DMM was monitored. Adult male mice were anesthetized, and then a microdialysis probe was inserted into the DMM. The mice were placed in a double-chamber plethysmograph. After recovery from anesthesia, the mice were exposed to hypoxic gas (7% O(2) in N(2)) for 5 min with or without a 5-HT2 receptor antagonist (LY-53857) perfused in the DMM. 5-HT release in the DMM was increased by hypoxia regardless of the presence of LY-53857. Immediate onset and the peak of initial hypoxic hyperventilatory responses were delayed. Subsequent ventilatory decline and airway dilation during initial hypoxic hyperventilation were suppressed with LY-53857. These results suggest that 5-HT release increased by hypoxia acts on 5-HT2 receptors in the DMM, which contributes to the immediate onset of initial hypoxic hyperventilation, airway dilation, and subsequent ventilatory decline. Hypoxic ventilatory and airway responses mediated via 5-HT2 receptors in the DMM may play roles in immediate rescue and defensive adaptation for hypoxia and may be included in periodic breathing and the pathogenesis of obstructive sleep apnea.

Raphé neurons stimulate respiratory circuit activity by multiple mechanisms via endogenously released serotonin and substance P

Brainstem serotonin (5-HT) neurons modulate activity of many neural circuits in the mammalian brain, but in many cases endogenous mechanisms have not been resolved. Here, we analyzed actions of raphé 5-HT neurons on respiratory network activity including at the level of the pre-Bötzinger complex (pre-BötC) in neonatal rat medullary slices in vitro, and in the more intact nervous system of juvenile rats in arterially perfused brainstem-spinal cord preparations in situ. At basal levels of activity, excitation of the respiratory network via simultaneous release of 5-HT and substance P (SP), acting at 5-HT(2A/2C), 5-HT(4), and/or neurokinin-1 receptors, was required to maintain inspiratory motor output in both the neonatal and juvenile systems. The midline raphé obscurus contained spontaneously active 5-HT neurons, some of which projected to the pre-BötC and hypoglossal motoneurons, colocalized 5-HT and SP, and received reciprocal excitatory connections from the pre-BötC. Experimentally augmenting raphé obscurus activity increased motor output by simultaneously exciting pre-BötC and motor neurons. Biophysical analyses in vitro demonstrated that 5-HT and SP modulated background cation conductances in pre-BötC and motor neurons, including a nonselective cation leak current that contributed to the resting potential, which explains the neuronal depolarization that augmented motor output. Furthermore, we found that 5-HT, but not SP, can transform the electrophysiological phenotype of some pre-BötC neurons to intrinsic bursters, providing 5-HT with an additional role in promoting rhythm generation. We conclude that raphé 5-HT neurons excite key circuit components required for generation of respiratory motor output.

Serotonin and NMDA receptors in respiratory long-term facilitation

Some have postulated that long-term facilitation (LTF), a persistent augmentation of respiratory activity after episodic hypoxia, may play a beneficial role in helping stabilize upper airway patency in obstructive sleep apnea (OSA) patients. However, the neuronal and cellular mechanisms underlying this plasticity of respiratory motor behavior are still poorly understood. The main purpose of this review is to summarize recent findings about serotonin and NMDA receptors involved in both LTF and its enhancement after chronic intermittent hypoxia (CIH). The potential roles of these receptors in the initiation, formation and/or maintenance of LTF, as well as the CIH effect on LTF, will be discussed. As background, different paradigms for the stimulus protocol, different patterns of LTF expression and their mechanistic implications in LTF will also be discussed.

Intermittent hypoxia and respiratory plasticity in humans and other animals: does exposure to intermittent hypoxia promote or mitigate sleep apnoea?

This review focuses on two phenomena that are initiated during and after exposure to intermittent hypoxia. The two phenomena are referred to as long-term facilitation and progressive augmentation of respiratory motor output. Both phenomena are forms of respiratory plasticity. Long-term facilitation is characterized by a sustained elevation in respiratory activity after exposure to intermittent hypoxia. Progressive augmentation is characterized by a gradual increase in respiratory activity from the initial to the final hypoxic exposure. There is much speculation that long-term facilitation may have a significant role in individuals with sleep apnoea because this disorder is characterized by periods of upper airway collapse accompanied by intermittent hypoxia, one stimulus known to induce long-term facilitation. It has been suggested that activation of long-term facilitation may serve to mitigate apnoea by facilitating ventilation and, more importantly, upper airway muscle activity. We examine the less discussed but equally plausible situation that exposure to intermittent hypoxia might ultimately lead to the promotion of apnoea. There are at least two scenarios in which apnoea might be promoted following exposure to intermittent hypoxia. In both scenarios, long-term facilitation of upper airway muscle activity is initiated but ultimately rendered ineffective because of other physiological conditions. Thus, one of the primary goals of this review is to discuss, with support from basic and clinical studies, whether various forms of respiratory motor neuronal plasticity have a beneficial and/or a detrimental impact on breathing stability in individuals with sleep apnoea.

Daily intermittent hypoxia augments spinal BDNF levels, ERK phosphorylation and respiratory long-term facilitation

Acute intermittent hypoxia (AIH) elicits a form of respiratory plasticity known as long-term facilitation (LTF). We hypothesized that: 1) daily AIH (dAIH) preconditioning enhances phrenic and hypoglossal (XII) LTF in a rat strain with low constitutive LTF expression; 2) dAIH induces brain-derived neurotrophic factor (BDNF), a critical protein for phrenic LTF (pLTF) in the cervical spinal cord; and 3) dAIH increases post-AIH extracellular regulated kinase (ERK) activation. Phrenic and XII motor output were monitored in anesthetized dAIH- or sham-treated Brown Norway rats with and without acute AIH. pLTF was observed in both sham (18+/-9% baseline; 60 min post-hypoxia; p<0.05; n=18) and dAIH treated rats (37+/-8%; p<0.05; n=14), but these values were not significantly different (p=0.13). XII LTF was not observed in sham-treated rats (4+/-5%), but was revealed in dAIH pretreated rats (48+/-18%; p<0.05). dAIH preconditioning increased basal ventral cervical BDNF protein levels (24+/-8%; p<0.05), but had no significant effect on ERK phosphorylation. AIH increased BDNF in sham (25+/-8%; p<0.05), but not dAIH-pretreated rats (-7+/-4%), and had complex effects on ERK phosphorylation (ERK2 increased in shams whereas ERK1 increased in dAIH-treated rats). Thus, dAIH elicits metaplasticity in LTF, revealing XII LTF in a rat strain with no constitutive XII LTF expression. Increased BDNF synthesis may no longer be necessary for phrenic LTF following dAIH preconditioning since BDNF concentration is already elevated.

HTR2A variation and sudden infant death syndrome: a case-control analysis

AIM

The serotonergic (5-HT) system functions in central autonomic regulation with homeostatic roles in cardiorespiratory control, thermoregulation, arousal and sleep-wake cycling. Altered function and development of this system in cases of sudden infant death syndrome (SIDS) have been established, but the aetiology of these disturbances remains unclear. The serotonin receptor, HTR2A, functions within this system with roles in the homeostatic response to hypoxia including excitatory effects on respiration, gasping and rhythm generation, all functions potentially compromised in SIDS. The objective of this study was to examine the relationship between SIDS risk and HTR2A variation.

METHODS

All coding regions, intron-exon boundaries and the promoter region of HTR2A were PCR amplified and analysed by standard sequencing in 96 SIDS cases and 96 matched controls.

RESULTS

Twenty-one HTR2A variations were identified in this case-control cohort, including four novel variations (c.C-1185A, c.T-923C, c.T-17C and c.C50T). None of the variations identified showed a significant association with SIDS.

CONCLUSION

This report provides evidence that despite known alterations of the 5-HT system in SIDS, and the logical role for the HTR2A receptor, genetic variation of HTR2A as studied in our cohort is not responsible for these alterations. These results represent a further step in the investigation of the aetiology of the altered serotonin system in SIDS cases.

Sleep fragmentation impairs ventilatory long-term facilitation via adenosine A1 receptors

Sleep fragmentation (SF), a primary feature of obstructive sleep apnoea (OSA), impairs hippocampal long-term potentiation and causes cognitive/attention deficits. However, its influence upon respiratory control has hardly been studied. This study examined the effect of SF on ventilatory long-term facilitation (LTF, a persistent augmentation of respiratory activity after episodic hypoxia) and the hypoxic ventilatory response (HVR), and investigated the role of adenosine A1 receptors in these SF effects in conscious adult male Sprague-Dawley rats. SF, confirmed by sleep architecture recordings, was achieved by periodic, forced locomotion in a rotating drum (30 s rotation/90 s stop for 24 h). LTF, elicited by five episodes of 5 min poikilocapnic hypoxia (10% O2) with 5 min intervals, was measured by plethysmography. Resting ventilation and metabolic rate were unchanged, HVR was reduced (150.6 +/- 3.5% versus 110.4 +/- 12.3%) and LTF was eliminated (22.6 +/- 0.5% versus -0.1 +/- 1.3%) shortly after 24 h SF. The SF-induced impairments were SF duration dependent, and completely reversible as HVR (< 24 h) and LTF (< 48 h) returned spontaneously to their pre-SF values. The SF-impaired HVR was improved (130.3 +/- 4.2%) and SF-eliminated LTF was restored (19.6 +/- 0.9%) by systemic injection of the adenosine A1 receptor antagonist 8-CPT (2.5 mg kg(-1)) approximately 30 min before LTF elicitation. Both HVR and LTF were also similarly impaired by 24 h total sleep deprivation or 24 h repeated cage tapping-induced SF, but not by a 24 h locomotion control protocol for SF. Collectively, these data suggest that: (1) 24 h SF impairs LTF and poikilocapnic HVR; (2) these impairments require A1 receptors; and (3) SF of OSA may exacerbate OSA via impaired ventilatory control mechanisms.

Determinants of frequency long-term facilitation following acute intermittent hypoxia in vagotomized rats

Acute intermittent (AIH), but not acute sustained hypoxia (ASH) elicits a form of respiratory plasticity known as long-term facilitation (LTF). In anesthetized rats, LTF is expressed as increased respiratory-related nerve burst amplitude, with variable effects on burst frequency. We analyzed a large data set from multiple investigators using the same experimental protocol to determine factors influencing frequency LTF. Our meta-analysis revealed that AIH elicits both phrenic amplitude and frequency LTF in anesthetized and vagotomized rats, but frequency LTF is small in comparison with amplitude LTF (12% versus 60%, respectively). ASH elicits a small, but significant frequency and amplitude LTF (8% and 10%, respectively) that is not significantly different than controls. Similar to all published reports, analysis of this large data set confirms that phrenic amplitude LTF following AIH is significantly greater than ASH. Multiple regression analysis revealed a strong correlation between baseline burst frequency and frequency LTF. Variations in baseline burst frequency may contribute to variation in frequency LTF and may underlie the apparent effects of some drug treatments.

Formation and maintenance of ventilatory long-term facilitation require NMDA but not non-NMDA receptors in awake rats

N-methyl-d-aspartate (NMDA) receptor antagonism in the phrenic motonucleus area eliminates phrenic long-term facilitation (pLTF; a persistent augmentation of phrenic nerve activity after episodic hypoxia) in anesthetized rats. However, whether NMDA antagonism can eliminate ventilatory LTF (vLTF) in awake rats is unclear. The role of non-NMDA receptors in LTF is also unknown. Serotonin receptor antagonism before, but not after, episodic hypoxia eliminates pLTF, suggesting that serotonin receptors are required for induction, but not maintenance, of pLTF. However, because NMDA and non-NMDA ionotropic glutamate receptors are directly involved in mediating the inspiratory drive to phrenic, hypoglossal, and intercostal motoneurons, we hypothesized that these receptors are required for both formation and maintenance of vLTF. vLTF, induced by five episodes of 5-min poikilocapnic hypoxia (10% O(2)) with 5-min normoxia intervals, was measured with plethysmography in conscious adult male Sprague-Dawley rats. Either (+/-)-2-amino-5-phosphonovaleric acid (APV; NMDA antagonist, 1.5 mg/kg) or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; non-NMDA antagonist, 10 mg/kg) was systemically (ip) injected approximately 30 min before hypoxia. APV was also injected immediately after or 20 min after episodic hypoxia in additional groups. As control, vehicle was similarly injected in each rat 1-2 days before. Regardless of being injected before or after episodic hypoxia, vehicle did not alter vLTF ( approximately 23%), whereas APV eliminated vLTF while having little effect on baseline ventilation or hypoxic ventilatory response. In contrast, CNQX enhanced vLTF ( approximately 34%) while decreasing baseline ventilation. Collectively, these results suggest that activation of NMDA but not non-NMDA receptors is necessary for formation and maintenance of vLTF in awake rats.

Functions of serotonin in hypoxic pulmonary vascular remodeling

In lung vasculature, reversible constriction of smooth muscle cells exists in response to acute decrease in oxygen levels (hypoxia). Progressive and irreversible structural remodeling that reduces blood vessel lumen takes place in response to chronic hypoxia and results in pulmonary hypertension. Several studies have shown a role of serotonin in regulating acute and chronic hypoxic responses. In this review the contribution of serotonin, its receptors and transporter in lung hypoxic responses is discussed. Hypoxic conditions modify plasma levels of serotonin, serotonin transporter activity, and expression of 5-HT1B and 5-HT2B receptors. These appear to be required for pulmonary vascular cell proliferation, which depends on the ratio between reactive oxygen species and nitric oxide. A heterozygous mutation was identified in the 5-HT2B receptor gene of a patient who developed pulmonary hypertension after fenfluramines anorexigen treatment. This C-terminus truncated 5-HT2B mutant receptor presents lower nitric oxide coupling, and higher cell proliferation capacity than the wild-type receptor. Under low oxygen tension, cells increase the transcription of specific genes via stabilization of the transcription factor hypoxia-inducible factor (HIF)-1. Factors such as angiotensin II or thrombin that can also control HIF-1 pathway contribute to pulmonary vascular remodeling. The 5-HT2B receptor via phosphatidylinositol-3 kinase/Akt activates nuclear factor-kappaB, which is involved in the regulation of HIF-1 expression. Acontrol of HIF- 1 by 5-HT2B receptors explains why expression of pulmonary vascular remodeling factors, such as endothelin-1 or transforming growth factor-beta, which is HIF-1-alpha regulated, is not modified in hypoxic 5-HT2B receptor mutant mice. Understanding the detailed mechanisms involved in lung hypoxic responses may provide general insight into pulmonary hypertension pathogenesis.

In hamsters the D1 receptor antagonist SCH23390 depresses ventilation during hypoxia

During exposure of animals to hypoxia, brain and blood dopamine levels increase stimulating dopaminergic receptors which influence the integrated ventilatory response to low oxygen. The purpose of the present study is to test the hypothesis that in conscious hamsters, systemic antagonism of D(1) receptors would depress their breathing in air and in response to hypoxic and hypercapnic challenges. Nine male hamsters were treated with saline or 0.25 mg/kg SCH-23390 (SCH), a D(1) receptor antagonist that crosses the blood-brain barrier. Ventilation was determined using the barometric method, and oxygen consumption and CO(2) production were evaluated utilizing the flow-through method. During exposure to air, SCH decreased frequency of breathing. During exposure to hypoxia (10% oxygen in nitrogen), relative to saline, SCH-treated hamsters decreased minute ventilation by decreasing tidal volume and oxygen consumption but not CO(2) production. During exposure to hypercapnia (5% CO(2) in 95% O(2)), frequency of breathing was decreased with SCH, but there was no significant effect on minute ventilation. Relative to saline treatment body temperature was lower in SCH-treated hamsters by 0.6 degrees C. These results demonstrate that in hamsters D(1) receptors can modulate control of ventilation in air and during hypoxia and hypercapnic exposures. Whether D(1) receptors located centrally or on carotid bodies modulate these effects is not clear from this study.

Inspiratory activation is not required for episodic hypoxia-induced respiratory long-term facilitation in postnatal rats

Episodic hypoxia causes repetitive inspiratory activation that induces a form of respiratory plasticity termed long-term facilitation (LTF). While LTF is a function of the hypoxic exposures and inspiratory activation, their relative importance in evoking LTF is unknown. The aims of this study were to: (1) dissociate the relative roles played by episodic hypoxia and respiratory activation in LTF; and (2) determine whether the magnitude of LTF varies as a function of hypoxic intensity. We did this by examining the effects of episodic hypoxia in postnatal rats (15-25 days old), which unlike adult rats exhibit a prominent hypoxia-induced respiratory depression. We quantified inspiratory phrenic nerve activity generated by the in situ working-heart brainstem before, during and for 60 min after episodic hypoxia. We demonstrate that episodic hypoxia evokes LTF despite the fact that it potently suppresses inspiratory activity during individual hypoxic exposures (P < 0.05). Specifically, we show that after episodic hypoxia (three 5 min periods of 10% O2) respiratory frequency increased to 40 +/- 3.3% above baseline values over the next 60 min (P < 0.001). Continuous hypoxia (15 min of 10% O2) had no lasting effects on respiratory frequency (P > 0.05). To determine if LTF magnitude was affected by hypoxic intensity, the episodic hypoxia protocol was repeated under three different O2 tensions. We demonstrate that the magnitude and time course of LTF depend on hypoxic severity, with more intense hypoxia inducing a more potent degree of LTF. We conclude that inspiratory activation is not required for LTF induction, and that hypoxia per se is the physiological stimulus for eliciting hypoxia-induced respiratory LTF.

Thyroid status affects 5-HT2A receptor modulation of breathing before, during, and following exposure of hamsters to acute intermittent hypoxia

The BIO 14.6 hamster (dystrophic), animal model of limb girdle muscular dystrophy, exhibits low plasma triiodothyronine levels, muscle weakness, and decreased breathing. After exposure to acute intermittent bouts of hypoxia, dystrophic hamsters depress ventilation relative to baseline resulting in ventilatory long-term depression (LTD). Control hamsters may increase ventilation relative to baseline resulting in ventilatory long-term facilitation (LTF). Serotonin (5-HT) receptors, especially the 5-HT(2A) subtype, are involved in the development of LTF. The purpose of this study was to evaluate the role of 5-HT(2A) receptors in ventilatory and metabolic responses before, during, and following intermittent hypoxia in eleven euthyroid, nine dystrophic, and eleven propylthiouracil (PTU)-induced hypothyroid male hamsters. Animals received subcutaneous injections of vehicle or 0.5 mg/kg MDL (5-HT(2A) receptor antagonist). Plethysmography was used to evaluate ventilatory responses of the three groups to air, five bouts of 5 min of 10% oxygen, each interspersed with 5 min of air, followed by 60 min of exposure to air. CO(2) production was measured using the flow-through method. Vehicle-treated dystrophic and PTU-treated hamsters exhibited LTD. MDL decreased body temperature in all groups. After MDL treatment, the euthyroid group exhibited LTD. MDL treatment in the dystrophic, but not in the PTU-treated hamsters, maintained tidal volume, but did not reverse LTD. CO(2) production was increased in the euthyroid group with MDL treatment. Thus, 5-HT(2A) receptors affect body temperature, ventilation, and metabolism in hamsters. The differential responses noted in this study may be in part dependent on thyroid hormone status.

In hamsters dopamine D2 receptors affect ventilation during and following intermittent hypoxia

We tested the hypothesis that in golden Syrian hamsters (Mesocricetus auratus) carotid body dopaminergic D2 receptors modulate ventilation in air, during exposure to intermittent hypoxia (IH) and reoxygenation. Ventilation was evaluated using the barometric method and CO2 production was determined using the flow through method. Hamsters (n=8) received either subcutaneous injections of vehicle, haloperidol (0.5 mg/kg) or domperidone (0.5 mg/kg). Ventilatory and metabolic variables were determined 30 min following injections, after each of 5 bouts of 5 min of 10% oxygen interspersed by normoxia (IH), and 15, 30, 45 and 60 min following IH when hamsters were exposed to air. Haloperidol, but not domperidone decreased body temperature in hamsters. Neither treatment affected CO2 production. Vehicle-treated hamsters exhibited ventilatory long-term facilitation (VLTF) following IH. Haloperidol or domperidone decreased ventilation in air, during IH and eliminated VLTF due to changes in tidal volume and not frequency of breathing. Thus, in hamsters D2 receptors are involved in control of body temperature and ventilation during and following IH.

Phrenic long-term facilitation is robust to hypercapnia and hypocapnia but not hyperventilatory hypotension under PEEP

Phrenic long-term facilitation (LTF) has been extensively studied in anesthetized animals under well-defined physiological conditions but the factors underlying its possible manifestation under clinically relevant conditions are not well understood. Here, we examine the stability of LTF in the face of hypercapnic or hypocapnic challenges in anesthetized, paralyzed and mechanically ventilated rats. Sixty minutes after induction of phrenic LTF by intermittent hypoxia the animal was exposed to one of four conditions for 5 min with or without positive end-expiratory pressure (PEEP, 3-4 cmH(2)O): hypocapnic apnea, hypocapnia (5 Torr below resting level), 5% CO(2) and 10% CO(2). LTF at 60 min post-intermittent hypoxia was approximately 39% above baseline. Following the above CO(2) tests, LTF almost invariably returned to the corresponding pre-test level after recovery for 20 min. The only exception was the combination of hypocapnic apnea and PEEP, which resulted in a marked decrease in mean arterial pressure (to 38-55mmHg) during test and a subsequent paradoxical sustained attenuation of LTF (to approximately 8% above baseline) even after mean arterial pressure had fully recovered. The results suggest that LTF, once developed, is highly robust to changes in CO(2) levels and is attenuated only after severe hypotension secondary to excessive hyperventilation under PEEP.

Acute intermittent hypoxia increases both phrenic and sympathetic nerve activities in the rat

The respiratory system expresses multiple forms of plasticity, defined as alterations in the breathing pattern that persist or develop after a stimulus. Stimulation of breathing with intermittent hypoxia (IH) elicits long-term facilitation (LTF), a type of plasticity in which respiratory motor activity progressively increases in anaesthetized animals, even after the stimuli have ceased and blood gases have normalized. It is unknown whether the sympathetic nervous system similarly expresses IH-induced plasticity, but we predicted that IH would evoke LTF in sympathetic nerve activity (SNA) because respiratory and sympathetic control systems are coupled. To test this idea, we recorded splanchnic (sSNA) and phrenic nerve activities (PNA) in equithesin-anaesthetized rats. Animals were exposed to 10 45 s episodes of 8% O(2)-92% N(2), separated by 5 min intervals of 100% O(2), and recordings were continued for 60 min following the last hypoxic exposure. Cycle-triggered averages of integrated PNA and sSNA from periods preceding, and 5 and 60 min following the hypoxic stimuli were compared. Intermittent hypoxia significantly increased both sSNA and PNA. Treatment with methysergide (3 mg kg(-1), i.v.) 20 min before the intermittent hypoxic exposures prevented the increases in integrated PNA and sSNA 60 min after IH, indicating a role of serotonergic pathways in this form of plasticity. No increases in PNA and sSNA occurred at comparable times (60 and 120 min) in rats not exposed to hypoxia. The increased sSNA was not simply tonic, but was correlated with respiratory bursts, and occurred predominantly during the first half of expiration. These findings support the hypothesis that sympathorespiratory coupling may underlie the sustained increase in SNA associated with the IH that occurs during sleep apnoea.

Is there a link between intermittent hypoxia-induced respiratory plasticity and obstructive sleep apnoea?

Although neuroplasticity is an important property of the respiratory motor control system, its existence has been appreciated only in recent years and, as a result, its functional significance is not completely understood. The most frequently studied models of respiratory plasticity is respiratory long-term facilitation (LTF) following acute intermittent hypoxia and enhanced LTF following chronic intermittent hypoxia. Since intermittent hypoxia is a prominent feature of sleep-disordered breathing, LTF and/or enhanced LTF may compensate for factors that predispose to sleep-disordered breathing, particularly during obstructive sleep apnoea (OSA). Long-term facilitation has been studied most frequently in rats, and exhibits interesting properties consistent with a role in stabilizing breathing during sleep. Specifically, LTF: (1) is prominent in upper airway respiratory motor activity, suggesting that it stabilizes upper airways and maintains airway patency; (2) is most prominent during sleep in unanaesthetized rats; and (3) exhibits sexual dimorphism (greatest in young male and middle-aged female rats; smallest in middle-aged male and young female rats). Although these features are consistent with the hypothesis that upper airway LTF minimizes the prevalence of OSA in humans, there is little direct evidence for such an effect. Here we review advances in our understanding of LTF and its underlying mechanisms and present evidence concerning a potential role for LTF in maintaining upper airway patency, stabilizing breathing and preventing OSA in humans. Regardless of the relationship between LTF and OSA, a detailed understanding of cellular and synaptic mechanisms that underlie LTF may guide the development of new drugs to regulate upper airway tone, thereby offsetting the tendency for upper airway collapse characteristic of heavy snoring and OSA.

Serotonin-induced in vitro long-term facilitation exhibits differential pattern sensitivity in cervical and thoracic inspiratory motor output

Intermittent hypoxia induces 5-HT-dependent, pattern-sensitive long-term facilitation (LTF) of spinal respiratory motor output. We used a split-bath in vitro neonatal rat brainstem-spinal cord preparation to test whether: 1) intermittent spinal 5-HT exposure (without hypoxia) is sufficient to induce LTF in phrenic and intercostal inspiratory motor outputs; 2) LTF magnitude is greater in intercostal versus phrenic activity; and 3) phrenic and intercostal motor output exhibits differential pattern sensitivity to 5-HT application. With a barrier at spinal segment C1, 5-HT (5 muM) was applied episodically (3 min 5-HT, 5 min wash, x3) to the spinal cord (C2-L1) while recording inspiratory bursts in cervical (C4 or C5) and thoracic (T5 or T6) ventral roots. Episodic 5-HT application increased cervical and thoracic burst amplitudes to 136+/-22% and 150+/-22% of baseline, respectively, at 120 min post-drug (P<0.01). Continuous 5-HT application (5 muM, 9 min) had no effect on cervical burst amplitude at 120 min post-drug, but increased thoracic burst amplitude to 142+/-11% of baseline at 120 min post-drug (P<0.001). Methysergide pretreatment abolished both cervical and thoracic 5-HT-induced LTF. Quantitative reverse transcriptase-polymerase chain reaction and immunocytochemistry revealed that 5-HT(2A) and 5-HT(7) receptor subtypes (receptors known to influence LTF expression in adult rats) are expressed in ventral cervical and thoracic spinal cord with no differences in expression levels due to spinal segment or age. Thus, 5-HT is sufficient to induce spinal LTF in neonatal rats and differences in pattern sensitivity suggest heterogeneity in underlying mechanisms.

Spinal synaptic enhancement with acute intermittent hypoxia improves respiratory function after chronic cervical spinal cord injury

Respiratory insufficiency is the leading cause of death after high-cervical spinal cord injuries (SCIs). Although respiratory motor recovery can occur with time after injury, the magnitude of spontaneous recovery is limited. We hypothesized that partial respiratory motor recovery after chronic cervical SCI could be strengthened using a known stimulus for spinal synaptic enhancement, intermittent hypoxia. Phrenic motor output was recorded before and after intermittent hypoxia from anesthetized, vagotomized, and pump-ventilated control and C2 spinally hemisected rats at 2, 4, and 8 weeks after injury. Weak spontaneous phrenic motor recovery was present in all C2-injured rats via crossed spinal synaptic pathways that convey bulbospinal inspiratory premotor drive to phrenic motoneurons on the side of injury. Intermittent hypoxia augmented crossed spinal synaptic pathways [phrenic long-term facilitation; pLTF] for up to 60 min after hypoxia at 8 weeks, but not 2 weeks, after injury. Ketanserin, a serotonin 2A receptor antagonist, administered before intermittent hypoxia at 8 weeks after injury prevented pLTF. Serotonergic innervation near phrenic motoneurons was assessed after injury. The limited magnitude of pLTF at 2 weeks was associated with an injury-induced reduction in serotonin-containing nerve terminals in the vicinity of phrenic motoneurons ipsilateral to C2 hemisection. Thereafter, pLTF magnitude progressively increased with the recovery of serotonergic innervation in the phrenic motor nucleus. Intermittent hypoxia (or pLTF) has intriguing possibilities as a therapeutic tool, because its greatest efficacy may be in patients with chronic SCI, a time when most patients have already achieved maximal spontaneous functional recovery.

Modulation of hypoglossal motoneuron excitability by intracellular signal transduction cascades

Motoneuronal excitability is highly modulated by various inputs; however, comparatively little is known about postsynaptic signal transduction cascades that affect motoneuron excitability. In this review, we discuss the role of intracellular signaling cascades in the modulation of respiratory motoneuronal excitability. In particular, protein kinases and phosphatases dynamically and constitutively modulate respiratory-modulated inputs to XII motoneurons: (i) activation of protein kinase A (PKA) potentiates both excitatory and inhibitory drive currents; (ii) protein kinase G (PKG) depresses excitatory currents, and (iii) inhibition of protein phosphatases potentiates excitatory drive currents. We also describe a novel form of persistent plasticity (in vitro long-term facilitation; ivLTF) of motoneuronal output. ivLTF is induced by episodic activation of 5-HT(2) or alpha(1)-adrenoreceptors and is manifested as an increase in the amplitude of XII nerve output due to an increase in alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-mediated motoneuronal drive currents. Blockade of Group 1 metabotropic glutamate receptors or protein kinase C (PKC) prevents the induction of ivLTF.

Phrenic long-term facilitation requires NMDA receptors in the phrenic motonucleus in rats

Exposure to episodic hypoxia induces a persistent augmentation of respiratory activity, known as long-term facilitation (LTF). LTF of phrenic nerve activity has been reported to require serotonin receptor activation and protein syntheses. However, the underlying cellular mechanism still remains poorly understood. NMDA receptors play key roles in synaptic plasticity (e.g. some forms of hippocampal long-term potentiation). The present study was designed to examine the role of NMDA receptors in phrenic LTF and test if the relevant receptors are located in the phrenic motonucleus. Integrated phrenic nerve activity was measured in anaesthetized, vagotomized, neuromuscularly blocked and artificially ventilated rats before, during and after three episodes of 5 min isocapnic hypoxia (P(a,O2) = 30-45 mmHg), separated by 5 min hyperoxia (50% O2). Either saline (as control) or the NMDA receptor antagonist MK-801 (0.2 mg kg(-1), i.p.) was systemically injected approximately 1 h before hypoxia. Phrenic LTF was eliminated by the MK-801 injection (vehicle, 32.8 +/- 3.7% above baseline in phrenic amplitude at 60 min post-hypoxia; MK-801, -0.5 +/- 4.1%, means +/- S.E.M.), with little change in both the CO2-apnoeic threshold and the hypoxic phrenic response (HPR). Vehicle (saline, 5 x 100 nl) or MK-801 (10 microM; 5 x 100 nl) was also microinjected into the phrenic motonucleus region in other groups. Phrenic LTF was eliminated by the MK-801 microinjection (vehicle, 34.2 +/- 3.4%; MK-801, -2.5 +/- 2.8%), with minimal change in HPR. Collectively, these results suggest that the activation of NMDA receptors in the phrenic motonucleus is required for the episodic hypoxia-induced phrenic LTF.

Ventilatory long-term facilitation is greater in 1- vs. 2-mo-old awake rats

Respiratory long-term facilitation (LTF) declines in middle-aged vs. adult male rats. Chronic intermittent hypoxia (CIH; 5 min 11–12% O2/5 min air, 12 h/night, 7 nights) enhances LTF in adult rats. However, LTF in immature rats and the effect of early CIH are unevaluated. The present study compared LTF in 1- and 2-mo-old rats and examined the effect of neonatal CIH (initiated at 2 days after birth) on the LTF. Ventilatory LTF, elicited by 5 ( protocol 1) or 10 ( protocol 2) episodes of poikilocapnic hypoxia (5 min 12% O2/5 min air), was measured twice by plethysmography on the same male conscious rat when it was 1 and 2 mo old. In untreated (without CIH) rats, both resting ventilation (54.7 ± 0.6 vs. 43.0 ± 0.2 ml·100 g−1·min−1) and hypoxic ventilatory response (131 ± 4 vs. 66 ± 3% above baseline) were greater in 1- vs. 2-mo-old rats. Protocol 1 elicited LTF in 1-mo-old (12.5 ± 1.0% above baseline) but not 2-mo-old rats. Protocol 2 elicited a greater LTF in 1-mo-old (24.3 ± 0.8%) vs. 2-mo-old rats (18.2 ± 0.5%). In CIH-treated rats, protocol 1 also elicited LTF in 1-mo-old (13.1 ± 1.5%) but not 2-mo-old rats. Protocol 2 elicited LTF in both age groups, but LTF was enhanced by the CIH only in 1-mo-old rats (28.8 ± 0.9%). These results suggest that ventilatory LTF and hypoxic ventilatory response are greater in male rats shortly before their sexual maturity and that the neonatal CIH somewhat enhances ventilatory LTF ∼3 wk after CIH, but this enhancement does not last to adulthood.

Serotonin receptor subtypes involved in vagus nerve stimulation-induced phrenic long-term facilitation in rats

Episodic vagus nerve stimulation (VNS) induces phrenic long-term facilitation (LTF, a persistent augmentation of phrenic nerve activity after the stimulation ends), sensitive to the serotonin 5-HT(1,2,5,6,7) receptor antagonist methysergide and similar to that elicited by episodic hypoxia or carotid sinus nerve stimulation. This study examined the effect of ketanserin (5-HT(2) antagonist) or clozapine (5-HT(2,6,7) antagonist) on VNS-induced LTF in anesthetized, vagotomized, paralyzed and ventilated rats to determine which receptor subtype(s) is involved. Three episodes of 5 min VNS (50 Hz, 0.1 ms, approximately 500 microA) with 5 min intervals elicited phrenic LTF in control (amplitude: 38% above baseline at 60 min post-VNS) and ketanserin (2 mg x kg(-1), i.p.) pre-treated rats (45%), but not clozapine (3 mg x kg(-1)) rats (8%). These data suggest that unlike hypoxia-induced LTF (5-HT(2) receptor-dependent), VNS-induced LTF requires non-5-HT(2) serotonin receptors, perhaps 5-HT(6) and/or 5-HT(7) subtype(s).