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

Adrenergic α₁ receptor activation is sufficient, but not necessary for phrenic long-term facilitation

Acute intermittent hypoxia (AIH; three 5-min hypoxic episodes) causes a form of phrenic motor facilitation (pMF) known as phrenic long-term facilitation (pLTF); pLTF is initiated by spinal activation of Gq protein-coupled 5-HT2 receptors. Because α1 adrenergic receptors are expressed in the phrenic motor nucleus and are also Gq protein-coupled, we hypothesized that α1 receptors are sufficient, but not necessary for AIH-induced pLTF. In anesthetized, paralyzed, and ventilated rats, episodic spinal application of the α1 receptor agonist phenylephrine (PE) elicited dose-dependent pMF (10 and 100 μM, P < 0.05; but not 1 μM). PE-induced pMF was blocked by the α1 receptor antagonist prazosin (1 mM; -20 ± 20% at 60 min, -5 ± 21% at 90 min; n = 6). Although α1 receptor activation is sufficient to induce pMF, it was not necessary for AIH-induced pLTF because intrathecal prazosin (1 mM) did not alter AIH-induced pLTF (56 ± 9% at 60 min, 78 ± 12% at 90 min; n = 9). Intravenous (iv) prazosin (150 μg/kg) appeared to reduce pLTF (21 ± 9% at 60 min, 26 ± 8% at 90 min), but this effect was not significant. Hypoglossal long-term facilitation was unaffected by intrathecal prazosin, but was blocked by iv prazosin (-4 ± 14% at 60 min, -13 ± 18% at 90 min), suggesting different LTF mechanisms in different motor neuron pools. In conclusion, Gq protein-coupled α1 adrenergic receptors evoke pMF, but they are not necessary for AIH-induced pLTF.

Mechanism of sympathetic activation and blood pressure elevation in humans and animals following acute intermittent hypoxia

Sleep apnea is associated with repeated episodes of hypoxemia, causing marked increase in sympathetic nerve activity and blood pressure. Considerable evidence suggests that intermittent hypoxia (IH) resulting from apnea is the primary stimulus for sympathetic overactivity in sleep apnea patients. Several IH protocols have been developed either in animals or in humans to investigate mechanisms underlying the altered autonomic regulation of the circulation. Most of these protocols involve several days (10-40 days) of IH exposure, that is, chronic intermittent hypoxia (CIH). Recent data suggest that a single session of IH exposure, that is, acute intermittent hypoxia (AIH), is already capable of increasing tonic sympathetic nerve output (sympathetic long-term facilitation, LTF) and altering chemo- and baroreflexes with or without elevation of blood pressure. This indicates that IH alters the autonomic neurocirculatory at a very early time point, although the mechanisms underlying this neuroplasticity have not been explored in detail. The purpose of this chapter is to briefly review the effects of AIH on sympathetic LTF and alteration of autonomic reflexes in comparison with the studies from CIH studies. We will also discuss the potential central and peripheral mechanism underlying sympathetic LTF.

Neuronal control of breathing: sex and stress hormones

There is a growing public awareness that hormones can have a significant impact on most biological systems, including the control of breathing. This review will focus on the actions of two broad classes of hormones on the neuronal control of breathing: sex hormones and stress hormones. The majority of these hormones are steroids; a striking feature is that both groups are derived from cholesterol. Stress hormones also include many peptides which are produced primarily within the paraventricular nucleus of the hypothalamus (PVN) and secreted into the brain or into the circulatory system. In this article we will first review and discuss the role of sex hormones in respiratory control throughout life, emphasizing how natural fluctuations in hormones are reflected in ventilatory metrics and how disruption of their endogenous cycle can predispose to respiratory disease. These effects may be mediated directly by sex hormone receptors or indirectly by neurotransmitter systems. Next, we will discuss the origins of hypothalamic stress hormones and their relationship with the respiratory control system. This relationship is 2-fold: (i) via direct anatomical connections to brainstem respiratory control centers, and (ii) via steroid hormones released from the adrenal gland in response to signals from the pituitary gland. Finally, the impact of stress on the development of neural circuits involved in breathing is evaluated in animal models, and the consequences of early stress on respiratory health and disease is discussed.

Spinal 5-HT7 receptors and protein kinase A constrain intermittent hypoxia-induced phrenic long-term facilitation

Phrenic long-term facilitation (pLTF) is a form of serotonin-dependent respiratory plasticity induced by acute intermittent hypoxia (AIH). pLTF requires spinal Gq protein-coupled serotonin-2 receptor (5-HT2) activation, new synthesis of brain-derived neurotrophic factor (BDNF) and activation of its high-affinity receptor, TrkB. Intrathecal injections of selective agonists for Gs protein-coupled receptors (adenosine 2A and serotonin-7; 5-HT7) also induce long-lasting phrenic motor facilitation via TrkB "trans-activation." Since serotonin released near phrenic motor neurons may activate multiple serotonin receptor subtypes, we tested the hypothesis that 5-HT7 receptor activation contributes to AIH-induced pLTF. A selective 5-HT7 receptor antagonist (SB-269970, 5mM, 12 μl) was administered intrathecally at C4 to anesthetized, vagotomized and ventilated rats prior to AIH (3, 5-min episodes, 11% O2). Contrary to predictions, pLTF was greater in SB-269970 treated versus control rats (80 ± 11% versus 45 ± 6% 60 min post-AIH; p<0.05). Hypoglossal LTF was unaffected by spinal 5-HT7 receptor inhibition, suggesting that drug effects were localized to the spinal cord. Since 5-HT7 receptors are coupled to protein kinase A (PKA), we tested the hypothesis that PKA inhibits AIH-induced pLTF. Similar to 5-HT7 receptor inhibition, spinal PKA inhibition (KT-5720, 100 μM, 15 μl) enhanced pLTF (99 ± 15% 60 min post-AIH; p<0.05). Conversely, PKA activation (8-br-cAMP, 100 μM, 15 μl) blunted pLTF versus control rats (16 ± 5% versus 45 ± 6% 60 min post-AIH; p<0.05). These findings suggest a novel mechanism whereby spinal Gs protein-coupled 5-HT7 receptors constrain AIH-induced pLTF via PKA activity.

Hypoxia-induced phrenic long-term facilitation: emergent properties

As in other neural systems, plasticity is a hallmark of the neural system controlling breathing. One spinal mechanism of respiratory plasticity is phrenic long-term facilitation (pLTF) following acute intermittent hypoxia. Although cellular mechanisms giving rise to pLTF occur within the phrenic motor nucleus, different signaling cascades elicit pLTF under different conditions. These cascades, referred to as Q and S pathways to phrenic motor facilitation (pMF), interact via cross-talk inhibition. Whereas the Q pathway dominates pLTF after mild to moderate hypoxic episodes, the S pathway dominates after severe hypoxic episodes. The biological significance of multiple pathways to pMF is unknown. This review will discuss the possibility that interactions between pathways confer emergent properties to pLTF, including pattern sensitivity and metaplasticity. Understanding these mechanisms and their interactions may enable us to optimize intermittent hypoxia-induced plasticity as a treatment for patients that suffer from ventilatory impairment or other motor deficits.

Drug effects on ventilatory control and upper airway physiology related to sleep apnea

Understanding the inter-relationship between pharmacological agents, ventilatory control, upper airway physiology and their consequent effects on sleep-disordered breathing may provide new directions for targeted drug therapy. Where available, this review focuses on human studies that contain both drug effects on sleep-disordered breathing and measures of ventilatory control or upper airway physiology. Many of the existing studies are limited in sample size or comprehensive methodology. At times, the presence of paradoxical findings highlights the complexity of drug therapy for OSA. The existing studies also highlight the importance of considering inter-individual pharmacokinetics and underlying causes of sleep apnea in interpreting drug effects on sleep-disordered breathing. Practical ways to assess an individual's ventilatory control and how it interacts with upper airway physiology is required for future targeted pharmacotherapy in sleep apnea.

Intermittent hypoxia, respiratory plasticity and sleep apnea in humans: present knowledge and future investigations

This review examines the role that respiratory plasticity has in the maintenance of breathing stability during sleep in individuals with sleep apnea. The initial portion of the review considers the manner in which repetitive breathing events may be initiated in individuals with sleep apnea. Thereafter, the role that two forms of respiratory plasticity, progressive augmentation of the hypoxic ventilatory response and long-term facilitation of upper airway and respiratory muscle activity, might have in modifying breathing events in humans is examined. In this context, present knowledge regarding the initiation of respiratory plasticity in humans during wakefulness and sleep is addressed. Also, published findings which reveal that exposure to intermittent hypoxia promotes breathing instability, at least in part, because of progressive augmentation of the hypoxic ventilatory response and the absence of long-term facilitation, are considered. Next, future directions are presented and are focused on the manner in which forms of plasticity that stabilize breathing might be promoted while diminishing destabilizing forms, concurrently. These future directions will consider the potential role of circadian rhythms in the promotion of respiratory plasticity and the role of respiratory plasticity in enhancing established treatments for sleep apnea.

Inactivity-induced phrenic and hypoglossal motor facilitation are differentially expressed following intermittent vs. sustained neural apnea

Reduced respiratory neural activity elicits a rebound increase in phrenic and hypoglossal motor output known as inactivity-induced phrenic and hypoglossal motor facilitation (iPMF and iHMF, respectively). We hypothesized that, similar to other forms of respiratory plasticity, iPMF and iHMF are pattern sensitive. Central respiratory neural activity was reversibly reduced in ventilated rats by hyperventilating below the CO2 apneic threshold to create brief intermittent neural apneas (5, ∼1.5 min each, separated by 5 min), a single brief massed neural apnea (7.5 min), or a single prolonged neural apnea (30 min). Upon restoration of respiratory neural activity, long-lasting (>60 min) iPMF was apparent following brief intermittent and prolonged, but not brief massed, neural apnea. Further, brief intermittent and prolonged neural apnea elicited an increase in the maximum phrenic response to high CO2, suggesting that iPMF is associated with an increase in phrenic dynamic range. By contrast, only prolonged neural apnea elicited iHMF, which was transient in duration (<15 min). Intermittent, massed, and prolonged neural apnea all elicited a modest transient facilitation of respiratory frequency. These results indicate that iPMF, but not iHMF, is pattern sensitive, and that the response to respiratory neural inactivity is motor pool specific.

Cardiorespiratory coupling in health and disease

Cardiac and respiratory activities are intricately linked both functionally as well as anatomically through highly overlapping brainstem networks controlling these autonomic physiologies that are essential for survival. Cardiorespiratory coupling (CRC) has many potential benefits creating synergies that promote healthy physiology. However, when such coupling deteriorates autonomic dysautonomia may ensue. Unfortunately there is still an incomplete mechanistic understanding of both normal and pathophysiological interactions that respectively give rise to CRC and cardiorespiratory dysautonomia. Moreover, there is also a need for better quantitative methods to assess CRC. This review addresses the current understanding of CRC by discussing: (1) the neurobiological basis of respiratory sinus arrhythmia (RSA); (2) various disease states involving cardiorespiratory dysautonomia; and (3) methodologies measuring heart rate variability and RSA.

Sleep apnoea syndrome as a risk for mortality in elderly inpatients

OBJECTIVE

The characteristics of sleep apnoea syndrome (SAS) in the elderly, including subtype classification and association with mortality, have not been fully elucidated. This study examined these factors in an elderly Japanese inpatient population.

METHODS

Overnight polysomnography was used to diagnose and classify SAS in 145 elderly inpatients (mean ± age 81 ± 8 years). Clinical data, including brain computerized tomography findings, were recorded. The study population included nine inpatients with obstructive SAS, 12 with central SAS, 25 with mixed SAS and 99 controls (no SAS).

RESULTS

Increased body mass index and grade of aortic arch calcification independently contributed to risk of all subtypes of SAS combined. There was an independent association between SAS and increased risk of mortality from all causes as well as from pneumonia and from cardiovascular disease. Only mixed SAS was independently and positively associated with increased risk of death from pneumonia.

CONCLUSIONS

Obstructive, central and mixed SAS were associated with increased risk of cardiovascular related and all-cause mortality. Mixed SAS was associated with an increase in mortality from pneumonia. There was no relationship between mortality and severity of SAS.

The impact of arousal state, sex, and sleep apnea on the magnitude of progressive augmentation and ventilatory long-term facilitation

We examined the impact of arousal state, sex, and obstructive sleep apnea (OSA) on the magnitude of progressive augmentation of the hypoxic ventilatory response and ventilatory long-term facilitation (vLTF). We also examined whether exposure to intermittent hypoxia during sleep has an impact on the apnea-hypopnea index (AHI) in individuals with OSA. Ten men and seven women with OSA, along with ten healthy men and ten healthy women, were exposed to twelve 2-min episodes of hypoxia (end-tidal PO(2): 50 Torr) in the presence of sustained hypercapnia (end-tidal PCO(2): 3 Torr above baseline), followed by a 30-min recovery period during wakefulness and sleep. The OSA participants completed an additional sham study during sleep. The AHI during the first hour of sleep following the intermittent hypoxia and sham protocols were compared. Progressive augmentation was only evident during wakefulness and was enhanced in the OSA participants. vLTF was evident during wakefulness and sleep. When standardized to baseline, vLTF was greater during wakefulness and was enhanced in the OSA group (men: wakefulness 1.39 ± 0.08 vs. sleep 1.14 ± 0.03; women: wakefulness 1.35 ± 0.03 vs. sleep 1.16 ± 0.05 fraction of baseline; P ≤ 0.001) compared with control (men: wakefulness 1.19 ± 0.03 vs. sleep 1.09 ± 0.03; women: wakefulness 1.26 ± 0.05 vs. sleep 1.08 ± 0.04 fraction of baseline; P ≤ 0.001). The AHI following exposure to intermittent hypoxia was increased (intermittent hypoxia 72.8 ± 7.3 vs. sham 56.5 ± 7.0 events/h; P ≤ 0.01). Sex-related differences were not observed for the primary measures. We conclude that progressive augmentation is not evident, and the magnitude of vLTF is diminished during sleep compared with wakefulness in men and women. However, when present, the phenomena are enhanced in individuals with OSA. The AHI data indicate that, under the prevailing experimental conditions, vLTF did not serve to mitigate apnea severity.

Spinal vascular endothelial growth factor (VEGF) and erythropoietin (EPO) induced phrenic motor facilitation after repetitive acute intermittent hypoxia

Vascular endothelial growth factor (VEGF) and erythropoietin (EPO) exert neurotrophic and neuroprotective effects in the CNS. We recently demonstrated that VEGF, EPO and their receptors (VEGF-R2, EPO-R) are expressed in phrenic motor neurons, and that cervical spinal VEGF-R2 and EPO-R activation elicit long-lasting phrenic motor facilitation (pMF). Since VEGF, VEGF-R, EPO, and EPO-R are hypoxia-regulated genes, and repetitive exposure to acute intermittent hypoxia (rAIH) up-regulates these molecules in phrenic motor neurons, we tested the hypothesis that 4 weeks of rAIH (10 episodes per day, 3 days per week) enhances VEGF- or EPO-induced pMF. We confirm that cervical spinal VEGF and EPO injections elicit pMF. However, neither VEGF- nor EPO-induced pMF was affected by rAIH pre-conditioning (4 wks). Although our data confirm that spinal VEGF and EPO may play an important role in respiratory plasticity, we provide no evidence that rAIH amplifies their impact. Further experiments with more robust protocols are warranted.

Hypersensitivity of lung vagal C fibers induced by acute intermittent hypoxia in rats: role of reactive oxygen species and TRPA1

Obstructive sleep apnea, manifested by intermittent hypoxia and excess production of reactive oxygen species (ROS) in airways, is associated with hyperreactive airway diseases, but the mechanism remains unclear. Sensitization of lung vagal C fibers (LVCFs) contributes to the airway hypersensitivity. We investigated the mechanisms underlying the sensitization of LVCFs with acute intermittent hypoxia (AIH), by 10 episodes of exposure to 30 s of hypoxic air (0%, 5%, or 10% O(2)) followed by 30 s of room air in anesthetized, open-chest, and artificially ventilated rats. Reflex apneic response to intravenous capsaicin (an LVCF stimulant), as measured by phrenic nerve activity, was concentration dependently augmented by AIH. Similarly, reflex apneic response to intravenous α,β-methylene-ATP (another LVCF stimulant) was augmented by AIH (0% O(2)). The reflex apnea evoked by these two stimulants was abolished by bilateral vagotomy, which suggests the involvement of lung vagal afferents. The AIH-augmented apneic response to these two stimulants was prevented by pretreatment with dimethylthiourea (a hydroxyl radical scavenger), N-acetyl-l-cysteine (an antioxidant) and HC-030031 [a transient receptor potential ankyrin 1 (TRPA1) receptor antagonist]. Consistently, electrophysiological study revealed the afferent responses of LVCFs to capsaicin or α,β-methylene-ATP were augmented by AIH, and this sensitization of LVCFs was prevented by dimethylthiourea, N-acetyl-l-cysteine, and HC-030031. In contrast, AIH did not alter the afferent response of LVCFs to mechanical stimulation by lung hyperinflation. We concluded that AIH sensitizes LVCFs in rats, thus resulting in exaggerated airway reflexogenic responses to chemical stimulants, possibly by ROS action and activation of TRPA1 receptors.

Phrenic long-term facilitation after acute intermittent hypoxia requires spinal ERK activation but not TrkB synthesis

Acute intermittent hypoxia (AIH) elicits a form of spinal respiratory plasticity known as phrenic long-term facilitation (pLTF). pLTF requires spinal serotonin receptor-2 activation, the synthesis of new brain-derived neurotrophic factor (BDNF), and the activation of its high-affinity receptor tyrosine kinase, TrkB. Spinal adenosine 2A receptor activation elicits a distinct pathway to phrenic motor facilitation (pMF); this BDNF synthesis-independent pathway instead requires new synthesis of an immature TrkB isoform. Since hypoxia increases extracellular adenosine levels, we tested the hypothesis that new synthesis of TrkB and BDNF contribute to AIH-induced pLTF. Furthermore, given that signaling mechanisms "downstream" from TrkB are unknown in either mechanism, we tested the hypothesis that pLTF requires MEK/ERK and/or phosphatidylinositol 3-kinase (PI3K)/Akt activation. In anesthetized Sprague-Dawley rats, an intrathecal catheter at cervical level 4 was used to deliver drugs near the phrenic motor nucleus. Since pLTF was blocked by spinal injections of small interfering RNAs targeting BDNF mRNA but not TrkB mRNA, only new BDNF synthesis is required for AIH-induced pLTF. Pretreatment with a MEK inhibitor (U0126) blocked pLTF, whereas a PI3K inhibitor (PI-828) had no effect. Thus, AIH-induced pLTF requires MEK/ERK (not PI3K/AKT) signaling pathways. When U0126 was injected post-AIH, pLTF development was halted but not reversed, suggesting that ERK is critical for the development but not maintenance of pLTF. Thus, there are clear mechanistic distinctions between AIH-induced pLTF (i.e., BDNF synthesis and MEK/ERK dependent) versus adenosine 2A receptor-induced pMF (i.e., TrkB synthesis and PI3K/Akt dependent).

Repetitive acute intermittent hypoxia increases expression of proteins associated with plasticity in the phrenic motor nucleus

Acute intermittent hypoxia (AIH) initiates plasticity in respiratory motor control, including phrenic long term facilitation (pLTF). Since pLTF is enhanced by preconditioning with repetitive exposure to AIH (rAIH), we hypothesized that a rAIH protocol consisting of 3 AIH exposures per week for 10 weeks (3×wAIH; AIH: 10, 5-min episodes of 10.5% O(2); 5-min normoxic intervals) would enhance expression of molecules that play key roles in pLTF within the phrenic motor nucleus. Immunohistochemical analyses revealed that 3×wAIH for 10 weeks increased serotonin terminal density in the C4 phrenic motor nucleus and serotonin 2A (5-HT(2A)) receptor expression in presumptive phrenic motor neurons. Immunoreactive brain derived neurotrophic factor (BDNF) and its high affinity receptor (TrkB) also increased following 3×wAIH. 3×wAIH also increased expression of another hypoxia-sensitive growth factor known to elicit phrenic motor facilitation, vascular endothelial growth factor (VEGF), and its receptor (VEGFR-2). Kinases "downstream" from TrkB and VEGFR-2 were up-regulated in or near presumptive phrenic motor neurons, including phosphorylated extracellular-signal regulated kinase (p-ERK) and protein kinase B (p-AKT). Thus, 3×wAIH up-regulates neurochemicals known to be associated with phrenic motor plasticity. Since 3×wAIH upregulates pro-plasticity molecules without evidence for CNS pathology, it may be a useful therapeutic tool in treating disorders that cause respiratory insufficiency, such as spinal injury or motor neuron disease.

Carotid chemoreceptor development and neonatal apnea

The premature transition from fetal to neonatal life is accompanied by an immature respiratory neural control system. Most preterm infants exhibit recurrent apnea, resulting in repetitive oscillations in O(2) saturation (intermittent hypoxia, IH). Numerous factors are likely to play a role in the etiology of apnea including inputs from the carotid chemoreceptors. Despite major advances in our understanding of carotid chemoreceptor function in the early neonatal period, however, their contribution to the initiation of an apneic event and its eventual termination are still largely speculative. Recent findings have provided a detailed account of the postnatal changes in the incidence of hypoxemic events associated with apnea, and there is anecdotal evidence for a positive correlation with carotid chemoreceptor maturation. Furthermore, studies on non-human animal models have shown that chronic IH sensitizes the carotid chemoreceptors, which has been proposed to perpetuate the occurrence of apnea. An alternative hypothesis is that sensitization of the carotid chemoreceptors could represent an important protective mechanism to defend against severe hypoxemia. The purpose of this review, therefore, is to discuss how the carotid chemoreceptors may contribute to the initiation and termination of an apneic event in the neonate and the use of xanthine therapy in the prevention of apnea.

Long-term facilitation of ventilation following acute continuous hypoxia in awake humans during sustained hypercapnia

In awake humans, long-term facilitation of ventilation (vLTF) following acute intermittent hypoxia (AIH) is only expressed if CO2 is maintained above normocapnic levels. vLTF has not been reported following acute continuous hypoxia (ACH) and it is not known whether this might be unmasked by elevated CO2. Twelve healthy participants completed three trials. In all trials end-tidal pressure of CO2 was elevated 4-5 mmHg above normocapnic levels. During Trial 1 (AIH) participants were exposed to eight 4 min episodes of hypoxia. During Trial 2 (ACH) participants were exposed to continuous hypoxia for 32 min. In Trial 3 (Control) participants were exposed to euoxia throughout. To assess the contribution of the carotid body (CB) in observed ventilatory responses, CB afferent discharge before and after each trial was transiently inhibited with hyperoxia. Minute ventilation ( ˙V E) increased following all trials, but was significantly greater in Trials 1 and 2 when compared with Trial 3 (Trial 1: 4.96 ± 0.87, Trial 2: 5.07 ± 0.7, Trial 3: 2.55 ± 0.98 l min-1, P < 0.05). Hyperoxia attenuated VE to a similar extent in baseline and recovery in all trials (Trial 1: 3.0 ± 0.57 vs. 3.27 ± 0.68, Trial 2: 1.97 ± 0.62 vs. 2.56 ± 0.62, Trial 3: 2.23 ± 0.49 vs. 2.15 ± 0.55 l min-1, P > 0.05). Data are means ± SEM. In awake humans with elevated CO2, ACH evokes a sustained increase in ventilation that is comparable to that evoked by AIH. However, a gradual positive drift in ventilation in response to elevated CO2 accounts for approximately half of this apparent vLTF. Additionally, our data support the view that the CB is not directly involved in maintaining vLTF.

Cervical spinal erythropoietin induces phrenic motor facilitation via extracellular signal-regulated protein kinase and Akt signaling

Erythropoietin (EPO) is typically known for its role in erythropoiesis but is also a potent neurotrophic/neuroprotective factor for spinal motor neurons. Another trophic factor regulated by hypoxia-inducible factor-1, vascular endothelial growth factor (VEGF), signals via ERK and Akt activation to elicit long-lasting phrenic motor facilitation (pMF). Because EPO also signals via ERK and Akt activation, we tested the hypothesis that EPO elicits similar pMF. Using retrograde labeling and immunohistochemical techniques, we demonstrate in adult, male, Sprague Dawley rats that EPO and its receptor, EPO-R, are expressed in identified phrenic motor neurons. Intrathecal EPO at C4 elicits long-lasting pMF; integrated phrenic nerve burst amplitude increased >90 min after injection (63 ± 12% baseline 90 min after injection; p < 0.001). EPO increased phosphorylation (and presumed activation) of ERK (1.6-fold vs controls; p < 0.05) in phrenic motor neurons; EPO also increased pAkt (1.6-fold vs controls; p < 0.05). EPO-induced pMF was abolished by the MEK/ERK inhibitor U0126 [1,4-diamino-2,3-dicyano-1,4-bis(o-aminophenylmercapto)butadiene] and the phosphatidylinositol 3-kinase/Akt inhibitor LY294002 [2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one], demonstrating that ERK MAP kinases and Akt are both required for EPO-induced pMF. Pretreatment with U0126 and LY294002 decreased both pERK and pAkt in phrenic motor neurons (p < 0.05), indicating a complex interaction between these kinases. We conclude that EPO elicits spinal plasticity in respiratory motor control. Because EPO expression is hypoxia sensitive, it may play a role in respiratory plasticity in conditions of prolonged or recurrent low oxygen.

Repetitive intermittent hypoxia induces respiratory and somatic motor recovery after chronic cervical spinal injury

Spinal injury disrupts connections between the brain and spinal cord, causing life-long paralysis. Most spinal injuries are incomplete, leaving spared neural pathways to motor neurons that initiate and coordinate movement. One therapeutic strategy to induce functional motor recovery is to harness plasticity in these spared neural pathways. Chronic intermittent hypoxia (CIH) (72 episodes per night, 7 nights) increases synaptic strength in crossed spinal synaptic pathways to phrenic motoneurons below a C2 spinal hemisection. However, CIH also causes morbidity (e.g., high blood pressure, hippocampal apoptosis), rendering it unsuitable as a therapeutic approach to chronic spinal injury. Less severe protocols of repetitive acute intermittent hypoxia may elicit plasticity without associated morbidity. Here we demonstrate that daily acute intermittent hypoxia (dAIH; 10 episodes per day, 7 d) induces motor plasticity in respiratory and nonrespiratory motor behaviors without evidence for associated morbidity. dAIH induces plasticity in spared, spinal pathways to respiratory and nonrespiratory motor neurons, improving respiratory and nonrespiratory (forelimb) motor function in rats with chronic cervical injuries. Functional improvements were persistent and were mirrored by neurochemical changes in proteins that contribute to respiratory motor plasticity after intermittent hypoxia (BDNF and TrkB) within both respiratory and nonrespiratory motor nuclei. Collectively, these studies demonstrate that repetitive acute intermittent hypoxia may be an effective and non-invasive means of improving function in multiple motor systems after chronic spinal injury.

Severe acute intermittent hypoxia elicits phrenic long-term facilitation by a novel adenosine-dependent mechanism

Acute intermittent hypoxia [AIH; 3, 5-min episodes; 35-45 mmHg arterial PO(2) (Pa(O(2)))] elicits serotonin-dependent phrenic long-term facilitation (pLTF), a form of phrenic motor facilitation (pMF) initiated by G(q) protein-coupled metabotropic 5-HT(2) receptors. An alternate pathway to pMF is induced by G(s) protein-coupled metabotropic receptors, including adenosine A(2A) receptors. AIH-induced pLTF is dominated by the serotonin-dependent pathway and is actually restrained via inhibition from the adenosine-dependent pathway. Here, we hypothesized that severe AIH shifts pLTF from a serotonin-dependent to an adenosine-dependent form of pMF. pLTF induced by severe (25-30 mmHg Pa(O(2))) and moderate (45-55 mmHg Pa(O(2))) AIH were compared in anesthetized rats, with and without intrathecal (C4) spinal A(2A) (MSX-3, 130 ng/kg, 12 μl) or 5-HT receptor antagonist (methysergide, 300 μg/kg, 15 μl) injections. During severe, but not moderate AIH, progressive augmentation of the phrenic response during hypoxic episodes was observed. Severe AIH (78% ± 8% 90 min post-AIH, n = 6) elicited greater pLTF vs. moderate AIH (41% ± 12%, n = 8; P < 0.05). MSX-3 (28% ± 6%; n = 6; P < 0.05) attenuated pLTF following severe AIH, but enhanced pLTF following moderate AIH (86% ± 26%; n = 8; P < 0.05). Methysergide abolished pLTF after moderate AIH (12% ± 5%; n = 6; P = 0.035), but had no effect after severe AIH (66 ± 13%; n = 5; P > 0.05). Thus severe AIH shifts pLTF from a serotonin-dependent to an adenosine-dependent mechanism; the adenosinergic pathway inhibits the serotonergic pathway following moderate AIH. Here we demonstrate a novel adenosine-dependent pathway to pLTF following severe AIH. Shifts in the mechanisms of respiratory plasticity provide the ventilatory control system greater flexibility as challenges that differ in severity are confronted.

Interactions between postnatal sustained hypoxia and intermittent hypoxia in the adulthood to alter brainstem structures and respiratory function

Neural plasticity is defined as a persistent change in the morphology and/or function based on prior experiences. Plasticity is well evident when the triggering experience occurs early in life, but in the case of respiratory control plasticity, it also can be triggered in adult life. We have combined a 10 days postnatal hypoxic (PH) (0-10 days of age;11% O(2)) and a 15 days intermittent hypoxia (IH) exposures in the adulthood (90-105 days of age; 5% O(2), 40 s/20% O(2), 80 s; 8 h/day) to test if early PH interacts with IH of the adulthood to generate detrimental plastic changes. After recording of ventilatory parameters, the brains were studied immunocytochemically for localization of the organization pattern of non-phosphorylated subunit of neurofilament H (NFH) and tyrosine hydroxylase (TH) expression in the nucleus tractus solitarius (Sol) and caudal (CVL) and rostral ventrolateral reticular (RVL) nuclei, areas related to central cardio-respiratory regulation. In comparison to control, PH male rats (but not females) at 1 month of age hyperventilated at rest, in response to moderate hypoxia (12% O(2)) and 5% CO(2), the effect being due to increased tidal volume. At 3.5 months sex differences in ventilation disappeared and it was indistinguishable between control and PH. IH tended to decrease ventilation in both control (C) and PH animals. PH augmented PENH values in air and in hypoxic conditions when compared with C group. IH in both groups, tended to decrease the PENH value, being statistically different in PH+IH. Results also show an increment of disorganization of NFH-positive labeled structures at the level of Sol and CVL/RVL nuclei in PH, IH and HP+HI groups. PH rats showed differences in the number of TH-positive neurons at the level of CVL/RVL nuclei, which was increased in the PH and PH+IH groups with respect to C one. In conclusion, PH alters the central morpho-physiological organization and the catecholaminergic components of cardio-respiratory nuclei, whose effects were enhanced after a period of IH in the adulthood.

Effect of chronic intermittent hypoxia on noradrenergic activation of hypoglossal motoneurons

In obstructive sleep apnea patients, elevated activity of the lingual muscles during wakefulness protects the upper airway against occlusions. A possibly related form of respiratory neuroplasticity is present in rats exposed to acute and chronic intermittent hypoxia (CIH). Since rats exposed to CIH have increased density of noradrenergic terminals and increased α(1)-adrenoceptor immunoreactivity in the hypoglossal (XII) nucleus, we investigated whether these anatomic indexes of increased noradrenergic innervation translate to increased sensitivity of XII motoneurons to noradrenergic activation. Adult male Sprague-Dawley rats were subjected to CIH for 35 days, with O(2) level varying between 24% and 7% with 180-s period for 10 h/day. They were then anesthetized, vagotomized, paralyzed, and artificially ventilated. The dorsal medulla was exposed, and phenylephrine (2 mM, 10 nl) and then the α(1)-adrenoceptor antagonist prazosin (0.2 mM, 3 × 40 nl) were microinjected into the XII nucleus while XII nerve activity (XIIa) was recorded. The area under integrated XIIa was measured before and at different times after microinjections. The excitatory effect of phenylephrine on XII motoneurons was similar in sham- and CIH-treated rats. In contrast, spontaneous XIIa was more profoundly reduced following prazosin injections in CIH- than sham-treated rats [to 21 ± 7% (SE) vs. 40 ± 8% of baseline, P < 0.05] without significant changes in central respiratory rate, arterial blood pressure, or heart rate. Thus, consistent with increased neuroanatomic measures of noradrenergic innervation of XII motoneurons following exposure to CIH, prazosin injections revealed a stronger endogenous noradrenergic excitatory drive to XII motoneurons in CIH- than sham-treated anesthetized rats.

Chronic intermittent hypoxia alters glutamatergic control of sympathetic and respiratory activities in the commissural NTS of rats

Sympathetic overactivity and altered respiratory control are commonly observed after chronic intermittent hypoxia (CIH) exposure. However, the central mechanisms underlying such neurovegetative dysfunctions remain unclear. Herein, we hypothesized that CIH (6% O(2) every 9 min, 8 h/day, 10 days) in juvenile rats alters glutamatergic transmission in the commissural nucleus tractus solitarius (cNTS), a pivotal site for integration of peripheral chemoreceptor inputs. Using an in situ working heart-brain stem preparation, we found that l-glutamate microinjections (1, 3, and 10 mM) into the cNTS of control rats (n = 8) evoked increases in thoracic sympathetic nerve (tSN) and central vagus nerve (cVN) activities combined with inhibition of phrenic nerve (PN) activity. Besides, the ionotropic glutamatergic receptor antagonism with kynurenic acid (KYN; 250 mM) in the cNTS of control group (n = 7) increased PN burst duration and frequency. In the CIH group (n = 10), the magnitude of l-glutamate-induced cVN excitation was smaller, and the PN inhibitory response was blunted (P < 0.05). In addition, KYN microinjections into the cNTS of CIH rats (n = 9) did not alter PN burst duration and produced smaller increases in its frequency compared with controls. Moreover, KYN microinjections into the cNTS attenuated the sympathoexcitatory response to peripheral chemoreflex activation in control but not in CIH rats (P < 0.05). These functional CIH-induced alterations were accompanied by a significant 10% increase of N-methyl-D-aspartate receptor 1 (NMDAR1) and glutamate receptor 2/3 (GluR2/3) receptor subunit density in the cNTS (n = 3-8, P < 0.05), evaluated by Western blot analysis. These data indicate that glutamatergic transmission is altered in the cNTS of CIH rats and may contribute to the sympathetic and respiratory changes observed in this experimental model.

Treatments to restore respiratory function after spinal cord injury and their implications for regeneration, plasticity and adaptation

Spinal cord injury (SCI) often leads to impaired breathing. In most cases, such severe respiratory complications lead to morbidity and death. However, in the last few years there has been extensive work examining ways to restore this vital function after experimental spinal cord injury. In addition to finding strategies to rescue breathing activity, many of these experiments have also yielded a great deal of information about the innate plasticity and capacity for adaptation in the respiratory system and its associated circuitry in the spinal cord. This review article will highlight experimental SCI resulting in compromised breathing, the various methods of restoring function after such injury, and some recent findings from our own laboratory. Additionally, it will discuss findings about motor and CNS respiratory plasticity and adaptation with potential clinical and translational implications.

Impact of repeated daily exposure to intermittent hypoxia and mild sustained hypercapnia on apnea severity

We examined whether exposure to intermittent hypoxia (IH) during wakefulness impacted on the apnea/hypopnea index (AHI) during sleep in individuals with sleep apnea. Participants were exposed to twelve 4-min episodes of hypoxia in the presence of sustained mild hypercapnia each day for 10 days. A control group was exposed to sustained mild hypercapnia for a similar duration. The intermittent hypoxia protocol was completed in the evening on day 1 and 10 and was followed by a sleep study. During all sleep studies, the change in esophageal pressure (ΔPes) from the beginning to the end of an apnea and the tidal volume immediately following apneic events were used to measure respiratory drive. Following exposure to IH on day 1 and 10, the AHI increased above baseline measures (day 1: 1.95 ± 0.42 fraction of baseline, P ≤ 0.01, vs. day 10: 1.53 ± 0.24 fraction of baseline, P < 0.06). The indexes were correlated to the hypoxic ventilatory response (HVR) measured during the IH protocol but were not correlated to the magnitude of ventilatory long-term facilitation (vLTF). Likewise, ΔPes and tidal volume measures were greater on day 1 and 10 compared with baseline (ΔPes: -8.37 ± 0.84 vs. -5.90 ± 1.30 cmH(2)0, P ≤ 0.04; tidal volume: 1,193.36 ± 101.85 vs. 1,015.14 ± 119.83 ml, P ≤ 0.01). This was not the case in the control group. Interestingly, the AHI on day 10 (0.78 ± 0.13 fraction of baseline, P ≤ 0.01) was significantly less than measures obtained during baseline and day 1 in the mild hypercapnia control group. We conclude that enhancement of the HVR initiated by exposure to IH may lead to increases in the AHI during sleep and that initiation of vLTF did not appear to impact on breathing stability. Lastly, our results suggest that repeated daily exposure to mild sustained hypercapnia may lead to a decrease in breathing events.

Postnatal loss of brainstem serotonin neurones compromises the ability of neonatal rats to survive episodic severe hypoxia

Pet-1(-/-) mice with a prenatal, genetically induced loss of 5-hydroxytryptamine (5-HT, serotonin) neurones are compromised in their ability to withstand episodic environmental anoxia via autoresuscitation. Given the prenatal role of 5-HT neurones in the development of neural networks, here we ask if a postnatal loss of 5-HT neurones also compromises autoresuscitation. We treated neonatal rat pups at postnatal day (P)2-3 with an intra-cisternal injection of 5,7-dihydroxytryptamine (5,7-DHT; ~40 μg; n = 8) to pharmacologically lesion the 5-HT system, or vehicle (control; n = 14). At P7-10 we exposed unanaesthetized treated and control pups to 15 episodes of environmental anoxia (97% N(2), 3% CO(2)). Medullary 5-HT content was reduced 80% by 5,7-DHT treatment (P < 0.001). Baseline ventilation (V(E)), metabolic rate (V(O(2))), ventilatory equivalent (V(E)/V(O(2))), heart rate (HR), heart rate variability (HRV) and arterial haemoglobin saturation (S(aO(2))) were no different in 5-HT-deficient pups compared to controls. However, only 25% of 5-HT-deficient pups survived all 15 episodes of environmental anoxia, compared to 79% of control littermates (P = 0.007). High mortality of 5,7-DHT-treated pups was associated with delayed onset of gasping (P < 0.001), delayed recovery of HR from hypoxic-induced bradycardia (P < 0.001), and delayed recovery of eupnoea from hypoxic-induced apnoea (P < 0.001). Treatment with 5,7-DHT affected neither the gasping pattern once initiated, nor HR, V(E)/V(O(2)) or S(aO(2)) during the intervening episodes of room air. A significant increase in HRV occurred in all animals with repeated exposure, and in 5-HT-deficient pups this increase occurred immediately prior to death. We conclude that a postnatal loss of brainstem 5-HT content compromises autoresuscitation in response to environmental anoxia. This report provides new evidence in rat pups that 5-HT neurones serve a physiological role in autoresuscitation. Our data may be relevant to understanding the aetiology of the sudden infant death syndrome (SIDS), in which there is medullary 5-HT deficiency and in some cases evidence of severe hypoxia and failed autoresuscitation.

Acute intermittent hypoxia induces phrenic long-term facilitation which is modulated by 5-HT1A receptor in the caudal raphe region of the rat

Obstructive sleep apnoea (OSA) is characterized by periods of upper airway collapse accompanied by repeated episodes of hypoxia. In experimental animals repeated bouts of hypoxia may evoke sustained augmentation of phrenic nerve activity, known as phrenic long-term facilitation (pLTF). This form of physiological compensation might contribute to stable breathing, minimizing the occurrence of apnoeas and/or hypopnoeas during sleep in patients with OSA. Serotonin (5-HT) has been shown to modulate respiratory neuronal activity, possibly via projections originating in the raphe nuclei. Our model focuses on the effects of 5-HT1A receptors blockade by selective antagonist WAY-100635 into the caudal raphe region on phrenic long-term facilitation after exposure to acute intermittent hypoxia (AIH) episodes. Adult, male, urethane-anaesthetized, vagotomized, paralyzed and mechanically ventilated Sprague-Dawley rats were exposed to AIH protocol. Experimental group received microinjection of WAY-100635 into the caudal raphe nucleus, whereas the control group received saline into the same site. Peak phrenic nerve activity and respiratory rhythm parameters were analysed during five hypoxic episodes, as well as at 15, 30 and 60 min after the end of hypoxias. In the control group, 1 h post-hypoxia pLTF was developed. Microinjections of selective 5-HT1A receptor antagonist WAY-100635 into the raphe nuclei prior to the AIH protocol prevented induction of pLTF. These results suggest that 5-HT1A receptor activation at supraspinal level is important for induction of pLTF, which is suggested to be an important respiratory neuroplasticity model in animal studies that possibly correlates with OSA in humans.

Exposure to acute intermittent hypoxia augments somatic motor function in humans with incomplete spinal cord injury

BACKGROUND

Neural plasticity may contribute to motor recovery following spinal cord injury (SCI). In rat models of SCI with respiratory impairment, acute intermittent hypoxia (AIH) strengthens synaptic inputs to phrenic motor neurons, thereby improving respiratory function by a mechanism known as respiratory long-term facilitation. Similar intermittent hypoxia-induced facilitation may be feasible in somatic motor pathways in humans.

OBJECTIVE

Using a randomized crossover design, the authors tested the hypothesis that AIH increases ankle strength in people with incomplete SCI.

METHODS

Ankle strength was measured in 13 individuals with chronic, incomplete SCI before and after AIH. Voluntary ankle strength was estimated using changes in maximum isometric ankle plantar flexion torque generation and plantar flexor electromyogram activity following 15 low oxygen exposures (Fio(2) = 0.09, 1-minute intervals). Results were compared with trials where subjects received sham exposure to room air.

RESULTS

AIH increased plantar flexion torque by 82 ± 33% (P < .003) immediately following AIH and was sustained above baseline for more than 90 minutes (P < .007). Increased ankle plantar flexor electromyogram activity (P = .01) correlated with increased torque (r(2) = .5; P < .001). No differences in plantar flexion strength or electromyogram activity were observed in sham experiments.

CONCLUSIONS

AIH elicits sustained increases in volitional somatic motor output in persons with chronic SCI. Thus, AIH has promise as a therapeutic tool to induce plasticity and enhance motor function in SCI patients.

Spinal vascular endothelial growth factor induces phrenic motor facilitation via extracellular signal-regulated kinase and Akt signaling

Although vascular endothelial growth factor (VEGFA-165) is primarily known for its role in angiogenesis, it also plays important neurotrophic and neuroprotective roles for spinal motor neurons. VEGFA-165 signals by activating its receptor tyrosine kinase VEGF receptor-2 (VEGFR-2). Because another growth/trophic factor that signals via a receptor tyrosine kinase (brain derived neurotrophic factor) elicits a long-lasting facilitation of respiratory motor activity in the phrenic nerve, we tested the hypothesis that VEGFA-165 elicits similar phrenic motor facilitation (pMF). Using immunohistochemistry and retrograde labeling techniques, we demonstrate that VEGFA-165 and VEGFR-2 are expressed in identified phrenic motor neurons. Furthermore, intrathecal VEGFA-165 administration at C4 elicits long-lasting pMF; intraspinal VEGFA-165 increased integrated phrenic nerve burst amplitude for at least 90 min after injection (53.1 ± 5.0% at 90 min; p < 0.001). Intrathecal VEGFA-165 increased phosphorylation (and presumed activation) of signaling molecules downstream from VEGFR-2 within the phrenic motor nucleus, including ERK (1.53 ± 0.13 vs 1.0 ± 0.05 arbitrary units in control rats; p < 0.05) and Akt (2.16 ± 0.41 vs 1.0 ± 0.41 arbitrary units in control rats; p < 0.05). VEGF-induced pMF was attenuated by the MEK/ERK inhibitor U0126 [1,4-diamino-2,3-dicyano-1,4-bis(o-aminophenylmercapto)butadiene] and was abolished by the phosphotidinositol 3 kinase/Akt inhibitor LY294002 [2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride], demonstrating that ERK mitogen-activated protein kinases and Akt are both required for full expression of VEGF-induced pMF. This is the first report that VEGFA-165 elicits plasticity in any motor system. Furthermore, because VEGFA-165 expression is hypoxia sensitive, it may play a role in respiratory plasticity after prolonged exposures to low oxygen.

Similarities and differences in mechanisms of phrenic and hypoglossal motor facilitation

Intermittent hypoxia-induced long-term facilitation (LTF) is variably expressed in the motor output of several inspiratory nerves, such as the phrenic and hypoglossal. Compared to phrenic LTF (pLTF), less is known about hypoglossal LTF (hLTF), although it is often assumed that cellular mechanisms are the same. While fundamental mechanisms appear to be similar, potentially important differences exist in the modulation of pLTF and hLTF. The primary objectives of this paper are to: (1) review similarities and differences in pLTF and hLTF, pointing out knowledge gaps and (2) present new data suggesting that reduced respiratory neural activity elicits differential plasticity in phrenic and hypoglossal output (inactivity-induced phrenic and hypoglossal motor facilitation, iPMF and iHMF), suggesting that these motor pool-specific differences are not unique to LTF. Differences in fundamental mechanisms or modulation of plasticity among motor pools may confer the capacity to mount a complex ventilatory response to specific challenges, particularly in motor pools with different "jobs" in the control of breathing.

Systemic inflammation impairs respiratory chemoreflexes and plasticity

Many lung and central nervous system disorders require robust and appropriate physiological responses to assure adequate breathing. Factors undermining the efficacy of ventilatory control will diminish the ability to compensate for pathology, threatening life itself. Although most of these same disorders are associated with systemic and/or neuroinflammation, and inflammation affects neural function, we are only beginning to understand interactions between inflammation and any aspect of ventilatory control (e.g. sensory receptors, rhythm generation, chemoreflexes, plasticity). Here we review available evidence, and present limited new data suggesting that systemic (or neural) inflammation impairs two key elements of ventilatory control: chemoreflexes and respiratory motor (versus sensory) plasticity. Achieving an understanding of mechanisms whereby inflammation undermines ventilatory control is fundamental since inflammation may diminish the capacity for natural, compensatory responses during pathological states, and the ability to harness respiratory plasticity as a therapeutic strategy in the treatment of devastating breathing disorders, such as during cervical spinal injury or motor neuron disease.

Diaphragm long-term facilitation following acute intermittent hypoxia during wakefulness and sleep

Acute intermittent hypoxia (AIH) elicits a form of respiratory plasticity known as long-term facilitation (LTF). Here, we tested four hypotheses in unanesthetized, spontaneously breathing rats using radiotelemetry for EEG and diaphragm electromyography (Dia EMG) activity: 1) AIH induces LTF in Dia EMG activity; 2) diaphragm LTF (Dia LTF) is more robust during sleep vs. wakefulness; 3) AIH (or repetitive AIH) disrupts natural sleep-wake architecture; and 4) preconditioning with daily AIH (dAIH) for 7 days enhances Dia LTF. Sleep-wake states and Dia EMG were monitored before (60 min), during, and after (60 min) AIH (10, 5-min hypoxic episodes, 5-min normoxic intervals; n = 9), time control (continuous normoxia, n = 8), and AIH following dAIH preconditioning for 7 days (n = 7). Dia EMG activities during quiet wakefulness (QW), rapid eye movement (REM), and non-REM (NREM) sleep were analyzed and normalized to pre-AIH values in the same state. During NREM sleep, diaphragm amplitude (25.1 ± 4.6%), frequency (16.4 ± 4.7%), and minute diaphragm activity (amplitude × frequency; 45.2 ± 6.6%) increased above baseline 0-60 min post-AIH (all P < 0.05). This Dia LTF was less robust during QW and insignificant during REM sleep. dAIH preconditioning had no effect on LTF (P > 0.05). We conclude that 1) AIH induces Dia LTF during NREM sleep and wakefulness; 2) Dia LTF is greater in NREM sleep vs. QW and is abolished during REM sleep; 3) AIH and repetitive AIH disrupt natural sleep patterns; and 4) Dia LTF is unaffected by dAIH. The capacity for plasticity in spinal pump muscles during sleep and wakefulness suggests an important role in the neural control of breathing.

Obstructive sleep apnea and type 2 diabetes

Type 2 diabetes and obstructive sleep apnea (OSA) are diseases with high prevalence and major public health impact. There is evidence that regular snoring and OSA are independently associated with alterations in glucose metabolism. Thus, OSA might be a risk factor for the development of type 2 diabetes. Possible causes might be intermittent hypoxia and sleep fragmentation, which are typical features of OSA. OSA might also be a reason of ineffective treatment of type 2 diabetes. There is further evidence that the treatment of OSA by continuous positive airway pressure (CPAP) therapy might correct metabolic abnormalities in glucose metabolism. It is assumed that this depends on therapy compliance to CPAP. On the other hand, there are also hints in the literature that type 2 diabetes per se might induce sleep apnea, especially in patients with autonomic neuropathy. Pathophysiological considerations open up new insights into that problem. Based on the current scientific data, clinicians have to be aware of the relations between the two diseases, both from the sleep medical and the diabetological point of view. The paper summarizes the most important issues concerning the different associations of OSA and type 2 diabetes.

5-HT induces enhanced phrenic nerve activity via 5-HT(2A) receptor/PKC mechanism in anesthetized rats

Respiratory behavior expresses diverse forms of plasticity by altering breathing patterns. Failure of respiratory neuroplasticity often leads to malfunctions. Long-term facilitation (LTF), the most frequently studied model induced by episodic hypoxia to produce long-lasting enhancement of phrenic motor output, is thought to be serotonin 2A (5-HT(2A)) receptor-dependent. Previous studies have described 5-HT-induced prompt apnea in intact animals. However, the role of exogenous 5-HT in mediating respiratory neuroplasticity is less attended in vivo study. We hypothesized that an in vivo 5-HT challenge contributes to respiratory neuroplasticity. Here, we found that systemic bolus administration of 5-HT exerted an initial transient inhibition followed by marked facilitation, forming a biphasic pattern of phrenic nerve activity in artificially ventilated, midcervically vagotomized, and anesthetized adult rats. The facilitatory phase corresponded to the enhanced phrenic nerve activity that lasted for at least one hour after drug exposure, characterized as phrenic LTF (pLTF). The 5-HT-induced biphasic pattern and pLTF were 5-HT(2A) receptor-dependent and coupled to protein kinase C (PKC) activation. The initial inhibition of phrenic nerve activity was found to be nodose ganglion-associated, whereas the subsequent facilitation was carotid body-associated, establishing a peripheral inhibitory-facilitatory afferent balance. Immunoreactive expressions of 5-HT/5-HT(2A) receptors and phospho-PKC isoforms/PKC substrate provide morphological evidence of existence of a 5-HT/5-HT(2A) receptor/PKC mechanism in the nodose ganglion and the carotid body. We speculate that 5-HT challenge in vivo may contribute to respiratory neuroplasticity, to yield pLTF or augmented pLTF in animals with reduced or absent peripheral inhibitory inputs.

Experimental protocols and preparations to study respiratory long term facilitation

Respiratory long-term facilitation is a form of neuronal plasticity that is induced following exposure to intermittent hypoxia. Long-term facilitation is characterized by a progressive increase in respiratory motor output during normoxic periods that separate hypoxic episodes and by a sustained elevation in respiratory activity for up to 90min after exposure to intermittent hypoxia. This phenomenon is associated with increases in phrenic, hypoglossal or carotid sinus nerve inspiratory-modulated discharge. The examination of long-term facilitation has been steadily ongoing for approximately 3 decades. During this period of time a variety of animal models (e.g. cats, rats and humans), experimental preparations and intermittent hypoxia protocols have been used to study long-term facilitation. This review is designed to summarize the strengths and weaknesses of the models, preparations and protocols that have been used to study LTF over the past 30 years. The review is divided into two primary sections. Initially, the models and protocols used to study LTF in animals other than humans will be discussed, followed by a section specifically focused on human studies. Each section will begin with a discussion of various factors that must be considered when selecting an experimental preparation and intermittent hypoxia protocol to examine LTF. Model and protocol design recommendations will follow, with the goal of presenting a prevailing model and protocol that will ultimately ensure standardized comparisons across studies.

Spinal 5-HT7 receptor activation induces long-lasting phrenic motor facilitation

Acute intermittent hypoxia elicits a form of serotonin-dependent respiratory plasticity known as phrenic long term facilitation (pLTF). Episodic spinal serotonin-2 (5-HT2) receptor activation on or near phrenic motor neurons is necessary for pLTF. A hallmark of pLTF is the requirement for serotonin-dependent synthesis of brain-derived neurotrophic factor (BDNF), and activation of its high affinity receptor, TrkB. Activation of spinal Gs protein-coupled adenosine 2A receptors (GsPCRs) elicits a unique form of long-lasting phrenic motor facilitation (PMF), but via unique mechanisms (BDNF independent TrkB trans-activation).We hypothesized that other GsPCRs elicit PMF, specifically serotonin-7 (5-HT7) receptors, which are expressed in phrenic motor neurons. Cervical spinal (C4) injections of a selective 5-HT7 receptor agonist, AS-19 (10 μM, 5 μl; 3 × 5 min), in anaesthetized, vagotomized and ventilated male Sprague-Dawley rats elicited long-lasting PMF (>120 min), an effect prevented by pretreatment with a 5-HT7 receptor antagonist (SB 269970; 5mM, 7 μl).GsPCR activation 'trans-activates'TrkB by increasing synthesis of an immature TrkB isoform. Spinal injection of a TrkB inhibitor (k252a) and siRNAs that prevent TrkB (but not BDNF) mRNA translation both blocked 5-HT7 agonist-induced PMF, confirming a requirement for TrkB synthesis and activity. k252a affected late PMF (≥ 90 min) only. Spinal inhibition of the PI3K/AKT pathway blocked 5-HT7 agonist-induced PMF, whereas MEK/ERK inhibition delayed, but did not block, PMF. An understanding of signalling mechanisms giving rise to PMF may guide development of novel therapeutic strategies to treat ventilatory control disorders associated with respiratory insufficiency, such as spinal injury and motor neuron disease.

The hypoxic ventilatory response and ventilatory long-term facilitation are altered by time of day and repeated daily exposure to intermittent hypoxia

This study examined whether time of day and repeated exposure to intermittent hypoxia have an impact on the hypoxic ventilatory response (HVR) and ventilatory long-term facilitation (vLTF). Thirteen participants with sleep apnea were exposed to twelve 4-min episodes of isocapnic hypoxia followed by a 30-min recovery period each day for 10 days. On days 1 (initial day) and 10 (final day) participants completed the protocol in the evening (PM); on the remaining days the protocol was completed in the morning (AM). The HVR was increased in the morning compared with evening on the initial (AM 0.83 ± 0.08 vs. PM 0.64 ± 0.11 l·min⁻¹·%SaO₂⁻¹; P ≤ 0.01) and final days (AM 1.0 ± 0.08 vs. PM 0.81 ± 0.09 l·min⁻¹·%SaO₂⁻¹; P ≤ 0.01, where %SaO₂ refers to percent arterial oxygen saturation). Moreover, the magnitude of the HVR was enhanced following daily exposure to intermittent hypoxia in the morning (initial day 0.83 ± 0.08 vs. final day 1.0 ± 0.08 l·min⁻¹·%SaO₂⁻¹; P ≤ 0.03) and evening (initial day 0.64 ± 0.11 vs. final day 0.81 ± 0.09 l·min⁻¹·%SaO₂⁻¹; P ≤ 0.03). vLTF was reduced in the morning compared with the evening on the initial (AM 19.03 ± 0.35 vs. PM 22.30 ± 0.49 l/min; P ≤ 0.001) and final (AM 20.54 ± 0.32 vs. PM 23.11 ± 0.54 l/min; P ≤ 0.01) days. Following daily exposure to intermittent hypoxia, vLTF was enhanced in the morning (initial day 19.03 ± 0.35 vs. final day 20.54 ± 0.32 l/min; P ≤ 0.01). We conclude that the HVR is increased while vLTF is decreased in the morning compared with the evening in individuals with sleep apnea and that the magnitudes of these phenomena are enhanced following daily exposure to intermittent hypoxia.

Short- and long-term modulation of the exercise ventilatory response

The importance of adaptive control strategies (modulation and plasticity) in the control of breathing during exercise has become recognized only in recent years. In this review, we discuss new evidence for modulation of the exercise ventilatory response in humans, specifically, short- and long-term modulation. Short-term modulation is proposed to be an important regulatory mechanism that helps maintain blood gas homeostasis during exercise.

Carotid chemoafferent activity is not necessary for all phrenic long-term facilitation following acute intermittent hypoxia

Phrenic long-term facilitation (pLTF) is a form of respiratory plasticity induced by acute intermittent hypoxia (AIH) or episodic carotid chemoafferent neuron activation. Surprisingly, residual pLTF is expressed in carotid denervated rats. However, since carotid denervation eliminates baroreceptor feedback and causes profound hypotension during hypoxia in anesthetized rats, potential contributions of these uncontrolled factors or residual chemoafferent neuron activity to residual pLTF cannot be ruled out. Since ATP is necessary for hypoxic carotid chemotransduction, we tested the hypothesis that functional peripheral chemoreceptor denervation (with intact baroreceptors) via systemic P2X receptor antagonism blocks hypoxic phrenic responses and AIH-induced pLTF in anesthetized rats. Pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS; 100 mg/kg i.v.), a non-selective P2X receptor antagonist, was administered to anesthetized, vagotomized, paralyzed and ventilated male Sprague-Dawley rats prior to AIH (3, 5 min episodes of 10% O(2); 5 min intervals). Although PPADS strongly attenuated the short-term hypoxic phrenic response (20 ± 4% vs. 113 ± 15% baseline; P < 0.001), pLTF was reduced but not eliminated 60 min post-AIH (25 ± 4% vs. 51 ± 11% baseline; n = 8 and 7, respectively; P < 0.002). Thus, AIH initiates residual pLTF out of proportion to the diminished hypoxic phrenic response and chemoafferent neuron activation. Although the mechanism of residual pLTF following functional chemo-denervation remains unclear, possible mechanisms involving direct effects of hypoxia on the CNS are discussed.

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.

Caffeine reduces apnea frequency and enhances ventilatory long-term facilitation in rat pups raised in chronic intermittent hypoxia

The mechanisms underlying the therapeutic function of caffeine on apneas in preterm neonates are not well determined. To better understand these effects, we exposed rat pups from postnatal d 3-12 to chronic intermittent hypoxia (5% O2/100 s every 10 min; 6 cycles/h followed by 1 h at 21% O2, 24 h/d), a model mimicking hypoxemic exposure in apneic neonates. Then, using whole-body plethysmography, we evaluated minute ventilation, apnea frequency, and duration after i.p injection of caffeine citrate (20 mg/kg) or saline under normoxia and in response to either sustained (FiO2 12%, 20 min) or brief (FiO2 5%, 60 s, total 10 episodes of 8 min each) hypoxia. These tests were used to assess peripheral and central components of hypoxic response. The latter also assessed the ventilatory long-term facilitation during recovery (2 h). Caffeine injection increased minute ventilation under baseline and during recovery. This effect was correlated with a decrease in apnea frequency (not duration). On the contrary, caffeine did not change the ventilatory response to sustained or brief hypoxic exposure. These results suggest that the effects of caffeine on apnea depend on increased central normoxic respiratory drive and enhancement of ventilatory long-term facilitation rather than on higher hypoxic ventilatory response.

Carotid sinus nerve stimulation, but not intermittent hypoxia, induces respiratory LTF in adult rats exposed to neonatal intermittent hypoxia

We tested the hypothesis that exposure to neonatal intermittent hypoxia (n-IH) in rat pups alters central integrative processes following acute and intermittent peripheral chemoreceptor activation in adults. Newborn male rats were exposed to n-IH or normoxia for 10 consecutive days after birth. We then used both awake and anesthetized 3- to 4-mo-old rats to record ventilation, blood pressure, and phrenic and splanchnic nerve activities to assess responses to peripheral chemoreflex activation (acute hypoxic response) and long-term facilitation (LTF, long-term response after intermittent hypoxia). In anesthetized rats, phrenic and splanchnic nerve activities and hypoxic responses were also recorded with or without intact carotid body afferent signal (bilateral chemodenervation) or in response to electrical stimulations of the carotid sinus nerve. In awake rats, n-IH alters the respiratory pattern (higher frequency and lower tidal volume) and increased arterial blood pressure in normoxia, but the ventilatory response to repeated hypoxic cycles was not altered. In anesthetized rats, phrenic nerve responses to repeated hypoxic cycles or carotid sinus nerve stimulation were not altered by n-IH; however, the splanchnic nerve response was suppressed by n-IH compared with control. In control rats, respiratory LTF was apparent in anesthetized but not in awake animals. In n-IH rats, respiratory LTF was not apparent in awake and anesthetized animals. Following intermittent electrical stimulation, however, phrenic LTF was clearly present in n-IH rats, being similar in magnitude to controls. We conclude that, in adult n-IH rats: 1) arterial blood pressure is elevated, 2) peripheral chemoreceptor responses to hypoxia and its central integration are not altered, but splanchnic nerve response is suppressed, 3) LTF is suppressed, and 4) the mechanisms involved in the generation of LTF are still present but are masked most probably as the result of an augmented inhibitory response to hypoxia in the central nervous system.

Acute intermittent hypoxia in rat in vivo elicits a robust increase in tonic sympathetic nerve activity that is independent of respiratory drive

Acute intermittent hypoxia (AIH) elicits long-term increases in respiratory and sympathetic outflow (long-term facilitation, LTF). It is still unclear whether sympathetic LTF is totally dependent on changes in respiration, even though respiratory drive modulates sympathetic nerve activity (SNA). In urethane-anaesthetized, vagotomized mechanically ventilated Sprague-Dawley rats, we investigated the effect of ten 45 s episodes of 10% O2-90% N(2) on splanchnic sympathetic nerve activity (sSNA) and phrenic nerve activity (PNA). We then tested whether or not hypoxic sympathetic chemoreceptor and baroreceptor reflexes were changed 60 min after AIH. We found that 17 animals manifested a sustained increase of sSNA (+51.2+/-4.7%) 60 min after AIH, but only 10 of these rats also expressed phrenic LTF compared with the time controls (rats not exposed to hypoxia, n=5). Inspiratory triggered averages of integrated sSNA showed respiratory modulation of SNA regardless of whether or not phrenic LTF had developed. The hypoxic chemoreceptor reflex was enhanced by 60 min after the development of AIH (peak change from 76.9+/-13.9 to 159.5+/-24.9%). Finally, sympathetic baroreceptor reflex sensitivity increased after sympathetic LTF was established (Gainmax from 1.79+/-0.18 to 2.60+/-0.28% mmHg1). Our findings indicate that respiratory-sympathetic coupling does contribute to sympathetic LTF, but that an additional tonic increase of sympathetic tone is also present that is independent of the level of PNA. Sympathetic LTF is not linked to the change in baroreflex function, since the baroreflex appears to be enhanced rather than impaired, but does play an important role in the enhancement of the hypoxic chemoreflex.

Spinal plasticity following intermittent hypoxia: implications for spinal injury

Plasticity is a fundamental property of the neural system controlling breathing. One frequently studied model of respiratory plasticity is long-term facilitation of phrenic motor output (pLTF) following acute intermittent hypoxia (AIH). pLTF arises from spinal plasticity, increasing respiratory motor output through a mechanism that requires new synthesis of brain-derived neurotrophic factor, activation of its high-affinity receptor, tropomyosin-related kinase B, and extracellular-related kinase mitogen-activated protein kinase signaling in or near phrenic motor neurons. Because intermittent hypoxia induces spinal plasticity, we are exploring the potential to harness repetitive AIH as a means of inducing functional recovery in conditions causing respiratory insufficiency, such as cervical spinal injury. Because repetitive AIH induces phenotypic plasticity in respiratory motor neurons, it may restore respiratory motor function in patients with incomplete spinal injury.

Atypical protein kinase C expression in phrenic motor neurons of the rat

Atypical protein kinase C (PKC) isoforms play important roles in many neural processes, including synaptic plasticity and neurodegenerative diseases. Although atypical PKCs are expressed throughout the brain, there are no reports concerning their expression in central neural regions associated with respiratory motor control. Therefore, we explored the neuroanatomical distribution of atypical PKCs in identified phrenic motor neurons, a motor pool that plays a key role in breathing. Diaphragm injections of cholera toxin B were used to retrogradely label and identify phrenic motor neurons; immunohistochemistry was used to localize atypical PKCs in and near labeled motor neurons (i.e. the phrenic motor nucleus). Atypical PKC expression in the phrenic motor nucleus appears specific to neurons; aPKC expression could not be detected in adjacent astrocytes or microglia. Strong atypical PKC labeling was observed within cholera toxin B labeled phrenic motor neurons. Documenting the expression of atypical PKCs in phrenic motor neurons provides a framework within which to assess their role in respiratory motor control, including novel forms of respiratory plasticity known to occur in this region.

Urethane inhibits genioglossal long-term facilitation in un-paralyzed anesthetized rats

For approximately 3 decades, urethane has been (partially or solely) used as a successful anesthetic in numerous respiratory long-term facilitation (LTF) studies, which were performed on anesthetized, paralyzed, vagotomized and artificially ventilated animals of several different species. However, things become complicated when LTF of muscle activity is studied in un-paralyzed animals. For example, a commonly used acute intermittent hypoxia (AIH) protocol failed to induce muscle LTF in anesthetized, spontaneously breathing rats. But muscle LTF could be induced when hypoxic episode number was increased and/or anesthetics other than urethane were used. In these studies however, neither anesthetic nor paralysis was mentioned as a potential factor influencing AIH-induced muscle LTF. This study tested whether urethane inhibits AIH-induced genioglossal LTF (gLTF) in un-paralyzed ventilated rats, and if so, determined whether reducing urethane dose reverses this inhibition. Three groups of adult male Sprague-Dawley rats were anesthetized (Group 1: approximately 1.6 g kg(-1) urethane; Group 2: 50 mg kg(-1) alpha-chloralose +0.9-1.2 g kg(-1) urethane; Group 3: 0.9 g kg(-1) urethane +200-400 microg kg(-1) min(-1) alphaxalone), vagotomized and mechanically ventilated. Integrated genioglossus activity was measured before, during and after AIH (5 episodes of 3-min isocapnic 12% O(2), separated by 3-min hyperoxic intervals). The AIH-induced gLTF was absent in Group 1 rats (success rate was only approximately 1/7), but was present in Group 2 (in 10/12 rats) and Group 3 (in 11/11 rats) rats. The genioglossal response to hypoxia was not significantly different among the 3 groups. Collectively, these data suggest that urethane dose-dependently inhibits gLTF in un-paralyzed anesthetized rats.