Respiratory plasticity following intermittent hypoxia: roles of protein phosphatases and reactive oxygen species

Intermittent but not sustained moderate hypoxia elicits long-term facilitation of hypoglossal motor output

Phrenic long-term facilitation (pLTF) is a form of serotonin-dependent respiratory motor plasticity induced by moderate acute intermittent hypoxia (AIH), but not by moderate acute sustained hypoxia (ASH) of similar cumulative duration. Thus, moderate AIH-induced pLTF is sensitive to the pattern of hypoxia. On the other hand, pLTF induced by severe AIH protocols is neither pattern sensitive nor serotonin dependent (it converts to an adenosine-dependent mechanism). Although moderate AIH also induces hypoglossal LTF (hLTF), no data are available concerning its sensitivity/insensitivity to the pattern of hypoxia. Since hLTF following moderate hypoxia is serotonin-dependent, we hypothesized that hLTF is pattern-sensitive, similar to serotonin-dependent pLTF. Integrated hypoglossal nerve activity was recorded in urethane-anesthetized, vagotomized, paralyzed, and ventilated rats exposed to isocapnic AIH (3, 5min episodes of 11% O₂) or ASH (a single 25min episode of 11% O₂). Similar to previous studies of pLTF, hypoglossal motor output was elevated for more than 1h following AIH (50±20%, p<0.01), but not ASH (-6±9%, p>0.05). Frequency LTF was not observed following either hypoxic exposure. Thus, in agreement with our hypothesis, hypoglossal LTF following moderate AIH is pattern-sensitive, similar to phrenic LTF.

Reactive Oxygen Species: Physiological and Physiopathological Effects on Synaptic Plasticity

In the mammalian central nervous system, reactive oxygen species (ROS) generation is counterbalanced by antioxidant defenses. When large amounts of ROS accumulate, antioxidant mechanisms become overwhelmed and oxidative cellular stress may occur. Therefore, ROS are typically characterized as toxic molecules, oxidizing membrane lipids, changing the conformation of proteins, damaging nucleic acids, and causing deficits in synaptic plasticity. High ROS concentrations are associated with a decline in cognitive functions, as observed in some neurodegenerative disorders and age-dependent decay of neuroplasticity. Nevertheless, controlled ROS production provides the optimal redox state for the activation of transductional pathways involved in synaptic changes. Since ROS may regulate neuronal activity and elicit negative effects at the same time, the distinction between beneficial and deleterious consequences is unclear. In this regard, this review assesses current research and describes the main sources of ROS in neurons, specifying their involvement in synaptic plasticity and distinguishing between physiological and pathological processes implicated.

Sustained Hypoxia Elicits Competing Spinal Mechanisms of Phrenic Motor Facilitation

Acute intermittent hypoxia (AIH) induces phrenic long-term facilitation (pLTF), a form of spinal motor plasticity. Competing mechanisms give rise to phrenic motor facilitation (pMF; a general term including pLTF) depending on the severity of hypoxia within episodes. In contrast, moderate acute sustained hypoxia (mASH) does not elicit pMF. By varying the severity of ASH and targeting competing mechanisms of pMF, we sought to illustrate why moderate AIH (mAIH) elicits pMF but mASH does not. Although mAIH elicits serotonin-dependent pLTF, mASH does not; thus, mAIH-induced pLTF is pattern sensitive. In contrast, severe AIH (sAIH) elicits pLTF through adenosine-dependent mechanisms, likely from greater extracellular adenosine accumulation. Because serotonin- and adenosine-dependent pMF interact via cross talk inhibition, we hypothesized that pMF is obscured because the competing mechanisms of pMF are balanced and offsetting during mASH. Here, we demonstrate the following: (1) blocking spinal A2A receptors with MSX-3 reveals mASH-induced pMF; and (2) sASH elicits A2A-dependent pMF. In anesthetized rats pretreated with intrathecal A2A receptor antagonist injections before mASH (PaO2 = 40-54 mmHg) or sASH (PaO2 = 25-36 mmHg), (1) mASH induced a serotonin-dependent pMF and (2) sASH induced an adenosine-dependent pMF, which was enhanced by spinal serotonin receptor inhibition. Thus, competing adenosine- and serotonin-dependent mechanisms contribute differentially to pMF depending on the pattern/severity of hypoxia. Understanding interactions between these mechanisms has clinical relevance as we develop therapies to treat severe neuromuscular disorders that compromise somatic motor behaviors, including breathing. Moreover, these results demonstrate how competing mechanisms of plasticity can give rise to pattern sensitivity in pLTF.

SIGNIFICANCE STATEMENT

Intermittent hypoxia elicits pattern-sensitive spinal plasticity and improves motor function after spinal injury or during neuromuscular disease. Specific mechanisms of pattern sensitivity in this form of plasticity are unknown. We provide evidence that competing mechanisms of phrenic motor facilitation mediated by adenosine 2A and serotonin 2 receptors are differentially expressed, depending on the pattern/severity of hypoxia. Understanding how these distinct mechanisms interact during hypoxic exposures differing in severity and duration will help explain interesting properties of plasticity, such as pattern sensitivity, and may help optimize therapies to restore motor function in patients with neuromuscular disorders that compromise movement.

Acute intermittent hypoxia induced phrenic long-term facilitation despite increased SOD1 expression in a rat model of ALS

Amyotrophic lateral sclerosis (ALS) is a progressive and fatal neurodegenerative disease characterized by motor neuron death. Since most ALS patients succumb to ventilatory failure from loss of respiratory motor neurons, any effective ALS treatment must preserve and/or restore breathing capacity. In rats over-expressing mutated super-oxide dismutase-1 (SOD1(G93A)), the capacity to increase phrenic motor output is decreased at disease end-stage, suggesting imminent ventilatory failure. Acute intermittent hypoxia (AIH) induces phrenic long-term facilitation (pLTF), a form of spinal respiratory motor plasticity with potential to restore phrenic motor output in clinical disorders that compromise breathing. Since pLTF requires NADPH oxidase activity and reactive oxygen species (ROS) formation, it is blocked by NADPH oxidase inhibition and SOD mimetics in normal rats. Thus, we hypothesized that SOD1(G93A) (mutant; MT) rats do not express AIH-induced pLTF due to over-expression of active mutant superoxide dismutase-1. AIH-induced pLTF and hypoglossal (XII) LTF were assessed in young, pre-symptomatic and end-stage anesthetized MT rats and age-matched wild-type littermates. Contrary to predictions, pLTF and XII LTF were observed in MT rats at all ages; at end-stage, pLTF was actually enhanced. SOD1 levels were elevated in young and pre-symptomatic MT rats, yet superoxide accumulation in putative phrenic motor neurons (assessed with dihydroethidium) was unchanged; however, superoxide accumulation significantly decreased at end-stage. Thus, compensatory mechanisms appear to maintain ROS homoeostasis until late in disease progression, preserving AIH-induced respiratory plasticity. Following intrathecal injections of an NADPH oxidase inhibitor (apocynin; 600 μM; 12 μL), pLTF was abolished in pre-symptomatic, but not end-stage MT rats, demonstrating that pLTF is NADPH oxidase dependent in pre-symptomatic, but NADPH oxidase independent in end-stage MT rats. Mechanisms preserving/enhancing the capacity for pLTF in MT rats are not known.

Human intermittent hypoxia-induced respiratory plasticity is not caused by inflammation

Ventilatory instability, reflected by enhanced acute hypoxic (AHVR) and hypercapnic (AHCVR) ventilatory responses is a fundamental component of obstructive sleep apnoea (OSA) pathogenesis. Intermittent hypoxia-induced inflammation is postulated to promote AHVR enhancement in OSA, although the role of inflammation in intermittent hypoxia-induced respiratory changes in humans has not been examined. Thus, this study assessed the role of inflammation in intermittent hypoxia-induced respiratory plasticity in healthy humans.In a double-blind, placebo-controlled, randomised crossover study design, 12 males were exposed to 6 h of intermittent hypoxia on three occasions. Prior to intermittent hypoxia exposures, participants ingested (for 4  days) either placebo or the nonsteroidal anti-inflammatory drugs indomethacin (nonselective cyclooxygenase (COX) inhibitor) and celecoxib (selective COX-2 inhibitor). Pre- and post-intermittent hypoxia resting ventilation, AHVR, AHCVR and serum concentration of the pro-inflammatory cytokine tumour necrosis factor (TNF)-α were assessed.Pre-intermittent hypoxia resting ventilation, AHVR, AHCVR and TNF-α concentrations were similar across all three conditions (p≥0.093). Intermittent hypoxia increased resting ventilation and the AHVR similarly across all conditions (p=0.827), while the AHCVR was increased (p=0.003) and TNF-α was decreased (p=0.006) with only selective COX-2 inhibition.These findings indicate that inflammation does not contribute to human intermittent hypoxia-induced respiratory plasticity. Moreover, selective COX-2 inhibition augmented the AHCVR following intermittent hypoxia exposure, suggesting that selective COX-2 inhibition could exacerbate OSA severity by increasing ventilatory instability.

Intermittent hypoxia and neurorehabilitation

In recent years, it has become clear that brief, repeated presentations of hypoxia [i.e., acute intermittent hypoxia (AIH)] can boost the efficacy of more traditional therapeutic strategies in certain cases of neurologic dysfunction. This hypothesis derives from a series of studies in animal models and human subjects performed over the past 35 yr. In 1980, Millhorn et al. (Millhorn DE, Eldridge FL, Waldrop TG. Respir Physiol 41: 87-103, 1980) showed that electrical stimulation of carotid chemoafferent neurons produced a persistent, serotonin-dependent increase in phrenic motor output that outlasts the stimulus for more than 90 min (i.e., a "respiratory memory"). AIH elicits similar phrenic "long-term facilitation" (LTF) by a mechanism that requires cervical spinal serotonin receptor activation and de novo protein synthesis. From 2003 to present, a series of studies demonstrated that AIH can induce neuroplasticity in the injured spinal cord, causing functional recovery of breathing capacity after cervical spinal injury. Subsequently, it was demonstrated that repeated AIH (rAIH) can induce recovery of limb function, and the functional benefits of rAIH are greatest when paired with task-specific training. Since uncontrolled and/or prolonged intermittent hypoxia can elicit pathophysiology, a challenge of intermittent hypoxia research is to ensure that therapeutic protocols are well below the threshold for pathogenesis. This is possible since many low dose rAIH protocols have induced functional benefits without evidence of pathology. We propose that carefully controlled rAIH is a safe and noninvasive modality that can be paired with other neurorehabilitative strategies including traditional activity-based physical therapy or cell-based therapies such as intraspinal transplantation of neural progenitors.

Chronic Intermittent Hypoxia Blunts the Expression of Ventilatory Long Term Facilitation in Sleeping Rats

We have previously reported that chronic intermittent hypoxia (CIH), a central feature of human sleep-disordered breathing, causes respiratory instability in sleeping rats (Edge D, Bradford A, O'halloran KD. Adv Exp Med Biol 758:359-363, 2012). Long term facilitation (LTF) of respiratory motor outputs following exposure to episodic, but not sustained, hypoxia has been described. We hypothesized that CIH would enhance ventilatory LTF during sleep. We examined the effects of 3 and 7 days of CIH exposure on the expression of ventilatory LTF in sleeping rats. Adult male Wistar rats were exposed to 20 cycles of normoxia and hypoxia (5 % O(2) at nadir; SaO(2) ~ 80 %) per hour, 8 h per day for 3 or 7 consecutive days (CIH, N = 7 per group). Corresponding sham groups (N = 7 per group) were subjected to alternating cycles of air under identical experimental conditions in parallel. Following gas exposures, breathing during sleep was assessed in unrestrained, unanaesthetized animals using the technique of whole-body plethysmography. Rats were exposed to room air (baseline) and then to an acute IH (AIH) protocol consisting of alternating periods of normoxia (7 min) and hypoxia (FiO(2) 0.1, 5 min) for 10 cycles. Breathing was monitored during the AIH exposure and for 1 h in normoxia following AIH exposure. Baseline ventilation was elevated after 3 but not 7 days of CIH exposure. The hypoxic ventilatory response was equivalent in sham and CIH animals after 3 days but ventilatory responses to repeated hypoxic challenges were significantly blunted following 7 days of CIH. Minute ventilation was significantly elevated following AIH exposure compared to baseline in sham but not in CIH exposed animals. LTF, determined as the % increase in minute ventilation from baseline following AIH exposure, was significantly blunted in CIH exposed rats. In summary, CIH leads to impaired ventilatory responsiveness to AIH. Moreover, CIH blunts ventilatory LTF. The physiological significance of ventilatory LTF is context-dependent but it is reasonable to consider that it can potentially destabilize respiratory control, in view of the potential for LTF to give rise to hypocapnia. CIH-induced blunting of LTF may represent a compensatory mechanism subserving respiratory homeostasis. Our results suggest that CIH-induced increase in apnoea index (Edge D, Bradford A, O'halloran KD. Adv Exp Med Biol 758:359-363, 2012) is not related to enhanced ventilatory LTF. We conclude that the mature adult respiratory system exhibits plasticity and metaplasticity with potential consequences for the control of respiratory homeostasis. Our results may have implications for human sleep apnoea.

Therapeutic potential of intermittent hypoxia: a matter of dose

Intermittent hypoxia (IH) has been the subject of considerable research in recent years, and triggers a bewildering array of both detrimental and beneficial effects in multiple physiological systems. Here, we review the extensive literature concerning IH and its impact on the respiratory, cardiovascular, immune, metabolic, bone, and nervous systems. One major goal is to define relevant IH characteristics leading to safe, protective, and/or therapeutic effects vs. pathogenesis. To understand the impact of IH, it is essential to define critical characteristics of the IH protocol under investigation, including potentially the severity of hypoxia within episodes, the duration of hypoxic episodes, the number of hypoxic episodes per day, the pattern of presentation across time (e.g., within vs. consecutive vs. alternating days), and the cumulative time of exposure. Not surprisingly, severe/chronic IH protocols tend to be pathogenic, whereas any beneficial effects are more likely to arise from modest/acute IH exposures. Features of the IH protocol most highly associated with beneficial vs. pathogenic outcomes include the level of hypoxemia within episodes and the number of episodes per day. Modest hypoxia (9-16% inspired O2) and low cycle numbers (3-15 episodes per day) most often lead to beneficial effects without pathology, whereas severe hypoxia (2-8% inspired O2) and more episodes per day (48-2,400 episodes/day) elicit progressively greater pathology. Accumulating evidence suggests that "low dose" IH (modest hypoxia, few episodes) may be a simple, safe, and effective treatment with considerable therapeutic potential for multiple clinical disorders.

Neither serotonin nor adenosine-dependent mechanisms preserve ventilatory capacity in ALS rats

In rats over-expressing SOD1G93A, ventilation is preserved despite significant loss of respiratory motor neurons. Thus, unknown forms of compensatory respiratory plasticity may offset respiratory motor neuron cell death. Although mechanisms of such compensation are unknown, other models of respiratory motor plasticity may provide a conceptual guide. Multiple cellular mechanisms give rise to phrenic motor facilitation; one mechanism requires spinal serotonin receptor and NADPH oxidase activity whereas another requires spinal adenosine receptor activation. Here, we studied whether these mechanisms contribute to compensatory respiratory plasticity in SOD1G93A rats. Using plethysmography, we assessed ventilation in end-stage SOD1G93A rats after: (1) serotonin depletion with parachlorophenylalanine (PCPA), (2) serotonin (methysergide) and A2A (MSX-3) receptor inhibition, (3) NADPH oxidase inhibition (apocynin), and (4) combined treatments. The ability to increase ventilation was not decreased by individual or combined treatments; thus, these mechanisms do not maintain breathing capacity at end-stage motor neuron disease. Possible mechanisms giving rise to enhanced breathing capacity with combined treatment in end-stage SOD1G93A rats are 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.

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.

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.

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.

Multiple pathways to long-lasting phrenic motor facilitation

Plasticity is a hallmark of neural systems, including the neural system controlling breathing (Mitchell and Johnson 2003). Despite its biological and potential clinical significance, our understanding of mechanisms giving rise to any form of respiratory plasticity remains incomplete. Here we discuss recent advances in our understanding of cellular mechanisms giving rise to phrenic long-term facilitation (pLTF), a long-lasting increase in phrenic motor output induced by acute intermittent hypoxia (AIH). Recently, we have come to realize that multiple, distinct mechanisms are capable of giving rise to long-lasting phrenic motor facilitation (PMF); we use PMF as a general term that includes AIH-induced pLTF. It is important to begin an appreciation and understanding of these diverse pathways. Hence, we introduce a nomenclature based on upstream steps in the signaling cascade leading to PMF. Two pathways are featured here: the "Q" and the "S" pathways, named because they are induced by metabotropic receptors coupled to Gq and Gs proteins, respectively. These pathways appear to interact in complex and interesting ways, thus providing a range of potential responses in the face of changing physiological conditions or the onset of disease.

Propofol abolished the phrenic long-term facilitation in rats

The aim was to investigate the effect of propofol anesthesia on the phrenic long-term facilitation (pLTF) in rats. We hypothesized that pLTF would be abolished during propofol-compared with urethane anesthesia. Fourteen adult, male, anesthetized, vagotomized, paralyzed, and mechanically ventilated Sprague-Dawley rats (seven per group), were exposed to the acute intermittent hypoxia (AIH) protocol. Peak phrenic nerve activity (PNA), burst frequency (f), and breathing rhythm parameters (Ti, Te, Ttot) were analyzed during the first hypoxia (TH1), as well as at 15 (T15), 30 (T30), and 60min (T60) after the final hypoxic episode, and compared to the baseline values. In propofol-anesthetized rats no significant changes of PNA were recorded after the last hypoxic episode, i.e. no pLTF was induced. There was a significant increase of PNA (59.4+/-6.6%, P<0.001) in urethane-anesthetized group at T60. AIH did not elicit significant changes in f, Ti, Te, Ttot in either group at T15, T30, and T60. The pLTF, elicited by AIH, was induced in the urethane-anesthetized rats. On the contrary, pLTF was abolished in the propofol-anesthetized rats.

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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Episodic spinal serotonin receptor activation elicits long-lasting phrenic motor facilitation by an NADPH oxidase-dependent mechanism

Phrenic long-term facilitation (pLTF) is a serotonin (5-HT)-dependent augmentation of phrenic motor output induced by acute intermittent hypoxia (AIH). AIH-induced pLTF requires spinal NADPH oxidase activity and reactive oxygen species (ROS) formation. Since 5-HT receptor activation stimulates NADPH oxidase activity in some cell types, we tested the hypothesis that episodic spinal 5-HT receptor activation (without AIH) is sufficient to elicit an NADPH oxidase-dependent facilitation of phrenic motor output (pMF). In anaesthetised, artificially ventilated adult male rats, episodic intrathecal 5-HT injections (3 x 6 microl injections at 5 min intervals) into the cerebrospinal fluid (CSF) near cervical spinal segments containing the phrenic motor nucleus elicited a progressive increase in integrated phrenic nerve burst amplitude (i.e. pMF) lasting at least 60 min post-5-HT administration. Hypoglossal (XII) nerve activity was unaffected, suggesting that effective doses of 5-HT did not reach the brainstem. A single 5-HT injection was without effect. 5-HT-induced pMF was dose dependent, but exhibited a bell-shaped dose-response curve. Activation of different 5-HT receptor subtypes, specifically 5-HT(2) versus 5-HT(7) receptors, may underlie the bell-shaped dose-response curve via a mechanism of 'cross-talk' inhibition. Pre-treatment with NADPH oxidase inhibitors, apocynin or diphenylenodium (DPI), blocked 5-HT induced pMF. Thus, episodic spinal 5-HT receptor activation is sufficient to elicit pMF by an NADPH oxidase-dependent mechanism, suggesting common mechanisms of ROS formation with AIH-induced pLTF. An understanding of the mechanisms giving rise to AIH-induced pLTF and 5-HT induced pMF may inspire novel therapeutic strategies for respiratory insufficiency in diverse conditions, such as sleep apnoea, cervical spinal injury or amyotrophic lateral sclerosis.

NADPH oxidase activity is necessary for acute intermittent hypoxia-induced phrenic long-term facilitation

Phrenic long-term facilitation (pLTF) following acute intermittent hypoxia (AIH) is a form of spinal, serotonin-dependent synaptic plasticity that requires reactive oxygen species (ROS) formation. We tested the hypothesis that spinal NADPH oxidase activity is a necessary source of ROS for pLTF. Sixty minutes post-AIH (three 5-min episodes of 11% O(2), 5 min intervals), integrated phrenic and hypoglossal (XII) nerve burst amplitudes were increased from baseline, indicative of phrenic and XII LTF. Intrathecal injections (approximately C(4)) of apocynin or diphenyleneiodonium chloride (DPI), two structurally and functionally distinct inhibitors of the NADPH oxidase complex, attenuated phrenic, but not XII, LTF. Immunoblots from soluble (cytosolic) and particulate (membrane) fractions of ventral C(4) spinal segments revealed predominantly membrane localization of the NADPH oxidase catalytic subunit, gp91(phox), whereas membrane and cytosolic expression were both observed for the regulatory subunits, p47(phox) and RAC1. Immunohistochemical analysis of fixed tissues revealed these same subunits in presumptive phrenic motoneurons of the C(4) ventral horn, but not in neighbouring astrocytes or microglia. Collectively, these data demonstrate that NADPH oxidase subunits localized within presumptive phrenic motoneurons are a major source of ROS necessary for AIH-induced pLTF. Thus, NADPH oxidase activity is a key regulator of spinal synaptic plasticity, and may be a useful pharmaceutical target in developing therapeutic strategies for respiratory insufficiency in patients with, for example, cervical spinal injury.

Respiratory long-term facilitation following intermittent hypoxia requires reactive oxygen species formation

Acute intermittent hypoxia (AIH) elicits a form of respiratory plasticity known as long-term facilitation (LTF). LTF is a progressive and sustained increase in respiratory motor output as expressed in phrenic and hypoglossal (XII) nerve activity. Since reactive oxygen species (ROS) play important roles in several forms of neuroplasticity, and ROS production is increased by intermittent hypoxia, we tested the hypothesis that ROS are necessary for phrenic and XII LTF following AIH. Urethane-anesthetized, paralyzed, vagotomized and pump-ventilated Sprague-Dawley rats were exposed to AIH (11% O2, 3, 5 min episodes, 5 min intervals), and both phrenic and XII nerve activity were monitored for 60 min post-AIH. Although phrenic and XII LTF were observed in control rats, i.v. manganese (III) tetrakis (1-methyl-4-pyridyl) porphyrin pentachloride (MnTMPyP), a superoxide anion scavenger, attenuated both phrenic and XII LTF in a dose dependent manner. Localized application of MnTMPyP (5.5 mM; 10 microl) to the intrathecal space of the cervical spinal cord (C4) abolished phrenic, but not XII LTF. Thus, ROS are necessary for AIH-induced respiratory LTF, and the relevant ROS appear to be localized near respiratory motor nuclei since cervical MnTMPyP injections impaired phrenic (and not XII) LTF. Phrenic LTF is a novel form of ROS-dependent neuroplasticity since its ROS-dependence resides in the spinal cord.

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.

Simulated apnoeas induce serotonin-dependent respiratory long-term facilitation in rats

Long-term facilitation (LTF) is a form of respiratory neuroplasticity frequently induced by acute intermittent isocapnic hypoxia (AIH, three 5 min isocapnic hypoxic episodes). Although repetitive apnoeas are a frequent natural occurrence producing brief (< 30 s) episodes of hypoxia and hypercapnia, it is unknown if repetitive apnoeas also elicit LTF. Apnoea-induced LTF may preserve upper airway patency during sleep, thereby limiting further apnoeic events. We tested the hypothesis that repeated, brief ventilator-induced apnoeas are sufficient to induce serotonin-dependent phrenic and hypoglossal (XII) LTF in anaesthetized rats. Anaesthetized, male Sprague-Dawley rats were exposed to three or six 25 s ventilator apnoeas with 5 min intervals, and compared to time control and AIH-treated rats. Three and six ventilator apnoeas induced phrenic and XII LTF with a magnitude similar to AIH. Both apnoea-induced and AIH-induced LTF were associated with a decreased CO(2) recruitment threshold for phrenic and XII activity (approximately 4 mmHg). Spinal methysergide, a serotonin receptor antagonist, blocked apnoea-induced LTF but not changes in the CO(2)-recruitment threshold. Thus, brief ventilator apnoeas elicit phrenic and XII LTF. Similar to AIH-induced LTF, apnoea-induced LTF is serotonin dependent, and the relevant serotonin receptors for phrenic LTF are located in the cervical spinal cord. Apnoea-induced LTF may have implications for the maintenance of breathing stability, particularly during sleep.