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

The O2-sensitive brain stem, hyperoxic hyperventilation, and CNS oxygen toxicity

Central nervous system oxygen toxicity (CNS-OT) is a complex disorder that presents, initially, as a sequence of cardio-respiratory abnormalities and nonconvulsive signs and symptoms (S/Sx) of brain stem origin that culminate in generalized seizures, loss of consciousness, and postictal cardiogenic pulmonary edema. The risk of CNS-OT and its antecedent “early toxic indications” are what limits the use of hyperbaric oxygen (HBO2) in hyperbaric and undersea medicine. The purpose of this review is to illustrate, based on animal research, how the temporal pattern of abnormal brain stem responses that precedes an “oxtox hit” provides researchers a window into the early neurological events underlying seizure genesis. Specifically, we focus on the phenomenon of hyperoxic hyperventilation, and the medullary neurons presumed to contribute in large part to this paradoxical respiratory response; neurons in the caudal Solitary complex (cSC) of the dorsomedial medulla, including putative CO2 chemoreceptor neurons. The electrophysiological and redox properties of O2-/CO2-sensitive cSC neurons identified in rat brain slice experiments are summarized. Additionally, evidence is summarized that supports the working hypothesis that seizure genesis originates in subcortical areas and involves cardio-respiratory centers and cranial nerve nuclei in the hind brain (brainstem and cerebellum) based on, respectively, the complex temporal pattern of abnormal cardio-respiratory responses and various nonconvulsive S/Sx that precede seizures during exposure to HBO2.

Respiratory neuroplasticity: Mechanisms and translational implications of phrenic motor plasticity

Widespread appreciation that neuroplasticity is an essential feature of the neural system controlling breathing has emerged only in recent years. In this chapter, we focus on respiratory motor plasticity, with emphasis on the phrenic motor system. First, we define related but distinct concepts: neuromodulation and neuroplasticity. We then focus on mechanisms underlying two well-studied models of phrenic motor plasticity: (1) phrenic long-term facilitation following brief exposure to acute intermittent hypoxia; and (2) phrenic motor facilitation after prolonged or recurrent bouts of diminished respiratory neural activity. Advances in our understanding of these novel and important forms of plasticity have been rapid and have already inspired translation in multiple respects: (1) development of novel therapeutic strategies to preserve/restore breathing function in humans with severe neurological disorders, such as spinal cord injury and amyotrophic lateral sclerosis; and (2) the discovery that similar plasticity also occurs in nonrespiratory motor systems. Indeed, the realization that similar plasticity occurs in respiratory and nonrespiratory motor neurons inspired clinical trials to restore leg/walking and hand/arm function in people living with chronic, incomplete spinal cord injury. Similar application may be possible to other clinical disorders that compromise respiratory and non-respiratory movements.

Whole-genome association analyses of sleep-disordered breathing phenotypes in the NHLBI TOPMed program

BACKGROUND

Sleep-disordered breathing is a common disorder associated with significant morbidity. The genetic architecture of sleep-disordered breathing remains poorly understood. Through the NHLBI Trans-Omics for Precision Medicine (TOPMed) program, we performed the first whole-genome sequence analysis of sleep-disordered breathing.

METHODS

The study sample was comprised of 7988 individuals of diverse ancestry. Common-variant and pathway analyses included an additional 13,257 individuals. We examined five complementary traits describing different aspects of sleep-disordered breathing: the apnea-hypopnea index, average oxyhemoglobin desaturation per event, average and minimum oxyhemoglobin saturation across the sleep episode, and the percentage of sleep with oxyhemoglobin saturation < 90%. We adjusted for age, sex, BMI, study, and family structure using MMSKAT and EMMAX mixed linear model approaches. Additional bioinformatics analyses were performed with MetaXcan, GIGSEA, and ReMap.

RESULTS

We identified a multi-ethnic set-based rare-variant association (p = 3.48 × 10-8) on chromosome X with ARMCX3. Additional rare-variant associations include ARMCX3-AS1, MRPS33, and C16orf90. Novel common-variant loci were identified in the NRG1 and SLC45A2 regions, and previously associated loci in the IL18RAP and ATP2B4 regions were associated with novel phenotypes. Transcription factor binding site enrichment identified associations with genes implicated with respiratory and craniofacial traits. Additional analyses identified significantly associated pathways.

CONCLUSIONS

We have identified the first gene-based rare-variant associations with objectively measured sleep-disordered breathing traits. Our results increase the understanding of the genetic architecture of sleep-disordered breathing and highlight associations in genes that modulate lung development, inflammation, respiratory rhythmogenesis, and HIF1A-mediated hypoxic response.

Divergent receptor utilization is necessary for phrenic long-term facilitation over the course of motor neuron loss following CTB-SAP intrapleural injections

Intrapleural injection of cholera toxin B conjugated to saporin (CTB-SAP) mimics respiratory motor neuron death and respiratory deficits observed in rat models of neuromuscular diseases. Seven-day CTB-SAP rats elicit enhanced phrenic long-term facilitation (pLTF) primarily through TrkB and PI3K/Akt-dependent mechanisms [i.e., Gs-pathway, which can be initiated by adenosine 2A (A2A) receptors in naïve rats], whereas 28-day CTB-SAP rats elicit moderate pLTF though BDNF- and MEK-/ERK-dependent mechanisms [i.e., Gq-pathway, which is typically initiated by serotonin (5-HT) receptors in naïve rats]. Here, we tested the hypothesis that pLTF following CTB-SAP is 1) A2A receptor-dependent at 7 days and 2) 5-HT receptor-dependent at 28 days. Adult Sprague-Dawley male rats were anesthetized, paralyzed, ventilated, and exposed to acute intermittent hypoxia (AIH; 3-, 5-min bouts of 10.5% O₂) following bilateral, intrapleural injections at 7 days and 28 days of 1) CTB-SAP (25 µg) or 2) unconjugated CTB and SAP (control). Intrathecal C₄ delivery included either the 1) A2A receptor antagonist (MSX-3; 10 µM; 12 µL) or 2) 5-HT receptor antagonist (methysergide; 20 mM; 15 µL). pLTF was abolished with A2A receptor inhibition in 7-day, not 28-day, CTB-SAP rats versus controls (P < 0.05), whereas pLTF was abolished following 5-HT receptor inhibition in 28-day, not 7-day, CTB-SAP rats versus controls (P < 0.05). In addition, 5-HT2A receptor expression was unchanged in CTB-SAP rats versus controls, whereas 5-HT2B receptor expression was decreased in CTB-SAP rats versus controls (P < 0.05). This study furthers our understanding of the contribution of differential receptor activation to pLTF and its implications for breathing following respiratory motor neuron death.NEW & NOTEWORTHY The current study investigates underlying receptor-dependent mechanisms contributing to phrenic long-term facilitation (pLTF) following CTB-SAP-induced respiratory motor neuron death at 7 days and 28 days. We found that A2A receptors are required for enhanced pLTF in 7-day CTB-SAP rats, whereas 5-HT receptors are required for moderate pLTF in 28-day CTB-SAP rats. Targeting these time-dependent mechanisms have implications for breathing maintenance over the course of many neuromuscular diseases.

Adenosine A2a receptors modulate TrkB receptor-dependent respiratory plasticity in neonatal rats

Neuroplasticity is a fundamental property of the respiratory control system, enabling critical adaptations in breathing to meet the challenges, but little is known whether neonates express neuroplasticity similar to adults. We tested the hypothesis that, similar to adults, tyrosine receptor kinase B (TrkB) or adenosine A2a receptor activation in neonates are independently sufficient to elicit respiratory motor facilitation, and that co-induction of TrkB and A2a receptor-dependent plasticity undermines respiratory motor facilitation. TrkB receptor activation with 7,8-dihydroxyflavone (DHF) in neonatal brainstem-spinal cord preparations induced a long-lasting increase in respiratory motor output in 55 % of preparations, whereas adenosine A2a receptor activation with CGS21680 only sporadically induced respiratory motor plasticity. CGS21680 and DHF co-application prevented DHF-dependent respiratory motor facilitation, whereas co-application of MSX-3 (adenosine A2a receptor antagonist) and DHF more rapidly induced respiratory motor plasticity. Collectively, these data suggest that mechanisms underlying respiratory neuroplasticity may be only partially operational in early neonatal life, and that adenosine A2a receptor activation undermines TrkB-induced respiratory plasticity.

Daily acute intermittent hypoxia enhances phrenic motor output and stimulus-evoked phrenic responses in rats

Plasticity is a hallmark of the respiratory neural control system. Phrenic long-term facilitation (pLTF) is one form of respiratory plasticity characterized by persistent increases in phrenic nerve activity following acute intermittent hypoxia (AIH). Although there is evidence that key steps in the cellular pathway giving rise to pLTF are localized within phrenic motor neurons (PMNs), the impact of AIH on the strength of breathing-related synaptic inputs to PMNs remains unclear. Further, the functional impact of AIH is enhanced by repeated/daily exposure to AIH (dAIH). Here, we explored the effects of AIH vs. 2 weeks of dAIH preconditioning on spontaneous and evoked responses recorded in anesthetized, paralyzed (with pancuronium bromide) and mechanically ventilated rats. Evoked phrenic potentials were elicited by respiratory cycle-triggered lateral funiculus stimulation at C2 delivered prior to- and 60 min post-AIH (or an equivalent time in controls). Charge-balanced biphasic pulses (100 µs/phase) of progressively increasing intensity (100 to 700 µA) were delivered during the inspiratory and expiratory phases of the respiratory cycle. Although robust pLTF (~60% from baseline) was observed after a single exposure to moderate AIH (3 x 5 min; 5 min intervals), there was no effect on evoked phrenic responses, contrary to our initial hypothesis. However, in rats preconditioned with dAIH, baseline phrenic nerve activity and evoked responses were increased, suggesting that repeated exposure to AIH enhances functional synaptic strength when assessed using this technique. The impact of daily AIH preconditioning on synaptic inputs to PMNs raises interesting questions that require further exploration.

The role of NADPH oxidase in chronic intermittent hypoxia-induced respiratory plasticity in adult male mice

Reactive oxygen species (ROS) are proposed as mediators of chronic intermittent hypoxia (CIH)-induced respiratory plasticity. We sought to determine if NADPH oxidase 2 (NOX2)-derived ROS underpin CIH-induced maladaptive changes in respiratory control. Adult male mice (C57BL/6 J) were assigned to one of three groups: normoxic controls (sham); chronic intermittent hypoxia-exposed (CIH, 12 cycles/hour, 8 h/day for 14 days); and CIH + apocynin (NOX2 inhibitor, 2 mM) given in the drinking water throughout exposure to CIH. In addition, we studied sham and CIH-exposed NOX2-null mice (B6.129S-Cybb^TM1Din/J). Whole-body plethysmography was used to measure breathing and metabolic parameters. Ventilation (V̇_I/V̇CO₂) during normoxia was unaffected by CIH, but apnoea index was increased, which was prevented by apocynin, but not by NOX2 deletion. The ventilatory response to hypercapnia following exposure to CIH was potentiated in NOX2-null mice. Our results reveal ROS-dependent influences on the control of breathing and point to antioxidant intervention as a potential adjunctive therapeutic strategy in respiratory control disorders.

A comprehensive review of respiratory, autonomic and cardiovascular responses to intermittent hypoxia in humans

This review explores forms of respiratory and autonomic plasticity, and associated outcome measures, that are initiated by exposure to intermittent hypoxia. The review focuses primarily on studies that have been completed in humans and primarily explores the impact of mild intermittent hypoxia on outcome measures. Studies that have explored two forms of respiratory plasticity, progressive augmentation of the hypoxic ventilatory response and long-term facilitation of ventilation and upper airway muscle activity, are initially reviewed. The role these forms of plasticity might have in sleep disordered breathing are also explored. Thereafter, the role of intermittent hypoxia in the initiation of autonomic plasticity is reviewed and the role this form of plasticity has in cardiovascular and hemodynamic responses during and following intermittent hypoxia is addressed. The role of these responses in individuals with sleep disordered breathing and spinal cord injury are subsequently addressed. Ultimately an integrated picture of the respiratory, autonomic and cardiovascular responses to intermittent hypoxia is presented. The goal of the integrated picture is to address the types of responses that one might expect in humans exposed to one-time and repeated daily exposure to mild intermittent hypoxia. This form of intermittent hypoxia is highlighted because of its potential therapeutic impact in promoting functional improvement and recovery in several physiological systems.

Mechanisms of severe acute intermittent hypoxia-induced phrenic long-term facilitation

Moderate acute intermittent hypoxia (mAIH; 35-55 mmHg Pa_O2) elicits phrenic long-term facilitation (pLTF) by a mechanism that requires activation of G_q protein-coupled serotonin type 2 receptors, MEK/ERK MAP kinase, and NADPH oxidase activity and is constrained by cAMP-PKA signaling. In contrast, severe AIH (sAIH; 25-35 mmHg Pa_O2) elicits G_s protein-coupled adenosine type 2 A receptor-dependent pLTF. Another G_s protein-coupled receptor, serotonin 7 receptors, elicits phrenic motor facilitation (pMF) by a mechanism that requires exchange protein activated by cyclic AMP (EPAC) and phosphatidylinositol 3-kinase/Akt (PI3K/Akt) activation and is constrained by NADPH oxidase activity. Here, we tested the hypothesis that the same downstream signaling mechanisms giving rise to serotonin 7 (vs. serotonin 2) receptor-induced pMF underlie sAIH-induced pLTF. In anesthetized rats, sAIH-induced pLTF was compared after pretreatment with intrathecal (C4) injections of inhibitors for: 1) EPAC (ESI-05); 2) MEK/ERK (UO126); 3) PKA (KT-5720); 4) PI3K/Akt (PI828); and 5) NADPH oxidase (apocynin). In partial agreement with our hypothesis, sAIH-induced pLTF was abolished by ESI-05 and PI828 and marginally enhanced by apocynin but, surprisingly, was abolished by UO126 and attenuated by KT-5720. Mechanisms of sAIH-induced pLTF reflect elements of both G_q and G_s pathways to pMF, likely as a consequence of the complex, cross-talk interactions between them.NEW & NOTEWORTHY Distinct mechanisms give rise to pLTF induced by moderate and severe AIH. We demonstrate that, unlike moderate AIH, severe AIH-induced pLTF requires EPAC and PI3K/Akt and is marginally constrained by NADPH oxidase activity. Surprisingly, sAIH-induced pLTF requires MEK/ERK activity similar to moderate AIH-induced pLTF and is reduced by PKA inhibition. We suggest sAIH-induced pLTF arises from complex interactions between dominant mechanisms characteristic of moderate versus severe AIH-induced pLTF.

Differential mechanisms are required for phrenic long-term facilitation over the course of motor neuron loss following CTB-SAP intrapleural injections

Selective elimination of respiratory motor neurons using intrapleural injections of cholera toxin B fragment conjugated to saporin (CTB-SAP) mimics motor neuron death and respiratory deficits observed in rat models of neuromuscular diseases. This CTB-SAP model allows us to study the impact of motor neuron death on the output of surviving phrenic motor neurons. After 7(d) days of CTB-SAP, phrenic long-term facilitation (pLTF, a form of respiratory plasticity) is enhanced, but returns towards control levels at 28d. However, the mechanism responsible for this difference in magnitude of pLTF is unknown. In naïve rats, pLTF predominately requires 5-HT2 receptors, the new synthesis of BDNF, and MEK/ERK signaling; however, pLTF can alternatively be induced via A2A receptors, the new synthesis of TrkB, and PI3K/Akt signaling. Since A2A receptor-dependent pLTF is enhanced in naïve rats, we suggest that 7d CTB-SAP treated rats utilize the alternative mechanism for pLTF. Here, we tested the hypothesis that pLTF following CTB-SAP is: 1) TrkB and PI3K/Akt, not BDNF and MEK/ERK, dependent at 7d; and 2) BDNF and MEK/ERK, not TrkB and PI3K/Akt, dependent at 28d. Adult Sprague Dawley male rats were anesthetized, paralyzed, ventilated, and were exposed to acute intermittent hypoxia (AIH; 3, 5 min bouts of 10.5% O2) following bilateral, intrapleural injections at 7d and 28d of: 1) CTB-SAP (25 μg), or 2) un-conjugated CTB and SAP (control). Intrathecal C4 delivery included either: 1) small interfering RNA that targeted BDNF or TrkB mRNA; 2) UO126 (MEK/ERK inhibitor); or 3) PI828 (PI3K/Akt inhibitor). Our data suggest that pLTF in 7d CTB-SAP treated rats is elicited primarily through TrkB and PI3K/Akt-dependent mechanisms, whereas BDNF and MEK/ERK-dependent mechanisms induce pLTF in 28d CTB-SAP treated rats. This project increases our understanding of respiratory plasticity and its implications for breathing following motor neuron death.

Competing mechanisms of plasticity impair compensatory responses to repetitive apnoea

KEY POINTS

Intermittent reductions in respiratory neural activity, a characteristic of many ventilatory disorders, leads to inadequate ventilation and arterial hypoxia. Both intermittent reductions in respiratory neural activity and intermittent hypoxia trigger compensatory enhancements in inspiratory output when experienced separately, forms of plasticity called inactivity-induced inspiratory motor facilitation (iMF) and long-term facilitation (LTF), respectively. Reductions in respiratory neural activity that lead to moderate, but not mild, arterial hypoxia occludes plasticity expression, indicating that concurrent induction of iMF and LTF impairs plasticity through cross-talk inhibition of their respective signalling pathways. Moderate hypoxia undermines iMF by enhancing NR2B-containing NMDA receptor signalling, which can be rescued by exogenous retinoic acid, a molecule necessary for iMF. These data suggest that in ventilatory disorders characterized by reduced inspiratory motor output, such as sleep apnoea, endogenous mechanisms of compensatory plasticity may be impaired, and that exogenously activating respiratory plasticity may be a novel strategy to improve breathing.

ABSTRACT

Many forms of sleep apnoea are characterized by recurrent reductions in respiratory neural activity, which leads to inadequate ventilation and arterial hypoxia. Both recurrent reductions in respiratory neural activity and hypoxia activate mechanisms of compensatory plasticity that augment inspiratory output and lower the threshold for apnoea, inactivity-induced inspiratory motor facilitation (iMF) and long-term facilitation (LTF), respectively. However, despite frequent concurrence of reduced respiratory neural activity and hypoxia, mechanisms that induce and regulate iMF and LTF have only been studied separately. Here, we demonstrate that recurrent reductions in respiratory neural activity ('neural apnoea') accompanied by cessations in ventilation that result in moderate (but not mild) hypoxaemia do not elicit increased inspiratory output, suggesting that concurrent induction of iMF and LTF occludes plasticity. A key role for NMDA receptor activation in impairing plasticity following concurrent neural apnoea and hypoxia is indicated since recurrent hypoxic neural apnoeas triggered increased phrenic inspiratory output in rats in which spinal NR2B-containing NMDA receptors were inhibited. Spinal application of retinoic acid, a key molecule necessary for iMF, bypasses NMDA receptor-mediated constraints, thereby rescuing plasticity following hypoxic neural apnoeas. These studies raise the intriguing possibility that endogenous mechanisms of compensatory plasticity may be impaired in some individuals with sleep apnoea, and that exogenously activating pathways giving rise to respiratory plasticity may be a novel pharmacological strategy to improve breathing.

Distinct metabolic adjustments arise from acclimation to constant hypoxia and intermittent hypoxia in estuarine killifish (Fundulus heteroclitus)

Many fish experience daily cycles of hypoxia in the wild, but the physiological strategies for coping with intermittent hypoxia are poorly understood. We examined how killifish adjust O2 supply and demand during acute hypoxia, and how these responses are altered after prolonged acclimation to constant or intermittent patterns of hypoxia exposure. We acclimated killifish to normoxia (∼20 kPa O2), constant hypoxia (2 kPa) or intermittent cycles of nocturnal hypoxia (12 h:12 h normoxia:hypoxia) for 28 days, and then compared whole-animal O2 consumption rates (ṀO2) and tissue metabolites during exposure to 12 h of hypoxia followed by reoxygenation in normoxia. Normoxia-acclimated fish experienced a pronounced 27% drop in ṀO2 during acute hypoxia, and modestly increased ṀO2 upon reoxygenation. They strongly recruited anaerobic metabolism during acute hypoxia, indicated by lactate accumulation in plasma, muscle, liver, brain, heart and digestive tract, as well as a transient drop in intracellular pH, and they increased hypoxia inducible factor (HIF)-1α protein abundance in muscle. Glycogen, glucose and glucose-6-phosphate levels suggested that glycogen supported brain metabolism in hypoxia, while the muscle used circulating glucose. Acclimation to constant hypoxia caused a stable ∼50% decrease in ṀO2 that persisted after reoxygenation, with minimal recruitment of anaerobic metabolism, suggestive of metabolic depression. By contrast, fish acclimated to intermittent hypoxia maintained sufficient O2 transport to support normoxic ṀO2, modestly recruited lactate metabolism and increased ṀO2 dramatically upon reoxygenation. Both groups of hypoxia-acclimated fish had similar glycogen, ATP, intracellular pH and HIF-1α levels as normoxic controls. We conclude that different patterns of hypoxia exposure favour distinct strategies for matching O2 supply and O2 demand.

Advances in cellular and integrative control of oxygen homeostasis within the central nervous system

Mammals must continuously regulate the levels of O2 and CO2 , which is particularly important for the brain. Failure to maintain adequate O2 /CO2 homeostasis has been associated with numerous disorders including sleep apnoea, Rett syndrome and sudden infant death syndrome. But, O2 /CO2 homeostasis poses major regulatory challenges, even in the healthy brain. Neuronal activities change in a differentiated, spatially and temporally complex manner, which is reflected in equally complex changes in O2 demand. This raises important questions: is oxygen sensing an emergent property, locally generated within all active neuronal networks, and/or the property of specialized O2 -sensitive CNS regions? Increasing evidence suggests that the regulation of the brain's redox state involves properties that are intrinsic to many networks, but that specialized regions in the brainstem orchestrate the integrated control of respiratory and cardiovascular functions. Although the levels of O2 in arterial blood and the CNS are very different, neuro-glial interactions and purinergic signalling are critical for both peripheral and CNS chemosensation. Indeed, the specificity of neuroglial interactions seems to determine the differential responses to O2 , CO2 and the changes in pH.

Pharmacological modulation of hypoxia-induced respiratory neuroplasticity

Hypoxia elicits complex cell signaling mechanisms in the respiratory control system that can produce long-lasting changes in respiratory motor output. In this article, we review experimental approaches used to elucidate signaling pathways associated with hypoxia, and summarize current hypotheses regarding the intracellular signaling pathways evoked by intermittent exposure to hypoxia. We review data showing that pharmacological treatments can enhance neuroplastic responses to hypoxia. Original data are included to show that pharmacological modulation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) function can reveal a respiratory neuroplastic response to a single, brief hypoxic exposure in anesthetized mice. Coupling pharmacologic treatments with therapeutic hypoxia paradigms may have rehabilitative value following neurologic injury or during neuromuscular disease. Depending on prevailing conditions, pharmacologic treatments can enable hypoxia-induced expression of neuroplasticity and increased respiratory motor output, or potentially could synergistically interact with hypoxia to more robustly increase motor output.

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.

Time Domains of the Hypoxic Ventilatory Response and Their Molecular Basis

Ventilatory responses to hypoxia vary widely depending on the pattern and length of hypoxic exposure. Acute, prolonged, or intermittent hypoxic episodes can increase or decrease breathing for seconds to years, both during the hypoxic stimulus, and also after its removal. These myriad effects are the result of a complicated web of molecular interactions that underlie plasticity in the respiratory control reflex circuits and ultimately control the physiology of breathing in hypoxia. Since the time domains of the physiological hypoxic ventilatory response (HVR) were identified, considerable research effort has gone toward elucidating the underlying molecular mechanisms that mediate these varied responses. This research has begun to describe complicated and plastic interactions in the relay circuits between the peripheral chemoreceptors and the ventilatory control circuits within the central nervous system. Intriguingly, many of these molecular pathways seem to share key components between the different time domains, suggesting that varied physiological HVRs are the result of specific modifications to overlapping pathways. This review highlights what has been discovered regarding the cell and molecular level control of the time domains of the HVR, and highlights key areas where further research is required. Understanding the molecular control of ventilation in hypoxia has important implications for basic physiology and is emerging as an important component of several clinical fields. © 2016 American Physiological Society. Compr Physiol 6:1345-1385, 2016.

N-Acetylcysteine reduces hyperacute intermittent hypoxia-induced sympathoexcitation in human subjects

NEW FINDINGS

What is the central question of this study? This study evaluated the following central question: does N-acetylcysteine (N-AC), an antioxidant that readily penetrates the blood-brain barrier, have the capability to reduce the increase in sympathetic nerve activity observed during hyperacute intermittent hypoxia? What is the main finding and its importance? We demonstrate that N-AC decreases muscle sympathetic nerve activity in response to hyperacute intermittent hypoxia versus placebo control. This finding suggests that antioxidants, such as N-AC, have therapeutic potential in obstructive sleep apnoea. This investigation tested the following hypotheses: that (i) N-acetylcysteine (N-AC) attenuates hyperacute intermittent hypoxia-induced sympathoexcitation, (ii) without elevating superoxide measured in peripheral venous blood. Twenty-eight healthy human subjects were recruited to the study. One hour before experimentation, each subject randomly ingested either 70 mg kg(-1) of N-AC (n = 16) or vehicle placebo (n = 12). Three-lead ECG and arterial blood pressure, muscle sympathetic nerve activity (n = 17) and whole-blood superoxide concentration (using electron paramagnetic resonance spectroscopy; n = 12) were measured. Subjects underwent a 20 min hyperacute intermittent hypoxia training (hAIHT) protocol that consisted of cyclical end-expiratory apnoeas with 100% nitrogen. N-AC decreased muscle sympathetic nerve activity after hAIHT compared with placebo (P < 0.02). However, N-AC did not alter superoxide concentrations in venous blood compared with placebo (P > 0.05). Moreover, hAIHT did not increase superoxide concentrations in the peripheral circulation as measured by electron paramagnetic resonance (P > 0.05). Based on these findings, we contend that (i) hAIHT and (ii) the actions of N-AC in hAIHT are primarily mediated centrally rather than peripherally, although central measurements of reactive oxygen species are difficult to obtain in human subjects, thus making this assertion difficult to verify. This investigation suggests the possibility of developing a pharmaceutical therapy to inhibit the sympathoexcitation associated with obstructive sleep apnoea.

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.

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.

Chemoreflexes, sleep apnea, and sympathetic dysregulation

Obstructive sleep apnea (OSA) and hypertension are closely linked conditions. Disordered breathing events in OSA are characterized by increasing efforts against an occluded airway while asleep, resulting in a marked sympathetic response. This is predominantly due to hypoxemia activating the chemoreflexes, resulting in reflex increases in sympathetic neural outflow. In addition, apnea - and the consequent lack of inhibition of the sympathetic system that occurs with lung inflation during normal breathing - potentiates central sympathetic outflow. Sympathetic activation persists into the daytime, and is thought to contribute to hypertension and other adverse cardiovascular outcomes. This review discusses chemoreflex physiology and sympathetic modulation during normal sleep, as well as the sympathetic dysregulation seen in OSA, its extension into wakefulness, and changes after treatment. Evidence supporting the role of the peripheral chemoreflex in the sympathetic dysregulation seen in OSA, including in the context of comorbid obesity, metabolic syndrome, and systemic hypertension, is reviewed. Finally, alterations in cardiovascular variability and other potential mechanisms that may play a role in the autonomic imbalance in OSA are also discussed.

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 nNOS regulates phrenic motor facilitation by a 5-HT2B receptor- and NADPH oxidase-dependent mechanism

Acute intermittent hypoxia (AIH) induces phrenic long-term facilitation (pLTF) by a mechanism that requires spinal serotonin (5-HT) receptor activation and NADPH oxidase (NOX) activity. Here, we investigated whether: (1) spinal nitric oxide synthase (NOS) activity is necessary for AIH-induced pLTF; (2) episodic exogenous nitric oxide (NO) is sufficient to elicit phrenic motor facilitation (pMF) without AIH (i.e. pharmacologically); and (3) NO-induced pMF requires spinal 5-HT2B receptor and NOX activation. In anesthetized, mechanically ventilated adult male rats, AIH (3 × 5-min episodes; 10% O2; 5 min) elicited a progressive increase in the amplitude of integrated phrenic nerve bursts (i.e. pLTF), which lasted 60 min post-AIH (45.1 ± 8.6% baseline). Pre-treatment with intrathecal (i.t.) injections of a neuronal NOS inhibitor (nNOS-inhibitor-1) near the phrenic motor nucleus attenuated pLTF (14.7 ± 2.5%), whereas an inducible NOS (iNOS) inhibitor (1400 W) had no effect (56.3 ± 8.0%). Episodic i.t. injections (3 × 5μl volume; 5 min) of a NO donor (sodium nitroprusside; SNP) elicited pMF similar in time-course and magnitude (40.4 ± 6.0%, 60 min post-injection) to AIH-induced pLTF. SNP-induced pMF was blocked by a 5-HT2B receptor antagonist (SB206553), a superoxide dismutase mimetic (MnTMPyP), and two NOX inhibitors (apocynin and DPI). Neither pLTF nor pMF was affected by pre-treatment with a protein kinase G (PKG) inhibitor (KT-5823). Thus, spinal nNOS activity is necessary for AIH-induced pLTF, and episodic spinal NO is sufficient to elicit pMF by a mechanism that requires 5-HT2B receptor activation and NOX-derived ROS formation, which indicates AIH (and NO) elicits spinal respiratory plasticity by a nitrergic-serotonergic mechanism.

The pattern and extent of retrograde transsynaptic transport of WGA-Alexa 488 in the phrenic motor system is dependent upon the site of application

The first aim of the study was to determine if WGA-Alexa 488 would undergo retrograde transsynaptic transport in the phrenic motor system as we have shown with WGA-HRP in a previous study. The advantage of using WGA-Alexa 488 is that labeled neurons could be isolated and analyzed for intracellular molecular mechanisms without exposing tissue sections to chemicals for histochemical staining. The second aim of the study was to investigate the pattern and extent of labeling that occurs when WGA-Alexa 488 is applied to the cervical phrenic nerve as compared to intradiaphragmatic injection. After injecting the hemidiaphragm ipsilateral to a C2 spinal cord hemisection, WGA-Alexa 488 presumably diffused to the contralateral hemidiaphragm and labeled the phrenic nuclei bilaterally. In all animals with hemidiaphragmatic injection, the rostral ventral respiratory group (rVRG) was also labeled bilaterally in the medulla. Thus, injection of WGA-Alexa 488 into the diaphragm results in retrograde transsynaptic transport in the phrenic motor system. After applying WGA-Alexa 488 to the ipsilateral intact cervical phrenic nerve in both C2 hemisected rats and rats with a sham hemisection, only ipsilateral phrenic neurons were labeled; there was no labeling of the rVRG or any other center in the medulla. These results suggest that WGA-Alexa 488 must be applied in the vicinity of the phrenic myoneural junction where there is a high concentration of WGA receptors in order for transsynaptic transport to occur. The present study provides investigators with a new tool to study plasticity in the respiratory system after spinal cord injury.

Signalling mechanisms of long term facilitation of breathing with intermittent hypoxia

Intermittent hypoxia causes long-term facilitation (LTF) of respiratory motor nerve activity and ventilation, which manifests as a persistent increase over the normoxic baseline for an hour or more after the acute hypoxic ventilatory response. LTF is likely involved in sleep apnea, but its exact role is uncertain. Previously, LTF was defined as a serotonergic mechanism, but new evidence shows that multiple signaling pathways can elicit LTF. This raises new questions about the interactions between signaling pathways in different time domains of the hypoxic ventilatory response, which can no longer be defined simply in terms of neurochemical mechanisms.

Baroreflex and chemoreflex controls of sympathetic activity following intermittent hypoxia

There is a large amount of evidence linking obstructive sleep apnea (OSA), and the associated intermittent hypoxia that accompanies it, with the development of hypertension. For example, cross-sectional studies demonstrate that the prevalence of hypertension increases with the severity of OSA (Bixler et al., 2000; Grote et al., 2001) and an initial determination of OSA is associated with a three-fold increase for future hypertension (Peppard et al., 2000). Interestingly, bouts of intermittent hypoxia have also been shown to affect sympathetic output associated with the baroreflex and chemoreflex, important mechanisms in the regulation of arterial blood pressure. As such, the possibility exists that changes in the baroreflex and chemoreflex may contribute to the development of chronic hypertension observed in OSA patients. The aim of the current article is to briefly review the response of the baroreflex and chemoreflex to intermittent hypoxic exposure and to evaluate evidence for the hypothesis that modification of these autonomic reflexes may, at least in part, support the comorbidity observed between chronic hypertension and OSA.

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.

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.

Reactive oxygen species are required for the hypothalamic osmoregulatory response

Free radicals, or reactive oxygen species (ROS), are highly reactive byproducts of oxygen degradation. They are well known for their cellular toxicity, but few studies have analyzed their potential role in homeostatic processes. We investigated ROS production and function during the arginine vasopressin (AVP) hypothalamic response to hyperosmolarity. Six-week-old male C3H/HeJ mice were subjected to salt loading for 2 or 8 d. The osmotic axis was progressively activated and reached a new steady-state status at 8 d as demonstrated by monitoring of plasmatic osmolality and c-Fos and AVP expression in the supraoptic nucleus (SON). Free radicals, visualized by dihydroethidine staining and measured by 2'-7'dichlorofluorescein diacetate assays, were detected after 2 d of salt loading. The activity and expression of superoxide dismutase 2 and catalase were concomitantly up-regulated in the SON, suggesting that free radicals are detoxified by endogenous antioxidant systems, thereby avoiding their deleterious effects. The early phase of the osmoregulatory response has been investigated using an acute hyperosmotic model; free radicals were produced 45 min after an ip injection of 1.5 m NaCl. This was followed by an increase in c-Fos and AVP expression and an increase in superoxide dismutase 2 and catalase activities. α-Lipoic acid, a ROS scavenger, administrated during the 3 d before the hypertonic ip injection, abolished the increase of AVP. These findings establish that hyperosmolarity causes ROS production in the SON, which is essential for AVP increase. This demonstrates the importance of free radicals as physiological signaling molecules in the regulation of body-fluid balance.

Tempol ameliorates pharyngeal dilator muscle dysfunction in a rodent model of chronic intermittent hypoxia

Respiratory muscle dysfunction is implicated in the pathophysiology of obstructive sleep apnea syndrome (OSAS), an oxidative stress disorder prevalent in men. Pharmacotherapy for OSAS is an attractive option, and antioxidant treatments may prove beneficial. We examined the effects of chronic intermittent hypoxia (CIH) on breathing and pharyngeal dilator muscle structure and function in male and female rats. Additionally, we tested the efficacy of antioxidant treatment in preventing (chronic administration) or reversing (acute administration) CIH-induced effects in male rats. Adult male and female Wistar rats were exposed to alternating cycles of normoxia and hypoxia (90 s each; Fi(O(2)) = 5% O(2) at nadir; Sa(O(2)) ∼ 80%) or sham treatment for 8 h/d for 9 days. Tempol (1 mM, superoxide dismutase mimetic) was administered to subgroups of sham- and CIH-treated animals. Breathing was assessed by whole-body plethysmography. Sternohyoid muscle contractile and endurance properties were examined in vitro. Muscle fiber type and cross-sectional area and the activity of key metabolic enzymes were determined. CIH decreased sternohyoid muscle force in male rats only. This was not attributable to fiber transitions or alterations in oxidative or glycolytic enzyme activity. Muscle weakness after CIH was prevented by chronic Tempol supplementation and was reversed by acute antioxidant treatment in vitro. CIH increased normoxic ventilation in male rats only. Sex differences exist in the effects of CIH on the respiratory system, which may contribute to the higher prevalence of OSAS in male subjects. Antioxidant treatment may be beneficial as an adjunct OSAS therapy.

Hydrogen peroxide differentially affects activity in the pre-Bötzinger complex and hippocampus

Reactive oxygen species (ROS) modulate neuronal excitability. In the present study we examined the effects of hydrogen peroxide (H(2)O(2)), a well established ROS, on neuronal activity from two neonatal mouse brain regions, i.e., the pre-Bötzinger complex (preBötC) within the ventral respiratory column (VRC) and the CA1 area of the hippocampus. In the preBötC, 2.2 mM H(2)O(2) evoked a transient depression followed by augmentation of neuronal activity. The iron chelator deferoxamine (500 μM) did not prevent H(2)O(2)-mediated neuronal augmentation but prevented the initial depression. Combined application of Fe(2+) and H(2)O(2) only caused depression of the preBötC rhythm. In contrast, H(2)O(2) suppressed neuronal activity in the CA1 region, and this effect was accentuated by coapplication of Fe(2+) and H(2)O(2), suggesting that hydroxyl radical generated by Fenton reaction mediates the effects of H(2)O(2) on CA1 neuronal activity. Malondialdehyde (MDA) levels were monitored as an index of lipid peroxidation in H(2)O(2)-treated preBötC and CA1 areas. MDA levels were unaltered in H(2)O(2)-treated preBötC, whereas MDA levels were markedly elevated in the CA1 region. These findings suggest that 1) exogenous administration of H(2)O(2) exerts differential effects on neuronal activities of preBötC versus CA1 neuronal populations and 2) H(2)O(2) is a potent modulator of respiratory rhythmogenesis from the preBötC without affecting global oxidative status.

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.

Lipopolysaccharide attenuates phrenic long-term facilitation following acute intermittent hypoxia

Lipopolysaccharide (LPS) induces inflammatory responses, including microglial activation in the central nervous system. Since LPS impairs certain forms of hippocampal and spinal neuroplasticity, we hypothesized that LPS would impair phrenic long-term facilitation (pLTF) following acute intermittent hypoxia (AIH) in outbred Sprague-Dawley (SD) and inbred Lewis (L) rats. Approximately 3h following a single LPS injection (i.p.), the phrenic response during hypoxic episodes is reduced in both rat strains versus vehicle treated, control rats (SD: 84 ± 7% vs. 128 ± 14% baseline for control, p < 0.05; L: 62 ± 10% vs. 90 ± 9% baseline for control, p < 0.05). At 60 min post-AIH, pLTF is also diminished by LPS in both strains: (SD: 22 ± 5% vs. 73.5 ± 14% baseline for control, p < 0.05; L: 18 ± 15% vs. 56 ± 8% baseline for control, p < 0.05). LPS alone does not affect phrenic burst frequency in either rat strain, suggesting that acute LPS injection has minimal effect on brainstem respiratory rhythm generation. Thus, systemic LPS injections and (presumptive) inflammation impair pLTF, a form of spinal neuroplasticity in respiratory motor control. These results suggest that ongoing infection or inflammation must be carefully considered in studies of respiratory plasticity, or during attempts to harness spinal plasticity as a therapeutic tool in the treatment of respiratory insufficiency, such as spinal cord injury.

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.

Potentiation of the hypoxic ventilatory response by 1 day of hyperoxia in neonatal rats

The O(2) sensitivity of the neonatal rat carotid body is increased after 1 day in moderate hyperoxia (60% O(2)) (Donnelly et al., 2009). We investigated whether this enhanced peripheral chemosensitivity increases the hypoxic ventilatory response (HVR) and tested the hypothesis that this plasticity is mediated by the superoxide anion. Neonatal rats (7 d old) were injected with saline or MnTMPyP, a superoxide scavenger, and placed into 60% O(2) for 23-28h. Baseline ventilation was reduced and the acute HVR (12% O(2)) was enhanced in hyperoxia-treated rats relative to age-matched controls; MnTMPyP did not block these effects. An additional group of rats was studied after only 30min in 60% O(2). This shorter exposure had no effect on normoxic ventilation or the HVR. We conclude that 1 d, but not 30min, of 60% O(2) augments the HVR of neonatal rats and that production of the superoxide anion does not contribute to this plasticity.

Serotonin 2A and 2B receptor-induced phrenic motor facilitation: differential requirement for spinal NADPH oxidase activity

Acute intermittent hypoxia (AIH) facilitates phrenic motor output by a mechanism that requires spinal serotonin (type 2) receptor activation, NADPH oxidase activity and formation of reactive oxygen species (ROS). Episodic spinal serotonin (5-HT) receptor activation alone, without changes in oxygenation, is sufficient to elicit NADPH oxidase-dependent phrenic motor facilitation (pMF). Here we investigated: (1) whether serotonin 2A and/or 2B (5-HT2A/B) receptors are expressed in identified phrenic motor neurons, and (2) which receptor subtype is capable of eliciting NADPH-oxidase-dependent pMF. In anesthetized, artificially ventilated adult rats, episodic C4 intrathecal injections (3×6 μl injections, 5 min intervals) of a 5-HT2A (DOI) or 5-HT2B (BW723C86) receptor agonist elicited progressive and sustained increases in integrated phrenic nerve burst amplitude (i.e. pMF), an effect lasting at least 90 min post-injection for both receptor subtypes. 5-HT2A and 5-HT2B receptor agonist-induced pMF were both blocked by selective antagonists (ketanserin and SB206553, respectively), but not by antagonists to the other receptor subtype. Single injections of either agonist failed to elicit pMF, demonstrating a need for episodic receptor activation. Phrenic motor neurons retrogradely labeled with cholera toxin B fragment expressed both 5-HT2A and 5-HT2B receptors. Pre-treatment with NADPH oxidase inhibitors (apocynin and diphenylenodium (DPI)) blocked 5-HT2B, but not 5-HT2A-induced pMF. Thus, multiple spinal type 2 serotonin receptors elicit pMF, but they act via distinct mechanisms that differ in their requirement for NADPH oxidase activity.

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.

Protein kinase G-dependent mechanisms modulate hypoglossal motoneuronal excitability and long-term facilitation

Since protein kinase-dependent modulation of motoneuronal excitability contributes to adaptive changes in breathing, we hypothesized that cGMP-dependent pathways activating protein kinase G (PKG) modulate motoneuronal inspiratory drive currents and long-term plasticity. In a medullary slice preparation from neonatal rat (postnatal days 0-4) generating spontaneous respiratory-related rhythm, hypoglossal (XII) motoneuronal inspiratory drive currents and respiratory-related XII nerve activity were recorded. Focal application of a PKG activator, 8-bromoguanosine-3',5'-cyclomonophosphate (8-Br-cGMP), to voltage-clamped XII motoneurones decreased inspiratory drive currents. In the presence of tetrodotoxin (TTX), 8-Br-cGMP decreased the exogenous postsynaptic inward currents induced by focal application of AMPA. Intracellular dialysis of XII motoneurones with an inhibitory peptide to PKG (PKGI) increased endogenous inspiratory-drive currents and exogenous AMPA-induced currents. Application of 8-Br-cGMP with PKGI had no further effect on spontaneous or evoked currents, confirming that the observed effects were induced by PKG. However, PKG differentially increased longer-term plasticity. Three 3 min applications (separated by 5 min) of the α(1)-adrenergic agonist phenylephrine (PE) in combination with 8-Br-cGMP yielded greater in vitro long-term facilitation than PE alone. These data indicate the presence of a cGMP/PKG-dependent signalling pathway in XII motoneurones that modulates inspiratory drive currents and plasticity of XII motoneurones, possibly contributing to their adaptation during physiological challenges, such as sleep and exercise.

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.