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

Protocol-Specific Effects of Intermittent Hypoxia Pre-Conditioning on Phrenic Motor Plasticity in Rats with Chronic Cervical Spinal Cord Injury

"Low-dose" acute intermittent hypoxia (AIH; 3-15 episodes/day) is emerging as a promising therapeutic strategy to improve motor function after incomplete cervical spinal cord injury (cSCI). Conversely, chronic "high-dose" intermittent hypoxia (CIH; > 80-100 episodes/day) elicits multi-system pathology and is a hallmark of sleep apnea, a condition highly prevalent in individuals with cSCI. Whereas daily AIH (dAIH) enhances phrenic motor plasticity in intact rats, it is abolished by CIH. However, there have been no direct comparisons of prolonged dAIH versus CIH on phrenic motor outcomes after chronic cSCI. Thus, phrenic nerve activity and AIH-induced phrenic long-term facilitation (pLTF) were assessed in anesthetized rats. Experimental groups included: 1) intact rats exposed to 28 days of normoxia (Nx28; 21% O2; 8 h/day), and three groups with chronic C2 hemisection (C2Hx) exposed to either: 2) Nx28; 3) dAIH (dAIH28; 10, 5-min episodes of 10.5% O2/day; 5-min intervals); or 4) CIH (IH28-2/2; 2-min episodes; 2-min intervals; 8 h/day). Baseline ipsilateral phrenic nerve activity was reduced in injured versus intact rats but unaffected by dAIH28 or IH28-2/2. There were no group differences in contralateral phrenic activity. pLTF was enhanced bilaterally by dAIH28 versus Nx28 but unaffected by IH28-2/2. Whereas dAIH28 enhanced pLTF after cSCI, it did not improve baseline phrenic output. In contrast, unlike shorter protocols in intact rats, CIH28-2/2 did not abolish pLTF in chronic C2Hx. Mechanisms of differential responses to dAIH versus CIH are not yet known, particularly in the context of cSCI. Further, it remains unclear whether enhanced phrenic motor plasticity can improve breathing after cSCI.

Acute intermittent hypoxia as a potential adjuvant to improve walking following spinal cord injury: evidence, challenges, and future directions

Purpose of Review

The reacquisition and preservation of walking ability are highly valued goals in spinal cord injury (SCI) rehabilitation. Recurrent episodes of breathing low oxygen (i.e., acute intermittent hypoxia, AIH) is a potential therapy to promote walking recovery after incomplete SCI via endogenous mechanisms of neuroplasticity. Here, we report on the progress of AIH, alone or paired with other treatments, on walking recovery in persons with incomplete SCI. We evaluate the evidence of AIH as a therapy ready for clinical and home use and the real and perceived challenges that may interfere with this possibility.

Recent Findings

Repetitive AIH is a safe and an efficacious treatment to enhance strength, walking speed and endurance, as well as, dynamic balance in persons with chronic, incomplete SCI.

Summary

The potential for AIH as a treatment for SCI remains high, but further research is necessary to understand treatment targets and effectiveness in a large cohort of persons with SCI.

Identification of different clinical faces of obstructive sleep apnea in children

OBJECTIVE

This study aimed to identify the heterogeneity of obstructive sleep apnea syndrome clinical presentation in children.

PARTICIPANTS

Children who were 3-14 years old and with obstructive sleep apnea syndrome after polysomnography monitoring (apnea and hypopnea index>5 or obstructive apnea index>1) in the sleep center of Beijing Children's Hospital were included.

METHODS

A sleep disorder questionnaire including different combinations of symptoms and co-morbidities of obstructive sleep apnea syndrome in children was used. A cluster analysis was used to classify the data.

RESULTS

The apnea hypopnea index alone is not adequate to predict clinical phenotypes. Based on symptoms and co-morbidities of obstructive sleep apnea syndrome, three distinct clusters were identified. They were "nocturnal snoring and daytime sleepiness group" (cluster 1), "hyperactivity group" (cluster 2), and "minimally symptomatic group" (cluster 3). A prediction model was built according to eight variables which showed statistical significance by pairwise comparison among clusters. Overall accuracy of the prediction model could reach 86%. Both the sensitivity and specificity of cluster 2 and 3 prediction were around 90%.

CONCLUSION

Children with obstructive sleep apnea syndrome have different patterns of clinical presentation and the identification of the different clinical profiles of obstructive sleep apnea syndrome can provide clues for more personalised diagnoses and therapies.

Short-term sleep fragmentation enhances anxiety-related behavior: The role of hormonal alterations

Introduction

The neuroendocrine background of acute sleep fragmentation in obstructive sleep apnea and sleep fragmentation involvement in psychiatric comorbidities, common in these patients, are still largely unknown. The aim of this study was to determine the effects of short-term experimental sleep fragmentation on anxiety -like behavior and hormonal status in rats.

Methods

Male rats were adapted to treadmill (ON and OFF mode with belt speed set on 0.02m/s and 0.00m/s) and randomized to: 1) treadmill control (TC, only OFF mode); 2) motion, activity control (AC, 10min ON and 30min OFF mode) and 3) sleep fragmentation (SF, 30s ON and 90s OFF mode) group. Six hours later, the animals were tested in the open field, elevated plus maze and light/dark test (n = 8/group). Testosterone, estradiol, progesterone and corticosterone were determined in separate animal cohort immediately upon sleep fragmentation (n = 6/group).

Results

SF rats showed decreased rearings number, decreased time spent in the central area and increased thigmotaxic index compared to TC and AC rats in the open field test. Similarly, increased anxiety upon sleep fragmentation was observed in the elevated plus maze and the light/dark test. Significantly lower testosterone, estradiol and progesterone levels were determined in SF in comparison to AC and TC groups, while there was no significant difference in the levels of corticosterone.

Conclusion

Short term sleep fragmentation enhances anxiety-related behavior in rats, which could be partly mediated by the observed hormonal changes presented in the current study in form of testosterone, estradiol and progesterone depletion.

Intermittent hypercapnic hypoxia during sleep does not induce ventilatory long-term facilitation in healthy males

Intermittent hypoxia-induced ventilatory neuroplasticity is likely important in obstructive sleep apnea pathophysiology. Although concomitant CO2levels and arousal state critically influence neuroplastic effects of intermittent hypoxia, no studies have investigated intermittent hypercapnic hypoxia effects during sleep in humans. Thus the purpose of this study was to investigate if intermittent hypercapnic hypoxia during sleep induces neuroplasticity (ventilatory long-term facilitation and increased chemoreflex responsiveness) in humans. Twelve healthy males were exposed to intermittent hypercapnic hypoxia (24 × 30 s episodes of 3% CO2and 3.0 ± 0.2% O2) and intermittent medical air during sleep after 2 wk washout period in a randomized crossover study design. Minute ventilation, end-tidal CO2, O2saturation, breath timing, upper airway resistance, and genioglossal and diaphragm electromyograms were examined during 10 min of stable stage 2 sleep preceding gas exposure, during gas and intervening room air periods, and throughout 1 h of room air recovery. There were no significant differences between conditions across time to indicate long-term facilitation of ventilation, genioglossal or diaphragm electromyogram activity, and no change in ventilatory response from the first to last gas exposure to suggest any change in chemoreflex responsiveness. These findings contrast with previous intermittent hypoxia studies without intermittent hypercapnia and suggest that the more relevant gas disturbance stimulus of concomitant intermittent hypercapnia frequently occurring in sleep apnea influences acute neuroplastic effects of intermittent hypoxia. These findings highlight the need for further studies of intermittent hypercapnic hypoxia during sleep to clarify the role of ventilatory neuroplasticity in the pathophysiology of sleep apnea.

NEW & NOTEWORTHY Both arousal state and concomitant CO2levels are known modulators of the effects of intermittent hypoxia on ventilatory neuroplasticity. This is the first study to investigate the effects of combined intermittent hypercapnic hypoxia during sleep in humans. The lack of neuroplastic effects suggests a need for further studies more closely replicating obstructive sleep apnea to determine the pathophysiological relevance of intermittent hypoxia-induced ventilatory neuroplasticity.

Obstructive Sleep Apnea Mimics Attention Deficit Disorder

Attention deficit and hyperactivity are known possible symptoms or correlates of obstructive sleep apnea (OSA). However, these associations may be missed in children, because children often fail to report excessive daytime sleepiness, and attention deficit disorder (ADD) and attention deficit hyperactivity disorder (ADHD) are common primary diagnoses in themselves. We report on a 17-year-old, slender, non-snoring male who presented to his pediatrician with a prolonged history of four complaints: inattention, fidgeting, frequent sinusitis, and somnolence. He was diagnosed with ADHD, while the somnolence, which often abated somewhat upon use of antibiotics for sinusitis, was attributed to the sinus infections. A later sleep study revealed OSA, and thorough additional testing proved that the original ADHD diagnosis was in error. All four conditions were allayed with proper use of a continuous positive airway pressure (CPAP) machine.

Chronical sleep interruption-induced cognitive decline assessed by a metabolomics method

Good sleep is necessary for optimal health, especially for mental health. Insomnia, sleep deprivation will make your ability to learn and memory impaired. Nevertheless, the underlying pathophysiological mechanism of sleep disorders-induced cognitive decline is still largely unknown. In this study, the sleep deprivation of animal model was induced by chronical sleep interruption (CSI), the behavioral tests, biochemical index determinations, and a liquid chromatography-mass spectrometry (LC-MS) based serum metabolic profiling analysis were performed to explore the effects of CSI on cognitive function and the underlying mechanisms. After 14-days CSI, the cognitive function of the mice was evaluated by new objects preference (NOP) task and temporal order judgment (TOJ) task. Serum corticosterone (CORT), and brain Malondialdehyde (MDA), Superoxide Dismutase (SOD), and Catalase (CAT) levels were determined by ELISA kits. Data were analyzed by Principal Component Analysis (PCA), Partial Least Squares project to latent structures-Discriminant Analysis (PLS-DA), and Student's t-test. We found that the cognitive function of the mice was significantly affected by CSI. Besides, levels of CORT and MDA were higher, and SOD and CAT were lower in CSI mice than those of control. Obvious body weight loss of CSI mice was also observed. Thirteen potential serum biomarkers including choline, valine, uric acid, allantoic acid, carnitines, and retinoids were identified. Affected metabolic pathways involve metabolism of purine, retinoid, lipids, and amino acid. These results showed that CSI can damage the cognitive performance notably. The cognitive decline may ascribe to excessive oxidative stress and a series of disturbed metabolic pathways.

Effect of age on long-term facilitation and chemosensitivity during NREM sleep

The reason for increased sleep-disordered breathing with a predominance of central apneas in the elderly is unknown. We speculate that ventilatory control instability may provide a link between aging and the onset of unstable breathing during sleep. We sought to investigate potential underlying mechanisms in healthy, elderly adults during sleep. We hypothesized that there is 1) a decline in respiratory plasticity or long-term facilitation (LTF) of ventilation and/or 2) increased ventilatory chemosensitivity in older adults during non-, this should be hyphenated, non-rapid rapid eye movement (NREM) sleep. Fourteen elderly adults underwent 15, 1-min episodes of isocapnic hypoxia (EH), nadir O2 saturation: 87.0 ± 0.8%. Measurements were obtained during control, hypoxia, and up to 20 min of recovery following the EH protocol, respectively, for minute ventilation (VI), timing, and inspiratory upper-airway resistances (RUA). The results showed the following. 1) Compared with baseline, there was a significant increase in VI (158 ± 11%, P < 0.05) during EH, but this was not accompanied by augmentation of VI during the successive hypoxia trials nor in VI during the recovery period (94.4 ± 3.5%, P = not significant), indicating an absence of LTF. There was no change in inspiratory RUA during the trials. This is in contrast to our previous findings of respiratory plasticity in young adults during sleep. Sham studies did not show a change in any of the measured parameters. 2) We observed increased chemosensitivity with increased isocapnic hypoxic ventilatory response and hyperoxic suppression of VI in older vs. young adults during NREM sleep. Thus increased chemosensitivity, unconstrained by respiratory plasticity, may explain increased periodic breathing and central apneas in elderly adults during NREM sleep.

Spinal metaplasticity in respiratory motor control

A hallmark feature of the neural system controlling breathing is its ability to exhibit plasticity. Less appreciated is the ability to exhibit metaplasticity, a change in the capacity to express plasticity (i.e., “plastic plasticity”). Recent advances in our understanding of cellular mechanisms giving rise to respiratory motor plasticity lay the groundwork for (ongoing) investigations of metaplasticity. This detailed understanding of respiratory metaplasticity will be essential as we harness metaplasticity to restore breathing capacity in clinical disorders that compromise breathing, such as cervical spinal injury, motor neuron disease and other neuromuscular diseases. In this brief review, we discuss key examples of metaplasticity in respiratory motor control, and our current understanding of mechanisms giving rise to spinal plasticity and metaplasticity in phrenic motor output; particularly after pre-conditioning with intermittent hypoxia. Progress in this area has led to the realization that similar mechanisms are operative in other spinal motor networks, including those governing limb movement. Further, these mechanisms can be harnessed to restore respiratory and non-respiratory motor function after spinal injury.

The efficacy of antihypertensive drugs in chronic intermittent hypoxia conditions

Sleep apnea/hypopnea disorders include centrally originated diseases and obstructive sleep apnea (OSA). This last condition is renowned as a frequent secondary cause of hypertension (HT). The mechanisms involved in the pathogenesis of HT can be summarized in relation to two main pathways: sympathetic nervous system stimulation mediated mainly by activation of carotid body (CB) chemoreflexes and/or asphyxia, and, by no means the least important, the systemic effects of chronic intermittent hypoxia (CIH). The use of animal models has revealed that CIH is the critical stimulus underlying sympathetic activity and hypertension, and that this effect requires the presence of functional arterial chemoreceptors, which are hyperactive in CIH. These models of CIH mimic the HT observed in humans and allow the study of CIH independently without the mechanical obstruction component. The effect of continuous positive airway pressure (CPAP), the gold standard treatment for OSA patients, to reduce blood pressure seems to be modest and concomitant antihypertensive therapy is still required. We focus this review on the efficacy of pharmacological interventions to revert HT associated with CIH conditions in both animal models and humans. First, we explore the experimental animal models, developed to mimic HT related to CIH, which have been used to investigate the effect of antihypertensive drugs (AHDs). Second, we review what is known about drug efficacy to reverse HT induced by CIH in animals. Moreover, findings in humans with OSA are cited to demonstrate the lack of strong evidence for the establishment of a first-line antihypertensive regimen for these patients. Indeed, specific therapeutic guidelines for the pharmacological treatment of HT in these patients are still lacking. Finally, we discuss the future perspectives concerning the non-pharmacological and pharmacological management of this particular type of HT.

Sustained sleep fragmentation affects brain temperature, food intake and glucose tolerance in mice

Sleep fragmentation is present in numerous sleep pathologies and constitutes a major feature of patients with obstructive sleep apnea. A prevalence of metabolic syndrome, diabetes and obesity has been shown to be associated to obstructive sleep apnea. While sleep fragmentation has been shown to impact sleep homeostasis, its specific effects on metabolic variables are only beginning to emerge. In this context, it is important to develop realistic animal models that would account for chronic metabolic effects of sleep fragmentation. We developed a 14-day model of instrumental sleep fragmentation in mice, and show an impact on both brain-specific and general metabolism. We first report that sleep fragmentation increases food intake without affecting body weight. This imbalance was accompanied by the inability to adequately decrease brain temperature during fragmented sleep. In addition, we report that sleep-fragmented mice develop glucose intolerance. We also observe that sleep fragmentation slightly increases the circadian peak level of glucocorticoids, a factor that may be involved in the observed metabolic effects. Our results confirm that poor-quality sleep with sustained sleep fragmentation has similar effects on general metabolism as actual sleep loss. Altogether, these results strongly suggest that sleep fragmentation is an aggravating factor for the development of metabolic dysfunctions that may be relevant for sleep disorders such as obstructive sleep apnea.

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.

Sleep fragmentation attenuates the hypercapnic (but not hypoxic) ventilatory responses via adenosine A1 receptors in awake rats

Sleep fragmentation (SF) and intermittent hypoxia and hypercapnia are the primary events associated with obstructive sleep apnea (OSA). We previously found that SF eliminates ventilatory long-term facilitation and attenuates poikilocapnic hypoxic ventilatory responses (HVR). This study examined the effect of SF on isocapnic HVR and hypercapnic ventilatory responses (HCVR), and investigated the time course of and the role of adenosine A1 receptors in these SF effects in conscious adult male Sprague-Dawley rats. SF was achieved by periodic, forced locomotion in a rotating drum (30 s rotation/90 s stop for 24 h). Ventilation during baseline, isocapnic hypoxia (11% O₂ plus 4% CO₂) and hypercapnia (6% CO₂) was measured using plethysmography. About 1h after 24h SF, resting ventilation, arterial blood gases and isocapnic HVR (control: 169.3 ± 11.5% vs. SF: 170.0 ± 10.3% above baseline) were not significantly changed, but HCVR was attenuated (control: 172.8 ± 17.5% vs. SF: 129.5 ± 9.6%; P = 0.003). This attenuated HCVR then returned spontaneously to the control level ∼4 h after SF (168.9 ± 12.1%). This HCVR attenuation was also reversed (184.0 ± 17.5%) by systemic injection of the adenosine A1 receptor antagonist 8-CPT (2.5 mg/kg) shortly after SF, while 8-CPT at this dose had little effect on HCVR in control rats (169.9 ± 11.8%). Collectively, these results suggest that: (1) 24 h SF does not change isocapnic HVR but causes an attenuation of HCVR; and (2) this attenuation lasts for only a few hours and requires activation of adenosine A1 receptors.

Sleep loss reduces respiratory motor plasticity

Sleep loss leads to deficits in neuroplasticity that underlie important physiological functions such as learning and memory. However, the influence of sleep loss on respiratory motor plasticity is unclear. In this study, we examined the influence of sleep loss on plasticity of upper airway motor outflow induced by repeated obstructive apneas. Here, we demonstrate that repeated airway obstructions, as experienced during obstructive apnea (OSA), induce a long-term enhancement of upper airway respiratory muscle activity, and that short-term sleep deprivation (6 hours) reduces the magnitude of this response. Our results suggest that respiratory motor plasticity may be reduced or minimized in disease conditions that are characterized by abnormal sleep disturbances (e.g., OSA).