Increased oxidative stress is associated with chronic intermittent hypoxia-mediated brain cortical neuronal cell apoptosis in a mouse model of sleep apnea

Childhood obstructive sleep-disordered breathing: a clinical update and discussion of technological innovations and challenges

Childhood sleep-disordered breathing (SDB) has been known to be associated with health and cognitive impacts for more than a century, and yet our understanding of this disorder is in its infancy. Neuropsychological consequences in children with snoring or subtle breathing disturbances not meeting the traditional definition of sleep apnea suggest that "benign, or primary snoring" may be clinically significant, and that the true prevalence of SDB might be underestimated. There is no standard definition of SDB in children. The polysomnographic technology used in many sleep laboratories may be inadequate to diagnose serious but subtle forms of clinically important airflow limitation. In the last several years, advances in digital technology as well as new observational studies of respiratory and arousal patterns in large populations of healthy children have led to alternative views of what constitutes sleep-related breathing and arousal abnormalities that may refine our diagnostic criteria. This article reviews our knowledge of childhood SDB, highlights recent advances in technology, and discusses diagnostic and treatment strategies that will advance the management of children with pediatric SDB.

Nitric oxide deficit in chronic intermittent hypoxia impairs large conductance calcium-activated potassium channel activity in rat hippocampal neurons

Sleep apnea associated with chronic intermittent hypoxia (IH) impairs hippocampal functions but the pathogenic mechanisms involving dysfunction of nitric oxide (NO) and ionic channels remain unclear. We examined the hypothesis that hippocampal NO deficit impairs the activity of large conductance calcium-activated potassium (BK) channels in rats with chronic IH, mimicking conditions in patients with sleep apnea. A patch-clamp study was performed on hippocampal CA1 neurons acutely dissociated from IH and control rats. The levels of endogenous NO and intracellular calcium in the CA1 region of the hippocampal slices were measured respectively by electrochemical microsensors and spectrofluorometry. We found that the open probability of BK channels remarkably decreased in the CA1 pyramidal neurons in a time-dependent manner with the IH treatment, without changes in the unitary conductance and reversal potential. NO donors, SNP or DETA/NO, significantly restored the activity of BK channels in the IH neurons, which was prevented by blockade of S-nitrosylation with NEM or MTSES but not by inhibition of the cGMP pathway with ODQ or 8-bromo-cGMP. Endogenous NO levels were substantially lowered in the IH hippocampus during resting and hypoxia. Also, the level of protein expression of neuronal NO synthase was markedly lessened in the IH neurons with decreased intracellular calcium response to hypoxia. Collectively, the results suggest that the IH-induced NO deficit mediated by a down-regulation of the expression of neuronal NO synthase plays a causative role in the impaired activity of BK channels, which could account for the hippocampal injury in patients with sleep apnea.

Pediatric obstructive sleep apnea: complications, management, and long-term outcomes

Obstructive sleep apnea (OSA) in children has emerged not only as a relatively prevalent condition but also as a disease that imposes a large array of morbidities, some of which may have long-term implications, well into adulthood. The major consequences of pediatric OSA involve neurobehavioral, cardiovascular, and endocrine and metabolic systems. The underlying pathophysiological mechanisms of OSA-induced end-organ injury are now being unraveled, and clearly involve oxidative and inflammatory pathways. However, the roles of individual susceptibility (as dictated by single-nucleotide polymorphisms), and of environmental and lifestyle conditions (such as diet, physical, and intellectual activity), may account for a substantial component of the variance in phenotype. Moreover, the clinical prototypic pediatric patient of the early 1990s has been insidiously replaced by a different phenotypic presentation that strikingly resembles that of adults afflicted by the disease. As such, analogous to diabetes, the terms type I and type II pediatric OSA have been proposed. The different manifestations of these two entities and their clinical course and approaches to management are reviewed.

Attenuation of heart rate control and neural degeneration in nucleus ambiguus following chronic intermittent hypoxia in young adult Fischer 344 rats

Chronic intermittent hypoxia (CIH) attenuates baroreflex control of heart rate (HR). In this study, we assessed whether CIH exposure reduced nucleus ambiguus (NA) control of HR and induced neural degeneration in the NA. Fischer 344 (age: 3-4 months) rats were exposed to either room air (RA: normoxia) or intermittent hypoxia for 35-50 days. At the end of these exposures, animals were anesthetized with pentobarbital. HR responses to arterial blood pressure (AP) changes induced by phenylephrine (PE) and sodium nitroprusside (SNP) were measured. In another set of rats, HR and AP responses to L-glutamate (L-Glu) microinjections (10 mM, 20 nl) into the left NA and electrical stimulation of the left cervical vagus nerve at 1-30 Hz (0.5 mA, 1 ms) for 20 s were measured. Brainstem slices at the level of -800, -400, 0, +400, +800 microm relative to the obex were processed in additional rats using Nissl staining. The NA was identified by retrogradely labeling vagal motoneurons using the tracer tetramethylrhodamine dextran (TMR-D) which was injected into the ipsilateral nodose ganglion. We found that CIH significantly 1) reduced the baroreflex control of HR (slope RA: -1.2+/-0.2 bpm/mmHg; CIH -0.5+/-0.1 bpm/mmHg; P<0.05); 2) attenuated the HR responses to l-Glu injections into the NA [HR: -280+/-15 (RA) vs. -235+/-16 (CIH) beats/min; P<0.05]; 3) augmented the HR responses to electrical stimulation of the vagus (P<0.05); 4) induced a significant cellular loss in the NA region (P<0.05). Thus, CIH induces a cell loss in the NA region which may contribute to attenuation of baroreflex sensitivity and NA control of HR following CIH.

Green tea catechin polyphenols attenuate behavioral and oxidative responses to intermittent hypoxia

RATIONALE

The intermittent hypoxia (IH) that characterizes sleep-disordered breathing impairs spatial learning and increases NADPH oxidase activity and oxidative stress in rodents. We hypothesized that green tea catechin polyphenols (GTPs) may attenuate IH-induced neurobehavioral deficits by reducing IH-induced NADPH oxidase expression, lipid peroxidation, and inflammation.

OBJECTIVES

To assess the effects of GTP administered in drinking water on the cognitive, inflammatory, and oxidative responses to long-term (>14 d) IH during sleep in male Sprague-Dawley rats.

METHODS

Cognitive assessments were conducted in the Morris water maze. We measured levels and expression of malondialdehyde (MDA), prostaglandin E(2), p47(phox) subunit of NADPH oxidase, receptor for advanced glycation end products (RAGE), and glial fibrillary acidic protein expression in rodent brain tissue.

MEASUREMENTS AND MAIN RESULTS

GTP treatment prevented IH-induced decreases in spatial bias for the hidden platform during the Morris water maze probe trails as well as IH-induced increases in p47phox expression within the hippocampal CA1 region. In untreated animals, IH exposure was associated with doubling of cortical MDA levels in comparison to room air control animals, and GTP-treated animals exposed to IH showed a 40% reduction in MDA levels. Increases in brain RAGE and glial fibrillary acidic protein expression were observed in IH-exposed animals, and these increases were attenuated in animals treated with GTP.

CONCLUSIONS

Oral GTP attenuates IH-induced spatial learning deficits and mitigates IH-induced oxidative stress through multiple beneficial effects on oxidant pathways. Because oxidative processes underlie neurocognitive deficits associated with IH, the potential therapeutic role of GTP in sleep-disordered breathing deserves further exploration.

Neurocognitive and behavioral morbidity in children with sleep disorders

PURPOSE OF REVIEW

This review examines in detail progress made regarding our understanding of the presence and pathophysiology of cognitive and behavioral morbidities among children with sleep disorders in general. Particular focus is given to pediatric obstructive sleep apnea.

RECENT FINDINGS

In recent years, increased awareness of the morbid consequences of respiratory sleep disturbances in children has emerged. Evidence suggesting a causal association of intermittent hypoxia and sleep fragmentation with alterations in memory, attention, and intelligence has accumulated. Research has also identified a link between sleep disorders, and problematic and hyperactive behaviors and mood disturbances. Furthermore, there is considerable inter-individual variability in the presence and magnitude of neurobehavioral morbidity at any given level of disease severity. This further suggests that, in addition to the disease per se, both genetic (individual susceptibility) and environmental modifiers play a role in determining morbidity.

SUMMARY

A more individually tailored approach to detecting morbidity associated with sleep disorders in children, employing biomarkers and gene-related single nucleotide polymorphisms, may ultimately be required to allow more rational prioritization of treatment.

Oxidative stress impairs upper airway muscle endurance in an animal model of sleep-disordered breathing

Obstructive sleep apnoea is characterised by intermittent hypoxia due to recurrent obstructions of the pharyngeal airway during sleep. We have shown that chronic intermittent hypoxia impairs respiratory muscle function and CNS control of upper airway patency. In this study, we tested the hypothesis that disruption of an endogenous antioxidant defence system exacerbates the effects of intermittent hypoxia on upper airway muscle contractile function. Thirty-two male Wistar rats were placed in restrainers with their heads in hoods in which the ambient oxygen concentration could be modified by controlling the gas supply to the hoods. Sixteen rats were exposed to alternating equal periods of hypoxia and normoxia, twice per minute, 8 hours per day for 1 week. The remaining 16 animals were exposed to normoxia continuously under identical experimental conditions. In both groups, half the animals received daily injections of buthionine sulfoxamine (BSO), an inhibitor of the rate-limiting enzyme in glutathione synthesis. The other half received daily vehicle injections. At the end of the 1-week treatment period, the sternohyoid muscles were removed and fatigue characteristics were determined in vitro. Intermittent hypoxia was associated with a decrease in sternohyoid muscle endurance, an effect that was exacerbated by treatment with BSO. In separate experiments, daily treatment with the antioxidant N-acetyl cysteine blocked the deleterious effects of intermittent hypoxia on respiratory muscle function. We suggest that oxidative stress contributes to impaired upper airway muscle endurance in our animal model and that endogenous glutathione may be especially important in limiting free radical-induced muscle dysfunction. Our results may have particular relevance to respiratory disorders associated with recurrent hypoxia, such as the sleep apnoea/hypopnoea syndrome.

Differential effects of chronic intermittent and chronic constant hypoxia on postnatal growth and development

Exposure to chronic constant or intermittent hypoxia (CCH or CIH) may have different effects on growth and development in early life. In this work, we exposed postnatal day 2 (P2) CD1 mice to CCH or CIH (11% O2) for 4 weeks and examined the effect of hypoxia on body and organ growth until P30. Regression analysis showed that weight increased in control, CCH and CIH cohorts with age with r2 values of 0.99, 0.97, and 0.94, respectively. Between days 2 and 30, slopes were 0.93+/-0.057, 0.76+/-0.108, and 0.63+/-0.061 (g/day, means+/-SEM) for control, CIH, and CCH, respectively and significantly different from each other (P<0.001). The slopes between P2 and P16 were 0.78+/-0.012, 0.46+/-0.002, and 0.47+/-0.019 for control, CCH and CIH, respectively. From P16 to 30, slopes were 1.12+/-0.033, 1.09+/-0.143, and 0.82+/-0.08 for control, CIH, and CCH, respectively with no significant difference from each other, suggesting a catch-up growth in the latter part of the hypoxic period. Slower weight gain resulted in a 12% and 23% lower body weight in CIH and CCH mice (P<0.001) by P30. Lung/body ratios were 0.010, 0.015, 0.015 for control, CIH, and CCH at P30, respectively. The decrease in liver, kidney, and brain weight were greater in CCH than CIH. Smaller liver weight was shown to be due to a reduction in cell size and cell number. Liver in CIH and CCH mice showed a 5% and 10% reduction in cell size (P<0.05) and a reduction of 28% in cell number (P<0.001) at P30. In contrast, CCH and CIH heart weight was 13% and 33% greater than control at P30 (P<0.05), respectively. This increase in the heart weight was due to an increase in the size of cardiomyocytes which showed an increase of 12% and 14% (P<0.001) for CIH and CCH, respectively as compared to control. Brain weight was 0.48 and 0.46 g for CIH and CCH, respectively (95% and 92% of normal). We concluded that (a) CIH and CCH follow different body and organ growth patterns; (b) mostly with CCH, the liver and kidneys are reduced in size in a proportionate way to body size but heart, lung, and brain are either spared or increased in size compared to body weight; and (c) the decrease in liver is secondary mostly to a decrease in cell number.

Left ventricular dysfunction and associated cellular injury in rats exposed to chronic intermittent hypoxia

Obstructive sleep apnea (OSA) increases cardiovascular morbidity and mortality. We have reported that chronic intermittent hypoxia (CIH), a direct consequence during OSA, leads to left ventricular (LV) remodeling and dysfunction in rats. The present study is to determine LV myocardial cellular injury that is possibly associated with LV global dysfunction. Fifty-six rats were exposed either to CIH (nadir O(2) 4-5%) or sham (handled normoxic controls, HC), 8 h/day for 6 wk. At the end of the exposure, we studied LV global function by cardiac catheterization, and LV myocardial cellular injury by in vitro analyses. Compared with HC, CIH animals demonstrated elevations in mean arterial pressure and LV end-diastolic pressure, but reductions in cardiac output (CIH 141.3 +/- 33.1 vs. HC 184.4 +/- 21.2 ml x min(-1) x kg(-1), P < 0.01), maximal rate of LV pressure rise in systole (+dP/dt), and maximal rate of LV pressure fall in diastole (-dP/dt). CIH led to significant cell injury in the left myocardium, including elevated LV myocyte size, measured by cell surface area (CIH 3,564 +/- 354 vs. HC 2,628 +/- 242 microm(2), P < 0.05) and cell length (CIH 148 +/- 23 vs. HC 115 +/- 16 microm, P < 0.05), elevated terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL)-stained positive cell number (CIH 98 +/- 45 vs. HC 15 +/- 13, P < 0.01), elevated caspase-3 activity (906 +/- 249 vs. 2,275 +/- 1,169 pmol x min(-1) x mg(-1), P < 0.05), and elevated expression of several remodeling gene markers, including c-fos, atrial natriuretic peptide, beta-myosin heavy chain, and myosin light chain-2. However, there was no difference between groups in sarcomere contractility of isolated LV myocytes, or in LV collagen deposition on trichrome-stained slices. In conclusion, CIH-mediated LV global dysfunction is associated with myocyte hypertrophy and apoptosis at the cellular level.

Inflammation and oxidative stress in obstructive sleep apnea syndrome

Similar to obesity, with which it is closely associated, obstructive sleep apnea syndrome (OSAS) is rapidly becoming a worldwide epidemic. Current knowledge of its pathogenesis has been significantly enriched by numerous experimental studies that have demonstrated an important role of oxidative stress and inflammation. Furthermore, new and exciting data strongly connect these two components in the perpetuation of the condition via the overexpression of nuclear factor kappaB. Experimental data support the hypothesis that nutrition might represent a promising future approach with antioxidants currently being good candidates for the modulation of cardiovascular sequelae, although weight reduction and controlled positive airway pressure remain the only established treatments for OSAS. We discuss herein the recent literature that illustrates these new paradigms and speculate on possible implications and future scenarios.

Acute Hypoxia, Diabetes, and Neuroimmune Dysregulation: Converging Mechanisms in the Brain

Acute hypoxia is experienced by a variety of individuals (neonates to the elderly) and in an assortment of conditions and diseases (terrorist bomb attack to decompensated heart failure). Increasingly, elaboration of inflammatory cytokines appears key to the brain-based response to hypoxia, as evidenced by the biobehaviors of malaise, fatigue, lethargy, and loss of interest in the physical and social environment. These sickness symptoms implicate hypoxia-dependent activation of the neuroimmune system as a key component of acute hypoxia. Type 2 diabetes (T2D) is associated with increased incidence, severity, and delayed recovery from hypoxic events. Why T2D negatively affects acute hypoxia is not well understood. Recent work, however, reveals that anti-inflammatory pathways tied to the interleukin (IL)-1β arm of the neuroimmune system may be critical. In this review, the authors examine the link between acute hypoxia, T2D, and neuroimmunity. NEUROSCIENTIST 14(3):235–239, 2008. DOI: 10.1177/1073858407309544

Cardiovascular morbidity in obstructive sleep apnea: oxidative stress, inflammation, and much more

Sleep-disordered breathing and obstructive sleep apnea (OSA) are highly prevalent disorders throughout the lifespan, which may affect up to 2-10% of the population, and have now been firmly associated with an increased risk for cardiovascular and neurobehavioral complications. Nevertheless, the overall pathophysiologic mechanisms mediating end-organ injury in OSA remain undefined, particularly due to the very frequent coexistence of other disease states, such as obesity, that clearly complicate the potential cause-effect relationships. Two major, and to some extent overlapping, mechanisms have been proposed to explain the morbid consequences of OSA, namely increased generation and propagation of reactive oxygen species and initiation and amplification of inflammatory processes. The evidence supporting the validity of these concepts as well as that detracting from such mechanisms will be critically reviewed in the context of clinical and laboratory-based approaches. In addition, some of the contradictory issues raised by such evaluation of the literature will be interpreted in the context of putative modifications of the individual responses to OSA, as determined by genetic variants among susceptibility-related genes, and also by potential environmental modulators of the phenotypic expression of any particular end-organ morbidity associated with OSA.

Manganese superoxide dismutase protects mouse cortical neurons from chronic intermittent hypoxia-mediated oxidative damage

Obstructive sleep apnea (OSA) syndrome has been recognized as a highly prevalent public health problem and is associated with major neurobehavioral morbidity. Chronic intermittent hypoxia (CIH), a major pathological component of OSA, increases oxidative damage to the brain cortex and decreases neurocognitive function in rodent models resembling human OSA. We employed in vitro and in vivo approaches to identify the specific phases and subcellular compartments in which enhanced reactive oxygen species (ROS) are generated during CIH. In addition, we utilized the cell culture and animal models to analyze the consequences of enhanced production of ROS on cortical neuronal cell damage and neurocognitive dysfunction. In a primary cortical neuron culture system, we demonstrated that the transition phase from hypoxia to normoxia (NOX) during CIH generates more ROS than the transition phase from NOX to hypoxia or hypoxia alone, all of which generate more ROS than NOX. Using selective inhibitors of the major pathways underlying ROS generation in the cell membrane, cytosol, and mitochondria, we showed that the mitochondria are the predominant source of enhanced ROS generation during CIH in mouse cortical neuronal cells. Furthermore, in both cell culture and transgenic mice, we demonstrated that overexpression of MnSOD-decreased CIH-mediated cortical neuronal apoptosis, and reduced spatial learning deficits measured with the Morris water maze assay. Together, the data from the in vitro and in vivo experiments indicate that CIH-mediated mitochondrial oxidative stress may play a major role in the neuronal cell loss and neurocognitive dysfunction in OSA. Thus, therapeutic strategies aiming at reducing ROS generation from mitochondria may improve the neurobehavioral morbidity in OSA.

Cellular mechanisms associated with intermittent hypoxia

Hypoxia, i.e. decreased availability of oxygen occurs under many different circumstances and can be either continuous or intermittent. Continuous hypoxia such as that experienced during periods of high altitude leads to physiological adaptations, whereas chronic IH (intermittent hypoxia) associated with sleep-disordered breathing manifested as recurrent apneas leads to morbidity. The purpose of the present chapter is to highlight recent findings on cellular responses to IH. Studies on cell culture models of IH revealed that for a given duration and intensity, IH is more potent than continuous hypoxia in evoking transcriptional activation. IH activates HIF-1 (hypoxia-inducible factor-1), the immediate early gene c-fos, activator protein-1, nuclear factor kappaB and cAMP-response-element-binding protein. Physiological studies showed that HIF-1 plays an important role in chronic IH-induced autonomic abnormalities in mice. IH affects expression of proteins associated with neuronal survival and apoptosis, as well as post-translational modifications of proteins resulting in increased biological activity. Comparisons between continuous hypoxia and IH revealed notable differences in the kinetics of protein kinase activation, type of protein kinase being activated and the downstream targets of protein kinases. IH increases ROS (reactive oxygen species) generation both in cell culture and in intact animals, and ROS-mediated signalling mechanisms contribute to cellular and systemic responses to IH. Future studies utilizing genomic and proteomic approaches may provide important clues to the mechanisms by which IH leads to morbidity as opposed to continuous hypoxia-induced adaptations. Cellular mechanisms associated with IH (other than recurrent apneas) such as repetitive, brief ascents to altitude, however, remain to be studied.

Selective loss of catecholaminergic wake active neurons in a murine sleep apnea model

The presence of refractory wake impairments in many individuals with severe sleep apnea led us to hypothesize that the hypoxia/reoxygenation events in sleep apnea permanently damage wake-active neurons. We now confirm that long-term exposure to hypoxia/reoxygenation in adult mice results in irreversible wake impairments. Functionality and injury were next assessed in major wake-active neural groups. Hypoxia/reoxygenation exposure for 8 weeks resulted in vacuolization in the perikarya and dendrites and markedly impaired c-fos activation response to enforced wakefulness in both noradrenergic locus ceruleus and dopaminergic ventral periaqueductal gray wake neurons. In contrast, cholinergic, histaminergic, orexinergic, and serotonergic wake neurons appeared unperturbed. Six month exposure to hypoxia/reoxygenation resulted in a 40% loss of catecholaminergic wake neurons. Having previously identified NADPH oxidase as a major contributor to wake impairments in hypoxia/reoxygenation, the role of NADPH oxidase in catecholaminergic vulnerability was next addressed. NADPH oxidase catalytic and cytosolic subunits were evident in catecholaminergic wake neurons, where hypoxia/reoxygenation resulted in translocation of p67(phox) to mitochondria, endoplasmic reticulum, and membranes. Treatment with a NADPH oxidase inhibitor, apocynin, throughout hypoxia/reoxygenation exposures conferred protection of catecholaminergic neurons. Collectively, these data show that select wake neurons, specifically the two catecholaminergic groups, can be rendered persistently impaired after long-term exposure to hypoxia/reoxygenation, modeling sleep apnea; wake impairments are irreversible; catecholaminergic neurons are lost; and neuronal NADPH oxidase contributes to this injury. It is anticipated that severe obstructive sleep apnea in humans destroys catecholaminergic wake neurons.

Reactive oxygen species contribute to sleep apnea-induced hypertension in rats

In clinical studies, sleep apnea is associated with hypertension, oxidative stress, and increased circulating endothelin-1 (ET-1). We previously developed a model of sleep apnea by exposing rats to eucapnic intermittent hypoxia (IH-C) during sleep, which increases both blood pressure and plasma levels of ET-1. Because similar protocols in mice increase tissue and plasma markers of oxidative stress, we hypothesized that IH-C generation of reactive oxygen species (ROS) contributes to the development of ET-1-dependent hypertension in IH-C rats. To test this, male Sprague-Dawley rats were instrumented with indwelling blood pressure telemeters and drank either plain water or water containing the superoxide dismutase mimetic, Tempol (4-hydroxy-2,2,6,6-tetramethyl-piperidine-1-oxyl, 1 mM). Mean arterial pressure (MAP) and heart rate (HR) were recorded for 3 control days and 14 treatment days with rats exposed 7 h/day to IH-C or air/air cycling (Sham). On day 14, MAP in IH-C rats treated with Tempol (107 +/- 2.29 mmHg) was significantly lower than in untreated IH-C rats (118 +/- 9 mmHg, P < 0.05). Tempol did not affect blood pressure in sham-operated rats (Tempol = 101 +/- 3, water = 101 +/- 2 mmHg). Immunoreactive ET-1 was greater in plasma from IH-C rats compared with plasma from sham-operated rats but was not different from Sham in Tempol-treated IH-C rats. Small mesenteric arteries from IH-C rats but not Tempol-treated IH-C rats had increased superoxide levels as measured by ferric cytochrome c reduction, lucigenin signaling, and dihydroethidium fluorescence. The data show that IH-C increases ET-1 production and vascular ROS levels and that scavenging superoxide prevents both. Thus oxidative stress appears to contribute to increases in ET-1 production and elevated arterial pressure in this rat model of sleep apnea-induced hypertension.

Urinary F2-isoprostane metabolite levels in children with sleep-disordered breathing

Oxidant stress-related mechanisms have been proposed as a major contributor to the increased prevalence of cardiovascular morbidity in adult patients with sleep-disordered breathing. Isoprostanes provide a reliable biomarker of oxidant injury in vivo. The purpose of the present study was to examine the hypothesis that oxidant stress, as evidenced by increased levels of F2-isoprostane metabolites (IsoP-m) in urine, is present in children with a spectrum of sleep-disordered breathing. Assays were performed on urinary samples obtained from each of 47 pediatric patients immediately upon awakening after standard overnight polysomnography. Of the subjects, 15% had mild, 9% had moderate, and 6% had severe sleep-disordered breathing. After controlling for correlations between BMI and IsoP-m and SpO2 values, IsoP-m values were unrelated to any polysomnographic measures. The absence of increased levels of urinary F2-isoprostane metabolites in children with sleep-disordered breathing suggests that oxidative stress is not a significant feature of pediatric sleep-disordered breathing.

Selective impairment of central mediation of baroreflex in anesthetized young adult Fischer 344 rats after chronic intermittent hypoxia

Baroreflex control of heart rate (HR) is impaired after chronic intermittent hypoxia (CIH). However, the location and nature of this response remain unclear. We examined baroreceptor afferent, vagal efferent, and central components of the baroreflex circuitry. Fischer 344 (F344) rats were exposed to room air (RA) or CIH for 35-50 days and were then anesthetized with isoflurane, ventilated, and catheterized for measurement of mean arterial blood pressure (MAP) and HR. Baroreceptor function was characterized by measuring percent changes of integrated aortic depressor nerve (ADN) activity (Int ADNA) relative to the baseline value in response to sodium nitroprusside- and phenylephrine-induced changes in MAP. Data were fitted to a sigmoid logistic function curve. HR responses to electrical stimulation of the left ADN and the right vagus nerve were assessed under ketamine-acepromazine anesthesia. Compared with RA controls, CIH significantly increased maximum baroreceptor gain or maximum slope, maximum Int ADNA, and Int ADNA range (maximum - minimum Int ADNA), whereas other parameters of the logistic function were unchanged. In addition, CIH increased the maximum amplitude of bradycardic response to vagal efferent stimulation and decreased the time from stimulus onset to peak response. In contrast, CIH significantly reduced the maximum amplitude of bradycardic response to left ADN stimulation and increased the time from stimulus onset to peak response. Therefore, CIH decreased central mediation of the baroreflex but augmented baroreceptor afferent function and vagal efferent control of HR.

Chronic intermittent hypoxia impairs baroreflex control of heart rate but enhances heart rate responses to vagal efferent stimulation in anesthetized mice

Chronic intermittent hypoxia (CIH) leads to increased sympathetic nerve activity and arterial hypertension. In this study, we tested the hypothesis that CIH impairs baroreflex (BR) control of heart rate (HR) in mice, and that decreased cardiac chronotropic responsiveness to vagal efferent activity contributes to such impairment. C57BL/6J mice were exposed to either room air (RA) or CIH (6-min alternations of 21% O(2) and 5.7% O(2), 12 h/day) for 90 days. After the treatment period, mice were anesthetized (Avertin) and arterial blood pressure (ABP) was measured from the femoral artery. Mean ABP (MABP) was significantly increased in mice exposed to CIH (98.7 +/- 2.5 vs. RA: 78.9 +/- 1.4 mmHg, P < 0.001). CIH increased HR significantly (584.7 +/- 8.9 beats/min; RA: 518.2 +/- 17.9 beats/min, P < 0.05). Sustained infusion of phenylephrine (PE) at different doses (0.1-0.4 microg/min) significantly increased MABP in both CIH and RA mice, but the ABP-mediated decreases in HR were significantly attenuated in mice exposed to CIH (P < 0.001). In contrast, decreases in HR in response to electrical stimulation of the left vagus nerve (30 microA, 2-ms pulses) were significantly enhanced in mice exposed to CIH compared with RA mice at low frequencies. We conclude that CIH elicits a sustained impairment of baroreflex control of HR in mice. The blunted BR-mediated bradycardia occurs despite enhanced cardiac chronotropic responsiveness to vagal efferent stimulation. This suggests that an afferent and/or a central defect is responsible for the baroreflex impairment following CIH.

Obstructive sleep apnea, cardiovascular disease, and inflammation—is NF-κB the key?

Obstructive sleep apnea (OSA) affects a large portion of the population and is associated with repeated airway collapse leading to chronic intermittent hypoxia, exaggerated swings in intrathoracic pressure and post apneic arousal. OSA is associated with heightened sympathoadrenal tone and is a risk factor for cardiovascular mortality and morbidity. In addition to well-known mechanical and autonomic effects, OSA appears to be associated with systemic inflammation. This could provide one mechanism leading to cardiovascular disease (CVD). A central factor in the inflammatory cascade is nuclear factor kappa B (NF-kappaB), which is involved in the transcription of numerous genes involved in the inflammatory cascade. The object of this article is to review recent literature on some of the aspects of OSA related to a proinflammatory state and the possible role of NF-kappaB as one mechanism providing a link between sleep apnea and CVD.

Sleep apnea is associated with bronchial inflammation and continuous positive airway pressure-induced airway hyperresponsiveness

BACKGROUND

Obstructive sleep apnea syndrome (OSA) is associated with systemic and upper airway inflammation. Pharyngeal inflammation has a potential role in upper airway collapse, whereas systemic inflammation relates to cardiovascular morbidity. However, the presence of an inflammatory involvement of lower airway has been poorly investigated.

OBJECTIVE

The aim of the study was to demonstrate an inflammatory process at the bronchial level in patients with OSA and to analyze effects of continuous positive airway pressure (CPAP) application and humidification on bronchial mucosa.

METHODS

The study was conducted by using sequential induced sputum for cell analysis and IL-8 production, nitric oxide exhalation measurement, and methacholine challenge before and after CPAP.

RESULTS

Bronchial neutrophilia and a high IL-8 concentration were observed in untreated OSA compared with controls (75% +/- 20% vs 43% +/- 12%, P < .05; and 25.02 +/- 9.43 ng/mL vs 8.6 +/- 3.7 ng/mL, P < .001, respectively). IL-8 in sputum supernatant was correlated to apnea hypopnea index (P < .01; r = 0.81). After 1 month of CPAP, this inflammatory pattern remained unchanged, and an increase in airway hyperresponsiveness (AHR) was observed (P < .001).

CONCLUSION

Obstructive sleep apnea syndrome is associated with bronchial inflammation. Our data demonstrate CPAP effect on the development of AHR, possibly facilitated by the pre-existing inflammation. Both issues should be evaluated during long-term CPAP use.

CLINICAL IMPLICATIONS

Results showing a spontaneous bronchial inflammation in OSA and the development of a CPAP-related AHR require a long-term follow-up to evaluate consequences on chronic bronchial obstruction.

Impaired spatial working memory and altered choline acetyltransferase (CHAT) immunoreactivity and nicotinic receptor binding in rats exposed to intermittent hypoxia during sleep

Exposure to intermittent hypoxia (IH), such as occurs in sleep-disordered breathing (SDB), is associated with cognitive impairment, neurodegeneration, oxidative stress, and inflammatory responses within rodent brain regions such as the basal forebrain. In this region, damage to cholinergic neurons correlates with working memory deficits in a number of neurodegenerative disorders, suggesting that degeneration of cholinergic systems may also contribute to the working memory impairments observed after IH exposures. We therefore examined basal forebrain choline acetyltransferase (CHAT) immunohistochemistry, nicotinic receptor binding in the prefrontal cortex (PFC), and working memory, in male rats tested on a delayed matching to place (DMP) task in the water maze following exposure to either room air (RA) or intermittent hypoxia (IH; alternating 90s epochs of 21% and 10% O(2) during sleep). IH-treated animals displayed impaired working memory with respect to controls, along with significant reductions in CHAT-stained neurons in the medial septal nucleus, in both the vertical and horizontal limbs of the diagonal band, and the substantia inominata after 14 days of IH exposure. In addition, increases in nicotinic binding and receptor affinity in the PFC were observed after 14 days of IH exposure. Thus, a loss of cholinergic neuronal phenotype in the basal forebrain may contribute to the cognitive impairments associated with CIH exposure. However, compensatory mechanisms may also be activated in other brain regions, and may provide potential therapeutic targets for the cognitive impairments associated with SDB.

Correlation between severity of sleep apnea and upper airway morphology based on advanced anatomical and functional imaging

Determination of the apnea hypopnea index (AHI) as a measure of the severity of obstructive sleep apnea/hypopnea syndrome (OSAHS) is a widely accepted methodology. However, the outcome of such a determination depends on the method used, is time consuming and insufficient for prediction of the effect of all treatment modalities. For these reasons more methods for evaluating the severity of OSAHS, based on different imaging modalities, have been looked into and recent studies have shown that anatomical properties determined from three-dimensional (3D) computed tomography (CT) images are good markers for the severity of the OSAHS. Therefore, we correlated anatomical measurements of a 3D reconstruction of the upper airway together with flow simulation results with the severity of OSAHS in order to find a combination of variables to indicate the severity of OSAHS in patients. The AHI of 20 non-selected, consecutive patients has been determined during a polysomnography. All patients also underwent a CT scan from which a 3D model of the upper airway geometry was reconstructed. This 3D model was used to evaluate the anatomical properties of the upper airway in OSAHS patients as well as to perform computational fluid dynamics (CFD) computations to evaluate the airflow and resistance of this upper airway. It has been shown that a combination of the smallest cross-sectional area and the resistance together with the body mass index (BMI) form a set of markers that predict very well the severity of OSAHS in patients within this study. We believe that these markers can be used to evaluate the outcome of an OSAHS treatment.

Reactive oxygen species mediate central cardiorespiratory network responses to acute intermittent hypoxia

Although oxidative stress and reactive oxygen species generation is typically associated with localized neuronal injury, reactive oxygen species have also recently been shown to act as a physiological signal in neuronal plasticity. Here we define an essential role for reactive oxygen species as a critical stimulus for cardiorespiratory reflex responses to acute episodic hypoxia in the brain stem. To examine central cardiorespiratory responses to episodic hypoxia, we used an in vitro medullary slice that allows simultaneous examination of rhythmic respiratory-related activity and synaptic neurotransmission to cardioinhibitory vagal neurons. We show that whereas continuous hypoxia does not stimulate excitatory neurotransmission to cardioinhibitory vagal neurons, acute intermittent hypoxia of equivalent duration incrementally recruits an inspiratory-evoked excitatory neurotransmission to cardioinhibitory vagal neurons during intermittent hypoxia. This recruitment was dependent on the generation of reactive oxygen species. Further, we demonstrate that reactive oxygen species are incrementally generated in glutamatergic neurons in the ventrolateral medulla during intermittent hypoxia. These results suggest a neurochemical basis for the pronounced bradycardia that protects the heart against injury during intermittent hypoxia and demonstrates a novel role of reactive oxygen species in the brain stem.

Changes in brain gene expression after long-term sleep deprivation

Long-term sleep deprivation in rats produces dramatic physiological changes including increase in energy expenditure, decrease in body weight, and death after 2-3 weeks. Despite several studies, the sleep deprivation syndrome remains largely unexplained. Here, to elucidate how prolonged sleep loss affects brain cells we used microarrays and screened the expression of > 26 000 transcripts in the cerebral cortex. Rats were sleep deprived using the disk-over-water method for 1 week. Seventy-five transcripts showed increased expression in these animals relative to controls that had been spontaneously awake or sleep deprived for a few hours. Most of them were induced as a result of chronic sleep loss and not non-specific effects of the disk stimulation. They include transcripts coding for several immunoglobulins, stress response proteins (macrophage inhibitor factor-related protein 14, heat-shock protein 27, alpha-B-crystallin), minoxidil sulfotransferase, globins and cortistatin. Twenty-eight transcripts decreased their expression in long-term sleep-deprived rats. Sixteen of them were specifically decreased as a result of chronic sleep loss, including those coding for type I procollagen and dihydrolipoamide acetyltransferase. We also compared sleeping rats to short-term and long-term sleep-deprived rats, and found that acute and chronic sleep loss led to some differences at the molecular level. Several plasticity-related genes were strongly induced after acute sleep deprivation only, and several glial genes were down-regulated in both sleep deprivation conditions, but to a different extent. These findings suggest that sustained sleep loss may trigger a generalized inflammatory and stress response in the brain.

Childhood obstructive sleep apnea associates with neuropsychological deficits and neuronal brain injury

BACKGROUND

Childhood obstructive sleep apnea (OSA) is associated with neuropsychological deficits of memory, learning, and executive function. There is no evidence of neuronal brain injury in children with OSA. We hypothesized that childhood OSA is associated with neuropsychological performance dysfunction, and with neuronal metabolite alterations in the brain, indicative of neuronal injury in areas corresponding to neuropsychological function.

METHODS AND FINDINGS

We conducted a cross-sectional study of 31 children (19 with OSA and 12 healthy controls, aged 6-16 y) group-matched by age, ethnicity, gender, and socioeconomic status. Participants underwent polysomnography and neuropsychological assessments. Proton magnetic resonance spectroscopic imaging was performed on a subset of children with OSA and on matched controls. Neuropsychological test scores and mean neuronal metabolite ratios of target brain areas were compared. Relative to controls, children with severe OSA had significant deficits in IQ and executive functions (verbal working memory and verbal fluency). Children with OSA demonstrated decreases of the mean neuronal metabolite ratio N-acetyl aspartate/choline in the left hippocampus (controls: 1.29, standard deviation [SD] 0.21; OSA: 0.91, SD 0.05; p = 0.001) and right frontal cortex (controls: 2.2, SD 0.4; OSA: 1.6, SD 0.4; p = 0.03).

CONCLUSIONS

Childhood OSA is associated with deficits of IQ and executive function and also with possible neuronal injury in the hippocampus and frontal cortex. We speculate that untreated childhood OSA could permanently alter a developing child's cognitive potential.

Neurocognitive dysfunction in children with sleep disorders

It is well known that adults with sleep disturbances frequently exhibit a wide range of neurocognitive decrements, and that these deficits are potentially reversible with effective treatment. However, the consequences of respiratory sleep disturbances on neurocognitive function in children have only recently been evaluated, and suggest a strong causal association between the episodic hypoxia and sleep fragmentation that characterize the disease and the emergence of reduced memory, attention and intelligence as well as a link to problematic and hyperactive behaviours and mood disturbances. This article takes a critical look at the current literature on these issues, reviews the major findings and discusses such findings in conjunction with those derived from pertinent animal models.

Oxidative stress and oxidant signaling in obstructive sleep apnea and associated cardiovascular diseases

Obstructive sleep apnea (OSA) has emerged as a major public health problem and increasing evidence indicates that untreated OSA can lead to the development of various cardiovascular disorders. One important mechanism by which OSA may promote cardiovascular diseases is intermittent hypoxia, in which patients are subjected to repeated episodes of brief oxygen desaturation in the blood, followed by reoxygenation. Such cycles of hypoxia/reoxygenation may result in the generation of reactive oxygen species. Some studies have demonstrated the presence of oxidative stress in OSA patients as well as in animals subjected to intermittent hypoxia. Further, modulations of nitric oxide and biothiol status might also play important roles in the pathogenesis of OSA-associated diseases. Reactive oxygen species and redox events are also involved in the regulation of signal transduction for oxygen-sensing mechanisms. This review summarizes currently available information on the evidence for and against the occurrence of oxidative stress in OSA and the role of reactive oxygen species in cardiovascular changes associated with OSA.

Neuropsychological morbidity linked to childhood sleep-disordered breathing

Understanding the long-term neuropsychological consequences of sleep disorders in children poses a significant challenge to researchers. Since children are in a state of rapidly changing cognition and neurobehavioral function, impacts on development may have profound consequences. Recent studies now demonstrate that mild sleep apnea and snoring, once considered within the spectrum of normal sleep patterns, are associated with deficits of neuropsychological function. Preliminary data suggest that some of these cognitive deficits may be reversible following treatment of mild sleep apnea in children; however, factors such as age at treatment, duration of sleep disordered breathing, pre-morbid intellectual level, socioeconomic status, or the effectiveness of treatment may adversely affect long-term outcome. Furthermore, it is imperative that researchers determine whether the developing brain exhibits critical periods of plasticity during which episodes of sleep-disordered breathing might cause long-term or permanent neuropsychological effects.

Oxidant stress and inflammation in the snoring child: confluent pathways to upper airway pathogenesis and end-organ morbidity

Snoring in children is increasingly being recognized as a highly prevalent condition, and indicates the presence of heightened upper airway resistance during sleep. In this paper, we present evidence to support the hypothesis that local inflammatory processes within the upper airway contribute to the pathophysiology of adenotonsillar hypertrophy and altered reflexes potentially leading to increased propensity for upper airway obstruction during sleep. Furthermore, the cumulative evidence supporting multiorgan morbidity for sleep-disordered breathing (SDB) is reviewed, and a unified hypothesis of a triple risk model proposing oxidative-inflammatory mechanisms as mediating the morbid consequences of SDB is presented. This hypothetical working model incorporates both dose-dependent disease severity components, as well as environmental and genetic elements of susceptibility.

The contribution of intermittent hypoxia, sleep debt and sleep disruption to daytime performance deficits in children: consideration of respiratory and non-respiratory sleep disorders

In children, the most abundant available information regarding the effects of paediatric sleep disturbance on daytime function has been obtained by studying children with sleep disordered breathing (SDB). The purported underlying pathophysiological mechanisms responsible for these deficits include hypoxia secondary to obstructive apneas/hypopneas and/or disrupted sleep architecture from frequent arousals during sleep. This review will present evidence that, while hypoxia is likely to play a role for many children with SDB, sleep disruption is an important and often overlooked factor that can contribute to daytime deficits in children with SDB. Indeed, sleep deprivation and disruption appear to have a potent impact on the daytime functioning of the much larger number of children with non-respiratory sleep disorders. It is concluded that sleep deprivation, sleep disruption, and intermittent hypoxia independently may be sufficient to cause daytime effects in vulnerable children, and the combination of two or more of these factors can result in particularly impaired daytime functioning. These conclusions have significant implications for research and clinical practice.

Neurobehavioral effects of obstructive sleep apnea: an overview and heuristic model

PURPOSE OF REVIEW

Obstructive sleep apnea can cause significant daytime behavioral and adaptive deficits, conventionally called 'neurobehavioral' because they are presumed to be mediated by the brain. The past few years have witnessed a marked increase in research into the neurobehavioral effects of obstructive sleep apnea, making it difficult for researchers and clinicians to stay abreast of the field. This article summarizes recent findings on the neurobehavioral effects of adult and pediatric obstructive sleep apnea, and presents a heuristic model to guide future research and clinical conceptualizations.

RECENT FINDINGS

Recent publications have suggested overlapping but distinct neurobehavioral deficits that tend to accompany obstructive sleep apnea in the adult versus childhood years. There have been exciting new developments into the mechanisms by which obstructive sleep apnea may result in these deficits, including findings based upon advanced neuroimaging tools and carefully controlled animal research. There has also been accumulating evidence for potential moderators of morbidity; that is, factors that alter the nature or severity of neurobehavioral deficit resulting from obstructive sleep apnea. Task- and context-related factors that may affect outcome have been identified, as have potential markers for individual risk and resiliency.

SUMMARY

The relation between obstructive sleep apnea and neurobehavioral deficit is probably not dependent on a single mediating mechanism, nor is it invariant across individuals. In outlining several potential moderating factors, the model presented here was designed to provide a guide for future research and a way of thinking about obstructive sleep apnea that better captures the wide variation in neurobehavioral outcome seen by practicing clinicians.

Immobilization stress induces cell death through production of reactive oxygen species in the mouse cerebral cortex

Prolonged stress has been shown to impair brain function and increase vulnerability to neuronal injury. To elucidate the in vivo response of neuronal cells to induced stress, we immobilized mice by binding their legs. Levels of reactive oxygen species (ROS) in the cerebral cortex were increased after stress induction. NADPH oxidase, interleukin-1beta (IL-1beta) and cyclooxygenase 2 mRNA (COX-2) expression levels were upregulated, and Fas levels were also increased. The increased expression of these factors was associated with neuronal death, which was confirmed by TUNEL and NeuN staining. OX42 staining was also evident around the TUNEL-stained lesions. From these findings, it appears that immobilization stress induces neuronal death in the mouse cerebral cortex, a process mediated by NADPH oxidase, IL-1beta, COX-2, ROS and Fas. However, this could be inhibited by pretreating the animals with antioxidants such as ebselen or pyrrolidine dithiocarbamate.

Secretion of brain-derived neurotrophic factor from PC12 cells in response to oxidative stress requires autocrine dopamine signaling

Expression of brain-derived neurotrophic factor (BDNF) is sensitive to changes in oxygen availability, suggesting that BDNF may be involved in adaptive responses to oxidative stress. However, it is unknown whether or not oxidative stress actually increases availability of BDNF by stimulating BDNF secretion. To approach this issue we examined BDNF release from PC12 cells, a well-established model of neurosecretion, in response to hypoxic stimuli. BDNF secretion from neuronally differentiated PC12 cells was strongly stimulated by exposure to intermittent hypoxia (IH). This response was inhibited by N-acetyl-l-cysteine, a potent scavenger of reactive oxygen species (ROS) and mimicked by exogenous ROS. IH-induced BDNF release requires activation of tetrodotoxin sensitive Na+ channels and Ca2+ influx through N- and L-type channels, as well as mobilization of internal Ca2+ stores. These results demonstrate that oxidative stress can stimulate BDNF release and that underlying mechanisms are similar to those previously described for activity-dependent BDNF secretion from neurons. Surprisingly, we also found that IH-induced secretion of BDNF was blocked by dopamine D2 receptor antagonists or by inhibition of dopamine synthesis with alpha-methyl-p-tyrosine. These data indicate that oxidative stress can stimulate BDNF release through an autocrine or paracrine loop that requires dopamine receptor activation.

Mild Intermittent Hypoxia Does Not Induce Stress Responses in the Neonatal Rat Brain

We previously demonstrated that intermittent hypoxia evokes persistent changes in extracellular striatal dopamine, locomotor activity and executive function, using a rodent model emulating apnea of prematurity in which rat pups are exposed to 20-second bursts of hypoxic gas mix containing 10% oxygen (60 events/h; 6 h/day) from postnatal days 7 to 11. To determine whether subtle repetitive hypoxic insults also induce expression of stress-related genes, we employed real-time RT-PCR to assay gene transcription in neonatal rats subjected to the same paradigm. In addition, we also measured expression of stress-induced transcripts in an age-matched cohort following a more severe oxidative stressor: permanent focal ischemia. Four transcripts were elevated following the ischemic insult: heat shock protein 70 (Hsp70), CL100, nurr77, and heme oxygenase-1. In contrast, these transcripts were not regulated in the majority of neonatal rats exposed to an intermittent hypoxia protocol. Hsp70 was strongly induced, and CL100 and nurr77 were slightly induced in only 2 of 11 post-hypoxic rats compared to controls. These data demonstrate that a single ischemic event elicits expression of specific stress-related genes, whereas 5 days of brief intermittent hypoxic insults typically do not. Thus, it is unlikely that the neurochemical and behavioral morbidity observed in juvenile and adult rodents exposed to intermittent hypoxia during a critical period of brain development are related to stress-induced changes in gene expression.

Development of respiratory control: Evolving concepts and perspectives

The mechanisms underlying respiratory system immaturity in newborns have been investigated, both in vivo and in vitro, in humans and in animals. Immaturity affects breathing rhythmicity and its modulation by suprapontine influences and by afferents from central and peripheral chemoreceptors. Recent research has moved from bedside tools to sophisticated technologies, bringing new insights into the plasticity and genetics of respiratory control development. Genetic research has benefited from investigations of newborn mice having targeted deletions of genes involved in respiratory control. Genetic variability may govern the normal programming of development and the processes underlying adaptation to homeostasis disturbances induced by prenatal and postnatal insults. Studies of plasticity have emphasized the role of neurotrophic factors. Improvements in our understanding of the mechanistic effects of these factors should lead to new neuroprotective strategies for infants at risk for early respiratory control disturbances, such as apnoeas of prematurity, sudden infant death syndrome and congenital central hypoventilation syndrome.

Does episodic hypoxia affect upper airway dilator muscle function? Implications for the pathophysiology of obstructive sleep apnoea

Obstructive sleep apnoea (OSA) is characterised by repetitive collapse of the upper airway during sleep owing to a sleep-related decrement in upper airway muscle activity with consequent failure of the pharyngeal dilator muscles to oppose the collapsing pressure that is generated by the diaphragm and accessory muscles during inspiration. The causes of upper airway obstruction during sleep are multi-factorial but there is evidence implicating intrinsic upper airway muscle function and impaired central regulation of the upper airway muscles in the pathophysiology of OSA. The condition is associated with episodic hypoxia due to recurrent apnoea. However, despite its obvious importance very little is known about the effects of episodic hypoxia on upper airway muscle function. In this review, we examine the evidence that chronic intermittent hypoxia can affect upper airway muscle structure and function and impair CNS control of the pharyngeal dilator muscles. We review the literature and discuss results from our laboratory showing that episodic hypoxia/asphyxia reduces upper airway muscle endurance and selectively impairs pharyngeal dilator EMG responses to physiological stimulation. Our observations lead us to speculate that episodic hypoxia--a consequence of periodic airway occlusion--is responsible for progression of OSA through impairment of the neural control systems that regulate upper airway patency and through altered respiratory muscle contractile function, leading to the establishment of a vicious cycle of further airway obstruction and hypoxic insult that chronically exacerbates and perpetuates the condition. We conclude that chronic intermittent hypoxia/asphyxia contributes to the pathophysiology of sleep-disordered breathing.

Postnatal intermittent hypoxia alters baroreflex function in adult rats

Chronic perinatal intermittent hypoxia (IH) could have long-term cardiovascular effects by altering baroreflex function. To examine this hypothesis, we exposed rats (n = 6/group) for postnatal days 1-30 or prenatal embryonic days 5-21 to IH (8% ambient O2 for 90 s after 90 s of 21% of O2, 12 h/day) or to normoxia (control). Baroreflex sensitivity (BRS) and cardiac chronotropic responses were examined in anesthetized animals 3.5-5 mo later by infusing phenylephrine or sodium nitroprusside (6-12 microg/min iv, 1-2 min) during normoxia and after 18 min of acute IH (IHA). In controls after IHA, baroreflex gain was 42% (P < 0.05) less than during normoxia. BRS in the postnatal IH group during normoxia was approximately 50% less than in control rats and similar to controls after IHA. The heart rate response to phenylephrine in the IH group was also less than in controls (P < 0.05) and was not changed by IHA. BRS and heart rate responses in the prenatal IH group were similar to the normoxic control group. Vagal efferent projections to atrial ganglia neurons in rats after postnatal IH (n = 4) were examined by injecting tracer into the left nucleus ambiguous. After 35 days of postnatal IH, basket ending density was reduced by 17% (P < 0.001) and vagal axon varicose contacts by 56% (P < 0.001) compared with controls. We conclude that reduction of vagal efferent projections in cardiac ganglia could be a cause of long-term modifications in baroreflex function.

Cobalt chloride induces neurite outgrowth in rat pheochromocytoma PC-12 cells through regulation of endothelin-2/vasoactive intestinal contractor

We investigated whether endothelin-2/vasoactive intestinal contractor (ET-2/VIC) gene expression, upregulated by hypoxia in cancer cells, was associated with differentiation in neuronal cells. RT-PCR analysis, morphological observations, and immunostaining revealed that CoCl2, a hypoxic mimetic agent, at 200 microM increased expression of the ET-2/VIC gene, decreased expression of the ET-1 gene, and induced neurite outgrowth in PC-12 rat pheochromocytoma cells. These effects induced by 200 microM CoCl2 were completely inhibited by the antioxidant N-acetyl cysteine at 20 mM. In addition, CoCl2 increased the level of intracellular reactive oxygen species (ROS) at an early stage. Furthermore, interleukin (IL)-6 gene expression was upregulated upon the differentiation induced by CoCl2. These results suggest that expression of ET-2/VIC and ET-1 mediated by ROS may be associated with neuronal differentiation through the regulation of IL-6. When the cells were treated with 500 microM CoCl2 for 24 hr, however, ET-2/VIC gene expression disappeared, IL-6 gene expression was downregulated, and necrosis was subsequently induced in the PC-12 cells.

Antioxidant responses to chronic hypoxia in the rat cerebellum and pons

Obstructive sleep apnea (OSA) is characterized by chronic intermittent hypoxia (CIH) and sleep fragmentation and deprivation. Exposure to CIH results in oxidative stress in the cortex, hippocampus and basal forebrain of rats and mice. We show that sustained and intermittent hypoxia induces antioxidant responses, an indicator of oxidative stress, in the rat cerebellum and pons. Increased glutathione reductase (GR) activity and thiobarbituric acid reactive substance (TBARS) levels were observed in the pons and cerebellum of rats exposed to CIH or chronic sustained hypoxia (CSH) compared with room air (RA) controls. Exposure to CIH or CSH increased GR activity in the pons, while exposure to CSH increased the level of TBARS in the cerebellum. The level of TBARS was increased to a greater extent after exposure to CSH than to CIH in the cerebellum and pons. Increased superoxide dismutase activity (SOD) and decreased total glutathione (GSHt) levels were observed after exposure to CIH compared with CSH only in the pons. We have previously shown that prolonged sleep deprivation decreased SOD activity in the rat hippocampus and brainstem, without affecting the cerebellum, cortex or hypothalamus. We therefore conclude that sleep deprivation and hypoxia differentially affect antioxidant responses in different brain regions.

Inducible nitric oxide synthase in long-term intermittent hypoxia: hypersomnolence and brain injury

RATIONALE

Long-term intermittent hypoxia (LTIH) exposure in adult mice, modeling oxygenation patterns of moderate-severe obstructive sleep apnea, results in lasting hypersomnolence and is associated with nitration and oxidation injuries in many brain regions, including wake-active regions.

OBJECTIVES

We sought to determine if LTIH activates inducible nitric oxide synthase (iNOS) in sleep/wake regions, and if this source of NO contributes to the LTIH-induced proinflammatory gene response, oxidative injury, and wake impairments.

METHODS

Mice with genetic absence of iNOS activity and wild-type control animals were exposed to 6 weeks of long-term hypoxia/reoxygenation before behavioral state recordings, molecular and biochemical assays, and a pharmacologic intervention.

MEASUREMENTS AND MAIN RESULTS

Two weeks after recovery from hypoxia/reoxygenation exposures, wild-type mice showed increased iNOS activity in representative wake-active regions, increased sleep times, and shortened sleep latencies. Mutant mice, with higher baseline sleep times, showed no effect of long-term hypoxia/reoxygenation on sleep time latencies and were resistant to hypoxia/reoxygenation increases in lipid peroxidation and proinflammatory gene responses (tumor necrosis factor alpha and cyclooxygenase 2). Inhibition of iNOS after long-term hypoxia/reoxygenation in wild-type mice was effective in reversing the proinflammatory gene response.

CONCLUSIONS

These data support a critical role for iNOS activity in the development of LTIH wake impairments, lipid peroxidation, and proinflammatory responses in wake-active brain regions, and suggest a potential role for inducible NO inhibition in protection from proinflammatory responses, oxidative injury, and residual hypersomnolence in obstructive sleep apnea.

Intermittent hypoxia damages cerebellar cortex and deep nuclei

Obstructive sleep apnea patients show cerebellar cortex and deep nuclei gray matter loss, a possible consequence of intermittent hypoxia (IH) accompanying the syndrome. We exposed Sprague-Dawley rats (n=24) to room air only or 10.3% O2, balance N2, alternating every 480 s (240 s duty cycle) with room air for 5, 10, 15, 20 or 30 h (7.5 h per day) during light periods. IH-exposed rats showed increased numbers of damaged Purkinje cells (31.1, 50.5, 54.7, 65.2, and 94.4% for 5, 10, 15, 20 and 30 h groups, respectively; p<0.001 for slopes of the total, swollen/autolysed, and shrunken/dark cell counts), as assessed by hematoxylin and eosin staining. Anti-caspase-3 antibody density increased in the fastigial nuclei subsequent to 5-h exposure. Short-term IH exposure elicits dose-dependent cerebellar Purkinje and fastigial neuron damage.