Neonatal exposure to intermittent hypoxia enhances mice performance in water maze and 8-arm radial maze tasks

Intermittent Hypobaric Hypoxia Ameliorates Autistic-Like Phenotypes in Mice

Autism spectrum disorder (ASD) is a neurodevelopmental condition characterized by persistent deficits in social communication and stereotyped behaviors. Although major advances in basic research on autism have been achieved in the past decade, and behavioral interventions can mitigate the difficulties that individuals with autism experience, little is known about the many fundamental issues of the interventions, and no specific medication has demonstrated efficiency for the core symptoms of ASD. Intermittent hypobaric hypoxia (IHH) is characterized by repeated exposure to lowered atmospheric pressure and oxygen levels, which triggers multiple physiological adaptations in the body. Here, using two mouse models of ASD, male Shank3B -/- and Fmr1 -/y mice, we found that IHH training at an altitude of 5,000 m for 4 h per day, for 14 consecutive days, ameliorated autistic-like behaviors. Moreover, IHH training enhanced hypoxia inducible factor (HIF) 1α in the dorsal raphe nucleus (DRN) and activated the DRN serotonergic neurons. Infusion of cobalt chloride into the DRN, to mimic IHH in increasing HIF1α expression or genetically knockdown PHD2 to upregulate HIF1α expression in the DRN serotonergic neurons, alleviated autistic-like behaviors in Shank3B -/- mice. In contrast, downregulation of HIF1α in DRN serotonergic neurons induced compulsive behaviors. Furthermore, upregulating HIF1α in DRN serotonergic neurons increased the firing rates of these neurons, whereas downregulation of HIF1α in DRN serotonergic neurons decreased their firing rates. These findings suggest that IHH activated DRN serotonergic neurons via upregulation of HIF1α, and thus ameliorated autistic-like phenotypes, providing a novel therapeutic option for ASD.

Neuroprotective Effects of Moderate Hypoxia: A Systematic Review

Emerging evidence highlights moderate hypoxia as a candidate treatment for brain disorders. This systematic review examines findings and the methodological quality of studies investigating hypoxia (10-16% O2) for ≥14 days in humans, as well as the neurobiological mechanisms triggered by hypoxia in animals, and suggests optimal treatment protocols to guide future studies. We followed the preferred reporting items for systematic reviews and meta-analysis (PRISMA) 2020. Searches were performed on PubMed/MEDLINE, PsycInfo, EMBASE, and the Cochrane Library, in May-September 2023. Two authors independently reviewed the human studies with the following tools: (1) revised Cochrane collaboration's risk of bias for randomized trials 2.0; (2) the risk of bias in nonrandomized studies of interventions. We identified 58 eligible studies (k = 8 human studies with N = 274 individuals; k = 48 animal studies) reporting the effects of hypoxia on cognition, motor function, neuroimaging, neuronal/synaptic morphology, inflammation, oxidative stress, erythropoietin, neurotrophins, and Alzheimer's disease markers. A total of 75% of human studies indicated cognitive and/or neurological benefits, although all studies were evaluated ashigh risk of bias due to a lack of randomization and assessor blinding. Low-dose intermittent or continuous hypoxia repeated for 30-240 min sessions, preferably in combination with motor-cognitive training, produced beneficial effects, and high-dose hypoxia with longer (≥6 h) durations and chronic exposure produced more adverse effects. Larger and methodologically stronger translational studies are warranted.

Intermittent Hypoxia in Preterm Neonates and Its Effect on Neonatal Morbidity and Mortality: A Systematic Review

The goal of the present systematic review was to investigate the occurrence patterns of intermittent hypoxemia in newborns throughout the early postnatal period as well as the link between neonatal intermittent hypoxemia exposure and harmful consequences such as neonatal morbidity and death. We collected data from 2014 to 2023 using several abstracting, referencing, and indexing database libraries in the field of medical sciences. A total of 715 papers were evaluated by both authors, and only seven articles met the specified review criteria after a thorough analysis. In preterm neonates with intermittent hypoxia (IH), severe morbidities such as bronchopulmonary dysplasia (BPD), retinopathy of prematurity (ROP), motor impairment, and cognitive delay were found. Only one study that extended to 18 months noted mortality. The length and occurrence of intermittent hypoxemia and the stage of premature neonates at the time of delivery are all closely associated with these morbidities. Therefore, it becomes important to continuously measure the patterns of occurrence of intermittent hypoxemia during early postnatal life to avoid its long-term morbidity and mortality impact.

Oxidative Stress Response Kinetics after 60 Minutes at Different Levels (10% or 15%) of Normobaric Hypoxia Exposure

In this study, the metabolic responses of hypoxic breathing for 1 h to inspired fractions of 10% and 15% oxygen were investigated. To this end, 14 healthy nonsmoking subjects (6 females and 8 males, age: 32.2 ± 13.3 years old (mean ± SD), height: 169.1 ± 9.9 cm, and weight: 61.6 ± 16.2 kg) volunteered for the study. Blood samples were taken before, and at 30 min, 2 h, 8 h, 24 h, and 48 h after a 1 h hypoxic exposure. The level of oxidative stress was evaluated by considering reactive oxygen species (ROS), nitric oxide metabolites (NOx), lipid peroxidation, and immune-inflammation by interleukin-6 (IL-6) and neopterin, while antioxidant systems were observed in terms of the total antioxidant capacity (TAC) and urates. Hypoxia abruptly and rapidly increased ROS, while TAC showed a U-shape pattern, with a nadir between 30 min and 2 h. The regulation of ROS and NOx could be explained by the antioxidant action of uric acid and creatinine. The kinetics of ROS allowed for the stimulation of the immune system translated by an increase in neopterin, IL-6, and NOx. This study provides insights into the mechanisms through which acute hypoxia affects various bodily functions and how the body sets up the protective mechanisms to maintain redox homeostasis in response to oxidative stress.

The human brain in a high altitude natural environment: A review

With the advancement of in vivo magnetic resonance imaging (MRI) technique, more detailed information about the human brain at high altitude (HA) has been revealed. The present review aimed to draw a conclusion regarding changes in the human brain in both unacclimatized and acclimatized states in a natural HA environment. Using multiple advanced analysis methods that based on MRI as well as electroencephalography, the modulations of brain gray and white matter morphology and the electrophysiological mechanisms underlying processing of cognitive activity have been explored in certain extent. The visual, motor and insular cortices are brain regions seen to be consistently affected in both HA immigrants and natives. Current findings regarding cortical electrophysiological and blood dynamic signals may be related to cardiovascular and respiratory regulations, and may clarify the mechanisms underlying some behaviors at HA. In general, in the past 10 years, researches on the brain at HA have gone beyond cognitive tests. Due to the sample size is not large enough, the current findings in HA brain are not very reliable, and thus much more researches are needed. Moreover, the histological and genetic bases of brain structures at HA are also needed to be elucidated.

Are we over-treating hypoxic spells in preterm infants?

A frequent challenge in Neonatology is the high frequency of spontaneously occurring hypoxemic events, a majority of which are associated with apnea or hypoventilation. These episodes present a challenge for caregivers and families as they frequently delay discharge of preterm infants. Supplemental oxygen, respiratory support, and caffeine therapy are widely used as therapeutic approaches, but challenges remain regarding their precise indications. Future clinical practice should be directed by an evidence-based approach including automated supplemental oxygen, minimizing the use of medications for gastroesophageal reflux, optimal timing and dosage of caffeine therapy, and standardization of alarm limits and discharge monitoring protocols.

The relationship between intermittent hypoxemia events and neural outcomes in neonates

This brief review examines 1) patterns of intermittent hypoxemia in extremely preterm infants during early postnatal life, 2) the relationship between neonatal intermittent hypoxemia exposure and outcomes in both human and animal models, 3) potential mechanistic pathways, and 4) future alterations in clinical care that may reduce morbidity. Intermittent hypoxemia events are pervasive in extremely preterm infants (<28 weeks gestation at birth) during early postnatal life. An increased frequency of intermittent hypoxemia events has been associated with a range of poor neural outcomes including language and cognitive delays, motor impairment, retinopathy of prematurity, impaired control of breathing, and intraventricular hemorrhage. Neonatal rodent models have shown that exposure to short repetitive cycles of hypoxia induce a pathophysiological cascade. However, not all patterns of intermittent hypoxia are deleterious and some may even improve neurodevelopmental outcomes. Therapeutic interventions include supplemental oxygen, pressure support and pharmacologic drugs but prolonged hyperoxia and pressure exposure have been associated with cardiopulmonary morbidity. Therefore, it becomes imperative to distinguish high risk from neutral and/or even beneficial patterns of intermittent hypoxemia during early postnatal life. Identification of such patterns could improve clinical care with targeted interventions for high-risk patterns and minimal or no exposure to treatment modalities for low-risk patterns.

Intermittent hypoxia modulates redox homeostasis, lipid metabolism associated inflammatory processes and redox post-translational modifications: Benefits at high altitude

Intermittent hypoxia, initially associated with adverse effects of sleep apnea, has now metamorphosed into a module for improved sports performance. The regimen followed for improved sports performance is milder intermittent hypoxic training (IHT) as compared to chronic and severe intermittent hypoxia observed in sleep apnea. Although several studies have indicated the mechanism and enough data on physiological parameters altered by IH is available, proteome perturbations remain largely unknown. Altitude induced hypobaric hypoxia is known to require acclimatization as it causes systemic redox stress and inflammation in humans. In the present study, a short IHT regimen consisting of previously reported physiologically beneficial FIO2 levels of 13.5% and 12% was administered to human subjects. These subjects were then airlifted to altitude of 3500 m and their plasma proteome along with associated redox parameters were analyzed on days 4 and 7 of high altitude stay. We observed that redox stress and associated post-translational modifications, perturbed lipid metabolism and inflammatory signaling were induced by IHT exposure at Baseline. However, this caused activation of antioxidants, energy homeostasis mechanisms and anti-inflammatory responses during subsequent high-altitude exposure. Thus, we propose IHT as a beneficial non-pharmacological intervention that benefits individuals venturing to high altitude areas.

Intermittent hypoxia improves cognition and reduces anxiety-related behavior in APP/PS1 mice

INTRODUCTION

Although hypoxia can exacerbate symptoms of various neurological disorders, accumulating evidence has indicated that intermittent hypoxia (IH) may exert protective effects against brain diseases. In the present study, we aimed to determine whether exposure to IH exerts beneficial effects in a transgenic murine model of Alzheimer's disease (AD). Because comorbid anxiety is prevalent among patients with AD, we explored the effects of IH on anxiety-like behaviors and associated factors in APP/PS1 mice.

METHODS

APP/PS1 mice were subjected to IH for two weeks. We assessed cognitive performance and anxiety-related behavior using standard behavioral assessments. Amyloid beta (Aβ) levels in the hippocampus were assessed using immunofluorescence and enzyme-linked immunosorbent assays (ELISA). We also assessed cell morphology and brain-derived neurotrophic factor (BDNF) expression in the hippocampus.

RESULTS

Exposure to IH significantly increased cognitive performance and decreased anxiety-related behaviors in APP/PS1 mice. Immunofluorescence and ELISA results revealed that IH pretreatment significantly lowered Aβ levels in the cortex and hippocampus. Morphological studies validated the neuroprotective effect of IH exposure on hippocampal neurogenesis. Molecular studies revealed IH-enhanced BDNF expression and inhibition of apoptosis-related protein expression in the hippocampus of APP/PS1 mice.

CONCLUSIONS

Our study demonstrates that IH improves cognition and reduces anxiety in a murine model of AD. Thus, further studies are required to determine whether IH can be used as a preventive/adjuvant therapy in patients with AD.

IGFBP-2 Signaling in the Brain: From Brain Development to Higher Order Brain Functions

Insulin-like growth factor-binding protein-2 (IGFBP-2) is a pleiotropic polypeptide that functions as autocrine and/or paracrine growth factors. IGFBP-2 is the most abundant of the IGFBPs in the cerebrospinal fluid (CSF), and developing brain showed the highest expression of IGFBP-2. IGFBP-2 expressed in the hippocampus, cortex, olfactory lobes, cerebellum, and amygdala. IGFBP-2 mRNA expression is seen in meninges, blood vessels, and in small cell-body neurons (interneurons) and astrocytes. The expression pattern of IGFBP-2 is often developmentally regulated and cell-specific. Biological activities of IGFBP-2 which are independent of their abilities to bind to insulin-like growth factors (IGFs) are mediated by the heparin binding domain (HBD). To execute IGF-independent functions, some IGFBPs have shown to bind with their putative receptors or to translocate inside the cells. Thus, IGFBP-2 functions can be mediated both via insulin-like growth factor receptor-1 (IGF-IR) and independent of IGF-Rs. In this review, I suggest that IGFBP-2 is not only involved in the growth, development of the brain but also with the regulation of neuronal plasticity to modulate high-level cognitive operations such as spatial learning and memory and information processing. Hence, IGFBP-2 serves as a neurotrophic factor which acts via metaplastic signaling from embryonic to adult stages.

Intermittent Hypoxemia in Preterm Infants

Intermittent hypoxemia (IH) events are common during early postnatal life, particularly in preterm infants. These events have been associated with multiple morbidities, including retinopathy of prematurity, sleep disordered breathing, neurodevelopmental impairment, and mortality. The relationship between IH and poor outcomes may depend on the patterns (frequency, duration, and timing) of the IH events. Current treatment modalities used in the clinical setting have been only partially successful in reducing the incidence of apnea and accompanying IH, but the risks and benefits of more aggressive interventions should include knowledge of the relationship between IH and morbidity.

Immediate and delayed decrease of long term potentiation and memory deficits after neonatal intermittent hypoxia

Apnea of prematurity is a common clinical condition that occurs in premature infants and results in intermittent hypoxia (IH) to brain and other organs. While short episodes of apnea are considered of no clinical significance, prolonged apnea with bradycardia and large oxygen desaturation is associated with adverse neurological and cognitive outcome. The mechanisms of cognitive deficits in IH are poorly understood. We hypothesized that brief but multiple episodes of severe oxygen desaturation accompanied by bradycardia may affect early and late synaptic plasticity and produce long-term cognitive deficits. C57BL/6 mouse pups were exposed to IH paradigm consisting of alternating cycles of 5% oxygen for 2.5 min and room air for 5-10 min, 2 h a day from P3 to P7. Long term potentiation (LTP) of synaptic strength in response to high frequency stimulation in hippocampal slices were examined 3 days and 6 weeks after IH. LTP was decreased in IH group relative to controls at both time points. That decrease was associated with deficits in spatial memory on Morris water maze and context fear conditioning test. Hypomyelination was observed in multiple gray and white matter areas on in vivo MRI using micromolecule proton fraction and ex vivo diffusion tensor imaging. No difference in caspase labeling was found between control and IH groups. We conclude that early changes in synaptic plasticity occurring during severe episodes of neonatal IH and persisting to adulthood may represent functional and structural substrate for long term cognitive deficits.

Intermittent hypoxia improves behavioral and adrenal gland dysfunction induced by posttraumatic stress disorder in rats

Nonpharmacological treatments of stress-induced disorders are promising, since they enhance endogenous stress defense systems, are free of side effects, and have few contraindications. The present study tested the hypothesis that intermittent hypoxia conditioning (IHC) ameliorates behavioral, biochemical, and morphological signs of experimental posttraumatic stress disorder (PTSD) induced in rats with a model of predator stress (10-day exposure to cat urine scent, 15 min daily followed by 14 days of stress-free rest). After the last day of stress exposure, rats were conditioned in an altitude chamber for 14 days at a 1,000-m simulated altitude for 30 min on day 1 with altitude and duration progressively increasing to 4,000 m for 4 h on day 5. PTSD was associated with decreased time spent in open arms and increased time spent in closed arms of the elevated X-maze, increased anxiety index, and increased rate of freezing responses. Functional and structural signs of adrenal cortex degeneration were also observed, including decreased plasma concentration of corticosterone, decreased weight of adrenal glands, reduced thickness of the fasciculate zone, and hydropic degeneration of adrenal gland cells. The thickness of the adrenal fasciculate zone negatively correlated with the anxiety index. IHC alleviated both behavioral signs of PTSD and morphological evidence of adrenal cortex dystrophy. Also, IHC alone exerted an antistress effect, which was evident from the increased time spent in open arms of the elevated X-maze and a lower number of rats displaying freezing responses. Therefore, IHC of rats with experimental PTSD reduced behavioral signs of the condition and damage to the adrenal glands.

NEW & NOTEWORTHY Intermittent hypoxia conditioning (IHC) has been shown to be cardio-, vaso-, and neuroprotective. For the first time, in a model of posttraumatic stress disorder (PTSD), this study showed that IHC alleviated both PTSD-induced behavioral disorders and functional and morphological damage to the adrenal glands. Also, IHC alone exerted an antistress effect. These results suggest that IHC may be a promising complementary treatment for PTSD-associated disorders.

Neural Network Reconfigurations: Changes of the Respiratory Network by Hypoxia as an Example

Neural networks, including the respiratory network, can undergo a reconfiguration process by just changing the number, the connectivity or the activity of their elements. Those elements can be either brain regions or neurons, which constitute the building blocks of macrocircuits and microcircuits, respectively. The reconfiguration processes can also involve changes in the number of connections and/or the strength between the elements of the network. These changes allow neural networks to acquire different topologies to perform a variety of functions or change their responses as a consequence of physiological or pathological conditions. Thus, neural networks are not hardwired entities, but they constitute flexible circuits that can be constantly reconfigured in response to a variety of stimuli. Here, we are going to review several examples of these processes with special emphasis on the reconfiguration of the respiratory rhythm generator in response to different patterns of hypoxia, which can lead to changes in respiratory patterns or lasting changes in frequency and/or amplitude.

Newborn blood gas derangements of children born extremely preterm and neurocognitive dysfunctions at age 10 years

Among 740 children born extremely preterm, we evaluated the relationship between the highest and lowest quartiles of the distributions of PaO₂ and PaCO₂, as well as the lowest quartile of pH on one day, and separately on two days, and the risk of neurocognitive, language, and behavioral dysfunctions at age 10 years. Children who had hypoxemia, hyperoxemia, hypocapnia, hypercapnia, and acidemia, sometimes on only one day, and sometimes on two or more days, were more likely than others to have a high illness severity score (within the first 12 postnatal hours), and 10 years later to have multiple dysfunctions. The tendency of blood gas derangements to be associated with high illness severity scores and with multiple dysfunctions 10 years later is compatible with the possibility that blood gas derangements are indicators of physiologic instability/vulnerability/immaturity rather than contributors to brain damage.

Alteration of Spontaneous Brain Activity After Hypoxia-Reoxygenation: A Resting-State fMRI Study

Zhang, Jiaxing, Ji Chen, Cunxiu Fan, Jinqiang Li, Jianzhong Lin, Tianhe Yang, and Ming Fan. Alteration of spontaneous brain activity after hypoxia-reoxygenation: A resting-state fMRI study. High Alt Med Biol. 18:20-26, 2017.-The present study was designed to investigate the effect of hypoxia-reoxygenation on the spontaneous neuronal activity in brain. Sixteen sea-level (SL) soldiers (20.5 ± 0.7 years), who garrisoned the frontiers in high altitude (HA) (2300-4400 m) for two years and subsequently descended to sea level for one to seven days, were recruited. Control group consisted of 16 matched SL natives. The amplitude of low-frequency fluctuations (ALFF) of regional brain functional magnetic resonance imaging signal in resting state and functional connectivity (FC) between brain regions was analyzed. HA subjects showed significant increases of ALFF at several sites within the bilateral occipital cortices and significant decreases of ALFF in the right anterior insula and extending to the caudate, putamen, inferior frontal orbital cortex, temporal pole, and superior temporal gyrus; lower ALFF values in the right insula were positively correlated with low respiratory measurements. The right insula in HA subjects had increases of FC with the right superior temporal gyrus, postcentral gyrus, rolandic operculum, supramarginal gyrus, and inferior frontal triangular area. We thus demonstrated that hypoxia-reoxygenation had influence on the spontaneous neuronal activity in brain. The decrease of insular neuronal activity may be related to the reduction of ventilatory drive, while the increase of FC with insula may indicate a central compensation.

Isoflurane-induced inactivation of CREB through histone deacetylase 4 is responsible for cognitive impairment in developing brain

Anesthetics including isoflurane are known to induce neuronal dysfunction in the developing brain, however, the underlying mechanism is mostly unknown. The transcriptional activation of CREB (cyclic AMP response element binding protein) and the alterations in acetylation of histones modulated by several histone deacetylases such as HDAC4 (histone deacetylase 4) are known to contribute to synaptic plasticity in the brain. Here we have shown that administration of isoflurane (1.4%) for 2h leads to transcriptional inactivation of CREB which results in loss of dendritic outgrowth and decreased expression level of proteins essential for memory and cognitive functions, such as BDNF, and c-fos in the developing brain of mice at postnatal day 7 (PND7). To elucidate the molecular mechanism, we found that exposure to isoflurane leads to an increase in nuclear translocation of HDAC4, which interacts with CREB in the nucleus. This event, in turn, results in a decrease in interaction between an acetyltransferase, CBP, and CREB that ultimately leads to transcriptional inactivation of CREB. As a result, the expression level of BDNF, and c-fos were significantly down-regulated after administration of isoflurane in PND7 brain. Depletion of HDAC4 in PND7 brain rescues the transcriptional activation of CREB along with augmentation in the level of the expression level of BDNF and c-fos. Moreover, administration of lentiviral particles of HDAC4 RNAi in primary neurons rescues neurite outgrowth following isoflurane treatment. Taken together, our study suggests that HDAC4-induced transcriptional inactivation of CREB is responsible for isoflurane-induced cognitive dysfunction in the brain.

Intermittent Hypoxia Does not Elicit Memory Impairment in Spinal Cord Injury Patients

There is a critical need for new therapeutic strategies to restore motor function in patients with spinal cord injuries (SCIs), without unwanted effects. Intermittent hypoxia (IH) induces plasticity in spared synaptic pathways to motor neurons below the level of injury, which can be harnessed to elicit motor recovery in incomplete SCI patients. However, there is conflicting evidence regarding the effects of IH on memory function. The aim of this study was to assess episodic verbal and visual memory function with the Complutense verbal learning test (TAVEC) and the Rey-Osterrieth Complex Figure Test (ROCF), respectively, before and after a 4-week protocol of repetitive IH combined with body weight-supported treadmill training (BWSTT) in incomplete ASIA C and D SCI subjects. Subjects received either IH (cycling 9%/21% FiO2 every 1.5 min, 15 cycles per day) or continued normoxia (Nx, 21% FiO2) combined with 45 min of BWSTT for 5 consecutive days, followed by 3 times per week IH and BWSTT for 3 additional weeks. ROCF Z scores between IH plus BWSTT and Nx plus BWSTT were not significantly different (p = .43). Compared with baseline, IH and BWSTT group showed a significantly greater (p < .05) verbal memory performance for immediate, short-term, and long-term recall; however, it was not different from Nx plus BWSTT group in all verbal memory components (p > .05). Our results suggest that a 4-week protocol of moderate IH does not elicit visual or verbal memory impairment. Thus, repetitive IH may be a safe therapeutic approach to incomplete spinal cord injury patients, without deleterious cognitive effects.

Protective effects of intermittent hypoxia on brain and memory in a mouse model of apnea of prematurity

Apnea of prematurity (AOP) is considered a risk factor for neurodevelopmental disorders in children based on epidemiological studies. This idea is supported by studies in newborn rodents in which exposure to intermittent hypoxia (IH) as a model of AOP significantly impairs development. However, the severe IH used in these studies may not fully reflect the broad spectrum of AOP severity. Considering that hypoxia appears neuroprotective under various conditions, we hypothesized that moderate IH would protect the neonatal mouse brain against behavioral stressors and brain damage. On P6, each pup in each litter was randomly assigned to one of three groups: a group exposed to IH while separated from the mother (IH group), a control group exposed to normoxia while separated from the mother (AIR group), and a group of untreated unmanipulated pups left continuously with their mother until weaning (UNT group). Exposure to moderate IH (8% O₂) consisted of 20 hypoxic events/hour, 6 h per day from postnatal day 6 (P6) to P10. The stress generated by maternal separation in newborn rodents is known to impair brain development, and we expected this effect to be smaller in the IH group compared to the AIR group. In a separate experiment, we combined maternal separation with excitotoxic brain lesions mimicking those seen in preterm infants. We analyzed memory, angiogenesis, neurogenesis and brain lesion size. In non-lesioned mice, IH stimulated hippocampal angiogenesis and neurogenesis and improved short-term memory indices. In brain-lesioned mice, IH decreased lesion size and prevented memory impairments. Contrary to common perception, IH mimicking moderate apnea may offer neuroprotection, at least in part, against brain lesions and cognitive dysfunctions related to prematurity. AOP may therefore have beneficial effects in some preterm infants. These results support the need for stratification based on AOP severity in clinical trials of treatments for AOP, to determine whether in patients with moderate AOP, these treatments are beneficial or deleterious.

Delivery of In Vivo Acute Intermittent Hypoxia in Neonatal Rodents to Prime Subventricular Zone-derived Neural Progenitor Cell Cultures

Extended culture of neural stem/progenitor cells facilitates in vitro analyses to understand their biology while enabling expansion of cell populations to adequate numbers prior to transplantation. Identifying approaches to refine this process, to augment the production of all CNS cell types (i.e., neurons), and to possibly contribute to therapeutic cell therapy protocols is a high research priority. This report describes an easily applied in vivo "pre-conditioning" stimulus which can be delivered to awake, non-anesthetized animals. Thus, it is a non-invasive and non-stressful procedure. Specifically described are the procedures for exposing mouse or rat pups (aged postnatal day 1-8) to a brief (40-80 min) period of intermittent hypoxia (AIH). The procedures included in this video protocol include calibration of the whole-body plethysmography chamber in which pups are placed during AIH and the technical details of AIH exposure. The efficacy of this approach to elicit tissue-level changes in the awake animal is demonstrated through the enhancement of subsequent in vitro expansion and neuronal differentiation in cells harvested from the subventricular zone (SVZ). These results support the notion that tissue level changes across multiple systems could be observed following AIH, and support the continued optimization and establishment of AIH as a priming or conditioning modality for therapeutic cell populations.

Extending injury- and disease-resistant CNS phenotypes by repetitive epigenetic conditioning

Significant reductions in the extent of acute injury in the CNS can be achieved by exposure to different preconditioning stimuli, but the duration of the induced protective phenotype is typically short-lasting, and thus is deemed as limiting its clinical applicability. Extending the period over which such adaptive epigenetic changes persist – in effect, expanding conditioning’s “therapeutic window” – would significantly broaden the potential applications of such a treatment approach in patients. The frequency of the conditioning stimulus may hold the key. While transient (1–3 days) protection against CNS ischemic injury is well established preclinically following a single preconditioning stimulus, repetitively presenting preconditioning stimuli extends the duration of ischemic tolerance by many weeks. Moreover, repetitive intermittent postconditioning enhances post-ischemic recovery metrics and improves long-term survival. Intermittent conditioning is also efficacious for preventing or delaying injury in preclinical models of chronic neurodegenerative disease, and for promoting long-lasting functional improvements in a number of other pathologies as well. Although the detailed mechanisms underlying these protracted kinds of neuroplasticity remain largely unstudied, accumulating empirical evidence supports the contention that all of these adaptive phenotypes are epigenetically mediated. Going forward, additional preclinical demonstrations of the ability to induce sustained beneficial phenotypes that reduce the burden of acute and chronic neurodegeneration, and experimental interrogations of the regulatory constructs responsible for these epigenetic responses, will accelerate the identification of not only efficacious but also practical, adaptive epigenetics-based treatments for individuals with neurological disease.

Unexpected benefits of intermittent hypoxia: enhanced respiratory and nonrespiratory motor function

Intermittent hypoxia (IH) is most often thought of for its role in morbidity associated with sleep-disordered breathing, including central nervous system pathology. However, recent evidence suggests that the nervous system fights back in an attempt to minimize pathology by increasing the expression of growth/trophic factors that confer neuroprotection and neuroplasticity. For example, even modest ("low dose") IH elicits respiratory motor plasticity, increasing the strength of respiratory contractions and breathing. These low IH doses upregulate hypoxia-sensitive growth/trophic factors within respiratory motoneurons but do not elicit detectable pathologies such as hippocampal cell death, neuroinflammation, or systemic hypertension. Recent advances have been made toward understanding cellular mechanisms giving rise to IH-induced respiratory plasticity, and attempts have been made to harness the benefits of low-dose IH to treat respiratory insufficiency after cervical spinal injury. Our recent realization that IH also upregulates growth/trophic factors in nonrespiratory motoneurons and improves limb (or leg) function after incomplete chronic spinal injuries suggests that IH-induced plasticity is a general feature of motor systems. Collectively, available evidence suggests that low-dose IH may represent a safe and effective treatment to restore lost motor function in diverse clinical disorders that impair motor function.

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.

Altitude acclimatization improves submaximal cognitive performance in mice and involves an imbalance of the cholinergic system

The aim of this work was to reveal a hypothetical improvement of cognitive abilities in animals acclimatized to altitude and performing under ground level conditions, when looking at submaximal performance, once seen that it was not possible when looking at maximal scores. We modified contrasted cognitive tasks (object recognition, operant conditioning, eight-arm radial maze, and classical conditioning of the eyeblink reflex), increasing their complexity in an attempt to find performance differences in acclimatized animals vs. untrained controls. In addition, we studied, through immunohistochemical quantification, the expression of choline acetyltransferase and acetyl cholinesterase, enzymes involved in the synthesis and degradation of acetylcholine, in the septal area, piriform and visual cortexes, and the hippocampal CA1 area of animals submitted to acute hypobaric hypoxia, or acclimatized to this simulated altitude, to find a relationship between the cholinergic system and a cognitive improvement due to altitude acclimatization. Results showed subtle improvements of the cognitive capabilities of acclimatized animals in all of the tasks when performed under ground-level conditions (although not before 24 h), in the three tasks used to test explicit memory (object recognition, operant conditioning in the Skinner box, and eight-arm radial maze) and (from the first conditioning session) in the classical conditioning task used to evaluate implicit memory. An imbalance of choline acetyltransferase/acetyl cholinesterase expression was found in acclimatized animals, mainly 24 h after the acclimatization period. In conclusion, altitude acclimatization improves cognitive capabilities, in a process parallel to an imbalance of the cholinergic system.

Minimal effects on human memory following long-term living at moderate altitude

A number of studies describe memory deficits at extremely high altitudes. However, little is known about the effect of long-term living at moderate altitude (MA). The subjects for this study were 52 college students originally from sea level (SL), but studying at a MA of 2260 m over a 7-month period, with a return to SL for 30 days in the middle of the period. Fifty-two matched college students who stayed at SL all the time were the control group. The neuropsychological battery of assessments included the Chinese revised version of Wechsler Memory Scale tests (WMS-CR), verbal and spatial two-back working memory tests, long-term explicit memory (word recall and recognition of words, faces, and pictures) tests, Rey Auditory Verbal Learning test, Rey-Osterrieth Complex Figure (ROCF) test, degraded picture naming test, and the Serial Reaction Time Test. We found that the MA subjects showed significantly poorer performances than SL controls only in short-term visual construction assessed in the visual reproduction test from WMS-CR and in the ROCF immediate test. There were no significant differences in all other tasks between the MA group and SL group. These findings suggest that long-term hypoxic exposure at moderate altitude has minimal effects on human memory.

Oxygen, a Key Factor Regulating Cell Behavior during Neurogenesis and Cerebral Diseases

Oxygen is vital to maintain the normal functions of almost all the organs, especially for brain which is one of the heaviest oxygen consumers in the body. The important roles of oxygen on the brain are not only reflected in the development, but also showed in the pathological processes of many cerebral diseases. In the current review, we summarized the oxygen levels in brain tissues tested by real-time measurements during the embryonic and adult neurogenesis, the cerebral diseases, or in the hyperbaric/hypobaric oxygen environment. Oxygen concentration is low in fetal brain (0.076-7.6 mmHg) and in adult brain (11.4-53.2 mmHg), decreased during stroke, and increased in hyperbaric oxygen environment. In addition, we reviewed the effects of oxygen tensions on the behaviors of neural stem cells (NSCs) in vitro cultures at different oxygen concentration (15.2-152 mmHg) and in vivo niche during different pathological states and in hyperbaric/hypobaric oxygen environment. Moderate hypoxia (22.8-76 mmHg) can promote the proliferation of NSCs and enhance the differentiation of NSCs into the TH-positive neurons. Next, we briefly presented the oxygen-sensitive molecular mechanisms regulating NSCs proliferation and differentiation recently found including the Notch, Bone morphogenetic protein and Wnt pathways. Finally, the future perspectives about the roles of oxygen on brain and NSCs were given.

Assessment of spatial memory in mice

Improvements in health care have greatly increased life span in the United States. The focus is now shifting from physical well-being to improvement in mental well-being or maintenance of cognitive function in old age. It is known that elderly people suffer from cognitive impairment, even without neurodegeneration, as a part of 'normal aging'. This 'age-associated memory impairment' (AAMI), can have a devastating impact on the social and economic life of an individual as well as the society. Scientists have been experimenting to find methods to prevent the memory loss associated with aging. The major factor involved in these experiments is the use of animal models to assess hippocampal-based spatial memory. This review describes the different types of memory including hippocampal-based memory that is vulnerable to aging. A detailed overview of various behavioral paradigms used to assess spatial memory including the T-maze, radial maze, Morris water maze, Barnes maze and others is presented. The review also describes the molecular basis of memory in hippocampus called as 'long-term potentiation'. The advantages and limitations of the behavioral models in assessing memory and the link to the long-term potentiation are discussed. This review should assist investigators in choosing suitable methods to assess spatial memory in mice.

Hippocampal spine-associated Rap-specific GTPase-activating protein induces enhancement of learning and memory in postnatally hypoxia-exposed mice

Spine-associated Rap-specific GTPase-activating protein (SPAR) is a postsynaptic protein that forms a complex with postsynaptic density (PSD)-95 and N-methyl-d-aspartate receptors (NMDARs), and morphologically regulates dendritic spines. Mild intermittent hypoxia (IH, 16.0% O(2), 4 h/day for 4 weeks) is known to markedly enhance spatial learning and memory in postnatal developing mice. Here, we report that this effect is correlated with persistent increases in SPAR expression as well as long-term potentiation (LTP) in the hippocampus of IH-exposed mice. Furthermore, an infusion of SPAR antisense oligonucleotides into the dorsal hippocampus disrupted elevation of SPAR expression, preventing enhanced hippocampal LTP in IH-exposed developing mice and also reducing LTP in normoxic mice, without altering basal synaptic transmission. In SPAR antisense-treated mice, acquisition of the Morris water maze spatial learning task was impaired, as was memory retention in probe trails following training. This study provides the first evidence that SPAR is functionally required for synaptic plasticity and contributes to the IH-induced enhancement of spatial learning and memory in postnatal developing mice.

Hypoxic preconditioning improves spatial cognitive ability in mice

Although it has been reported in a lot of studies that hypoxic preconditioning could protect the brain from hypoxic/ischemic injury, it is not clear whether hypoxic preconditioning could affect brain functions such as cognitive ability. This work aims at investigating the effect of hypoxic preconditioning on spatial cognitive ability in mice after acute and repeated hypoxic exposures. The mice were randomly divided into 3 groups: a control group in which mice were not exposed to hypoxia (H0) and experimental groups in which mice encountered hypoxia either once (H1) or 4 times (H4). Neural cell adhesion molecule (NCAM) expression, long-term potentiation (LTP) recording and Morris water maze test were used to measure the animals' cognitive ability. The tolerance time was progressively prolonged as exposure went on. The expression of both NCAM mRNA and NCAM protein as well as the LTP induction rate decreased in group H1, but recovered to control level in group H4. The performance of mice in the maze test was improved in H4 in comparison with that in both H1 and H0. These findings may indicate that spatial cognitive ability is improved in adult mice by their hypoxic preconditioning.

Classical eyeblink conditioning during acute hypobaric hypoxia is improved in acclimatized mice and involves Fos expression in selected brain areas

This work attempts to evaluate the cognitive aspects of the acclimatization ability of mice submitted to simulated altitude. Critical altitudes were detected by evaluating open field activity, combined or not with object recognition tasks, at different acute simulated altitudes. Results showed impaired cognitive abilities at approximately 3,733 m and above. To evaluate acclimatization capabilities, mice submitted to hypobaric hypoxia at approximately 5,000 m for 1 wk were tested for learning and memory performances with classical eyeblink conditioning at the same altitude or at land altitude. Results showed total acclimatization in mice conditioned at approximately 5,000 m but no improved performance in those conditioned at land altitudes compared with controls. Selected brain sites of conditioned animals were analyzed by immunohistochemistry to detect expression of the protein product of the protooncogene c-fos (Fos) in relation to both motor learning processes and hypobaric conditions. In the nucleus of the solitary tract, a higher expression of Fos was found in the acute hypobaric conditioned animals than in control conditioned and nonconditioned animals. Similar patterns between groups were found in the other brain areas, mainly in the piriform cortex and area 1 of the cingulate cortex and in the hippocampus. Differences between hemispheres were detected only in acute hypobaric animals. The present results show that acclimatization to high altitude prevents the impairment of classical eyeblink conditioning evoked by hypobaric hypoxic conditions but does not improve this task when acquired under land conditions, although it could diminish the activation requirements for its performance.

The contribution of intermittent hypoxemia to late neurological handicap in mice with hyperoxia-induced lung injury

Bronchopulmonary dysplasia (BPD) is considered by many to be an independent risk factor for poor neurodevelopment in premature infants. However, infants with BPD experience intermittent hypoxic episodes. This study was undertaken to determine whether intermittent hypoxic stress associated with BPD contributes to the development of neurological deficit. The model of BPD was produced in neonatal mice by exposure to hyperoxia (65% O(2)) for 4 weeks. Arterial blood gases, pulmonary mechanics, and histopathology were used to define the degree of lung injury. The mice were subjected to brief (10 min/day) and intermittent (10 days) hypoxic stress (8% O(2)) at different stages of the development of hyperoxia-induced lung injury. At 8 weeks of life, the neurofunction was assessed by water maze and rota-rod tests followed by cerebral morphological analysis using Nissl, bromodeoxyuridine, and caspase-3 immunostaining. Data were compared to naïve normoxic littermates and those mice that were exposed only to hyperoxia or intermittent hypoxia alone. Mice with BPD subjected to brief/intermittent hypoxia demonstrated a significantly poorer navigational memory performance as compared with normoxic mice and mice with BPD that were not subjected to intermittent hypoxia. The neurofunctional handicap in these mice was associated with significantly decreased brain weight and increased cerebral expression of caspase-3. Our results suggest that intermittent hypoxia associated with hyperoxia-induced lung injury, but not lung injury itself, results in significant neurological handicap in neonatal mice with BPD.

Postnatal inflammatory rat model for cerebral palsy: too different from humans

OBJECTIVE

In humans, cerebral palsy (CP) may originate from inflammation during the second and third trimesters of gestation when preoligodendrocytes (Pre-OL) are most vulnerable to an inflammatory insult. We studied a postnatal CP model to evaluate injury that would correlate with presence of Pre-OL in human pregnancy.

STUDY DESIGN

On postnatal (P) days 2, 3, 4, 5 and 6, pups were treated with (lipopolysaccharide [LPS]) (n = 7; 30, 30, 60, 60, 120 microg/Kg) or saline (n = 7). Neonates were tested for motor and cognitive development. Adult offspring performed beam walking and rotarod for motor activity. White matter damage was assessed with immunohistochemical Pre-OL markers (CNP, PLP). Statistical analysis included Mann-Whitney U and analysis of variance.

RESULTS

LPS-treated animals performed negative geotaxis (P = .009) and surface righting (P = .01) earlier than controls. No differences were observed for other neonatal tests. Adult LPS-treated offspring performed better in tests of motor control: rotarod (P = .01) and beam walking (P = .02). Pre-OL markers were altered in LPS-treated animals at both P22 (CNP and PLP increased in LPS, P < .01 and P < .001, respectively) and 12 weeks (CNP and PLP decreased in LPS, P < .0001 and P < .03, respectively).

CONCLUSION

Neonatal exposure to LPS induced white matter damage in the brain, accelerated neurodevelopment and motor tasks in adulthood. These are similar to findings from a postnatal hypoxic model suggesting that in the rodent, targeting the Pre-OL does not result in a CP phenotype.

Intermittent hypoxia impairs performance of adult mice in the two-way shuttle box but not in the Morris water maze

We have previously found that neonatal intermittent hypobaric hypoxia exposure enhanced mouse spatial, but impaired associative, cognition. This study sought to investigate the effects of hypobaric hypoxia on adult mice cognition. Mice were exposed to 2, 5, 10, 15, or 25 days of intermittent hypoxia (IH; 4 hr/day) at 2 km (16.0% O2) or 5 km (10.8% O2) altitudes in a hypobaric chamber for the Morris water maze (MWM) test and exposed to IH for 2, 10, or 25 days for the shuttle-box test. Amino acid dynamics in vivo in the hippocampus and amygdala of mice exposed to 2 km hypoxia were analyzed by high-pressure liquid chromatography. The results in MWM task showed that IH-2d to -25d at 2 km or 5 km did not change the escape latencies of mice in the training test or the retention of platform in the probe test. In the shuttle-box task, however, IH-10d at 5 km significantly reduced mouse avoidances in the acquisition test on day 4, and IH-10d at 2 km reduced avoidances in the retention test; IH-25d at 5 km significantly reduced avoidances of mice throughout the acquisition days. Glutamate in the amygdala persisted in declining to 69% of baseline at 8 hr posthypoxia (P = 0.040 vs. GLU released during 30 min before hypoxia) during the posthypoxia stage. These results suggest that adult hypobaric IH impairs the hippocampal-independent, but not the hippocampal-dependent, task in mice. The different GLU releases in the hippocampus and amygdala in response to hypoxia are involved in the different behaviors.

Early life modulators and predictors of adult synaptic plasticity

Early life experience can induce long-lasting changes in brain and behaviour that are opposite in direction, such as enhancement or impairment in regulation of stress response, structural and functional integrity of the hippocampus, and learning and memory. To explore how multiple early life events jointly determine developmental outcome, we investigated the combined effects of neonatal trauma (anoxia on postnatal day 1, P1) and neonatal novelty exposure (P2-21) on adult social recognition memory (3 months of age) and synaptic plasticity in the CA1 of the rat hippocampus (4.5-8 months of age). While neonatal anoxia selectively reduced post-tetanic potentiation (PTP), neonatal novel exposure selectively increased long-term potentiation (LTP). No interaction between anoxia and novelty exposure was found on either PTP or LTP. These findings suggest that the two contrasting neonatal events have selective and distinct effects on two different forms of synaptic plasticity. At the level of behaviour, the effect of novelty exposure on LTP was associated with increased social memory, and the effect of anoxia on PTP was not accompanied by changes in social memory. Such a finding suggests a bias toward the involvement of LTP over PTP in social memory. Finally, we report a surprising finding that an early behavioural measure of emotional response to a novel environment obtained at 25 days of age can predict adult LTP measured several months later. Therefore, individual differences in emotional responses present during the juvenile stage may contribute to adult individual differences in cellular mechanisms that underlie learning and memory.