Gender differentiation of the chemoreflex during growth at high altitude: functional and neurochemical studies

Chronic intermittent hypoxia attenuates noradrenergic innervation of hypoglossal motor nucleus

The state-dependent noradrenergic activation of hypoglossal motoneurons plays an important role in the maintenance of upper airway patency and pathophysiology of obstructive sleep apnea (OSA). Chronic intermittent hypoxia (CIH), a major pathogenic factor of OSA, contributes to the risk for developing neurodegenerative disorders in OSA patients. Using anterograde tracer, channelrhodopsin-2, we mapped axonal projections from noradrenergic A7 and SubCoeruleus neurons to hypoglossal nucleus in DBH-cre mice and assessed the effect of CIH on these projections. We found that CIH significantly reduced the number of axonal projections from SubCoeruleus neurons to both dorsal (by 68%) and to ventral (by73%) subregions of the hypoglossal motor nucleus compared to sham-treated animals. The animals' body weight was also negatively affected by CIH. Both effects, the decrease in axonal projections and body weight, were more pronounced in male than female mice, which was likely caused by less sensitivity of female mice to CIH as compared to males. The A7 neurons appeared to have limited projections to the hypoglossal nucleus. Our findings suggest that CIH-induced reduction of noradrenergic innervation of hypoglossal motoneurons may exacerbate progression of OSA, especially in men.

The ultrasound-based cardiac output monitoring is a useful tool to define baseline hemodynamic parameters in healthy permanent residents at high altitude: results of a monocentric pilot study

Previous studies on the cardiac data of healthy permanent residents living in high-altitude regions such as Tibet and the Andes have yielded inconsistent findings and significant disparities. These discrepancies can be mainly attributed to the invasive methods conventionally used for parameter evaluation. However, with the introduction of cutting-edge ultrasound technology, there is now an innovative approach to addressing and reconciling these variations. In this pilot study, we employed an ultrasound-based cardiac output monitoring (USCOM) device to evaluate cardiac output and related hemodynamic variables in a group of 20 healthy high-altitude Andean residents (comprising 10 men and 10 women) aged between 26 and 35 years old. The monocentric study was carried out in La Paz, Bolivia, located between at an altitude of 3,600-4,000 m. A total of 60 hemodynamic measurements were evaluated, accounting for three technical replicates per subject. Our results showed strong intrasubject reproducibility and revealed important differences related to both sex and hemodynamic parameters in highlanders compared to individuals residing at sea level. We conclude that USCOM represents a highly reliable technology for performing hemodynamic measurements in high-altitude residents. Our preliminary findings underscore the need for larger studies, encompassing larger sample sizes, specifically tailored to gender considerations, and extendable to broader highland populations. These findings have special significant implications for the management of hemodynamics in intensive care and postoperative settings, warranting further comprehensive research efforts.

COVID-19 Smell Impairment and Crosstalk with Hypoxia Physiology

Since its apomorphic appearance in 2019, severe acute respiratory syndrome Coronavirus type 2 (SARS-CoV-2) nowadays circulates as a plesiomorphic human virus in several synapomorphic variants. The respiratory tract is the most important site of infection, the viral effects in the lungs are well described, and more than half of the patients could develop shortness of breath and dyspnea and require ventilatory support. The physiological sign of this condition is the decrease in the partial pressure of oxygen in the blood, leading to acute hypoxia, which could be a factor in the disease. In severe patients, we recorded several physiological parameters: breath frequency (BF), partial pressure of oxygen in the blood (pO2), partial pressure of carbon dioxide in the blood (pCO2), hemoglobin (Hb), heart rate (HR), and blood pressure in correlation with the olfactory threshold. We found significant correlations between reduced olfactory threshold with pO2 and hemoglobin levels, changes in heart rate, and increased HR and pCO2. These results suggest that COVID-19 causes an impaired sense of smell that decreases in threshold corresponding to the disease severity.

The role of sex and ovarian hormones in hippocampal damage and cognitive deficits induced by chronic exposure to hypobaric hypoxia

Purpose

This study aims to investigate the role of sex and ovarian hormones in hippocampal damage and cognitive deficits and behavioral dysfunction in rats induced by chronic exposure to hypobaric hypoxia.

Methods

Six-week-old male and female SD rats were housed for 3 months either in a real altitude (4,250 m) environment as the model of chronic hypobaric-hypoxia (CHH) or in a plain as controls. The animal behavioral and hippocampal neurons at subcellular, molecular, and ultrastructural levels were characterized after CHH exposure.

Results

After 3 months of CHH exposure, (1) male CHH rats’ serum testosterone level was lower than male controls’ whereas female CHH rats’ serum estradiol level was higher than female controls’; (2) Morris water maze test finds that male rats showed more learning and spatial memory deficits than female rats; (3) male rats showed more severe hippocampal damage, hippocampal inflammation, oxidative stress and decreased hippocampal integrity (neurogenesis and dendritic spine density) than female rats; (4) Western blot analysis shows that, compared with the male control group, in male CHH group’s hippocampus, expression of nNOS, HO-1, and Bax protein increased whereas that of Bcl-2 protein decreased; (5) Expression of PON2 protein in male rats (CHH and controls) was lower than female rats (CHH and controls). In addition, CHH exposure decreased the expression of PON2 protein in both male and female rats; (6) qPCR analysis reveals that CHH exposure reduced the gene expression of N-methyl-D-aspartate receptor NR2A and NR2B subunits in male rats’ hippocampus. In addition, compared with the sham CHH group, the expression level of PON2 protein decreased in the OVX-CHH group’s hippocampus whereas oxidative stress, neuroinflammation, and degeneration of hippocampal neurons increased in the OVX-CHH group’s hippocampus.

Conclusion

After CHH exposure, male rats were significantly more likely than female rats to develop hippocampal damage, hippocampal neuroinflammation, and cognitive decline and deficits, suggesting that sex and ovarian hormones were significantly involved in regulating the rats’ susceptibility to CHH exposure-induced hippocampal damage.

Genetic variation in HIF-2α attenuates ventilatory sensitivity and carotid body growth in chronic hypoxia in high-altitude deer mice

KEY POINTS

High-altitude natives of many species have experienced natural selection on the gene encoding HIF-2α, Epas1, including high-altitude populations of deer mice. HIF-2α regulates ventilation and carotid body growth in hypoxia, so the genetic variants in Epas1 in high-altitude natives may underlie evolved changes in control of breathing. Deer mice from controlled crosses between high- and low-altitude populations were used to examine the effects of Epas1 genotype on an admixed genomic background. The high-altitude variant was associated with reduced ventilatory chemosensitivity and carotid body growth in chronic hypoxia, but had no effects on haematology. The results help us better understand the genetic basis for the unique physiological phenotype of high-altitude natives.

ABSTRACT

The gene encoding HIF-2α, Epas1, has experienced a history of natural selection in many high-altitude taxa, but the functional role of mutations in this gene are still poorly understood. We investigated the influence of the high-altitude variant of Epas1 in North American deer mice (Peromyscus maniculatus) on control of breathing and carotid body growth during chronic hypoxia. We created hybrids between high- and low-altitude populations of deer mice to disrupt linkages between genetic loci so physiological effects of Epas1 alleles (Epas1^H and Epas1^L , respectively) could be examined on an admixed genomic background. In general, chronic hypoxia (4 weeks at 12 kPa O₂ ) enhanced ventilatory chemosensitivity (assessed as the acute ventilatory response to hypoxia), increased total ventilation and arterial O₂ saturation during progressive poikilocapnic hypoxia, and increased haematocrit and blood haemoglobin content across genotypes. However, effects of chronic hypoxia on ventilatory chemosensitivity were attenuated in mice that were homozygous for the high-altitude Epas1 allele (Epas1^H/H ). Carotid body growth and glomus cell hyperplasia, which was strongly induced in Epas1^L/L mice in chronic hypoxia, was not observed in Epas1^H/H mice. Epas1 genotype also modulated the effects of chronic hypoxia on metabolism and body temperature depression in hypoxia, but had no effects on haematological traits. These findings confirm the important role of HIF-2α in modulating ventilatory sensitivity and carotid body growth in chronic hypoxia, and show that genetic variation in Epas1 is responsible for evolved changes in the control of breathing and metabolism in high-altitude deer mice. Abstract figure legend ventilation and carotid body growth in hypoxia, so we investigated the role genetic variants in Epas1 in highaltitude deer mice on the control of breathing. In the lab, hybrids between high- and lowaltitude populations of deer mice were created to disrupt linkages between genetic loci so physiological effects of Epas1 alleles (Epas1H and Epas1L, respectively) could be examined on an admixed genomic background. The high-altitude variant was associated with reduced ventilatory chemosensitivity and carotid body growth after 4 weeks of chronic hypoxia, compared to mice homozygous for the low-altitude allele (Epas1LL). These results help us better understand the genetic basis for the unique physiological phenotype of high-altitude natives. This article is protected by copyright. All rights reserved.

Oxygen therapy limiting peripheral oxygen saturation to 89-93% is associated with a better survival prognosis for critically ill COVID-19 patients at high altitudes

Patients admitted to the Intensive Care Unit (ICU) with acute hypoxemic respiratory failure automatically receive oxygen therapy to improve inspiratory oxygen fraction (FiO₂). Supplemental oxygen is the most prescribed drug for critically ill patients regardless of altitude of residence. In high altitude dwellers (i.e. in La Paz [≈3,400 m] and El Alto [≈4,150 m] in Bolivia), a peripheral oxygen saturation (SatpO₂) of 89-95% and an arterial partial pressure of oxygen (PaO₂) of 50-67 mmHg (lower as altitude rises), are considered normal values ​​for arterial blood. Consequently, it has been suggested that limiting oxygen therapy to maintain SatpO₂ around normoxia may help avoid episodes of hypoxemia, hyperoxemia, intermittent hypoxemia, and ultimately, mortality. In this study, we evaluated the impact of oxygen therapy on the mortality of critically ill COVID-19 patients who permanently live at high altitudes. A multicenter cross-sectional descriptive observational study was performed on 100 patients admitted to the ICU at the "Clinica Los Andes" (in La Paz city) and "Agramont" and "Del Norte" Hospitals (in El Alto city). Our results show that: 1) as expected, fatal cases were detected only in patients who required intubation and connection to invasive mechanical ventilation as a last resort to overcome their life-threatening desaturation; 2) among intubated patients, prolonged periods in normoxia are associated with survival, prolonged periods in hypoxemia are associated with death, and time spent in hyperoxemia shows no association with survival or mortality; 3) the oxygenation limits required to effectively support the intubated patients' survival in the ICU are between 89% and 93%; 4) among intubated patients with similar periods of normoxemic oxygenation, those with better SOFA scores survive; and 5) a lower frequency of observable reoxygenation events is not associated with survival. In conclusion, our findings indicate that high-altitude patients entering an ICU at altitudes of 3,400 - 4,150 m should undergo oxygen therapy to maintain oxygenation levels between 89 and 93 %.

Coping with hypoxemia: Could erythropoietin (EPO) be an adjuvant treatment of COVID-19?

A very recent epidemiological study provides preliminary evidence that living in habitats located at 2500 m above sea level (masl) might protect from the development of severe respiratory symptoms following infection with the novel SARS-CoV-2 virus. This epidemiological finding raises the question of whether physiological mechanisms underlying the acclimatization to high altitude identifies therapeutic targets for the effective treatment of severe acute respiratory syndrome pivotal to the reduction of global mortality during the COVID-19 pandemic. This article compares the symptoms of acute mountain sickness (AMS) with those of SARS-CoV-2 infection and explores overlapping patho-physiological mechanisms of the respiratory system including impaired oxygen transport, pulmonary gas exchange and brainstem circuits controlling respiration. In this context, we also discuss the potential impact of SARS-CoV-2 infection on oxygen sensing in the carotid body. Finally, since erythropoietin (EPO) is an effective prophylactic treatment for AMS, this article reviews the potential benefits of implementing FDA-approved erythropoietin-based (EPO) drug therapies to counteract a variety of acute respiratory and non-respiratory (e.g. excessive inflammation of vascular beds) symptoms of SARS-CoV-2 infection.

Ventilatory and carotid body responses to acute hypoxia in rats exposed to chronic hypoxia during the first and second postnatal weeks

Chronic hypoxia (CH) during postnatal development causes a blunted hypoxic ventilatory response (HVR) in neonatal mammals. The magnitude of the HVR generally increases with age, so CH could blunt the HVR by delaying this process. Accordingly, we predicted that CH would have different effects on the respiratory control of neonatal rats if initiated at birth versus initiated later in postnatal development (i.e., after the HVR has had time to mature). Rats had blunted ventilatory and carotid body responses to hypoxia whether CH (12 % O₂) occurred for the first postnatal week (P0 to P7) or second postnatal week (P7 to P14). However, if initiated at P0, CH also caused the HVR to retain the "biphasic" shape characteristic of newborn mammals; CH during the second postnatal week did not result in a biphasic HVR. CH from birth delayed the transition from a biphasic HVR to a sustained HVR until at least P9-11, but the HVR attained a sustained (albeit blunted) phenotype by P13-15. Since delayed maturation of the HVR did not completely explain the blunted HVR, we tested the alternative hypothesis that the blunted HVR was caused by an inflammatory response to CH. Daily administration of the anti-inflammatory drug ibuprofen (4 mg kg^-1, i.p.) did not alter the effects of CH on the HVR. Collectively, these data suggest that CH blunts the HVR in neonatal rats by impairing carotid body responses to hypoxia and by delaying (but not preventing) postnatal maturation of the biphasic HVR. The mechanisms underlying this plasticity require further investigation.

An age- and sex-dependent role of catecholaminergic neurons in the control of breathing and hypoxic chemoreflex during postnatal development

The respiratory system undergoes significant development during the postnatal phase. Maturation of brainstem catecholaminergic (CA) neurons is important for the control and modulation of respiratory rhythmogenesis, as well as for chemoreception in early life. We demonstrated an inhibitory role for CA neurons in CO₂ chemosensitivity in neonatal and juvenile male and female rats, but information regarding their role in the hypoxic ventilatory response (HVR) is lacking. We evaluated the contribution of brainstem CA neurons in the HVR during postnatal (P) development (P7-8, P14-15 and P20-21) in male and female rats through chemical injury with conjugated saporin anti-dopamine beta-hydroxylase (DβH-SAP, 420 ng·μL^-1) injected in the fourth ventricle. Ventilation (V̇_E) and oxygen consumption were recorded one week after the lesion in unanesthetized rats during exposure to normoxia and hypoxia. Hypoxia reduced breathing variability in P7-8 control rats of both sexes. At P7-8, the HVR for lesioned males and females increased 27% and 24%, respectively. Additionally, the lesion reduced the normoxic breathing variability in both sexes at P7-8, but hypoxia partially reverted this effect. For P14-15, the increase in V̇_E during hypoxia was 30% higher for male and 24% higher for female lesioned animals. A sex-specific difference was detected at P20-21, as lesioned males exhibited a 24% decrease in the HVR, while lesioned females experienced a 22% increase. Furthermore, the hypoxia-induced body temperature reduction was attenuated in P20-21 lesioned females. We conclude that brainstem CA neurons modulate the HRV during the postnatal phase, and possibly thermoregulation during hypoxia.

Sex differences in breathing

Breathing is a vital behavior that ensures both the adequate supply of oxygen and the elimination of CO₂, and it is influenced by many factors. Despite that most of the studies in respiratory physiology rely heavily on male subjects, there is much evidence to suggest that sex is an important factor in the respiratory control system, including the susceptibility for some diseases. These different respiratory responses in males and females may be related to the actions of sex hormones, especially in adulthood. These hormones affect neuromodulatory systems that influence the central medullary rhythm/pontine pattern generator and integrator, sensory inputs to the integrator and motor output to the respiratory muscles. In this article, we will first review the sex dependence on the prevalence of some respiratory-related diseases. Then, we will discuss the role of sex and gonadal hormones in respiratory control under resting conditions and during respiratory challenges, such as hypoxia and hypercapnia, and whether hormonal fluctuations during the estrous/menstrual cycle affect breathing control. We will then discuss the role of the locus coeruleus, a sexually dimorphic CO₂/pH-chemosensitive nucleus, on breathing regulation in males and females. Next, we will highlight the studies that exist regarding sex differences in respiratory control during development. Finally, the few existing studies regarding the influence of sex on breathing control in non-mammalian vertebrates will be discussed.

Ventilatory and Autonomic Regulation in Sleep Apnea Syndrome: A Potential Protective Role for Erythropoietin?

Obstructive sleep apnea (OSA) is the most common form of sleep disordered breathing and is associated with wide array of cardiovascular morbidities. It has been proposed that during OSA, the respiratory control center (RCC) is affected by exaggerated afferent signals coming from peripheral/central chemoreceptors which leads to ventilatory instability and may perpetuate apnea generation. Treatments focused on decreasing hyperactivity of peripheral/central chemoreceptors may be useful to improving ventilatory instability in OSA patients. Previous studies indicate that oxidative stress and inflammation are key players in the increased peripheral/central chemoreflex drive associated with OSA. Recent data suggest that erythropoietin (Epo) could also be involved in modulating chemoreflex activity as functional Epo receptors are constitutively expressed in peripheral and central chemoreceptors cells. Additionally, there is some evidence that Epo has anti-oxidant/anti-inflammatory effects. Accordingly, we propose that Epo treatment during OSA may reduce enhanced peripheral/central chemoreflex drive and normalize the activity of the RCC which in turn may help to abrogate ventilatory instability. In this perspective article we discuss the potential beneficial effects of Epo administration on ventilatory regulation in the setting of OSA.

Brief ethanol exposure and stress-related factors disorganize neonatal breathing plasticity during the brain growth spurt period in the rat

RATIONALE

The effects of early ethanol exposure upon neonatal respiratory plasticity have received progressive attention given a multifactorial perspective related with sudden infant death syndrome or hypoxia-associated syndromes. The present preclinical study was performed in 3-9-day-old pups, a stage in development characterized by a brain growth spurt that partially overlaps with the 3rd human gestational trimester.

METHODS

Breathing frequencies and apneas were examined in pups receiving vehicle or a relatively moderate ethanol dose (2.0 g/kg) utilizing a whole body plethysmograph. The experimental design also considered possible associations between drug administration stress and exteroceptive cues (plethysmographic context or an artificial odor). Ethanol exposure progressively exerted a detrimental effect upon breathing frequencies. A test conducted at PD9 when pups were under the state of sobriety confirmed ethanol's detrimental effects upon respiratory plasticity (breathing depression).

RESULTS

Pre-exposure to the drug also resulted in a highly disorganized respiratory response following a hypoxic event, i.e., heightened apneic episodes. Associative processes involving drug administration procedures and placement in the plethysmographic context also affected respiratory plasticity. Pups that experienced intragastric administrations in close temporal contiguity with such a context showed diminished hyperventilation during hypoxia. In a 2nd test conducted at PD9 while pups were intoxicated and undergoing hypoxia, an attenuated hyperventilatory response was observed. In this test, there were also indications that prior ethanol exposure depressed breathing frequencies during hypoxia and a recovery normoxia phase.

CONCLUSION

As a whole, the results demonstrated that brief ethanol experience and stress-related factors significantly disorganize respiratory patterns as well as arousal responses linked to hypoxia in neonatal rats.

Ageing of the carotid body

The ageing process is characterized by a decline in several physiological functions resulting in a reduced capability to maintain homeostasis. The lowered homeostatic capacity seems to involve the carotid body (CB), whose role is to modulate ventilation and tissue oxygen supply. It thus plays a prime role in all ageing processes. Ageing causes marked changes in CB morphology. In older animals, it is enlarged and shows a concomitant decrease in the percentage of chemoreceptor tissue, as well as a proliferation of type II cells. The carotid glomitis is present with aggregates of lymphocytes and fibrosis of the lobules. Type I cells are dehydrated, with a profound vacuolization, a shrinking nucleus, and lipofuscin accumulation. With increased age, human CB shows a reduction in the number and volume of mitochondria, fewer synaptic junctions between glomi, along with a reduction in CB content of neurotransmitters, leading to a sort of 'physiological denervation'. Ageing could be interpreted as a cumulative result of oxidative damage to cells, which derives from aerobic metabolism. Moreover, metabolic rate is tightly correlated with life duration; thus a loss in mitochondrial function is one of the prime factors affecting CB ageing processes. The age-related reduction in synaptic junctions might be a self-protective mechanism through which cells buffer themselves against an accumulation of reactive oxygen species. The correlation between hypoxia and the life duration of CB cells remains an open question until how and why cells sense oxygen is understood.

Increased ventilation in female erythropoietin-deficient mouse line is not progesterone and estrous stage-dependent

Previous studies suggest that chronic erythropoietin (Epo) deficiency in male mice does not alter normoxic/hypoxic ventilation. As effects of Epo are sex specific and as progesterone could be a respiratory stimulant, we evaluated the impact of Epo deficiency and its possible interaction with progesterone in ventilatory control in female mice during estrous cycle phases. Compared to wild type (WT) animals, Epo-TAg^h female mice exhibited higher ventilation in hypoxia. However, when data were separated into luteal and follicular phases of the estrous cycle, basal ventilation and hypoxic ventilation were not different in both mice strains. As progesterone is known to be a potent respiratory stimulant, additional experiments were performed to elucidate its role. Interestingly, after mifepristone treatment, HVR was not modified in WT and Epo-TAg^h mice, showing that the ventilatory stimulation observed in females was not directly mediated by progesterone. We conclude that Epo-TAg^h female mice show no estrous stage-dependent increase of ventilatory control and progesterone independent response to hypoxia.

Blunted Respiratory Responses in the Streptozotocin-Induced Alzheimer's Disease Rat Model

Alzheimer's disease (AD) is known for the progressive decline of cognition and memory. In addition to these disease-defining symptoms, impairment of respiratory function is frequently observed and often expressed by sleep-disordered breathing or reduced ability to adjust respiration when oxygen demand is elevated. The mechanisms for this are widely unknown. Postmortem analysis from the brainstem of AD patients reveals pathological alterations, including in nuclei responsible for respiratory control. In this study, we analyzed respiratory responses and morphological changes in brainstem nuclei following intracerebroventricular (ICV) injections of streptozotocin (STZ), a rat model commonly used to mimic sporadic AD. ICV-STZ induced significant astrogliosis in the commissural part of the nucleus tractus solitarii, an area highly involved in respiration control. The astrogliosis was identified by a significant increase in S100B-immunofluorescence that is similar to the astrogliosis found in the CA1 region of the hippocampus. Using plethysmography, the control group displayed a typical age-dependent decrease of ventilation that was absent in the STZ rat group. This is indicative of elevated minute ventilation at rest after STZ treatment. Peripheral chemoreflex responses were significantly blunted in STZ rats as seen by a reduced respiratory rate and minute ventilation to hypoxia. Central chemoreflex responses to hypercapnia, on the other hand, only decreased in respiratory rate following STZ treatment. Overall, our results show that ICV-STZ induces respiratory dysfunction at rest and in response to hypoxia. This provides a new tool to study the underlying mechanisms of breathing disorders in clinical AD.

Efficient breathing at neonatal ages: A sex and Epo-dependent issue

During postnatal life, the respiratory control system undergoes intense development and is highly responsive to stimuli emerging from the environment. In fact, interruption of breathing prevents gas exchange and results in systemic hypoxia that, if prolonged, can lead to cardio-respiratory failure or sudden infant death. Moreover, in newborns and infants, respiratory disorders related to neural control dysfunction show significant sexual dimorphism with a higher prevalence in males. To this day, the therapeutic tools available to alleviate these respiratory disorders remain limited. Furthermore, the factors explaining the sexual dimorphism in newborns and during infancy remain unknown. Erythropoietin (Epo) was originally discovered as a cytokine able to increase the production of red blood cells upon conditions of reduced oxygen availability. We now know that Epo is a cytokine also secreted by neurons and astrocytes that protects the brain during trauma or hypoxic stress in a sex dependent manner. In this novel line of research, our previous studies demonstrated at adult ages that cerebral Epo acts as a respiratory stimulant in rodents and humans. These results provided a strong rationale for exploring the role of cerebral Epo in neuronal respiratory control during postnatal development. The objective of this review is to summarize our recent findings showing that cerebral Epo is a potent sex-specific respiratory stimulant at neonatal ages. Keeping in mind that Epo is routinely and safely administrated in newborn humans for anemia and neonatal asphyxia, we predict that our research provides the basis necessary to promote the clinical use of Epo against neonatal respiratory disorders related to neural control dysfunction.

Microglia modulate brainstem serotonergic expression following neonatal sustained hypoxia exposure: implications for sudden infant death syndrome

KEY POINTS

Neonatal sustained hypoxia exposure modifies brainstem microglia and serotonin expression. The altered brainstem neurochemistry is associated with impaired ventilatory responses to acute hypoxia and mortality. The deleterious effects of sustained hypoxia exposure can be prevented by an inhibitor of activated microglia. These observations demonstrate a potential cause of the brainstem serotonin abnormalities thought to be involved in sudden infant death syndrome.

ABSTRACT

We showed previously that the end of the second postnatal week (days P11-15) represents a period of development during which the respiratory neural control system exhibits a heightened vulnerability to sustained hypoxia (SH, 11% O2 , 5 days) exposure. In the current study, we investigated whether the vulnerability to SH during the same developmental time period is associated with changes in brainstem serotonin (5-HT) expression and whether it can be prevented by the microglia inhibitor minocycline. Using whole-body plethysmography, SH attenuated the acute (5 min) hypoxic ventilatory response (HVR) and caused a high incidence of mortality compared to normoxia rats. SH also increased microglia cell numbers and decreased 5-HT immunoreactivity in the nucleus of the solitary tract (nTS) and dorsal motor nucleus of the vagus (DMNV). The attenuated HVR, mortality, and changes in nTS and DMNV immunoreactivity was prevented by minocycline (25 mg kg(-1) /2 days during SH). These data demonstrate that the 5-HT abnormalities in distinct respiratory neural control regions can be initiated by prolonged hypoxia exposure and may be modulated by microglia activity. These observations share several commonalities with the risk factors thought to underlie the aetiology of sudden infant death syndrome, including: (1) a vulnerable neonate; (2) a critical period of development; (3) evidence of hypoxia; (4) brainstem gliosis (particularly the nTS and DMNV); and (5) 5-HT abnormalities.

Gokyo Khumbu/Ama Dablam Trek 2012: effects of physical training and high-altitude exposure on oxidative metabolism, muscle composition, and metabolic cost of walking in women

PURPOSE

We investigated the effects of moderate-intensity training at low and high altitude on VO2 and QaO2 kinetics and on myosin heavy-chain expression (MyHC) in seven women (36.3 yy ± 7.1; 65.8 kg ± 11.7; 165 cm ± 8) who participated in two 12- to 14-day trekking expeditions at low (598 m) and high altitude (4132 m) separated by 4 months of recovery.

METHODS

Breath-by-breath VO2 and beat-by-beat QaO2 at the onset of moderate-intensity cycling exercise and energy cost of walking (Cw) were assessed before and after trekking. MyHC expression of vastus lateralis was evaluated before and after low-altitude and after high-altitude trekking; muscle fiber high-resolution respirography was performed at the beginning of the study and after high-altitude trekking.

RESULTS

Mean response time of VO2 kinetics was faster (P = 0.002 and P = 0.001) and oxygen deficit was smaller (P = 0.001 and P = 0.0004) after low- and high-altitude trekking, whereas ˙ QaO2 kinetics and Cw did not change. Percentages of slow and fast isoforms of MyHC and mitochondrial mass were not affected by low- and high-altitude training. After training altitude, muscle fiber ADP-stimulated mitochondrial respiration was decreased as compared with the control condition (P = 0.016), whereas leak respiration was increased (P = 0.031), leading to a significant increase in the respiratory control ratio (P = 0.016).

CONCLUSIONS

Although training did not significantly modify muscle phenotype, it induced beneficial adaptations of the oxygen transport-utilization systems witnessed by faster VO2 kinetics at exercise onset.

Echinacea-Based Dietary Supplement Does Not Increase Maximal Aerobic Capacity in Endurance-Trained Men and Women

PURPOSE

To determine if an echinacea-based dietary supplement (EBS) provided at two different doses (a regular dose (RD), 8,000 mg/day, vs. a double dose (DD), 16,000 mg/day) would increase erythropoietin (EPO) and other blood markers involved in improving aerobic capacity and maximal oxygen consumption (VO2max) in endurance-trained men. Secondly, to determine if any sex differences exist between male and female endurance-trained athletes.

METHODS

Forty-five endurance athletes completed three visits during a 35-day intervention. Participants were randomized into placebo (PLA; n = 8 men, n = 7 women), RD of EBS (n = 7 men, n = 8 women), or DD of EBS (n = 15 men) for the 35-day intervention period. At baseline, weight, body composition, and VO2max were measured. Blood was drawn to measure EPO, ferritin, red blood cells, white blood cells, hemoglobin, and hematocrit. At the mid-intervention visit, blood was collected. At the post-intervention visit, all measurements from the baseline visit were obtained once again.

RESULTS

There was a significant increase in VO2max for endurance-trained men in PLA (increase of 2.8 ± 1.5 ml kg(-1) min(-1), p = .01) and RD of EBS (increase of 2.6 ± 1.8 ml kg(-1) min(-1), p = .04), but not in DD of EBS (p = .96). Importantly, there was no difference in the change in VO2max between PLA and RD of EBS. For endurance-trained women, VO2max did not change in either treatment (PLA: -0.7 ± 1.7 ml kg(-1) min(-1), p = .31; RD of EBS: -0.2 ± 2.4 ml kg(-1) min(-1), p = .80). There were no significant changes in any blood parameter across visits for any treatment group.

CONCLUSIONS

This EBS should not be recommended as a means to improve performance in endurance athletes.

Perinatal hypoxia increases susceptibility to high-altitude polycythemia and attendant pulmonary vascular dysfunction

Perinatal exposures exert a profound influence on physiological function, including developmental processes vital for efficient pulmonary gas transfer throughout the lifespan. We extend the concept of developmental programming to chronic mountain sickness (CMS), a debilitating syndrome marked by polycythemia, ventilatory impairment, and pulmonary hypertension that affects ∼10% of male high-altitude residents. We hypothesized that adverse perinatal oxygenation caused abnormalities of ventilatory and/or pulmonary vascular function that increased susceptibility to CMS in adulthood. Subjects were 67 male high-altitude (3,600-4,100 m) residents aged 18-25 yr with excessive erythrocytosis (EE, Hb concentration ≥18.3 g/dl), a preclinical form of CMS, and 66 controls identified from a community-based survey (n = 981). EE subjects not only had higher Hb concentrations and erythrocyte counts, but also lower alveolar ventilation, impaired pulmonary diffusion capacity, higher systolic pulmonary artery pressure, lower pulmonary artery acceleration time, and more frequent right ventricular hypertrophy, than controls. Compared with controls, EE subjects were more often born to mothers experiencing hypertensive complications of pregnancy and hypoxia during the perinatal period, with each increasing the risk of developing EE (odds ratio = 5.25, P = 0.05 and odds ratio = 6.44, P = 0.04, respectively) after other factors known to influence EE status were taken into account. Adverse perinatal oxygenation is associated with increased susceptibility to EE accompanied by modest abnormalities of the pulmonary circulation that are independent of increased blood viscosity. The association between perinatal hypoxia and EE may be due to disrupted alveolarization and microvascular development, leading to impaired gas exchange and/or pulmonary hypertension.

Vulnerability of neonatal respiratory neural control to sustained hypoxia during a uniquely sensitive window of development

The first postnatal weeks represent a period of development in the rat during which the respiratory neural control system may be vulnerable to aberrant environmental stressors. In the present study, we investigated whether sustained hypoxia (SH; 11% O2) exposure starting at different postnatal ages differentially modifies the acute hypoxic (HVR) and hypercapnic ventilatory response (HCVR). Three different groups of rat pups were exposed to 5 days of SH, starting at either postnatal age 1 (SH1-5), 11 (SH11-15), or 21 (SH21-25) days. Whole body plethysmography was used to assess the HVR and HCVR the day after SH exposure ended. The primary results indicated that 1) the HVR and HCVR of SH11-15 rats were absent or attenuated (respectively) compared with age-matched rats raised in normoxia; 2) there was a profoundly high (∼84% of pups) incidence of unexplained mortality in the SH11-15 rats; and 3) these phenomena were unique to the SH11-15 group with no comparable effect of the SH exposure on the HVR, HCVR, or mortality in the younger (SH1-5) or older (SH21-25) rats. These results share several commonalities with the risk factors thought to underlie the etiology of sudden infant death syndrome, including 1) a vulnerable neonate; 2) a critical period of development; and 3) an environmental stressor.

Gender considerations in ventilatory and metabolic development in rats: special emphasis on the critical period

In rats, a critical period exists around postnatal day (P) 12-13, when an imbalance between heightened inhibition and suppressed excitation led to a weakened ventilatory and metabolic response to acute hypoxia. An open question was whether the two genders follow the same or different developmental trends throughout the first 3 postnatal weeks and whether the critical period exists in one or both genders. The present large-scale, in-depth ventilatory and metabolic study was undertaken to address this question. Our data indicated that: (1) the ventilatory and metabolic rates in both normoxia and acute hypoxia were comparable between the two genders from P0 to P21; thus, gender was never significant as a main effect; and (2) the age effect was highly significant in all parameters studies for both genders, and both genders exhibited a significantly weakened response to acute hypoxia during the critical period. Thus, the two genders have comparable developmental trends, and the critical period exists in both genders in rats.

Using plethysmography to determine erythropoietin's impact on neural control of ventilation

The evaluation of respiratory parameters often requires the use of anesthetics (that depress the neural -network controlling respiration), and/or ways to restrain the animal's mobility (that produces a stress-dependent increase of respiration). Consequently, the establishment of plethysmography represented an invaluable technique in respiratory physiology. Plethysmography, indeed, allows the assessment of ventilatory parameters on living, unanesthetized, and unrestrained animals. The conception of the barometric plethysmography relies on the fact that an animal placed inside a hermetically closed chamber generates through its breathing a fluctuation of pressure in the chamber than can be recorded. Thus, the respiratory frequency and the tidal volume can be directly measured, while the animal's ventilation is calculated indirectly by the multiplication of these two parameters. In our hands, plethysmography was a key tool to investigate the impact of erythropoietin (Epo) on the neural control of hypoxic ventilation in mice.

Age and sex differences in the ventilatory response to hypoxia and hypercapnia in awake neonatal, pre-pubertal and young adult rats

There is evidence for a "sensitive period" in respiratory development in rats around postnatal age (P) 12-13d. Little is known about sex differences during that time. The purpose of this study was to assess the effect of sex on breathing development, specifically around the "sensitive period". We used whole-body plethysmography to study breathing in normoxic, hypoxic and hypercapnic gases in non-anesthetized male and female neonatal rats from P10 to P15, juvenile (P30) and young adult (P90) rats. Compared to other neonatal ages, P12-13 male rats had significantly lower ventilation during normoxia, hypoxia, and hypercapnia. Compared to age-matched females, P12-13 male rats had lower ventilation in normoxia and hypoxia and a lower O(2) saturation during hypoxia. Circulating estradiol was greater in P12-13 male vs. female rats. Estradiol and ventilatory responses to hypoxia and hypercapnia were negatively correlated in neonatal male, but not female rats. Our results suggest that P10-15 includes a critical developmental period in male but not female rats.

Life-long consequences of postnatal normoxia exposure in rats raised at high altitude

We tested the hypothesis that exposure of high-altitude (HA) rats to a period of postnatal normoxia has long-term consequences on the ventilatory and hematological acclimatization in adults. Male and female HA rats (3,600 m, Po(2) ≃ 100 Torr; La Paz, Bolivia) were exposed to normal room air [HA control (HACont)] or enriched oxygen (32% O(2); Po(2) ≃ 160 Torr) from 1 day before to 15 days after birth [HA postnatal normoxia (HApNorm)]. Hematocrit and hemoglobin values were assessed at 2, 12, and 32 wk of age. Cardiac and lung morphology were assessed at 12 wk by measuring right ventricular hypertrophy (pulmonary hypertension index) and lung air space-to-tissue ratio (indicative of alveolarization). Respiratory parameters under baseline conditions and in response to 32% O(2) for 10 min (relieving the ambient hypoxic stimulus) were measured by whole body plethysmography at 12 wk. Finally, we performed a survival analysis up to 600 days of age. Compared with HACont, HApNorm rats had reduced hematocrit and hemoglobin levels at all ages (both sexes); reduced right ventricular hypertrophy (both sexes); lower air space-to-tissue ratio in the lungs (males only); reduced CO(2) production rate, but higher oxygen uptake (males only); and similar respiratory frequency, tidal volume, and minute ventilation. When breathing 32% O(2), HApNorm male rats had a stronger decrease of minute ventilation than HACont. HApNorm rats had a marked tendency toward longer survival throughout the study. We conclude that exposure to ambient hypoxia during postnatal development in HA rats has deleterious consequences on acclimatization to hypoxia as adults.

Humans at high altitude: hypoxia and fetal growth

High-altitude studies offer insight into the evolutionary processes and physiological mechanisms affecting the early phases of the human lifespan. Chronic hypoxia slows fetal growth and reduces the pregnancy-associated rise in uterine artery (UA) blood flow. Multigenerational vs. shorter-term high-altitude residents are protected from the altitude-associated reductions in UA flow and fetal growth. Presently unknown is whether this fetal-growth protection is due to the greater delivery or metabolism of oxygen, glucose or other substrates or to other considerations such as mechanical factors protecting fragile fetal villi, the creation of a reserve protecting against ischemia/reperfusion injury, or improved placental O(2) transfer as the result of narrowing the A-V O(2) difference and raising uterine P(v)O₂. Placental growth and development appear to be normal or modified at high altitude in ways likely to benefit diffusion. Much remains to be learned concerning the effects of chronic hypoxia on embryonic development. Further research is required for identifying the fetoplacental and maternal mechanisms responsible for transforming the maternal vasculature and regulating UA blood flow and fetal growth. Genomic as well as epigenetic studies are opening new avenues of investigation that can yield insights into the basic pathways and evolutionary processes involved.

Lowland origin women raised at high altitude are not protected against lower uteroplacental O2 delivery during pregnancy or reduced birth weight

OBJECTIVE

Maternal physiologic responses to pregnancy promoting fetal oxygen and nutrient delivery are important determinants of reproductive success. Incomplete physiologic compensation for reduced oxygen availability at high altitude (≥2,500 m) compromises fetal growth. Populations of highland (e.g., Andeans, Tibetans) compared with lowland origin groups (e.g., Europeans, Han Chinese) are protected from this altitude-associated decrease in birth weight; here we sought to determine whether maternal development at high altitude-rather than highland ancestry-contributed to the protection of birth weight and uterine artery (UA) blood flow during pregnancy.

METHODS

In women of lowland ancestry who were either raised at high altitude in La Paz, Bolivia (3,600-4,100 m) ("lifelong," n = 18) or who had migrated there as adults ("newcomers," n = 40) we compared maternal O(2) transport during pregnancy and their infant's birth weight.

RESULTS

Pregnancy raised maternal ventilation and arterial O(2) saturation equally, with the result that arterial O(2) content was similarly maintained at nonpregnant levels despite a fall in hemoglobin. UA blood flow and uteroplacental O(2) delivery were lower in lifelong than newcomer residents (main effect). Birth weight was similar in lifelong residents versus newcomers (2,948 ± 93 vs. 3,090 ± 70 gm), with both having values below those of a subset of eight high-altitude residents who descended to deliver at low altitude (3,418 ± 133 gm, P < 0.05).

CONCLUSION

Lifelong compared with newcomer high-altitude residents have lower uteroplacental O(2) delivery and similar infant birth weights, suggesting that developmental factors are likely not responsible for the protective effect of highland ancestry.

Impaired ventilatory acclimatization to hypoxia in female mice overexpressing erythropoietin: unexpected deleterious effect of estradiol in carotid bodies

Apart from enhancing the production of red blood cells, erythropoietin (Epo) alters the ventilatory response when oxygen supply is reduced. We recently demonstrated that Epo's beneficial effect on the ventilatory response to acute hypoxia is sex dependent, with female mice being better able to cope with reduced oxygenation. In the present work, we hypothesized that ventilatory acclimatization to chronic hypoxia (VAH) in transgenic female mice (Tg6) harboring high levels of Epo in the brain and blood will also be improved compared with wild-type (WT) animals. Surprisingly, VAH was blunted in Tg6 female mice. To define whether this phenomenon had a central (brain stem respiratory centers) and/or peripheral (carotid bodies) origin, a bilateral transection of carotid sinus nerve (chemodenervation) was performed. This procedure allowed the analysis of the central response in the absence of carotid body information. Interestingly, chemodenervation restored the VAH in Tg6 mice, suggesting that carotid bodies were responsible for the blunted response. Coherently with this observation, the sensitivity to oxygen alteration in arterial blood (Dejour test) after chronic hypoxia was lower in transgenic carotid bodies compared with the WT control. As blunted VAH occurred in female but not male transgenic mice, the involvement of sex female steroids was obvious. Indeed, measurement of sexual female hormones revealed that the estradiol serum level was 4 times higher in transgenic mice Tg6 than in WT animals. While ovariectomy decreased VAH in WT females, this treatment restored VAH in Tg6 female mice. In line with this observation, injections of estradiol in ovariectomized Tg6 females dramatically reduced the VAH. We concluded that during chronic hypoxia, estradiol in carotid bodies suppresses the Epo-mediated elevation of ventilation. Considering the increased application of recombinant Epo for a variety of disorders, our data imply the need to take the patient's hormonal status into consideration.

The Ventilatory Response to Hypoxia in Mammals: Mechanisms, Measurement, and Analysis

The respiratory response to hypoxia in mammals develops from an inhibition of breathing movements in utero into a sustained increase in ventilation in the adult. This ventilatory response to hypoxia (HVR) in mammals is the subject of this review. The period immediately after birth contains a critical time window in which environmental factors can cause long-term changes in the structural and functional properties of the respiratory system, resulting in an altered HVR phenotype. Both neonatal chronic and chronic intermittent hypoxia, but also chronic hyperoxia, can induce such plastic changes, the nature of which depends on the time pattern and duration of the exposure (acute or chronic, episodic or not, etc.). At adult age, exposure to chronic hypoxic paradigms induces adjustments in the HVR that seem reversible when the respiratory system is fully matured. These changes are orchestrated by transcription factors of which hypoxia-inducible factor 1 has been identified as the master regulator. We discuss the mechanisms underlying the HVR and its adaptations to chronic changes in ambient oxygen concentration, with emphasis on the carotid bodies that contain oxygen sensors and initiate the response, and on the contribution of central neurotransmitters and brain stem regions. We also briefly summarize the techniques used in small animals and in humans to measure the HVR and discuss the specific difficulties encountered in its measurement and analysis.

Breathing at high altitude

Acclimatization to long-term hypoxia takes place at high altitude and allows gradual improvement of the ability to tolerate the hypoxic environment. An important component of this process is the hypoxic ventilatory acclimatization (HVA) that develops over several days. HVA reveals profound cellular and neurochemical re-organization occurring both in the peripheral chemoreceptors and in the central nervous system (in brainstem respiratory groups). These changes lead to an enhanced activity of peripheral chemoreceptor and re-inforce the central translation of peripheral inputs to efficient respiratory motor activity under the steady low O(2) pressure. We will review the cellular processes underlying these changes with a particular emphasis on changes of neurotransmitter function and ion channel properties in peripheral chemoreceptors, and present evidence that low O(2) level acts directly on brainstem nuclei to induce cellular changes contributing to maintain a high tonic respiratory drive under chronic hypoxia.

Impaired acclimatization to chronic hypoxia in adult male and female rats following neonatal hypoxia

We tested the hypothesis that neonatal exposure to hypoxia alters acclimatization to chronic hypoxia later in life. Rat pups were exposed to normobaric hypoxia (12% O(2); nHx group) in a sealed chamber, or to normoxia (21% O(2); nNx group) from the day before birth to postnatal day 10. The animals were then raised in normal conditions until reaching 12 wk of age. At this age, we assessed ventilatory and hematological acclimatization to chronic hypoxia by exposing male and female nHx and nNx rats for 2 wk to 10% O(2). Minute ventilation, metabolic rate, hypoxic ventilatory response, hematocrit, and hemoglobin levels were measured both before and after acclimatization. We also quantified right ventricular hypertrophy as an index of pulmonary hypertension both before and after acclimatization. There was a significant effect of neonatal hypoxia that decreases ventilatory response (relative to metabolic rate, VE/VCO(2)) to acute hypoxia before acclimatization in males but not in females. nHx rats had an impaired acclimatization to chronic hypoxia characterized by altered respiratory pattern and elevated hematocrit and hemoglobin levels after acclimatization, in both males and females. Right ventricular hypertrophy was present before and after acclimatization in nHx rats, indicating that neonatal hypoxia results in pulmonary hypertension in adults. We conclude that neonatal hypoxia impairs acclimatization to chronic hypoxia in adults and may be a factor contributing to the establishment of chronic mountain sickness in humans living at high altitude.

Sex steroidal hormones and respiratory control

There is a growing public awareness that sex hormones can have an impact on a variety of physiological processes. Yet, despite almost a century of research, we still do not have a clear picture as to the effects of sex hormones on the regulation of breathing. Considerable data has accumulated showing that estrogen, progesterone and testosterone can influence respiratory function in animals and humans. Several disorders of breathing such as obstructive sleep apnea (OSA) and sudden infant death syndrome (SIDS) show clear sex differences in their prevalence, lending weight to the importance of sex hormones in respiratory control. This review focuses on questions such as: how early do sex hormones influence breathing? Which is the most effective? Where do sex hormones exert their effects? What mechanisms are involved? Are there age-associated changes? A clearer understanding of how sex hormones influence the control of breathing could enable sex- and age-specific therapeutic interventions for diseases of the respiratory control system.

Novel neural correlates of operant conditioning in normal and differentially reared Lymnaea

The aerial respiratory behaviour of the mollusc Lymnaea stagnalisis an important homeostatic behaviour that can be operantly conditioned. The central pattern generator underlying this behaviour, as well as motorneurons innervating the respiratory orifice, the pneumostome, have been identified and their activity can be monitored in the semi-intact preparation using electrophysiological recordings. In this study, we used both intact animals and semi-intact preparations to identify novel changes in the respiratory central pattern generator following operant conditioning. In addition, we reared animals in the absence of this respiratory behaviour throughout development, to investigate whether previous experience and activity-dependent plasticity during development are essential to allow neural plasticity in the adult. We found that animals raised normally (allowed to perform aerial respiratory behaviour) exhibited the expected reduction in aerial respiratory behaviour following operant conditioning. Then, using the semi-intact preparation, we identified novel neural changes within the network as a result of the conditioning. These included specific changes at the level of the central pattern generator interneurons, as well as the motor output. In the differentially reared intact animals, there was no behavioural reduction as a result of operant conditioning, although their baseline respiratory behaviour was already significantly reduced as a result of their differential rearing. There were, however, significant differences found in the network parameters in the semi-intact preparation, similar to those observed in normally reared animals. We thus provide evidence for neural plasticity within the network in the absence of significant behavioural changes in differentially reared animals, and show that plasticity was not dependent on previous activity of the network during development.

Sex-dependent regulation of hypoxic ventilation in mice and humans is mediated by erythropoietin

Acclimatization to hypoxic exposure relies on an elevated ventilation and erythropoietic activity. We recently proposed that erythropoietin (Epo) links both responses: apart from red blood cell production, cerebral and plasma Epo interact with the central and peripheral respiratory centers. Knowing that women cope better than men with reduced oxygen supply (as observed at high altitude), we analyzed the hypoxic ventilatory response in Epo-overexpressing transgenic male and female mice with high Epo levels in brain and plasma (Tg6) or in wild-type animals injected with recombinant human Epo (rhEpo). Exposure to moderate and severe hypoxia as well as to hyperoxia and injection of domperidone, a potent peripheral ventilatory stimulant, revealed that the presence of transgenic or rhEpo extensively increased the hypoxic ventilatory response in female mice compared with their corresponding male siblings. Alterations of catecholamines in the brain stem's respiratory centers were also sex dependent. In a proof-of-concept study, human volunteers were intravenously injected with 5,000 units rhEpo and subsequently exposed to 10% oxygen. Compared with men, the hypoxic ventilatory response was significantly increased in women. We conclude that Epo exerts a sex-dependent impact on hypoxic ventilation improving the response in female mice and in women that most probably involves sexual hormones. Our data provides an explanation as to why women are less susceptible to hypoxia-associated syndromes than men.

Ventilatory responses to acute and chronic hypoxia are altered in female but not male Paskin-deficient mice

Proteins harboring a Per-Arnt-Sim (PAS) domain are versatile and allow archaea, bacteria, and plants to sense oxygen partial pressure, as well as light intensity and redox potential. A PAS domain associated with a histidine kinase domain is found in FixL, the oxygen sensor molecule of Rhizobium species. PASKIN is the mammalian homolog of FixL, but its function is far from being understood. Using whole body plethysmography, we evaluated the ventilatory response to acute and chronic hypoxia of homozygous deficient male and female PASKIN mice ( Paskin −/−). Although only slight ventilatory differences were found in males, female Paskin −/− mice increased ventilatory response to acute hypoxia. Unexpectedly, females had an impaired ability to reach ventilatory acclimatization in response to chronic hypoxia. Central control of ventilation occurs in the brain stem respiratory centers and is modulated by catecholamines via tyrosine hydroxylase (TH) activity. We observed that TH activity was altered in male and female Paskin −/− mice. Peripheral chemoreceptor effects on ventilation were evaluated by exposing animals to hyperoxia (Dejours test) and domperidone, a peripheral ventilatory stimulant drug directly affecting the carotid sinus nerve discharge. Male and female Paskin −/− had normal peripheral chemosensory (carotid bodies) responses. In summary, our observations suggest that PASKIN is involved in the central control of hypoxic ventilation, modulating ventilation in a gender-dependent manner.

Ventilation and phrenic output following high cervical spinal hemisection in male vs. female rats

Female sex hormones influence the neural control of breathing and may impact neurologic recovery from spinal cord injury. We hypothesized that respiratory recovery after C2 spinal hemisection (C2HS) differs between males and females and is blunted by prior ovariectomy (OVX) in females. Inspiratory tidal volume (VT), frequency (fR), and ventilation (VE) were quantified during quiet breathing (baseline) and 7% CO2 challenge before and after C2HS in unanesthetized adult rats via plethysmography. Baseline breathing was similarly altered in all rats (reduced VT, elevated fR) but during hypercapnia females had relatively higher VT (i.e. compared to pre-injury) than male or OVX rats (p<0.05). Phrenic neurograms recorded in anesthetized rats indicated that normalized burst amplitude recorded ipsilateral to C2HS (i.e. the crossed phrenic phenomenon) is greater in females during respiratory challenge (p<0.05 vs. male and OVX). We conclude that sex differences in recovery of VT and phrenic output are present at 2 weeks post-C2HS. These differences are consistent with the hypothesis that ovarian sex hormones influence respiratory recovery after cervical spinal cord injury.

Prenatal nicotine alters maturation of breathing and neural circuits regulating respiratory control

While perinatal nicotine effects on ventilation have been widely investigated, the prenatal impact of nicotine treatment during gestation on both breathing and neural circuits involved in respiratory control remains unknown. We examined the effects of nicotine, from embryonic day 5 (E5) to E20, on baseline ventilation, the two hypoxic ventilatory response components and in vivo tyrosine hydroxylase (TH) activity in carotid bodies and brainstem areas, assessed at postnatal day 7 (P7), P11 and P21. In pups prenatally exposed to nicotine, baseline ventilation and hypoxic ventilatory response were increased at P7 (+48%) and P11 (+46%), with increased tidal volume (p<0.05). Hypoxia blunted frequency response at P7 and revealed unstable ventilation at P11. In carotid bodies, TH activity increased by 20% at P7 and decreased by 48% at P11 (p<0.05). In most brainstem areas it was reduced by 20-33% until P11. Changes were resolved by P21. Prenatal nicotine led to postnatal ventilatory sequelae, partly resulting from impaired maturation of peripheral chemoreceptors and brainstem integrative sites.

Developmental plasticity of the carotid chemoafferent pathway in rats that are hypoxic during the prenatal period

The chemoreflex pathway undergoes postnatal maturation, and the perinatal environment plays a critical role in shaping respiratory control system. We investigated the role of prenatal hypoxia on the maturation of the chemoreflex neural circuits regulating ventilation in rat. Effects of hypoxia (10% O2) from the 5th to the 20th day of gestation were studied on male offspring at birth and on postnatal days 3, 7, 21 and 68. Maturation of the respiratory control system was assessed by in vivo tyrosine hydroxylase (TH) activity measurement in peripheral chemoreceptors (carotid bodies, petrosal ganglia), and in brainstem catecholaminergic cell groups (A2C2c and A1C1 areas in the medulla, A5 and A6 areas in the pons). Resting ventilation and ventilatory response to hypoxia were evaluated as functional sequelae. In peripheral structures, prenatal hypoxia reduced TH activity within the first postnatal week and enhanced it later. In contrast, in central areas, prenatal hypoxia upregulated TH activity within the first postnatal week and downregulated it later. The in vivo TH activity impairment is therefore tissue specific, with an opposite effect on the peripheral and central neural circuits. A shift of the effect of prenatal hypoxia occurred between 1 and 3 weeks, indicating a postnatal temporal effect of prenatal hypoxia. An important period in the development of the chemoafferent pathway occurred between the first and the third postnatal week. Functionally, prenatal hypoxia impaired resting ventilation and ventilatory response to hypoxia. The alterations of the catecholaminergic components of the chemoafferent pathway resulting from prenatal hypoxia might contribute to impair postnatal respiratory behaviour.

Acute and chronic exposure to hypoxia alters ventilatory pattern but not minute ventilation of mice overexpressing erythropoietin

Apart from enhancing red blood cell production, erythropoietin (Epo) has been shown to modulate the ventilatory response to reduced oxygen supply. Both functions are crucial for the organism to cope with increased oxygen demand. In the present work, we analyzed the impact of Epo and the resulting excessive erythrocytosis in the neural control of normoxic and hypoxic ventilation. To this end, we used our transgenic mouse line (Tg6) that shows high levels of human Epo in brain and plasma, the latter leading to a hematocrit of ∼80%. Interestingly, while normoxic and hypoxic ventilation in Tg6 mice was similar to WT mice, Tg6 mice showed an increased respiratory frequency but a decreased tidal volume. Knowing that Epo modulates catecholaminergic activity, the altered catecholaminergic metabolism measured in brain stem suggested that the increased respiratory frequency in Tg6 mice was related to the overexpression of Epo in brain. In the periphery, higher response to hyperoxia (Dejours test), as well as reduced tyrosine hydroxylase activity in carotid bodies, revealed a higher chemosensitivity to oxygen in transgenic mice. Moreover, in line with the decreased activity of the rate-limiting enzyme for dopamine synthesis, the intraperitoneal injection of a highly specific peripheral ventilatory stimulant, domperidone, did not stimulate hypoxic ventilatory response in Tg6 mice. These results suggest that high Epo plasma levels modulate the carotid body's chemotransduction. All together, these findings are relevant for understanding the cross-talk between the ventilatory and erythropoietic systems exposed to hypoxia.

Does chronic mountain sickness (CMS) have perinatal origins?

Chronic mountain sickness (CMS) occurs in approximately 10% of male high-altitude residents. It is characterized by hypoventilation and hypoxemia but its underlying cause remains unknown. We hypothesized that CMS' origins reside in exaggerated perinatal hypoxia that serves, in turn, to impair the development of pulmonary structure and/or respiratory control. As a preliminary test, we asked if birth weights were low and other signs of perinatal hypoxia were present in 12 young men with excessive erythrocytosis (EE, Hb>or=18.3g/dL), a condition thought to be a preclinical phase of CMS. Their birth weights were uniformly low (2571+/-243g) and all but one demonstrated perinatal hypoxia as manifested either by being small for their gestational age (SGA, 8%), preterm (67%), born to a preeclamptic (PE) mother (50%), or diagnosed with neonatal hypoxia (83%). Impaired growth in utero has been shown to raise susceptibility to adult disease; these are the first data to demonstrate a possible influence of reduced fetal growth and/or exaggerated perinatal hypoxia on increasing the susceptibility to CMS. Future studies, with more detailed testing in larger samples of control as well as EE subjects, with longitudinal follow-up, are required to determine the role of perinatal hypoxia in the development of CMS.

Soluble erythropoietin receptor is present in the mouse brain and is required for the ventilatory acclimatization to hypoxia

While erythropoietin (Epo) and its receptor (EpoR) have been widely investigated in brain, the expression and function of the soluble Epo receptor (sEpoR) remain unknown. Here we demonstrate that sEpoR, a negative regulator of Epo's binding to the EpoR, is present in the mouse brain and is down-regulated by 62% after exposure to normobaric chronic hypoxia (10% O2 for 3 days). Furthermore, while normoxic minute ventilation increased by 58% in control mice following hypoxic acclimatization, sEpoR infusion in brain during the hypoxic challenge efficiently reduced brain Epo concentration and abolished the ventilatory acclimatization to hypoxia (VAH). These observations imply that hypoxic downregulation of sEpoR is required for adequate ventilatory acclimatization to hypoxia, thereby underlying the function of Epo as a key factor regulating oxygen delivery not only by its classical activity on red blood cell production, but also by regulating ventilation.

Postnatal cardiopulmonary adaptations to high altitude

Postnatal cardiopulmonary adaptations to high altitude constitute a key component of any set of responses developed to face high altitude hypoxia. Such responses are required ultimately to meet the energy demands necessary for adequate functioning at cell and organism level. After a brief insight on general and cardiopulmonary comparative studies in growing and adult organisms, differences and possible explanations for varying cardiopulmonary pathology, pulmonary artery hypertension, persistent right ventricular predominance and subacute high altitude pulmonary hypertension in different populations of children living at high altitude are discussed. Potential long-term implications of early chronic hypoxic exposure on later diseases are also presented. It is hoped that this review will help the practicing physician working at high altitude to make informed decisions concerning individual pediatric patients, specifically with regard to diagnosis and management of altitude-related cardiopulmonary pathology. Finally, plausibility and the knowledge-base of public health interventions to reduce the risks posed by suboptimal or inadequate postnatal cardiopulmonary responses to high altitude are discussed.

Neonatal maternal separation induces sex-specific augmentation of the hypercapnic ventilatory response in awake rat

Neonatal maternal separation (NMS) is a form of stress that exerts persistent, sex-specific effects on the hypoxic ventilatory response. Adult male rats previously subjected to NMS show a 25% increase in the response, whereas NMS females show a response 30% lower than controls ( 8 ). To assess the extent to which NMS affects ventilatory control development, we tested the hypothesis that NMS alters the ventilatory response to hypercapnia in awake, unrestrained rats. Pups subjected to NMS were placed in a temperature- and humidity-controlled incubator 3 h/day for 10 consecutive days (P3 to P12). Control pups were undisturbed. At adulthood (8 to 10 wk old), rats were placed in a plethysmography chamber for measurement of ventilatory parameters under baseline and hypercapnic conditions (inspired CO2 fraction = 0.05). After 20 min of hypercapnia, the minute ventilation response measured in NMS males was 47% less than controls, owing to a lower tidal volume response (22%). Conversely, females previously subjected to NMS showed minute ventilation and tidal volume responses 63 and 18% larger than controls respectively. Although a lower baseline minute ventilation contributes to this effect, the higher minute ventilation/CO2 production response observed in NMS females suggests a greater responsiveness to CO2/H+ in this group. We conclude that NMS exerts sex-specific effects on the hypercapnic ventilatory response and that the neural mechanisms affected by NMS likely differ from those involved in the hypoxic chemoreflex.

Postnatal nicotine and/or intermittent hypercapnic hypoxia effects on apoptotic markers in the developing piglet brainstem medulla

The most important risk factors currently identified for the sudden infant death syndrome (SIDS) are prone sleeping and cigarette smoke exposure. In this study, we investigated the neuropathological sequelae of these risk factors by exposing piglets to intermittent hypercapnic-hypoxia (IHH) and/or nicotine (nic) in the early postnatal period. Our hypothesis was that either nic or IHH exposure could increase neuronal cell death, and that combined exposure (nic+IHH) would be additive. Four exposure patterns were studied: controls (n=14), IHH (n=10), nic (n=14), and nic+IHH (n=14). All groups had equal gender ratios. Nic exposure via an implanted osmotic minipump commenced within 48 h of birth and continued until age 13-14 days when animals were killed and brains collected. A total of 48 min of hypercapnic-hypoxia was delivered on the day immediately prior to killing in a pattern comprising 6 min of HH (8% O(2), 7% CO(2), balance N(2)) alternating with 6 min of air. Immunohistochemistry was performed to identify neurons positive for active caspase-3 and DNA fragmentation (terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling, TUNEL) in seven nuclei of the caudal medulla. Staining quantification showed that: 1. IHH induced neuronal death (increased both TUNEL and casapse-3) in more brainstem nuclei than nicotine. 2. Females were more severely affected by IHH than males. 3. Where IHH and nicotine were combined, TUNEL expression was approximately 5% less than IHH alone, but changes in caspase-3 were variable. We conclude that acute exposure to IHH in the postnatal period is more neurotoxic than exposure to nicotine alone. Combined exposure to IHH and nicotine produced variable responses with some results suggesting that nicotine can be neuroprotective. These results indicate that environmental insults attributable to prone sleeping can produce neurotoxic sequelae in SIDS, with some regional specificity in the response. However, no consistent relationship is evident when combining the two insults.

Activation of NMDA receptors prevents excessive metabolic decrease in hypoxic rat pups

We tested the hypothesis that glutamate NMDA receptors may help maintain metabolic rate and body temperature during acute or chronic hypoxic exposure in newborn rats. We recorded ventilation, metabolism ((.)V(O(2)) -- ((.)V(CO(2)) and rectal temperature, under normoxia, acute hypoxia (30 min -- 12% O(2)), or following 10 days of chronic hypoxia, in 10 days old male and female rats, receiving saline i.p. injection or the NMDA receptor antagonist MK-801. Acute hypoxia decreased rectal temperature and metabolism, and increased ventilation, and (.)V(E)/((.)V(O(2) and (.)V(E)/((.)V(CO(2) to the same extent in males and females. MK-801 injection amplified the metabolic decrease under acute (in males and females) and chronic (in males) hypoxia, prevented the increase of minute ventilation, while (.)V(E)/((.)V(O(2) or (.)V(E)/((.)V(CO(2)remained constant. Hence, NMDA glutamate receptors help to maintain metabolic rate, minute ventilation and body temperature at a determined level in acute (males and females) and chronic hypoxia (males only).

Long-term consequences of neonatal caffeine on ventilation, occurrence of apneas, and hypercapnic chemoreflex in male and female rats

Caffeine is an adenosine receptor antagonist commonly used as a respiratory stimulant to treat neonatal apneas of premature newborn. Neonatal caffeine treatment (NCT) has long-term effects on adenosine receptor expression and distribution; however, the potential effects of NCT on respiratory control development are unknown. To address this issue, rat pups received orally each day from postnatal d 3-12, 15 mg/kg of caffeine (NCT), water (vehicle), or were undisturbed during early life (control). Measurements of resting ventilation, apnea index, and ventilatory response to moderate hypercapnia (FiCO2 = 0.05) were made using whole-body plethysmography at postnatal d 20 (juvenile) and adulthood. At d 20, resting respiratory variables were not affected by the treatments. Juvenile NCT male rats showed a 22% higher minute ventilation response to hypercapnia than vehicle rats. However, oral gavage alone increased the frequency component of the response by 11%. In adult males, caffeine increased the resting respiratory frequency by 15%. In these animals, the tidal volume response to hypercapnia was increased by 15%, whereas the frequency response was decreased by 20%. In juvenile and adult females, no differences were observed between treatments. In juvenile rats of both sexes, gavage increased the apnea index by at least 200%. These results show that NCT and gavage influence respiratory control during early life and that these effects persist until adulthood. The underlying mechanisms are unclear, but may be related to persistent changes in adenosinergic neurotransmission because neonatal caffeine administration increases A1 adenosine receptor density in adult rats.