Central nervous system mechanisms of ventilatory acclimatization to hypoxia

Recovery of ventilatory and metabolic responses to hypoxia in neonatal rats after chronic hypoxia

Chronic hypoxia (CH) during postnatal development attenuates the hypoxic ventilatory response (HVR) in mammals, but there are conflicting reports on whether this plasticity is permanent or reversible. This study tested the hypothesis that CH-induced respiratory plasticity is reversible in neonatal rats and investigated whether the initial plasticity or recovery differs between sexes. Rat pups were exposed to 3 d of normobaric CH (12 % O2) beginning shortly after birth. Ventilation and metabolic CO2 production were then measured in normoxia and during an acute hypoxic challenge (12 % O2) immediately following CH and after 1, 4-5, and 7 d in room air. CH pups hyperventilated when returned to normoxia immediately following CH, but normoxic ventilation was similar to age-matched control rats within 7 d after return to room air. The early phase of the HVR (minute 1) was only blunted immediately following the CH exposure, while the late phase of the HVR (minute 15) remained blunted after 1 and 4-5 d in room air; recovery appeared complete by 7 d. However, when normalized to CO2 production, the late phase of the hypoxic response recovered within only 1 d. The initial blunting of the HVR and subsequent recovery were similar in female and male rats. Carotid body responses to hypoxia (in vitro) were also normal in CH pups after approximately one week in room air. Collectively, these data indicate that ventilatory and metabolic responses to hypoxia recover rapidly in both female and male neonatal rats once normoxia is restored following CH.

Changes in hypoxic and hypercapnic ventilatory responses at high altitude measured using rebreathing methods

Ventilatory responses to hypoxia and hypercapnia play a vital role in maintaining gas exchange homeostasis and in adaptation to high-altitude environments. This study investigates the mechanisms underlying sensitization of hypoxic and hypercapnic ventilatory response (HVR and HCVR, respectively) in individuals acclimatized to moderate high altitude (3,800 m). Thirty-one participants underwent chemoreflex testing using the Duffin-modified rebreathing technique. Measures were taken at sea level and after 2 days of acclimatization to high altitude. Ventilatory recruitment threshold (VRT), HCVR-Hyperoxia, HCVR-Hypoxia, and HVR were quantified. Acclimatization to high altitude resulted in increased HVR (P < 0.001) and HCVR-Hyperoxia (P < 0.001), as expected. We also observed that the decrease in VRT under hypoxic test conditions significantly contributed to the elevated HVR at high altitude since the change in VRT across hyperoxic and hypoxic test conditions was greater at high altitudes compared to baseline sea-level tests (P = 0.043). Pre-VRT, or basal, ventilation also increased at high altitudes (P < 0.001), but the change did not differ between oxygen conditions. Taken together, these data suggest that the increase in HVR at high altitude is at least partially driven by a larger decrease in the VRT in hypoxia versus hyperoxia at high altitude compared to sea level. This study highlights the intricacies of respiratory adaptations during acclimatization to moderate high altitude, shedding light on the roles of the VRT, baseline respiratory drive, and two-slope HCVR in this process. These findings contribute to our understanding of how human respiratory control responds to hypoxic and hypercapnic challenges at high altitude.NEW & NOTEWORTHY We report the first measurements of the hypoxic ventilatory response (HVR) after 2 days at high altitude using a CO2 rebreathing technique. We evaluated mechanisms by which the HVR becomes elevated with acclimatization (increased hypercapnic ventilatory response sensitivity in hypoxia, increased baseline respiratory drive in hypoxia, or lower ventilatory recruitment thresholds in hypoxia). For the first time, we report that decreases in the ventilatory recruitment threshold in hypoxia contribute to elevated HVR at high altitude.

The hypoxic respiratory response of the pre-Bötzinger complex

Since the discovery of the pre-Bötzinger Complex (preBötC) as a crucial region for generating the main respiratory rhythm, our understanding of its cellular and molecular aspects has rapidly increased within the last few decades. It is now apparent that preBötC is a highly flexible neuronal network that reconfigures state-dependently to produce the most appropriate respiratory output in response to various metabolic challenges, such as hypoxia. However, the responses of the preBötC to hypoxic conditions can be varied based on the intensity, pattern, and duration of the hypoxic challenge. This review discusses the preBötC response to hypoxic challenges at the cellular and network level. Particularly, the involvement of preBötC in the classical biphasic response of the respiratory network to acute hypoxia is illuminated. Furthermore, the article discusses the functional and structural changes of preBötC neurons following intermittent and sustained hypoxic challenges. Accumulating evidence shows that the preBötC neural circuits undergo substantial changes following hypoxia and contribute to several types of the respiratory system's hypoxic ventilatory responses.

Influence of chronic hypoxia on the hypoxic ventilatory response of juvenile and adult rats

Chronic hypoxia (CH) from birth attenuates the acute hypoxic ventilatory response (HVR) in rats and other mammals, but CH is often reported to augment the HVR in adult mammals. To test the hypothesis that this transition - from blunting to augmenting the HVR - occurs in the third or fourth postnatal week in rats, juvenile and adult rats were exposed to normobaric CH (12% O2) for 7 days and the HVR was assessed by whole-body plethysmography. No transition was observed, however, and the acute HVR was reduced by 61 - 85% across all ages studied. The failure to observe an augmented HVR in adult rats could not be explained by the substrain of Sprague Dawley rats used, the duration of the CH exposure, the order in which test gases were presented, the level of hypoxia used for CH and to assess the HVR, or the effects of CH on the metabolic response to hypoxia and the hypercapnic ventilatory response. A literature survey revealed several distinct patterns of ventilatory acclimatization to hypoxia (VAH) in adult rats, with most studies (77%) revealing a decrease or no change in the acute HVR after CH. In conclusion, the effects of CH on respiratory control are qualitatively similar across age groups, at least within the populations of Sprague Dawley rats used in the present study, and there does not appear to be one "typical" pattern for VAH in adult rats.

Sex-Specific Effects of Stress on Respiratory Control: Plasticity, Adaptation, and Dysfunction

As our understanding of respiratory control evolves, we appreciate how the basic neurobiological principles of plasticity discovered in other systems shape the development and function of the respiratory control system. While breathing is a robust homeostatic function, there is growing evidence that stress disrupts respiratory control in ways that predispose to disease. Neonatal stress (in the form of maternal separation) affects "classical" respiratory control structures such as the peripheral O₂ sensors (carotid bodies) and the medulla (e.g., nucleus of the solitary tract). Furthermore, early life stress disrupts the paraventricular nucleus of the hypothalamus (PVH), a structure that has emerged as a primary determinant of the intensity of the ventilatory response to hypoxia. Although underestimated, the PVH's influence on respiratory function is a logical extension of the hypothalamic control of metabolic demand and supply. In this article, we review the functional and anatomical links between the stress neuroendocrine axis and the medullary network regulating breathing. We then present the persistent and sex-specific effects of neonatal stress on respiratory control in adult rats. The similarities between the respiratory phenotype of stressed rats and clinical manifestations of respiratory control disorders such as sleep-disordered breathing and panic attacks are remarkable. These observations are in line with the scientific consensus that the origins of adult disease are often found among developmental and biological disruptions occurring during early life. These observations bring a different perspective on the structural hierarchy of respiratory homeostasis and point to new directions in our understanding of the etiology of respiratory control disorders. © 2021 American Physiological Society. Compr Physiol 11:1-38, 2021.

Perioperative Opioid Consumption is Not Reduced in Cyanotic Patients Presenting for the Fontan Procedure

Adequate pain control is a critical component of the perioperative approach to children undergoing repair of congenital heart disease (CHD). The impact of specific anatomic and physiologic disturbances on the management of analgesia has been largely unexplored at the present time. Studies in other pediatric populations have found an association between chronic hypoxemia and an increased sensitivity to the effects of opioid medications. The purpose of this retrospective study was to examine perioperative opioid administration and opioid-associated adverse effects in children undergoing surgical repair of CHD, with a comparison between patients with and without chronic preoperative cyanosis. Patients between the ages of 2 and 5 years whose tracheas were extubated in the operating room were included and were classified in the cyanotic group if they presented for the Fontan completion. The primary outcomes of interest were intraoperative and postoperative opioid administration. Secondary outcomes included pain scores and opioid-related side effects. The study cohort included 156 patients. Seventy-one underwent the Fontan procedure, twelve of which were fenestrated. Fontan patients received fewer opioids intraoperatively (11.33 µg/kg fentanyl equivalents versus 12.56 µg/kg, p = 0.03). However, there were no differences with regards to opioid consumption postoperatively and no correlation between preoperative oxygen saturation and total opioid administration. There were no differences between groups with regards to the respiratory rate nadir, postoperative pain scores, or the incidence of opioid-related side effects. In contrast to other populations with chronic hypoxemia exposure, children with cyanotic CHD did not appear to have increased sensitivity to the effects of opioid medications.

The role of astrocytes in the nucleus tractus solitarii in maintaining central control of autonomic function

The nucleus tractus solitarii (nTS) is the first central site for the termination and integration of autonomic and respiratory sensory information. Sensory afferents terminating in the nTS as well as the embedded nTS neurocircuitry release and utilize glutamate that is critical for maintenance of baseline cardiorespiratory parameters and initiating cardiorespiratory reflexes, including those activated by bouts of hypoxia. nTS astrocytes contribute to synaptic and neuronal activity through a variety of mechanisms, including gliotransmission and regulation of glutamate in the extracellular space via membrane-bound transporters. Here, we aim to highlight recent evidence for the role of astrocytes within the nTS and their regulation of autonomic and cardiorespiratory processes under normal and hypoxic conditions.

Distinct cardiovascular and respiratory responses to short-term sustained hypoxia in juvenile Sprague Dawley and Wistar Hannover rats

Short-term sustained hypoxia (SH) elicits active expiration, augmented late-expiratory (late-E) sympathetic activity, increased arterial pressure and ventilation, and amplified sympathetic and abdominal expiratory responses to chemoreflex activation in rats of the Wistar-Ribeirão Preto (WRP) strain. Herein, we investigated whether SH can differentially affect the cardiovascular and respiratory outcomes of Sprague-Dawley (SD) and Wistar Hannover (WH) rats and compared the results with previous data using WRP rats. For this, we exposed SD and WH rats to SH (FiO₂ = 0.1) for 24 h and evaluated arterial pressure, sympathetic activity, and respiratory pattern. SD rats presented increased arterial pressure, respiratory rate and tidal volume, as well as augmented late-E expiratory motor output and increased sympathetic outflow due to post-inspiratory and late-E sympathetic overactivity. WH rats presented reduced changes, suggesting lower responsiveness of this strain to this SH protocol. The magnitudes of changes in sympathetic and abdominal expiratory motor activities to chemoreflex activation in SD rats were reduced by SH. Pressor responses to chemoreflex activation were shown to be blunted in SD and WH rats after SH. The data are showing that SD, WH, and WRP rat strains exhibit marked differences in their cardiovascular, autonomic and respiratory responses to 24-h SH and draw attention to the importance of rat strain for studies exploring the underlying mechanisms involved in the neuronal changes induced by the experimental model of SH.

Neuronal HIF-1α in the nucleus tractus solitarius contributes to ventilatory acclimatization to hypoxia

KEY POINTS

We hypothesized that hypoxia inducible factor 1α (HIF-1α) in CNS respiratory centres is necessary for ventilatory acclimatization to hypoxia (VAH); VAH is a time-dependent increase in baseline ventilation and the hypoxic ventilatory response (HVR) occurring over days to weeks of chronic sustained hypoxia (CH). Constitutive deletion of HIF-1α in CNS neurons in transgenic mice tended to blunt the increase in HVR that occurs in wild-type mice with CH. Conditional deletion of HIF-1α in glutamatergic neurons of the nucleus tractus solitarius during CH significantly decreased ventilation in acute hypoxia but not normoxia in CH mice. These effects are not explained by changes in metabolic rate, nor CO2 , and there were no changes in the HVR in normoxic mice. HIF-1α mediated changes in gene expression in CNS respiratory centres are necessary in addition to plasticity of arterial chemoreceptors for normal VAH.

ABSTRACT

Chronic hypoxia (CH) produces a time-dependent increase of resting ventilation and the hypoxic ventilatory response (HVR) that is called ventilatory acclimatization to hypoxia (VAH). VAH involves plasticity in arterial chemoreceptors and the CNS [e.g. nucleus tractus solitarius (NTS)], although the signals for this plasticity are not known. We hypothesized that hypoxia inducible factor 1α (HIF-1α), an O2 -sensitive transcription factor, is necessary in the NTS for normal VAH. We tested this in two mouse models using loxP-Cre gene deletion. First, HIF-1α was constitutively deleted in CNS neurons (CNS-HIF-1α-/- ) by breeding HIF-1α floxed mice with mice expressing Cre-recombinase driven by the calcium/calmodulin-dependent protein kinase IIα promoter. Second, HIF-1α was deleted in NTS neurons in adult mice (NTS-HIF-1α-/- ) by microinjecting adeno-associated virus that expressed Cre-recombinase in HIF-1α floxed mice. In normoxic control mice, HIF-1α deletion in the CNS or NTS did not affect ventilation, nor the acute HVR (10-15 min hypoxic exposure). In mice acclimatized to CH for 1 week, ventilation in hypoxia was blunted in CNS-HIF-1α-/- and significantly decreased in NTS-HIF-1α-/- compared to control mice (P < 0.0001). These changes were not explained by differences in metabolic rate or CO2 . Immunofluorescence showed that HIF-1α deletion in NTS-HIF-1α-/- was restricted to glutamatergic neurons. The results indicate that HIF-1α is a necessary signal for VAH and the previously described plasticity in glutamatergic neurotransmission in the NTS with CH. HIF-1α deletion had no effect on the increase in normoxic ventilation with acclimatization to CH, indicating this is a distinct mechanism from the increased HVR with VAH.

Sustained Hypoxia Alters nTS Glutamatergic Signaling and Expression and Function of Excitatory Amino Acid Transporters

Glutamate is the major excitatory neurotransmitter in the nucleus tractus solitarii (nTS) and mediates chemoreflex function during periods of low oxygen (i.e. hypoxia). We have previously shown that nTS excitatory amino acid transporters (EAATs), specifically EAAT-2, located on glia modulate neuronal activity, cardiorespiratory and chemoreflex function under normal conditions via its tonic uptake of extracellular glutamate. Chronic sustained hypoxia (SH) elevates nTS synaptic transmission and chemoreflex function. The goal of this study was to determine the extent to which glial EAAT-2 contributes to SH-induced nTS synaptic alterations. To do so, male Sprague-Dawley rats (4-7 weeks) were exposed to either 1, 3, or 7 days of SH (10% O2, 24 h/day) and compared to normoxic controls (21% O2, 24 h/day, i.e., 0 days SH). After which, the nTS was harvested for patch clamp electrophysiology, quantitative real-time PCR, immunohistochemistry and immunoblots. SH induced time- and parameter-dependent increases in excitatory postsynaptic currents (EPSCs). TS-evoked EPSC amplitude increased after 1D SH which returned at 3D and 7D SH. Spontaneous EPSC frequency increased only after 3D SH, which returned to normoxic levels at 7D SH. EPSC enhancement occurred primarily by presynaptic mechanisms. Inhibition of EAAT-2 with dihydrokainate (DHK, 300 µM) did not alter EPSCs following 1D SH but induced depolarizing inward currents (Ihold). After 3D SH, DHK decreased TS-EPSC amplitude yet its resulting Ihold was eliminated. EAAT-2 mRNA and protein increased after 3D and 7D SH, respectively. These data suggest that SH alters the expression and function of EAAT-2 which may have a neuroprotective effect.

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.

Glutamate receptor plasticity in brainstem respiratory nuclei following chronic hypercapnia in goats

Patients that retain CO2 in respiratory diseases such as chronic obstructive pulmonary disease (COPD) have worse prognoses and higher mortality rates than those with equal impairment of lung function without hypercapnia. We recently characterized the time-dependent physiologic effects of chronic hypercapnia in goats, which suggested potential neuroplastic shifts in ventilatory control mechanisms. However, little is known about how chronic hypercapnia affects brainstem respiratory nuclei (BRN) that control multiple physiologic functions including breathing. Since many CNS neuroplastic mechanisms include changes in glutamate (AMPA (GluR) and NMDA (GluN)) receptor expression and/or phosphorylation state to modulate synaptic strength and network excitability, herein we tested the hypothesis that changes occur in glutamatergic signaling within BRN during chronically elevated inspired CO2 (InCO2 )-hypercapnia. Healthy goats were euthanized after either 24 h or 30 days of chronic exposure to 6% InCO2 or room air, and brainstems were rapidly extracted for western blot analyses to assess GluR and GluN receptor expression within BRN. Following 24-hr exposure to 6% InCO2 , GluR or GluN receptor expression were changed from control (P < 0.05) in the solitary complex (NTS & DMV),ventrolateral medulla (VLM), medullary raphe (MR), ventral respiratory column (VRC), hypoglossal motor nucleus (HMN), and retrotrapezoid nucleus (RTN). These neuroplastic changes were not found following 30 days of chronic hypercapnia. However, at 30 days of chronic hypercapnia, there was overall increased (P < 0.05) expression of glutamate receptors in the VRC and RTN. We conclude that time- and site-specific glutamate receptor neuroplasticity may contribute to the concomitant physiologic changes that occur during chronic hypercapnia.

Carotid Body Type-I Cells Under Chronic Sustained Hypoxia: Focus on Metabolism and Membrane Excitability

Chronic sustained hypoxia (CSH) evokes ventilatory acclimatization characterized by a progressive hyperventilation due to a potentiation of the carotid body (CB) chemosensory response to hypoxia. The transduction of the hypoxic stimulus in the CB begins with the inhibition of K+ currents in the chemosensory (type-I) cells, which in turn leads to membrane depolarization, Ca2+ entry and the subsequent release of one- or more-excitatory neurotransmitters. Several studies have shown that CSH modifies both the level of transmitters and chemoreceptor cell metabolism within the CB. Most of these studies have been focused on the role played by such putative transmitters and modulators of CB chemoreception, but less is known about the effect of CSH on metabolism and membrane excitability of type-I cells. In this mini-review, we will examine the effects of CSH on the ion channels activity and excitability of type-I cell, with a particular focus on the effects of CSH on the TASK-like background K+ channel. We propose that changes on TASK-like channel activity induced by CSH may contribute to explain the potentiation of CB chemosensory activity.

Short-Term Sustained Hypoxia Elevates Basal and Hypoxia-Induced Ventilation but Not the Carotid Body Chemoreceptor Activity in Rats

Exposure to chronic sustained hypoxia (SH), as experienced in high altitudes, elicits an increase in ventilation, named ventilatory acclimatization to hypoxia (VAH). We previously showed that rats exposed to short-term (24 h) SH exhibit enhanced abdominal expiratory motor activity at rest, accompanied by augmented baseline sympathetic vasoconstrictor activity. In the present study, we investigated whether the respiratory and sympathetic changes elicited by short-term SH are accompanied by carotid body chemoreceptor sensitization. Juvenile male Holtzman rats (60-80 g) were exposed to SH (10% O₂ for 24 h) or normoxia (control) to examine basal and hypoxic-induced ventilatory parameters in unanesthetized conditions, as well as the sensory response of carotid body chemoreceptors in artificially perfused in situ preparations. Under resting conditions (normoxia/normocapnia), SH rats (n = 12) exhibited higher baseline respiratory frequency, tidal volume, and minute ventilation compared to controls (n = 11, P < 0.05). SH group also showed greater hypoxia ventilatory response than control group (P < 0.05). The in situ preparations of SH rats (n = 8) exhibited augmented baseline expiratory and sympathetic activities under normocapnia, with additional bursts in abdominal and thoracic sympathetic nerves during late expiratory phase that were not seen in controls (n = 8, P < 0.05). Interestingly, basal and potassium cyanide-induced afferent activity of carotid sinus nerve (CSN) was similar between SH and control rats. Our findings indicate that the maintenance of elevated resting ventilation, baseline sympathetic overactivity, and enhanced ventilatory responses to hypoxia in rats exposed to 24 h of SH are not dependent on increased basal and sensorial activity of carotid body chemoreceptors.

Naked mole rats exhibit metabolic but not ventilatory plasticity following chronic sustained hypoxia

Naked mole rats are among the most hypoxia-tolerant mammals identified and live in chronic hypoxia throughout their lives. The physiological mechanisms underlying this tolerance, however, are poorly understood. Most vertebrates hyperventilate in acute hypoxia and exhibit an enhanced hyperventilation following acclimatization to chronic sustained hypoxia (CSH). Conversely, naked mole rats do not hyperventilate in acute hypoxia and their response to CSH has not been examined. In this study, we explored mechanisms of plasticity in the control of the hypoxic ventilatory response (HVR) and hypoxic metabolic response (HMR) of freely behaving naked mole rats following 8-10 days of chronic sustained normoxia (CSN) or CSH. Specifically, we investigated the role of the major inhibitory neurotransmitter γ-amino butyric acid (GABA) in mediating these responses. Our study yielded three important findings. First, naked mole rats did not exhibit ventilatory plasticity following CSH, which is unique among adult animals studied to date. Second, GABA receptor (GABAR) antagonism altered breathing patterns in CSN and CSH animals and modulated the acute HVR in CSN animals. Third, naked mole rats exhibited GABAR-dependent metabolic plasticity following long-term hypoxia, such that the basal metabolic rate was approximately 25% higher in normoxic CSH animals than CSN animals, and GABAR antagonists modulated this increase.

Ventilatory acclimatization to hypoxia in mice: Methodological considerations

We examined ventilatory acclimatization to hypoxia (VAH) in CD1 mice, and contrasted results obtained using the barometric method on unrestrained mice with pneumotachography and pulse oximetry on restrained mice. Responses to progressive step reductions in O₂ fraction (21%-8%) were assessed in mice acclimated to normoxia and hypobaric hypoxia (barometric pressure of 60kPa for 6-8 weeks). Hypoxia acclimation increased the hypoxic ventilatory response (primarily by increasing breathing frequency rather than tidal volume), arterial O₂ saturation (Sa_O2) and heart rate in deep hypoxia, hypoxic chemosensitivity (ventilatory O₂/CO₂ equivalents versus Sa_O2), and respiratory water loss, and it blunted the hypoxic depression of metabolism and body temperature. Although some effects of hypoxia acclimation were qualitatively similar between methods, the effects were often greater in magnitude when assessed using pneumotachography. Furthermore, whereas hypoxia acclimation reduced ventilatory O₂ equivalent and increased pulmonary O₂ extraction in barometric experiments, it had the opposite effects in pneumotachography experiments. Our findings highlight the importance of considering the impact of how breathing is measured on the apparent responses to hypoxia.

Time Domains of the Hypoxic Ventilatory Response and Their Molecular Basis

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

Erythropoietin and the use of a transgenic model of erythropoietin-deficient mice

Despite its well-known role in red blood cell production, it is now accepted that erythropoietin (Epo) has other physiological functions. Epo and its receptors are expressed in many tissues, such as the brain and heart. The presence of Epo/Epo receptors in these organs suggests other roles than those usually assigned to this protein. Thus, the aim of this review is to describe the effects of Epo deficiency on adaptation to normoxic and hypoxic environments and to suggest a key role of Epo on main physiological adaptive functions. Our original model of Epo-deficient (Epo-TAg^h) mice allowed us to improve our knowledge of the possible role of Epo in O₂ homeostasis. The use of anemic transgenic mice revealed Epo as a crucial component of adaptation to hypoxia. Epo-TAg^h mice survive well in hypoxic conditions despite low hematocrit. Furthermore, Epo plays a key role in neural control of ventilatory acclimatization and response to hypoxia, in deformability of red blood cells, in cerebral and cardiac angiogenesis, and in neuro- and cardioprotection.

Chronic intermittent hyperoxia alters the development of the hypoxic ventilatory response in neonatal rats

Chronic exposure to sustained hyperoxia alters the development of the respiratory control system, but the respiratory effects of chronic intermittent hyperoxia have rarely been investigated. We exposed newborn rats to short, repeated bouts of 30% O2 or 60% O2 (5 bouts h(-1)) for 4-15 days and then assessed their hypoxic ventilatory response (HVR; 10 min at 12% O2) by plethysmography. The HVR tended to be enhanced by intermittent hyperoxia at P4 (early phase of the HVR), but it was significantly reduced at P14-15 (primarily late phase of the HVR) compared to age-matched controls; the HVR recovered when individuals were returned to room air and re-studied as adults. To investigate the role of carotid body function in this plasticity, single-unit carotid chemoafferent activity was recorded in vitro. Intermittent hyperoxia tended to decrease spontaneous action potential frequency under normoxic conditions but, contrary to expectations, hypoxic responses were only reduced at P4 (not at P14) and only in rats exposed to higher O2 levels (i.e., intermittent 60% O2). Rats exposed to intermittent hyperoxia had smaller carotid bodies, and this morphological change may contribute to the blunted HVR. In contrast to rats exposed to intermittent hyperoxia beginning at birth, two weeks of intermittent 60% O2 had no effect on the HVR or carotid body size of rats exposed beginning at P28; therefore, intermittent hyperoxia-induced respiratory plasticity appears to be unique to development. Although both intermittent and sustained hyperoxia alter carotid body development and the HVR of rats, the specific effects and time course of this plasticity differs.

Role of glutamate and serotonin on the hypoxic ventilatory response in high-altitude-adapted plateau Pika

The highland "plateau Pika" is considered to be adapted to chronic hypoxia. We hypothesized that glutamate N-methyl-D-aspartate (NMDA) and non-NMDA receptors, nitric oxide (NO) synthase, and serotonin are involved in hypoxic ventilatory response (HVR) in Pikas. We tested the effects of NMDA (memantine) and non-NMDA receptors (DNQX) antagonists, NO synthase inhibitor (L-NAME), and selective serotonin reuptake inhibitors (fluoxetine) on ventilation and HVR in Pikas. Ventilatory parameters were measured before and after drug (or vehicle) injections in conscious Pikas at their natural living altitude (PIO2 86 mmHg) and after a hypoxic challenge (PIO2 57 mmHg, 3 min) to assess the influence of peripheral chemoreceptor on HVR. Minute ventilation (VI) and tidal volume (Vt) increased during hypoxic challenge after vehicle injection, whereas the Ti/Ttot ratio remained unchanged. The increase in VI and Vt observed with vehicle at PIO2-57, when compared with PIO2-86, was inhibited after memantine and fluoxetine injection, whereas the DNQX injection increased HVR. At PIO2-57, L-NAME induced an increase in the Ti/Ttot ratio when compared with vehicle. Therefore, the glutamate through NMDA-R/AMPA receptor bindings and serotonin pathway are implicated at the peripheral chemoreceptor level in HVR in Pikas. However, NO influences the ventilatory pattern of Pikas at their habitual living altitude.

Control of breathing and the circulation in high-altitude mammals and birds

Hypoxia is an unremitting stressor at high altitudes that places a premium on oxygen transport by the respiratory and cardiovascular systems. Phenotypic plasticity and genotypic adaptation at various steps in the O2 cascade could help offset the effects of hypoxia on cellular O2 supply in high-altitude natives. In this review, we will discuss the unique mechanisms by which ventilation, cardiac output, and blood flow are controlled in high-altitude mammals and birds. Acclimatization to high altitudes leads to some changes in respiratory and cardiovascular control that increase O2 transport in hypoxia (e.g., ventilatory acclimatization to hypoxia). However, acclimatization or development in hypoxia can also modify cardiorespiratory control in ways that are maladaptive for O2 transport. Hypoxia responses that arose as short-term solutions to O2 deprivation (e.g., peripheral vasoconstriction) or regional variation in O2 levels in the lungs (i.e., hypoxic pulmonary vasoconstriction) are detrimental at in chronic high-altitude hypoxia. Evolved changes in cardiorespiratory control have arisen in many high-altitude taxa, including increases in effective ventilation, attenuation of hypoxic pulmonary vasoconstriction, and changes in catecholamine sensitivity of the heart and systemic vasculature. Parallel evolution of some of these changes in independent highland lineages supports their adaptive significance. Much less is known about the genomic bases and potential interactive effects of adaptation, acclimatization, developmental plasticity, and trans-generational epigenetic transfer on cardiorespiratory control. Future work to understand these various influences on breathing and circulation in high-altitude natives will help elucidate how complex physiological systems can be pushed to their limits to maintain cellular function in hypoxia.

Two-week normobaric intermittent hypoxia exposures enhance oxyhemoglobin equilibrium and cardiac responses during hypoxemia

Intermittent hypoxia (IH) is extensively applied to challenge cardiovascular and respiratory function, and to induce physiological acclimatization. The purpose of this study was to test the hypothesis that oxyhemoglobin equilibrium and tachycardiac responses during hypoxemia were enhanced after 14-day IH exposures. Normobaric-poikilocapnic hypoxia was induced with inhalation of 10% O2 for 5-6 min interspersed with 4 min recovery on eight nonsmokers. Heart rate (HR), arterial O2 saturation (SaO 2), and end-tidal O2 (PetO 2) were continuously monitored during cyclic normoxia and hypoxia. These variables were compared during the first and fifth hypoxic bouts between day 1 and day 14. There was a rightward shift in the oxyhemoglobin equilibrium response following 14-day IH exposures, as indicated by the greater PetO 2 (an index of arterial Po2) at 50% of SaO 2 on day 14 compared with day 1 [33.9 ± 1.5 vs. 28.2 ± 1.3 mmHg (P = 0.005) during the first hypoxic bout and 39.4 ± 2.4 vs. 31.4 ± 1.5 mmHg (P = 0.006) during the fifth hypoxic bout] and by the augmented gains of ΔSaO 2/ΔPetO 2 (i.e., deoxygenation) during PetO 2 from 65 to 40 mmHg in the first (1.12 ± 0.08 vs. 0.80 ± 0.02%/mmHg, P = 0.001) and the fifth (1.76 ± 0.31 vs. 1.05 ± 0.06%/mmHg, P = 0.024) hypoxic bouts. Repetitive IH exposures attenuated (P = 0.049) the tachycardiac response to hypoxia while significantly enhancing normoxic R-R interval variability in low-frequency and high-frequency spectra without changes in arterial blood pressure at rest or during hypoxia. We conclude that 14-day IH exposures enhance arterial O2 delivery and improve vagal control of HR during hypoxic hypoxemia.

Short-term sustained hypoxia induces changes in the coupling of sympathetic and respiratory activities in rats

Individuals experiencing sustained hypoxia (SH) exhibit adjustments in the respiratory and autonomic functions by neural mechanisms not yet elucidated. In the present study we evaluated the central mechanisms underpinning the SH-induced changes in the respiratory pattern and their impact on the sympathetic outflow. Using a decerebrated arterially perfused in situ preparation, we verified that juvenile rats exposed to SH (10% O2) for 24 h presented an active expiratory pattern, with increased abdominal, hypoglossal and vagal activities during late-expiration (late-E). SH also enhanced the activity of augmenting-expiratory neurones and depressed the activity of post-inspiratory neurones of the Bötzinger complex (BötC) by mechanisms not related to changes in their intrinsic electrophysiological properties. SH rats exhibited high thoracic sympathetic activity and arterial pressure levels associated with an augmented firing frequency of pre-sympathetic neurones of the rostral ventrolateral medulla (RVLM) during the late-E phase. The antagonism of ionotropic glutamatergic receptors in the BötC/RVLM abolished the late-E bursts in expiratory and sympathetic outputs of SH rats, indicating that glutamatergic inputs to the BötC/RVLM are essential for the changes in the expiratory and sympathetic coupling observed in SH rats. We also observed that the usually silent late-E neurones of the retrotrapezoid nucleus/parafacial respiratory group became active in SH rats, suggesting that this neuronal population may provide the excitatory drive essential to the emergence of active expiration and sympathetic overactivity. We conclude that short-term SH induces the activation of medullary expiratory neurones, which affects the pattern of expiratory motor activity and its coupling with sympathetic activity.

Integrative regulation of human brain blood flow

Herein, we review mechanisms regulating cerebral blood flow (CBF), with specific focus on humans. We revisit important concepts from the older literature and describe the interaction of various mechanisms of cerebrovascular control. We amalgamate this broad scope of information into a brief review, rather than detailing any one mechanism or area of research. The relationship between regulatory mechanisms is emphasized, but the following three broad categories of control are explicated: (1) the effect of blood gases and neuronal metabolism on CBF; (2) buffering of CBF with changes in blood pressure, termed cerebral autoregulation; and (3) the role of the autonomic nervous system in CBF regulation. With respect to these control mechanisms, we provide evidence against several canonized paradigms of CBF control. Specifically, we corroborate the following four key theses: (1) that cerebral autoregulation does not maintain constant perfusion through a mean arterial pressure range of 60-150 mmHg; (2) that there is important stimulatory synergism and regulatory interdependence of arterial blood gases and blood pressure on CBF regulation; (3) that cerebral autoregulation and cerebrovascular sensitivity to changes in arterial blood gases are not modulated solely at the pial arterioles; and (4) that neurogenic control of the cerebral vasculature is an important player in autoregulatory function and, crucially, acts to buffer surges in perfusion pressure. Finally, we summarize the state of our knowledge with respect to these areas, outline important gaps in the literature and suggest avenues for future research.

Catalyzing role of erythropoietin on the nitric oxide central pathway during the ventilatory responses to hypoxia

The N‐Methyl‐d‐Aspartate (NMDA) receptors – neuronal nitric oxide synthase (nNOS) pathway is involved in the ventilatory response to hypoxia. The objective was to assess the possible effect of erythropoietin deficiency and chronic exposure to hypoxia on this pathway during ventilatory response to acute hypoxia. Wild‐type (WT) and erythropoietin‐deficient (Epo‐TAg^h) male mice were exposed (14 days) either to hypobaric hypoxia (Pb = 435 mmHg) or to normoxia. The ventilation was measured at 21% or 8% O₂ after injection of vehicle (NaCl), nNOS inhibitor (SMTC) or NMDA receptor antagonist (MK‐801). Nitric oxide production and the expression of NMDA receptor and nNOS were assessed by real‐time RT‐PCR and Western blot analyses in the medulla. At rest, Epo‐TAg^h mice displayed normal ventilatory parameters at 21% O₂ but did not respond to acute hypoxia despite a larger expression of NMDA receptors and nNOS in the medulla. Ventilatory acclimatization to hypoxia was observed in WT but was absent in Epo‐TAg^h mice. nNOS inhibition blunted the hypoxic ventilatory acclimatization of WT mice without any effect in Epo‐TAg^h mice. Acute hypoxic ventilatory response (HVR) was increased after chronic hypoxia in WT but remained unchanged in Epo‐TAg^h mice. Ventilatory response to acute hypoxia was modified by MK‐801 injection in WT and Epo‐TAg^h mice. The results confirm that adequate erythropoietin level is necessary to obtain an appropriate HVR and a significant ventilatory acclimatization to hypoxia. Furthermore, erythropoietin plays a potential catalyzing role in the NMDA‐NO central pathway during the ventilatory response and acclimatization to hypoxia.

e00223

Adequate erythropoietin level is necessary to obtain an appropriate hypoxic ventilatory response and a significant ventilatory acclimatization to hypoxia. Erythropoietin plays a potential catalyzing role on the N‐Methyl‐d‐Aspartate (NMDA)‐nNOS central pathway during the ventilatory response and acclimatization to hypoxia.

Neuronal control of breathing: sex and stress hormones

There is a growing public awareness that hormones can have a significant impact on most biological systems, including the control of breathing. This review will focus on the actions of two broad classes of hormones on the neuronal control of breathing: sex hormones and stress hormones. The majority of these hormones are steroids; a striking feature is that both groups are derived from cholesterol. Stress hormones also include many peptides which are produced primarily within the paraventricular nucleus of the hypothalamus (PVN) and secreted into the brain or into the circulatory system. In this article we will first review and discuss the role of sex hormones in respiratory control throughout life, emphasizing how natural fluctuations in hormones are reflected in ventilatory metrics and how disruption of their endogenous cycle can predispose to respiratory disease. These effects may be mediated directly by sex hormone receptors or indirectly by neurotransmitter systems. Next, we will discuss the origins of hypothalamic stress hormones and their relationship with the respiratory control system. This relationship is 2-fold: (i) via direct anatomical connections to brainstem respiratory control centers, and (ii) via steroid hormones released from the adrenal gland in response to signals from the pituitary gland. Finally, the impact of stress on the development of neural circuits involved in breathing is evaluated in animal models, and the consequences of early stress on respiratory health and disease is discussed.

Hypoxia-induced phrenic long-term facilitation: emergent properties

As in other neural systems, plasticity is a hallmark of the neural system controlling breathing. One spinal mechanism of respiratory plasticity is phrenic long-term facilitation (pLTF) following acute intermittent hypoxia. Although cellular mechanisms giving rise to pLTF occur within the phrenic motor nucleus, different signaling cascades elicit pLTF under different conditions. These cascades, referred to as Q and S pathways to phrenic motor facilitation (pMF), interact via cross-talk inhibition. Whereas the Q pathway dominates pLTF after mild to moderate hypoxic episodes, the S pathway dominates after severe hypoxic episodes. The biological significance of multiple pathways to pMF is unknown. This review will discuss the possibility that interactions between pathways confer emergent properties to pLTF, including pattern sensitivity and metaplasticity. Understanding these mechanisms and their interactions may enable us to optimize intermittent hypoxia-induced plasticity as a treatment for patients that suffer from ventilatory impairment or other motor deficits.

Inactivity-induced phrenic and hypoglossal motor facilitation are differentially expressed following intermittent vs. sustained neural apnea

Reduced respiratory neural activity elicits a rebound increase in phrenic and hypoglossal motor output known as inactivity-induced phrenic and hypoglossal motor facilitation (iPMF and iHMF, respectively). We hypothesized that, similar to other forms of respiratory plasticity, iPMF and iHMF are pattern sensitive. Central respiratory neural activity was reversibly reduced in ventilated rats by hyperventilating below the CO2 apneic threshold to create brief intermittent neural apneas (5, ∼1.5 min each, separated by 5 min), a single brief massed neural apnea (7.5 min), or a single prolonged neural apnea (30 min). Upon restoration of respiratory neural activity, long-lasting (>60 min) iPMF was apparent following brief intermittent and prolonged, but not brief massed, neural apnea. Further, brief intermittent and prolonged neural apnea elicited an increase in the maximum phrenic response to high CO2, suggesting that iPMF is associated with an increase in phrenic dynamic range. By contrast, only prolonged neural apnea elicited iHMF, which was transient in duration (<15 min). Intermittent, massed, and prolonged neural apnea all elicited a modest transient facilitation of respiratory frequency. These results indicate that iPMF, but not iHMF, is pattern sensitive, and that the response to respiratory neural inactivity is motor pool specific.

Chronic hyperoxia and the development of the carotid body

Preterm infants often experience hyperoxia while receiving supplemental oxygen. Prolonged exposure to hyperoxia during development is associated with pathologies such as bronchopulmonary dysplasia and retinopathy of prematurity. Over the last 25 years, however, experiments with animal models have revealed that moderate exposures to hyperoxia (e.g., 30-60% O(2) for days to weeks) can also have profound effects on the developing respiratory control system that may lead to hypoventilation and diminished responses to acute hypoxia. This plasticity, which is generally inducible only during critical periods of development, has a complex time course that includes both transient and permanent respiratory deficits. Although the molecular mechanisms of hyperoxia-induced plasticity are only beginning to be elucidated, it is clear that many of the respiratory effects are linked to abnormal morphological and functional development of the carotid body, the principal site of arterial O(2) chemoreception for respiratory control. Specifically, developmental hyperoxia reduces carotid body size, decreases the number of chemoafferent neurons, and (at least transiently) diminishes the O(2) sensitivity of individual carotid body glomus cells. Recent evidence suggests that hyperoxia may also directly or indirectly impact development of the central neural control of breathing. Collectively, these findings emphasize the vulnerability of the developing respiratory control system to environmental perturbations.

Chronic hypoxia increases the gain of the hypoxic ventilatory response by a mechanism in the central nervous system

We studied the effects of the ventilatory stimulant doxapram to test the hypothesis that chronic hypoxia increases the translation of carotid body afferent input into ventilatory motor efferent output by the central nervous system. Chronic hypoxia (inspired Po(2) = 70 Torr, 2 days) significantly increased the ventilatory response to an intravenous infusion of a high dose of doxapram in conscious, unrestrained rats breathing normoxic or hypoxic gas. The in vitro carotid body response to hypoxia increased with chronic hypoxia, but the response was not increased with a high dose of doxapram. Similarly, the phrenic nerve response to doxapram in anesthetized rats with carotid bodies denervated did not change with 7 days of chronic hypoxia. The results support the hypothesis that chronic hypoxia causes plasticity in the central component of the carotid chemoreceptor ventilatory reflex, which increases the hypoxic ventilatory response. We conclude that doxapram provides a promising tool to study the time course of changes in the central gain of the hypoxic ventilatory response during chronic hypoxia in awake animals and humans.

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.

Chronic hypoxia suppresses the CO2 response of solitary complex (SC) neurons from rats

We studied the effect of chronic hypobaric hypoxia (CHx; 10-11% O(2)) on the response to hypercapnia (15% CO(2)) of individual solitary complex (SC) neurons from adult rats. We simultaneously measured the intracellular pH and firing rate responses to hypercapnia of SC neurons in superfused medullary slices from control and CHx-adapted adult rats using the blind whole cell patch clamp technique and fluorescence imaging microscopy. We found that CHx caused the percentage of SC neurons inhibited by hypercapnia to significantly increase from about 10% up to about 30%, but did not significantly alter the percentage of SC neurons activated by hypercapnia (50% in control vs. 35% in CHx). Further, the magnitudes of the responses of SC neurons from control rats (chemosensitivity index for activated neurons of 166+/-11% and for inhibited neurons of 45+/-15%) were the same in SC neurons from CHx-adapted rats. This plasticity induced in chemosensitive SC neurons by CHx appears to involve intrinsic changes in neuronal properties since they were the same in synaptic blockade medium.

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.

Chronic sustained hypoxia enhances both evoked EPSCs and norepinephrine inhibition of glutamatergic afferent inputs in the nucleus of the solitary tract

The nucleus of the solitary tract (NTS) receives inputs from both arterial chemoreceptors and central noradrenergic neural structures activated during hypoxia. We investigated norepinephrine (NE) modulation of chemoreceptor afferent integration after a chronic exposure to sustained hypoxia (CSH) (7-8 d at 10% FIO(2)). Whole-cell recordings of NTS second-order neurons identified by DiA (1,1'-dilinoleyl-3,3,3',3'-tetra-methylindocarbocyanine, 4-chlorobenzenesulphonate) labeling of carotid bodies were obtained in a brain slice. Electrical stimulation of the solitary tract was used to evoke EPSCs. CSH exposure increased evoked EPSC (eEPSC) amplitude via both presynaptic and postsynaptic mechanisms. NE dose dependently decreased the amplitude of eEPSCs. NE increased the paired-pulse ratio of eEPSCs and reduced the frequency of miniature EPSCs, suggesting a presynaptic mechanism. EC(50) of NE inhibition of eEPSCs was lower in CSH cells (3.0 +/- 0.9 microM; n = 5) than in normoxic (NORM) cells (7.6 +/- 1.0 microM; n = 7; p < 0.01). NE (10 microM) elicited greater inhibition of eEPSCs in CSH cells (63 +/- 2%; n = 16) than NORM cells (45 +/- 3%; n = 21; p < 0.01). The alpha-adrenoreceptor antagonist phentolamine abolished NE inhibition of eEPSCs. CSH enhanced the alpha2-adrenoreceptor agonist clonidine-mediated inhibition (3 microM; NORM, 23 +/- 2%, n = 5 vs CSH, 44 +/- 5%, n = 4; p < 0.05) but attenuated alpha1-adrenoreceptor agonist phenylephrine-mediated inhibition (40 microM; NORM, 36 +/- 2%, n = 11 vs CSH, 26 +/- 4%, n = 6; p < 0.05). The alpha2-adrenoreceptor antagonist yohimbine abolished CSH-induced enhancement of NE inhibition of eEPSCs. These results demonstrate that CSH increases evoked excitatory inputs to NTS neurons receiving arterial chemoreceptor inputs. CSH also enhances NE inhibition of glutamate release from inputs to these neurons via presynaptic alpha2-adrenoreceptors. These changes represent central neural adaptations to CSH.

Oxygen sensing in the brain--invited article

Carotid body arterial chemoreceptors are essential for a normal hypoxic ventilatory response (HVR) and ventilatory acclimatization to hypoxia (VAH). However, recent results show that O(2)-sensing in the brain is involved in these responses also. O(2)-sensing in the rostral ventrolateral medulla, the posterior hypothalamus, the pre-Bötzinger complex and the nucleus tractus solitarius contribute to the acute HVR. Chronic hypoxia causes plasticity in the brain that contributes to VAH and represents another time domain of central O(2)-sensing. The cellular and molecular mechanisms of acute O(2)-sensing in the brain remain to be determined but they appear to involve O(2)-sensitive ion channels and heme oxygenase-2, which acts by a different mechanism than has been described for the carotid body. It is not known if plasticity in such mechanisms of acute central O(2)-sensitivity contributes to VAH. However, O(2)-sensitive changes in gene expression in the brain do contribute to VAH and demonstrate another mechanism of O(2)-sensing that is important for ventilatory control. This time domain of O(2)-sensing in the brain involves gene expression under the control of hypoxia inducible factor-1+/- (HIF-1+/- and potentially several HIF-1+/- targets, such as erythropoietin, endothelin-1, heme oxygenase and tyrosine hydroxylase.

Brain stem NO modulates ventilatory acclimatization to hypoxia in mice

The objective of our study was to assess the role of neuronal nitric oxide synthase (nNOS) in the ventilatory acclimatization to hypoxia. We measured the ventilation in acclimatized Bl6/CBA mice breathing 21% and 8% oxygen, used a nNOS inhibitor, and assessed the expression of N-methyl-d-aspartate (NMDA) glutamate receptor and nNOS (mRNA and protein). Two groups of Bl6/CBA mice (n = 60) were exposed during 2 wk either to hypoxia [barometric pressure (PB) = 420 mmHg] or normoxia (PB = 760 mmHg). At the end of exposure the medulla was removed to measure the concentration of nitric oxide (NO) metabolites, the expression of NMDA-NR1 receptor, and nNOS by real-time RT-PCR and Western blot. We also measured the ventilatory response [fraction of inspired O(2) (Fi(O(2))) = 0.21 and 0.08] before and after S-methyl-l-thiocitrulline treatment (SMTC, nNOS inhibitor, 10 mg/kg ip). Chronic hypoxia caused an increase in ventilation that was reduced after SMTC treatment mainly through a decrease in tidal volume (Vt) in normoxia and in acute hypoxia. However, the difference observed in the magnitude of acute hypoxic ventilatory response [minute ventilation (Ve) 8% - Ve 21%] in acclimatized mice was not different. Acclimatization to hypoxia induced a rise in NMDA receptor as well as in nNOS and NO production. In conclusion, our study provides evidence that activation of nNOS is involved in the ventilatory acclimatization to hypoxia in mice but not in the hypoxic ventilatory response (HVR) while the increased expression of NMDA receptor expression in the medulla of chronically hypoxic mice plays a role in acute HVR. These results are therefore consistent with central nervous system plasticity, partially involved in ventilatory acclimatization to hypoxia through nNOS.

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.

Chronic hypoxia attenuates central respiratory-related pH/CO2 chemosensitivity in the cane toad

This study examined the effects of chronic hypoxia (CH) and mid-brain transection on central respiratory-related pH/CO(2) chemosensitivity in cane toads (Bufo marinus). Toads were exposed to 10 days of CH (10% O(2)) following which in vitro brainstem-spinal cord preparations, with the mid-brain attached, were used to examine central pH/CO(2) chemosensitivity. A reduction in artificial cerebral spinal fluid (aCSF) pH increased fictive breathing frequency (fR) and total fictive ventilation. CH reduced fictive fR and total fictive ventilation, compared to controls. Mid-brain transection caused an increase in fictive fR, at the lower aCSF pH levels, in both control and CH preparations. In the CH preparations, mid-brain transection restored fictive breathing to control levels. In both groups, mid-brain transection eliminated fictive breath clustering. The data indicate that CH attenuates central pH/CO(2)-sensitive fictive breathing but a mid-brain transection in the middle of the optic lobes abolishes this attenuation. The results suggest that CH induces inhibition of central pH/CO(2) chemoreceptor function via descending inputs from the mid-brain region.

Adaptive depression in synaptic transmission in the nucleus of the solitary tract after in vivo chronic intermittent hypoxia: evidence for homeostatic plasticity

The respiratory system is highly pliable in its adaptation to low-oxygen (hypoxic) environments. After chronic intermittent hypoxia (CIH), alterations in the regulation of cardiorespiratory system become persistent because of changes in the peripheral chemoreceptor reflex. We present evidence for the induction of a novel form of homeostatic plasticity in this reflex pathway in the nucleus tractus solitarius (NTS), the site of termination of the chemosensory afferent fibers. CIH induces an increase in NTS postsynaptic cell activity initiated by spontaneous presynaptic transmitter release that is counterbalanced by a reduction in evoked synaptic transmission between sensory afferents and NTS second-order cells. This is accomplished via presynaptic mechanisms involving changes in neurotransmitter release and calcium/calmodulin-dependent kinase II activation.

The influence of chronic hypoxia upon chemoreception

Carotid body chemoreceptors are essential for time-dependent changes in ventilatory control during chronic hypoxia. Early theories of ventilatory acclimatization to hypoxia focused on time-dependent changes in known ventilatory stimuli, such as small changes in arterial pH that may play a significant role in some species. However, plasticity in the cellular and molecular mechanisms of carotid body chemoreception play a major role in ventilatory acclimatization to hypoxia in all species studied. Chronic hypoxia causes changes in (a) ion channels (potassium, sodium, calcium) to increase glomus cell excitability, and (b) neurotransmitters (dopamine, acetylcholine, ATP) and neuromodulators (endothelin-1) to increase carotid body afferent activity for a given PO(2) and optimize O(2)-sensitivity. O(2)-sensing heme-containing molecules in the carotid body have not been studied in chronic hypoxia. Plasticity in medullary respiratory centers processing carotid body afferent input also contributes to ventilatory acclimatization to hypoxia. It is not known if the same mechanisms occur in patients with chronic hypoxemia from lung disease or high altitude natives.

5-HT evokes sensory long-term facilitation of rodent carotid body via activation of NADPH oxidase

5-Hydroxytryptamine (5-HT) evokes long-term activation of neuronal activity in the nervous system. Carotid bodies, the sensory organs for detecting arterial oxygen, express 5-HT. In the present study we examined whether 5-HT evokes sensory long-term facilitation (LTF) of the carotid body, and if so by what mechanism(s). Experiments were performed on anaesthetized adult rats and mice. Sensory activity was recorded from carotid bodies ex vivo. Spaced (3 x 15 s of 100 nm at 5 min intervals) but not mass (300 nm, 45 s) application of 5-HT elicited LTF, whereas both modes of 5-HT application evoked initial sensory excitation of the carotid bodies in rats. Ketanserin, a 5-HT(2) receptor antagonist prevented sensory LTF but not the initial sensory excitation. Spaced application of 5-HT activated protein kinase C (PKC) as evidenced by increased phosphorylations of PKC at Thr(514) and myristoylated alanine-rich C kinase substrate (MARCKS) and these effects were abolished by ketanserin as well as bisindolylmaleimide (Bis-1), an inhibitor of PKC. Bis-1 prevented 5-HT-evoked sensory LTF. 5-HT increased NADPH oxidase activity and PKC-dependent phosphorylation of p47(phox) subunit of the oxidase complex. NADPH oxidase inhibitors (apocynin and diphenyl iodinium), as well as an anti-oxidant (N-acetyl cysteine), prevented 5-HT-evoked sensory LTF. Mice deficient in gp91(phox), the membrane subunit of the NADPH oxidase complex, showed no sensory LTF, although responding to 5-HT with initial afferent nerve activation, whereas both LTF and initial excitation by 5-HT were seen in wild-type mice. These results demonstrate that spaced but not mass application of 5-HT elicits sensory LTF of the carotid body via activation of 5-HT(2) receptors, which involves a novel signalling mechanism coupled to PKC-dependent activation of NADPH oxidase.

Changes in ventilatory adaptations associated with long-term intermittent hypoxia across the age spectrum in the rat

Intermittent hypoxia (IH) induces alterations in respiratory control that reflect various types of ventilatory plasticity. In freely behaving rats, acute exposure to IH elicits enhancements in normoxic minute ventilation (VE), termed ventilatory long-term facilitation. Exposure to longer time periods of IH induces unique ventilatory adaptations to intermittent hypoxia (VAIH). We hypothesized that long-term IH-induced ventilatory plasticity may be developmentally regulated and thus, IH exposures at progressively later post-natal ages may elicit differential effects on the magnitude of VAIH. To examine this issue, male Sprague-Dawley rats were exposed to 30 continuous days of IH beginning at post-natal ages 1, 10, 30, 60, 180, 360, and 540 days. Control animals were exposed to normoxic conditions with room air. Normoxic VE was significantly higher in IH-exposed rats (p < 0.01) except for the group in which IH was initiated at post-natal age 540 days (p = NS). The magnitude of VAIH was greatest in rats exposed in the immediate post-natal period and gradually diminished with advancing post-natal age. Enhanced normoxic VE was due to significant contributions from both frequency (p < 0.01) and tidal volume (p < 0.01), and could not be accounted for by changes in metabolic rate. We conclude that the magnitude of IH-induced ventilatory plasticity is age-dependent with progressive declines becoming apparent with advancing post-natal age.

Does central nitric oxide chronically modulate the acute hypoxic ventilatory response in conscious rats?

AIM

Hypoxia initiates an increase in ventilation (VE) through a cascade of events of which central nitric oxide (NO) has been implicated as an important neuromodulator. There have not been any reports describing the consequences of long-term imbalances in the central NO pathways on the modulation of the acute hypoxic ventilatory response (HVR). Chronic hypoxia (CH) can potentially modify the HVR, and so we hypothesized that central NO may be involved. In this study we describe the long-term role of central NO in the modulation of HVR before and after CH.

METHODS

Male Sprague-Dawley rats (BW c. 200-320 g; n = 21) were implanted with an osmotic pump for continuous intracerebroventricular administration of either artificial cerebrospinal fluid (control), Nomega-nitro-L-arginine methyl ester (L-NAME) (150 microg kg(-1) day(-1)) or the NO-donor, 3-[4-morpholinyl]-sydnonimine-hydrochloride (SIN-1) (100 microg kg(-1) day(-1)). The VE response to acute poikilocapnic hypoxia (8% O2 for 20 min) was measured by plethysmography seven days after surgery, in normoxia, and again after 14 days of exposure to CH (CH = 12% O2).

RESULTS

The magnitude of the HVR (c. 230% increase in VE) was unaltered by centrally infusing either L-NAME or SIN-1 for 1 week. CH did not modify the HVR, although baseline VE and HVR were shifted downward by L-NAME during CH - because of a reduction in the frequency component.

CONCLUSIONS

These results suggest that long-term alterations in central NO levels may not alter the HVR under moderate CH, presumably because of the onset/development of compensatory mechanisms. However, NO appears to be an important component of the HVR following CH.

Developmental plasticity of the hypoxic ventilatory response after perinatal hyperoxia and hypoxia

Both genetic and environmental factors influence the normal development of the respiratory control system. This review examines the role perinatal O2 plays in the development of normoxic breathing and the hypoxic ventilatory response in mammals. Hyperoxia and hypoxia elicit plasticity in respiratory control that is unique to development and may persist weeks to years after return to normoxia. Specifically, both hyperoxia and hypoxia during early postnatal development attenuate the adult hypoxic ventilatory response, but the underlying mechanisms for this plasticity differ. Hyperoxia attenuates the hypoxic ventilatory response through potentially life-long changes in carotid body function. Neonatal hypoxia appears to have short-term effects on carotid body function, but persistent changes in the hypoxic ventilatory response may instead reflect changes in respiratory mechanics or related neural pathways. Overall, it appears that a relatively narrow range of environmental O2 is consistent with "normal" postnatal respiratory control development, predisposing animals to potentially maladaptive plasticity in the face of disease or atypical environmental conditions.

Effects of chronic hypoxia on MK-801-induced changes in the acute hypoxic ventilatory response

Chronic hypoxia increases the sensitivity of the central nervous system to afferent input from carotid body chemoreceptors. We hypothesized that this process involves N-methyl-D-aspartate (NMDA) receptor-mediated mechanisms and predicted that chronic hypoxia would change the effect of the NMDA receptor blocker dizocilpine (MK-801) on the poikilocapnic hypoxic ventilatory response (HVR). Male Sprague-Dawley rats were studied before and after acclimatization to hypoxia (70 Torr inspiratory Po(2) for 9 days). We measured ventilation (VI) and the HVR before and after systemic MK-801 treatment (3 mg/kg ip). MK-801 resulted in a constant respiratory frequency (approximately 175 min(-1)) during acute exposure to 10% and 30% O(2) before and after acclimatization. MK-801 had no effect on tidal volume (VT) before acclimatization, but it significantly decreased Vt when the animals were breathing 10% O(2) after acclimatization. The net effect of MK-801 was to eliminate the O(2) sensitivity of Vi before (via changes in respiratory frequency) and after (via changes in VT) acclimatization. Hence, chronic hypoxia altered the effect of MK-801 on the acute HVR, primarily because of increased effects on Vt. This indicates that changes in NMDA receptor-mediated neurotransmission may be involved in ventilatory acclimatization to hypoxia. However, further experiments are necessary to determine the precise location of such plasticity in the central nervous system.