Excitation of phrenic and sympathetic output during acute hypoxia: contribution of medullary oxygen detectors

Hypoxia evokes a sequence of raphe-pontomedullary network operations for inspiratory drive amplification and gasping

Hypoxia can trigger a sequence of breathing-related behaviors, from tachypnea to apneusis to apnea and gasping, an autoresuscitative behavior that, via large tidal volumes and altered intrathoracic pressure, can enhance coronary perfusion, carotid blood flow, and sympathetic activity, and thereby coordinate cardiac and respiratory functions. We tested the hypothesis that hypoxia-evoked gasps are amplified through a disinhibitory microcircuit within the inspiratory neuron chain and a distributed efference copy mechanism that generates coordinated gasp-like discharges concurrently in other circuits of the raphe-pontomedullary respiratory network. Data were obtained from 6 decerebrate, vagotomized, neuromuscularly-blocked, and artificially ventilated adult cats. Arterial blood pressure, phrenic nerve activity, end-tidal CO2, and other parameters were monitored. Hypoxia was produced by ventilation with a gas mixture of 5% O2 in nitrogen (N2). Neuron spike trains were recorded at multiple pontomedullary sites simultaneously and evaluated for firing rate modulations and short-time scale correlations indicative of functional connectivity. Experimental perturbations evoked reconfiguration of raphe-pontomedullary circuits during tachypnea, apneusis and augmented bursts, apnea, and gasping. The functional connectivity, altered firing rates, efference copy of gasp drive, and coordinated step increments in blood pressure reported here support a distributed brain stem network model for amplification and broadcasting of inspiratory drive during autoresuscitative gasping that begins with a reduction in inhibition by expiratory neurons and an initial loss of inspiratory drive during hypoxic apnea.

Role of microglia in blood pressure and respiratory responses to acute hypoxic exposure in rats

Microglia modulate cardiorespiratory activities during chronic hypoxia. It has not been clarified whether microglia are involved in the cardiorespiratory responses to acute hypoxia. Here we investigated this issue by comparing cardiorespiratory responses to two levels of acute hypoxia (13% O₂ for 4 min and 7% O₂ for 5 min) in conscious unrestrained rats before and after systemic injection of minocycline (MINO), an inhibitor of microglia activation. MINO increased blood pressure but not lung ventilation in the control normoxic condition. Acute hypoxia stimulated cardiorespiratory responses in MINO-untreated rats. MINO failed to significantly affect the magnitude of hypoxia-induced blood pressure elevation. In contrast, MINO tended to suppress the ventilatory responses to hypoxia. We conclude that microglia differentially affect cardiorespiratory regulation depending on the level of blood oxygenation. Microglia suppressively contribute to blood pressure regulation in normoxia but help maintain ventilatory augmentation in hypoxia, which underscores the dichotomy of central regulatory pathways for both systems.

Ethnic Differences on Cardiac Rhythms and Autonomic Nervous System Responses During a High-Altitude Trek: A Pilot Study Comparing Italian Trekkers to Nepalese Porters

Altitude hypoxia exposure results in increased sympathetic activity and heart rate due to several mechanisms. Recent studies have contested the validity of heart rate variability (HRV) analysis on sympathetic activity measurement. But the plethora of HRV metrics may provide meaningful insights, particularly if linked with cardiovascular and autonomic nervous system parameters. However, the population-specific nature of HRV and cardiorespiratory response to altitude hypoxia are still missing. Six Italian trekkers and six Nepalese porters completed 300 km of a Himalayan trek. The ECG analysis was conducted at baseline, and before (bBC) and after (aBC) the high-altitude (HA) circuit. Urine was collected before and after the expedition in Italians, for assessing catecholamines. Heart rate increased with altitude significantly (p < 0.001) in the Italian group; systolic (p = 0.030) and diastolic (p = 0.012) blood pressure, and mean arterial pressure (p = 0.004) increased with altitude. Instead, pulse pressure did not change, although the Nepalese group showed lower baseline values than the Italians. As expected, peripheral oxygen saturation decreased with altitude (p < 0.001), independently of the ethnic groups. Nepalese had a higher respiratory rate (p = 0.007), independent of altitude. The cardiac vagal index increased at altitude, from baseline to bBC (p = 0.008). Higuchi fractal dimension (HFD) showed higher basal values in the Nepalese group (p = 0.041), and a tendency for the highest values at bBC. Regarding the urinary catecholamine response, exposure to HA increased urinary levels, particularly of norepinephrine (p = 0.005, d = 1.623). Our findings suggest a better cardiovascular resilience of the Nepalese group when compared with Italians, which might be due to an intrinsic adaptation to HA, resulting from their job.

Neural mechanisms underlying respiratory regulation within the preBötzinger complex of the rabbit

The preBötzinger complex (preBötC) is a medullary area essential for normal breathing and widely recognized as necessary and sufficient to generate the inspiratory phase of respiration. It has been studied mainly in rodents. Here we report the main results of our studies revealing the characteristics of the rabbit preBötC identified by means of neuronal recordings, D,L-homocysteic acid microinjections and histological controls. A crucial role in the respiratory rhythmogenesis within this neural substrate is played by excitatory amino acids, but also GABA and glycine display important contributions. Increases in respiratory frequency are induced by microinjections of neurokinins, somatostatin as well by serotonin (5-HT) through an action on 5-HT_1A and 5-HT₃ receptors or the disinhibition of a GABAergic circuit. Respiratory depression is observed in response to microinjections of the μ-opioid receptor agonist DAMGO. Our results show similarities and differences with the rodent preBötC and emphasize the importance of comparative studies on the mechanisms underlying respiratory rhythmogenesis in different animal species.

The central role of protein kinase C epsilon in cyanide cardiotoxicity and its treatment

In adult mouse myocytes, brief exposure to sodium cyanide (CN) in the presence of glucose does not decrease ATP levels, yet produces profound reduction in contractility, intracellular Ca2+ concentration ([Ca2+]i) transient and L-type Ca2+ current (ICa) amplitudes. We analyzed proteomes from myocytes exposed to CN, focusing on ionic currents associated with excitation-contraction coupling. CN induced phosphorylation of α1c subunit of L-type Ca2+ channel and α2 subunit of Na+-K+-ATPase. Methylene blue (MB), a CN antidote that we previously reported to ameliorate CN-induced reduction in contraction, [Ca2+]i transient and ICa amplitudes, was able to reverse this phosphorylation. CN decreased Na+-K+-ATPase current contributed by α2 but not α1 subunit, an effect that was also counteracted by MB. Peptide consensus sequences suggested CN-induced phosphorylation was mediated by protein kinase C epsilon (PKCε). Indeed, CN stimulated PKC kinase activity and induced PKCε membrane translocation, effects that were prevented by MB. Pre-treatment with myristoylated PKCε translocation activator or inhibitor peptides mimicked and inhibited the effects of CN on ICa and myocyte contraction, respectively. We conclude that CN activates PKCε, which phosphorylates L-type Ca2+ channel and Na+-K+-ATPase, resulting in depressed cardiac contractility. We hypothesize that this inhibition of ion fluxes represents a novel mechanism by which the cardiomyocyte reduces its ATP demand (decreased ion fluxes and contractility), diminishes ATP turnover and preserves cell viability. However, this cellular protective effect translates into life-threatening cardiogenic shock in vivo, thereby creating a profound disconnect between survival mechanisms at the cardiomyocyte level from those at the level of the whole organism.

Respiratory frequency plasticity during development

Respiratory frequency plasticity is a long-lasting increase in breathing frequency due to a perturbation. Mechanisms underlying respiratory frequency are poorly understood, and there is little evidence of frequency plasticity in neonates. This hybrid review/research article discusses available literature regarding frequency plasticity and highlights potential research opportunities. Also, we include data demonstrating a model of frequency plasticity using isolated neonatal rat brainstem-spinal cord preparations. Specifically, substance P (SubP) application induced a long-lasting (>60 min) increase in spontaneous respiratory motor burst frequency, particularly in brainstem-spinal cords with the pons attached; there were no male/female differences. SubP-induced frequency plasticity is dependent on the application pattern, such that intermittent (rather than sustained) SubP applications induce more frequency plasticity. SubP-induced frequency plasticity was blocked by a neurokinin-1 receptor antagonist. Thus, the newborn rat respiratory control system has the capacity to express frequency plasticity. Identifying mechanisms that induce frequency plasticity may lead to novel methods to safely treat breathing disorders in premature and newborn infants.

Isolated adult turtle brainstems exhibit central hypoxic chemosensitivity

During hypoxia, red-eared slider turtles increase ventilation and decrease episodic breathing, but whether these responses are due to central mechanisms is not known. To test this question, isolated adult turtle brainstems were exposed to 240 min of hypoxic solution (bath PO₂ = 32.6 ± 1.2 mmHg) and spontaneous respiratory-related motor bursts (respiratory event) were recorded on hypoglossal nerve roots. During hypoxia, burst frequency increased during the first 15 min, and then decreased during the remaining 35-240 min of hypoxia. Burst amplitude was maintained for 120 min, but then decreased during the last 120 min. The number of bursts/respiratory event decreased within 30 min and remained decreased. Pretreatment with either prazosin (α₁-adrenergic antagonist) or MDL7222 (5-HT₃ antagonist) blocked the hypoxia-induced short-term increase and the longer duration decrease in burst frequency. MDL7222, but not prazosin, blocked the hypoxia-induced decrease in bursts/respiratory event. Thus, during bath hypoxia, isolated turtle brainstems continued to produce respiratory motor output, but the frequency and pattern were altered in a manner that required endogenous α₁-adrenergic and serotonin 5-HT₃ receptor activation. This is the first example of isolated reptile brainstems exhibiting central hypoxic chemosensitivity similar to other vertebrate species.

Methylene blue counteracts cyanide cardiotoxicity: cellular mechanisms

In adult left ventricular mouse myocytes, exposure to sodium cyanide (NaCN) in the presence of glucose dose-dependently reduced contraction amplitude, with ~80% of maximal inhibitory effect attained at 100 µM. NaCN (100 µM) exposure for 10 min significantly decreased contraction and intracellular Ca2+ concentration ([Ca2+]i) transient amplitudes, systolic but not diastolic [Ca2+]i, and maximal L-type Ca2+ current ( ICa) amplitude, indicating acute alteration of [Ca2+]i homeostasis largely accounted for the observed excitation-contraction abnormalities. In addition, NaCN depolarized resting membrane potential ( Em), reduced action potential (AP) amplitude, prolonged AP duration at 50% (APD50) and 90% repolarization (APD90), and suppressed depolarization-activated K+ currents but had no effect on Na+-Ca2+ exchange current ( INaCa). NaCN did not affect cellular adenosine triphosphate levels but depolarized mitochondrial membrane potential (ΔΨm) and increased superoxide (O2·-) levels. Methylene blue (MB; 20 µg/ml) added 3 min after NaCN restored contraction and [Ca2+]i transient amplitudes, systolic [Ca2+]i, Em, AP amplitude, APD50, APD90, ICa, depolarization-activated K+ currents, ΔΨm, and O2·- levels toward normal. We conclude that MB reversed NaCN-induced cardiotoxicity by preserving intracellular Ca2+ homeostasis and excitation-contraction coupling ( ICa), minimizing risks of arrhythmias ( Em, AP configuration, and depolarization-activated K+ currents), and reducing O2·- levels. NEW & NOTEWORTHY Cyanide poisoning due to industrial exposure, smoke inhalation, and bioterrorism manifests as cardiogenic shock and requires rapidly effective antidote. In the early stage of cyanide exposure, adenosine triphosphate levels are normal but myocyte contractility is reduced, largely due to alterations in Ca2+ homeostasis because of changes in oxidation-reduction environment of ion channels. Methylene blue, a drug approved by the U.S. Food and Drug Administration, ameliorates cyanide toxicity by normalizing oxidation-reduction state and Ca2+ channel function.

Partial Raphe Dysfunction in Neurotransmission Is Sufficient to Increase Mortality after Anoxic Exposures in Mice at a Critical Period in Postnatal Development

Sudden infant death syndrome (SIDS) cases often have abnormalities of the brainstem raphe serotonergic (5-HT) system. We hypothesize that raphe dysfunction contributes to a failure to autoresuscitate from multiple hypoxic events, leading to SIDS. We studied autoresuscitation in two transgenic mouse models in which exocytic neurotransmitter release was impaired via conditional expression of the light chain from tetanus toxin (tox) in raphe neurons expressing serotonergic bacterial artificial chromosome drivers Pet1 or Slc6a4. These used recombinase drivers targeted different portions of medullary raphe serotonergic, tryptophan hydroxylase 2 (Tph2)(+) neurons by postnatal day (P) 5 through P12: approximately one-third in triple transgenic Pet1::Flpe, hβactin::cre, RC::PFtox mice; approximately three-fourths inSlc6a4::cre, RC::Ptox mice; with the first model capturing a near equal number of Pet1(+),Tph2(+) versus Pet1(+),Tph2(low or negative) raphe cells. At P5, P8, and P12, "silenced" mice and controls were exposed to five, ∼37 s bouts of anoxia. Mortality was 5-10 times greater in "silenced" pups compared with controls at P5 and P8 (p = 0.001) but not P12, with cumulative survival not differing between experimental transgenic models. "Silenced" pups that eventually died took longer to initiate gasping (p = 0.0001), recover heart rate (p = 0.0001), and recover eupneic breathing (p = 0.011) during the initial anoxic challenges. Variability indices for baseline breathing distinguished "silenced" from controls but did not predict mortality. We conclude that dysfunction of even a portion of the raphe, as observed in many SIDS cases, can impair ability to autoresuscitate at critical periods in postnatal development and that baseline indices of breathing variability can identify mice at risk.

SIGNIFICANCE STATEMENT

Many sudden infant death syndrome (SIDS) cases exhibit a partial (∼26%) brainstem serotonin deficiency. Using recombinase drivers, we targeted different fractions of serotonergic and raphe neurons in mice for tetanus toxin light chain expression, which prevented vesicular neurotransmitter release. In one model, approximately one-third of medullary Tph2(+) neurons are silenced by postnatal (P) days 5 and 12, along with some Pet1(+),Tph2(low or negative) raphe cells; in the other, approximately three-fourths of medullary Tph2(+) neurons, also with some Tph2(low or negative) cells. Both models demonstrated excessive mortality to anoxia (a postulated SIDS stressor) at P5 and P8. We demonstrated fatal vulnerability to anoxic stress at a specific time in postnatal life induced by a partial defect in raphe function. This models features of SIDS.

Chemosensitivity, Cardiovascular Risk, and the Ventilatory Response to Exercise in COPD

COPD is associated with elevated cardiovascular risk and a potentiated ventilatory response to exercise. Enhanced carotid chemoreceptor (CC) activity/sensitivity is present in other clinical conditions, has been shown to contribute to sympathetic vasoconstrictor outflow, and is predictive of mortality. CC activity/sensitivity, and the resulting functional significance, has not been well examined in COPD. We hypothesized that CC activity/sensitivity would be elevated in COPD, and related to increased pulse wave velocity (a marker of CV risk) and the ventilatory response to exercise.

METHODS

30 COPD patients and 10 healthy age-matched controls were examined. Participants performed baseline cardiopulmonary exercise and pulmonary function testing. CC activity was later evaluated by the drop in ventilation with breathing 100% O2, and CC sensitivity was then assessed by the ventilatory response to hypoxia (ΔVE/ΔSpO2). Peripheral arterial stiffness was subsequently evaluated by measurement of pulse wave velocity (PWV) using applanation tonometry while the subjects were breathing room air, and then following chemoreceptor inhibition by breathing 100% O2 for 2 minutes.

RESULTS

CC activity, CC sensitivity, PWV and the ventilatory response to exercise were all increased in COPD relative to controls. CC sensitivity was related to PWV; however, neither CC activity nor CC sensitivity was related to the ventilatory response to exercise in COPD. CC inhibition by breathing 100% O2 normalized PWV in COPD, while no effect was observed in controls.

CONCLUSION

CC activity and sensitivity are elevated in COPD, and appear related to cardiovascular risk; however, CC activity/sensitivity does not contribute to the potentiated ventilatory response to exercise.

Intermittent severe hypoxia induces plasticity within serotonergic and catecholaminergic neurons in the neonatal rat ventrolateral medulla

5-HT neurons contribute to autoresuscitation and survival during intermittent severe hypoxia (IsH). In adults, catecholaminergic neurons in the ventrolateral medulla (VLM) contribute to the autonomic response to hypoxia. We hypothesized that 1) catecholaminergic neurons in the neonatal VLM are activated following IsH, 2) this activation is compromised following an acute loss of brain stem 5-HT, and 3) IsH induces cellular and/or transcriptomic plasticity within catecholaminergic and serotonergic neurons that are within or project to the VLM, respectively. To test these hypotheses, we treated rat pups with 6-fluorotryptophan, a tryptophan hydroxylase (TPH) inhibitor, and then exposed treated and vehicle controls to IsH or air. Along with immunohistochemistry to detect tyrosine hydroxylase (TH)- or Fos-positive neurons, we used RNA sequencing to resolve the effects of IsH and 5-HT deficiency on the expression of serotonergic and catecholaminergic system genes in the VLM. 5-HT deficiency compromised autoresuscitation and survival. IsH significantly increased the number of identifiable TH-positive VLM neurons, an effect enhanced by 5-HT deficiency (P = 0.003). Contrary to our hypothesis, 5-HT-deficient pups had significantly more Fos-positive neurons following IsH (P = 0.008) and more activated TH-positive neurons following IsH or air (P = 0.04). In both groups the expression of the 5-HT transporter and TPH2 was increased following IsH. In 5-HT-deficient pups, the expression of the inhibitory 5-HT1A receptor was decreased following IsH, while the expression of DOPA decarboxylase was increased. These data show that the serotonergic and catecholaminergic systems in the VLM of the neonatal rat are dynamically upregulated by IsH, potentially adapting cardiorespiratory responses to severe hypoxia.

Brain stem serotonin protects blood pressure in neonatal rats exposed to episodic anoxia

In neonatal rodents, a loss of brain stem serotonin [5-hydroxytryptamine (5-HT)] in utero or at birth compromises anoxia-induced gasping and the recovery of heart rate (HR) and breathing with reoxygenation (i.e., autoresuscitation). How mean arterial pressure (MAP) is influenced after an acute loss of brain stem 5-HT content is unknown. We hypothesized that a loss of 5-HT for ∼1 day would compromise MAP during episodic anoxia. We injected 6-fluorotryptophan (20 mg/kg ip) into rat pups (postnatal days 9-10 or 11-13, n = 22 treated, 24 control), causing a ∼70% loss of brain stem 5-HT. Pups were exposed to a maximum of 15 anoxic episodes, separated by 5 min of room air to allow autoresuscitation. In younger pups, we measured breathing frequency and tidal volume using "head-out" plethysmography and HR from the electrocardiogram. In older pups, we used whole body plethysmography to detect gasping, while monitoring MAP. Gasp latency and the time required for respiratory, HR, and MAP recovery following each episode were determined. Despite normal gasp latency, breathing frequency and a larger tidal volume (P < 0.001), 5-HT-deficient pups survived one-half the number of episodes as controls (P < 0.001). The anoxia-induced decrease in MAP experienced by 5-HT-deficient pups was double that of controls (P = 0.017), despite the same drop in HR (P = 0.48). MAP recovery was delayed ∼10 s by 5-HT deficiency (P = 0.001). Our data suggest a loss of brain stem 5-HT leads to a pronounced, premature loss of MAP in response to episodic anoxia. These data may help explain why some sudden infant death syndrome cases die from what appears to be cardiovascular collapse during apparent severe hypoxia.

A key circulatory defence against asphyxia in infancy--the heart of the matter!

A resumption of, and escalation in, breathing efforts (hyperpnoea) reflexively accelerates heart rate (HR) and may facilitate cardiac and circulatory recovery from apnoea. We analysed whether this mechanism can produce a sustained rise in HR (tachycardia) when a sleeping infant is confronted by mild, rapidly worsening asphyxia, simulating apnoea. Twenty-seven healthy term-born infants aged 1-8 days rebreathed the expired gas for 90 s during quiet sleep to stimulate breathing and heart rate. To discriminate cardio-excitatory effects of central respiratory drive, lung inflation, hypoxia, hypercapnia and asphyxia, we varied the inspired O(2) level and compared temporal changes in response profiles as respiratory sensitivity to hypoxia and asphyxia 'reset' after birth. We demonstrate that asphyxia-induced hyperpnoea and tachycardia strengthen dramatically over the first week with different time courses and via separate mechanisms. Cardiac excitation by hypercapnia improves first, followed by a slower improvement in respiratory hypoxic drive. A rise in CO(2) consequently elicits stronger, longer lasting tachycardia than moderate increases in respiratory drive or lung expansion. We suggest that without a strong facilitating action of CO(2) on the immature heart, respiratory manoeuvres may be unable to reflexively counteract strong vagal bradycardia. This may increase the vulnerability of some infants to apnoea-asphyxia.

Postnatal loss of brainstem serotonin neurones compromises the ability of neonatal rats to survive episodic severe hypoxia

Pet-1(-/-) mice with a prenatal, genetically induced loss of 5-hydroxytryptamine (5-HT, serotonin) neurones are compromised in their ability to withstand episodic environmental anoxia via autoresuscitation. Given the prenatal role of 5-HT neurones in the development of neural networks, here we ask if a postnatal loss of 5-HT neurones also compromises autoresuscitation. We treated neonatal rat pups at postnatal day (P)2-3 with an intra-cisternal injection of 5,7-dihydroxytryptamine (5,7-DHT; ~40 μg; n = 8) to pharmacologically lesion the 5-HT system, or vehicle (control; n = 14). At P7-10 we exposed unanaesthetized treated and control pups to 15 episodes of environmental anoxia (97% N(2), 3% CO(2)). Medullary 5-HT content was reduced 80% by 5,7-DHT treatment (P < 0.001). Baseline ventilation (V(E)), metabolic rate (V(O(2))), ventilatory equivalent (V(E)/V(O(2))), heart rate (HR), heart rate variability (HRV) and arterial haemoglobin saturation (S(aO(2))) were no different in 5-HT-deficient pups compared to controls. However, only 25% of 5-HT-deficient pups survived all 15 episodes of environmental anoxia, compared to 79% of control littermates (P = 0.007). High mortality of 5,7-DHT-treated pups was associated with delayed onset of gasping (P < 0.001), delayed recovery of HR from hypoxic-induced bradycardia (P < 0.001), and delayed recovery of eupnoea from hypoxic-induced apnoea (P < 0.001). Treatment with 5,7-DHT affected neither the gasping pattern once initiated, nor HR, V(E)/V(O(2)) or S(aO(2)) during the intervening episodes of room air. A significant increase in HRV occurred in all animals with repeated exposure, and in 5-HT-deficient pups this increase occurred immediately prior to death. We conclude that a postnatal loss of brainstem 5-HT content compromises autoresuscitation in response to environmental anoxia. This report provides new evidence in rat pups that 5-HT neurones serve a physiological role in autoresuscitation. Our data may be relevant to understanding the aetiology of the sudden infant death syndrome (SIDS), in which there is medullary 5-HT deficiency and in some cases evidence of severe hypoxia and failed autoresuscitation.

Failed heart rate recovery at a critical age in 5-HT-deficient mice exposed to episodic anoxia: implications for SIDS

Mice deficient in the transcription factor Pet-1⁻/⁻ have a ∼70% deficiency of brainstem serotonin [5-hydroxytryptamine (5-HT)] neurons and exhibit spontaneous bradycardias in room air at postnatal day (P)5 and P12 and delayed gasping in response to a single episode of anoxia at P4.5 and P9.5 (Cummings KJ, Li A, Deneris ES, Nattie EE. Am J Physiol Regul Integr Comp Physiol 298: R1333-R1342, 2010; and Erickson JT, Sposato BC. J Appl Physiol 106: 1785-1792, 2009). We hypothesized that at a critical age Pet-1⁻/⁻ mice will fail to autoresuscitate during episodic anoxia, ultimately dying from a failure of gasping to restore heart rate (HR). We exposed P5, P8, and P12 Pet-1⁻/⁻ mice and wild-type littermates (WT) to four 30-s episodes of anoxia (97% N₂-3% CO₂), separated by 5 min of room air. We observed excess mortality in Pet-1⁻/⁻ only at P8: 43% of Pet-1⁻/⁻ animals survived past the third episode of anoxia while ∼95% of WT survived all four episodes (P = 0.004). No deaths occurred at P5 and at P12, and one of six Pet-1⁻/⁻ mice died after the fourth episode, while all WT animals survived. At P8, dying Pet-1⁻/⁻ animals had delayed gasping, recovery of HR, and eupnea after the first two episodes of anoxia (P < 0.001 for each); death ultimately occurred when gasping failed to restore HR. Both high- and low-frequency components of HR variability were abnormally elevated in dying Pet-1⁻/⁻ animals following the first episode of anoxia. Dying P8 Pet-1⁻/⁻ animals had significantly fewer 5-HT neurons in the raphe magnus than surviving animals (P < 0.001). Our data indicate a critical developmental window at which a brainstem 5-HT deficiency increases the risk of death during episodes of anoxia. They may apply to the sudden infant death syndrome, which occurs at a critical age and is associated with 5-HT deficiency.

Decreased GABAB receptor function in the cerebellum and brain stem of hypoxic neonatal rats: role of glucose, oxygen and epinephrine resuscitation

Background-

Hypoxia during the first week of life can induce neuronal death in vulnerable brain regions usually associated with an impairment of cognitive function that can be detected later in life. The neurobiological changes mediated through neurotransmitters and other signaling molecules associated with neonatal hypoxia are an important aspect in establishing a proper neonatal care.

Methods-

The present study evaluated total GABA, GABA_B receptor alterations, gene expression changes in GABA_B receptor and glutamate decarboxylase in the cerebellum and brain stem of hypoxic neonatal rats and the resuscitation groups with glucose, oxygen and epinephrine. Radiolabelled GABA and baclofen were used for receptor studies of GABA and GABA_B receptors respectively and Real Time PCR analysis using specific probes for GABA_B receptor and GAD mRNA was done for gene expression studies.

Results-

The adaptive response of the body to hypoxic stress resulted in a reduction in total GABA and GABA_B receptors along with decreased GABA_B receptor and GAD gene expression in the cerebellum and brain stem. Hypoxic rats supplemented with glucose alone and with oxygen showed a reversal of the receptor alterations and changes in GAD. Resuscitation with oxygen alone and epinephrine was less effective in reversing the receptor alterations.

Conclusions-

Being a source of immediate energy, glucose can reduce the ATP-depletion-induced changes in GABA and oxygenation, which helps in encountering hypoxia. The present study suggests that reduction in the GABA_B receptors functional regulation during hypoxia plays an important role in central nervous system damage. Resuscitation with glucose alone and glucose and oxygen to hypoxic neonatal rats helps in protecting the brain from severe hypoxic damage.

Hypoxia-induced cellular and vascular changes in the nucleus tractus solitarius and ventrolateral medulla

Major changes in arterial pressure, autonomic, and respiratory activity occur in response to hypoxia. We analyzed structural damage and increased vascular permeability in the ventrolateral medulla and nucleus tractus solitarius, which control autonomic, respiratory, and cardiovascular functions in adult Wistar rats subjected to 2 hours of hypoxia (7% oxygen + 93% nitrogen) for up to 14 days after hypoxicexposure. Brainstem tissue levels of vascular endothelial growth factor (VEGF), nitric oxide (NO), and glutamate were significantly increased over control levels after hypoxic injury. By electron microscopy, swollen neurons and dendrites, degenerating axons, disrupted myelin sheaths, and swollen astrocyte processes were observed in the nucleus tractus solitarius and ventrolateral medulla. Leakage of intravenously administered horseradish peroxidase was observed through vascular walls in hypoxic rats. These results suggest that increased VEGF and NO production in hypoxia resulted in increased vascular permeability, which, along with increased levels of glutamate, may have induced structural alterations of the neurons, dendrites, and axons. Administration of the antioxidant neurohormone melatonin (10mg/kg) before and after the hypoxia reduced VEGF, NO, and glutamate levels and improved ultrastructural abnormalities induced by hypoxia exposure, suggesting that it may have a therapeutic potential in reducing hypoxia-associated brainstem damage.

Respiratory responses induced by blockades of GABA and glycine receptors within the Bötzinger complex and the pre-Bötzinger complex of the rabbit

The respiratory role of GABA(A), GABA(B) and glycine receptors within the Bötzinger complex (BötC) and the pre-Bötzinger complex (preBötC) was investigated in alpha-chloralose-urethane anesthetized, vagotomized, paralysed and artificially ventilated rabbits by using bilateral microinjections (30-50 nl) of GABA and glycine receptor agonists and antagonists. GABA(A) receptor blockade by bicuculline (5mM) or gabazine (2mM) within the BötC induced strong depression of respiratory activity up to apnea. The latter was reversed by hypercapnia. Glycine receptor blockade by strychnine (5mM) within the BötC decreased the frequency and amplitude of phrenic bursts. Bicuculline microinjections into the preBötC caused decreases in respiratory frequency and the appearance of two alternating different levels of peak phrenic activity. Strychnine microinjections into the preBötC increased respiratory frequency and decreased peak phrenic amplitude. GABA(A), but not glycine receptor antagonism within the preBötC restored respiratory rhythmicity during apnea due to bicuculline or gabazine applied to the BötC. GABA(B) receptor blockade by CGP-35348 (50mM) within the BötC and the preBötC did not affect baseline respiratory activity, though microinjections of the GABA(B) receptor agonist baclofen (1mM) into the same regions altered respiratory activity. The results show that only GABA(A) and glycine receptors within the BötC and the preBötC mediate a potent control on both the intensity and frequency of inspiratory activity during eupneic breathing. This study is the first to provide evidence that these inhibitory receptors have a respiratory function within the BötC.

An interdependent model of central/peripheral chemoreception: evidence and implications for ventilatory control

In this review we discuss the implications for ventilatory control of newer evidence suggesting that central and peripheral chemoreceptors are not functionally separate but rather that they are dependent upon one another such that the sensitivity of the medullary chemoreceptors is critically determined by input from the carotid body chemoreceptors and vice versa i.e., they are interdependent. We examine potential interactions of the interdependent central and carotid body (CB) chemoreceptors with other ventilatory-related inputs such as central hypoxia, lung stretch, and exercise. The limitations of current approaches addressing this question are discussed and future studies are suggested.

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.

Reconfiguration of the pontomedullary respiratory network: a computational modeling study with coordinated in vivo experiments

A large body of data suggests that the pontine respiratory group (PRG) is involved in respiratory phase-switching and the reconfiguration of the brain stem respiratory network. However, connectivity between the PRG and ventral respiratory column (VRC) in computational models has been largely ad hoc. We developed a network model with PRG-VRC connectivity inferred from coordinated in vivo experiments. Neurons were modeled in the "integrate-and-fire" style; some neurons had pacemaker properties derived from the model of Breen et al. We recapitulated earlier modeling results, including reproduction of activity profiles of different respiratory neurons and motor outputs, and their changes under different conditions (vagotomy, pontine lesions, etc.). The model also reproduced characteristic changes in neuronal and motor patterns observed in vivo during fictive cough and during hypoxia in non-rapid eye movement sleep. Our simulations suggested possible mechanisms for respiratory pattern reorganization during these behaviors. The model predicted that network- and pacemaker-generated rhythms could be co-expressed during the transition from gasping to eupnea, producing a combined "burst-ramp" pattern of phrenic discharges. To test this prediction, phrenic activity and multiple single neuron spike trains were monitored in vagotomized, decerebrate, immobilized, thoracotomized, and artificially ventilated cats during hypoxia and recovery. In most experiments, phrenic discharge patterns during recovery from hypoxia were similar to those predicted by the model. We conclude that under certain conditions, e.g., during recovery from severe brain hypoxia, components of a distributed network activity present during eupnea can be co-expressed with gasp patterns generated by a distinct, functionally "simplified" mechanism.

Acetylcholine esterase activity and behavioral response in hypoxia induced neonatal rats: effect of glucose, oxygen and epinephrine supplementation

Brain damage due to an episode of hypoxia remains a major problem in infants causing deficit in motor and sensory function. Hypoxia leads to neuronal functional failure, cerebral palsy and neuro-developmental delay with characteristic biochemical and molecular alterations resulting in permanent or transitory neurological sequelae or even death. During neonatal hypoxia, traditional resuscitation practices include the routine administration of 100% oxygen, epinephrine and glucose. In the present study, we assessed the changes in the cholinergic system by measuring the acetylcholinesterase (AChE) activity and the behavioral responses shown by hypoxia induced neonatal rats and hypoxic rats supplemented with glucose, oxygen and epinephrine using elevated plus-maze and open-field test. The acetylcholine esterase enzyme activity showed a significant decrease in cerebral cortex, whereas it increased significantly in the muscle of experimental rats when compared to control. Hypoxic rats supplemented with glucose, glucose and oxygen showed a reversal to the control status. Behavioral studies were carried out in experimental rats with elevated plus-maze test and open-field test. Hypolocomotion and anxiogenic behavioral responses were observed in all experimental rats when compared to control, hypoxic rats supplemented with glucose, glucose and oxygen. Thus, our results suggest that brain damage due to hypoxia, oxygen and epinephrine supplementation in the neonatal rats cause acetylcholine-neuromuscular-defect leading to hypolocomotion and anxiogenic behavioral response. Glucose and glucose with oxygen supplementation to hypoxic neonates protect the brain damage for a better functional status in the later life.

Lack of histamine type-1 receptors impairs the thermal response of respiration during hypoxia in mice (Mus musculus)

Thermoregulation and the hypoxic ventilatory response are modulated by histamine type-1 (H1) receptors in the brain. In this study, we tested the hypothesis that activation of H1 receptors is required for the thermal control of ventilation during normoxia and hypoxia, using conscious male wild-type and H1 receptor-knockout (H1RKO) mice (Mus musculus). Under normoxic conditions, hyperthermia (39 degrees C) decreased minute ventilation (V (E)) and oxygen consumption [Formula: see text] in both genotypes, suggesting that H1 receptors are not involved in thermal ventilatory control during normoxia. Pa(CO2) was unchanged in both hyperthermia and normothermia, suggesting that the thermal decrease in V (E) is optimized by metabolic demand. Acute hypoxic gas exposure (7% O(2)+3% CO(2) in N(2)) increased, and then decreased, V (E) in wild-type mice; this increase was augmented and sustained by hyperthermia. Hypoxic gas exposure reduced [Formula: see text] and [Formula: see text] in wild-type mice at both body temperatures; the reduced [Formula: see text] during combined hyperthermia and hypoxia was higher than during normothermia and hypoxia. In H1RKO mice, hyperthermia did not augment the V (E) response to hypoxia, and did not affect [Formula: see text] and [Formula: see text] during hypoxia. In conclusion, histamine participates in the thermal increase of ventilation during hypoxia by activating H1 receptors.

Accuracy of a third (Dolphin Voyager) versus first generation pulse oximeter (Nellcor N-180) in predicting arterial oxygen saturation and pulse rate in the anesthetized dog

OBJECTIVE

To compare the accuracy of a 3rd (Dolphin Voyager) versus 1st generation pulse oximeter (Nellcor N-180).

STUDY DESIGN

Prospective laboratory investigation.

ANIMALS

Eight adult dogs.

METHODS

In anesthetized dogs, arterial oxygen saturation (SpO(2)) was recorded simultaneously with each pulse oximeter. The oxygen fraction in inspired gas (FiO(2)) was successively reduced from 1.00 to 0.09, with re-saturation (FiO(2) 0.40) after each breathe-down step. After each 3-minute FiO(2) plateau, SpO(2) and pulse rate (PR) were compared with the fractional arterial saturation (SaO(2)) and PR determined by co-oximetry and invasive blood pressure monitoring, respectively. Data analysis included Bland-Altman (B-A) plots, Lin's concordance correlation factor (rho(c)), and linear regression models.

RESULTS

Over a SaO(2) range of 33-99%, the overall bias (mean SpO(2) - SaO(2)), precision (SD of bias), and accuracy (A(rms)) for the Dolphin Voyager and Nellcor N-180 were 4.3%, 4.4%, and 6.1%, and 3.2%, 3.0%, and 4.3%, respectively. Bias increased at SaO(2) < 90%, more so with the Dolphin Voyager (from 1.6% to 8.6%) than Nellcor N-180 (from 3.2% to 4.5%). The SpO(2) readings correlated significantly with SaO(2) for both the Dolphin Voyager (rho(c) = 0.94) and Nellcor N-180 (rho(c) = 0.97) (p < 0.001). Regarding PR, bias, precision, and accuracy (A(rms)) for the Dolphin Voyager and Nellcor N-180 were -0.5, 4.6, and 4.6 and 1.38, 4.3, and 4.5 beats minute(-1), respectively. Significant correlation existed between pulse oximeter and directly measured PR (Dolphin Voyager: rho(c) = 0.98; Nellcor N-180: rho(c) = 0.99) (p < 0.001).

CONCLUSIONS AND CLINICAL RELEVANCE

In anesthetized dogs with adequate hemodynamic function, both instruments record SaO(2) relatively accurately over a wide range of normal saturation values. However, there is an increasing overestimation at SaO(2) < 90%, particularly with the Dolphin Voyager, indicating that 3rd generation pulse oximeters may not perform better than older instruments. The 5.4-fold increase in bias with the Dolphin Voyager at SaO(2) < 90% stresses the importance of a 93-94% SpO(2) threshold to ensure an arterial saturation of >or=90%. In contrast, PR monitoring with both devices is very reliable.

Medullary lateral tegmental field neurons influence the timing and pattern of phrenic nerve activity in cats

In an effort to characterize the role of the medullary lateral tegmental field (LTF) in regulating respiration, we tested the effects of selective blockade of excitatory (EAA) and inhibitory amino acid (IAA) receptors in this region on phrenic nerve activity (PNA) of vagus-intact and vagotomized cats anesthetized with dial-urethane. We found distinct patterns of changes in central respiratory rate, duration of inspiratory and expiratory phases of PNA (Ti and Te, respectively), and I-burst amplitude after selective blockade of EAA and IAA receptors in the LTF. First, blockade of N-methyl-D-aspartate (NMDA) receptors significantly (P < 0.05) decreased central respiratory rate primarily by increasing Ti but did not alter I-burst amplitude. Second, blockade of non-NMDA receptors significantly reduced I-burst amplitude without affecting central respiratory rate. Third, blockade of GABAA receptors significantly decreased central respiratory rate by increasing Te and significantly reduced I-burst amplitude. Fourth, blockade of glycine receptors significantly decreased central respiratory rate by causing proportional increases in Ti and Te and significantly reduced I-burst amplitude. These changes in PNA were markedly different from those produced by blockade of EAA or IAA receptors in the pre-Bötzinger complex. We propose that a proper balance of excitatory and inhibitory inputs to several functionally distinct pools of LTF neurons is essential for maintaining the normal pattern of PNA in anesthetized cats.

Development of the respiratory response to hypoxia in the isolated brainstem of the bullfrog Rana catesbeiana

The aim of this study was to examine the effects of cellular hypoxia, and the contribution of anaerobic metabolism, on respiratory activity in bullfrogs at different stages of development. Respiratory-related neural activity was recorded from cranial nerve rootlets in isolated brainstem preparations from pre-metamorphic (Taylor-Kollros (T-K) stages VIII-XVI) and postmetamorphic tadpoles (T-K stages XXIV-XXV) and adults. Changes in fictive gill/lung activity in brainstems from pre-metamorphic tadpoles and lung activity in postmetamorphic tadpoles and adults were examined during superfusion with control (98% O(2)/2% CO(2)) or hypoxic (98% N(2)/2% CO(2)) artificial cerebrospinal fluid (aCSF). Iodoacetate (IAA; 100 micromol l(-1)) was used in conjunction with hypoxic aCSF to inhibit glycolysis. Gill burst frequency in pre-metamorphic brainstems did not change over a 3 h exposure to hypoxia and fictive lung burst frequency slowed significantly, but only after 3 h hypoxia. Blockade of glycolysis with IAA during hypoxia significantly reduced the time respiratory activity could be maintained in pre-metamorphic, but not in adult, brainstems. In brainstems from post-metamorphic tadpoles and adults, lung burst frequency became significantly more episodic within 5-15 min hypoxic exposure, but respiratory neural activity was subsequently abolished in every preparation. The cessation of fictive breathing was restored to control levels upon reoxygenation. Neither tadpole nor adult brainstems exhibited changes in neural bursts resembling 'gasping' that is observed in mammalian brainstems exposed to severe hypoxia. There was also a significant increase in the frequency of 'non-respiratory' bursts in hypoxic postmetamorphic and adult brainstems, but not in pre-metamorphic brainstems. These results indicate that pre-metamorphic tadpoles are capable of maintaining respiratory activity for 3 h or more during severe hypoxia and rely to a great extent upon anaerobic metabolism to maintain respiratory motor output. Upon metamorphosis, however, hypoxia results in significant changes in respiratory frequency and pattern, including increased lung burst episodes, non-ventilatory bursts and a reversible cessation of respiratory activity. Adults have little or no ability to maintain respiratory activity through glycolysis but, instead, stop respiratory activity until oxygen is available. This 'switch' in the respiratory response to hypoxia coincides morphologically with the loss of gills and obligate air-breathing in the postmetamorphic frog. We hypothesize that the cessation of respiratory activity in post-metamorphic tadpoles and adults is an adaptive, energy-saving response to low oxygen.

Glutamate neurotransmission is not required for, but may modulate, hypoxic sensitivity of pre-Bötzinger complex in vivo

Focal hypoxia in the pre-Bötzinger complex (pre-BötC) in vivo elicits excitation of inspiratory motor output by modifying the patterning and timing of phrenic bursts. Hypoxia, however, has been reported to enhance glutamate release in some regions of the brain, including the medullary ventral respiratory column; thus the pre-BötC-mediated hypoxic respiratory excitation may result from, or be influenced by, hypoxia-induced activation of ionotropic glutamate [i.e., excitatory amino acid (EAA)] receptors. To test this possibility, the effects of focal pre-BötC hypoxia [induced by sodium cyanide (NaCN)] were examined before and after blockade of ionotropic EAA receptors [using kynurenic acid (KYN)] in this region in chloralose-anesthetized, vagotomized, mechanically ventilated cats. Before blockade of ionotropic EAA receptors, unilateral microinjection of NaCN (1 mM; 10-20 nl) into the pre-BötC produced either phasic or tonic excitation of phrenic nerve discharge. Unilateral microinjection of KYN (50-100 mM; 40 nl) decreased the amplitude and frequency of basal phrenic nerve discharge; however, subsequent microinjection of NaCN, but not DL-homocysteic acid (DLH, a glutamate analog), still produced excitation of phrenic motor output. Under these conditions, the NaCN-induced excitation included frequency modulation (FM) of phasic phrenic bursts, and in many cases, augmented and/or fractionated phrenic bursts. These findings show that the hypoxia-sensing function of the in vivo pre-BötC, which produces excitation of phrenic nerve discharge, is not dependent on activation of ionotropic glutamate receptors, but ionotropic glutamate receptor activation may modify the expression of the focal hypoxia-induced response. Thus these findings provide additional support to the concept of intrinsic hypoxic sensitivity of the pre-BötC.

Ionotropic excitatory amino acid receptors in pre-Bötzinger complex play a modulatory role in hypoxia-induced gasping in vivo

Activation of ionotropic excitatory amino acid (EAA) receptors in pre-Bötzinger complex (pre-BötC) not only influences the eupneic pattern of phrenic motor output but also modifies hypoxia-induced gasping in vivo by increasing gasp frequency. Although ionotropic EAA receptor activation in this region appears to be required for the generation of eupneic breathing, it remains to be determined whether similar activation is necessary for the production and/or expression of hypoxia-induced gasping. Therefore, we examined the effects of severe brain hypoxia before and after blockade of ionotropic EAA receptors in the pre-BötC in eight chloralose-anesthetized, deafferented, mechanically ventilated cats. In each experiment, before blockade of ionotropic EAA receptors in the pre-BötC, severe brain hypoxia (6% O2 in a balance of N2 for 3-6 min) produced gasping. Although bilateral microinjection of the broad-spectrum ionotropic EAA receptor antagonist kynurenic acid (20-100 mM; 40 nl) into the pre-BötC eliminated basal phrenic nerve discharge, severe brain hypoxia still produced gasping. Under these conditions, however, the onset latency to gasping was increased ( P < 0.05), the number of gasps was reduced for the same duration of hypoxic gas exposure ( P < 0.05), the duration of gasps was prolonged ( P < 0.05), and the duration between gasps was increased ( P < 0.05). These findings demonstrate that hypoxia-induced gasping in vivo does not require activation of ionotropic EAA receptors in the pre-BötC, but ionotropic EAA receptor activation in this region may modify the expression of the hypoxia-induced response. The present findings also provide additional support for the pre-BötC as the primary locus of respiratory rhythm generation.

BDNF is necessary and sufficient for spinal respiratory plasticity following intermittent hypoxia

Intermittent hypoxia causes a form of serotonin-dependent synaptic plasticity in the spinal cord known as phrenic long-term facilitation (pLTF). Here we show that increased synthesis of brain-derived neurotrophic factor (BDNF) in the spinal cord is necessary and sufficient for pLTF in adult rats. We found that intermittent hypoxia elicited serotonin-dependent increases in BDNF synthesis in ventral spinal segments containing the phrenic nucleus, and the magnitude of these BDNF increases correlated with pLTF magnitude. We used RNA interference (RNAi) to interfere with BDNF expression, and tyrosine kinase receptor inhibition to block BDNF signaling. These disruptions blocked pLTF, whereas intrathecal injection of BDNF elicited an effect similar to pLTF. Our findings demonstrate new roles and regulatory mechanisms for BDNF in the spinal cord and suggest new therapeutic strategies for treating breathing disorders such as respiratory insufficiency after spinal injury. These experiments also illustrate the potential use of RNAi to investigate functional consequences of gene expression in the mammalian nervous system in vivo.

Two types of rhythm in the respiratory network output in the isolated ventrolateral medulla in the neonatal rats

Effects of substance P and extracellular [K(+)](o) on respiratory motor activity in the ventrolateral medulla in neonatal rat (0-4 days old) brainstem-spinal cord preparation were studied. In addition to fictive eupneic rhythm (8-13 bursts/minute), the respiratory motor output was composed of biphasic bursts which might underlie the sigh pattern in vivo. These bursts had considerably lower frequency (0.15-0.86 bursts/minute) and appeared when inspiratory neurons generated augmented biphasic discharges. The two rhythms were differently affected when the respiratory network excitability was increased by substance P or decreased by lowering external [K(+)](o), the effects on biphasic burst frequency being considerably greater. The augmented bursts could suppress inspiratory, but not pre-inspiratory neuron discharge, suggesting that pre-inspiratory neurons formed a supplementary rhythmic network which was not directly affected by biphasic burst generation.

Effect of domperidone on ventilation and polycythemia after 5 weeks of chronic hypoxia in rats

Chronically hypoxic humans and some mammals have attenuated ventilatory responses, which have been associated with high dopamine level in carotid bodies. Alveolar hypoventilation and blunted ventilatory response have been recognized to be at the basis of Chronic Mountain Sickness by generating arterial hypoxemia and polycythemia. To investigate whether dopamine antagonism could decrease the hemoglobin concentration by stimulating resting ventilation (VE) and/or hypoxic ventilatory response, 18 chronically hypoxic rats (5 weeks, PB=433 Torr) were studied with and without domperidone treatment (a peripheral dopamine antagonist). Acute and prolonged treatment significantly increased poikilocapnic ventilatory response to hypoxia (RVE ml/min/kg=VE at 0.1 FI(O(2))-VE at 0.21 FI(O(2))), from 506+/-36 to 697+/-48; and from 394+/-37 to 660+/-81, respectively. In addition, Domperidone treatment decreased hemoglobin concentration from 21.6+/-0.29 to 18.9+/-0.19 (P<0.01) in rats chronically exposed to hypobaric hypoxia. Our study suggests that the stimulant effect of D(2)-R blockade on ventilatory response to hypoxia seems to compensate the low hypoxic peripheral chemosensitivity after chronic exposure and the latter in turn decrease hemoglobin concentration.

Sympathetic neural overactivity in healthy humans after prolonged exposure to hypobaric hypoxia

Acute exposure to hypoxia causes chemoreflex activation of the sympathetic nervous system. During acclimatization to high altitude hypoxia, arterial oxygen content recovers, but it is unknown to what degree sympathetic activation is maintained or normalized during prolonged exposure to hypoxia. We therefore measured sympathetic nerve activity directly by peroneal microneurography in eight healthy volunteers (24 +/- 2 years of age) after 4 weeks at an altitude of 5260 m (Chacaltaya, Bolivian Andes) and at sea level (Copenhagen). The subjects acclimatized well to altitude, but in every subject sympathetic nerve activity was highly elevated at altitude vs. sea level (48 +/- 5 vs. 16 +/- 3 bursts min(-1), respectively, P < 0.05), coinciding with increased mean arterial blood pressure (87 +/- 3 vs. 77 +/- 2 mmHg, respectively, P < 0.05). To examine the underlying mechanisms, we administered oxygen (to eliminate chemoreflex activation) and saline (to reduce cardiopulmonary baroreflex deactivation). These interventions had minor effects on sympathetic activity (48 +/- 5 vs. 38 +/- 4 bursts min(-1), control vs. oxygen + saline, respectively, P < 0.05). Moreover, sympathetic activity was still markedly elevated (37 +/- 5 bursts min(-1)) when subjects were re-studied under normobaric, normoxic and hypervolaemic conditions 3 days after return to sea level. In conclusion, acclimatization to high altitude hypoxia is accompanied by a striking and long-lasting sympathetic overactivity. Surprisingly, chemoreflex activation by hypoxia and baroreflex deactivation by dehydration together could account for only a small part of this response, leaving the major underlying mechanisms unexplained.

Intermittent hypoxia induces phrenic long-term facilitation in carotid-denervated rats

Episodic hypoxia elicits a long-lasting augmentation of phrenic inspiratory activity known as long-term facilitation (LTF). We investigated the respective contributions of carotid chemoafferent neuron activation and hypoxia to the expression of LTF in urethane-anesthetized, vagotomized, paralyzed, and ventilated Sprague-Dawley rats. One hour after three 5-min isocapnic hypoxic episodes [arterial Po(2) (Pa(O(2))) = 40 +/- 5 Torr], integrated phrenic burst amplitude was greater than baseline in both carotid-denervated (n = 8) and sham-operated (n = 7) rats (P < 0.05), indicating LTF. LTF was reduced in carotid-denervated rats relative to sham (P < 0.05). In this and previous studies, rats were ventilated with hyperoxic gas mixtures (inspired oxygen fraction = 0.5) under baseline conditions. To determine whether episodic hyperoxia induces LTF, phrenic activity was recorded under normoxic (Pa(O(2)) = 90-100 Torr) conditions before and after three 5-min episodes of isocapnic hypoxia (Pa(O(2)) = 40 +/- 5 Torr; n = 6) or hyperoxia (Pa(O(2)) > 470 Torr; n = 6). Phrenic burst amplitude was greater than baseline 1 h after episodic hypoxia (P < 0.05), but episodic hyperoxia had no detectable effect. These data suggest that hypoxia per se initiates LTF independently from carotid chemoafferent neuron activation, perhaps through direct central nervous system effects.

The murine neurokinin NK1 receptor gene contributes to the adult hypoxic facilitation of ventilation

Substance P and neurokinin-1 receptors (NK1) modulate the respiratory activity and are expressed early during development. We tested the hypothesis that NK1 receptors are involved in prenatal development of the respiratory network by comparing the resting respiratory activity and the respiratory response to hypoxia of control mice and mutant mice lacking the NK1 receptor (NK1-/-). In vitro and in vivo experiments were conducted on neonatal, young and adult mice from wild-type and NK1-/- strains. In the wild strain, immunohistological, pharmacological and electrophysiological studies showed that NK1 receptors were expressed within medullary respiratory areas prior to birth and that their activation at birth modulated central respiratory activity and the membrane properties of phrenic motoneurons. Both the membrane properties of phrenic motoneurons and the respiratory activity generated in vitro by brainstem-spinal cord preparation from NK1-/- neonate mice were similar to that from the wild strain. In addition, in vivo ventilation recordings by plethysmography did not reveal interstrain differences in resting breathing parameters. The facilitation of ventilation by short-lasting hypoxia was similar in wild and NK1-/- neonates but was significantly weaker in adult NK1-/- mice. Results demonstrate that NK1 receptors do appear to be necessary for a normal respiratory response to short-lasting hypoxia in the adult. However, NK1 receptors are not obligatory for the prenatal development of the respiratory network, for the production of the rhythm, or for the regulation of breathing by short-lasting hypoxia in neonates.

Neurogenesis of gasping does not require inhibitory transmission using GABA(A) or glycine receptors

We evaluated the hypothesis that the neurogenesis of gasping is not dependent upon inhibitory synaptic transmission involving GABA(A) or glycine receptors. Activity of the phrenic nerve was recorded in a perfused juvenile rat preparation. The pattern of phrenic activity was altered from eupnea to gasping in severe hypoxia or ischaemia. To block GABA(A) receptors, bicuculline or picrotoxin was administered. Strychnine was used to block transmission by glycine. Following administrations of bicuculline, picrotoxin or strychnine, the eupneic rhythm was greatly distorted whereas the decrementing pattern of the gasp was maintained. At high concentrations of these antagonists, the frequency of gasps was increased and the peak height of gasps fell. We conclude that the neurogenesis of gasping is not dependent upon fast, chloride-mediated inhibitory synaptic transmission.

The process of cardiorespiratory autoresuscitation in intact newborn rats

To examine the process of spontaneous autoresuscitation and the recovery of the hypoxic ventilatory response (HVR) after prolonged anoxia, we monitored respiratory frequency (f, by body plethysmography) and heart rate (HR, by ECG) in intact newborn rats (n = 12, day 2–4) before, during, and after 100% N2 exposure. The rat before anoxia showed signs of HVR: f changes at acute hypoxia (10% O2) and hyperoxia (100% O2). During anoxia, the spontaneous respiratory movement "gasping" appeared for 21 min (mean). At O2 restoration (with 100% O2), gasping stopped and no respiratory flow was detected for 1 min. One rat failed to autoresuscitate and had heart arrhythmia during the transient apnea, but 11 rats recovered respiration after the HR acceleration. Despite the successful autoresuscitation, the rats did not show HVR at 10 min into the recovery period and the recovery of HVR required more than 30 min. The results indicate that O2 inhalation is useful to trigger autoresuscitation even when the rat has already been in a state of profound hypoxic depression, but the rat becomes transiently insensitive to HVR after autoresuscitation. We estimate that reform of the respiratory control system in newborn rats is not yet firmly established to track HVR early in the recovery phase after prolonged anoxia.Key words: anoxia, hypoxic ventilatory response, cardiopulmonary resuscitation (CPR), sudden infant death (SID).