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.
Anoxic insults cause hyperexcitability and cell death in mammalian neurons. Conversely, in anoxia-tolerant turtle brain, spontaneous electrical activity is suppressed by anoxia (i.e., spike arrest; SA) and cell death does not occur. The mechanism(s) of SA is unknown but likely involves GABAergic synaptic transmission, because GABA concentration increases dramatically in anoxic turtle brain. We investigated this possibility in turtle cortical neurons exposed to anoxia and/or GABA A/B receptor (GABAR) modulators. Anoxia increased endogenous slow phasic GABAergic activity, and both anoxia and GABA reversibly induced SA by increasing GABA A Rmediated postsynaptic activity and Cl − conductance, which eliminated the Cl − driving force by depolarizing membrane potential (∼8 mV) to GABA receptor reversal potential (∼−81 mV), and dampened excitatory potentials via shunting inhibition. In addition, both anoxia and GABA decreased excitatory postsynaptic activity, likely via GABA B R-mediated inhibition of presynaptic glutamate release. In combination, these mechanisms increased the stimulation required to elicit an action potential >20-fold, and excitatory activity decreased >70% despite membrane potential depolarization. In contrast, anoxic neurons cotreated with GABA A+B R antagonists underwent seizure-like events, deleterious Ca 2+ influx, and cell death, a phenotype consistent with excitotoxic cell death in anoxic mammalian brain. We conclude that increased endogenous GABA release during anoxia mediates SA by activating an inhibitory postsynaptic shunt and inhibiting presynaptic glutamate release. This represents a natural adaptive mechanism in which to explore strategies to protect mammalian brain from low-oxygen insults.western painted turtle | cerebral cortex | channel arrest | pyramidal neurons | natural anesthetic mechanism W hen deprived of oxygen, mammalian neurons are unable to produce sufficient ATP to meet cellular demands (1, 2). As a result, the Na + /K + ATPase (Na + pump) fails and neuronal membrane potential (V m ) becomes unsustainable and anoxic depolarization (AD) follows, causing electrical hyperexcitability, deleterious Ca 2+ influx, and spreading depression in the penumbral region (2, 3). Numerous studies have focused on the role of glutamatergic N-methyl-D-aspartate receptors (NMDARs) in this mechanism, and although NMDAR blockade prevents glutamatergic excitotoxicity (4), it does not prevent AD-mediated injury or postinsult apoptotic cell death (5). Thus, it is not surprising that clinical interventions targeting glutamate receptors alone have been largely ineffective against anoxic or ischemic damage (6), and therefore examination of alternative mechanisms to limit excitability during such insults is necessary.A potential therapeutic alternative to directly antagonizing excitatory pathways is to up-regulate inhibitory mechanisms such as those mediated by γ-aminobutyric acid (GABA), the primary inhibitory neurotransmitter in the mature mammalian CNS (7). GABAergic mechanisms are not strongly recr...
Naked mole-rats are long-lived animals that show unusual resistance to hypoxia, cancer and ageing. Protein deimination is an irreversible post-translational modification caused by the peptidylarginine deiminase (PAD) family of enzymes, which convert arginine into citrulline in target proteins. Protein deimination can cause structural and functional protein changes, facilitating protein moonlighting, but also leading to neo-epitope generation and effects on gene regulation. Furthermore, PADs have been found to regulate cellular release of extracellular vesicles (EVs), which are lipid-vesicles released from cells as part of cellular communication. EVs carry protein and genetic cargo and are indicative biomarkers that can be isolated from most body fluids. This study was aimed at profiling deiminated proteins in plasma and EVs of naked mole-rat. Key immune and metabolic proteins were identified to be post-translationally deiminated, with 65 proteins specific for plasma, while 42 proteins were identified to be deiminated in EVs only. Using protein-protein interaction network analysis, deiminated plasma proteins were found to belong to KEEG (Kyoto Encyclopedia of Genes and Genomes) pathways of immunity, infection, cholesterol and drug metabolism, while deiminated proteins in EVs were also linked to KEEG pathways of HIF-1 signalling and glycolysis. The mole-rat EV profiles showed a poly-dispersed population of 50–300 nm, similar to observations of human plasma. Furthermore, the EVs were assessed for three key microRNAs involved in cancer, inflammation and hypoxia. The identification of post-translational deimination of critical immunological and metabolic markers contributes to the current understanding of protein moonlighting functions, via post-translational changes, in the longevity and cancer resistance of naked mole-rats.
Key pointsr Ventilation increases more with chronic than acute hypoxia and does not return to control levels when normoxia is restored, indicating plasticity in the reflexes that control breathing.r Glutamate is the primary excitatory neurotransmitter between arterial chemoreceptors that sense hypoxia and neural circuits that control breathing in the brainstem.r We microinjected specific glutamate receptor antagonists into the brainstem of awake unrestrained rats and found NMDA-type glutamate receptors explain increased ventilatory sensitivity to hypoxia after chronic hypoxia. AMPA-type glutamate receptors mediate increased ventilatory drive in normoxia after chronic hypoxia, as well as increased ventilation in acute hypoxia after chronic hypoxia and in control conditions. r Phosphorylation of AMPA and NMDA receptors is increased by chronic hypoxia. r The results indicate that plasticity in different glutamate receptors have unique effects on the reflexes that control breathing in chronic hypoxia and may share cellular mechanisms with other models of neural plasticity.Abstract When exposed to a hypoxic environment the body's first response is a reflex increase in ventilation, termed the hypoxic ventilatory response (HVR). With chronic sustained hypoxia (CSH), such as during acclimatization to high altitude, an additional time-dependent increase in ventilation occurs, which increases the HVR. This secondary increase persists after exposure to CSH and involves plasticity within the circuits in the central nervous system that control breathing. Currently these mechanisms of HVR plasticity are unknown and we hypothesized that they involve glutamatergic synapses in the nucleus tractus solitarius (NTS), where afferent endings from arterial chemoreceptors terminate. To test this, we treated rats held in normoxia (CON) or 10% O 2 (CSH) for 7 days and measured ventilation in conscious, unrestrained animals before and after microinjecting glutamate receptor agonists and antagonists into the NTS. In normoxia, AMPA increased ventilation 25% and 50% in CON and CSH, respectively, while NMDA doubled ventilation in both groups (P < 0.05). Specific AMPA and NMDA receptor antagonists (NBQX and MK801, respectively) abolished these effects. MK801 significantly decreased the HVR in CON rats, and completely blocked the acute HVR in CSH rats but had no effect on ventilation in normoxia. NBQX decreased ventilation whenever it was increased relative to normoxic controls; i.e. acute hypoxia in CON and CSH, and normoxia in CSH. These results support our hypothesis that glutamate receptors in the NTS contribute to plasticity in the HVR with CSH. The mechanism underlying this synaptic plasticity is probably glutamate receptor modification, as in CSH rats the expression of phosphorylated NR1 and GluR1 proteins in the NTS increased 35% and 70%, respectively, relative to that in CON rats.
Naked mole‐rats (NMRs) are mouse‐sized mammals that exhibit an exceptionally long lifespan (>30 vs. <4 years for mice), and resist aging‐related pathologies such as cardiovascular and pulmonary diseases, cancer, and neurodegeneration. However, the mechanisms underlying this exceptional longevity and disease resistance remain poorly understood. The oxidative stress theory of aging posits that (a) senescence results from the accumulation of oxidative damage inflicted by reactive oxygen species (ROS) of mitochondrial origin, and (b) mitochondria of long‐lived species produce less ROS than do mitochondria of short‐lived species. However, comparative studies over the past 28 years have produced equivocal results supporting this latter prediction. We hypothesized that, rather than differences in ROS generation, the capacity of mitochondria to consume ROS might distinguish long‐lived species from short‐lived species. To test this hypothesis, we compared mitochondrial production and consumption of hydrogen peroxide (H2O2; as a proxy of overall ROS metabolism) between NMR and mouse skeletal muscle and heart. We found that the two species had comparable rates of mitochondrial H2O2 generation in both tissues; however, the capacity of mitochondria to consume ROS was markedly greater in NMRs. Specifically, maximal observed consumption rates were approximately two and fivefold greater in NMRs than in mice, for skeletal muscle and heart, respectively. Our results indicate that differences in matrix ROS detoxification capacity between species may contribute to their divergence in lifespan.
In severe hypoxia, most vertebrates increase anaerobic energy production, which results in the development of a metabolic acidosis and an O2 debt that must be repaid during reoxygenation. Naked mole rats (NMRs) are among the most hypoxia-tolerant mammals, capable of drastically reducing their metabolic rate in acute hypoxia; while staying active and alert. We hypothesized that a key component of remaining active is an increased reliance on anaerobic metabolism during severe hypoxia. To test this hypothesis, we exposed NMRs to progressive reductions in inspired O2 (9 to 3% O2) followed by reoxygenation (21% O2) and measured breathing frequency, heart rate, behavioural activity, body temperature, metabolic rate, and also metabolic substrates and pH in blood and tissues. We found that NMRs exhibit robust metabolic rate depression in acute hypoxia, accompanied by declines in all physiological and behavioural variables examined. However, blood and tissue pH were unchanged and tissue [ATP] and [phosphocreatine] were maintained. Naked mole rats increased their reliance on carbohydrates in hypoxia, and glucose was mobilized from the liver to the blood. Upon reoxygenation NMRs entered into a coma-like state for∼15-20 mins during which metabolic rate was negligible and body temperature remained suppressed. However, an imbalance in the rates at which V̇O2 and V̇CO2 returned to normoxic levels during reoxygenation hint at the possibility that NMRs do utilize anaerobic metabolism during hypoxia but have a tissue and/or blood buffering capacity that mask typical markers of metabolic acidosis, and prioritize the synthesis of glucose from lactate during recovery.
] c 8.9 ± 0.7% and 3.8 ± 0.3%, while decreasing normoxic whole-cell NMDAR currents by 41.1 ± 6.7% and 55.4 ± 10.2%, respectively. These changes were also blocked by 5HD or glibenclamide, BAPTA, or spermine. Blockade of mitochondrial Ca 2+ -uptake decreased normoxic NMDAR currents 47.0 ± 3.1% and this change was blocked by BAPTA but not by 5HD. Taken together, these data suggest mK ATP channel activation in the anoxic turtle cortex uncouples mitochondria and reduces mitochondrial Ca 2+ uptake via the uniporter, subsequently increasing [Ca 2+ ] c and decreasing NMDAR activity.
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