Matthew E. Pamenter

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.

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.

Mitochondrial responses to prolonged anoxia in brain of red-eared slider turtles

Mitochondria are central to aerobic energy production and play a key role in neuronal signalling. During anoxia, however, the mitochondria of most vertebrates initiate deleterious cell death cascades. Nonetheless, a handful of vertebrate species, including some freshwater turtles, are remarkably tolerant of low oxygen environments and survive months of anoxia without apparent damage to brain tissue. This tolerance suggests that mitochondria in the brains of such species are adapted to withstand prolonged anoxia, but little is known about potential neuroprotective responses. In this study, we address such mechanisms by comparing mitochondrial function between brain tissues isolated from cold-acclimated red-eared slider turtles (Trachemys scripta elegans) exposed to two weeks of either normoxia or anoxia. We found that brain mitochondria from anoxia-acclimated turtles exhibited a unique phenotype of remodelling relative to normoxic controls, including: (i) decreased citrate synthase and F1FO-ATPase activity but maintained protein content, (ii) markedly reduced aerobic capacity, and (iii) mild uncoupling of the mitochondrial proton gradient. These data suggest that turtle brain mitochondria respond to low oxygen stress with a unique suite of changes tailored towards neuroprotection.

Naked mole rat brain mitochondria electron transport system flux and H+ leak are reduced during acute hypoxia

ABSTRACT Mitochondrial respiration and ATP production are compromised by hypoxia. Naked mole rats (NMRs) are among the most hypoxia-tolerant mammals and reduce metabolic rate in hypoxic environments; however, little is known regarding mitochondrial function during in vivo hypoxia exposure in this species. To address this knowledge gap, we asked whether the function of NMR brain mitochondria exhibits metabolic plasticity during acute hypoxia. Respirometry was utilized to assess whole-animal oxygen consumption rates and high-resolution respirometry was utilized to assess electron transport system (ETS) function in saponin-permeabilized NMR brain. We found that NMR whole-animal oxygen consumption rate reversibly decreased by ∼85% in acute hypoxia (4 h at 3% O2). Similarly, relative to untreated controls, permeabilized brain respiratory flux through the ETS was decreased by ∼90% in acutely hypoxic animals. Relative to carbonyl cyanide p-trifluoro-methoxyphenylhydrazone-uncoupled total ETS flux, this functional decrease was observed equally across all components of the ETS except for complex IV (cytochrome c oxidase), at which flux was further reduced, supporting a regulatory role for this enzyme during acute hypoxia. The maximum enzymatic capacities of ETS complexes I–V were not altered by acute hypoxia; however, the mitochondrial H+ gradient decreased in step with the decrease in ETS respiration. Taken together, our results indicate that NMR brain ETS flux and H+ leak are reduced in a balanced and regulated fashion during acute hypoxia. Changes in NMR mitochondrial metabolic plasticity mirror whole-animal metabolic responses to hypoxia. Summary: The function of naked mole rat brain mitochondria is downregulated during acute hypoxia and matches whole-animal metabolic rate depression.

Divergent behavioural responses to acute hypoxia between individuals and groups of naked mole rats

Most small rodents reduce energy demand in hypoxia via behavioural strategies. For example, animals may reduce their activity, and/or move to colder environments or alter huddling strategies to take advantage of anapyretical energy savings. Naked mole rats (NMRs) are among the most hypoxia tolerant mammals and are highly social; social interactions also have a significant impact on behaviour. Therefore, this species offers a fascinating model in which to study trade-offs between social interactions and energy conservation in hypoxia. We hypothesized that the need to conserve energy in hypoxia supersedes the impetus of sociality in this species and predicted that, in hypoxia, behaviour would not differ between individuals or groups of NMRs. To test this hypothesis, we placed awake, freely behaving NMRs, alone or in groups of 2 or 4, into a temperature-controlled apparatus and measured behavioural activity during 1 h each of normoxia (21% O2), acute hypoxia (7% O2), and normoxic recovery. We found that in normoxia, groups of 4 NMRs were significantly more active in all temperatures than were groups of 1-2 NMRs. When exposed to hypoxia, individual NMRs were ~50% less active and their speed was reduced relative to normoxic levels. Conversely, groups of 2 or 4 NMRs exhibited minor or insignificant decreases in time spent active and speed in hypoxia and huddling behaviour was not altered. Our findings suggest that social interactions influence behavioural strategies employed by NMRs in hypoxia.

Comparative insights into mitochondrial adaptations to anoxia in brain

Numerous diseases and pathologies impair the delivery of oxygen to brain, with rapid and deleterious consequences. For example, diseases related to systemic hypoxemia (e.g., chronic pulmonary disorders, cystic fibrosis), decreased oxygen carrying capacity of blood (e.g., anemia), or decreased transport (e.g., heart attack, stroke) can all reduce or entirely prevent the delivery of oxygen to brain cells, resulting in the initiation of programmed cell death pathways, necrosis, or excitotoxic cell death in brain (Pamenter, 2014). However, oxygen-limited environments are common on earth and many organisms naturally experience periods of intermittent or prolonged hypoxia or anoxia in their daily and/or annual life cycles (Bickler and Buck, 2007). This exposure exerts a strong selective pressure that has driven the evolution of a wide range of adaptations that are neuroprotective against low oxygen stress. The brains of such hypoxia-tolerant species therefore offer a working model of neuronal survival in the absence of oxygen and their study may inform the development of novel therapeutics to protect mammalian brain against diseases and pathologies related to hypoxia.

Neuroprotective Interactions Between Delta-Opioid Receptors and Glutamatergic Signaling Mediate Hypoxia-Tolerance in Brain

Delta-opioid receptors are a class of membrane proteins found throughout the nervous system. They have traditionally been associated with the transmission of sensations related to pain via neuromodulation of excitatory glutamatergic synaptic signaling. Research examining these interactions in nocireception and related anesthesia applications has demonstrated that delta-opioid receptors are capable of mediating glutamatergic signaling via both pre-synaptic and post-synaptic mechanisms. In addition to normal neurotransmission functions, derangements in glutamatergic signaling are also associated with pathological brain damage due to low oxygen stresses, such as hypoxia or ischemic stroke; delta-opioid receptors are capable of mediating neuroprotective responses to such stresses via the inhibition of deleterious excitatory glutamatergic signaling. Specifically, studies of the mechanisms of hypoxic or ischemic preconditioning have demonstrated that delta-opioid receptors are central triggers that mediate inducible neuroprotective mechanisms against acute hypoxic, ischemic, and glutamatergic stresses in mammalian brain, and recent evidence points to the modulation of glutamate receptors as a critical component of this neuroprotective mechanism. In addition, a recent study has demonstrated that endogenous activation of similar mechanisms contributes to the innate anoxia-tolerance of the brain of one of the most hypoxia-tolerant vertebrates identified—the Western Painted turtle, and that the mechanism of neuroprotection in this organism involves the direct inhibition of neuronal glutamatergic signaling at the post-synapse. This chapter will focus on the putative neuroprotective effects of delta-opioid receptor signaling in models of hypoxic preconditioning in mammal brain and also in glutamatergic channel arrest in turtle brain. Similarities between the underlying neuroprotective mechanisms against hypoxia and the mechanistic interactions in nocireception and analgesia will also be discussed.