Critical oxygen tension in rat brain: a combined (31)P-NMR and EPR oximetry study

Isoflurane lowers the cerebral metabolic rate of oxygen and prevents hypoxia during cortical spreading depolarization in vitro: An integrative experimental and modeling study

Cortical spreading depolarization (SD) imposes a massive increase in energy demand and therefore evolves as a target for treatment following acute brain injuries. Anesthetics are empirically used to reduce energy metabolism in critical brain conditions, yet their effect on metabolism during SD remains largely unknown. We investigated oxidative metabolism during SD in brain slices from Wistar rats. Extracellular potassium ([K+]o), local field potential and partial tissue oxygen pressure (ptiO2) were measured simultaneously. The cerebral metabolic rate of oxygen (CMRO2) was calculated using a reaction-diffusion model. By that, we tested the effect of clinically relevant concentrations of isoflurane on CMRO2 during SD and modeled tissue oxygenation for different capillary pO2 values. During SD, CMRO2 increased 2.7-fold, resulting in transient hypoxia in the slice core. Isoflurane decreased CMRO2, reduced peak [K+]o, and prolonged [K+]o clearance, which indicates reduced synaptic transmission and sodium-potassium ATPase inhibition. Modeling tissue oxygenation during SD illustrates the need for increased capillary pO2 levels to prevent hypoxia. In the absence thereof, isoflurane could improve tissue oxygenation by lowering CMRO2. Therefore, isoflurane is a promising candidate for pre-clinical studies on neuronal survival in conditions involving SD.

Gypenosides (GPs) alleviates hypoxia-induced injury in PC12 cells and enhances tolerance to anoxia in C57BL/6 mice

Oxygen is a necessary substance for life activities, but reduced oxygen utilization due to high altitude exposure and respiratory dysfunction diseases could lead to pathological changes in the organisms. Herein gypenosides, the active ingredients in the food and medicine resource plant Gynostemma pentaphyllum (Thunb.) Makino were found to alleviate hypoxia-induced injury in PC12 cells. Moreover, hypoxia induced an increase in Ca²⁺ and reactive oxygen species content, and such patterns were both significantly reduced by gypenosides treatment. At the same time, gypenosides significantly blocked the decrease of both NO content and mitochondrial membrane potential caused by hypoxia. Furthermore, gypenosides gavage treatment significantly prolonged the survival time of C57BL/6 mice in confinement up to 24.3% and enhanced the locomotor ability of mice. Therefore, gypenosides have good neuroprotective effects and hypoxia tolerance activity and have the prospect of being developed as a preventive and therapeutic drug for hypoxia-related diseases. PRACTICAL APPLICATIONS: Gypenosides can enhance tolerance of cells and mice to hypoxia and have the potential to be developed into hypoxia-resistant health food and drugs.

Brain tissue oxygen dynamics while mimicking the functional deficiency of interneurons

The dynamic interaction between excitatory and inhibitory activity in the brain is known as excitatory-inhibitory balance (EIB). A significant shift in EIB toward excitation has been observed in numerous pathological states and diseases, such as autism or epilepsy, where interneurons may be dysfunctional. The consequences of this on neurovascular interactions remains to be elucidated. Specifically, it is not known if there is an elevated metabolic consumption of oxygen due to increased excitatory activity. To investigate this, we administered microinjections of picrotoxin, a gamma aminobutyric acid (GABA) antagonist, to the rabbit cortex in the awake state to mimic the functional deficiency of GABAergic interneurons. This caused an observable shift in EIB toward excitation without the induction of seizures. We used chronically implanted electrodes to measure both neuronal activity and brain tissue oxygen concentrations (PO₂) simultaneously and in the same location. Using a high-frequency recording rate for PO₂, we were able to detect two important phenomena, (1) the shift in EIB led to a change in the power spectra of PO₂ fluctuations, such that higher frequencies (8-15 cycles per minute) were suppressed and (2) there were brief periods (dips with a duration of less than 100 ms associated with neuronal bursts) when PO₂ dropped below 10 mmHg, which we defined as the threshold for hypoxia. The dips were followed by an overshoot, which indicates either a rapid vascular response or decrease in oxygen consumption. Our results point to the essential role of interneurons in brain tissue oxygen regulation in the resting state.

Cognitive impairment caused by hypoxia: from clinical evidences to molecular mechanisms

Hypoxia is a state of reduced oxygen supply and excessive oxygen consumption. According to the duration of hypoxic period, it can be classified as acute and chronic hypoxia. Both acute and chronic hypoxia could induce abundant neurological deficits. Although there have been significant advances in the pathophysiological injuries, few studies have focused on the cognitive dysfunction. In this review, we focused on the clinical evidences and molecular mechanisms of cognitive impairment under acute and chronic hypoxia. Hypoxia can impair several cognitive domains such as attention, learning and memory, procession speed and executive function, which are similar in acute and chronic hypoxia. The severity of cognitive deficit correlates with the duration and degree of hypoxia. Recovery can be achieved after acute hypoxia, while sequelae or even dementia can be observed after chronic hypoxia, perhaps due to the different molecular mechanisms. Cardiopulmonary compensatory response, glycolysis, oxidative stress, calcium overload, adenosine, mitochondrial disruption, inflammation and excitotoxicity contribute to the molecular mechanisms of cognitive deficit after acute hypoxia. During the chronic stage of hypoxia, different adaptive responses, impaired neurovascular coupling, apoptosis, transcription factors-mediated inflammation, as well as Aβ accumulation and tau phosphorylation account for the neurocognitive deficit. Moreover, brain structural changes with hippocampus and cortex atrophy, ventricle enlargement, senile plaque and neurofibrillary tangle deposition can be observed under chronic hypoxia rather than acute hypoxia.

Early Development of the GABAergic System and the Associated Risks of Neonatal Anesthesia

Human and animal studies have elucidated the apparent neurodevelopmental effects resulting from neonatal anesthesia. Observations of learning and behavioral deficits in children, who were exposed to anesthesia early in development, have instigated a flurry of studies that have predominantly utilized animal models to further interrogate the mechanisms of neonatal anesthesia-induced neurotoxicity. Specifically, while neonatal anesthesia has demonstrated its propensity to affect multiple cell types in the brain, it has shown to have a particularly detrimental effect on the gamma aminobutyric acid (GABA)ergic system, which contributes to the observed learning and behavioral deficits. The damage to GABAergic neurons, resulting from neonatal anesthesia, seems to involve structure-specific changes in excitatory-inhibitory balance and neurovascular coupling, which manifest following a significant interval after neonatal anesthesia exposure. Thus, to better understand how neonatal anesthesia affects the GABAergic system, we first review the early development of the GABAergic system in various structures that have been the focus of neonatal anesthesia research. This is followed by an explanation that, due to the prolonged developmental curve of the GABAergic system, the entirety of the negative effects of neonatal anesthesia on learning and behavior in children are not immediately evident, but instead take a substantial amount of time (years) to fully develop. In order to address these concerns going forward, we subsequently offer a variety of in vivo methods which can be used to record these delayed effects.

Oxidative Stress and Energy Metabolism in the Brain: Midlife as a Turning Point

Neural tissue is one of the main oxygen consumers in the mammalian body, and a plentitude of metabolic as well as signaling processes within the brain is accompanied by the generation of reactive oxygen (ROS) and nitrogen (RNS) species. Besides the important signaling roles, both ROS and RNS can damage/modify the self-derived cellular components thus promoting neuroinflammation and oxidative stress. While previously, the latter processes were thought to progress linearly with age, newer data point to midlife as a critical turning point. Here, we describe (i) the main pathways leading to ROS/RNS generation within the brain, (ii) the main defense systems for their neutralization and (iii) summarize the recent literature about considerable changes in the energy/ROS homeostasis as well as activation state of the brain's immune system at midlife. Finally, we discuss the role of calorie restriction as a readily available and cost-efficient antiaging and antioxidant lifestyle intervention.

A Review of Low-Frequency EPR Technology for the Measurement of Brain pO2 and Oxidative Stress

EPR can uniquely measure paramagnetic species. Although commercial EPR was introduced in 1950s, the early studies were mostly restricted to chemicals in solution or cellular experiments using X-band EPR equipment. Due to its limited penetration (<1 mm), experiments with living animals were almost impossible. To overcome these difficulties, Swartz group, along with several other leaders in field, pioneered the technology of low frequency EPR (e.g., L-band, 1-2 GHz). The development of low frequency EPR and the associated probes have dramatically expanded the application of EPR technology into the biomedical research field, providing answers to important scientific questions by measuring specific parameters that are impossible or very difficult to obtain by other approaches. In this review, which is aimed at highlighting the seminal contribution from Swartz group over the last several decades, we will focus on the development of EPR technology that was designed to deal with the potential challenges arising from conducting EPR spectroscopy in living animals. The second half of the review will be concentrated on the application of low frequency EPR in measuring cerebral tissue pO₂ changes and oxidative stress in various physiological and pathophysiological conditions in the brain of animal disease models.

The brains of six African mole-rat species show divergent responses to hypoxia

Mole-rats are champions of self-preservation, with increased longevity compared with other rodents their size, strong antioxidant capabilities and specialized defenses against endogenous oxidative stress. However, how the brains of these subterranean mammals handle acute in vivo hypoxia is poorly understood. This study is the first to examine the molecular response to low oxygen in six different species of hypoxia-tolerant mole-rats from sub-Saharan Africa. Protein carbonylation, a known marker of DNA damage (hydroxy-2'-deoxyguanosine), and antioxidant capacity did not change following hypoxia but HIF-1 protein levels increased significantly in the brains of two species. Nearly 30 miRNAs known to play roles in hypoxia tolerance were differentially regulated in a species-specific manner. The miRNAs exhibiting the strongest response to low oxygen stress inhibit apoptosis and regulate neuroinflammation, likely providing neuroprotection. A principal component analysis (PCA) using a subset of the molecular targets assessed herein revealed differences between control and hypoxic groups for two solitary species (Georychus capensis and Bathyergus suillus), which are ecologically adapted to a normoxic environment, suggesting a heightened sensitivity to hypoxia relative to species that may experience hypoxia more regularly in nature. By contrast, all molecular data were included in the PCA to detect a difference between control and hypoxic populations of eusocial Heterocephalus glaber, indicating they may require many lower-fold changes in signaling pathways to adapt to low oxygen settings. Finally, none of the Cryptomys hottentotus subspecies showed a statistical difference between control and hypoxic groups, presumably due to hypoxia tolerance derived from environmental pressures associated with a subterranean and social lifestyle.

Small Vessel Disease-Related Dementia: An Invalid Neurovascular Coupling?

The arteriosclerosis-dependent alteration of brain perfusion is one of the major determinants in small vessel disease, since small vessels have a pivotal role in the brain's autoregulation. Nevertheless, as far as we know, endothelium distress can potentiate the flow dysregulation and lead to subcortical vascular dementia that is related to small vessel disease (SVD), also being defined as subcortical vascular dementia (sVAD), as well as microglia activation, chronic hypoxia and hypoperfusion, vessel-tone dysregulation, altered astrocytes, and pericytes functioning blood-brain barrier disruption. The molecular basis of this pathology remains controversial. The apparent consequence (or a first event, too) is the macroscopic alteration of the neurovascular coupling. Here, we examined the possible mechanisms that lead a healthy aging process towards subcortical dementia. We remarked that SVD and white matter abnormalities related to age could be accelerated and potentiated by different vascular risk factors. Vascular function changes can be heavily influenced by genetic and epigenetic factors, which are, to the best of our knowledge, mostly unknown. Metabolic demands, active neurovascular coupling, correct glymphatic process, and adequate oxidative and inflammatory responses could be bulwarks in defense of the correct aging process; their impairments lead to a potentially catastrophic and non-reversible condition.

Local oxygen homeostasis during various neuronal network activity states in the mouse hippocampus

Cortical information processing comprises various activity states emerging from timed synaptic excitation and inhibition. However, the underlying energy metabolism is widely unknown. We determined the cerebral metabolic rate of oxygen (CMRO₂) along a tissue depth of <0.3 mm in the hippocampal CA3 region during various network activities, including gamma oscillations and sharp wave-ripples that occur during wakefulness and sleep. These physiological states associate with sensory perception and memory formation, and critically depend on perisomatic GABA inhibition. Moreover, we modelled vascular oxygen delivery based on quantitative microvasculature analysis. (1) Local CMRO₂ was highest during gamma oscillations (3.4 mM/min), medium during sharp wave-ripples, asynchronous activity and isoflurane application (2.0-1.6 mM/min), and lowest during tetrodotoxin application (1.4 mM/min). (2) Energy expenditure of axonal and synaptic signaling accounted for >50% during gamma oscillations. (3) CMRO₂ positively correlated with number and synchronisation of activated synapses, and neural multi-unit activity. (4) The median capillary distance was 44 µm. (5) The vascular oxygen partial pressure of 33 mmHg was needed to sustain oxidative phosphorylation during gamma oscillations. We conclude that gamma oscillations featuring high energetics require a hemodynamic response to match oxygen consumption of respiring mitochondria, and that perisomatic inhibition significantly contributes to the brain energy budget.

Brain Energy and Oxygen Metabolism: Emerging Role in Normal Function and Disease

Dynamic metabolic changes occurring in neurons are critically important in directing brain plasticity and cognitive function. In other tissue types, disruptions to metabolism and the resultant changes in cellular oxidative state, such as increased reactive oxygen species (ROS) or induction of hypoxia, are associated with cellular stress. In the brain however, where drastic metabolic shifts occur to support physiological processes, subsequent changes to cellular oxidative state and induction of transcriptional sensors of oxidative stress likely play a significant role in regulating physiological neuronal function. Understanding the role of metabolism and metabolically-regulated genes in neuronal function will be critical in elucidating how cognitive functions are disrupted in pathological conditions where neuronal metabolism is affected. Here, we discuss known mechanisms regulating neuronal metabolism as well as the role of hypoxia and oxidative stress during normal and disrupted neuronal function. We also summarize recent studies implicating a role for metabolism in regulating neuronal plasticity as an emerging neuroscience paradigm.

Comparative Response of Brain to Chronic Hypoxia and Hyperoxia

Two antithetic terms, hypoxia and hyperoxia, i.e., insufficient and excess oxygen availability with respect to needs, are thought to trigger opposite responses in cells and tissues. This review aims at summarizing the molecular and cellular mechanisms underlying hypoxia and hyperoxia in brain and cerebral tissue, a context that may prove to be useful for characterizing not only several clinically relevant aspects, but also aspects related to the evolution of oxygen transport and use by the tissues. While the response to acute hypoxia/hyperoxia presumably recruits only a minor portion of the potentially involved cell machinery, focusing into chronic conditions, instead, enables to take into consideration a wider range of potential responses to oxygen-linked stress, spanning from metabolic to genic. We will examine how various brain subsystems, including energetic metabolism, oxygen sensing, recruitment of pro-survival pathways as protein kinase B (Akt), mitogen-activated protein kinases (MAPK), neurotrophins (BDNF), erythropoietin (Epo) and its receptors (EpoR), neuroglobin (Ngb), nitric oxide (NO), carbon monoxide (CO), deal with chronic hypoxia and hyperoxia to end-up with the final outcomes, oxidative stress and brain damage. A more complex than expected pattern results, which emphasizes the delicate balance between the severity of the stress imposed by hypoxia and hyperoxia and the recruitment of molecular and cellular defense patterns. While for certain functions the expectation that hypoxia and hyperoxia should cause opposite responses is actually met, for others it is not, and both emerge as dangerous treatments.

Hyperoxia results in increased aerobic metabolism following acute brain injury

Acute brain injury is associated with depressed aerobic metabolism. Below a critical mitochondrial pO₂ cytochrome c oxidase, the terminal electron acceptor in the mitochondrial respiratory chain, fails to sustain oxidative phosphorylation. After acute brain injury, this ischaemic threshold might be shifted into apparently normal levels of tissue oxygenation. We investigated the oxygen dependency of aerobic metabolism in 16 acutely brain-injured patients using a 120-min normobaric hyperoxia challenge in the acute phase (24-72 h) post-injury and multimodal neuromonitoring, including transcranial Doppler ultrasound-measured cerebral blood flow velocity, cerebral microdialysis-derived lactate-pyruvate ratio (LPR), brain tissue pO₂ (p_brO₂), and tissue oxygenation index and cytochrome c oxidase oxidation state (oxCCO) measured using broadband spectroscopy. Increased inspired oxygen resulted in increased p_brO₂ [Δp_brO₂ 30.9 mmHg p < 0.001], reduced LPR [ΔLPR -3.07 p = 0.015], and increased cytochrome c oxidase (CCO) oxidation (Δ[oxCCO] + 0.32 µM p < 0.001) which persisted on return-to-baseline (Δ[oxCCO] + 0.22 µM, p < 0.01), accompanied by a 7.5% increase in estimated cerebral metabolic rate for oxygen ( p = 0.038). Our results are consistent with an improvement in cellular redox state, suggesting oxygen-limited metabolism above recognised ischaemic p_brO₂ thresholds. Diffusion limitation or mitochondrial inhibition might explain these findings. Further investigation is warranted to establish optimal oxygenation to sustain aerobic metabolism after acute brain injury.

Experimental Models Combining Traumatic Brain Injury and Hypoxia

Traumatic brain injury (TBI) is one of the most common causes of death and disability, and cerebral hypoxia is a frequently occurring harmful secondary event in TBI patients. The hypoxic conditions that occur on the scene of accident, where the airways are often obstructed or breathing is in other ways impaired, could be reproduced using animal TBI models where oxygen delivery is strictly controlled throughout the entire experimental procedure. Monitoring physiological parameters of the animal is of utmost importance in order to maintain an adequate quality of the experiment. Peripheral oxygen saturation, O2 pressure (pO2) in the blood, or fraction of inhaled O2 (FiO2) could be used as goals to validate the hypoxic conditions. Different models of traumatic brain injury could be used to inflict desired injury type, whereas effects then could be studied using radiological, physiological and functional tests. In order to confirm that the brain has been affected by a hypoxic injury, appropriate substances in the affected cerebral tissue, cerebrospinal fluid, or serum should be analyzed.

Application of Electron Paramagnetic Resonance (EPR) Oximetry to Monitor Oxygen in Wounds in Diabetic Models

A lack of oxygen is classically described as a major cause of impaired wound healing in diabetic patients. Even if the role of oxygen in the wound healing process is well recognized, measurement of oxygen levels in a wound remains challenging. The purpose of the present study was to assess the value of electron paramagnetic resonance (EPR) oximetry to monitor pO2 in wounds during the healing process in diabetic mouse models. Kinetics of wound closure were carried out in streptozotocin (STZ)-treated and db/db mice. The pO2 was followed repeatedly during the healing process by 1 GHz EPR spectroscopy with lithium phthalocyanine (LiPc) crystals used as oxygen sensor in two different wound models: a full-thickness excisional skin wound and a pedicled skin flap. Wound closure kinetics were dramatically slower in 12-week-old db/db compared to control (db/+) mice, whereas kinetics were not statistically different in STZ-treated compared to control mice. At the center of excisional wounds, measurements were highly influenced by atmospheric oxygen early in the healing process. In pedicled flaps, hypoxia was observed early after wounding. While reoxygenation occurred over time in db/+ mice, hypoxia was prolonged in the diabetic db/db model. This observation was consistent with impaired healing and microangiopathies observed using intravital microscopy. In conclusion, EPR oximetry using LiPc crystals as the oxygen sensor is an appropriate technique to follow wound oxygenation in acute and chronic wounds, in normal and diabetic animals. Nevertheless, the technique is limited for measurements in pedicled skin flaps and cannot be applied to excisional wounds in which diffusion of atmospheric oxygen significantly affects the measurements.

Electron paramagnetic resonance: a powerful tool to support magnetic resonance imaging research

The purpose of this paper is to describe some of the areas where electron paramagnetic resonance (EPR) has provided unique information to MRI developments. The field of application mainly encompasses the EPR characterization of MRI paramagnetic contrast agents (gadolinium and manganese chelates, nitroxides) and superparamagnetic agents (iron oxide particles). The combined use of MRI and EPR has also been used to qualify or disqualify sources of contrast in MRI. Illustrative examples are presented with attempts to qualify oxygen sensitive contrast (i.e. T1 - and T2 *-based methods), redox status or melanin content in tissues. Other areas are likely to benefit from the combined EPR/MRI approach, namely cell tracking studies. Finally, the combination of EPR and MRI studies on the same models provides invaluable data regarding tissue oxygenation, hemodynamics and energetics. Our description will be illustrative rather than exhaustive to give to the readers a flavour of 'what EPR can do for MRI'.

A tale of two methods: combining near-infrared spectroscopy with MRI for studies of brain oxygenation and metabolism

Combining magnetic resonance imaging (MRI) with near-infrared spectroscopy (NIRS) leads to excellent synergies which can improve the interpretation of either method and can provide novel data with respect to measuring brain oxygenation and metabolism. MRI has good spatial resolution, can detect a range of physiological parameters and is sensitive to changes in deoxyhemoglobin content. NIRS has lower spatial resolution, but can detect, and with specific technologies, quantify, deoxyhemoglobin, oxyhemoglobin, total hemoglobin and cytochrome oxidase. This paper reviews the application of both methods, as a multimodal technology, for assessing changes in brain oxygenation that may occur with changes in functional activation state or metabolic rate. Examples of hypoxia and ischemia are shown. Data support the concept of reduced metabolic rate resulting from hypoxia/ischemia and that metabolic rate in brain is not close to oxygen limitation during normoxia. We show that multimodal MRI and NIRS can provide novel information for studies of brain metabolism.

Hypoxia-induced inflammation in the lung: a potential therapeutic target in acute lung injury?

Acute lung injury (ALI) is a severe form of hypoxic lung disease responsible for a large number of deaths worldwide. Despite recent advances in supportive care, no reduction in mortality has been evident since the introduction of a standard consensus definition almost two decades ago. New strategies are urgently required to help design effective therapies for this condition. A key pathological feature of ALI involves regional alveolar hypoxia. Because alveolar hypoxia in isolation, such as that encountered at high altitude, causes an inflammatory pulmonary phenotype in the absence of any other pathogenic stimuli, these regions may not be passive bystanders but may actually contribute to the pathogenesis and progression of lung injury. Unique transcriptional responses to hypoxia in the lung apparently allow it to express an inflammatory phenotype at levels of hypoxia that would not produce such a response in other organs. We will review recent advances in our understanding of these unique transcriptional responses to moderate levels of alveolar hypoxia, which may provide new insights into the pathogenesis of ALI.

Modulation of network pacemaker neurons by oxygen at the anaerobic threshold

Previous in vitro and in vivo studies showed that the frequency of rhythmic pyloric network activity in the lobster is modulated directly by oxygen partial pressure (PO(2)). We have extended these results by (1) increasing the period of exposure to low PO(2) and by (2) testing the sensitivity of the pyloric network to changes in PO(2) that are within the narrow range normally experienced by the lobster (1 to 6 kPa). We found that the pyloric network rhythm was indeed altered by changes in PO(2) within the range typically observed in vivo. Furthermore, a previous study showed that the lateral pyloric constrictor motor neuron (LP) contributes to the O(2) sensitivity of the pyloric network. Here, we expanded on this idea by testing the hypothesis that pyloric pacemaker neurons also contribute to pyloric O(2) sensitivity. A 2-h exposure to 1 kPa PO(2), which was twice the period used previously, decreased the frequency of an isolated group of pacemaker neurons, suggesting that changes in the rhythmogenic properties of these cells contribute to pyloric O(2) sensitivity during long-term near-anaerobic (anaerobic threshold, 0.7-1.2 kPa) conditions.

Notch4 normalization reduces blood vessel size in arteriovenous malformations

Abnormally enlarged blood vessels underlie many life-threatening disorders including arteriovenous (AV) malformations (AVMs). The core defect in AVMs is high-flow AV shunts, which connect arteries directly to veins, "stealing" blood from capillaries. Here, we studied mouse brain AV shunts caused by up-regulation of Notch signaling in endothelial cells (ECs) through transgenic expression of constitutively active Notch4 (Notch4*). Using four-dimensional two-photon imaging through a cranial window, we found that normalizing Notch signaling by repressing Notch4* expression converted large-caliber, high-flow AV shunts to capillary-like vessels. The structural regression of the high-flow AV shunts returned blood to capillaries, thus reversing tissue hypoxia. This regression was initiated by vessel narrowing without the loss of ECs and required restoration of EphB4 receptor expression by venous ECs. Normalization of Notch signaling resulting in regression of high-flow AV shunts, and a return to normal blood flow suggests that targeting the Notch pathway may be useful therapeutically for treating diseases such as AVMs.

Cerebral oxygenation in awake rats during acclimation and deacclimation to hypoxia: an in vivo electron paramagnetic resonance study

Exposure to high altitude or hypobaric hypoxia results in a series of metabolic, physiologic, and genetic changes that serve to acclimate the brain to hypoxia. Tissue Po(2) (Pto(2)) is a sensitive index of the balance between oxygen delivery and utilization and can be considered to represent the summation of such factors as cerebral blood flow, capillary density, hematocrit, arterial Po(2), and metabolic rate. As such, it can be used as a marker of the extent of acclimation. We developed a method using electron paramagnetic resonance (EPR) to measure Pto(2) in unanesthetized subjects with a chronically implanted sensor. EPR was used to measure rat cortical tissue Pto(2) in awake rats during acute hypoxia and over a time course of acclimation and deacclimation to hypobaric hypoxia. This was done to simulate the effects on brain Pto(2) of traveling to altitude for a limited period. Acute reduction of inspired O(2) to 10% caused a decline from 26.7 ± 2.2 to 13.0 ± 1.5 mmHg (mean ± SD). Addition of 10% CO(2) to animals breathing 10% O(2) returned Pto(2) to values measured while breathing 21% O(2,) indicating that hypercapnia can reverse the effects of acute hypoxia. Pto(2) in animals acclimated to 10% O(2) was similar to that measured preacclimation when breathing 21% O(2). Using a novel, individualized statistical model, it was shown that the T(1/2) of the Pto(2) response during exposure to chronic hypoxia was approximately 2 days. This indicates a capacity for rapid adaptation to hypoxia. When subjects were returned to normoxia, there was a transient hyperoxygenation, followed by a return to lower values with a T(1/2) of deacclimation of 1.5 to 3 days. These data indicate that exposure to hypoxia results in significant improvements in steady-state oxygenation for a given inspired O(2) and that both acclimation and deacclimation can occur within days.

Redox reactions of the α-synuclein-Cu(2+) complex and their effects on neuronal cell viability

α-Synuclein (α-syn), a presynaptic protein believed to play an important role in neuropathology in Parkinson's disease (PD), is known to bind Cu(2+). Cu(2+) has been shown to accelerate the aggregation of α-syn to form various toxic aggregates in vitro. Copper is also a redox-active metal whose complexes with amyloidogenic proteins/peptides have been linked to oxidative stress in major neurodegenerative diseases. In this work, the formation of the Cu(2+) complex with α-syn or with an N-terminal peptide, α-syn(1-19), was confirmed with electrospray-mass spectrometry (ES-MS). The redox potentials of the Cu(2+) complex with α-syn (α-syn-Cu(2+)) and α-syn(1-19) were determined to be 0.018 and 0.053 V, respectively. Furthermore, the Cu(2+) center(s) can be readily reduced to Cu(+), and possible reactions of α-syn-Cu(2+) with cellular species (e.g., O(2), ascorbic acid, and dopamine) were investigated. The occurrence of a redox reaction can be rationalized by comparing the redox potential of the α-syn-Cu(2+) complex to that of the specific cellular species. For example, ascorbic acid can directly reduce α-syn-Cu(2+) to α-syn-Cu(+), setting up a redox cycle in which O(2) is reduced to H(2)O(2) and cellular redox species is continuously exhausted. In addition, the H(2)O(2) generated was demonstrated to reduce viability of the neuroblastoma SY-HY5Y cells. Although our results ruled out the direct oxidation of dopamine by α-syn-Cu(2+), the H(2)O(2) generated in the presence of α-syn-Cu(2+) can oxidize dopamine. Our results suggest that oxidative stress is at least partially responsible for the loss of dopaminergic cells in PD brain and reveal the multifaceted role of the α-syn-Cu(2+) complex in oxidative stress associated with PD symptoms.

A method for measuring brain partial pressure of oxygen in unanesthetized unrestrained subjects: the effect of acute and chronic hypoxia on brain tissue PO(2)

The level of tissue oxygenation provides information related to the balance between oxygen delivery, oxygen utilization, tissue reactivity and morphology during physiological conditions. Tissue partial pressure of oxygen (PtO(2)) is influenced by the use of anesthesia or restraint. These factors may impact the absolute level of PtO(2). In this study we present a novel fiber optic method to measure brain PtO(2). This method can be used in unanesthetized, unrestrained animals, provides absolute values for PO(2), has a stable calibration, does not consume oxygen and is MRI compatible. Brain PtO(2) was studied during acute hypoxia, as well as before and after 28 days of high altitude acclimatization. A sensor was chronically implanted in the frontal cortex of eight Wistar rats. It is comprised of a fiber optic probe with a tip containing material that fluoresces with an oxygen dependent lifetime. Brain PtO(2) declines by 80% and 76% pre- and post-acclimatization, respectively, when the fraction of inspired oxygen declines from 0.21 to 0.08. In addition, a linear relationship between brain PtO(2) and inspired O(2) levels was demonstrated r(2)=0.98 and r(2)=0.99 (pre- and post-acclimatization). Hypoxia acclimatization resulted in an increase in the overall brain PtO(2) by approximately 35%. This paper demonstrates the use of a novel chronically implanted fiber optic based sensor for measuring absolute PtO(2). It shows a very strong linear relationship in awake animals between inspired O(2) and tissue O(2), and shows that there is a proportional increase in PtO(2) over a range of inspired values after exposure to chronic hypoxia.

Nimodipine prevents memory impairment caused by nitroglycerin-induced hypotension in adult mice

BACKGROUND

Hypotension and a resultant decrease in cerebral blood flow have been implicated in the development of cognitive dysfunction. We tested the hypothesis that nimodipine (NIMO) administered at the onset of nitroglycerin (NTG)-induced hypotension would preserve long-term associative memory.

METHODS

The passive avoidance (PA) paradigm was used to assess memory retention. For PA training, latencies (seconds) were recorded for entry from a suspended platform into a Plexiglas tube where a shock was automatically delivered. Latencies were recorded 48 h later for a testing trial. Ninety-six Swiss-Webster mice (30-35 g, 6-8 wk), were randomized into 6 groups 1) saline (control), 2) NTG immediately after learning, 3) NTG 3 h after learning, 4) NTG and NIMO, 5) vehicle, and 6) NIMO alone. The extent of hypotension and changes in brain tissue oxygenation (PbtO(2)) and in cerebral blood flow were studied in a separate group of animals.

RESULTS

All groups exhibited similar training latencies (17.0 +/- 4.6 s). Mice subjected to hypotensive episodes showed a significant decrease in latency time (178 +/- 156 s) compared with those injected with saline, NTG + NIMO, or delayed NTG (580 +/- 81 s, 557 +/- 67 s, and 493 +/- 146 s, respectively). A Kruskal-Wallis 1-way analysis of variance indicated a significant difference among the 4 treatment groups (H = 15.34; P < 0.001). In a separate group of mice not subjected to behavioral studies, the same dose of NTG (n = 3) and NTG + NIMO (n = 3) caused mean arterial blood pressure to decrease from 85.9 +/- 3.8 mm Hg sem to 31.6 +/- 0.8 mm Hg sem and from 86.2 +/- 3.7 mm Hg sem to 32.6 +/- 0.2 mm Hg sem, respectively. Mean arterial blood pressure in mice treated with NIMO alone decreased from 88.1 +/- 3.8 mm Hg to 80.0 +/- 2.9 mm Hg. The intergroup difference was statistically significant (P < 0.05). PbtO(2) decreased from 51.7 +/- 4.5 mm Hg sem to 33.8 +/- 5.2 mm Hg sem in the NTG group and from 38.6 +/- 6.1 mm Hg sem to 25.4 +/- 2.0 mm Hg sem in the NTG + NIMO groups, respectively. There were no significant differences among groups.

CONCLUSION

In a PA retention paradigm, the injection of NTG immediately after learning produced a significant impairment of long-term associative memory in mice, whereas delayed induced hypotension had no effect. NIMO attenuated the disruption in consolidation of long-term memory caused by NTG but did not improve latency in the absence of hypotension. The observed effect of NIMO may have been attributable to the preservation of calcium homeostasis during hypotension, because there were no differences in the PbtO(2) indices among groups.

Binding of alpha-synuclein with Fe(III) and with Fe(II) and biological implications of the resultant complexes

Parkinson's disease (PD) is hallmarked by the abnormal intracellular inclusions (Lewy bodies or LBs) in dopaminergic cells. Amyloidogenic protein alpha-synuclein (alpha-syn) and iron (including both Fe(III) and Fe(II)) are both found to be present in LBs. The interaction between iron and alpha-syn might have important biological relevance to PD etiology. Previously, a moderate binding affinity between alpha-syn and Fe(II) (5.8x10(3)M(-1)) has been measured, but studies on the binding between alpha-syn and Fe(III) have not been reported. In this work, electrospray mass spectrometry (ES-MS), cyclic voltammetry (CV), and fluorescence spectroscopy were used to study the binding between alpha-syn and Fe(II) and the redox property of the resultant alpha-syn-Fe(II) complex. The complex is of a 1:1 stoichiometry and can be readily oxidized electrochemically and chemically (by O(2)) to the putative alpha-syn-Fe(III) complex, with H(2)O(2) as a co-product. The reduction potential was estimated to be 0.025V vs. Ag/AgCl, which represents a shift by -0.550V vs. the standard reduction potential of the free Fe(III)/Fe(II) couple. Such a shift allows a binding constant between alpha-syn and Fe(III), 1.2x10(13)M(-1), to be deduced. Despite the relatively high binding affinity, alpha-syn-Fe(III) generated from the oxidation of alpha-syn-Fe(II) still dissociates due to the stronger tendency of Fe(III) to hydrolyze to Fe(OH)(3) and/or ferrihydrite gel. The roles of alpha-syn and its interaction with Fe(III) and/or Fe(II) are discussed in the context of oxidative stress, metal-catalyzed alpha-syn aggregation, and iron transfer processes.

Dynamic changes in brain bioenergetics during obstructive sleep apnea

Repetitive collapse of the upper airway during obstructive sleep apnea/hypopnea (OSA) exposes the brain of sufferers to frequent, transient, hypoxic episodes. The loss of cerebrovascular reactivity in sleep, and particularly in OSA, means that physiologic compensatory mechanisms may not ensure adequate brain oxygen levels. This (31)P magnetic resonance spectroscopy study, of 13 males with severe, untreated OSA undertaken after overnight sleep deprivation, represents the first, seconds time-scale analysis of human brain bioenergetics during transient hypoxia and demonstrates that a moderate degree of oxygen desaturation during sleep has significant effects on brain bioenergetic status. Oxygen desaturation >10% of sleeping baseline resulted in decreases in brain adenosine triphosphate levels (P<0.01), and increases in inorganic phosphate (P<0.0001) with no concomitant changes in phosphocreatine or brain pH. This indicates that the mechanism of adenosine triphosphate depletion in these patients is different to that observed in normoxic, awake working brain. These data show that the buffering capacity of phosphocreatine and the creatine kinase system is not active in mild transient hypoxia and that cerebrovascular compensatory mechanisms are not adequate to prevent decrements in brain high-energy phosphates in OSA. Transient hypoxia experienced during sleep may impair brain function more than previously thought.

Tissue pO2 of orthotopic 9L and C6 gliomas and tumor-specific response to radiotherapy and hyperoxygenation

PURPOSE

Tumor hypoxia is a well-known therapeutic problem; however, a lack of methods for repeated measurements of glioma partial pressure of oxygen (pO(2)) limits the ability to optimize the therapeutic approaches. We report the effects of 9.3 Gy of radiation and carbogen inhalation on orthotopic 9L and C6 gliomas and on the contralateral brain pO(2) in rats using a new and potentially widely useful method, multisite in vivo electron paramagnetic resonance oximetry.

METHODS AND MATERIALS

Intracerebral 9L and C6 tumors were established in the left hemisphere of syngeneic rats, and electron paramagnetic resonance oximetry was successfully used for repeated tissue pO(2) measurements after 9.3 Gy of radiation and during carbogen breathing for 5 consecutive days.

RESULTS

Intracerebral 9L gliomas had a pO(2) of 30-32 mm Hg and C6 gliomas were relatively hypoxic, with a pO(2) of 12-14 mm Hg (p < 0.05). The tissue pO(2) of the contralateral brain was 40-45 mm Hg in rats with either 9L or C6 gliomas. Irradiation resulted in a significant increase in pO(2) of the 9L gliomas only. A significant increase in the pO(2) of the 9L and C6 gliomas was observed in rats breathing carbogen, but this effect decreased during 5 days of repeated experiments in the 9L gliomas.

CONCLUSION

These results highlight the tumor-specific effect of radiation (9.3.Gy) on tissue pO(2) and the different responses to carbogen inhalation. The ability of electron paramagnetic resonance oximetry to provide direct repeated measurements of tissue pO(2) could have a vital role in understanding the dynamics of hypoxia during therapy that could then be optimized by scheduling doses at times of improved tumor oxygenation.

Gamma oscillations and spontaneous network activity in the hippocampus are highly sensitive to decreases in pO2 and concomitant changes in mitochondrial redox state

Gamma oscillations have been implicated in higher cognitive processes and might critically depend on proper mitochondrial function. Using electrophysiology, oxygen sensor microelectrode, and imaging techniques, we investigated the interactions of neuronal activity, interstitial pO2, and mitochondrial redox state [NAD(P)H and FAD (flavin adenine dinucleotide) fluorescence] in the CA3 subfield of organotypic hippocampal slice cultures. We find that gamma oscillations and spontaneous network activity decrease significantly at pO2 levels that do not affect neuronal population responses as elicited by moderate electrical stimuli. Moreover, pO2 and mitochondrial redox states are tightly coupled, and electrical stimuli reveal transient alterations of redox responses when pO2 decreases within the normoxic range. Finally, evoked redox responses are distinct in somatic and synaptic neuronal compartments and show different sensitivity to changes in pO2. We conclude that the threshold of interstitial pO2 for robust CA3 network activities and required mitochondrial function is clearly above the "critical" value, which causes spreading depression as a result of generalized energy failure. Our study highlights the importance of a functional understanding of mitochondria and their implications on activities of individual neurons and neuronal networks.

Hypoxia-induced acute mountain sickness is associated with intracellular cerebral edema: a 3 T magnetic resonance imaging study

Acute mountain sickness is common among not acclimatized persons ascending to high altitude; the underlying mechanism is unknown, but may be related to cerebral edema. Nine healthy male students were studied before and after 6-h exposure to isobaric hypoxia. Subjects inhaled room air enriched with N(2) to obtain arterial O(2) saturation values of 75 to 80%. Acute mountain sickness was assessed with the environmental symptom questionnaire, and cerebral edema with 3 T magnetic resonance imaging in 18 regions of interest in the cerebral white matter. The main outcome measures were development of intra- and extracellular cerebral white matter edema assessed by visual inspection and quantitative analysis of apparent diffusion coefficients derived from diffusion-weighted imaging, and B0 signal intensities derived from T2-weighted imaging. Seven of nine subjects developed acute mountain sickness. Mean apparent diffusion coefficient increased 2.12% (baseline, 0.80+/-0.09; 6 h hypoxia, 0.81+/-0.09; P=0.034), and mean B0 signal intensity increased 4.56% (baseline, 432.1+/-98.2; 6 h hypoxia, 450.7+/-102.5; P<0.001). Visual inspection of magnetic resonance images failed to reveal cerebral edema. Cerebral acute mountain sickness scores showed a negative correlation with relative changes of apparent diffusion coefficients (r=-0.83, P=0.006); there was no correlation with relative changes of B0 signal intensities. In conclusion, isobaric hypoxia is associated with mild extracellular (vasogenic) cerebral edema irrespective of the presence of acute mountain sickness in most subjects, and severe acute mountain sickness with additional mild intracellular (cytotoxic) cerebral edema.

Brain tissue oxygen concentration measurements

Brain function depends exquisitely on oxygen for energy metabolism. Measurements of brain tissue oxygen tension, by a variety of quantitative and qualitative techniques, going back for >50 years, have led to a number of significant conclusions. These conclusions have important consequences for understanding brain physiology as it is now being explored by techniques such as blood-oxygen-level-dependent functional magnetic resonance imaging (BOLD fMRI) and near-infrared spectroscopy (NIRS). It has been known for some time that most of the measured oxygen tensions are less than venous pO2 and are distributed in a spatially and temporally heterogeneous manner on a microregional scale. Although certain large-scale methods can provide reproducible average brain pO2 measurements, no useful concept of a characteristic oxygen tension or meaningful average value for brain tissue oxygen can be known on a microregional level. Only an oxygen field exists with large local gradients due to local tissue respiration, and the most useful way to express this is with a pO2 distribution curve or histogram. The neurons of the brain cortex normally exist in a low-oxygen environment and on activation are oxygenated by increases in local capillary blood flow that lead to increases in hemoglobin saturation and tissue oxygen.

Uniform distribution of projection data for improved reconstruction quality of 4D EPR imaging

In continuous wave (CW) electron paramagnetic resonance imaging (EPRI), high quality of reconstruction in a limited acquisition time is a high priority. It has been shown for the case of 3D EPRI, that a uniform distribution of the projection data generally enhances reconstruction quality. In this work, we have suggested two data acquisition techniques for which the gradient orientations are more evenly distributed over the 4D acquisition space as compared to the existing methods. The first sampling technique is based on equal solid angle partitioning of 4D space, while the second technique is based on Fekete points estimation in 4D to generate a more uniform distribution of data. After acquisition, filtered backprojection (FBP) is applied to carry out the reconstruction in a single stage. The single-stage reconstruction improves the spatial resolution by eliminating the necessity of data interpolation in multi-stage reconstructions. For the proposed data distributions, the simulations and experimental results indicate a higher fidelity to the true object configuration. Using the uniform distribution, we expect about 50% reduction in the acquisition time over the traditional method of equal linear angle acquisition.

Differences in O2 availability resolve the apparent discrepancies in metabolic intrinsic optical signals in vivo and in vitro

Monitoring changes in the fluorescence of metabolic chromophores, reduced nicotinamide adenine dinucleotide and flavin adenine dinucleotide, and the absorption of cytochromes, is useful to study neuronal activation and mitochondrial metabolism in the brain. However, these optical signals evoked by stimulation, seizures and spreading depression in intact brain differ from those observed in vitro. The responses in vivo consist of a persistent oxidized state during neuronal activity followed by mild reduction during recovery. In vitro, however, brief oxidation is followed by prolonged and heightened reduction, even during persistent neuronal activation. In normally perfused, oxygenated and activated brain tissue in vivo, partial pressure of oxygen (P(O2)) levels often undergo a brief 'dip' that is always followed by an overshoot above baseline, due to increased blood flow (neuronal-vascular coupling). By contrast, in the absence of blood circulation, tissue P(O2)in vitro decreases more markedly and recovers slowly to baseline without overshooting. Although oxygen is abundant in vivo, it is diffusion-limited in vitro. The disparities in mitochondrial and tissue oxygen availability account for the different redox responses.

Oxygen dependence of tyrosine hydroxylase

The effects of dioxygen on tyrosine hydroxylase (TH) activity was studied, measuring the formation of DOPA from tyrosine, (3)H(2)O from 3,5-(3)H-tyrosine, or by direct oxygraphic determination of oxygen consumption. A high enzyme activity was observed during the initial 1-2 min of the reactions, followed by a decline in activity, possibly related to a turnover dependent substoichiometrical oxidation of enzyme bound Fe(II) to the inactive Fe(III) state. During the initial reaction phase, apparent K (m)-values of 29-45 microM for dioxygen were determined for all human TH isoforms, i.e. 2-40 times higher than previously reported for TH isolated from animal tissues. After 8 min incubation, the K (m) (O(2))-values had declined to an average of 20 +/- 4 microM. Thus, TH activity may be severely limited by oxygen availability even at moderate hypoxic conditions, and the enzyme is rapidly and turnover dependent inactivated at the experimental conditions commonly employed to measure in vitro activities.

A parametric approach to spectral-spatial EPR imaging

Continuous wave electron paramagnetic resonance imaging for in vivo mapping of spin distribution and spectral shape requires rapid data acquisition. A spectral-spatial imaging technique is presented that provides an order of magnitude reduction in acquisition time, compared to iterative tomographic reprojection. The proposed approach assumes that spectral shapes in the sample are well-approximated by members from a parametric family of functions. A model is developed for the spectra measured with magnetic field modulation. Parameters defining the spin distribution and spectral shapes are then determined directly from the measurements using maximum a posteriori probability estimation. The approach does not suffer approximation error from limited sweep width of the main magnetic field and explicitly incorporates the variability in signal-to-noise ratio versus strength of magnetic field gradient. The processing technique is experimentally demonstrated on a one-dimensional phantom containing a nitroxide spin label with constant g-factor. Using an L-band EPR spectrometer, spectral shapes and spin distribution are accurately recovered from two projections and a spectral window which is comparable to the maximum linewidth of the sample.

Enhanced resolution for EPR imaging by two-step deblurring

The broad spectrum of spin probes used for electron paramagnetic resonance imaging (EPRI) result in poor spatial resolution of the reconstructed images. Conventional deconvolution procedures can enhance the resolution to some extent but obtaining high resolution EPR images is still a challenge. In this work, we have implemented and analyzed the performance of a postacquisition deblurring technique to enhance the spatial resolution of the EPR images. The technique consists of two steps; noniterative deconvolution followed by iterative deconvolution of the acquired projections which are then projected back using filtered backprojection (FBP) to reconstruct a high resolution image. Further, we have proposed an analogous technique for iterative reconstruction algorithms such as multiplicative simultaneous iterative reconstruction technique (MSIRT) which can be a method of choice for many applications. The performance of the suggested deblurring approach is evaluated using computer simulations and EPRI experiments. Results suggest that the proposed procedure is superior to the standard FBP and standard iterative reconstruction algorithms in terms of mean-square-error (MSE), spatial resolution, and visual judgment. Although the procedure is described for 2D imaging, it can be readily extended to 3D imaging.

Quasi Monte Carlo-based isotropic distribution of gradient directions for improved reconstruction quality of 3D EPR imaging

In continuous wave (CW) electron paramagnetic resonance imaging (EPRI), high quality of reconstructed image along with fast and reliable data acquisition is highly desirable for many biological applications. An accurate representation of uniform distribution of projection data is necessary to ensure high reconstruction quality. The current techniques for data acquisition suffer from nonuniformities or local anisotropies in the distribution of projection data and present a poor approximation of a true uniform and isotropic distribution. In this work, we have implemented a technique based on Quasi-Monte Carlo method to acquire projections with more uniform and isotropic distribution of data over a 3D acquisition space. The proposed technique exhibits improvements in the reconstruction quality in terms of both mean-square-error and visual judgment. The effectiveness of the suggested technique is demonstrated using computer simulations and 3D EPRI experiments. The technique is robust and exhibits consistent performance for different object configurations and orientations.

Automated on-the-fly detection and correction procedure for EPR imaging data acquisition

Fast and reliable data acquisition is a major requirement for successful and useful biological electron paramagnetic resonance imaging (EPRI) experiments. Even a technologically advanced and professionally supervised EPRI system can exhibit instabilities initiated by perturbations such as animal motion, microphonics, and temperature changes. As a result, part of an acquired data set may become corrupted with excessive noise and distortions, which in turn may degrade the quality of the reconstructed image. In this work an automated scheme to monitor the system performance and stability over the course of an experiment is demonstrated. This method ensures that the quality of the acquired data is maintained during the experiment. For this purpose, four parameters including noise content and integration of each acquired projection are quantified and measured against those of the zero-gradient (ZG) projection, which is set as a quality benchmark. Projections with parameter values that differ substantially from the expected values are identified as damaged and consequently are reacquired. Therefore, the proposed technique not only effectively monitors the quality of acquisition, it also saves a substantial amount of acquisition time because it eliminates the necessity of repeating the entire experiment in cases in which only a small fraction of the data are corrupted.

A generic model of electron transport in mitochondria

In this paper, a simplified, generic model of mitochondrial metabolism is explored. In particular the following question is addressed: To what extent are phenomena observed in experiments and simulations of mitochondrial metabolism generic, in the sense that they must occur in all models with this basic structure? Of particular interest are the electron transport chain and oxidative phosphorylation, and how flux through the system and the redox states of intermediates respond to physiologically important stimuli. These stimuli include changes in substrate supply (NADH/FADH(2)), in oxygenation, and in membrane proton gradient/ATP demand. Analytical techniques are used to show that certain experimentally observed effects must occur in the generic model. These include the responses of both flux and redox states to changed substrate and oxygen concentrations. At the same time other effects, such as the responses of redox states to changes in proton gradient, are dependent on the details of the model, and are not common to every model with the same basic structure. The phenomenon of saturation in response to large inputs is also discussed.

Mitochondrial membrane potential and ischemic neuronal death

Mitochondria are intracellular organelles in which high energy phosphate is produced. Ischemia causes depletion of the materials necessary to produce this phosphate and strongly affects the electron transport chain. Apoptosis commences during and after ischemia. As such, it is likely that a significant relationship exists between inactivation of electron transport and apoptosis. Mitochondrial membrane potential (MMP) reflects performance of the electron transport chain and can indicate a pathological disorder of this system. In an experimental setting, oxygen-glucose depletion (OGD) in neuronal cell culture has been employed to simulate an ischemic condition. The relationship between MMP and subsequent neuronal death during and after OGD has been examined. MMP dissipation and concomitant neuronal death have been reported, but recent studies have demonstrated mitochondrial hyperpolarization preceding neuronal death. The direction of MMP polarization depends on the extent of OGD. Long OGD results in depolarization, while shorter OGD induces hyperpolarization. Neurons are still viable during hyperpolarization, but the process may switch on the apoptotic cascade. Meanwhile, dissipation of MMP seems to be a consequence of severe energy deficit, leading to necrosis. MMP may be a marker of subsequent apoptosis, although a causal relationship remains to be determined.

Cerebral and muscle tissue oxygenation in acute hypoxic ventilatory response test

Eight men were exposed to progressive isocapnic hypoxia for 10 min to test the hypothesis that (i) cerebral and muscle tissue would follow similar deoxygenation profiles during an acute hypoxic ventilatory response (AHVR) test; and (ii) strong cerebrovascular responsiveness to hypoxia would be related to attenuated cerebral deoxygenation. End-tidal O(2) concentration was reduced from normoxia (approximately 102 mmHg) to approximately 45 mmHg while arterial oxygen saturation (SpO2 %) declined from 98+/-1% to 77+/-7% (P<0.001). Near-infrared spectroscopy (NIRS)-derived local cerebral tissue (frontal lobe) deoxyhemoglobin increased 5.55+/-2.22 microM, while oxyhemoglobin and tissue oxygenation index decreased 2.57+/-1.99 microM and 6.2+/-3.4%, respectively (all P<0.001). In muscle (m. vastus lateralis) the NIRS changes from the initial normoxic level were non-significant. Cerebral blood velocity (V(mean), transcranial Doppler) in the middle cerebral artery increased from 53.4+/-10.4 to 60.6+/-11.6 cms(-1) (P<0.001). In relation to the decline in SpO2 % the mean rate of increase of V(mean) and AHVR were 0.33+/-0.19 cms(-1)%(-1) and 0.52+/-0.20l min(-1)%(-1), respectively. We conclude that cerebral, but not muscle, tissue shows changes reflecting a greater deoxygenation during acute hypoxia. However, the changes in NIRS parameters were not related to cerebrovascular responsiveness or ventilatory chemosensitivity during graded hypoxia.

Impaired oxidative metabolism precipitates delirium: a study of 101 ICU patients

Data from 101 consecutively admitted intensive care unit (ICU) patients were examined to determine whether oxidative metabolic stress existed within the 48 hours before delirium onset. The occurrence of pneumonia and sepsis at any time during hospitalization was also recorded. Delirium was defined retrospectively with the Confusion Assessment Method (CAM). Older patients were found to develop delirium more frequently than younger patients. There were no differences in illness severity (APACHE II) between those who developed delirium and those who did not. Three measures of oxygenation (hemoglobin, hematocrit, pulse oximetry) were worse in the patients who later developed delirium. Two measures of oxidative stress (sepsis, pneumonia) occurred more frequently among those diagnosed with delirium. Hence, patients with indicators of oxidative dysfunction developed delirium more frequently, and this was not linked to illness severity.

Reduction and oxidation of the active site iron in tyrosine hydroxylase: kinetics and specificity

Tyrosine hydroxylase (TyrH) is a pterin-dependent enzyme that catalyzes the hydroxylation of tyrosine to form dihydroxyphenylalanine. The oxidation state of the active site iron atom plays a central role in the regulation of the enzyme. The kinetics of reduction of ferric TyrH by several reductants were determined by anaerobic stopped-flow spectroscopy. Anaerobic rapid freeze-quench EPR confirmed that the change in the near-UV absorbance of TyrH upon adding reductant corresponded to iron reduction. Tetrahydrobiopterin reduces wild-type TyrH following a simple second-order mechanism with a rate constant of 2.8 +/- 0.1 mM(-)(1) s(-)(1). 6-Methyltetrahydropterin reduces the ferric enzyme with a second-order rate constant of 6.1 +/- 0.1 mM(-)(1) s(-)(1) and exhibits saturation kinetics. No EPR signal for a radical intermediate was detected. Ascorbate, glutathione, and 1,4-benzoquinone all reduce ferric TyrH, but much more slowly than tetrahydrobiopterin, suggesting that the pterin is a physiological reductant. E332A TyrH, which has an elevated K(m) for tetrahydropterin in the catalytic reaction, is reduced by tetrahydropterins with the same kinetic parameters as those of the wild-type enzyme, suggesting that BH(4) does not bind in the catalytic conformation during the reduction. Oxidation of ferrous TyrH by molecular oxygen can be described as a single-step second-order reaction, with a rate constant of 210 mM(-)(1) s(-)(1). S40E TyrH, which mimics the phosphorylated state of the enzyme, has oxidation and reduction kinetics similar to those of the wild-type enzyme, suggesting that phosphorylation does not directly regulate the interconversion of the ferric and ferrous forms.

Effect of acute hypoxia on microcirculatory and tissue oxygen levels in rat cremaster muscle

Repeated exposure to brief periods of hypoxia leads to pathophysiological changes in experimental animals similar to those seen in sleep apnea. To determine the effects of such exposure on oxygen levels in vivo, we used an optical method to measure Po2in microcirculatory vessels and tissue of the rat cremaster muscle during a 1-min step reduction of inspired oxygen fraction from 0.21 to 0.07. Under control conditions, Po2was 98.1 ± 1.9 Torr in arterial blood, 52.2 ± 2.8 Torr in 29.0 ± 2.7-μm arterioles, 26.8 ± 1.7 Torr in the tissue interstitium near venous capillaries, and 35.1 ± 2.6 Torr in 29.7 ± 1.9-μm venules. The initial fall in Po2during hypoxia was significantly greater in arterial blood, being 93% complete in the first 10 s, whereas it was 68% complete in arterioles, 47% at the tissue sites, and 38% in venules. In the 10- to 30-s period, the fall in normalized tissue and venular Po2was significantly greater than in arterial Po2. At the end of hypoxic exposure, Po2at all measurement sites had fallen very nearly in proportion to that in the inspired gas, but tissue oxygen levels did not reach critical Po2. Significant differences in oxyhemoglobin desaturation rate were also observed between arterial and microcirculatory vessels during hypoxia. In conclusion, the fall in microcirculatory and tissue oxygen levels in resting skeletal muscle is significantly slower than in arterial blood during a step reduction to an inspired oxygen fraction of 0.07, and tissue Po2does not reach anaerobic levels.

Cerebral PtO2, acute hypoxia, and volatile anesthetics in the rat brain

We describe our results on the effect in rats of two commonly used, volatile anesthetics on cerebral tissue PO2 (PtO2) and other physiological parameters at FiO2 levels ranging from 0.35 to 0.1. The study was performed in 12 rats that had lithium phthalocyanine (LiPc) crystals implanted in the left cerebral cortex. FiO2 was maintained at 0.35 during surgical manipulation and baseline EPR measurements, after which time, each animal was exposed to varying levels of FiO2 (0.26, 0.21, 0.15, and 0.10) for 30 minutes at each level. No significant difference in PtO2 was observed between the isoflurane and halothane groups at any FiO2 level, and the cerebral arterial PO2 (PaO2) also was similar for both groups. However, the cerebral PtO2 under both isoflurane and halothane anesthesia was lower during hypoxia (FiO2 < or = 0.15) than under normoxia (FiO2 = 0.21) and there was a significant difference in mean arterial blood pressure (MABP) between isoflurane and halothane groups under both mild and severe hypoxia. The pH and cerebral arterial PCO2 (PaCO2) were similar for the halothane and isoflurane groups during normoxia (FiO2 = 0.21) and mild hypoxia (FiO2 = 0,15), but following severe hypoxia (FiO2 = 0.10), both parameters were lower in the halothane anesthetized animals. These results confirm that cerebral PO2 cannot be inferred directly from measurements of other parameters, indicating that methodology incorporating continuous direct measurement of brain oxygen will lead to a better understanding of cerebral oxygenation under anesthesia and hypoxia.

Responses to GABA(A) receptor activation are altered in NTS neurons isolated from chronic hypoxic rats

The inhibitory amino acid GABA is released within the nucleus of the solitary tract (NTS) during hypoxia and modulates the respiratory response to hypoxia. To determine if responses of NTS neurons to activation of GABA(A) receptors are altered following exposure to chronic hypoxia, GABA(A) receptor-evoked whole cell currents were measured in enzymatically dispersed NTS neurons from normoxic and chronic hypoxic rats. Chronic hypoxic rats were exposed to 10% O(2) for 9-12 days. Membrane capacitance was the same in neurons from normoxic (6.9+/-0.5 pF, n=16) and hypoxic (6.3+/-0.5 pF, n=15) rats. The EC(50) for peak GABA-evoked current density was significantly greater in neurons from hypoxic (21.7+/-2.2 microM) compared to normoxic rats (12.2+/-0.9 microM) (p<0.001). Peak and 5-s adapted GABA currents evoked by 1, 3 and 10 microM were greater in neurons from normoxic compared to hypoxic rats (p<0.05) whereas peak and 5-s adapted responses to 30 and 100 microM GABA were not different comparing normoxic to hypoxic rats. Desensitization of GABA(A)-evoked currents was observed at concentrations greater than 3 microM and, measured as the ratio of the current 5 s after the onset of 100 microM GABA application to the peak GABA current, was the same in neurons from normoxic (0.37+/-0.03) and hypoxic rats (0.33+/-0.04). Reduced sensitivity to GABA(A) receptor-evoked inhibition in chronic hypoxia could influence chemoreceptor afferent integration by NTS neurons.