Activity-induced tissue oxygenation changes in rat cerebellar cortex: interplay of postsynaptic activation and blood flow

Electro-metabolic signaling

Precise matching of energy substrate delivery to local metabolic needs is essential for the health and function of all tissues. Here, we outline a mechanistic framework for understanding this critical process, which we refer to as electro-metabolic signaling (EMS). All tissues exhibit changes in metabolism over varying spatiotemporal scales and have widely varying energetic needs and reserves. We propose that across tissues, common signatures of elevated metabolism or increases in energy substrate usage that exceed key local thresholds rapidly engage mechanisms that generate hyperpolarizing electrical signals in capillaries that then relax contractile elements throughout the vasculature to quickly adjust blood flow to meet changing needs. The attendant increase in energy substrate delivery serves to meet local metabolic requirements and thus avoids a mismatch in supply and demand and prevents metabolic stress. We discuss in detail key examples of EMS that our laboratories have discovered in the brain and the heart, and we outline potential further EMS mechanisms operating in tissues such as skeletal muscle, pancreas, and kidney. We suggest that the energy imbalance evoked by EMS uncoupling may be central to cellular dysfunction from which the hallmarks of aging and metabolic diseases emerge and may lead to generalized organ failure states-such as diverse flavors of heart failure and dementia. Understanding and manipulating EMS may be key to preventing or reversing these dysfunctions.

Light conditions influence optic nerve OCT angiography parameter in healthy subjects with neutral pupils

Optical coherence tomography angiography measurements are influenced by a range of environmental factors as blood pressure and physical fitness. The present study aimed to evaluate the effects of light and dark exposure in eyes with neutral and mydriatic pupils on vessel density in the macular and optic nerve head regions, as measured using optical coherence tomography angiography (OCTA). 55 eyes of 55 healthy volunteers (28 patients with neutral pupils; 27.18 ± 4.33 years) were examined using a high-speed and high-resolution spectral-domain OCT XR Avanti system with a split-spectrum amplitude de-correlation angiography algorithm. OCTA imaging was performed after dark adaptation and after exposure to light. The vessel density data of the superficial and deep retinal macular and optic nerve head region OCT-angiogram were analyzed for these two light conditions. Through Bonferroni correction for multiple testing, the p- value was adapted from 0.05 to 0.017. In eyes with neutral pupils, a significant increase was found in the capillary region of the optic nerve head region (p = 0.002), comparing dark- and light-adaptation. In the macular region of eyes with neutral (p = 0.718) and mydriatic pupils (p = 0.043), no significant differences were observed, as were any in the optic nerve head region of the mydriatic eyes (p = 0.797). This observation suggests that light conditions could be a possible factor influencing OCTA measurements. After dark exposure, vessel density data were significantly different between eyes with neutral and mydriatic pupils (nerve head region: p < 0.0001, superficial macula: p < 0.0001, deep macula: p = 0.0025). These data warn for the effect of mydriatic drops on vessel density measurements.

Glucose Metabolism Impairment in Parkinson's Disease

Impairments in systematic and regional glucose metabolism exist in patients with Parkinson's disease (PD) at every stage of the disease course, and such impairments are associated with the incidence, progression, and special phenotypes of PD, which affect each physiological process of glucose metabolism including glucose uptake, glycolysis, tricarboxylic acid cycle, oxidative phosphorylation, and pentose phosphate shunt pathway. These impairments may be attributed to various mechanisms, such as insulin resistance, oxidative stress, abnormal glycated modification, blood-brain-barrier dysfunction, and hyperglycemia-induced damages. These mechanisms could subsequently cause excessive methylglyoxal and reactive oxygen species production, neuroinflammation, abnormal aggregation of protein, mitochondrial dysfunction, and decreased dopamine, and finally result in energy supply insufficiency, neurotransmitter dysregulation, aggregation and phosphorylation of α-synuclein, and dopaminergic neuron loss. This review discusses the glucose metabolism impairment in PD and its pathophysiological mechanisms, and briefly summarized the currently-available therapies targeting glucose metabolism impairment in PD, including glucagon-likepeptide-1 (GLP-1) receptor agonists and dual GLP-1/gastric inhibitory peptide receptor agonists, metformin, and thiazoledinediones.

Neurovascular coupling changes in patients with magnetic resonance imaging negative focal epilepsy

Brain neuron activity is closely related to cerebral blood flow (CBF) changes. Alterations in the regional homogeneity (ReHo) and CBF occur in patients with magnetic resonance imaging negative focal epilepsy (FEP-MRI^-). However, the coupling alterations of ReHo and CBF in FEP-MRI^- remain unclear. The study aims to explore neurovascular coupling alterations and their clinical implication in FEP-MRI^-. We collected resting-state magnetic resonance imaging (MRI) data from 31 healthy controls (HCs) and 48 patients with FEP-MRI^-,including three-dimensional (3D) T1-weighted imaging, 3D arterial spin labeling (ASL) imaging,and resting-state functional MRI (rs-fMRI). The CBF and ReHo values were calculated from the ASL and rs-fMRI data, respectively. The CBF/ReHo ratio per voxel and whole-brain CBF-ReHo coupling were compared between the two groups. Correlation analysis involved the CBF/ReHo ratio and clinical indicators in FEP-MRI^-. Patients with FEP-MRI^- showed significantly increased cross-subject CBF-ReHo and global cross-voxel CBF-ReHo coupling. The CBF/ReHo ratio was higher in the bilateral orbitofrontal gyrus, right parietal lobe, and right middle frontal gyrus of patients with FEP-MRI^-. Nevertheless, this ratio was lower in the bilateral supplementary motor areas, the left middle and posterior cingulate gyrus, and the right central sulcus cover. The CBF/ReHo ratio was markedly correlated with cognitive function, memory, intelligence, and epilepsy duration in the above abnormal brain regions. CBF/ReHo ratio may be useful as an indicator of neuropathological mechanisms. These results support the hypothesis that CBF/ReHo ratio relates to the neuropathological mechanisms of FEP-MRI^-. Furthermore, it offers new perspectives for studying the mechanisms of MRI-negative epilepsy.

Assaying activity-dependent arteriole and capillary responses in brain slices

Significance: Neurovascular coupling (NVC) is the process that increases cerebral blood flow in response to neuronal activity. NVC is orchestrated by signaling between neurons, glia, and vascular cells. Elucidating the mechanisms underlying NVC at different vascular segments and in different brain regions is imperative for understanding of brain function and mechanisms of dysfunction. Aim: Our goal is to describe a protocol for concurrently monitoring stimulation-evoked neuronal activity and resultant vascular responses in acute brain slices. Approach: We describe a step-by-step protocol that allows the study of endogenous NVC mechanisms engaged by neuronal activity in a controlled, reduced preparation. Results: This ex vivo NVC assay allows researchers to disentangle the mechanisms regulating the contractile responses of different vascular segments in response to neuronal firing independent of flow and pressure mediated effects from connected vessels. It also enables easy pharmacological manipulations in a simplified, reduced system and can be combined with Ca 2 + imaging or broader electrophysiology techniques to obtain multimodal data during NVC. Conclusions: The ex vivo NVC assay will facilitate investigations of cellular and molecular mechanisms that give rise to NVC and should serve as a valuable complement to in vivo imaging methods.

Emerging Roles of Microfluidics in Brain Research: From Cerebral Fluids Manipulation to Brain-on-a-Chip and Neuroelectronic Devices Engineering

Remarkable progress made in the past few decades in brain research enables the manipulation of neuronal activity in single neurons and neural circuits and thus allows the decipherment of relations between nervous systems and behavior. The discovery of glymphatic and lymphatic systems in the brain and the recently unveiled tight relations between the gastrointestinal (GI) tract and the central nervous system (CNS) further revolutionize our understanding of brain structures and functions. Fundamental questions about how neurons conduct two-way communications with the gut to establish the gut-brain axis (GBA) and interact with essential brain components such as glial cells and blood vessels to regulate cerebral blood flow (CBF) and cerebrospinal fluid (CSF) in health and disease, however, remain. Microfluidics with unparalleled advantages in the control of fluids at microscale has emerged recently as an effective approach to address these critical questions in brain research. The dynamics of cerebral fluids (i.e., blood and CSF) and novel in vitro brain-on-a-chip models and microfluidic-integrated multifunctional neuroelectronic devices, for example, have been investigated. This review starts with a critical discussion of the current understanding of several key topics in brain research such as neurovascular coupling (NVC), glymphatic pathway, and GBA and then interrogates a wide range of microfluidic-based approaches that have been developed or can be improved to advance our fundamental understanding of brain functions. Last, emerging technologies for structuring microfluidic devices and their implications and future directions in brain research are discussed.

The interplay of neurovasculature and adult hippocampal neurogenesis

The subgranular zone of the dentate gyrus provides a local microenvironment (niche) for neural stem cells. In the adult brain, it has been established that the vascular compartment of such niches has a significant role in regulating adult hippocampal neurogenesis. More recently, evidence showed that neurovascular coupling, the relationship between blood flow and neuronal activity, also regulates hippocampal neurogenesis. Here, we review the most recent articles on addressing the intricate relationship between neurovasculature and adult hippocampal neurogenesis and a novel pathway where functional hyperemia enhances hippocampal neurogenesis. In the end, we have further reviewed recent research showing that impaired neurovascular coupling may cause declined neurogenesis and contribute to brain damage in neurodegenerative diseases.

Retinotopic variations of the negative blood-oxygen-level dependent hemodynamic response function in human primary visual cortex

Functional magnetic resonance imaging (fMRI) measures blood-oxygen-level-dependent (BOLD) contrast that is generally assumed to be linearly related to excitatory neural activity. The positive hemodynamic response function (pHRF) is the positive BOLD response (PBR) evoked by a brief neural stimulation; the pHRF is often used as the impulse response for linear analysis of neural excitation. Many fMRI studies have observed a negative BOLD response (NBR) that is often associated with neural suppression. However, the temporal dynamics of the NBR evoked by a brief stimulus, the negative HRF (nHRF), remains unclear. Here, a unilateral visual stimulus was presented in a slow event-related design to elicit both pHRFs in the stimulus representation (SR), and nHRFs elsewhere. The observed nHRFs were not inverted versions of the pHRF previously reported. They were characterized by a stronger initial negative response followed by a significantly later positive peak. In contralateral primary visual cortex (V1), these differences varied with eccentricity from the SR. Similar nHRFs were observed in ipsilateral V1 with less eccentricity variation. Experiments with the blocked version of the same stimulus confirmed that brain regions presenting the unexpected nHRF dynamics correspond to those presenting a strong NBR. These data demonstrated that shift-invariant temporal linearity did not hold for the NBR while confirming that the PBR maintained rough linearity. Modeling indicated that the observed nHRFs can be created by suppression of both blood flow and oxygen metabolism. Critically, the nHRF can be misinterpreted as a pHRF due to their similarity, which could confound linear analysis for event-related fMRI experiments.NEW & NOTEWORTHY We investigate dynamics of the negative hemodynamic response function (nHRF), the negative blood-oxygen-level-dependent (BOLD) response (NBR) evoked by a brief stimulus, in human early visual cortex. Here, we show that the nHRFs are not inverted versions of the corresponding pHRFs. The nHRF has complex dynamics that varied significantly with eccentricity. The results also show shift-invariant temporal linearity does not hold for the NBR.

The bioactivity of neuronal-derived nitric oxide in aging and neurodegeneration: Switching signaling to degeneration

The small and diffusible free radical nitric oxide (^•NO) has fascinated biological and medical scientists since it was promoted from atmospheric air pollutant to biological ubiquitous signaling molecule. Its unique physical chemical properties expand beyond its radical nature to include fast diffusion in aqueous and lipid environments and selective reactivity in a biological setting determined by bioavailability and reaction rate constants with biomolecules. In the brain, ^•NO is recognized as a key player in numerous physiological processes ranging from neurotransmission/neuromodulation to neurovascular coupling and immune response. Furthermore, changes in its bioactivity are central to the molecular pathways associated with brain aging and neurodegeneration. The understanding of ^•NO bioactivity in the brain, however, requires the knowledge of its concentration dynamics with high spatial and temporal resolution upon stimulation of its synthesis. Here we revise our current understanding of the role of neuronal-derived ^•NO in brain physiology, aging and degeneration, focused on changes in the extracellular concentration dynamics of this free radical and the regulation of bioenergetic metabolism and neurovascular coupling.

The mass transfer coefficient for oxygen transport from blood to tissue in cerebral cortex

The functioning of cerebral cortex depends on adequate tissue oxygenation. MRI-based techniques allow estimation of blood oxygen levels, tissue perfusion, and oxygen consumption rate (CMRO₂), but do not directly measure partial pressure of oxygen (PO₂) in tissue. To address the estimation of tissue PO₂, the oxygen mass transfer coefficient (KTO₂) is here defined as the CMRO₂ divided by the difference in spatially averaged PO₂ between blood and tissue, and is estimated by analyzing Krogh-cylinder type models. Resistance to radial diffusion of oxygen from microvessels to tissue is distributed within vessels and in the extravascular tissue. The value of KTO₂ is shown to depend strongly on vascular length density and also on microvessel tube hematocrits and diameters, but to be insensitive to blood flow rate and to transient changes in flow or oxygen consumption. Estimated values of KTO₂ are higher than implied by previous studies, implying smaller declines in PO₂ from blood to tissue. Average tissue PO₂ can be estimated from MRI-based measurements as average blood PO₂ minus the product of KTO₂ and CMRO₂. For oxygen consumption rates and vascular densities typical of mouse cortex, the predicted difference between average blood and tissue PO₂ is about 10 mmHg.

Spreading depolarizations in the rat endothelin-1 model of focal cerebellar ischemia

Focal brain ischemia is best studied in neocortex and striatum. Both show highly vulnerable neurons and high susceptibility to spreading depolarization (SD). Therefore, it has been hypothesized that these two variables generally correlate. However, this hypothesis is contradicted by findings in cerebellar cortex, which contains highly vulnerable neurons to ischemia, the Purkinje cells, but is said to be less susceptible to SD. Here, we found in the rat cerebellar cortex that elevated K⁺ induced a long-lasting depolarizing event superimposed with SDs. Cerebellar SDs resembled those in neocortex, but negative direct current (DC) shifts and regional blood flow responses were usually smaller. The K⁺ threshold for SD was higher in cerebellum than in previous studies in neocortex. We then topically applied endothelin-1 (ET-1) to the cerebellum, which is assumed to cause SD via vasoconstriction-induced focal ischemia. Although the blood flow decrease was similar to that in previous studies in neocortex, the ET-1 threshold for SD was higher. Quantitative cell counting found that the proportion of necrotic Purkinje cells was significantly higher in ET-1-treated rats than sham controls even if ET-1 had not caused SDs. Our results suggest that ischemic death of Purkinje cells does not require the occurrence of SD.

Development and validation of a real-time microelectrochemical sensor for clinical monitoring of tissue oxygenation/perfusion

Oxygen is of critical importance to tissue viability and there is increasing demand for its reliable real-time clinical monitoring in order to prevent, diagnose, and treat several pathological disorders, including hypoxia, stroke and reperfusion injury. Herein we report the development and characterisation of a prototype clinical O2 sensor, and its validation in vivo, including proof-of-concept monitoring in patients undergoing surgery for carpal tunnel release. An integrated platinum-based microelectrochemical device was custom designed and controlled using a miniaturised telemetry-operated single channel clinical potentiostat. The in vitro performance of different sensor configurations is presented, with the best sensor design (S2) displaying appropriate linearity (R2 = 0.994) and sensitivity (0.569 ± 0.022 nA μM-1). Pre-clinical validation of S2 was performed in the hind limb muscle of anaesthetised rats; tourniquet application resulted in a significant rapid decrease in signal (90 ± 27%, [ΔO2] ca. 140 ± 18 μM), with a return to baseline within a period of ca. 3 min following tourniquet release. Similar trends were observed in the clinical study; an immediate decrease in signal (39 ± 3%, [ΔO2] ca. 30 ± 20 μM), with basal levels re-established within 2 min of tourniquet release. These results confirm that continuous real-time monitoring of dynamic changes in tissue O2 can serve as an indicator of reperfusion status in patients undergoing carpal tunnel surgery, and suggests the potential usefulness of the developed microelectrochemical sensor for other medical conditions where clinical monitoring of O2 and perfusion is important.

What is the key mediator of the neurovascular coupling response?

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Cellular and molecular mechanisms underlying increases in regional blood flow in response to neuronal activity are not fully understood.

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We have compared the effects of 79 in vivo and 36 in vitro experimental attempts to inhibit the neurovascular response.

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Blockade of neuronal NO synthase (nNOS) has the largest effect of any individual target, reducing the neurovascular response by 64%.

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This points to the existence of an unknown key signalling mechanism which accounts for approximately one third of the neurovascular response.

The mechanisms of neurovascular coupling contribute to ensuring brain energy supply is sufficient to meet demand. Despite significant research interest, the mechanisms underlying increases in regional blood flow that follow heightened neuronal activity are not completely understood. This article presents a systematic review and analysis of published data reporting the effects of pharmacological or genetic blockade of all hypothesised signalling pathways of neurovascular coupling. Our primary outcome measure was the percent reduction of the neurovascular response assessed using in vivo animal models. Selection criteria were met by 50 primary sources reporting the effects of 79 treatments. Experimental conditions were grouped into categories targeting mechanisms mediated by nitric oxide (NO), prostanoids, purines, potassium, amongst others. Blockade of neuronal NO synthase was found to have the largest effect of inhibiting any individual target, reducing the neurovascular response by 64% (average of 11 studies). Inhibition of multiple targets in combination with nNOS blockade had no further effect. This analysis points to the existence of an unknown signalling mechanism accounting for approximately one third of the neurovascular response.

Dissociation between CSD-Evoked Metabolic Perturbations and Meningeal Afferent Activation and Sensitization: Implications for Mechanisms of Migraine Headache Onset

The onset of the headache phase during attacks of migraine with aura, which occur in ∼30% of migraineurs, is believed to involve cortical spreading depression (CSD) and the ensuing activation and sensitization of primary afferent neurons that innervate the intracranial meninges, and their related large vessels. The mechanism by which CSD enhances the activity and mechanosensitivity of meningeal afferents remains poorly understood, but may involve cortical metabolic perturbations. We used extracellular single-unit recording of meningeal afferent activity and monitored changes in cortical blood flow and tissue partial pressure of oxygen (tpO₂) in anesthetized male rats to test whether the prolonged cortical hypoperfusion and reduction in tissue oxygenation that occur in the wake of CSD contribute to meningeal nociception. Suppression of CSD-evoked cortical hypoperfusion with the cyclooxygenase inhibitor naproxen blocked the reduction in cortical tpO₂, but had no effect on the activation of meningeal afferents. Naproxen, however, distinctly prevented CSD-induced afferent mechanical sensitization. Counteracting the CSD-evoked persistent hypoperfusion and reduced tpO₂ by preemptively increasing cortical blood flow using the ATP-sensitive potassium [K(ATP)] channel opener levcromakalim did not inhibit the sensitization of meningeal afferents, but prevented their activation. Our data show that the cortical hypoperfusion and reduction in tpO₂ that occur in the wake of CSD can be dissociated from the activation and mechanical sensitization of meningeal afferent responses, suggesting that the metabolic changes do not contribute directly to these neuronal nociceptive responses.SIGNIFICANCE STATEMENT Cortical spreading depression (CSD)-evoked activation and mechanical sensitization of meningeal afferents is thought to mediate the headache phase in migraine with aura. We report that blocking the CSD-evoked cortical hypoperfusion and reduced tissue partial pressure of oxygen by cyclooxygenase inhibition is associated with the inhibition of the afferent sensitization, but not their activation. Normalization of these CSD-evoked metabolic perturbations by activating K(ATP) channels is, however, associated with the inhibition of afferent activation but not sensitization. These results question the contribution of cortical metabolic perturbations to the triggering mechanism underlying meningeal nociception and the ensuing headache in migraine with aura, further point to distinct mechanisms underlying the activation and sensitization of meningeal afferents in migraine, and highlight the need to target both processes for an effective migraine therapy.

Assessment of brain oxygenation imbalance following soman exposure in rats

Nerve agents (NAs) are potent organophosphorus (OP) compounds with applications in chemical warfare. OP compounds act by inhibiting acetylcholinesterase (AChE). Soman (O-pinacolyl methylphosphonofluoridate) is one of the most potent NAs. It is well known that small doses of NAs can be lethal, and that even non-lethal exposure leads to long-term mental debilitation/neurological damage. However, the neuropathology following exposure to sub-lethal nerve agents is not well understood. In this study, we examined changes in tissue oxygenation (pO₂) in the cortex and hippocampus after a sub-lethal dose of soman [80-90 μg/kg; subcutaneous]. pO₂ changes can provide information regarding oxygen delivery and utilization and may be indicative of a disruption in cerebral blood flow and/or metabolism. Changes in oxygenation were measured with chronically implanted oxygen sensors in awake and freely moving rats. Measurements were taken before, during, and after soman-induced convulsive seizures. Soman exposure resulted in an immediate increase in pO₂ in the cortex, followed by an even greater increase that precedes the onset of soman-induced convulsive seizures. The rise in hippocampus pO₂ was delayed relative to the cortex, although the general pattern of brain oxygenation between these two regions was similar. After convulsive seizures began, pO₂ levels declined but usually remained hyperoxygenated. Following the decline in pO₂, low frequency cycles of large amplitude changes were observed in both the cortex and hippocampus. This pattern is consistent with recurring seizures. Measuring real-time changes in brain pO₂ provides new information on the physiological status of the brain following soman exposure. These results highlight that the measurement of brain oxygenation could provide a sensitive marker of nerve agent exposure and serve as a biomarker for treatment studies.

The roadmap for estimation of cell-type-specific neuronal activity from non-invasive measurements

The computational properties of the human brain arise from an intricate interplay between billions of neurons connected in complex networks. However, our ability to study these networks in healthy human brain is limited by the necessity to use non-invasive technologies. This is in contrast to animal models where a rich, detailed view of cellular-level brain function with cell-type-specific molecular identity has become available due to recent advances in microscopic optical imaging and genetics. Thus, a central challenge facing neuroscience today is leveraging these mechanistic insights from animal studies to accurately draw physiological inferences from non-invasive signals in humans. On the essential path towards this goal is the development of a detailed 'bottom-up' forward model bridging neuronal activity at the level of cell-type-specific populations to non-invasive imaging signals. The general idea is that specific neuronal cell types have identifiable signatures in the way they drive changes in cerebral blood flow, cerebral metabolic rate of O2 (measurable with quantitative functional Magnetic Resonance Imaging), and electrical currents/potentials (measurable with magneto/electroencephalography). This forward model would then provide the 'ground truth' for the development of new tools for tackling the inverse problem-estimation of neuronal activity from multimodal non-invasive imaging data.This article is part of the themed issue 'Interpreting BOLD: a dialogue between cognitive and cellular neuroscience'.

Neural-metabolic coupling in the central visual pathway

Studies are described which are intended to improve our understanding of the primary measurements made in non-invasive neural imaging. The blood oxygenation level-dependent signal used in functional magnetic resonance imaging (fMRI) reflects changes in deoxygenated haemoglobin. Tissue oxygen concentration, along with blood flow, changes during neural activation. Therefore, measurements of tissue oxygen together with the use of a neural sensor can provide direct estimates of neural-metabolic interactions. We have used this relationship in a series of studies in which a neural microelectrode is combined with an oxygen micro-sensor to make simultaneous co-localized measurements in the central visual pathway. Oxygen responses are typically biphasic with small initial dips followed by large secondary peaks during neural activation. By the use of established visual response characteristics, we have determined that the oxygen initial dip provides a better estimate of local neural function than the positive peak. This contrasts sharply with fMRI for which the initial dip is unreliable. To extend these studies, we have examined the relationship between the primary metabolic agents, glucose and lactate, and associated neural activity. For this work, we also use a Doppler technique to measure cerebral blood flow (CBF) together with neural activity. Results show consistent synchronously timed changes such that increases in neural activity are accompanied by decreases in glucose and simultaneous increases in lactate. Measurements of CBF show clear delays with respect to neural response. This is consistent with a slight delay in blood flow with respect to oxygen delivery during neural activation.This article is part of the themed issue 'Interpreting BOLD: a dialogue between cognitive and cellular neuroscience'.

Effects of Glutamate Receptor Activation on Local Oxygen Changes

Glutamate is ubiquitous throughout the brain and serves as the primary excitatory neurotransmitter. Neurons require energy to fire, and energetic substrates (i.e., O₂, glucose) are renewed via cerebral blood flow (CBF) to maintain metabolic homeostasis. Magnetic resonance brain functionality studies rely on the assumption that CBF and neuronal activity are coupled consistently throughout the brain; however, the origin of neuronal activity does not always coincide with signals indicative of energy consumption (e.g., O₂ decreases) at high spatial resolutions. Therefore, relationships between excitatory neurotransmission and energy use must be evaluated at higher resolutions. In this study, we showed that both endogenously released and exogenously ejected glutamate decrease local tissue O₂ concentrations, but whether hyperemic O₂ restoration followed depended on the stimulus method. Electrically stimulating the glutamatergic corticostriatal pathway evoked biphasic O₂ responses at striatal terminals: first O₂ decreased, then concentrations increased above baseline. Using iontophoresis to locally eject ionotropic glutamate receptor antagonists revealed that these receptors only influenced the O₂ decrease. We compared electrical stimulation to iontophoretic glutamate stimulation, and measured concurrent single-unit activity and O₂ to limit both stimulation and recordings to <50 μm radius from our sensor. Similarly, iontophoretic glutamate delivery elicited monophasic O₂ decreases without subsequent increases.

Increase of reactive oxygen species generation in cerebral cortex slices after the transiently enhanced metabolic activity

Under certain conditions such as hypoxia-reoxygenation, the generation of reactive oxygen species (ROS) increases following hypoxia caused by a decreased oxygen supply. As another hypoxic condition, an excess neural activity status including epileptic seizure induces a decrease in tissue oxygen partial pressure (pO₂) caused by enhanced oxygen utilization; however, whether ROS generation increases following the hypoxic status induced by transiently enhanced energy metabolism in brain tissue currently remains unknown. We herein investigated ROS-dependent chemiluminescence in cerebral cortex slices during the restoration of transiently enhanced energy metabolism induced by a high-potassium treatment with tissue pO₂ changes and redox balance. ROS generation in the tissue was enhanced after high-potassium-induced hypoxia, but not by the reversed order of the treatment: control-potassium then high-potassium treatment, high-potassium treatment alone, and control-potassium treatment alone. The high-potassium treatment induced a transient decrease in tissue pO₂ and a shift in the tissue redox balance towards reduction. The transient shift in the tissue redox balance towards reduction with enhanced metabolic activity and its recovery may correlate with ROS generation. This phenomenon may mimic ROS generation following the hypoxic status induced by transiently enhanced energy metabolism.

Binaural blood flow control by astrocytes: listening to synapses and the vasculature

Astrocytes are the most common glial cells in the brain with fine processes and endfeet that intimately contact both neuronal synapses and the cerebral vasculature. They play an important role in mediating neurovascular coupling (NVC) via several astrocytic Ca²⁺ -dependent signalling pathways such as K⁺ release through B_K channels, and the production and release of arachidonic acid metabolites. They are also involved in maintaining the resting tone of the cerebral vessels by releasing ATP and COX-1 derivatives. Evidence also supports a role for astrocytes in maintaining blood pressure-dependent change in cerebrovascular tone, and perhaps also in blood vessel-to-neuron signalling as posited by the 'hemo-neural hypothesis'. Thus, astrocytes are emerging as new stars in preserving the intricate balance between the high energy demand of active neurons and the supply of oxygen and nutrients from the blood by maintaining both resting blood flow and activity-evoked changes therein. Following neuropathology, astrocytes become reactive and many of their key signalling mechanisms are altered, including those involved in NVC. Furthermore, as they can respond to changes in vascular pressure, cardiovascular diseases might exert previously unknown effects on the central nervous system by altering astrocyte function. This review discusses the role of astrocytes in neurovascular signalling in both physiology and pathology, and the impact of these findings on understanding BOLD-fMRI signals.

Modeling of Cerebral Oxygen Transport Based on In vivo Microscopic Imaging of Microvascular Network Structure, Blood Flow, and Oxygenation

Oxygen is delivered to brain tissue by a dense network of microvessels, which actively control cerebral blood flow (CBF) through vasodilation and contraction in response to changing levels of neural activity. Understanding these network-level processes is immediately relevant for (1) interpretation of functional Magnetic Resonance Imaging (fMRI) signals, and (2) investigation of neurological diseases in which a deterioration of neurovascular and neuro-metabolic physiology contributes to motor and cognitive decline. Experimental data on the structure, flow and oxygen levels of microvascular networks are needed, together with theoretical methods to integrate this information and predict physiologically relevant properties that are not directly measurable. Recent progress in optical imaging technologies for high-resolution in vivo measurement of the cerebral microvascular architecture, blood flow, and oxygenation enables construction of detailed computational models of cerebral hemodynamics and oxygen transport based on realistic three-dimensional microvascular networks. In this article, we review state-of-the-art optical microscopy technologies for quantitative in vivo imaging of cerebral microvascular structure, blood flow and oxygenation, and theoretical methods that utilize such data to generate spatially resolved models for blood flow and oxygen transport. These “bottom-up” models are essential for the understanding of the processes governing brain oxygenation in normal and disease states and for eventual translation of the lessons learned from animal studies to humans.

Mapping oxygen concentration in the awake mouse brain

Although critical for brain function, the physiological values of cerebral oxygen concentration have remained elusive because high-resolution measurements have only been performed during anesthesia, which affects two major parameters modulating tissue oxygenation: neuronal activity and blood flow. Using measurements of capillary erythrocyte-associated transients, fluctuations of oxygen partial pressure (Po₂) associated with individual erythrocytes, to infer Po₂ in the nearby neuropil, we report the first non-invasive micron-scale mapping of cerebral Po₂ in awake, resting mice. Interstitial Po₂ has similar values in the olfactory bulb glomerular layer and the somatosensory cortex, whereas there are large capillary hematocrit and erythrocyte flux differences. Awake tissue Po₂ is about half that under isoflurane anesthesia, and within the cortex, vascular and interstitial Po₂ values display layer-specific differences which dramatically contrast with those recorded under anesthesia. Our findings emphasize the importance of measuring energy parameters non-invasively in physiological conditions to precisely quantify and model brain metabolism.

DOI:http://dx.doi.org/10.7554/eLife.12024.001

Brain cells need a constant supply of oxygen to fuel their activities. This oxygen is delivered by the flow of blood through the vessels in the brain. If the blood flow to brain tissue is cut off as happens in stroke, or if an individual stops breathing, the brain becomes deprived of oxygen and brain cells will be damaged and die. To better understand how the brain works in health and disease, scientists need to learn how much oxygen the blood must deliver to the brain tissue to adequately support the activities of brain cells.

Many studies have measured oxygen levels in the brain. However, these studies have looked only roughly and taken measurements from large areas of the brain, or they have involved animals receiving anesthesia, which can alter blood flow and oxygen use in the brain. Recently, scientists discovered that they could measure oxygen concentration at high detail in the brain of anesthetized rodents with a specialized microscope, by using molecules that emit light at a rate that depends on the local oxygen concentration.

Now, Lyons et al. have shown that this same technique can be used in mice that are awake. First, a piece of the skull was replaced with glass to create a small transparent window. Then, the animals were allowed to recover for a few weeks, and were trained to get them used to how they would be handled during the experiments. After this period, the oxygen concentrations and blood flow in different parts of the mouse brains were measured in fine detail using the microscope while the animals were awake and relaxed.

The experiments showed that oxygen levels in awake resting mice are actually lower than in anesthetized mice, and that oxygen levels differ between different parts of the mouse brain. This first detailed look at oxygen levels in the brain of awake animals will likely lead to more studies. For example, future studies may look at how quickly the brain uses oxygen under normal conditions and what happens in the brain during disease.

DOI:http://dx.doi.org/10.7554/eLife.12024.002

Functional magnetic resonance imaging

Functional magnetic resonance imaging (fMRI) maps the spatiotemporal distribution of neural activity in the brain under varying cognitive conditions. Since its inception in 1991, blood oxygen level-dependent (BOLD) fMRI has rapidly become a vital methodology in basic and applied neuroscience research. In the clinical realm, it has become an established tool for presurgical functional brain mapping. This chapter has three principal aims. First, we review key physiologic, biophysical, and methodologic principles that underlie BOLD fMRI, regardless of its particular area of application. These principles inform a nuanced interpretation of the BOLD fMRI signal, along with its neurophysiologic significance and pitfalls. Second, we illustrate the clinical application of task-based fMRI to presurgical motor, language, and memory mapping in patients with lesions near eloquent brain areas. Integration of BOLD fMRI and diffusion tensor white-matter tractography provides a road map for presurgical planning and intraoperative navigation that helps to maximize the extent of lesion resection while minimizing the risk of postoperative neurologic deficits. Finally, we highlight several basic principles of resting-state fMRI and its emerging translational clinical applications. Resting-state fMRI represents an important paradigm shift, focusing attention on functional connectivity within intrinsic cognitive networks.

Bidirectional Control of Blood Flow by Astrocytes: A Role for Tissue Oxygen and Other Metabolic Factors

Altering cerebral blood flow through the control of cerebral vessel diameter is critical so that the delivery of molecules important for proper brain functioning is matched to the activity level of neurons. Although the close relationship of brain glia known as astrocytes with cerebral blood vessels has long been recognized, it is only recently that these cells have been demonstrated to translate information on the activity level and energy demands of neurons to the vasculature. In particular, astrocytes respond to elevations in extracellular glutamate as a consequence of synaptic transmission through the activation of group 1 metabotropic glutamate receptors. These Gq-protein coupled receptors elevate intracellular calcium via IP3 signaling. A close examination of astrocyte endfeet calcium signals has been shown to cause either vasoconstriction or vasodilation. Common to both vasomotor responses is the generation of arachidonic acid in astrocytes by calcium sensitive phospholipase A2. Vasoconstriction ensues from the conversion of arachidonic acid to 20-hydroxyeicosatetraenoic acid, while vasodilation ensues from the production of epoxyeicosatrienoic acids or prostaglandins. Factors that determine whether constrictor or dilatory pathways predominate include brain oxygen, lactate, adenosine as well as nitric oxide. Changing the oxygen level itself leads to many downstream changes that facilitate the switch from vasoconstriction at high oxygen to vasodilation at low oxygen. These findings highlight the importance of astrocytes as sensors of neural activity and metabolism to coordinate the delivery of essential nutrients via the blood to the working cells.

Relating Translational Neuroimaging and Amperometric Endpoints: Utility for Neuropsychiatric Drug Discovery

Measures of neuronal activation are a natural and parsimonious translational biomarker to consider in the context of neuropsychiatric drug discovery studies. In this regard, functional neuroimaging using the BOLD fMRI technique is becoming more frequently employed to not only probe aberrant brain regions and circuits in disease, but also to assess the effects of novel pharmacological agents on these processes. In the ideal situation, these types of studies would first be conducted pre-clinically in rodents to confirm a measurable functional response on relevant brain circuits before seeking to replicate the findings in an analogous fMRI paradigm in humans. However, the need for animal immobilization during the scanning procedure precludes all but the simplest behavioural task-based paradigms in rodent BOLD fMRI. This chapter considers how in vivo oxygen amperometry may represent a viable and valid proxy for BOLD fMRI in freely moving rodents engaged in behavioural tasks. The amperometric technique and several examples of emerging evidence are described to show how the technique can deliver results that translate to pharmacological, event-related and functional connectivity variants of fMRI. In vivo oxygen amperometry holds great promise as a technique that may help to bridge the gap between basic drug discovery research in rodents and applied efficacy testing in humans.

Astrocyte regulation of blood flow in the brain

Neuronal activity results in increased blood flow in the brain, a response named functional hyperemia. Astrocytes play an important role in mediating this response. Neurotransmitters released from active neurons evoke Ca(2+) increases in astrocytes, leading to the release of vasoactive metabolites of arachidonic acid from astrocyte endfeet onto blood vessels. Synthesis of prostaglandin E2 (PGE2) and epoxyeicosatrienoic acids (EETs) dilate blood vessels, whereas 20-hydroxyeicosatetraenoic acid (20-HETE) constricts vessels. The release of K(+) from astrocyte endfeet may also contribute to vasodilation. Oxygen modulates astrocyte regulation of blood flow. Under normoxic conditions, astrocytic Ca(2+) signaling results in vasodilation, whereas under hyperoxic conditions, vasoconstriction is favored. Astrocytes also contribute to the generation of vascular tone. Tonic release of both 20-HETE and ATP from astrocytes constricts vascular smooth muscle cells, generating vessel tone. Under pathological conditions, including Alzheimer's disease and diabetic retinopathy, disruption of normal astrocyte physiology can compromise the regulation of blood flow.

Encoding of graded changes in spatial specificity of prior cues in human visual cortex

Prior information about the relevance of spatial locations can vary in specificity; a single location, a subset of locations, or all locations may be of potential importance. Using a contrast-discrimination task with four possible targets, we asked whether performance benefits are graded with the spatial specificity of a prior cue and whether we could quantitatively account for behavioral performance with cortical activity changes measured by blood oxygenation level-dependent (BOLD) imaging. Thus we changed the prior probability that each location contained the target from 100 to 50 to 25% by cueing in advance 1, 2, or 4 of the possible locations. We found that behavioral performance (discrimination thresholds) improved in a graded fashion with spatial specificity. However, concurrently measured cortical responses from retinotopically defined visual areas were not strictly graded; response magnitude decreased when all 4 locations were cued (25% prior probability) relative to the 100 and 50% prior probability conditions, but no significant difference in response magnitude was found between the 100 and 50% prior probability conditions for either cued or uncued locations. Also, although cueing locations increased responses relative to noncueing, this cue sensitivity was not graded with prior probability. Furthermore, contrast sensitivity of cortical responses, which could improve contrast discrimination performance, was not graded. Instead, an efficient-selection model showed that even if sensory responses do not strictly scale with prior probability, selection of sensory responses by weighting larger responses more can result in graded behavioral performance benefits with increasing spatial specificity of prior information.

Aversive prediction error signals in the amygdala

Prediction error signals are fundamental to learning. Here, in mice, we show that aversive prediction signals are found in the hemodynamic responses and theta oscillations recorded from the basolateral amygdala. During fear conditioning, amygdala responses evoked by footshock progressively decreased, whereas responses evoked by the auditory cue that predicted footshock concomitantly increased. Unexpected footshock evoked larger amygdala responses than expected footshock. The magnitude of the amygdala response to the footshock predicted behavioral responses the following day. The omission of expected footshock led to a decrease below baseline in the amygdala response suggesting a negative aversive prediction error signal. Thus, in mice, amygdala activity conforms to temporal difference models of aversive learning.

Neurovascular coupling in hippocampus is mediated via diffusion by neuronal-derived nitric oxide

The coupling between neuronal activity and cerebral blood flow (CBF) is essential for normal brain function. The mechanisms behind this neurovascular coupling process remain elusive, mainly because of difficulties in probing dynamically the functional and coordinated interaction between neurons and the vasculature in vivo. Direct and simultaneous measurements of nitric oxide (NO) dynamics and CBF changes in hippocampus in vivo support the notion that during glutamatergic activation nNOS-derived NO induces a time-, space-, and amplitude-coupled increase in the local CBF, later followed by a transient increase in local O2 tension. These events are dependent on the activation of the NMDA-glutamate receptor and nNOS, without a significant contribution of endothelial-derived NO or astrocyte-neuron signaling pathways. Upon diffusion of NO from active neurons, the vascular response encompasses the activation of soluble guanylate cyclase. Hence, in the hippocampus, neurovascular coupling is mediated by nNOS-derived NO via a diffusional connection between active glutamatergic neurons and blood vessels.

Brain tissue hypoxia and oxidative stress induced by obstructive apneas is different in young and aged rats

STUDY OBJECTIVES

To test the hypotheses that brain oxygen partial pressure (PtO2) in response to obstructive apneas changes with age and that it might lead to different levels of cerebral tissue oxidative stress.

DESIGN

Prospective controlled animal study.

SETTING

University laboratory.

PARTICIPANTS

Sixty-four male Wistar rats: 32 young (3 mo old) and 32 aged (18 mo).

INTERVENTIONS

Protocol 1: Twenty-four animals were subjected to obstructive apneas (50 apneas/h, lasting 15 sec each) or to sham procedure for 50 min. Protocol 2: Forty rats were subjected to obstructive apneas or sham procedure for 4 h.

MEASUREMENTS AND RESULTS

Protocol 1: Real-time PtO2 measurements were performed using a fast-response oxygen microelectrode. During successive apneas cerebral cortex PtO2 presented a different pattern in the two age groups; there was a fast increase in young rats, whereas it remained without significant changes between the beginning and the end of the protocol in the aged group. Protocol 2: Brain oxidative stress assessed by lipid peroxidation increased after apneas in young rats (1.34 ± 0.17 nmol/mg of protein) compared to old ones (0.63 ± 0.03 nmol/mg), where a higher expression of antioxidant enzymes was observed.

CONCLUSIONS

The results suggest that brain oxidative stress in aged rats is lower than in young rats in response to recurrent apneas, mimicking obstructive sleep apnea. This could be due to the different PtO2 response observed between age groups and the increased antioxidant expression in aged rats.

CITATION

Dalmases M, Torres M, Márquez-Kisinousky L, Almendros I, Planas AM, Embid C, Martínez-Garcia MA, Navajas D, Farré R, Montserrat JM. Brain tissue hypoxia and oxidative stress induced by obstructive apneas is different in young and aged rats.

The contribution of astrocytes to the regulation of cerebral blood flow

In order to maintain normal brain function, it is critical that cerebral blood flow (CBF) is matched to neuronal metabolic needs. Accordingly, blood flow is increased to areas where neurons are more active (a response termed functional hyperemia). The tight relationships between neuronal activation, glial cell activity, cerebral energy metabolism, and the cerebral vasculature, known as neurometabolic and neurovascular coupling, underpin functional MRI (fMRI) signals but are incompletely understood. As functional imaging techniques, particularly BOLD fMRI, become more widely used, their utility hinges on our ability to accurately and reliably interpret the findings. A growing body of data demonstrates that astrocytes can serve as a "bridge," relaying information on the level of neural activity to blood vessels in order to coordinate oxygen and glucose delivery with the energy demands of the tissue. It is widely assumed that calcium-dependent release of vasoactive substances by astrocytes results in arteriole dilation and the increased blood flow which accompanies neuronal activity. However, the signaling molecules responsible for this communication between astrocytes and blood vessels are yet to be definitively confirmed. Indeed, there is controversy over whether activity-induced changes in astrocyte calcium are widespread and fast enough to elicit such functional hyperemia responses. In this review, I will summarize the evidence which has convincingly demonstrated that astrocytes are able to modify the diameter of cerebral arterioles. I will discuss the prevalence, presence, and timing of stimulus-induced astrocyte calcium transients and describe the evidence for and against the role of calcium-dependent formation and release of vasoactive substances by astrocytes. I will also review alternative mechanisms of astrocyte-evoked changes in arteriole diameter and consider the questions which remain to be answered in this exciting area of research.

Integrative regulation of human brain blood flow

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

Quantitative characterization of functional anatomical contributions to cognitive control under uncertainty

Although much evidence indicates that RT increases as a function of computational load in many cognitive tasks, quantification of changes in neural activity related to increasing demand of cognitive control has rarely been attempted. In this fMRI study, we used a majority function task to quantify the effect of computational load on brain activation, reflecting the mental processes instantiated by cognitive control under conditions of uncertainty. We found that the activation of the frontoparieto-cingulate system as well as the deactivation of the anticorrelated default mode network varied parametrically as a function of information uncertainty, estimated as entropy with an information theoretic model. The current findings suggest that activity changes in the dynamic networks of the brain (especially the frontoparieto-cingulate system) track with information uncertainty, rather than only conflict or other commonly proposed targets of cognitive control.

The oxygen paradox of neurovascular coupling

The coupling of cerebral blood flow (CBF) to neuronal activity is well preserved during evolution. Upon changes in the neuronal activity, an incompletely understood coupling mechanism regulates diameter changes of supplying blood vessels, which adjust CBF within seconds. The physiologic brain tissue oxygen content would sustain unimpeded brain function for only 1 second if continuous oxygen supply would suddenly stop. This suggests that the CBF response has evolved to balance oxygen supply and demand. Surprisingly, CBF increases surpass the accompanying increases of cerebral metabolic rate of oxygen (CMRO2). However, a disproportionate CBF increase may be required to increase the concentration gradient from capillary to tissue that drives oxygen delivery. However, the brain tissue oxygen content is not zero, and tissue pO2 decreases could serve to increase oxygen delivery without a CBF increase. Experimental evidence suggests that CMRO2 can increase with constant CBF within limits and decreases of baseline CBF were observed with constant CMRO2. This conflicting evidence may be viewed as an oxygen paradox of neurovascular coupling. As a possible solution for this paradox, we hypothesize that the CBF response has evolved to safeguard brain function in situations of moderate pathophysiological interference with oxygen supply.

Blocking NMDA receptors delays death in rats with acute liver failure by dual protective mechanisms in kidney and brain

Treatment of patients with acute liver failure (ALF) is unsatisfactory and mortality remains unacceptably high. Blocking NMDA receptors delays or prevents death of rats with ALF. The underlying mechanisms remain unclear. Clarifying these mechanisms will help to design more efficient treatments to increase patient's survival. The aim of this work was to shed light on the mechanisms by which blocking NMDA receptors delays rat's death in ALF. ALF was induced by galactosamine injection. NMDA receptors were blocked by continuous MK-801 administration. Edema and cerebral blood flow were assessed by magnetic resonance. The time course of ammonia levels in brain, muscle, blood, and urine; of glutamine, lactate, and water content in brain; of glomerular filtration rate and kidney damage; and of hepatic encephalopathy (HE) and intracranial pressure was assessed. ALF reduces kidney glomerular filtration rate (GFR) as reflected by reduced inulin clearance. GFR reduction is due to both reduced renal perfusion and kidney tubular damage as reflected by increased Kim-1 in urine and histological analysis. Blocking NMDA receptors delays kidney damage, allowing transient increased GFR and ammonia elimination which delays hyperammonemia and associated changes in brain. Blocking NMDA receptors does not prevent cerebral edema or blood-brain barrier permeability but reduces or prevents changes in cerebral blood flow and brain lactate. The data show that dual protective effects of MK-801 in kidney and brain delay cerebral alterations, HE, intracranial pressure increase and death. NMDA receptors antagonists may increase survival of patients with ALF by providing additional time for liver transplantation or regeneration.

Binocular activation elicits differences in neurometabolic coupling in visual cortex

Non-invasive brain imaging requires comprehensive interpretation of hemodynamic signals. In functional magnetic resonance imaging, blood oxygen level dependent (BOLD) signals are used to infer neural processes. This necessitates a clear understanding of how BOLD signals and neural activity are related. One fundamental question concerns the relative importance of synaptic activity and spiking discharge. Although these two components are related, most previous work shows that synaptic activity is better reflected in the BOLD signal. However, the mechanisms of this relationship are not clear. The BOLD signal depends on relative changes in cerebral blood flow and cerebral metabolic rate of oxygen. Oxygen metabolism changes are difficult to measure with current imaging techniques, but it is possible to obtain direct quantitative simultaneous in vivo measurement of tissue oxygen and co-localized underlying neural activity. Here, we use this approach with a specific binocular stimulus protocol in order to activate inhibitory and excitatory neuronal pathways in the visual cortex. During excitatory binocular interaction, we find that metabolic, spiking, and local field potential responses are correlated. However, during suppressive binocular interaction, spiking activity and local field potentials (LFP) are dissociated while only the latter is coupled with metabolic response. These results suggest that inhibitory connections may be a key factor in the dissociation between LFP and spiking activity, which may contribute substantially to the close coupling between the BOLD signal and synchronized synaptic activity in the brain.

Oxygen consumption rates during three different neuronal activity states in the hippocampal CA3 network

The brain is an organ with high metabolic rate. However, little is known about energy utilization during different activity states of neuronal networks. We addressed this issue in area CA3 of hippocampal slice cultures under well-defined recording conditions using a 20% O(2) gas mixture. We combined recordings of local field potential and interstitial partial oxygen pressure (pO(2)) during three different activity states, namely fast network oscillations in the gamma-frequency band (30 to 100 Hz), spontaneous network activity and absence of spiking (action potentials). Oxygen consumption rates were determined by pO(2) depth profiles with high spatial resolution and a mathematical model that considers convective transport, diffusion, and activity-dependent consumption of oxygen. We show that: (1) Relative oxygen consumption rate during cholinergic gamma oscillations was 2.2-fold and 5.3-fold higher compared with spontaneous activity and absence of spiking, respectively. (2) Gamma oscillations were associated with a similar large decrease in pO(2) as observed previously with a 95% O(2) gas mixture. (3) Sufficient oxygenation during fast network oscillations in vivo is ensured by the calculated critical radius of 30 to 40 μm around a capillary. We conclude that the structural and biophysical features of brain tissue permit variations in local oxygen consumption by a factor of about five.

Imaging local neuronal activity by monitoring PO₂ transients in capillaries

Two-photon phosphorescence lifetime microscopy (2PLM) has been used recently for depth measurements of oxygen partial pressure (PO(2)) in the rodent brain. In capillaries of olfactory bulb glomeruli, 2PLM has also allowed simultaneous measurements of PO(2) and blood flow and revealed the presence of erythrocyte-associated transients (EATs), which are PO(2) gradients that are associated with individual erythrocytes. We investigated the extent to which EAT properties in capillaries report local neuronal activity. We find that at rest, PO(2) at EAT peaks overestimates the mean PO(2) by 35 mm Hg. PO(2) between two EAT peaks is at equilibrium with, and thus reports, PO(2) in the neuropil. During odor stimulation, there is a small PO(2) decrease before functional hyperemia, showing that the initial dip in PO(2) is present at the level of capillaries. We conclude that imaging oxygen dynamics in capillaries provides a unique and noninvasive approach to map neuronal activity.

Hemodynamic responses in amygdala and hippocampus distinguish between aversive and neutral cues during Pavlovian fear conditioning in behaving rats

Lesion and electrophysiological studies in rodents have identified the amygdala and hippocampus (HPC) as key structures for Pavlovian fear conditioning, but human functional neuroimaging studies have not consistently found activation of these structures. This could be because hemodynamic responses cannot detect the sparse neuronal activity proposed to underlie conditioned fear. Alternatively, differences in experimental design or fear levels could account for the discrepant findings between rodents and humans. To help distinguish between these alternatives, we used tissue oxygen amperometry to record hemodynamic responses from the basolateral amygdala (BLA), dorsal HPC (dHPC) and ventral HPC (vHPC) in freely-moving rats during the acquisition and extinction of conditioned fear. To enable specific comparison with human studies we used a discriminative paradigm, with one auditory cue [conditioned stimulus (CS)+] that was always followed by footshock, and another auditory cue (CS-) that was never followed by footshock. BLA tissue oxygen signals were significantly higher during CS+ than CS- trials during training and early extinction. In contrast, they were lower during CS+ than CS- trials by the end of extinction. dHPC and vHPC tissue oxygen signals were significantly lower during CS+ than CS- trials throughout extinction. Thus, hemodynamic signals in the amygdala and HPC can detect the different patterns of neuronal activity evoked by threatening vs. neutral stimuli during fear conditioning. Discrepant neuroimaging findings may be due to differences in experimental design and/or fear levels evoked in participants. Our methodology offers a way to improve translation between rodent models and human neuroimaging.

Oxidative phosphorylation, not glycolysis, powers presynaptic and postsynaptic mechanisms underlying brain information processing

Neural activity has been suggested to initially trigger ATP production by glycolysis, rather than oxidative phosphorylation, for three reasons: glycolytic enzymes are associated with ion pumps; neurons may increase their energy supply by activating glycolysis in astrocytes to generate lactate; and activity increases glucose uptake more than O₂ uptake. In rat hippocampal slices, neuronal activity rapidly decreased the levels of extracellular O₂ and intracellular NADH (reduced nicotinamide adenine dinucleotide), even with lactate dehydrogenase blocked to prevent lactate generation, or with only 20% superfused O₂ to mimic physiological O₂ levels. Pharmacological analysis revealed an energy budget in which 11% of O₂ use was on presynaptic action potentials, 17% was on presynaptic Ca²⁺ entry and transmitter release, 46% was on postsynaptic glutamate receptors, and 26% was on postsynaptic action potentials, in approximate accord with theoretical brain energy budgets. Thus, the major mechanisms mediating brain information processing are all initially powered by oxidative phosphorylation, and an astrocyte-neuron lactate shuttle is not needed for this to occur.

Extracellular levels of lactate, but not oxygen, reflect sleep homeostasis in the rat cerebral cortex

STUDY OBJECTIVE

It is well established that brain metabolism is higher during wake and rapid eye movement (REM) sleep than in nonrapid eye movement (NREM) sleep. Most of the brain's energy is used to maintain neuronal firing and glutamatergic transmission. Recent evidence shows that cortical firing rates, extracellular glutamate levels, and markers of excitatory synaptic strength increase with time spent awake and decline throughout NREM sleep. These data imply that the metabolic cost of each behavioral state is not fixed but may reflect sleep-wake history, a possibility that is investigated in the current report.

DESIGN

Chronic (4d) electroencephalographic (EEG) recordings in the rat cerebral cortex were coupled with fixed-potential amperometry to monitor the extracellular concentration of oxygen ([oxy]) and lactate ([lac]) on a second-by-second basis across the spontaneous sleep-wake cycle and in response to sleep deprivation.

SETTING

Basic sleep research laboratory.

PATIENTS OR PARTICIPANTS

Wistar Kyoto (WKY) adult male rats.

INTERVENTIONS

N/A.

MEASUREMENTS AND RESULTS

Within 30-60 sec [lac] and [oxy] progressively increased during wake and REM sleep and declined during NREM sleep (n = 10 rats/metabolite), but with several differences. [Oxy], but not [lac], increased more during wake with high motor activity and/or elevated EEG high-frequency power. Meanwhile, only the NREM decline of [lac] reflected sleep pressure as measured by slow-wave activity, mirroring previous results for cortical glutamate.

CONCLUSIONS

The observed state-dependent changes in cortical [lac] and [oxy] are consistent with higher brain metabolism during waking and REM sleep in comparison with NREM sleep. Moreover, these data suggest that glycolytic activity, most likely through its link with glutamatergic transmission, reflects sleep homeostasis.

Evolution of the dynamic changes in functional cerebral oxidative metabolism from tissue mitochondria to blood oxygen

The dynamic properties of the cerebral metabolic rate of oxygen consumption (CMR(O2)) during changes in brain activity remain unclear. Therefore, the spatial and temporal evolution of functional increases in CMR(O2) was investigated in the rat somato-sensory cortex during forelimb stimulation under a suppressed blood flow response condition. Temporally, stimulation elicited a fast increase in tissue mitochondria CMR(O2) described by a time constant of ~1 second measured using flavoprotein autofluorescence imaging. CMR(O2)-driven changes in the tissue oxygen tension measured using an oxygen electrode and blood oxygenation measured using optical imaging of intrinsic signal followed; however, these changes were slow with time constants of ~5 and ~10 seconds, respectively. This slow change in CMR(O2)-driven blood oxygenation partly explains the commonly observed post-stimulus blood oxygen level-dependent (BOLD) undershoot. Spatially, the changes in mitochondria CMR(O2) were similar to the changes in blood oxygenation. Finally, the increases in CMR(O2) were well correlated with the evoked multi-unit spiking activity. These findings show that dynamic CMR(O2) calculations made using only blood oxygenation data (e.g., BOLD functional magnetic resonance imaging (fMRI)) do not directly reflect the temporal changes in the tissue's mitochondria metabolic rate; however, the findings presented can bridge the gap between the changes in cellular oxidative rate and blood oxygenation.

Cellular Links between Neuronal Activity and Energy Homeostasis

Neuronal activity, astrocytic responses to this activity, and energy homeostasis are linked together during baseline, conscious conditions, and short-term rapid activation (as occurs with sensory or motor function). Nervous system energy homeostasis also varies during long-term physiological conditions (i.e., development and aging) and with adaptation to pathological conditions, such as ischemia or low glucose. Neuronal activation requires increased metabolism (i.e., ATP generation) which leads initially to substrate depletion, induction of a variety of signals for enhanced astrocytic function, and increased local blood flow and substrate delivery. Energy generation (particularly in mitochondria) and use during ATP hydrolysis also lead to considerable heat generation. The local increases in blood flow noted following neuronal activation can both enhance local substrate delivery but also provides a heat sink to help cool the brain and removal of waste by-products. In this review we highlight the interactions between short-term neuronal activity and energy metabolism with an emphasis on signals and factors regulating astrocyte function and substrate supply.

Changes in reward-related signals in the rat nucleus accumbens measured by in vivo oxygen amperometry are consistent with fMRI BOLD responses in man

Real-time in vivo oxygen amperometry, a technique that allows measurement of regional brain tissue oxygen (O(2)) has been previously shown to bear relationship to the BOLD signal measured with functional magnetic resonance imaging (fMRI) protocols. In the present study, O(2) amperometry was applied to the study of reward processing in the rat nucleus accumbens to validate the technique with a behavioural process known to cause robust signals in human neuroimaging studies. After acquisition of a cued-lever pressing task a robust increase in O(2) tissue levels was observed in the nucleus accumbens specifically following a correct lever press to the rewarded cue. This O(2) signal was modulated by cue reversal but not lever reversal, by differences in reward magnitudes and by the motivational state of the animal consistent with previous reports of the role of the nucleus accumbens in both the anticipation and representation of reward value. Moreover, this modulation by reward value was related more to the expected incentive value rather than the hedonic value of reward, also consistent with previous reports of accumbens coding of "wanting" of reward. Altogether, these results show striking similarities to those obtained in human fMRI studies suggesting the use of oxygen amperometry as a valid surrogate for fMRI in animals performing cognitive tasks, and a powerful approach to bridge between different techniques of measurement of brain function.

Activity-dependent increases in local oxygen consumption correlate with postsynaptic currents in the mouse cerebellum in vivo

Evoked neural activity correlates strongly with rises in cerebral metabolic rate of oxygen (CMRO(2)) and cerebral blood flow (CBF). Activity-dependent rises in CMRO(2) fluctuate with ATP turnover due to ion pumping. In vitro studies suggest that increases in cytosolic Ca(2+) stimulate oxidative metabolism via mitochondrial signaling, but whether this also occurs in the intact brain is unknown. Here we applied a pharmacological approach to dissect the effects of ionic currents and cytosolic Ca(2+) rises of neuronal origin on activity-dependent rises in CMRO(2). We used two-photon microscopy and current source density analysis to study real-time Ca(2+) dynamics and transmembrane ionic currents in relation to CMRO(2) in the mouse cerebellar cortex in vivo. We report a direct correlation between CMRO(2) and summed (i.e., the sum of excitatory, negative currents during the whole stimulation period) field EPSCs (∑fEPSCs) in Purkinje cells (PCs) in response to stimulation of the climbing fiber (CF) pathway. Blocking stimulus-evoked rises in cytosolic Ca(2+) in PCs with the P/Q-type channel blocker ω-agatoxin-IVA (ω-AGA), or the GABA(A) receptor agonist muscimol, did not lead to a time-locked reduction in CMRO(2), and excitatory synaptic or action potential currents. During stimulation, neither ω-AGA or (μ-oxo)-bis-(trans-formatotetramine-ruthenium) (Ru360), a mitochondrial Ca(2+) uniporter inhibitor, affected the ratio of CMRO(2) to fEPSCs or evoked local field potentials. However, baseline CBF and CMRO(2) decreased gradually with Ru360. Our data suggest that in vivo activity-dependent rises in CMRO(2) are correlated with synaptic currents and postsynaptic spiking in PCs. Our study did not reveal a unique role of neuronal cytosolic Ca(2+) signals in controlling CMRO(2) increases during CF stimulation.

Neuronal inhibition and excitation, and the dichotomic control of brain hemodynamic and oxygen responses

Brain's electrical activity correlates strongly to changes in cerebral blood flow (CBF) and the cerebral metabolic rate of oxygen (CMRO(2)). Subthreshold synaptic processes correlate better than the spike rates of principal neurons to CBF, CMRO(2) and positive BOLD signals. Stimulation-induced rises in CMRO(2) are controlled by the ATP turnover, which depends on the energy used to fuel the Na,K-ATPase to reestablish ionic gradients, while stimulation-induced CBF responses to a large extent are controlled by mechanisms that depend on Ca(2+) rises in neurons and astrocytes. This dichotomy of metabolic and vascular control explains the gap between the stimulation-induced rises in CMRO(2) and CBF, and in turn the BOLD signal. Activity-dependent rises in CBF and CMRO(2) vary within and between brain regions due to differences in ATP turnover and Ca(2+)-dependent mechanisms. Nerve cells produce and release vasodilators that evoke positive BOLD signals, while the mechanisms that control negative BOLD signals by activity-dependent vasoconstriction are less well understood. Activation of both excitatory and inhibitory neurons produces rises in CBF and positive BOLD signals, while negative BOLD signals under most conditions correlate to excitation of inhibitory interneurons, but there are important exceptions to that rule as described in this paper. Thus, variations in the balance between synaptic excitation and inhibition contribute dynamically to the control of metabolic and hemodynamic responses, and in turn the amplitude and polarity of the BOLD signal. Therefore, it is not possible based on a negative or positive BOLD signal alone to decide whether the underlying activity goes on in principal or inhibitory neurons.