The contribution of astrocyte signalling to neurovascular coupling

Astrocyte-Mediated Neuroinflammation in Neurological Conditions

Astrocytes are one of the key glial types of the central nervous system (CNS), accounting for over 20% of total glial cells in the brain. Extensive evidence has established their indispensable functions in the maintenance of CNS homeostasis, as well as their broad involvement in neurological conditions. In particular, astrocytes can participate in various neuroinflammatory processes, e.g., releasing a repertoire of cytokines and chemokines or specific neurotrophic factors, which result in both beneficial and detrimental effects. It has become increasingly clear that such astrocyte-mediated neuroinflammation, together with its complex crosstalk with other glial cells or immune cells, designates neuronal survival and the functional integrity of neurocircuits, thus critically contributing to disease onset and progression. In this review, we focus on the current knowledge of the neuroinflammatory responses of astrocytes, summarizing their common features in neurological conditions. Moreover, we highlight several vital questions for future research that promise novel insights into diagnostic or therapeutic strategies against those debilitating CNS diseases.

Astroglial activation: Current concepts and future directions

Astrocytes are abundantly and ubiquitously expressed cell types with diverse functions throughout the central nervous system. Astrocytes show remarkable plasticity and exhibit morphological, molecular, and functional remodeling in response to injury, disease, or infection of the central nervous system, as evident in neurodegenerative diseases. Astroglial mediated inflammation plays a prominent role in the pathogenesis of neurodegenerative diseases. This review focus on the role of astrocytes as essential players in neuroinflammation and discuss their morphological and functional heterogeneity in the normal central nervous system and explore the spatial and temporal variations in astroglial phenotypes observed under different disease conditions. This review discusses the intimate relationship of astrocytes to pathological hallmarks of neurodegenerative diseases. Finally, this review considers the putative therapeutic strategies that can be deployed to modulate the astroglial functions in neurodegenerative diseases. HIGHLIGHTS: Astroglia mediated neuroinflammation plays a key role in the pathogenesis of neurodegenerative diseases. Activated astrocytes exhibit diverse phenotypes in a region-specific manner in brain and interact with β-amyloid, tau, and α-synuclein species as well as with microglia and neuronal circuits. Activated astrocytes are likely to influence the trajectory of disease progression of neurodegenerative diseases, as determined by the stage of disease, individual susceptibility, and state of astroglial priming. Modulation of astroglial activation may be a therapeutic strategy at various stages in the trajectory of neurodegenerative diseases to modify the disease course.

Ion Channels and Ionotropic Receptors in Astrocytes: Physiological Functions and Alterations in Alzheimer's Disease and Glioblastoma

The human brain is composed of nearly one hundred billion neurons and an equal number of glial cells, including macroglia, i.e., astrocytes and oligodendrocytes, and microglia, the resident immune cells of the brain. In the last few decades, compelling evidence has revealed that glial cells are far more active and complex than previously thought. In particular, astrocytes, the most abundant glial cell population, not only take part in brain development, metabolism, and defense against pathogens and insults, but they also affect sensory, motor, and cognitive functions by constantly modulating synaptic activity. Not surprisingly, astrocytes are actively involved in neurodegenerative diseases (NDs) and other neurological disorders like brain tumors, in which they rapidly become reactive and mediate neuroinflammation. Reactive astrocytes acquire or lose specific functions that differently modulate disease progression and symptoms, including cognitive impairments. Astrocytes express several types of ion channels, including K+, Na+, and Ca2+ channels, transient receptor potential channels (TRP), aquaporins, mechanoreceptors, and anion channels, whose properties and functions are only partially understood, particularly in small processes that contact synapses. In addition, astrocytes express ionotropic receptors for several neurotransmitters. Here, we provide an extensive and up-to-date review of the roles of ion channels and ionotropic receptors in astrocyte physiology and pathology. As examples of two different brain pathologies, we focus on Alzheimer's disease (AD), one of the most diffuse neurodegenerative disorders, and glioblastoma (GBM), the most common brain tumor. Understanding how ion channels and ionotropic receptors in astrocytes participate in NDs and tumors is necessary for developing new therapeutic tools for these increasingly common neurological conditions.

Pathophysiology, Management, and Therapeutics in Subarachnoid Hemorrhage and Delayed Cerebral Ischemia: An Overview

Subarachnoid hemorrhage (SAH) is a type of hemorrhagic stroke resulting from the rupture of an arterial vessel within the brain. Unlike other stroke types, SAH affects both young adults (mid-40s) and the geriatric population. Patients with SAH often experience significant neurological deficits, leading to a substantial societal burden in terms of lost potential years of life. This review provides a comprehensive overview of SAH, examining its development across different stages (early, intermediate, and late) and highlighting the pathophysiological and pathohistological processes specific to each phase. The clinical management of SAH is also explored, focusing on tailored treatments and interventions to address the unique pathological changes that occur during each stage. Additionally, the paper reviews current treatment modalities and pharmacological interventions based on the evolving guidelines provided by the American Heart Association (AHA). Recent advances in our understanding of SAH will facilitate clinicians' improved management of SAH to reduce the incidence of delayed cerebral ischemia in patients.

Modeling the relationship between neuronal activity and the BOLD signal: contributions from astrocyte calcium dynamics

Functional magnetic resonance imaging relies on the coupling between neuronal and vascular activity, but the mechanisms behind this coupling are still under discussion. Recent experimental evidence suggests that calcium signaling may play a significant role in neurovascular coupling. However, it is still controversial where this calcium signal is located (in neurons or elsewhere), how it operates and how relevant is its role. In this paper we introduce a biologically plausible model of the neurovascular coupling and we show that calcium signaling in astrocytes can explain main aspects of the dynamics of the coupling. We find that calcium signaling can explain so-far unrelated features such as the linear and non-linear regimes, the negative vascular response (undershoot) and the emergence of a (calcium-driven) Hemodynamic Response Function. These features are reproduced here for the first time by a single model of the detailed neuronal-astrocyte-vascular pathway. Furthermore, we analyze how information is coded and transmitted from the neuronal to the vascular system and we predict that frequency modulation of astrocytic calcium dynamics plays a key role in this process. Finally, our work provides a framework to link neuronal activity to the BOLD signal, and vice-versa, where neuronal activity can be inferred from the BOLD signal. This opens new ways to link known alterations of astrocytic calcium signaling in neurodegenerative diseases (e.g. Alzheimer's and Parkinson's diseases) with detectable changes in the neurovascular coupling.

Challenging neurovascular coupling through complex and variable duration cognitive paradigms: A subcomponent analysis

A similar pattern of cerebral blood velocity (CBv) response has been observed for neurovascular coupling (NVC) assessment with cognitive tasks of varying complexity and duration. This lack of specificity could result from parallel changes in arterial blood pressure (BP) and PaCO₂, which could confound the estimates of NVC integrity. Healthy participants (n = 16) underwent recordings at rest (5 min sitting) and during randomized paradigms of different complexity (naming words (NW) beginning with P-, R-, V- words and serial subtractions (SS) of 100-2, 100-7, 1000-17, with durations of 5, 30 and 60 s). Bilateral CBv (middle cerebral arteries, transcranial Doppler), end-tidal CO₂ (EtCO₂, capnography), blood pressure (BP, Finapres) and heart rate (HR, ECG) were recorded continuously. The bilateral CBv response to all paradigms was classified under objective criteria to select only responders, then the repeated data were averaged between visits. Bilateral CBv change to tasks was decomposed into the relative contributions (subcomponents) of arterial BP (V_BP; neurogenic), critical closing pressure (V_CrCP; metabolic) and resistance area product (V_RAP; myogenic). A temporal effect was demonstrated in bilateral V_BP and V_RAP during all tasks (p<0.002), increased V_BP early (between 0 and 10 s) and followed by decreases of V_RAP late (25-35 s) in the response. V_CrCP varied by complexity and duration (p<0.046). The main contributions to CBv responses to cognitive tasks of different complexity and duration were V_BP and V_RAP, whilst a smaller contribution from V_CrCP would suggest sensitivity to metabolic demands. Further studies are needed to assess the influence of different paradigms, ageing and cerebrovascular conditions.

Brain Energy Metabolism: Astrocytes in Neurodegenerative Diseases

Astrocytes are the most abundant cells in the brain. They have many important functions in the central nervous system (CNS), including the maintenance of glutamate and ion homeostasis, the elimination of oxidative stress, energy storage in glycogen, tissue repair, regulating synaptic activity by releasing neurotransmitters, and participating in synaptic formation. Astrocytes have special highly ramified structure. Their branches contact with synapses of neurons inwardly, with fine structure and wrapping synapses; their feet contact with blood vessels of brain parenchyma outward, almost wrapping the whole brain. The adjacent astrocytes rarely overlap and communicate with each other through gap junction channels. The ideal location of astrocytes enables them to sense the weak changes of their surroundings and provide the structural basis for the energy supply of neurons. Neurons and astrocytes are closely coupled units of energy metabolism in the brain. Neurons consume a lot of ATPs in the process of neurotransmission. Astrocytes provide metabolic substrates for neurons, maintain high activity of neuron, and facilitate information transmission of neurons. This article reviews the characteristics of glucose metabolism, lipid metabolism, and amino acid metabolism of astrocytes. The metabolic interactions between astrocytes and neurons, astrocytes and microglia were also detailed discussed. Finally, we classified analyzed the role of metabolic disorder of astrocytes in the occurrence and development of neurodegenerative diseases.

The glymphatic system: Current understanding and modeling

We review theoretical and numerical models of the glymphatic system, which circulates cerebrospinal fluid and interstitial fluid around the brain, facilitating solute transport. Models enable hypothesis development and predictions of transport, with clinical applications including drug delivery, stroke, cardiac arrest, and neurodegenerative disorders like Alzheimer's disease. We sort existing models into broad categories by anatomical function: Perivascular flow, transport in brain parenchyma, interfaces to perivascular spaces, efflux routes, and links to neuronal activity. Needs and opportunities for future work are highlighted wherever possible; new models, expanded models, and novel experiments to inform models could all have tremendous value for advancing the field.

Altered linear coupling between stimulus-evoked blood flow and oxygen metabolism in the aging human brain

Neural-vascular coupling (NVC) is the process by which oxygen and nutrients are delivered to metabolically active neurons by blood vessels. Murine models of NVC disruption have revealed its critical role in healthy neural function. We hypothesized that, in humans, aging exerts detrimental effects upon the integrity of the neural-glial-vascular system that underlies NVC. To test this hypothesis, calibrated functional magnetic resonance imaging (cfMRI) was used to characterize age-related changes in cerebral blood flow (CBF) and oxygen metabolism during visual cortex stimulation. Thirty-three younger and 27 older participants underwent cfMRI scanning during both an attention-controlled visual stimulation task and a hypercapnia paradigm used to calibrate the blood-oxygen-level-dependent signal. Measurement of stimulus-evoked blood flow and oxygen metabolism permitted calculation of the NVC ratio to assess the integrity of neural-vascular communication. Consistent with our hypothesis, we observed monotonic NVC ratio increases with increasing visual stimulation frequency in younger adults but not in older adults. Age-related changes in stimulus-evoked cerebrovascular and neurometabolic signal could not fully explain this disruption; increases in stimulus-evoked neurometabolic activity elicited corresponding increases in stimulus-evoked CBF in younger but not in older adults. These results implicate age-related, demand-dependent failures of the neural-glial-vascular structures that comprise the NVC system.

Microvascular Changes in Parkinson’s Disease- Focus on the Neurovascular Unit

Vascular alterations emerge as a common denominator for several neurodegenerative diseases. In Parkinson’s disease (PD), a number of observations have been made suggesting that the occurrence of vascular pathology is an important pathophysiological aspect of the disease. Specifically, pathological activation of pericytes, blood-brain barrier (BBB) disruption, pathological angiogenesis and vascular regression have been reported. This review summarizes the current evidence for the different vascular alterations in patients with PD and in animal models of PD. We suggest a possible sequence of vascular pathology in PD ranging from early pericyte activation and BBB leakage to an attempt for compensatory angiogenesis and finally vascular rarefication. We highlight different pathogenetic mechanisms that play a role in these vascular alterations including perivascular inflammation and concomitant metabolic disease. Awareness of the contribution of vascular events to the pathogenesis of PD may allow the identification of targets to modulate those mechanisms. In particular the BBB has for decades only been viewed as an obstacle for drug delivery, however, preservation of its integrity and/or modulation of the signaling at this interface between the blood and the brain may prove to be a new avenue to take in order to develop disease-modifying strategies for neurodegenerative disorders.

Unraveling Aβ-Mediated Multi-Pathway Calcium Dynamics in Astrocytes: Implications for Alzheimer's Disease Treatment From Simulations

The accumulation of amyloid β peptide (Aβ) in the brain is hypothesized to be the major factor driving Alzheimer's disease (AD) pathogenesis. Mounting evidence suggests that astrocytes are the primary target of Aβ neurotoxicity. Aβ is known to interfere with multiple calcium fluxes, thus disrupting the calcium homeostasis regulation of astrocytes, which are likely to produce calcium oscillations. Ca²⁺ dyshomeostasis has been observed to precede the appearance of clinical symptoms of AD; however, it is experimentally very difficult to investigate the interactions of many mechanisms. Given that Ca²⁺ disruption is ubiquitously involved in AD progression, it is likely that focusing on Ca²⁺ dysregulation may serve as a potential therapeutic approach to preventing or treating AD, while current hypotheses concerning AD have so far failed to yield curable therapies. For this purpose, we derive and investigate a concise mathematical model for Aβ-mediated multi-pathway astrocytic intracellular Ca²⁺ dynamics. This model accounts for how Aβ affects various fluxes contributions through voltage-gated calcium channels, Aβ-formed channels and ryanodine receptors. Bifurcation analysis of Aβ level, which reflected the corresponding progression of the disease, revealed that Aβ significantly induced the increasing [Ca²⁺] _i and frequency of calcium oscillations. The influence of inositol 1,4,5-trisphosphate production (IP₃) is also investigated in the presence of Aβ as well as the impact of changes in resting membrane potential. In turn, the Ca²⁺ flux can be considerably changed by exerting specific interventions, such as ion channel blockers or receptor antagonists. By doing so, a "combination therapy" targeting multiple pathways simultaneously has finally been demonstrated to be more effective. This study helps to better understand the effect of Aβ, and our findings provide new insight into the treatment of AD.

Soluble Guanylate Cyclase Stimulator Vericiguat Enhances Long-Term Memory in Rats without Altering Cerebral Blood Volume

Vascular cognitive impairment (VCI) is characterized by impairments in cerebral blood flow (CBF), endothelial function and blood-brain barrier (BBB) integrity. These processes are all physiologically regulated by the nitric oxide (NO)-soluble guanylate cyclase (sGC)-cGMP signaling pathway. Additionally, cGMP signaling plays an important role in long-term potentiation (LTP) underlying memory formation. Therefore, targeting the NO-sGC-cGMP pathway may be a therapeutic strategy for treating VCI. Hence, in this study we investigated whether sGC stimulator vericiguat has potential as a cognitive enhancer. The effects of vericiguat on long-term memory were measured in rats using an object location task. Due to the low brain-penetrance of vericiguat found in this study, it was investigated whether in the absence of BBB limitations, vericiguat enhanced hippocampal plasticity using an ex vivo memory acquisition-like chemical LTP model. Finally, peripheral effects were measured by means of blood pressure and cerebral blood volume. Vericiguat successfully enhanced long-term memory and increased hippocampal plasticity via enhanced translocation of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors to the cell membrane, while blood pressure and cerebral blood volume were unaltered. Although the memory enhancing effects in this study are likely due to peripheral effects on the cerebral microvasculature, sGC stimulation may provide a new therapeutic strategy for treating VCI, especially when BBB integrity is reduced.

Age-related alterations in the cerebrovasculature affect neurovascular coupling and BOLD fMRI responses: Insights from animal models of aging

The present and future research efforts in cognitive neuroscience and psychophysiology rely on the measurement, understanding, and interpretation of blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) to effectively investigate brain function. Aging and age-associated pathophysiological processes change the structural and functional integrity of the cerebrovasculature which can significantly alter how the BOLD signal is recorded and interpreted. In order to gain an improved understanding of the benefits, drawbacks, and methodological implications for BOLD fMRI in the context of cognitive neuroscience, it is crucial to understand the cellular and molecular mechanism of age-related vascular pathologies. This review discusses the multifaceted effects of aging and the contributions of age-related pathologies on structural and functional integrity of the cerebral microcirculation as they has been investigated in animal models of aging, including age-related alterations in neurovascular coupling responses, cellular and molecular mechanisms involved in microvascular damage, vascular rarefaction, blood-brain barrier disruption, senescence, humoral deficiencies as they relate to, and potentially introduce confounding factors in the interpretation of BOLD fMRI.

Astrocyte-Neuron Metabolic Crosstalk in Neurodegeneration: A Mitochondrial Perspective

Converging evidence made clear that declining brain energetics contribute to aging and are implicated in the initiation and progression of neurodegenerative disorders such as Alzheimer's and Parkinson's disease. Indeed, both pathologies involve instances of hypometabolism of glucose and oxygen in the brain causing mitochondrial dysfunction, energetic failure and oxidative stress. Importantly, recent evidence suggests that astrocytes, which play a key role in supporting neuronal function and metabolism, might contribute to the development of neurodegenerative diseases. Therefore, exploring how the neuro-supportive role of astrocytes may be impaired in the context of these disorders has great therapeutic potential. In the following, we will discuss some of the so far identified features underlining the astrocyte-neuron metabolic crosstalk. Thereby, special focus will be given to the role of mitochondria. Furthermore, we will report on recent advancements concerning iPSC-derived models used to unravel the metabolic contribution of astrocytes to neuronal demise. Finally, we discuss how mitochondrial dysfunction in astrocytes could contribute to inflammatory signaling in neurodegenerative diseases.

Vascular Dysfunction in the Brain; Implications for Heavy Metal Exposures

Normal or diseased conditions that alter the brain's requirement for oxygen and nutrients via alterations to neurovascular coupling have an impact on the level of the neurovascular unit; comprising neuronal, glial and vascular components. The communications between the components of the neurovascular unit are precise and accurate for its functions; hence a minute disturbance can result in neurovascular dysfunction. Heavy metals such as cadmium, mercury, and lead have been identified to increase the vulnerability of the neurovascular unit to damage. This review examines the role of heavy metals in neurovascular dysfunctions and the possible mechanisms by which these metals act. Risk factors ranging from lifestyle, environment, genetics, infections, and physiologic ageing involved in neurological dysfunctions were highlighted, while stroke was discussed as the prevalent consequence of neurovascular dysfunctions. Furthermore, the role of these heavy metals in the pathogenesis of stroke consequently pinpoints the importance of understanding the mechanisms of neurovascular damage in a bid to curb the occurrence of neurovascular dysfunctions.

In-depth quantitative proteomic characterization of organotypic hippocampal slice culture reveals sex-specific differences in biochemical pathways

Sex differences in the brain of mammals range from neuroarchitecture through cognition to cellular metabolism. The hippocampus, a structure mostly associated with learning and memory, presents high vulnerability to neurodegeneration and aging. Therefore, we explored basal sex-related differences in the proteome of organotypic hippocampal slice culture, a major in vitro model for studying the cellular and molecular mechanisms related to neurodegenerative disorders. Results suggest a greater prevalence of astrocytic metabolism in females and significant neuronal metabolism in males. The preference for glucose use in glycolysis, pentose phosphate pathway and glycogen metabolism in females and high abundance of mitochondrial respiration subunits in males support this idea. An overall upregulation of lipid metabolism was observed in females. Upregulation of proteins responsible for neuronal glutamate and GABA synthesis, along with synaptic associated proteins, were observed in males. In general, the significant spectrum of pathways known to predominate in neurons or astrocytes, together with the well-known neuronal and glial markers observed, revealed sex-specific metabolic differences in the hippocampus. TEM qualitative analysis might indicate a greater presence of mitochondria at CA1 synapses in females. These findings are crucial to a better understanding of how sex chromosomes can influence the physiology of cultured hippocampal slices and allow us to gain insights into distinct responses of males and females on neurological diseases that present a sex-biased incidence.

Cerebral blood microcirculation measurement in APP/PS1 double transgenic mice at the preclinical stage of Alzheimer's disease: preliminary data on the early intervention of anodal transcranial direct current stimulation

Anodal transcranial direct current stimulation (AtDCS) can improve memory and cognitive dysfunction in patients with Alzheimer's disease (AD), which has been proven in basic and clinical studies. Intervention of AD in preclinical stage is important to prevent progression of AD in the aging society. At the same time, there is increasing evidence that a close link exists between cerebrovascular dysfunction and AD disease. Here we investigated the changes of local cerebral blood microcirculation in preclinical AD mouse model after AtDCS based on the previous studies. Twenty-four 6-month-old male APP/PS1 double transgenic mice were randomly divided into three groups: a model group (AD), a model sham stimulation (ADST) group and a model stimulation group (ATD). Eight 6-month-old male C57 wild-type mice served as a control group (CTL). Mice in the ATD group received 10 AtDCS sessions. Two months after the end of AtDCS in the ATD group, the microcirculation parameters of the frontal cortex of the mice in each group, including cerebral blood flow (CBF), blood flow velocity (Velo), oxygen saturation (SO₂) and relative hemoglobin content (rHb), were obtained by the non-invasive laser-Doppler spectrophotometry system "Oxygen-to-See (O2C)". The results showed that AtDCS increased CBF, Velo and SO₂, and reduce rHb in APP/PS1 double transgenic mice at the preclinical stage of AD.Clinical Relevance-This shows the positive effect of AtDCS on preclinical AD in cerebrovascular function, and provides effective basic research facts for AtDCS to intervene and delay the clinical application of AD disease.

Role of Delayed Neuroglial Activation in Impaired Cerebral Blood Flow Restoration Following Comorbid Injury

Besides other causes, ischemia and Alzheimer's disease pathology is also linked to decreased cerebral blood flow (CBF). There is little or no consensus about the role of neuroglial cells in maintaining CBF in various neuropathologies. This consensus becomes scarcer when it comes to clinical and experimental cases of comorbid Abeta-amyloid (Aβ) toxicity and ischemia. Here, a comorbid rat model of Aβ toxicity and endothelin-1 induced ischemia (ET1) not only demonstrated the appearance of axotomized phagocytosed pyknotic neurons (NeuN) immediately after the injury, but also showed a diversity of continuously changing neuroglia (MHC Class II/OX6, Iba1) and macrophage (Iba1/CD68) phenotypes with round, stout somas, and retracted processes. This is indicative of a response to a concomitant increase in large fluid-filled spaces due to the vascular leakage. Ironically 4 weeks after the injury despite a conclusive reduction in neurons, CBF restoration in ET1 rats was associated with a massive increase in neuroglial cell numbers, hypertrophy, ramification, and soma sizes bordering the continuously reducing lesion core and inflamed vasculature, possibly to shield their leaky phenotype. Astrocytes were also found to be releasing matrix metalloproteinase9 (MMP9), which stabilized matrix ligand β-dystroglycan (β-DG) in repaired or functional vessels. Changing neuroglia phenotypes, responses, motility, astrocytic recruitment of MMP9, and β-DG stabilization implies the role of communication between neuroglia and endothelium in recovering CBF, in the absence of neurons, in ET1 rats compared to Aβ+ET1 rats, which showed characteristics delayed neuroglial activation. Stimulation of timely neuroglial reactivity may serve as a viable strategy to compensate for the neuronal loss in restoring CBF in comorbid cases of ischemia and Aβ toxicity.

Comparing dynamic causal models of neurovascular coupling with fMRI and EEG/MEG

This technical note presents a dynamic causal modelling (DCM) procedure for evaluating different models of neurovascular coupling in the human brain - using combined electromagnetic (M/EEG) and functional magnetic resonance imaging (fMRI) data. This procedure compares the evidence for biologically informed models of neurovascular coupling using Bayesian model comparison. First, fMRI data are used to localise regionally specific neuronal responses. The coordinates of these responses are then used as the location priors in a DCM of electrophysiological responses elicited by the same paradigm. The ensuing estimates of model parameters are then used to generate neuronal drive functions, which model pre- or post-synaptic activity for each experimental condition. These functions form the input to a model of neurovascular coupling, whose parameters are estimated from the fMRI data. Crucially, this enables one to evaluate different models of neurovascular coupling, using Bayesian model comparison - asking, for example, whether instantaneous or delayed, pre- or post-synaptic signals mediate haemodynamic responses. We provide an illustrative application of the procedure using a single-subject auditory fMRI and MEG dataset. The code and exemplar data accompanying this technical note are available through the statistical parametric mapping (SPM) software.

The Neurovascular Unit: Effects of Brain Insults During the Perinatal Period

The neurovascular unit (NVU) is a relatively recent concept in neuroscience that broadly describes the relationship between brain cells and their blood vessels. The NVU incorporates cellular and extracellular components involved in regulating cerebral blood flow and blood-brain barrier function. The NVU within the adult brain has attracted strong research interest and its structure and function is well described, however, the NVU in the developing brain over the fetal and neonatal period remains much less well known. One area of particular interest in perinatal brain development is the impact of known neuropathological insults on the NVU. The aim of this review is to synthesize existing literature to describe structure and function of the NVU in the developing brain, with a particular emphasis on exploring the effects of perinatal insults. Accordingly, a brief overview of NVU components and function is provided, before discussion of NVU development and how this may be affected by perinatal pathologies. We have focused this discussion around three common perinatal insults: prematurity, acute hypoxia, and chronic hypoxia. A greater understanding of processes affecting the NVU in the perinatal period may enable application of targeted therapies, as well as providing a useful basis for research as it expands further into this area.

AMPA Receptor-Mediated Ca2+ Transients in Mouse Olfactory Ensheathing Cells

Ca²⁺ signaling in glial cells is primarily triggered by metabotropic pathways and the subsequent Ca²⁺ release from internal Ca²⁺ stores. However, there is upcoming evidence that various ion channels might also initiate Ca²⁺ rises in glial cells by Ca²⁺ influx. We investigated AMPA receptor-mediated inward currents and Ca²⁺ transients in olfactory ensheathing cells (OECs), a specialized glial cell population in the olfactory bulb (OB), using whole-cell voltage-clamp recordings and confocal Ca²⁺ imaging. By immunohistochemistry we showed immunoreactivity to the AMPA receptor subunits GluA1, GluA2 and GluA4 in OECs, suggesting the presence of AMPA receptors in OECs. Kainate-induced inward currents were mediated exclusively by AMPA receptors, as they were sensitive to the specific AMPA receptor antagonist, GYKI53655. Moreover, kainate-induced inward currents were reduced by the selective Ca²⁺-permeable AMPA receptor inhibitor, NASPM, suggesting the presence of functional Ca²⁺-permeable AMPA receptors in OECs. Additionally, kainate application evoked Ca²⁺ transients in OECs which were abolished in the absence of extracellular Ca²⁺, indicating that Ca²⁺ influx via Ca²⁺-permeable AMPA receptors contribute to kainate-induced Ca²⁺ transients. However, kainate-induced Ca²⁺ transients were partly reduced upon Ca²⁺ store depletion, leading to the conclusion that Ca²⁺ influx via AMPA receptor channels is essential to trigger Ca²⁺ transients in OECs, whereas Ca²⁺ release from internal stores contributes in part to the kainate-evoked Ca²⁺ response. Endogenous glutamate release by OSN axons initiated Ca²⁺ transients in OECs, equally mediated by metabotropic receptors (glutamatergic and purinergic) and AMPA receptors, suggesting a prominent role for AMPA receptor mediated Ca²⁺ signaling in axon-OEC communication.

Improving Understanding and Outcomes of Traumatic Brain Injury Using Bidirectional Translational Research

Recent clinical trials in traumatic brain injury (TBI) have failed to demonstrate therapeutic effects even when there appears to be good evidence for efficacy in one or more appropriate pre-clinical models. While existing animal models mimic the injury, difficulties in translating promising therapeutics are exacerbated by the lack of alignment of discrete measures of the underlying injury pathology between the animal models and human subjects. To address this mismatch, we have incorporated reverse translation of bedside experience to inform pre-clinical studies in a large animal (pig) model of TBI that mirror practical clinical assessments. Cerebral autoregulation is impaired after TBI, contributing to poor outcome. Cerebral perfusion pressure (CPP) is often normalized by use of vasoactive agents to increase mean arterial pressure (MAP) and thereby limit impairment of cerebral autoregulation and neurological deficits. Vasoactive agents clinically used to elevate MAP to increase CPP after TBI, such as phenylephrine (Phe), dopamine (DA), norepinephrine (NE), and epinephrine (EPI), however, have not been compared sufficiently regarding effect on CPP, autoregulation, and survival after TBI, and clinically, current vasoactive agent use is variable. The cerebral effects of these clinically commonly used vasoactive agents are not known. This review will emphasize pediatric work and will describe bidirectional translational studies using a more human-like animal model of TBI to identify better therapeutic strategies to improve outcome post-injury. These studies in addition investigated the mechanism(s) involved in improvement of outcome in the setting of TBI.

Preserved canonicality of the BOLD hemodynamic response reflects healthy cognition: Insights into the healthy brain through the window of Multiple Sclerosis

The hemodynamic response function (HRF), a model of brain blood-flow changes in response to neural activity, reflects communication between neurons and the vasculature that supplies these neurons in part by means of glial cell intermediaries (e.g., astrocytes). Intact neural-vascular communication might play a central role in optimal cognitive performance. This hypothesis can be tested by comparing healthy individuals to those with known white-matter damage and impaired performance, as seen in Multiple Sclerosis (MS). Glial cell intermediaries facilitate the ability of neurons to adequately convey metabolic needs to cerebral vasculature for sufficient oxygen and nutrient perfusion. In this study, we isolated measurements of the HRF that could quantify the extent to which white-matter affects neural-vascular coupling and cognitive performance. HRFs were modeled from multiple brain regions during multiple cognitive tasks using piecewise cubic spline functions, an approach that minimized assumptions regarding HRF shape that may not be valid for diseased populations, and were characterized using two shape metrics (peak amplitude and time-to-peak). Peak amplitude was reduced, and time-to-peak was longer, in MS patients relative to healthy controls. Faster time-to-peak was predicted by faster reaction time, suggesting an important role for vasodilatory speed in the physiology underlying processing speed. These results support the hypothesis that intact neural-glial-vascular communication underlies optimal neural and cognitive functioning.

Panglial gap junctions between astrocytes and olfactory ensheathing cells mediate transmission of Ca2+ transients and neurovascular coupling

Astrocytes are arranged in highly organized gap junction-coupled networks, communicating via the propagation of Ca²⁺ waves. Astrocytes are gap junction-coupled not only to neighboring astrocytes, but also to oligodendrocytes, forming so-called panglial syncytia. It is not known, however, whether glial cells in panglial syncytia transmit information using Ca²⁺ signaling. We used confocal Ca²⁺ imaging to study intercellular communication between astrocytes and olfactory ensheathing glial cells (OECs) in in-toto preparations of the mouse olfactory bulb. Our results demonstrate that Ca²⁺ transients in juxtaglomerular astrocytes, evoked by local photolysis of "caged" ATP and "caged" tACPD, led to subsequent Ca²⁺ responses in OECs. This transmission of Ca²⁺ responses from astrocytes to OECs persisted in the presence of neuronal inhibition, but was absent when gap junctional coupling was suppressed with carbenoxolone. When Ca²⁺ transients were directly evoked in OECs by puff application of DHPG, they resulted in delayed Ca²⁺ responses in juxtaglomerular astrocytes, indicating that panglial transmission of Ca²⁺ signals occurred in a bidirectional manner. In addition, panglial transmission of Ca²⁺ signals from astrocytes to OECs resulted in vasoconstriction of OEC-associated blood vessels in the olfactory nerve layer. Our results demonstrate functional transmission of Ca²⁺ signals between different classes of glial cells within gap junction-coupled panglial networks and the resulting regulation of blood vessel diameter in the olfactory bulb.

Frequency-specific functional connectivity related to the rehabilitation task of stroke patients

PURPOSES

Stroke survivors suffering from deficits in motor control typically show persistent motor symptoms and limited functional abilities, which affect their functional independence in daily life. Active rehabilitation training is commonly applied for stroke patients to recover from motor dysfunction. The global connectivity reflects the synchronization of cardiac and respiratory activities in the cerebral regions. However, the understanding of the patterns of frequency-specific global connectivity (GC) and functional connectivity (FC) when performing active rehabilitation training is still far from comprehensive. This study was conducted to investigate the brain network patterns of stroke patients while performing a four-limb linkage rehabilitation training using the functional near-infrared spectroscopy (fNIRS) method.

METHODS

Two groups of stroke patients (LH, left hemiplegia; RH, right hemiplegia) and one healthy group were recruited to participate in this study. The wavelet phase coherence (WPCO) method was used to calculate the frequency-specific GC and FC of the brain network in four frequency bands: I, 0.6-2 Hz; II, 0.145-0.6 Hz; III, 0.052-0.145 Hz; and IV, 0.021-0.052 Hz.

RESULTS

Results showed that the healthy group exhibited lower WPCO in the four frequency bands during the task state than during the resting state (P < 0.05). Interestingly, the stroke groups showed increased WPCO in the frequency band II during the task state than during the resting state (P < 0.05). Moreover, significantly lower WPCO values in the frequency bands III (P < 0.05) were found during task state in the RH and LH groups compared with the healthy group. The RH group showed increased WPCO values in the frequency band II during the task state compared with the healthy group (P < 0.05). In addition, the RH group showed increased WPCO in the frequency bands I (P < 0.05) and II (P < 0.05) than the LH group. Notably, the rehabilitation task did not induce significant changes in stroke groups in the frequency band IV, which implied the neurogenic activity.

CONCLUSIONS

The reductions in FC suggested that the brain impairments caused a disturbed neurovascular coupling regulation in stroke patients. Results in frequency band IV suggested that the limb movement rehabilitation task intrinsically may not produce remarkable effect on the neurogenic activity of stroke patients. Significant differences in WPCO between the LH and RH groups suggested that the rehabilitation task should be specifically designed for individual rehabilitation. The frequency-specific FC methods based on WPCO would provide a potential approach to quantitatively assess the effect of rehabilitation tasks.

Astrocytes in Flavivirus Infections

Virus infections of the central nervous system (CNS) can manifest in various forms of inflammation, including that of the brain (encephalitis) and spinal cord (myelitis), all of which may have long-lasting deleterious consequences. Although the knowledge of how different viruses affect neural cells is increasing, understanding of the mechanisms by which cells respond to neurotropic viruses remains fragmented. Several virus types have the ability to infect neural tissue, and astrocytes, an abundant and heterogeneous neuroglial cell type and a key element providing CNS homeostasis, are one of the first CNS cell types to get infected. Astrocytes are morphologically closely aligned with neuronal synapses, blood vessels, and ventricle cavities, and thereby have the capacity to functionally interact with neurons and endothelial cells. In this review, we focus on the responses of astrocytes to infection by neurotropic flaviviruses, including tick-borne encephalitis virus (TBEV), Zika virus (ZIKV), West Nile virus (WNV), and Japanese encephalitis virus (JEV), which have all been confirmed to infect astrocytes and cause multiple CNS defects. Understanding these mechanisms may help design new strategies to better contain and mitigate virus- and astrocyte-dependent neuroinflammation.

Astrogliopathology in the infectious insults of the brain

Astroglia, a heterogeneous type of neuroglia, play key homeostatic functions in the central nervous system (CNS) and represent an important defence system. Impaired homeostatic capacity of astrocytes manifests in diseases and this is mirrored in various astrocyte-based pathological features including reactive astrogliosis, astrodegeneration with astroglial atrophy and pathological remodelling of astrocytes. All of these manifestations are most prominently associated with infectious insults, mediated by bacteria, protozoa and viruses. Here we focus onto neurotropic viruses such as tick-borne encephalitis (TBEV) and Zika virus (ZIKV), both belonging to Flaviviridae and both causing severe neurological impairments. We argue that astrocytes provide a route through which neurotropic infectious agents attack the CNS, since they are anatomically associated with the blood-brain barrier and exhibit aerobic glycolysis, a metabolic specialisation of highly morphologically dynamic cells, which may provide a suitable metabolic milieu for proliferation of infectious agents, including viral bodies.

Computational Models for Calcium-Mediated Astrocyte Functions

The computational neuroscience field has heavily concentrated on the modeling of neuronal functions, largely ignoring other brain cells, including one type of glial cell, the astrocytes. Despite the short history of modeling astrocytic functions, we were delighted about the hundreds of models developed so far to study the role of astrocytes, most often in calcium dynamics, synchronization, information transfer, and plasticity in vitro, but also in vascular events, hyperexcitability, and homeostasis. Our goal here is to present the state-of-the-art in computational modeling of astrocytes in order to facilitate better understanding of the functions and dynamics of astrocytes in the brain. Due to the large number of models, we concentrated on a hundred models that include biophysical descriptions for calcium signaling and dynamics in astrocytes. We categorized the models into four groups: single astrocyte models, astrocyte network models, neuron-astrocyte synapse models, and neuron-astrocyte network models to ease their use in future modeling projects. We characterized the models based on which earlier models were used for building the models and which type of biological entities were described in the astrocyte models. Features of the models were compared and contrasted so that similarities and differences were more readily apparent. We discovered that most of the models were basically generated from a small set of previously published models with small variations. However, neither citations to all the previous models with similar core structure nor explanations of what was built on top of the previous models were provided, which made it possible, in some cases, to have the same models published several times without an explicit intention to make new predictions about the roles of astrocytes in brain functions. Furthermore, only a few of the models are available online which makes it difficult to reproduce the simulation results and further develop the models. Thus, we would like to emphasize that only via reproducible research are we able to build better computational models for astrocytes, which truly advance science. Our study is the first to characterize in detail the biophysical and biochemical mechanisms that have been modeled for astrocytes.

Which of the branched-chain amino acids increases cerebral blood flow in hepatic encephalopathy? A double-blind randomized trial

Branched-chain amino acids increase the brain perfusion of patients with hepatic encephalopathy (HE), but the amino acid and the mechanisms involved are still unknown. This study compared brain perfusion and clinical improvement during leucine or isoleucine supplementation. After randomization, 27 subjects with cirrhosis and HE received leucine or isoleucine supplements for one year. Brain single Photon Emission Computed Tomography (SPECT) and dynamic brain scintigraphy (DBS) were performed pretreatment and at 1, 8 and 12 months of supplementation. Brain perfusion was increased only in the isoleucine group at 8 months of treatment by both SPECT and DBS (p < 0.001 and p = 0.05, respectively) and by SPECT at the 12th month (p < 0.05). This was associated with hepatic encephalopathy improvement at 8 and 12 months (p = 0.008 and 0.004, respectively), which was not observed in the leucine group (p = 0.313 and 0.055, respectively). Isoleucine supplementation achieved a better impact on brain perfusion restoration in HE.

Impact of Metabolic Diseases on Cerebral Circulation: Structural and Functional Consequences

Metabolic diseases including obesity, insulin resistance, and diabetes have profound effects on cerebral circulation. These diseases not only affect the architecture of cerebral blood arteries causing adverse remodeling, pathological neovascularization, and vasoregression but also alter the physiology of blood vessels resulting in compromised myogenic reactivity, neurovascular uncoupling, and endothelial dysfunction. Coupled with the disruption of blood brain barrier (BBB) integrity, changes in blood flow and microbleeds into the brain rapidly occur. This overview is organized into sections describing cerebrovascular architecture, physiology, and BBB in these diseases. In each section, we review these properties starting with larger arteries moving into smaller vessels. Where information is available, we review in the order of obesity, insulin resistance, and diabetes. We also tried to include information on biological variables such as the sex of the animal models noted since most of the information summarized was obtained using male animals. © 2018 American Physiological Society. Compr Physiol 8:773-799, 2018.

Astroglial vesicular network: evolutionary trends, physiology and pathophysiology

Intracellular organelles, including secretory vesicles, emerged when eukaryotic cells evolved some 3 billion years ago. The primordial organelles that evolved in Archaea were similar to endolysosomes, which developed, arguably, for specific metabolic tasks, including uptake, metabolic processing, storage and disposal of molecules. In comparison with prokaryotes, cell volume of eukaryotes increased by several orders of magnitude and vesicle traffic emerged to allow for communication between distant intracellular locations. Lysosomes, first described in 1955, a prominent intermediate of endo- and exocytotic pathways, operate virtually in all eukaryotic cells including astroglia, the most heterogeneous type of homeostatic glia in the central nervous system. Astrocytes support neuronal network activity in particular through elaborated secretion, based on a complex intracellular vesicle network dynamics. Deranged homeostasis underlies disease and astroglial vesicle traffic contributes to the pathophysiology of neurodegenerative (Alzheimer's disease, Huntington's disease), neurodevelopmental diseases (intellectual deficiency, Rett's disease) and neuroinfectious (Zika virus) disorders. This review addresses astroglial cell-autonomous vesicular traffic network, as well as its into primary and secondary vesicular network defects in diseases, and considers this network as a target for developing new therapies for neurological conditions.

Turing-like structures in a functional model of cortical spreading depression

Cortical spreading depression (CSD) along with migraine waves and spreading depolarization events with stroke or injures are the front-line examples of extreme physiological behaviors of the brain cortex which manifest themselves via the onset and spreading of localized areas of neuronal hyperactivity followed by their depression. While much is known about the physiological pathways involved, the dynamical mechanisms of the formation and evolution of complex spatiotemporal patterns during CSD are still poorly understood, in spite of the number of modeling studies that have been already performed. Recently we have proposed a relatively simple mathematical model of cortical spreading depression which counts the effects of neurovascular coupling and cerebral blood flow redistribution during CSD. In the present study, we address the main dynamical consequences of newly included pathways, namely, the changes in the formation and propagation speed of the CSD front and the pattern formation features in two dimensions. Our most notable finding is that the combination of vascular-mediated spatial coupling with local regulatory mechanisms results in the formation of stationary Turing-like patterns during a CSD event.

Trigeminal, Visceral and Vestibular Inputs May Improve Cognitive Functions by Acting through the Locus Coeruleus and the Ascending Reticular Activating System: A New Hypothesis

It is known that sensory signals sustain the background discharge of the ascending reticular activating system (ARAS) which includes the noradrenergic locus coeruleus (LC) neurons and controls the level of attention and alertness. Moreover, LC neurons influence brain metabolic activity, gene expression and brain inflammatory processes. As a consequence of the sensory control of ARAS/LC, stimulation of a sensory channel may potential influence neuronal activity and trophic state all over the brain, supporting cognitive functions and exerting a neuroprotective action. On the other hand, an imbalance of the same input on the two sides may lead to an asymmetric hemispheric excitability, leading to an impairment in cognitive functions. Among the inputs that may drive LC neurons and ARAS, those arising from the trigeminal region, from visceral organs and, possibly, from the vestibular system seem to be particularly relevant in regulating their activity. The trigeminal, visceral and vestibular control of ARAS/LC activity may explain why these input signals: (1) affect sensorimotor and cognitive functions which are not directly related to their specific informational content; and (2) are effective in relieving the symptoms of some brain pathologies, thus prompting peripheral activation of these input systems as a complementary approach for the treatment of cognitive impairments and neurodegenerative disorders.

Changes in fMRI BOLD dynamics reflect anticipation to moving objects

The human brain is thought to respond differently to novel versus predictable neural input. In human visual cortex, neural response amplitude to visual input might be determined by the degree of predictability. We investigated how fMRI BOLD responses in human early visual cortex reflect the anticipation of a single moving bar's trajectory. We found that BOLD signals decreased linearly from onset to offset of the stimulus trajectory. Moreover, decreased amplitudes of BOLD responses coincided with an increased initial dip as the stimulus moved along its trajectory. Importantly, motion anticipation effects were absent, when motion coherence was disrupted by means of stimulus contrast reversals. These results show that human early visual cortex anticipates the trajectory of a coherently moving object at the initial stages of visual motion processing. The results can be explained by suppression of predictable input, plausibly underlying the formation of stable visual percepts.

Microdevice Platform for In Vitro Nervous System and Its Disease Model

The development of precise microdevices can be applied to the reconstruction of in vitro human microenvironmental systems with biomimetic physiological conditions that have highly tunable spatial and temporal features. Organ-on-a-chip can emulate human physiological functions, particularly at the organ level, as well as its specific roles in the body. Due to the complexity of the structure of the central nervous system and its intercellular interaction, there remains an urgent need for the development of human brain or nervous system models. Thus, various microdevice models have been proposed to mimic actual human brain physiology, which can be categorized as nervous system-on-a-chip. Nervous system-on-a-chip platforms can prove to be promising technologies, through the application of their biomimetic features to the etiology of neurodegenerative diseases. This article reviews the microdevices for nervous system-on-a-chip platform incorporated with neurobiology and microtechnology, including microfluidic designs that are biomimetic to the entire nervous system. The emulation of both neurodegenerative disorders and neural stem cell behavior patterns in micro-platforms is also provided, which can be used as a basis to construct nervous system-on-a-chip.

Glutamate-Mediated Blood-Brain Barrier Opening: Implications for Neuroprotection and Drug Delivery

The blood-brain barrier is a highly selective anatomical and functional interface allowing a unique environment for neuro-glia networks. Blood-brain barrier dysfunction is common in most brain disorders and is associated with disease course and delayed complications. However, the mechanisms underlying blood-brain barrier opening are poorly understood. Here we demonstrate the role of the neurotransmitter glutamate in modulating early barrier permeability in vivo Using intravital microscopy, we show that recurrent seizures and the associated excessive glutamate release lead to increased vascular permeability in the rat cerebral cortex, through activation of NMDA receptors. NMDA receptor antagonists reduce barrier permeability in the peri-ischemic brain, whereas neuronal activation using high-intensity magnetic stimulation increases barrier permeability and facilitates drug delivery. Finally, we conducted a double-blind clinical trial in patients with malignant glial tumors, using contrast-enhanced magnetic resonance imaging to quantitatively assess blood-brain barrier permeability. We demonstrate the safety of stimulation that efficiently increased blood-brain barrier permeability in 10 of 15 patients with malignant glial tumors. We suggest a novel mechanism for the bidirectional modulation of brain vascular permeability toward increased drug delivery and prevention of delayed complications in brain disorders.

SIGNIFICANCE STATEMENT

In this study, we reveal a new mechanism that governs blood-brain barrier (BBB) function in the rat cerebral cortex, and, by using the discovered mechanism, we demonstrate bidirectional control over brain endothelial permeability. Obviously, the clinical potential of manipulating BBB permeability for neuroprotection and drug delivery is immense, as we show in preclinical and proof-of-concept clinical studies. This study addresses an unmet need to induce transient BBB opening for drug delivery in patients with malignant brain tumors and effectively facilitate BBB closure in neurological disorders.

Losartan improves cerebrovascular function in a mouse model of Alzheimer's disease with combined overproduction of amyloid-β and transforming growth factor-β1

Alterations of the renin-angiotensin system have been implicated in the pathogenesis of Alzheimer's disease. We tested the efficacy of losartan (10 mg/kg/day for three months), a selective angiotensin II type 1 receptor antagonist, in alleviating cerebrovascular and cognitive deficits in double-transgenic mice (six months at endpoint) that overexpress a mutated form of the human amyloid precursor protein (APP_Swe,Ind) and a constitutively active form of the transforming growth factor-β1, thereafter named A/T mice. Losartan rescued cerebrovascular reactivity, particularly the dilatory responses, but failed to attenuate astroglial activation and to normalize the neurovascular uncoupling response to sensory stimulation. The cognitive deficits of A/T mice were not restored by losartan nor were the increased brain levels of soluble and insoluble Aβ_1-40 and Aβ_1-42 peptides normalized. Our results are the first to demonstrate the capacity of losartan to improve cerebrovascular reactivity in an Alzheimer's disease mouse model of combined Aβ-induced vascular oxidative stress and transforming growth factor-β1-mediated vascular fibrosis. These data suggest that losartan may be promising for restoring cerebrovascular function in patients with vascular diseases at risk for vascular dementia or Alzheimer's disease. However, a combined therapy may be warranted for rescuing both vascular and cognitive deficits in a multifaceted pathology like Alzheimer's disease.

Dynamic causal modelling revisited

This paper revisits the dynamic causal modelling of fMRI timeseries by replacing the usual (Taylor) approximation to neuronal dynamics with a neural mass model of the canonical microcircuit. This provides a generative or dynamic causal model of laminar specific responses that can generate haemodynamic and electrophysiological measurements. In principle, this allows the fusion of haemodynamic and (event related or induced) electrophysiological responses. Furthermore, it enables Bayesian model comparison of competing hypotheses about physiologically plausible synaptic effects; for example, does attentional modulation act on superficial or deep pyramidal cells – or both? In this technical note, we describe the resulting dynamic causal model and provide an illustrative application to the attention to visual motion dataset used in previous papers. Our focus here is on how to answer long-standing questions in fMRI; for example, do haemodynamic responses reflect extrinsic (afferent) input from distant cortical regions, or do they reflect intrinsic (recurrent) neuronal activity? To what extent do inhibitory interneurons contribute to neurovascular coupling? What is the relationship between haemodynamic responses and the frequency of induced neuronal activity? This paper does not pretend to answer these questions; rather it shows how they can be addressed using neural mass models of fMRI timeseries.

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This paper describes a DCM for fMRI based on neural mass models and canonical microcircuits.

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This enables the (Bayesian) fusion of EEG and fMRI data.

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That encompasses the formal modelling of neurovascular coupling.

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Offers a surprising insight into the relationship between haemodynamic and electrophysiological responses.

Intracranial mechanisms for preserving brain blood flow in health and disease

The brain is an exceptionally energetically demanding organ with little metabolic reserve, and multiple systems operate to protect and preserve the brain blood supply. But how does the brain sense its own perfusion? In this review, we discuss how the brain may harness the cardiovascular system to counter threats to cerebral perfusion sensed via intracranial pressure (ICP), cerebral oxygenation and ischaemia. Since the work of Cushing over 100 years ago, the existence of brain baroreceptors capable of eliciting increases in sympathetic outflow and blood pressure has been hypothesized. In the clinic, this response has generally been thought to occur only in extremis, to perfuse the severely ischaemic brain as cerebral autoregulation fails. We review evidence that pressor responses may also occur with smaller, physiologically relevant increases in ICP. The incoming brain oxygen supply is closely monitored by the carotid chemoreceptors; however, hypoxia and other markers of ischaemia are also sensed intrinsically by astrocytes or other support cells within brain tissue itself and elicit reactive hyperaemia. Recent studies suggest that astrocytic oxygen signalling within the brainstem may directly affect sympathetic nerve activity and blood pressure. We speculate that local cerebral oxygen tension is a major determinant of the mean level of arterial pressure and discuss recent evidence that this may be the case. We conclude that intrinsic intra- and extra-cranial mechanisms sense and integrate information about hypoxia/ischaemia and ICP and play a major role in determining the long-term level of sympathetic outflow and arterial pressure, to optimize cerebral perfusion.

Inflammatory Activation of Microglia and Astrocytes in Manganese Neurotoxicity

Neurotoxicity due to excessive exposure to manganese (Mn) has been described as early as 1837 (Couper, Br Ann Med Pharm Vital Stat Gen Sci 1:41-42, 1837). Extensive research over the past two decades has revealed that Mn-induced neurological injury involves complex pathophysiological signaling mechanisms between neurons and glial cells. Glial cells are an important target of Mn in the brain, both for sequestration of the metal, as well as for activating inflammatory signaling pathways that damage neurons through overproduction of numerous reactive oxygen and nitrogen species and inflammatory cytokines. Understanding how these pathways are regulated in glial cells during Mn exposure is critical to determining the mechanisms underlying permanent neurological dysfunction stemming from excess exposure. The subject of this review will be to delineate mechanisms by which Mn interacts with glial cells to perturb neuronal function, with a particular emphasis on neuroinflammation and neuroinflammatory signaling between distinct populations of glial cells.

Fluid and ion transfer across the blood-brain and blood-cerebrospinal fluid barriers; a comparative account of mechanisms and roles

The two major interfaces separating brain and blood have different primary roles. The choroid plexuses secrete cerebrospinal fluid into the ventricles, accounting for most net fluid entry to the brain. Aquaporin, AQP1, allows water transfer across the apical surface of the choroid epithelium; another protein, perhaps GLUT1, is important on the basolateral surface. Fluid secretion is driven by apical Na+-pumps. K+ secretion occurs via net paracellular influx through relatively leaky tight junctions partially offset by transcellular efflux. The blood-brain barrier lining brain microvasculature, allows passage of O2, CO2, and glucose as required for brain cell metabolism. Because of high resistance tight junctions between microvascular endothelial cells transport of most polar solutes is greatly restricted. Because solute permeability is low, hydrostatic pressure differences cannot account for net fluid movement; however, water permeability is sufficient for fluid secretion with water following net solute transport. The endothelial cells have ion transporters that, if appropriately arranged, could support fluid secretion. Evidence favours a rate smaller than, but not much smaller than, that of the choroid plexuses. At the blood-brain barrier Na+ tracer influx into the brain substantially exceeds any possible net flux. The tracer flux may occur primarily by a paracellular route. The blood-brain barrier is the most important interface for maintaining interstitial fluid (ISF) K+ concentration within tight limits. This is most likely because Na+-pumps vary the rate at which K+ is transported out of ISF in response to small changes in K+ concentration. There is also evidence for functional regulation of K+ transporters with chronic changes in plasma concentration. The blood-brain barrier is also important in regulating HCO3- and pH in ISF: the principles of this regulation are reviewed. Whether the rate of blood-brain barrier HCO3- transport is slow or fast is discussed critically: a slow transport rate comparable to those of other ions is favoured. In metabolic acidosis and alkalosis variations in HCO3- concentration and pH are much smaller in ISF than in plasma whereas in respiratory acidosis variations in pHISF and pHplasma are similar. The key similarities and differences of the two interfaces are summarized.

Neuronal networks and mediators of cortical neurovascular coupling responses in normal and altered brain states

Brain imaging techniques that use vascular signals to map changes in neuronal activity, such as blood oxygenation level-dependent functional magnetic resonance imaging, rely on the spatial and temporal coupling between changes in neurophysiology and haemodynamics, known as 'neurovascular coupling (NVC)'. Accordingly, NVC responses, mapped by changes in brain haemodynamics, have been validated for different stimuli under physiological conditions. In the cerebral cortex, the networks of excitatory pyramidal cells and inhibitory interneurons generating the changes in neural activity and the key mediators that signal to the vascular unit have been identified for some incoming afferent pathways. The neural circuits recruited by whisker glutamatergic-, basal forebrain cholinergic- or locus coeruleus noradrenergic pathway stimulation were found to be highly specific and discriminative, particularly when comparing the two modulatory systems to the sensory response. However, it is largely unknown whether or not NVC is still reliable when brain states are altered or in disease conditions. This lack of knowledge is surprising since brain imaging is broadly used in humans and, ultimately, in conditions that deviate from baseline brain function. Using the whisker-to-barrel pathway as a model of NVC, we can interrogate the reliability of NVC under enhanced cholinergic or noradrenergic modulation of cortical circuits that alters brain states.This article is part of the themed issue 'Interpreting BOLD: a dialogue between cognitive and cellular neuroscience'.

The importance of the negative blood-oxygenation-level-dependent (BOLD) response in the somatosensory cortex

In recent years, multiple studies have shown task-induced negative blood-oxygenation-level-dependent responses (NBRs) in multiple brain regions in humans and animals. Converging evidence suggests that task-induced NBRs can be interpreted in terms of decreased neuronal activity. However, the vascular and metabolic dynamics and functional importance of the NBR are highly debated. Here, we review studies investigating the origin and functional importance of the NBR, with special attention to the somatosensory cortex.

Is Vasomotion in Cerebral Arteries Impaired in Alzheimer's Disease?

A substantial body of evidence supports the hypothesis of a vascular component in the pathogenesis of Alzheimer’s disease (AD). Cerebral hypoperfusion and blood-brain barrier dysfunction have been indicated as key elements of this pathway. Cerebral amyloid angiopathy (CAA) is a cerebrovascular disorder, frequent in AD, characterized by the accumulation of amyloid-β (Aβ) peptide in cerebral blood vessel walls. CAA is associated with loss of vascular integrity, resulting in impaired regulation of cerebral circulation, and increased susceptibility to cerebral ischemia, microhemorrhages, and white matter damage. Vasomotion— the spontaneous rhythmic modulation of arterial diameter, typically observed in arteries/arterioles in various vascular beds including the brain— is thought to participate in tissue perfusion and oxygen delivery regulation. Vasomotion is impaired in adverse conditions such as hypoperfusion and hypoxia. The perivascular and glymphatic pathways of Aβ clearance are thought to be driven by the systolic pulse. Vasomotion produces diameter changes of comparable amplitude, however at lower rates, and could contribute to these mechanisms of Aβ clearance. In spite of potential clinical interest, studies addressing cerebral vasomotion in the context of AD/CAA are limited. This study reviews the current literature on vasomotion, and hypothesizes potential paths implicating impaired cerebral vasomotion in AD/CAA. Aβ and oxidative stress cause vascular tone dysregulation through direct effects on vascular cells, and indirect effects mediated by impaired neurovascular coupling. Vascular tone dysregulation is further aggravated by cholinergic deficit and results in depressed cerebrovascular reactivity and (possibly) impaired vasomotion, aggravating regional hypoperfusion and promoting further Aβ and oxidative stress accumulation.

Metabolic Changes Following Perinatal Asphyxia: Role of Astrocytes and Their Interaction with Neurons

Perinatal Asphyxia (PA) represents an important cause of severe neurological deficits including delayed mental and motor development, epilepsy, major cognitive deficits and blindness. The interaction between neurons, astrocytes and endothelial cells plays a central role coupling energy supply with changes in neuronal activity. Traditionally, experimental research focused on neurons, whereas astrocytes have been more related to the damage mechanisms of PA. Astrocytes carry out a number of functions that are critical to normal nervous system function, including uptake of neurotransmitters, regulation of pH and ion concentrations, and metabolic support for neurons. In this work, we aim to review metabolic neuron-astrocyte interactions with the purpose of encourage further research in this area in the context of PA, which is highly complex and its mechanisms and pathways have not been fully elucidated to this day.

Beyond neurovascular coupling, role of astrocytes in the regulation of vascular tone

The brain possesses two intricate mechanisms that fulfill its continuous metabolic needs: cerebral autoregulation, which ensures constant cerebral blood flow over a wide range of arterial pressures and functional hyperemia, which ensures rapid delivery of oxygen and glucose to active neurons. Over the past decade, a number of important studies have identified astrocytes as key intermediaries in neurovascular coupling (NVC), the mechanism by which active neurons signal blood vessels to change their diameter. Activity-dependent increases in astrocytic Ca(2+) activity are thought to contribute to the release of vasoactive substances that facilitate arteriole vasodilation. A number of vasoactive signals have been identified and their role on vessel caliber assessed both in vitro and in vivo. In this review, we discuss mechanisms implicating astrocytes in NVC-mediated vascular responses, limitations encountered as a result of the challenges in maintaining all the constituents of the neurovascular unit intact and deliberate current controversial findings disputing a main role for astrocytes in NVC. Finally, we briefly discuss the potential role of pericytes and microglia in NVC-mediated processes.