Unveiling astrocytic control of cerebral blood flow with optogenetics

Characterizing astrocyte-mediated neurovascular coupling by combining optogenetics and biophysical modeling

Vasoactive signaling from astrocytes is an important contributor to the neurovascular coupling (NVC), which aims at providing energy to neurons during brain activation by increasing blood perfusion in the surrounding vasculature. Pharmacological manipulations have been previously combined with experimental techniques (e.g., transgenic mice, uncaging, and multiphoton microscopy) and stimulation paradigms to isolate in vivo individual pathways of the astrocyte-mediated NVC. Unfortunately, these pathways are highly nonlinear and non-additive. To separate these pathways in a unified framework, we combine a comprehensive biophysical model of vasoactive signaling from astrocytes with a unique optogenetic stimulation method that selectively induces astrocytic Ca2+ signaling in a large population of astrocytes. We also use a sensitivity analysis and an optimization technique to estimate key model parameters. Optogenetically-induced Ca2+ signals in astrocytes cause a cerebral blood flow (CBF) response with two major components. Component-1 was rapid and smaller (ΔCBF∼13%, 18 seconds), while component-2 was slowest and highest (ΔCBF ∼18%, 45 seconds). The proposed biophysical model was adequate in reproducing component-2, which was validated with a pharmacological manipulation. Model's predictions were not in contradiction with previous studies. Finally, we discussed scenarios accounting for the existence of component-1, which once validated might be included in our model.

Neuropsychiatric Manifestations of COVID-19 Disease and Post COVID Syndrome: The Role of N-acetylcysteine and Acetyl-L-carnitine

COVID-19 is associated with neuropsychiatric symptoms, such as anosmia, anxiety, depression, stress-related reactions, and psychoses. The illness can cause persistent cognitive impairment and "brain fog", suggesting chronic brain involvement. Clinical entities of ongoing symptomatic COVID-19 and Post COVID Syndrome (PCS) mainly present neuropsychiatric symptoms such as dysgeusia, headache, fatigue, anxiety, depression, sleep disturbances, and post-traumatic stress disorder. The pathophysiology of COVID-19-related brain damage is unclear, but it is linked to various mechanisms such as inflammation, oxidative stress, immune dysregulation, impaired glutamate homeostasis, glial and glymphatic damage, and hippocampal degeneration. Noteworthy is that the metabotropic receptor mGluR2 was discovered as a mechanism of internalisation of SARS-CoV-2 in Central Nervous System (CNS) cells. N-acetylcysteine (NAC) and acetyl-L-carnitine (ALC) are two supplements that have already been found effective in treating psychiatric conditions. Furthermore, NAC showed evidence in relieving cognitive symptomatology in PCS, and ALC was found effective in treating depressive symptomatology of PCS. The overlapping effects on the glutamatergic system of ALC and NAC could help treat COVID-19 psychiatric symptoms and PCS, acting through different mechanisms on the xc-mGluR2 network, with potentially synergistic effects on chronic pain and neuro-astrocyte protection. This paper aims to summarise the current evidence on the potential therapeutic role of NAC and ALC, providing an overview of the underlying molecular mechanisms and pathophysiology. It proposes a pathophysiological model explaining the effectiveness of NAC and ALC in treating COVID-19-related neuropsychiatric symptoms.

Modeling of Blood Flow Dynamics in Rat Somatosensory Cortex

Background: The cerebral microvasculature forms a dense network of interconnected blood vessels where flow is modulated partly by astrocytes. Increased neuronal activity stimulates astrocytes to release vasoactive substances at the endfeet, altering the diameters of connected vessels. Methods: Our study simulated the coupling between blood flow variations and vessel diameter changes driven by astrocytic activity in the rat somatosensory cortex. We developed a framework with three key components: coupling between the vasculature and synthesized astrocytic morphologies, a fluid dynamics model to compute flow in each vascular segment, and a stochastic process replicating the effect of astrocytic endfeet on vessel radii. Results: The model was validated against experimental flow values from the literature across cortical depths. We found that local vasodilation from astrocyte activity increased blood flow, especially in capillaries, exhibiting a layer-specific response in deeper cortical layers. Additionally, the highest blood flow variability occurred in capillaries, emphasizing their role in cerebral perfusion regulation. We discovered that astrocytic activity impacted blood flow dynamics in a localized, clustered manner, with most vascular segments influenced by two to three neighboring endfeet. Conclusions: These insights enhance our understanding of neurovascular coupling and guide future research on blood flow-related diseases.

Ginsenoside Rg1 regulates astrocytes to promote angiogenesis in spinal cord injury via the JAK2/STAT3 signaling pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Ginseng (Panax ginseng C. A. Mey) is a common traditional Chinese medicine used for anti-inflammation, anti-apoptosis, anti-oxidative stress, and neuroprotection. Ginsenosides Rg1, the main active components isolated from ginseng, may be a feasible therapy for spinal cord injury (SCI).

AIMS OF THE STUDY

SCI causes endothelial cell death and blood vessel rupture, ultimately resulting in long-term neurological impairment. As a result, encouraging spinal angiogenesis may be a feasible therapy for SCI. This investigation aimed to validate the capacity of ginsenoside Rg1 in stimulating angiogenesis within the spinal cord.

MATERIALS AND METHODS

Rats with SCI were injected intraperitoneally with ginsenoside Rg1. The effectiveness of ginsenoside Rg1 was assessed using the motor function score and the motor-evoked potential (MEP). Immunofluorescence techniques were applied to identify the spinal cord's angiogenesis. Angiogenic factors were examined through Western Blot (WB) and Immunohistochemistry. Oxygen-glucose deprivation (OGD) was employed to establish the hypoxia-ischemia model in vitro, and astrocytes (As) were given ginsenoside Rg1 and co-cultured with spinal cord microvascular endothelial cells (SCMECs). Immunofluorescence, wound healing test, and tube formation assay were used to identify the co-cultured SCMECs' activity. Finally, network pharmacology analysis and siRNA transfection were applied to verify the mechanism of ginsenoside Rg1 promoting angiogenesis.

RESULTS

The rats with SCI treated with ginsenoside Rg1 indicated more significant functional recovery, more pronounced angiogenesis, and higher levels of angiogenic factor expression. In vitro, the co-culture system with ginsenoside Rg1 intervention improved SCMECs' capacity for proliferating, migrating, and forming tubes, possibly by promoting the expression of vascular endothelial growth factor (VEGF) in As via the janus kinase 2 (JAK2)/signal transducer and activator of transcription 3 (STAT3) signaling pathway.

CONCLUSION

Ginsenoside Rg1 can regulate As to promote angiogenesis, which may help to understand the mechanism of promoting SCI recovery.

Intermittent Metabolic Switching and Vascular Cognitive Impairment

Intermittent fasting (IF), a dietary pattern alternating between eating and fasting periods within a 24-hour cycle, has garnered recognition for its potential to enhance both healthspan and lifespan in animal models and humans. It also shows promise in alleviating age-related diseases, including neurodegeneration. Vascular cognitive impairment (VCI) spans a severity range from mild cognitive deficits to severe cognitive deficits and loss of function in vascular dementia. Chronic cerebral hypoperfusion has emerged as a significant contributor to VCI, instigating vascular pathologies such as microbleeds, blood-brain barrier dysfunction, neuronal loss, and white matter lesions. Preclinical studies in rodents strongly suggest that IF has the potential to attenuate pathological mechanisms, including excitotoxicity, oxidative stress, inflammation, and cell death pathways in VCI models. Hence, this supports evaluating IF in clinical trials for both existing and at-risk VCI patients. This review compiles existing data supporting IF's potential in treating VCI-related vascular and neuronal pathologies, emphasizing the mechanisms by which IF may mitigate these issues. Hence providing a comprehensive overview of the available data supporting IF's potential in treating VCI by emphasizing the underlying mechanisms that make IF a promising intervention for VCI.

Analysis of potassium ion diffusion from neurons to capillaries: Effects of astrocyte endfeet geometry

Neurovascular coupling (NVC) refers to a local increase in cerebral blood flow in response to increased neuronal activity. Mechanisms of communication between neurons and blood vessels remain unclear. Astrocyte endfeet almost completely cover cerebral capillaries, suggesting that astrocytes play a role in NVC by releasing vasoactive substances near capillaries. An alternative hypothesis is that direct diffusion through the extracellular space of potassium ions (K+ ) released by neurons contributes to NVC. Here, the goal is to determine whether astrocyte endfeet present a barrier to K+ diffusion from neurons to capillaries. Two simplified 2D geometries of extracellular space, clefts between endfeet, and perivascular space are used: (i) a source 1 μm from a capillary; (ii) a neuron 15 μm from a capillary. K+ release is modelled as a step increase in [K+ ] at the outer boundary of the extracellular space. The time-dependent diffusion equation is solved numerically. In the first geometry, perivascular [K+ ] approaches its final value within 0.05 s. Decreasing endfeet cleft width or increasing perivascular space width slows the rise in [K+ ]. In the second geometry, the increase in perivascular [K+ ] occurs within 0.5 s and is insensitive to changes in cleft width or perivascular space width. Predicted levels of perivascular [K+ ] are sufficient to cause vasodilation, and the rise time is within the time for flow increase in NVC. These results suggest that direct diffusion of K+ through the extracellular space is a possible NVC signalling mechanism.

Sex, hormones and cerebrovascular function: from development to disorder

Proper cerebrovascular development and neurogliovascular unit assembly are essential for brain growth and function throughout life, ensuring the continuous supply of nutrients and oxygen. This involves crucial events during pre- and postnatal stages through key pathways, including vascular endothelial growth factor (VEGF) and Wnt signaling. These pathways are pivotal for brain vascular growth, expansion, and blood-brain barrier (BBB) maturation. Interestingly, during fetal and neonatal life, cerebrovascular formation coincides with the early peak activity of the hypothalamic-pituitary-gonadal axis, supporting the idea of sex hormonal influence on cerebrovascular development and barriergenesis.Sex hormonal dysregulation in early development has been implicated in neurodevelopmental disorders with highly sexually dimorphic features, such as autism spectrum disorder (ASD) and attention-deficit/hyperactivity disorder (ADHD). Both disorders show higher prevalence in men, with varying symptoms between sexes, with boys exhibiting more externalizing behaviors, such as aggressivity or hyperactivity, and girls displaying higher internalizing behaviors, including anxiety, depression, or attention disorders. Indeed, ASD and ADHD are linked to high prenatal testosterone exposure and reduced aromatase expression, potentially explaining sex differences in prevalence and symptomatology. In line with this, high estrogen levels seem to attenuate ADHD symptoms. At the cerebrovascular level, sex- and region-specific variations of cerebral blood flow perfusion have been reported in both conditions, indicating an impact of gonadal hormones on the brain vascular system, disrupting its ability to respond to neuronal demands.This review aims to provide an overview of the existing knowledge concerning the impact of sex hormones on cerebrovascular formation and maturation, as well as the onset of neurodevelopmental disorders. Here, we explore the concept of gonadal hormone interactions with brain vascular and BBB development to function, with a particular focus on the modulation of VEGF and Wnt signaling. We outline how these pathways may be involved in the underpinnings of ASD and ADHD. Outstanding questions and potential avenues for future research are highlighted, as uncovering sex-specific physiological and pathological aspects of brain vascular development might lead to innovative therapeutic approaches in the context of ASD, ADHD and beyond.

Glymphatic System and Psychiatric Disorders: A Rapid Comprehensive Scoping Review

BACKGROUND

Since discovering the glymphatic system, there has been a looming interest in exploring its relationship with psychiatric disorders. Recently, increasing evidence suggests an involvement of the glymphatic system in the pathophysiology of psychiatric disorders. However, clear data are still lacking. In this context, this rapid comprehensive PRISMA-ScR (Preferred Reporting Items for Systematic Reviews and Meta-Analyses extension for Scoping Reviews) scoping review aims to identify and analyze current evidence about the relation between the glymphatic system and psychiatric disorders.

METHODS

We conducted a comprehensive review of the literature and then proceeded to discuss the findings narratively. Tables were then constructed and articles were sorted according to authors, year, title, location of study, sample size, psychiatric disorder, the aim of the study, principal findings, implications.

RESULTS

Twenty papers were identified as eligible, among which 2 articles on Schizophrenia, 1 on Autism Spectrum Disorders, 2 on Depression, 1 on Depression and Trauma-related Disorders, 1 on Depression and Anxiety, 2 on Anxiety and Sleep Disorders, 8 on Sleep Disorders, 2 on Alcohol use disorder and 1 on Cocaine Use Disorder.

CONCLUSION

This review suggests a correlation between the glymphatic system and several psychiatric disorders: Schizophrenia, Depression, Anxiety Disorders, Sleep Disorders, Alcohol Use Disorder, Cocaine Use Disorder, Trauma-Related Disorders, and Autism Spectrum Disorders. Impairment of the glymphatic system could play a role in Trauma-Related Disorders, Alcohol Use Disorders, Cocaine Use Disorders, Sleep Disorders, Depression, and Autism Spectrum Disorders. It is important to implement research on this topic and adopt standardized markers and radio diagnostic tools.

Optoα1AR activation in astrocytes modulates basal hippocampal synaptic excitation and inhibition in a stimulation-specific manner

Astrocytes play active roles at synapses and can monitor, respond, and adapt to local synaptic activity. While there is abundant evidence that astrocytes modulate excitatory transmission in the hippocampus, evidence for astrocytic modulation of hippocampal synaptic inhibition remains more limited. Furthermore, to better investigate roles for astrocytes in modulating synaptic transmission, more tools that can selectively activate native G protein signaling pathways in astrocytes with both spatial and temporal precision are needed. Here, we utilized AAV8-GFAP-Optoα1AR-eYFP (Optoα1AR), a viral vector that enables activation of Gq signaling in astrocytes via light-sensitive α1-adrenergic receptors. To determine if stimulating astrocytic Optoα1AR modulates hippocampal synaptic transmission, recordings were made in CA1 pyramidal cells with surrounding astrocytes expressing Optoα1AR, channelrhodopsin (ChR2), or GFP. Both high-frequency (20 Hz, 45-ms light pulses, 5 mW, 5 min) and low-frequency (0.5 Hz, 1-s pulses at increasing 1, 5, and 10 mW intensities, 90 s per intensity) blue light stimulation were tested. 20 Hz Optoα1AR stimulation increased both inhibitory and excitatory postsynaptic current (IPSC and EPSC) frequency, and the effect on miniature IPSCs (mIPSCs) was largely reversible within 20 min. However, low-frequency stimulation of Optoα1AR did not modulate either IPSCs or EPSCs, suggesting that astrocytic Gq -dependent modulation of basal synaptic transmission in the hippocampus is stimulation-dependent. By contrast, low-frequency stimulation of astrocytic ChR2 was effective in increasing both synaptic excitation and inhibition. Together, these data demonstrate that Optoα1AR activation in astrocytes changes basal GABAergic and glutamatergic transmission, but only following high-frequency stimulation, highlighting the importance of temporal dynamics when using optical tools to manipulate astrocyte function.

Application of optogenetic glial cells to neuron-glial communication

Optogenetic techniques combine optics and genetics to enable cell-specific targeting and precise spatiotemporal control of excitable cells, and they are increasingly being employed. One of the most significant advantages of the optogenetic approach is that it allows for the modulation of nearby cells or circuits with millisecond precision, enabling researchers to gain a better understanding of the complex nervous system. Furthermore, optogenetic neuron activation permits the regulation of information processing in the brain, including synaptic activity and transmission, and also promotes nerve structure development. However, the optimal conditions remain unclear, and further research is required to identify the types of cells that can most effectively and precisely control nerve function. Recent studies have described optogenetic glial manipulation for coordinating the reciprocal communication between neurons and glia. Optogenetically stimulated glial cells can modulate information processing in the central nervous system and provide structural support for nerve fibers in the peripheral nervous system. These advances promote the effective use of optogenetics, although further experiments are needed. This review describes the critical role of glial cells in the nervous system and reviews the optogenetic applications of several types of glial cells, as well as their significance in neuron-glia interactions. Together, it briefly discusses the therapeutic potential and feasibility of optogenetics.

Astrocytes: new evidence, new models, new roles

Astrocytes have been in the limelight of active research for about 3 decades now. Over this period, ideas about their function and role in the nervous system have evolved from simple assistance in energy supply and homeostasis maintenance to a complex informational and metabolic hub that integrates data on local neuronal activity, sensory and arousal context, and orchestrates many crucial processes in the brain. Rapid progress in experimental techniques and data analysis produces a growing body of data, which can be used as a foundation for formulation of new hypotheses, building new refined mathematical models, and ultimately should lead to a new level of understanding of the contribution of astrocytes to the cognitive tasks performed by the brain. Here, we highlight recent progress in astrocyte research, which we believe expands our understanding of how low-level signaling at a cellular level builds up to processes at the level of the whole brain and animal behavior. We start our review with revisiting data on the role of noradrenaline-mediated astrocytic signaling in locomotion, arousal, sensory integration, memory, and sleep. We then briefly review astrocyte contribution to the regulation of cerebral blood flow regulation, which is followed by a discussion of biophysical mechanisms underlying astrocyte effects on different brain processes. The experimental section is closed by an overview of recent experimental techniques available for modulation and visualization of astrocyte dynamics. We then evaluate how the new data can be potentially incorporated into the new mathematical models or where and how it already has been done. Finally, we discuss an interesting prospect that astrocytes may be key players in important processes such as the switching between sleep and wakefulness and the removal of toxic metabolites from the brain milieu.

Optogenetic Activation of Astrocytes Reduces Blood-Brain Barrier Disruption via IL-10 In Stroke

Optogenetics has been used to regulate astrocyte activity and modulate neuronal function after brain injury. Activated astrocytes regulate blood-brain barrier functions and are thereby involved in brain repair. However, the effect and molecular mechanism of optogenetic-activated astrocytes on the change in barrier function in ischemic stroke remain obscure. In this study, adult male GFAP-ChR2-EYFP transgenic Sprague-Dawley rats were stimulated by optogenetics at 24, 36, 48, and 60 h after photothrombotic stroke to activate ipsilateral cortical astrocytes. The effects of activated astrocytes on barrier integrity and the underlying mechanisms were explored using immunostaining, western blotting, RT-qPCR, and shRNA interference. Neurobehavioral tests were performed to evaluate therapeutic efficacy. The results demonstrated that IgG leakage, gap formation of tight junction proteins, and matrix metallopeptidase 2 expression were reduced after optogenetic activation of astrocytes (p<0.05). Moreover, photo-stimulation of astrocytes protected neurons against apoptosis and improved neurobehavioral outcomes in stroke rats compared to controls (p<0.05). Notably, interleukin-10 expression in optogenetic-activated astrocytes significantly increased after ischemic stroke in rats. Inhibition of interleukin-10 in astrocytes compromised the protective effects of optogenetic-activated astrocytes (p<0.05). We found for the first time that interleukin-10 derived from optogenetic-activated astrocytes protected blood-brain barrier integrity by decreasing the activity of matrix metallopeptidase 2 and attenuated neuronal apoptosis, which provided a novel therapeutic approach and target in the acute stage of ischemic stroke.

Astrocytic GPCR-Induced Ca2+ Signaling Is Not Causally Related to Local Cerebral Blood Flow Changes

Activation of Gq-type G protein-coupled receptors (GPCRs) gives rise to large cytosolic Ca2+ elevations in astrocytes. Previous in vitro and in vivo studies have indicated that astrocytic Ca2+ elevations are closely associated with diameter changes in the nearby blood vessels, which astrocytes enwrap with their endfeet. However, the causal relationship between astrocytic Ca2+ elevations and blood vessel diameter changes has been questioned, as mice with diminished astrocytic Ca2+ signaling show normal sensory hyperemia. We addressed this controversy by imaging cortical vasculature while optogenetically elevating astrocyte Ca2+ in a novel transgenic mouse line, expressing Opto-Gq-type GPCR Optoα1AR (Astro-Optoα1AR) in astrocytes. Blue light illumination on the surface of the somatosensory cortex induced Ca2+ elevations in cortical astrocytes and their endfeet in mice under anesthesia. Blood vessel diameter did not change significantly with Optoα1AR-induced Ca2+ elevations in astrocytes, while it was increased by forelimb stimulation. Next, we labeled blood plasma with red fluorescence using AAV8-P3-Alb-mScarlet in Astro-Optoα1AR mice. We were able to identify arterioles that display diameter changes in superficial areas of the somatosensory cortex through the thinned skull. Photo-stimulation of astrocytes in the cortical area did not result in noticeable changes in the arteriole diameters compared with their background strain C57BL/6. Together, compelling evidence for astrocytic Gq pathway-induced vasodiameter changes was not observed. Our results support the notion that short-term (<10 s) hyperemia is not mediated by GPCR-induced astrocytic Ca2+ signaling.

Do astrocytes act as immune cells after pediatric TBI?

Astrocytes are in contact with the vasculature, neurons, oligodendrocytes and microglia, forming a local network with various functions critical for brain homeostasis. One of the primary responders to brain injury are astrocytes as they detect neuronal and vascular damage, change their phenotype with morphological, proteomic and transcriptomic transformations for an adaptive response. The role of astrocytic responses in brain dysfunction is not fully elucidated in adult, and even less described in the developing brain. Children are vulnerable to traumatic brain injury (TBI), which represents a leading cause of death and disability in the pediatric population. Pediatric brain trauma, even with mild severity, can lead to long-term health complications, such as cognitive impairments, emotional disorders and social dysfunction later in life. To date, the underlying pathophysiology is still not fully understood. In this review, we focus on the astrocytic response in pediatric TBI and propose a potential immune role of the astrocyte in response to trauma. We discuss the contribution of astrocytes in the local inflammatory cascades and secretion of various immunomodulatory factors involved in the recruitment of local microglial cells and peripheral immune cells through cerebral blood vessels. Taken together, we propose that early changes in the astrocytic phenotype can alter normal development of the brain, with long-term consequences on neurological outcomes, as described in preclinical models and patients.

Acceleration of the Development of Microcirculation Embolism in the Brain due to Capillary Narrowing

BACKGROUND

Cerebral microvascular obstruction is critically involved in recurrent stroke and decreased cerebral blood flow with age. The obstruction must occur in the capillary with a greater resistance to perfusion pressure through the microvascular networks. However, little is known about the relationship between capillary size and embolism formation. This study aimed to determine whether the capillary lumen space contributes to the development of microcirculation embolism.

METHODS

To spatiotemporally manipulate capillary diameters in vivo, transgenic mice expressing the light-gated cation channel protein ChR2 (channelrhodopsin-2) in mural cells were used. The spatiotemporal changes in the regional cerebral blood flow in response to the photoactivation of ChR2 mural cells were first characterized using laser speckle flowgraphy. Capillary responses to optimized photostimulation were then examined in vivo using 2-photon microscopy. Finally, microcirculation embolism due to intravenously injected fluorescent microbeads was compared under conditions with or without photoactivation of ChR2 mural cells.

RESULTS

Following transcranial photostimulation, the stimulation intensity-dependent decrease in cerebral blood flow centered at the irradiation was observed (14%-49% decreases relative to the baseline). The cerebrovascular response to photostimulation showed significant constriction of the cerebral arteries and capillaries but not of the veins. As a result of vasoconstriction, a temporal stall of red blood cell flow occurred in the capillaries of the venous sides. The 2-photon excitation of a single ChR2 pericyte demonstrated the partial shrinkage of capillaries (7% relative to the baseline) around the stimulated cell. With the intravenous injection of microbeads, the occurrence of microcirculation embolism was significantly enhanced (11% increases compared to the control) with photostimulation.

CONCLUSIONS

Capillary narrowing increases the risk of developing microcirculation embolism in the venous sides of the cerebral capillaries.

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.

Modern Microscopic Approaches to Astrocytes

Microscopy started as the histological analysis based on intrinsic optical properties of tissues such as the refractive index and light absorption, and is expanding to include the visualization of organelles by chemical staining, localization of molecules by immunostaining, physiological measurements such as Ca²⁺ imaging, functional manipulation by optogenetics, and comprehensive analysis of chemical composition by Raman spectra. The microscope is one of the most important tools in neuroscience, which aims to reveal the complex intercellular communications underlying brain function and pathology. Many aspects of astrocytes, including the structures of their fine processes and physiological activities in concert with neurons and blood vessels, were revealed in the course of innovations in modern microscopy. The evolution of modern microscopy is a consequence of breakthroughs in spatiotemporal resolutions and expansions in molecular and physiological targets due to the progress in optics and information technology, as well as the inventions of probes using organic chemistry and molecular biology. This review overviews the modern microscopic approach to astrocytes.

Local brain environment changes associated with epileptogenesis

Plastic change of the neuronal system has traditionally been assumed to be governed primarily by the long-term potentiation/depression mechanisms of synaptic transmission. However, a rather simple shift in the ambient ion, transmitter and metabolite concentrations could have a pivotal role in generating plasticity upon the physiological process of learning and memory. Local brain environment and metabolic changes could also be the cause and consequences of the pathogenesis leading to epilepsy. Governing of the local brain environment is the primal function of astrocytes. The metabolic state of the entire brain is strongly linked to the activity of the lateral hypothalamus. In this study, plastic change of astrocyte reactions in the lateral hypothalamus was examined using epileptogenesis as an extreme form of plasticity. Fluorescent sensors for calcium or pH expressed in astrocytes were examined for up to one week by in vivo fibre photometry in freely moving transgenic male mice. Optical fluctuations on a timescale of seconds is difficult to assess because these signals are heavily influenced by local brain blood volume changes and pH changes. Using a newly devised method for the analysis of the optical signals, changes in Ca2+ and pH in astrocytes and changes in local brain blood volume associated with hippocampal-stimulated epileptic seizures were extracted. Following a transient alkaline shift in the astrocyte triggered by neuronal hyperactivity, a prominent acidic shift appeared in response to intensified seizure which developed with kindling. The acidic shift was unexpected as transient increase in local brain blood volume was observed in response to intensified seizures, which should lead to efficient extrusion of the acidic CO2. The acidic shift could be a result of glutamate transporter activity and/or due to the increased metabolic load of astrocytes leading to increased CO2 and lactate production. This acidic shift may trigger additional gliotransmitter release from astrocytes leading to the exacerbation of epilepsy. As all cellular enzymic reactions are influenced by Ca2+ and pH, changes in these parameters could also have an impact on the neuronal circuit activity. Thus, controlling the astrocyte pH and/or Ca2+ could be a new therapeutic target for treatment of epilepsy or prevention of undesired plasticity associated with epileptogenesis.

Separate optogenetic manipulation of Nerve/glial antigen 2 (NG2) glia and mural cells using the NG2 promoter

Nerve/glial antigen 2 (NG2) is a protein marker of NG2 glia and mural cells, and NG2 promoter activity is utilized to target these cells. However, the NG2 promoter cannot target NG2 glia and mural cells separately. This has been an obstacle for NG2 glia-specific manipulation. Here, we developed transgenic mice in which either cell type can be targeted using the NG2 promoter. We selected a tetracycline-controllable gene induction system for cell type-specific transgene expression, and generated NG2-tetracycline transactivator (tTA) transgenic lines. We crossed tTA lines with the tetO-ChR2 (channelrhodopsin-2)-EYFP line to characterize tTA-dependent transgene induction. We isolated two unique NG2-tTA mouse lines: one that induced ChR2-EYFP only in mural cells, likely due to the chromosomal position effect of NG2-tTA insertion, and the other that induced it in both cell types. We then applied a Cre-mediated set-subtraction strategy to the latter case and eliminated ChR2-EYFP from mural cells, resulting in NG2 glia-specific transgene induction. We further demonstrated that tTA-dependent ChR2 expression could manipulate cell function. Optogenetic mural cell activation decreased cerebral blood flow, as previously reported, indicating that tTA-mediated ChR2 expression was sufficient to impact cellular function. ChR2-mediated depolarization was observed in NG2 glia in acute hippocampal slices. In addition, ChR2-mediated depolarization of NG2 glia inhibited their proliferation but promoted their differentiation in juvenile mice. Since the tTA-tetO combination is expandable, the mural cell-specific NG2-tTA line and the NG2 glia-specific NG2-tTA line will permit us to conduct observational and manipulation studies to examine in vivo function of these cells separately.

MicroRNA-mediated regulation of reactive astrocytes in central nervous system diseases

Astrocytes (AST) are abundant glial cells in the human brain, accounting for approximately 20-50% percent of mammalian central nervous system (CNS) cells. They display essential functions necessary to sustain the physiological processes of the CNS, including maintaining neuronal structure, forming the blood-brain barrier, coordinating neuronal metabolism, maintaining the extracellular environment, regulating cerebral blood flow, stabilizing intercellular communication, participating in neurotransmitter synthesis, and defending against oxidative stress et al. During the pathological development of brain tumors, stroke, spinal cord injury (SCI), neurodegenerative diseases, and other neurological disorders, astrocytes undergo a series of highly heterogeneous changes, which are called reactive astrocytes, and mediate the corresponding pathophysiological process. However, the pathophysiological mechanisms of reactive astrocytes and their therapeutic relevance remain unclear. The microRNAs (miRNAs) are essential for cell differentiation, proliferation, and survival, which play a crucial role in the pathophysiological development of CNS diseases. In this review, we summarize the regulatory mechanism of miRNAs on reactive astrocytes in CNS diseases, which might provide a theoretical basis for the diagnosis and treatment of CNS diseases.

Cerebral Microcirculation and Oxygenation Modulation by Transcranial Alternating Current Stimulation in Awake and Anesthetized Mice

Transcranial alternating current stimulation (tACS) is a novel non-invasive electrical stimulation technique where a sinusoidal oscillating low-voltage electric current is applied to the brain. TACS is being actively investigated in practice for cognition and behavior modulation and for treating brain disorders. However, the physiological mechanisms of tACS are underinvestigated and poorly understood. Previously, we have shown that transcranial direct current stimulation (tDCS) facilitates cerebral microcirculation and oxygen supply in a mouse brain through nitric oxide-dependent vasodilatation of arterioles. Considering that the effects of tACS and tDCS might be both similar and dissimilar, we tested the effects of tACS on regional cerebral blood flow and oxygen saturation in anesthetized and awake mice using laser speckle contrast imaging and multispectral intrinsic optical signal imaging. The anesthetized mice were imaged under isoflurane anesthesia ∼1.0% in 30% O2 and 70% N2O. The awake mice were pre-trained on the rotating ball for awake imaging. Baseline imaging with further tACS was followed by post-stimulation imaging for ~3 h. Differences between groups were determined using a two-way ANOVA analysis for multiple comparisons and post hoc testing using the Mann-Whitney U test. TACS increased cerebral blood flow and oxygen saturation. In awake mice, rCBF and oxygen saturation responses were more robust and prolonged as opposed to anesthetized, where the response was weaker and shorter with overshoot. The significant difference between anesthetized and awake mice emphasizes the importance of the experiments on the latter as anesthesia is not typical for human stimulation and significantly alters the results.

Astrocytes amplify neurovascular coupling to sustained activation of neocortex in awake mice

Functional hyperemia occurs when enhanced neuronal activity signals to increase local cerebral blood flow (CBF) to satisfy regional energy demand. Ca2+ elevation in astrocytes can drive arteriole dilation to increase CBF, yet affirmative evidence for the necessity of astrocytes in functional hyperemia in vivo is lacking. In awake mice, we discovered that functional hyperemia is bimodal with a distinct early and late component whereby arteriole dilation progresses as sensory stimulation is sustained. Clamping astrocyte Ca2+ signaling in vivo by expressing a plasma membrane Ca2+ ATPase (CalEx) reduces sustained but not brief sensory-evoked arteriole dilation. Elevating astrocyte free Ca2+ using chemogenetics selectively augments sustained hyperemia. Antagonizing NMDA-receptors or epoxyeicosatrienoic acid production reduces only the late component of functional hyperemia, leaving brief increases in CBF to sensory stimulation intact. We propose that a fundamental role of astrocyte Ca2+ is to amplify functional hyperemia when neuronal activation is prolonged.

Astrocyte-derived adenosine excites sleep-promoting neurons in the ventrolateral preoptic nucleus: Astrocyte-neuron interactions in the regulation of sleep

Although ATP and/or adenosine derived from astrocytes are known to regulate sleep, the precise mechanisms underlying the somnogenic effects of ATP and adenosine remain unclear. We selectively expressed channelrhodopsin-2 (ChR2), a light-sensitive ion channel, in astrocytes within the ventrolateral preoptic nucleus (VLPO), which is an essential brain nucleus involved in sleep promotion. We then examined the effects of photostimulation of astrocytic ChR2 on neuronal excitability using whole-cell patch-clamp recordings in two functionally distinct types of VLPO neurons: sleep-promoting GABAergic projection neurons and non-sleep-promoting local GABAergic neurons. Optogenetic stimulation of VLPO astrocytes demonstrated opposite outcomes in the two types of VLPO neurons. It led to the inhibition of non-sleep-promoting neurons and excitation of sleep-promoting neurons. These responses were attenuated by blocking of either adenosine A₁ receptors or tissue-nonspecific alkaline phosphatase (TNAP). In contrast, exogenous adenosine decreased the excitability of both VLPO neuron populations. Moreover, TNAP was expressed in galanin-negative VLPO neurons, but not in galanin-positive sleep-promoting projection neurons. Taken together, these results suggest that astrocyte-derived ATP is converted into adenosine by TNAP in non-sleep-promoting neurons. In turn, adenosine decreases the excitability of local GABAergic neurons, thereby increasing the excitability of sleep-promoting GABAergic projection neurons. We propose a novel mechanism involving astrocyte-neuron interactions in sleep regulation, wherein endogenous adenosine derived from astrocytes excites sleep-promoting VLPO neurons, and thus decreases neuronal excitability in arousal-related areas of the brain.

Toolbox for studying neurovascular coupling in vivo, with a focus on vascular activity and calcium dynamics in astrocytes

Significance: Insights into the cellular activity of each member of the neurovascular unit (NVU) is critical for understanding their contributions to neurovascular coupling (NVC)-one of the key control mechanisms in cerebral blood flow regulation. Advances in imaging and genetic tools have enhanced our ability to observe, manipulate and understand the cellular activity of NVU components, namely neurons, astrocytes, microglia, endothelial cells, vascular smooth muscle cells, and pericytes. However, there are still many unresolved questions. Since astrocytes are considered electrically unexcitable,Ca 2 + signaling is the main parameter used to monitor their activity. It is therefore imperative to study astrocyticCa 2 + dynamics simultaneously with vascular activity using tools appropriate for the question of interest. Aim: To highlight currently available genetic and imaging tools for studying the NVU-and thus NVC-with a focus on astrocyteCa 2 + dynamics and vascular activity, and discuss the utility, technical advantages, and limitations of these tools for elucidating NVC mechanisms. Approach: We draw attention to some outstanding questions regarding the mechanistic basis of NVC and emphasize the role of astrocyticCa 2 + elevations in functional hyperemia. We further discuss commonly used genetic, and optical imaging tools, as well as some newly developed imaging modalities for studying NVC at the cellular level, highlighting their advantages and limitations. Results: We provide an overview of the current state of NVC research, focusing on the role of astrocyticCa 2 + elevations in functional hyperemia; summarize recent advances in genetically engineeredCa 2 + indicators, fluorescence microscopy techniques for studying NVC; and discuss the unmet challenges for future imaging development. Conclusions: Advances in imaging techniques together with improvements in genetic tools have significantly contributed to our understanding of NVC. Many pieces of the puzzle have been revealed, but many more remain to be discovered. Ultimately, optimizing NVC research will require a concerted effort to improve imaging techniques, available genetic tools, and analytical software.

Activation of Glutamate Transport Increases Arteriole Diameter in v ivo: Implications for Neurovascular Coupling

In order to meet the energetic demands of cell-to-cell signaling, increases in local neuronal signaling are matched by a coordinated increase in local blood flow, termed neurovascular coupling. Multiple different signals from neurons, astrocytes, and pericytes contribute to this control of blood flow. Previously, several groups demonstrated that inhibition/ablation of glutamate transporters attenuates the neurovascular response. However, it was not determined if glutamate transporter activation was sufficient to increase blood flow. Here, we used multiphoton imaging to monitor the diameter of fluorescently labeled cortical arterioles in anesthetized C57/B6J mice. We delivered vehicle, glutamate transporter substrates, or a combination of a glutamate transporter substrate with various pharmacologic agents via a glass micropipette while simultaneously visualizing changes in arteriole diameter. We developed a novel image analysis method to automate the measurement of arteriole diameter in these time-lapse analyses. Using this workflow, we first conducted pilot experiments in which we focally applied L-glutamate, D-aspartate, or L-threo-hydroxyaspartate (L-THA) and measured arteriole responses as proof of concept. We subsequently applied the selective glutamate transport substrate L-THA (applied at concentrations that do not activate glutamate receptors). We found that L-THA evoked a significantly larger dilation than that observed with focal saline application. This response was blocked by co-application of the potent glutamate transport inhibitor, L-(2S,3S)-3-[3-[4-(trifluoromethyl)-benzoylamino]benzyloxy]-aspartate (TFB-TBOA). Conversely, we were unable to demonstrate a reduction of this effect through co-application of a cocktail of glutamate and GABA receptor antagonists. These studies provide the first direct evidence that activation of glutamate transport is sufficient to increase arteriole diameter. We explored potential downstream mechanisms mediating this transporter-mediated dilation by using a Ca2+ chelator or inhibitors of reversed-mode Na+/Ca2+ exchange, nitric oxide synthetase, or cyclo-oxygenase. The estimated effects and confidence intervals suggested some form of inhibition for a number of these inhibitors. Limitations to our study design prevented definitive conclusions with respect to these downstream inhibitors; these limitations are discussed along with possible next steps. Understanding the mechanisms that control blood flow are important because changes in blood flow/energy supply are implicated in several neurodegenerative disorders and are used as a surrogate measure of neuronal activity in widely used techniques such as functional magnetic resonance imaging (fMRI).

Impact of multisession 40Hz tACS on hippocampal perfusion in patients with Alzheimer's disease

BACKGROUND

Alzheimer's disease (AD) is associated with alterations in cortical perfusion that correlate with cognitive impairment. Recently, neural activity in the gamma band has been identified as a driver of arteriolar vasomotion while, on the other hand, gamma activity induction on preclinical models of AD has been shown to promote protein clearance and cognitive protection.

METHODS

In two open-label studies, we assessed the possibility to modulate cerebral perfusion in 15 mild to moderate AD participants via 40Hz (gamma) transcranial alternating current stimulation (tACS) administered 1 h daily for 2 or 4 weeks, primarily targeting the temporal lobe. Perfusion-sensitive MRI scans were acquired at baseline and right after the intervention, along with electrophysiological recording and cognitive assessments.

RESULTS

No serious adverse effects were reported by any of the participants. Arterial spin labeling MRI revealed a significant increase in blood perfusion in the bilateral temporal lobes after the tACS treatment. Moreover, perfusion changes displayed a positive correlation with changes in episodic memory and spectral power changes in the gamma band.

CONCLUSIONS

Results suggest 40Hz tACS should be further investigated in larger placebo-controlled trials as a safe, non-invasive countermeasure to increase fast brain oscillatory activity and increase perfusion in critical brain areas in AD patients.

TRIAL REGISTRATION

Studies were registered separately on ClinicalTrials.gov ( NCT03290326 , registered on September 21, 2017; NCT03412604 , registered on January 26, 2018).

Contribution of animal models toward understanding resting state functional connectivity

Functional connectivity, which reflects the spatial and temporal organization of intrinsic activity throughout the brain, is one of the most studied measures in human neuroimaging research. The noninvasive acquisition of resting state functional magnetic resonance imaging (rs-fMRI) allows the characterization of features designated as functional networks, functional connectivity gradients, and time-varying activity patterns that provide insight into the intrinsic functional organization of the brain and potential alterations related to brain dysfunction. Functional connectivity, hence, captures dimensions of the brain's activity that have enormous potential for both clinical and preclinical research. However, the mechanisms underlying functional connectivity have yet to be fully characterized, hindering interpretation of rs-fMRI studies. As in other branches of neuroscience, the identification of the neurophysiological processes that contribute to functional connectivity largely depends on research conducted on laboratory animals, which provide a platform where specific, multi-dimensional investigations that involve invasive measurements can be carried out. These highly controlled experiments facilitate the interpretation of the temporal correlations observed across the brain. Indeed, information obtained from animal experimentation to date is the basis for our current understanding of the underlying basis for functional brain connectivity. This review presents a compendium of some of the most critical advances in the field based on the efforts made by the animal neuroimaging community.

Differential pial and penetrating arterial responses examined by optogenetic activation of astrocytes and neurons

A variety of brain cells participates in neurovascular coupling by transmitting and modulating vasoactive signals. The present study aimed to probe cell type-dependent cerebrovascular (i.e., pial and penetrating arterial) responses with optogenetics in the cortex of anesthetized mice. Two lines of the transgenic mice expressing a step function type of light-gated cation channel (channelrhodopsine-2; ChR2) in either cortical neurons (muscarinic acetylcholine receptors) or astrocytes (Mlc1-positive) were used in the experiments. Photo-activation of ChR2-expressing astrocytes resulted in a widespread increase in cerebral blood flow (CBF), extending to the nonstimulated periphery. In contrast, photo-activation of ChR2-expressing neurons led to a relatively localized increase in CBF. The differences in the spatial extent of the CBF responses are potentially explained by differences in the involvement of the vascular compartments. In vivo imaging of the cerebrovascular responses revealed that ChR2-expressing astrocyte activation led to the dilation of both pial and penetrating arteries, whereas ChR2-expressing neuron activation predominantly caused dilation of the penetrating arterioles. Pharmacological studies showed that cell type-specific signaling mechanisms participate in the optogenetically induced cerebrovascular responses. In conclusion, pial and penetrating arterial vasodilation were differentially evoked by ChR2-expressing astrocytes and neurons.

Optical manipulation of local cerebral blood flow in the deep brain of freely moving mice

An artificial tool for manipulating local cerebral blood flow (CBF) is necessary for understanding how CBF controls brain function. Here, we generate vascular optogenetic tools whereby smooth muscle cells and endothelial cells express optical actuators in the brain. The illumination of channelrhodopsin-2 (ChR2)-expressing mice induces a local reduction in CBF. Photoactivated adenylyl cyclase (PAC) is an optical protein that increases intracellular cyclic adenosine monophosphate (cAMP), and the illumination of PAC-expressing mice induces a local increase in CBF. We target the ventral striatum, determine the temporal kinetics of CBF change, and optimize the illumination intensity to confine the effects to the ventral striatum. We demonstrate the utility of this vascular optogenetic manipulation in freely and adaptively behaving mice and validate the task- and actuator-dependent behavioral readouts. The development of vascular optogenetic animal models will help accelerate research linking vasculature, circuits, and behavior to health and disease.

In vivo brain ischemia-reperfusion model induced by hypoxia-reoxygenation using zebrafish larvae

Cerebral infarct is caused by cerebrovascular occlusion and results in brain damage. Although many rodent models of cerebral infarct exist, there is none based on zebrafish. In this study, we developed a novel ischemia-reperfusion model induced by hypoxic treatment using zebrafish. We first examined the changes in blood flow under hypoxic conditions. Hypoxic treatment interrupted the blood flow in 4 dpf (days post fertilization) zebrafish larvae. To quantify the trunk and cerebral blood flow, we selected the middle mesencephalic central artery (MMCtA) as a cerebral blood vessel and the dorsal aorta (DA) as a blood vessel of the trunk. Interestingly, the interruption of blood flow in MMCtA preceded that in DA. Considering these results, we hypothesized that reoxygenation immediately after hypoxia-induced cerebral ischemia leads to reperfusion. As a result, hypoxia-reoxygenation (H/R) treatment induced ischemia-reperfusion in cerebral vessels. Furthermore, brain cell death was increased 24 h after H/R treatment. Transgenic zebrafish (HuC:kaede), with neuronal cells expressing the kaede fluorescent protein, was used to investigate the effect of H/R on neuronal cells. The H/R treatment reduced the fluorescence intensity of kaede. Besides, glial fibrillary acidic protein immunoreactivity in H/R-treated larvae was significantly increased. In conclusion, H/R-treated zebrafish larvae may provide a novel ischemia-reperfusion model.

Unraveling ChR2-driven stochastic Ca2+ dynamics in astrocytes: A call for new interventional paradigms

Optogenetic targeting of astrocytes provides a robust experimental model to differentially induce Ca2+ signals in astrocytes in vivo. However, a systematic study quantifying the response of optogenetically modified astrocytes to light is yet to be performed. Here, we propose a novel stochastic model of Ca2+ dynamics in astrocytes that incorporates a light sensitive component-channelrhodopsin 2 (ChR2). Utilizing this model, we investigated the effect of different light stimulation paradigms on cells expressing select variants of ChR2 (wild type, ChETA, and ChRET/TC). Results predict that depending on paradigm specification, astrocytes might undergo drastic changes in their basal Ca2+ level and spiking probability. Furthermore, we performed a global sensitivity analysis to assess the effect of variation in parameters pertinent to the shape of the ChR2 photocurrent on astrocytic Ca2+ dynamics. Results suggest that directing variants towards the first open state of the ChR2 photocycle (o1) enhances spiking activity in astrocytes during optical stimulation. Evaluation of the effect of Ca2+ buffering and coupling coefficient in a network of ChR2-expressing astrocytes demonstrated basal level elevations in the stimulated region and propagation of calcium activity to unstimulated cells. Buffering reduced the diffusion range of Ca2+ within the network, thereby limiting propagation and influencing the activity of astrocytes. Collectively, the framework presented in this study provides valuable information for the selection of light stimulation paradigms that elicit desired astrocytic activity using existing ChR2 constructs, as well as aids in the engineering of future application-oriented optogenetic variants.

Optogenetic and chemogenetic modulation of astroglial secretory phenotype

Astrocytes play a major role in brain function and alterations in astrocyte function that contribute to the pathogenesis of many brain disorders. The astrocytes are attractive cellular targets for neuroprotection and brain tissue regeneration. Development of novel approaches to monitor and to control astroglial function is of great importance for further progress in basic neurobiology and in clinical neurology, as well as psychiatry. Recently developed advanced optogenetic and chemogenetic techniques enable precise stimulation of astrocytes in vitro and in vivo, which can be achieved by the expression of light-sensitive channels and receptors, or by expression of receptors exclusively activated by designer drugs. Optogenetic stimulation of astrocytes leads to dramatic changes in intracellular calcium concentrations and causes the release of gliotransmitters. Optogenetic and chemogenetic protocols for astrocyte activation aid in extracting novel information regarding the function of brain's neurovascular unit. This review summarizes current data obtained by this approach and discusses a potential mechanistic connection between astrocyte stimulation and changes in brain physiology.

Toward three-dimensional in vitro models to study neurovascular unit functions in health and disease

The high metabolic demands of the brain require an efficient vascular system to be coupled with neural activity to supply adequate nutrients and oxygen. This supply is coordinated by the action of neurons, glial and vascular cells, known collectively as the neurovascular unit, which temporally and spatially regulate local cerebral blood flow through a process known as neurovascular coupling. In many neurodegenerative diseases, changes in functions of the neurovascular unit not only impair neurovascular coupling but also permeability of the blood-brain barrier, cerebral blood flow and clearance of waste from the brain. In order to study disease mechanisms, we need improved physiologically-relevant human models of the neurovascular unit. Advances towards modeling the cellular complexity of the neurovascular unit in vitro have been made using stem-cell derived organoids and more recently, vascularized organoids, enabling intricate studies of non-cell autonomous processes. Engineering and design innovations in microfluidic devices and tissue engineering are progressing our ability to interrogate the cerebrovasculature. These advanced models are being used to gain a better understanding of neurodegenerative disease processes and potential therapeutics. Continued innovation is required to build more physiologically-relevant models of the neurovascular unit encompassing both the cellular complexity and designed features to interrogate neurovascular unit functionality.

Rapid, Dose-Dependent Enhancement of Cerebral Blood Flow by transcranial AC Stimulation in Mouse

BACKGROUND

Transcranial electrical stimulation at an appropriate dose may demonstrate intracranial effects, including neuronal stimulation and cerebral blood flow responses.

OBJECTIVE

We performed in vivo experiments on mouse cortex using transcranial alternating current [AC] stimulation to assess whether cerebral blood flow can be reliably altered by extracranial stimulation.

METHODS

We performed transcranial AC electrical stimulation transversely across the closed skull in anesthetized mice, measuring transcranial cerebral blood flow with a laser Doppler probe and intracranial electrical responses as endpoint biomarkers. We calculated a stimulation dose-response function between intracranial electric field and cerebral blood flow.

RESULTS

Stimulation at electric field amplitudes of 5-20 mV/mm at 10-20 Hz rapidly increased cerebral blood flow (within 100 ms), which then quickly decreased with no residual effects. The time to peak and blood flow shape varied with stimulation intensity and duration, showing a linear correlation between stimulation dose and peak blood flow increase. Neither afterdischarges nor spreading depression occurred from this level of stimulation.

CONCLUSIONS

Extracranial stimulation amplitudes sufficient to evoke reliable blood flow changes require electric field strengths higher than what is tolerable in unanesthetized humans (<1 mV/mm), but less than electroconvulsive therapy levels (>40 mV/mm). However, anesthesia effects, spontaneous blood flow fluctuations, and sampling error may accentuate the apparent field strength needed for enhanced blood flow. The translation to a human dose-response function to augment cerebral blood flow (i.e., in stroke recovery) will require significant modification, potentially to pericranial, focused, multi-electrode application or intracranial stimulation.

Activation of endothelial Wnt/β-catenin signaling by protective astrocytes repairs BBB damage in ischemic stroke

The role of astrocytes in dysregulation of blood-brain barrier (BBB) function following ischemic stroke is not well understood. Here, we investigate the effects of restoring the repair properties of astrocytes on the BBB after ischemic stroke. Mice deficient for NHE1, a pH-sensitive Na⁺/H⁺ exchanger 1, in astrocytes have reduced BBB permeability after ischemic stroke, increased angiogenesis and cerebral blood flow perfusion, in contrast to wild-type mice. Bulk RNA-sequencing transcriptome analysis of purified astrocytes revealed that ∼177 genes were differentially upregulated in mutant astrocytes, with Wnt7a mRNA among the top genes. Using a Wnt reporter line, we confirmed that the pathway was upregulated in cerebral vessels of mutant mice after ischemic stroke. However, administration of the Wnt/β-catenin inhibitor, XAV-939, blocked the reparative effects of Nhe1-deficient astrocytes. Thus, astrocytes lacking pH-sensitive NHE1 protein are transformed from injurious to "protective" by inducing Wnt production to promote BBB repair after ischemic stroke.

Varying perivascular astroglial endfoot dimensions along the vascular tree maintain perivascular-interstitial flux through the cortical mantle

The glymphatic system is a recently defined brain-wide network of perivascular spaces along which cerebrospinal fluid (CSF) and interstitial solutes exchange. Astrocyte endfeet encircling the perivascular space form a physical barrier in between these two compartments, and fluid and solutes that are not taken up by astrocytes move out of the perivascular space through the junctions in between astrocyte endfeet. However, little is known about the anatomical structure and the physiological roles of the astrocyte endfeet in regulating the local perivascular exchange. Here, visualizing astrocyte endfoot-endfoot junctions with immunofluorescent labeling against the protein megalencephalic leukoencephalopathy with subcortical cysts-1 (MLC1), we characterized endfoot dimensions along the mouse cerebrovascular tree. We observed marked heterogeneity in endfoot dimensions along vessels of different sizes, and of different types. Specifically, endfoot size was positively correlated with the vessel diameters, with large vessel segments surrounded by large endfeet and small vessel segments surrounded by small endfeet. This association was most pronounced along arterial, rather than venous segments. Computational modeling simulating vascular trees with uniform or varying endfeet dimensions demonstrates that varying endfoot dimensions maintain near constant perivascular-interstitial flux despite correspondingly declining perivascular pressures along the cerebrovascular tree through the cortical depth. These results describe a novel anatomical feature of perivascular astroglial endfeet and suggest that endfoot heterogeneity may be an evolutionary adaptation to maintain perivascular CSF-interstitial fluid exchange through deep brain structures.

Cortical Hemodynamic Response to Multi-afferent Stimulation: an optical imaging study

Multisensory stimulation plays an important role in the recovery of ischemic stroke. However, little is known about the interactions between neuronal activities with multi-afferent stimulations and their effects on hemodynamic responses. Optogenetics has been a useful tool in neuroscience research to unravel the mechanisms of neurovascular coupling at cell-specific level. In this study, we applied laser speckle contrast imaging (LSCI) to map the cortical hemodynamic response with high spatiotemporal resolution. The results showed that optogenetic inhibition of pyramidal neurons in sensorimotor cortex induced both local and distant increases of cerebral blood flow (CBF) with dual peaks, and the full width at half maximum (FWHM) was significantly larger than that of the CBF response to optogenetic excitation. Furthermore, optogenetic excitation of pyramidal neurons could significantly increase the local CBF response to sensory stimulation, whereas optogenetic inhibition of pyramidal neurons decreased the local CBF response at the early stage after sensory stimulation and increased the distant CBF response during the recovery period. Our work provided useful insights into the mechanisms of brain stimulation, which might help in clinical neurological applications.

Transient, Consequential Increases in Extracellular Potassium Ions Accompany Channelrhodopsin2 Excitation

Channelrhodopsin2 (ChR2) optogenetic excitation is widely used to study neurons, astrocytes, and circuits. Using complementary approaches in situ and in vivo, we found that ChR2 stimulation leads to significant transient elevation of extracellular potassium ions by ~5 mM. Such elevations were detected in ChR2-expressing mice, following local in vivo expression of ChR2(H134R) with adeno-associated viruses (AAVs), in different brain areas and when ChR2 was expressed in neurons or astrocytes. In particular, ChR2-mediated excitation of striatal astrocytes was sufficient to increase medium spiny neuron (MSN) excitability and immediate early gene expression. The effects on MSN excitability were recapitulated in silico with a computational MSN model and detected in vivo as increased action potential firing in awake, behaving mice. We show that transient, physiologically consequential increases in extracellular potassium ions accompany ChR2 optogenetic excitation. This coincidental effect may be important to consider during astrocyte studies employing ChR2 to interrogate neural circuits and animal behavior.

Using multiple approaches, Octeau et al. discover that optogenetic excitation of ChR2-expressing cells leads to significant transient extracellular potassium ion elevations that increase neuronal excitability and immediate early gene expression in neurons following in vivo stimulation.

Melanopsin for Time-Controlling Activation of Astrocyte -Neuron Networks

Melanopsin, a mammalian G-protein-coupled photopigment, is a novel optical tool which enables studying astrocyte-neuron networks. Here, we describe the required guidelines to take advantage of this promising optical tool for functional neuron-glia studies. The selective expression of melanopsin in astrocytes allows triggering astrocytic Ca²⁺ signaling, changes in synaptic transmission, and modifying behavioral responses.

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.

Optogenetic manipulation of astrocytes from synapses to neuronal networks: A potential therapeutic strategy for neurodegenerative diseases

Astrocytes are the most widespread and heterogeneous glial cells in the central nervous system and key regulators for brain development. They are capable of receiving neurotransmitters produced by synaptic activities and regulating synaptic functions by releasing gliotransmitters as part of the tripartite synapse. In addition to communicating with neurons at synaptic levels, astrocytes can integrate into inhibitory neural networks to interact with neurons in neuronal circuits. Astrocytes are closely related to the pathogenesis and pathological processes of neurodegenerative diseases (NDs). Recently, optogenetics has now been applied to reveal the function of astrocytes in physiology and pathology. Herein, we discuss the possibility whether optogenetics could be used to control the release of gliotransmitters and regulate astrocytic membrane channels. Thus, the capability of modulating the bidirectional interactions between astrocytes and neurons in both synaptic and neuronal networks via optogenetics is evaluated. Furthermore, we discuss that manipulating astrocytes via optogenetics might be an effective way to investigate the potential therapeutic strategy for NDs.

Spatiotemporal dynamics of red blood cells in capillaries in layer I of the cerebral cortex and changes in arterial diameter during cortical spreading depression and response to hypercapnia in anesthetized mice

OBJECTIVE

Control of red blood cell velocity in capillaries is essential to meet local neuronal metabolic requirements, although changes of capillary diameter are limited. To further understand the microcirculatory response during cortical spreading depression, we analyzed the spatiotemporal changes of red blood cell velocity in intraparenchymal capillaries.

METHODS

In urethane-anesthetized Tie2-green fluorescent protein transgenic mice, the velocity of fluorescence-labeled red blood cells flowing in capillaries in layer I of the cerebral cortex was automatically measured with our Matlab domain software (KEIO-IS2) in sequential images obtained with a high-speed camera laser-scanning confocal fluorescence microscope system.

RESULTS

Cortical spreading depression repeatedly increased the red blood cell velocity prior to arterial constriction/dilation. During the first cortical spreading depression, red blood cell velocity significantly decreased, and sluggishly moving or retrograde-moving red blood cells were observed, concomitantly with marked arterial constriction. The velocity subsequently returned to around the basal level, while oligemia after cortical spreading depression with slight vasoconstriction remained. After several passages of cortical spreading depression, hypercapnia-induced increase of red blood cell velocity, regional cerebral blood flow and arterial diameter were all significantly reduced, and the correlations among them became extremely weak.

CONCLUSIONS

Taken together with our previous findings, these simultaneous measurements of red blood cell velocity in multiple capillaries, arterial diameter and regional cerebral blood flow support the idea that red blood cell flow might be altered independently, at least in part, from arterial regulation, that neuro-capillary coupling plays a role in rapidly meeting local neural demand.

Selected Topics Relating to Functional MRI Study of the Brain

The authors selected some interesting current topics among many in the field of functional MRI (fMRI) of the brain. The selection was based on authours' immediate interests in exploring these aspects further; the topics are presented and discussed along with their perspectives. If progress can be made in these areas, it would be very advantageous to the field of brain research. The topics are (I) Detectable MRI signals in response to functional activity of the brain, including the current status of neurocurrent MRI; (II) Vascular-dependent and vascular-independent MRI signals, leading to the distinction of functional and structural MRI; (III) Functional specificity and functional connectivity of local sites, including differences between task-fMRI and resting state fMRI; (IV) Functional networks: an example of application to assessing the vocational aptitude test by fMRI; (V) Neural oscillation relevant to the formation of fMRI signals and of networks; (VI) Upgrading fMRI to "information-content-reflecting" fMRI, discussed as one of the prospects of near-future fMRI.

In vivo imaging for neurovascular disease research

Connections between various cell types in the brain enable cognitive function. The neurovascular unit is a structure composed of different cell types that regulate neurovascular coupling, blood-brain barrier permeability, and other interactions with peripheral systems. The relationship among the components of the neurovascular unit is complex and difficult to study without the use of in vivo neurovascular disease imaging. In this review, we introduce principles and examples of various in vivo optical imaging techniques including laser Doppler flowmetry, laser speckle contrast imaging, intrinsic optical signal imaging, optical coherence tomography, and two-photon microscopy. Furthermore, we introduce recent advances of in vivo imaging and future directions for promoting neurovascular disease research.

Astrocytic endfeet re-cover blood vessels after removal by laser ablation

The astrocyte, one of the glial cells, plays many functional roles. These include provision of nutrients from blood vessels to neurons, supply of neurotransmitters and support of blood-brain barrier (BBB) integrity. Astrocytes are known to support the integrity of BBB through maintenance of the tight junction between endothelial cells of blood vessels. However, evidence of its direct contribution to BBB is lacking owing to technical limitations. In this study, astrocytic endfeet covering blood vessels were removed by the laser ablation method with two photon laser scanning microscopy in in vivo mouse brain, and the re-covering of blood vessels with the astrocytic endfeet was observed in about half of the cases. Blood vessels kept their integrity without astrocytic endfoot covers: leakage of plasma marker dyes, Evans Blue or dextran-conjugated fluorescein, was not observed from stripped blood vessels, while ablation of vascular walls induced extravasation of Evans Blue. These results suggest that the astrocytic endfeet covering blood vessels do not contribute to the immediate BBB barrier.

Melanopsin for precise optogenetic activation of astrocyte-neuron networks

Optogenetics has been widely expanded to enhance or suppress neuronal activity and it has been recently applied to glial cells. Here, we have used a new approach based on selective expression of melanopsin, a G-protein-coupled photopigment, in astrocytes to trigger Ca²⁺ signaling. Using the genetically encoded Ca²⁺ indicator GCaMP6f and two-photon imaging, we show that melanopsin is both competent to stimulate robust IP3-dependent Ca²⁺ signals in astrocyte fine processes, and to evoke an ATP/Adenosine-dependent transient boost of hippocampal excitatory synaptic transmission. Additionally, under low-frequency light stimulation conditions, melanopsin-transfected astrocytes can trigger long-term synaptic changes. In vivo, melanopsin-astrocyte activation enhances episodic-like memory, suggesting melanopsin as an optical tool that could recapitulate the wide range of regulatory actions of astrocytes on neuronal networks in behaving animals. These results describe a novel approach using melanopsin as a precise trigger for astrocytes that mimics their endogenous G-protein signaling pathways, and present melanopsin as a valuable optical tool for neuron-glia studies.