Coupling of neural activity to blood flow in olfactory glomeruli is mediated by astrocytic pathways

Influence of age on nicotinic cholinergic regulation of blood flow in rat's olfactory bulb and neocortex

The olfactory bulb receives cholinergic basal forebrain inputs as does the neocortex. With a focus on nicotinic acetylcholine receptors (nAChRs), this review article provides an overview and discussion of the following findings: (1) the nAChRs-mediated regulation of regional blood flow in the neocortex and olfactory bulb, (2) the nAChR subtypes that mediate their responses, and (3) their activity in old rats. The activation of the α4β2-like subtype of nAChRs produces vasodilation in the neocortex, and potentiates olfactory bulb vasodilation induced by olfactory stimulation. The nAChR activity producing neocortical vasodilation was similarly maintained in 2-year-old rats as in adult rats, but was clearly reduced in 3-year-old rats. In contrast, nAChR activity in the olfactory bulb was reduced already in 2-year-old rats. Thus, age-related impairment of α4β2-like nAChR function may occur earlier in the olfactory bulb than in the neocortex. Given the findings, the vasodilation induced by α4β2-like nAChR activation may be beneficial for neuroprotection in the neocortex and the olfactory bulb.

A2A adenosine receptor-driven cAMP signaling in olfactory bulb astrocytes is unaffected in experimental autoimmune encephalomyelitis

Introduction

The cyclic nucleotide cyclic adenosine monophosphate (cAMP) is a ubiquitous second messenger, which is known to play an important anti-inflammatory role. Astrocytes in the central nervous system (CNS) can modulate inflammation but little is known about the significance of cAMP in their function.

Methods

We investigated cAMP dynamics in mouse olfactory bulb astrocytes in brain slices prepared from healthy and experimental autoimmune encephalomyelitis (EAE) mice.

Results

The purinergic receptor ligands adenosine and adenosine triphosphate (ATP) both induced transient increases in cAMP in astrocytes expressing the genetically encoded cAMP sensor Flamindo2. The A2A receptor antagonist ZM241385 inhibited the responses. Similar transient increases in astrocytic cAMP occurred when olfactory receptor neurons were stimulated electrically, resulting in ATP release from the stimulated axons that increased cAMP, again via A2A receptors. Notably, A2A-mediated responses to ATP and adenosine were not different in EAE mice as compared to healthy mice.

Discussion

Our results indicate that ATP, synaptically released by afferent axons in the olfactory bulb, is degraded to adenosine that acts on A2A receptors in astrocytes, thereby increasing the cytosolic cAMP concentration. However, this pathway is not altered in the olfactory bulb of EAE mice.

The "molecular soldiers" of the CNS: Astrocytes, a comprehensive review on their roles and molecular signatures

For a long time, neurons held the position of central players in the nervous system. Since there are far more astrocytes than neurons in the brain, it makes us wonder if these cells just take up space and support the neurons or if they are actively participating in central nervous system (CNS) homeostasis. Now, astrocytes' contribution to CNS physiology is appreciated as they are known to regulate ion and neurotransmitter levels, synapse formation and elimination, blood-brain barrier integrity, immune function, cerebral blood flow, and many more. In many neurological and psychiatric disorders, astrocyte functions are altered. Advancements in microscopic and transcriptomic tools revealed populations of astrocytes with varied morphology, electrophysiological properties, and transcriptomic profiles. Neuron-circuit-specific functions and neuron-specific interactions of astroglial subpopulations are found, which suggests that diversity is essential in carrying out diverse region-specific CNS functions. Investigations on heterogeneous astrocyte populations are revealing new astrocyte functions and their role in pathological conditions, opening a new therapeutic avenue for targeting neurological conditions. The true extent of astrocytic heterogeneity and its functional implications are yet to be fully explored. This review summarizes essential astrocytic functions and their relevance in pathological conditions and discusses astrocytic diversity in relation to morphology, function, and gene expression throughout the CNS.

Neurovascular Compression-Induced Intracranial Allodynia May Be the True Nature of Migraine Headache: an Interpretative Review

PURPOSE OF REVIEW

Surgical deactivation of migraine trigger sites by extracranial neurovascular decompression has produced encouraging results and challenged previous understanding of primary headaches. However, there is a lack of in-depth discussions on the pathophysiological basis of migraine surgery. This narrative review provides interpretation of relevant literature from the perspective of compressive neuropathic etiology, pathogenesis, and pathophysiology of migraine.

RECENT FINDINGS

Vasodilation, which can be asymptomatic in healthy subjects, may produce compression of cranial nerves in migraineurs at both extracranial and intracranial entrapment-prone sites. This may be predetermined by inherited and acquired anatomical factors and may include double crush-type lesions. Neurovascular compression can lead to sensitization of the trigeminal pathways and resultant cephalic hypersensitivity. While descending (central) trigeminal activation is possible, symptomatic intracranial sensitization can probably only occur in subjects who develop neurovascular entrapment of cranial nerves, which can explain why migraine does not invariably afflict everyone. Nerve compression-induced focal neuroinflammation and sensitization of any cranial nerve may neurogenically spread to other cranial nerves, which can explain the clinical complexity of migraine. Trigger dose-dependent alternating intensity of sensitization and its synchrony with cyclic central neural activities, including asymmetric nasal vasomotor oscillations, may explain the laterality and phasic nature of migraine pain. Intracranial allodynia, i.e., pain sensation upon non-painful stimulation, may better explain migraine pain than merely nociceptive mechanisms, because migraine cannot be associated with considerable intracranial structural changes and consequent painful stimuli. Understanding migraine as an intracranial allodynia could stimulate research aimed at elucidating the possible neuropathic compressive etiology of migraine and other primary headaches.

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.

PI3K block restores age-dependent neurovascular coupling defects associated with cerebral small vessel disease

Neurovascular coupling (NVC), a vital physiological process that rapidly and precisely directs localized blood flow to the most active regions of the brain, is accomplished in part by the vast network of cerebral capillaries acting as a sensory web capable of detecting increases in neuronal activity and orchestrating the dilation of upstream parenchymal arterioles. Here, we report a Col4a1 mutant mouse model of cerebral small vessel disease (cSVD) with age-dependent defects in capillary-to-arteriole dilation, functional hyperemia in the brain, and memory. The fundamental defect in aged mutant animals was the depletion of the minor membrane phospholipid phosphatidylinositol 4,5 bisphosphate (PIP2) in brain capillary endothelial cells, leading to the loss of inwardly rectifying K+ (Kir2.1) channel activity. Blocking phosphatidylinositol-3-kinase (PI3K), an enzyme that diminishes the bioavailability of PIP2 by converting it to phosphatidylinositol (3, 4, 5)-trisphosphate (PIP3), restored Kir2.1 channel activity, capillary-to-arteriole dilation, and functional hyperemia. In longitudinal studies, chronic PI3K inhibition also improved the memory function of aged Col4a1 mutant mice. Our data suggest that PI3K inhibition is a viable therapeutic strategy for treating defective NVC and cognitive impairment associated with cSVD.

Virally induced CRISPR/Cas9-based knock-in of fluorescent albumin allows long-term visualization of cerebral circulation in infant and adult mice

Albumin, a protein produced by liver hepatocytes, represents the most abundant protein in blood plasma. We have previously engineered a liver-targeting adeno-associated viral vector (AAV) that expresses fluorescent protein-tagged albumin to visualize blood plasma in mice. While this approach is versatile for imaging in adult mice, transgene expression vanishes when AAV is administered in neonates due to dilution of the episomal AAV genome in the rapidly growing liver. Here, we use CRISPR/Cas9 genome editing to insert the fluorescent protein mNeonGreen (mNG) gene into the albumin (Alb) locus of hepatocytes to produce fluorescently labeled albumin (Alb-mNG). We constructed a CRISPR AAV that includes ∼1 kb homologous arms around Alb exon 14 to express Alb-mNG. Subcutaneous injection of this AAV with AAV-CMV-Cas9 in postnatal day 3 mice resulted in two-photon visualization of the cerebral cortex vasculature within ten days. The expression levels of Alb-mNG were persistent for at least three months and were so robust that vasomotion and capillary blood flow could be assessed transcranially in early postnatal mice. This knock-in approach provides powerful means for micro- and macroscopic imaging of cerebral vascular dynamics in postnatal and adult mice.

PI3K block restores age-dependent neurovascular coupling defects associated with cerebral small vessel disease

Neurovascular coupling (NVC), a vital physiological process that rapidly and precisely directs localized blood flow to the most active regions of the brain, is accomplished in part by the vast network of cerebral capillaries acting as a sensory web capable of detecting increases in neuronal activity and orchestrating the dilation of upstream parenchymal arterioles. Here, we report a Col4a1 mutant mouse model of cerebral small vessel disease (cSVD) with age-dependent defects in capillary-to-arteriole dilation, functional hyperemia in the brain, and memory. The fundamental defect in aged mutant animals was the depletion of the minor membrane phospholipid phosphatidylinositol 4,5 bisphosphate (PIP 2 ) in brain capillary endothelial cells, leading to the loss of inwardly rectifier K + (Kir2.1) channel activity. Blocking phosphatidylinositol-3-kinase (PI3K), an enzyme that diminishes the bioavailability of PIP 2 by converting it to phosphatidylinositol (3,4,5)-trisphosphate (PIP 3 ), restored Kir2.1 channel activity, capillary-to-arteriole dilation, and functional hyperemia. In longitudinal studies, chronic PI3K inhibition also improved the memory function of aged Col4a1 mutant mice. Our data suggest that PI3K inhibition is a viable therapeutic strategy for treating defective NVC and cognitive impairment associated with cSVD.

One-sentence summary

PI3K inhibition rescues neurovascular coupling defects in cerebral small vessel disease.

Astrocytes Transplanted during Early Postnatal Development Integrate, Mature, and Survive Long Term in Mouse Cortex

Astrocytes have complex structural, molecular, and physiological properties and form specialized microenvironments that support circuit-specific functions in the CNS. To better understand how astrocytes acquire their unique features, we transplanted immature mouse cortical astrocytes into the developing cortex of male and female mice and assessed their integration, maturation, and survival. Within days, transplanted astrocytes developed morphologies and acquired territories and tiling behavior typical of cortical astrocytes. At 35-47 d post-transplantation, astrocytes appeared morphologically mature and expressed levels of EAAT2/GLT1 similar to nontransplanted astrocytes. Transplanted astrocytes also supported excitatory/inhibitory (E/I) presynaptic terminals within their territories, and displayed normal Ca2+ events. Transplanted astrocytes showed initially reduced expression of aquaporin 4 (AQP4) at endfeet and elevated expression of EAAT1/GLAST, with both proteins showing normalized expression by 110 d and one year post-transplantation, respectively. To understand how specific brain regions support astrocytic integration and maturation, we transplanted cortical astrocytes into the developing cerebellum. Cortical astrocytes interlaced with Bergmann glia (BG) in the cerebellar molecular layer to establish discrete territories. However, transplanted astrocytes retained many cortical astrocytic features including higher levels of EAAT2/GLT1, lower levels of EAAT1/GLAST, and the absence of expression of the AMPAR subunit GluA1. Collectively, our findings demonstrate that immature cortical astrocytes integrate, mature, and survive (more than one year) following transplantation and retain cortical astrocytic properties. Astrocytic transplantation can be useful for investigating cell-autonomous (intrinsic) and non-cell-autonomous (environmental) mechanisms contributing to astrocytic development/diversity, and for determining the optimal timing for transplanting astrocytes for cellular delivery or replacement in regenerative medicine.SIGNIFICANCE STATEMENT The mechanisms that enable astrocytes to acquire diverse molecular and structural properties remain to be better understood. In this study, we systematically analyzed the properties of cortical astrocytes following their transplantation to the early postnatal brain. We found that immature cortical astrocytes transplanted into cerebral cortex during early postnatal mouse development integrate and establish normal astrocytic properties, and show long-term survival in vivo (more than one year). In contrast, transplanted cortical astrocytes display reduced or altered ability to integrate into the more mature cerebral cortex or developing cerebellum, respectively. This study demonstrates the developmental potential of transplanted cortical astrocytes and provides an approach to tease apart cell-autonomous (intrinsic) and non-cell-autonomous (environmental) mechanisms that determine the structural, molecular, and physiological phenotype of astrocytes.

Role of P2Y receptors in astrocyte physiology and pathophysiology

Astrocytes are active constituents of the brain that manage ion homeostasis and metabolic support of neurons and directly tune synaptic transmission and plasticity. Astrocytes express all known P2Y receptors. These regulate a multitude of physiological functions such as cell proliferation, Ca²⁺ signalling, gliotransmitter release and neurovascular coupling. In addition, P2Y receptors are fundamental in the transition of astrocytes into reactive astrocytes, as occurring in many brain disorders such as neurodegenerative diseases, neuroinflammation and epilepsy. This review summarizes the current literature addressing the function of P2Y receptors in astrocytes in the healthy brain as well as in brain diseases.

From nociception to pain perception, possible implications of astrocytes

Astrocytes are determinants for the functioning of the CNS. They respond to neuronal activity with calcium increases and can in turn modulate synaptic transmission, brain plasticity as well as cognitive processes. Astrocytes display sensory-evoked calcium responses in different brain structures related to the discriminative system of most sensory modalities. In particular, noxious stimulation evoked calcium responses in astrocytes in the spinal cord, the hippocampus, and the somatosensory cortex. However, it is not clear if astrocytes are involved in pain. Pain is a private, personal, and complex experience that warns us about potential tissue damage. It is a perception that is not linearly associated with the amount of tissue damage or nociception; instead, it is constructed with sensory, cognitive, and affective components and depends on our previous experiences. However, it is not fully understood how pain is created from nociception. In this perspective article, we provide an overview of the mechanisms and neuronal networks that underlie the perception of pain. Then we proposed that coherent activity of astrocytes in the spinal cord and pain-related brain areas could be important in binding sensory, affective, and cognitive information on a slower time scale.

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 astrocytic Ca 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 astrocyte Ca 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 astrocytic Ca 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 astrocytic Ca 2 + elevations in functional hyperemia; summarize recent advances in genetically engineered Ca 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).

Fear learning induces α7-nicotinic acetylcholine receptor-mediated astrocytic responsiveness that is required for memory persistence

Memory persistence is a fundamental cognitive process for guiding behaviors and is considered to rely mostly on neuronal and synaptic plasticity. Whether and how astrocytes contribute to memory persistence is largely unknown. Here, by using two-photon Ca²⁺ imaging in head-fixed mice and fiber photometry in freely moving mice, we show that aversive sensory stimulation activates α7-nicotinic acetylcholine receptors (nAChRs) in a subpopulation of astrocytes in the auditory cortex. We demonstrate that fear learning causes the de novo induction of sound-evoked Ca²⁺ transients in these astrocytes. The astrocytic responsiveness persisted over days along with fear memory and disappeared in animals that underwent extinction of learned freezing behavior. Conditional genetic deletion of α7-nAChRs in astrocytes significantly impaired fear memory persistence. We conclude that learning-acquired, α7-nAChR-dependent astrocytic responsiveness is an integral part of the cellular substrate underlying memory persistence.

Distinct Characteristics of Odor-evoked Calcium and Electrophysiological Signals in Mitral/Tufted Cells in the Mouse Olfactory Bulb

Fiber photometry is a recently-developed method that indirectly measures neural activity by monitoring Ca²⁺ signals in genetically-identified neuronal populations. Although fiber photometry is widely used in neuroscience research, the relationship between the recorded Ca²⁺ signals and direct electrophysiological measurements of neural activity remains elusive. Here, we simultaneously recorded odor-evoked Ca²⁺ and electrophysiological signals [single-unit spikes and local field potentials (LFPs)] from mitral/tufted cells in the olfactory bulb of awake, head-fixed mice. Odors evoked responses in all types of signal but the response characteristics (e.g., type of response and time course) differed. The Ca²⁺ signal was correlated most closely with power in the β-band of the LFP. The Ca²⁺ signal performed slightly better at odor classification than high-γ oscillations, worse than single-unit spikes, and similarly to β oscillations. These results provide new information to help researchers select an appropriate method for monitoring neural activity under specific conditions.

Calcium signaling in neuroglia

Glial cells exploit calcium (Ca²⁺) signals to perceive the information about the activity of the nervous tissue and the tissue environment to translate this information into an array of homeostatic, signaling and defensive reactions. Astrocytes, the best studied glial cells, use several Ca²⁺ signaling generation pathways that include Ca²⁺ entry through plasma membrane, release from endoplasmic reticulum (ER) and from mitochondria. Activation of metabotropic receptors on the plasma membrane of glial cells is coupled to an enzymatic cascade in which a second messenger, InsP₃ is generated thus activating intracellular Ca²⁺ release channels in the ER endomembrane. Astrocytes also possess store-operated Ca²⁺ entry and express several ligand-gated Ca²⁺ channels. In vivo astrocytes generate heterogeneous Ca²⁺ signals, which are short and frequent in distal processes, but large and relatively rare in soma. In response to neuronal activity intracellular and inter-cellular astrocytic Ca²⁺ waves can be produced. Astrocytic Ca²⁺ signals are involved in secretion, they regulate ion transport across cell membranes, and are contributing to cell morphological plasticity. Therefore, astrocytic Ca²⁺ signals are linked to fundamental functions of the central nervous system ranging from synaptic transmission to behavior. In oligodendrocytes, Ca²⁺ signals are generated by plasmalemmal Ca²⁺ influx, or by release from intracellular stores, or by combination of both. Microglial cells exploit Ca²⁺ permeable ionotropic purinergic receptors and transient receptor potential channels as well as ER Ca²⁺ release. In this contribution, basic morphology of glial cells, glial Ca²⁺ signaling toolkit, intracellular Ca²⁺ signals and Ca²⁺-regulated functions are discussed with focus on astrocytes.

Norepinephrine-Induced Calcium Signaling and Store-Operated Calcium Entry in Olfactory Bulb Astrocytes

It is well-established that astrocytes respond to norepinephrine with cytosolic calcium rises in various brain areas, such as hippocampus or neocortex. However, less is known about the effect of norepinephrine on olfactory bulb astrocytes. In the present study, we used confocal calcium imaging and immunohistochemistry in mouse brain slices of the olfactory bulb, a brain region with a dense innervation of noradrenergic fibers, to investigate the calcium signaling evoked by norepinephrine in astrocytes. Our results show that application of norepinephrine leads to a cytosolic calcium rise in astrocytes which is independent of neuronal activity and mainly mediated by PLC/IP3-dependent internal calcium release. In addition, store-operated calcium entry (SOCE) contributes to the late phase of the response. Antagonists of both α1- and α2-adrenergic receptors, but not β-receptors, largely reduce the adrenergic calcium response, indicating that both α-receptor subtypes mediate norepinephrine-induced calcium transients in olfactory bulb astrocytes, whereas β-receptors do not contribute to the calcium transients.

VasoMetrics: unbiased spatiotemporal analysis of microvascular diameter in multi-photon imaging applications

BACKGROUND

Multi-photon imaging of the cerebrovasculature provides rich data on the dynamics of cortical arterioles, capillaries, and venules. Vascular diameter is the major determinant of blood flow resistance, and is the most commonly quantified metric in studies of the cerebrovasculature. However, there is a lack of accessible and easy-to-use methods to quantify vascular diameter in imaging data.

METHODS

We created VasoMetrics, a macro written in ImageJ/Fiji for spatiotemporal analysis of microvascular diameter. The key feature of VasoMetrics is rapid analysis of many evenly spaced cross-sectional lines along the vessel of interest, permitting the extraction of numerous diameter measurements from individual vessels. Here we demonstrated the utility of VasoMetrics by analyzing in vivo multi-photon imaging stacks and movies collected from lightly sedated mice, as well as data from optical coherence tomography angiography (OCTA) of human retina.

RESULTS

Compared to the standard approach, which is to measure cross-sectional diameters at arbitrary points along a vessel, VasoMetrics accurately reported spatiotemporal features of vessel diameter, reduced measurement bias and time spent analyzing data, and improved the reproducibility of diameter measurements between users. VasoMetrics revealed the dynamics in pial arteriole diameters during vasomotion at rest, as well as changes in capillary diameter before and after pericyte ablation. Retinal arteriole diameter was quantified from a human retinal angiogram, providing proof-of-principle that VasoMetrics can be applied to contrast-enhanced clinical imaging of microvasculature.

CONCLUSIONS

VasoMetrics is a robust macro for spatiotemporal analysis of microvascular diameter in imaging applications.

Elevated neuroinflammation contributes to the deleterious impact of iron overload on brain function in aging

Intracellular iron is essential for many neurobiological mechanisms. However, at high concentrations, iron may induce oxidative stress and inflammation. Brain iron overload has been shown in various neurodegenerative disorders and in normal aging. Elevated brain iron in old age may trigger brain dysfunction and concomitant cognitive decline. However, the exact mechanism underlying the deleterious impact of iron on brain function in aging is unknown. Here, we investigated the role of iron on brain function across the adult lifespan from 187 healthy participants (20-79 years old, 99 women) who underwent fMRI scanning while performing a working-memory n-back task. Iron content was quantified using R2* relaxometry, whereas neuroinflammation was estimated using myo-inositol measured by magnetic resonance spectroscopy. Striatal iron increased non-linearly with age, with linear increases at both ends of adulthood. Whereas higher frontostriatal activity was related to better memory performance independent of age, the link between brain activity and iron differed across age groups. Higher striatal iron was linked to greater frontostriatal activity in younger, but reduced activity in older adults. Further mediation analysis revealed that, after age 40, iron provided unique and shared contributions with neuroinflammation to brain activations, such that neuroinflammation partly mediated brain-iron associations. These findings promote a novel mechanistic understanding of how iron may exert deleterious effects on brain function and cognition with advancing age.

Postsynaptic activity of inhibitory neurons evokes hemodynamic fMRI responses

Functional MRI responses are localized to the synaptic sites of evoked inhibitory neurons, but it is unknown whether, or by what mechanisms, these neurons initiate functional hyperemia. Here, the neuronal origins of these hemodynamic responses were investigated by fMRI or local field potential and blood flow measurements during topical application of pharmacological agents when GABAergic granule cells in the rat olfactory bulb were synaptically targeted. First, to examine if postsynaptic activation of these inhibitory neurons was required for neurovascular coupling, we applied an NMDA receptor antagonist during cerebral blood volume-weighted fMRI acquisition and found that responses below the drug application site (up to ~1.5 mm) significantly decreased within ~30 min. Similarly, large decreases in granule cell postsynaptic activities and blood flow responses were observed when AMPA or NMDA receptor antagonists were applied. Second, inhibition of nitric oxide synthase preferentially decreased the initial, fast component of the blood flow response, while inhibitors of astrocyte-specific glutamate transporters and vasoactive intestinal peptide receptors did not decrease blood flow responses. Third, inhibition of GABA release with a presynaptic GABAB receptor agonist caused less reduction of neuronal and blood flow responses compared to the postsynaptic glutamate receptor antagonists. In conclusion, local hyperemia by synaptically-evoked inhibitory neurons was primarily driven by their postsynaptic activities, possibly through NMDA receptor-dependent calcium signaling that was not wholly dependent on nitric oxide.

Obesity in Midlife Hampers Resting and Sensory-Evoked Cerebral Blood Flow in Mice

OBJECTIVE

This study aimed to investigate the effects of a high-fat diet (HFD) and aging on resting and activity-dependent cerebral blood flow (CBF).

METHODS

To run a comparison between obese and age-matched control animals, 6-week-old mice were fed either with regular chow or an HFD for 3 months or 8 months. Glucose tolerance and insulin sensitivity were assessed for metabolic phenotyping. Resting and odor-evoked CBF at the microvascular scale in the olfactory bulb (OB) was investigated by multiexposure speckle imaging. Immunolabeling-enabled imaging of solvent-cleared organs was used to analyze vascular density. The ejection fraction was studied by using cardioechography. Olfactory sensitivity was tested by using a buried-food test.

RESULTS

Glucose intolerance and compromised odor-evoked CBF were observed in obese mice in the younger group. Prolonged HFD feeding triggered insulin resistance and stronger impairment in activity-dependent CBF. Aging had a specific negative impact on resting CBF. There was no decrease in vascular density in the OB of obese mice, although cardiac function was impaired at both ages. In addition, decreased olfactory sensitivity was observed only in the older, middle-aged obese mice.

CONCLUSIONS

OB microvasculature in obese mice showed a specific functional feature characterized by impaired sensory-evoked CBF and a specific deleterious effect of aging on resting CBF.

Distinct Features of Cerebral Blood Flow and Spontaneous Neural Activity as Integrated Predictors of Early Response to Antidepressants

AIMS

The purpose of this study is to explore whether pre-treatment features of brain function can discriminate non-responders to antidepressant medication in the early phase.

METHODS

Forty-four treatment-responsive depressed (RD) patients, 36 non-responsive depressed (NRD) patients, and 42 healthy controls (HCs) were recruited. Regional cerebral blood flow (CBF) and amplitude of low-frequency fluctuation (ALFF) values were calculated for all subjects. Correlation analyses were used to explore the relationship between symptom improvement and CBF/ALFF. Receiver operating characteristics (ROC) and the 10-fold cross-validation support vector machine (SVM) classifier were applied for the discrimination of treatment response.

RESULTS

Compared with the HCs, the RD and NRD groups exhibited lower CBF and ALFF in the right posterior lobe of the cerebellum. Compared with the NRD group, the RD group showed distinct CBF patterns in the left frontal striatal regions and right frontal cerebellar regions, as well as distinct ALFF features in the left frontoparietal striatum and right frontotemporal striatal cerebellar regions. The ROC and SVM classifier revealed the optimal power to distinguish the RD and NRD groups based on the combined measures (i.e., CBF and ALFF).

CONCLUSION

Distinct features of CBF and ALFF in the frontal striatal network may serve as promising neuroimaging predictors for identifying patients with blunted responsiveness, which may facilitate personalized antidepressant treatment.

Heterogeneity of Sensory-Induced Astrocytic Ca2+ Dynamics During Functional Hyperemia

Astrocytic Ca²⁺ fluctuations associated with functional hyperemia have typically been measured from large cellular compartments such as the soma, the whole arbor and the endfoot. The most prominent Ca²⁺ event is a large magnitude, delayed signal that follows vasodilation. However, previous work has provided little information about the spatio-temporal properties of such Ca²⁺ transients or their heterogeneity. Here, using an awake, in vivo two-photon fluorescence-imaging model, we performed detailed profiling of delayed astrocytic Ca²⁺ signals across astrocytes or within individual astrocyte compartments using small regions of interest next to penetrating arterioles and capillaries along with vasomotor responses to vibrissae stimulation. We demonstrated that while a 5-s air puff that stimulates all whiskers predominantly generated reproducible functional hyperemia in the presence or absence of astrocytic Ca²⁺ changes, whisker stimulation inconsistently produced astrocytic Ca²⁺ responses. More importantly, these Ca²⁺ responses were heterogeneous among subcellular structures of the astrocyte and across different astrocytes that resided within the same field of view. Furthermore, we found that whisker stimulation induced discrete Ca²⁺ “hot spots” that spread regionally within the endfoot. These data reveal that astrocytic Ca²⁺ dynamics associated with the microvasculature are more complex than previously thought, and highlight the importance of considering the heterogeneity of astrocytic Ca²⁺ activity to fully understanding neurovascular coupling.

High-speed volumetric two-photon fluorescence imaging of neurovascular dynamics

Understanding the structure and function of vasculature in the brain requires us to monitor distributed hemodynamics at high spatial and temporal resolution in three-dimensional (3D) volumes in vivo. Currently, a volumetric vasculature imaging method with sub-capillary spatial resolution and blood flow-resolving speed is lacking. Here, using two-photon laser scanning microscopy (TPLSM) with an axially extended Bessel focus, we capture volumetric hemodynamics in the awake mouse brain at a spatiotemporal resolution sufficient for measuring capillary size and blood flow. With Bessel TPLSM, the fluorescence signal of a vessel becomes proportional to its size, which enables convenient intensity-based analysis of vessel dilation and constriction dynamics in large volumes. We observe entrainment of vasodilation and vasoconstriction with pupil diameter and measure 3D blood flow at 99 volumes/second. Demonstrating high-throughput monitoring of hemodynamics in the awake brain, we expect Bessel TPLSM to make broad impacts on neurovasculature research.

Metabolic tuning of inhibition regulates hippocampal neurogenesis in the adult brain

Hippocampus-engaged behaviors stimulate neurogenesis in the adult dentate gyrus by largely unknown means. To explore the underlying mechanisms, we used tetrode recording to analyze neuronal activity in the dentate gyrus of freely moving adult mice during hippocampus-engaged contextual exploration. We found that exploration induced an overall sustained increase in inhibitory neuron activity that was concomitant with decreased excitatory neuron activity. A mathematical model based on energy homeostasis in the dentate gyrus showed that enhanced inhibition and decreased excitation resulted in a similar increase in neurogenesis to that observed experimentally. To mechanistically investigate this sustained inhibitory regulation, we performed metabolomic and lipidomic profiling of the hippocampus during exploration. We found sustainably increased signaling of sphingosine-1-phosphate, a bioactive metabolite, during exploration. Furthermore, we found that sphingosine-1-phosphate signaling through its receptor 2 increased interneuron activity and thus mediated exploration-induced neurogenesis. Taken together, our findings point to a behavior-metabolism circuit pathway through which experience regulates adult hippocampal neurogenesis.

Erythrocytic α-synuclein contained in microvesicles regulates astrocytic glutamate homeostasis: a new perspective on Parkinson's disease pathogenesis

Parkinson's disease is a neurodegenerative disorder characterized by the transmission and accumulation of toxic species of α-synuclein (α-syn). Extracellular vesicles (EVs) are believed to play a vital role in the spread of toxic α-syn species. Recently, peripheral α-syn pathology has been investigated, but little attention has been devoted to erythrocytes, which contain abundant α-syn. In this study, we first demonstrated that erythrocyte-derived EVs isolated from Parkinson's disease patients carried elevated levels of oligomeric α-syn, compared to those from healthy controls. Moreover, human erythrocyte-derived EVs, when injected into peripheral blood in a mouse model of Parkinson's disease, were found to readily cross the blood-brain barrier (BBB). These EVs accumulated in astrocyte endfeet, a component of the BBB, where they impaired glutamate uptake, likely via interaction between excitatory amino acid transporter 2 (EAAT2) and oligomeric α-syn. These data suggest that erythrocyte-derived EVs and the oligomeric α-syn carried in them may play critical roles in the progression or even initiation of Parkinson's disease. Additionally, the mechanisms involved are attributable at least in part to dysfunction of astrocytes induced by these EVs. These observations provide new insight into the understanding of the mechanisms involved in Parkinson's disease.

The pericyte connectome: spatial precision of neurovascular coupling is driven by selective connectivity maps of pericytes and endothelial cells and is disrupted in diabetes

Functional hyperemia, or the matching of blood flow with activity, directs oxygen and nutrients to regionally firing neurons. The mechanisms responsible for this spatial accuracy remain unclear but are critical for brain function and establish the diagnostic resolution of BOLD-fMRI. Here, we described a mosaic of pericytes, the vasomotor capillary cells in the living retina. We then tested whether this net of pericytes and surrounding neuroglia predicted a connectivity map in response to sensory stimuli. Surprisingly, we found that these connections were not only selective across cell types, but also highly asymmetric spatially. First, pericytes connected predominantly to other neighboring pericytes and endothelial cells, and less to arteriolar smooth muscle cells, and not to surrounding neurons or glia. Second, focal, but not global stimulation evoked a directional vasomotor response by strengthening connections along the feeding vascular branch. This activity required local NO signaling and occurred by means of direct coupling via gap junctions. By contrast, bath application of NO or diabetes, a common microvascular pathology, not only weakened the vascular signaling but also abolished its directionality. We conclude that the exclusivity of neurovascular interactions may thus establish spatial accuracy of blood delivery with the precision of the neuronal receptive field size, and is disrupted early in diabetes.

Effects of nicotine on regional blood flow in the olfactory bulb in response to olfactory nerve stimulation

This study examined the effect of olfactory nerve stimulation on regional cerebral blood flow and assessed the effect of intravenous nicotine administration on this response in anesthetized rats. Regional cerebral blood flow was measured with laser Doppler flowmetry or laser speckle contrast imaging. Unilateral olfactory nerve stimulation for 5 s produced current (≥ 100 μA) and frequency-dependent (≥ 5 Hz) increases in blood flow in the olfactory bulb ipsilateral to the stimulus. The increased olfactory bulb blood flow peaked at 30 ± 7% using stimulus parameters of 300 μA and 20 Hz. Nerve stimulation did not change frontal cortical blood flow or mean arterial pressure. The intravenous injection of nicotine (30 μg/kg) augmented the olfactory bulb blood flow response to nerve stimulation (20 Hz, 300 μA) by approximately 1.5-fold (60-s area after the stimulation). These results indicate that olfactory nerve stimulation increases olfactory bulb blood flow, and the response is potentiated by the activation of nicotinic cholinergic transmission.

Dopamine-induced calcium signaling in olfactory bulb astrocytes

It is well established that astrocytes respond to the major neurotransmitters glutamate and GABA with cytosolic calcium rises, whereas less is known about the effect of dopamine on astroglial cells. In the present study, we used confocal calcium imaging in mouse brain slices of the olfactory bulb, a brain region with a large population of dopaminergic neurons, to investigate calcium signaling evoked by dopamine in astrocytes. Our results show that application of dopamine leads to a dose-dependent cytosolic calcium rise in astrocytes (EC₅₀ = 76 µM) which is independent of neuronal activity and mainly mediated by PLC/IP₃-dependent internal calcium release. Antagonists of both D₁- and D₂-class dopamine receptors partly reduce the dopaminergic calcium response, indicating that both receptor classes contribute to dopamine-induced calcium transients in olfactory bulb astrocytes.

Sublamina-specific organization of the blood brain barrier in the mouse olfactory nerve layer

Astrocytes constitute the main glial component of the mammalian blood brain barrier (BBB). However, in the olfactory bulb (OB), the olfactory nerve layer (ONL) is almost devoid of astrocytes, raising the question which glial cells are part of the BBB. We used mice expressing EGFP in astrocytes and tdTomato in olfactory ensheathing cells (OECs), a specialized type of glial cells in the ONL, to unequivocally identify both glial cell types and investigate their contribution to the BBB in the olfactory bulb. OECs were located exclusively in the ONL, while somata of astrocytes were located in deeper layers and extended processes in the inner sublamina of the ONL. These processes surrounded blood vessels and contained aquaporin-4, an astrocytic protein enriched at the BBB. In the outer sublamina of the ONL, in contrast, blood vessels were surrounded by aquaporin-4-negative processes of OECs. Transcardial perfusion of blood vessels with lanthanum and subsequent visualization by electron microscopy showed that blood vessels enwrapped by OECs possessed intact tight junctions. In acute olfactory bulb preparations, injection of fluorescent glucose 6-NBDG into blood vessels resulted in labeling of OECs, indicating glucose transport from the perivascular space into OECs. In addition, Ca²⁺ transients in OECs in the outer sublamina evoked vasoconstriction, whereas Ca²⁺ signaling in OECs of the inner sublamina had no effect on adjacent blood vessels. Our results demonstrate that the BBB in the inner sublamina of the ONL contains astrocytes, while in the outer ONL OECs are part of the BBB.

Neurophysiological effects of multiple mood episodes in bipolar disorder

OBJECTIVES

Bipolar disorder is marked by progressive symptomatic changes, which have been linked with episode-related structural findings-particularly in the prefrontal cortex. However, few studies have examined neurofunctional and neurochemical effects of disease burden. In this study, we compared first- and multi-episode bipolar individuals. We hypothesized that the latter would demonstrate evidence of neurophysiological differences consistent with a model of progressive functional degradation of these networks.

METHODS

First- and multi-episode manic bipolar subjects participated in functional magnetic resonance imaging (fMRI) including a continuous performance task with emotional distractors, and in single-voxel (¹ H) magnetic resonance spectroscopy (MRS). A priori fMRI regions-of-interest (ROI) included structures comprising prefrontal-striatal-amygdala networks; (¹ H)MRS voxels were placed within bilateral ventrolateral prefrontal (VLPFC) and anterior cingulate cortex (ACC). Both ROI and voxel-based brain activation in response to emotional stimuli, and neurochemical concentrations derived from (¹ H)MRS were compared across bipolar groups.

RESULTS

Multi-episode bipolar subjects showed relatively lower regional activation across prefrontal-striatal-amygdala networks, including bilateral VLPFC, orbitofrontal cortex, ACC, putamen, caudate, and amygdala. Exploratory whole-brain, voxel-based analysis suggested additional areas of lower activation extending into Brodmann area 22, posterior parietal regions, and right thalamus. Glutamate and N-acetylaspartate (NAA) concentrations were also relatively lower in the ACC of multi-episode subjects.

CONCLUSIONS

Disease burden, exemplified by multiple affective episodes is associated with evidence of widespread decrements in affective network activity. Lower ACC NAA concentration is similarly consistent with a model of progressive functional deficits. These findings support the functional significance of previously observed progressive structural changes throughout these regions.

Purinergic Signaling in the Vertebrate Olfactory System

Adenosine 5'-triphosphate (ATP) is an ubiquitous co-transmitter in the vertebrate brain. ATP itself, as well as its breakdown products ADP and adenosine are involved in synaptic transmission and plasticity, neuron-glia communication and neural development. Although purinoceptors have been demonstrated in the vertebrate olfactory system by means of histological techniques for many years, detailed insights into physiological properties and functional significance of purinergic signaling in olfaction have been published only recently. We review the current literature on purinergic neuromodulation, neuron-glia interactions and neurogenesis in the vertebrate olfactory system.

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.

Astrocytes drive cortical vasodilatory signaling by activating endothelial NMDA receptors

Astrocytes express neurotransmitter receptors that serve as sensors of synaptic activity and initiate signals leading to activity-dependent local vasodilation and increases in blood flow. We previously showed that arteriolar vasodilation produced by activation of cortical astrocytes is dependent on endothelial nitric oxide synthase (eNOS) and endogenous agonists of N-methyl-D-aspartate (NMDA) receptors. Here, we tested the hypothesis that these effects are mediated by NMDA receptors expressed by brain endothelial cells. Primary endothelial cultures expressed NMDA receptor subunits and produced nitric oxide in response to co-agonists, glutamate and D-serine. In cerebral cortex in situ, immunoelectron microscopy revealed that endothelial cells express the GluN1 NMDA receptor subunit at basolateral membrane surfaces in an orientation suitable for receiving intercellular messengers from brain cells. In cortical slices, activation of astrocytes by two-photon flash photolysis of a caged Ca²⁺ compound or application of a metabotropic glutamate receptor agonist caused endothelial NO generation and local vasodilation. These effects were mitigated by NMDA receptor antagonists and conditional gene silencing of endothelial GluN1, indicating at least partial dependence on endothelial NMDA receptors. Our observations identify a novel astrocyte-endothelial vasodilatory signaling axis that could contribute to endothelium-dependent vasodilation in brain functional hyperemia.

Optical imaging and modulation of neurovascular responses

The cerebral microvasculature consists of pial vascular networks, parenchymal descending arterioles, ascending venules and parenchymal capillaries. This vascular compartmentalization is vital to precisely deliver blood to balance continuously varying neural demands in multiple brain regions. Optical imaging techniques have facilitated the investigation of dynamic spatial and temporal properties of microvascular functions in real time. Their combination with transgenic animal models encoding specific genetic targets have further strengthened the importance of optical methods for neurovascular research by allowing for the modulation and monitoring of neuro vascular function. Image analysis methods with three-dimensional reconstruction are also helping to understand the complexity of microscopic observations. Here, we review the compartmentalized cerebral microvascular responses to global perturbations as well as regional changes in response to neural activity to highlight the differences in vascular action sites. In addition, microvascular responses elicited by optical modulation of different cell-type targets are summarized with emphasis on variable spatiotemporal dynamics of microvascular responses. Finally, long-term changes in microvascular compartmentalization are discussed to help understand potential relationships between CBF disturbances and the development of neurodegenerative diseases and cognitive decline.

What is the key mediator of the neurovascular coupling response?

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

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

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

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

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

Spontaneous astrocytic Ca2+ activity abounds in electrically suppressed ischemic penumbra of aged mice

Experimental focal cortical ischemic lesions consist of an ischemic core and a potentially salvageable peri-ischemic region, the ischemic penumbra. The activity of neurons and astrocytes is assumed to be suppressed in the penumbra because the electrical function is interrupted, but this is incompletely elucidated. Most experimental stroke studies used young adult animals, whereas stroke is prevalent in the elderly population. Using two-photon imaging in vivo, we here demonstrate extensive but electrically silent, spontaneous Ca²⁺ activity in neurons and astrocytes in the ischemic penumbra of 18- to 24-month-old mice 2-4 hr after middle cerebral artery occlusion. In comparison, stroke reduced spontaneous Ca²⁺ activity in neurons and astrocytes in adult mice (3-4 months of age). In aged mice, stroke increased astrocytic spontaneous Ca²⁺ activity considerably while neuronal spontaneous Ca²⁺ activity was unchanged. Blockade of action potentials and of purinergic receptors strongly reduced spontaneous Ca²⁺ activity in both neurons and astrocytes in the penumbra of old stroke mice. This indicates that stroke had a direct influence on mechanisms in presynaptic terminals and on purinergic signaling. Thus, highly dynamic variations in spontaneous Ca²⁺ activity characterize the electrically compromised penumbra, with remarkable differences between adult and old mice. The data are consistent with the notion that aged neurons and astrocytes take on a different phenotype than young mice. The increased activity of the aged astrocyte phenotype may be harmful to neurons. We suggest that the abundant spontaneous Ca²⁺ activity in astrocytes in the ischemic penumbra of old mice may be a novel target for neuroprotection strategies. A video abstract of this article can be found at https://youtu.be/AKlwKFsz1qE.

Sparsened neuronal activity in an optogenetically activated olfactory glomerulus

Glomeruli are the functional units of olfactory information processing but little remains known about their individual unit function. This is due to their widespread activation by odor stimuli. We expressed channelrhodopsin-2 in a single olfactory sensory neuron type, and used laser stimulation and simultaneous in vivo calcium imaging to study the responses of a single glomerulus to optogenetic stimulation. Calcium signals in the neuropil of this glomerulus were representative of the sensory input and nearly identical if evoked by intensity-matched odor and laser stimuli. However, significantly fewer glomerular layer interneurons and olfactory bulb output neurons (mitral cells) responded to optogenetic versus odor stimuli, resulting in a small and spatially compact optogenetic glomerular unit response. Temporal features of laser stimuli were represented with high fidelity in the neuropil of the glomerulus and the mitral cells, but not in interneurons. Increases in laser stimulus intensity were encoded by larger signal amplitudes in all compartments of the glomerulus, and by the recruitment of additional interneurons and mitral cells. No spatial expansion of the glomerular unit response was observed in response to stronger input stimuli. Our data are among the first descriptions of input-output transformations in a selectively activated olfactory glomerulus.

Astrocyte Activation and the Calcineurin/NFAT Pathway in Cerebrovascular Disease

Calcineurin (CN) is a Ca2+/calmodulin-dependent protein phosphatase with high abundance in nervous tissue. Though enriched in neurons, CN can become strongly induced in subsets of activated astrocytes under different pathological conditions where it interacts extensively with the nuclear factor of activated T cells (NFATs). Recent work has shown that regions of small vessel damage are associated with the upregulation of a proteolized, highly active form of CN in nearby astrocytes, suggesting a link between the CN/NFAT pathway and chronic cerebrovascular disease. In this Mini Review article, we discuss CN/NFAT signaling properties in the context of vascular disease and use previous cell type-specific intervention studies in Alzheimer's disease and traumatic brain injury models as a framework to understand how astrocytic CN/NFATs may couple vascular pathology to neurodegeneration and cognitive loss.

Selective deletion of glutamine synthetase in the mouse cerebral cortex induces glial dysfunction and vascular impairment that precede epilepsy and neurodegeneration

Glutamate-ammonia ligase (glutamine synthetase; Glul) is enriched in astrocytes and serves as the primary enzyme for ammonia detoxification and glutamate inactivation in the brain. Loss of astroglial Glul is reported in hippocampi of epileptic patients, but the mechanism by which Glul deficiency might cause disease remains elusive. Here we created a novel mouse model by selectively deleting Glul in the hippocampus and neocortex. The Glul deficient mice were born without any apparent malformations and behaved unremarkably until postnatal week three. There were reductions in tissue levels of aspartate, glutamate, glutamine and GABA and in mRNA encoding glutamate receptor subunits GRIA1 and GRIN2A as well as in the glutamate transporter proteins EAAT1 and EAAT2. Adult Glul-deficient mice developed progressive neurodegeneration and spontaneous seizures which increased in frequency with age. Importantly, progressive astrogliosis occurred before neurodegeneration and was first noted in astrocytes along cerebral blood vessels. The responses to CO₂-provocation were attenuated at four weeks of age and dilated microvessels were observed histologically in sclerotic areas of cKO. Thus, the abnormal glutamate metabolism observed in this model appeared to cause epilepsy by first inducing gliopathy and disrupting the neurovascular coupling.

Peripapillary and parafoveal vascular network assessment by optical coherence tomography angiography in aquaporin-4 antibody-positive neuromyelitis optica spectrum disorders

BACKGROUND/AIMS

Current understanding of the alterations in the retinal vascular network in neuromyelitis optica spectrum disorders (NMOSDs) is limited. We aim to assess the peripapillary and parafoveal vessel density in aquaporin-4 antibody-positive NMOSD patients by optical coherence tomography (OCT) angiography.

METHODS

A total of 55 aquaporin-4 antibody-positive NMOSD patients with or without a history of optic neuritis (ON) and 33 healthy controls underwent spectral domain OCT and OCT angiography. Clinical histories, Expanded Disability Status Scale score, visual functional system score (VFSS) and disease duration were collected.

RESULTS

Peripapillary and parafoveal vessel density was significantly decreased in NMOSD eyes with or without a history of ON. The decrease in retinal vessel density could occur before ON and retinal nerve fibre layer (RNFL) atrophy. Peripapillary vessel density correlated well with the spectral domain OCT measurements and VFSS in NMOSD eyes with a history of ON.

CONCLUSION

Subclinical primary retinal vasculopathy may occur in NMOSD prior to ON and RNFL atrophy. Peripapillary vessel density might be a sensitive predictor of visual outcomes in NMOSD patients with ON.

fMRI study of the role of glutamate NMDA receptor in the olfactory processing in monkeys

Studies in rodents show that olfactory processing in the principal neurons of olfactory bulb (OB) and piriform cortex (PC) is controlled by local inhibitory interneurons, and glutamate NMDA receptor plays a role in this inhibitory control. It is not clear if findings from studies in rodents translate to olfactory processing in nonhuman primates (NHPs). In this study, the effect of the glutamate NMDA receptor antagonist MK801 on odorant-induced olfactory responses in the OB and PC of anesthetized NHPs (rhesus monkeys) was investigated by cerebral blood volume (CBV) fMRI. Isoamyl-acetate was used as the odor stimulant. For each NHP, sixty fMRI measurements were made during a 4-h period, with each 4-min measurement consisting of a 1-min baseline period, a 1-min odor stimulation period, and a 2-min recovery period. MK801 (0.3 mg/kg) was intravenously delivered 1 hour after starting fMRI. Before MK801 injection, olfactory fMRI activations were observed only in the OB, not in the PC. After MK801 injection, olfactory fMRI activations in the OB increased, and robust olfactory fMRI activations were observed in the PC. The data indicate that MK801 enhances the olfactory responses in both the OB and PC. The enhancement effects of MK801 are most likely from its blockage of NMDA receptors on local inhibitory interneurons and the attenuation of the inhibition onto principal neurons. This study suggests that the mechanism of local inhibitory control of principal neurons in the OB and PC derived from studies in rodents translates to NHPs.

Astrocytic Ca2+ responses in the spinal dorsal horn by noxious stimuli to the skin

The role of astrocytes in the spinal dorsal horn (SDH) for sensory information processing under normal conditions is poorly understood. In this study, we investigated whether SDH astrocytes respond to noxious and innocuous stimuli to the skin of normal mice using in vivo two-photon Ca²⁺ imaging under anesthesia. We found that noxious stimulation evoked by intraplantar formalin injection provoked an elevation in intracellular Ca²⁺ levels in SDH astrocytes. By contrast, neither instantaneous noxious pinching nor innocuous stimuli (cooling or brushing) to the hindpaw elicited astrocytic Ca²⁺ responses. Thus, SDH astrocytes could respond preferentially to a strong and/or sustained noxious stimulus.

Linear and inverted U-shaped dose-response functions describe estrogen effects on hippocampal activity in young women

In animals, 17-beta-estradiol (E2) enhances hippocampal plasticity in a dose-dependent, monotonically increasing manner, but this relationship can also exhibit an inverted U-shaped function. To investigate E2’s dose-response function in the human hippocampus, we pharmacologically increased E2 levels in 125 naturally cycling women (who were in their low-hormone menstruation phase) to physiological (equivalent to menstrual cycle peak) and supraphysiological (equivalent to levels during early pregnancy) concentrations in a placebo-controlled design. Twenty-four hours after first E2 intake, we measured brain activity during encoding of neutral and negative pictures and then tested recognition memory 24 h after encoding. Here we report that E2 exhibits both a monotonically increasing relationship with hippocampal activity as well as an inverted U-shaped relationship, depending on the hippocampal region. Hippocampal activity exhibiting a U-shaped relationship inflects at supraphysiological E2 levels, suggesting that while E2 within physiological ranges stimulates hippocampal activity, supraphysiological ranges show opposite effects.

While estrogen is known to change hippocampal activity in animals, it is not known if this effect extends to humans. Here, authors vary the doses of estrogen in young women and show that the effects on hippocampal activity can be described by linear and inverted-U shaped dose-response functions.

Glial Calcium Waves are Triggered by Seizure Activity and Not Essential for Initiating Ictal Onset or Neurovascular Coupling

It has been postulated that glia play a critical role in modifying neuronal activity, mediating neurovascular coupling, and in seizure initiation. We investigated the role of glia in ictogenesis and neurovascular coupling through wide-field multicell and 2-photon single cell imaging of calcium and intrinsic signal imaging of cerebral blood volume in an in vivo rat model of focal neocortical seizures. Ictal events triggered a slowly propagating glial calcium wave that was markedly delayed after both neuronal and hemodynamic onset. Glial calcium waves exhibited a stereotypical spread that terminated prior to seizure offset and propagated to an area ~60% greater than the propagation area of neural and vascular signals. Complete blockage of glial activity with fluoroacetate resulted in no change in either neuronal or hemodynamic activity. These ictal glial waves were blocked by carbenoxolone, a gap junction blocker. Our in vivo data reveal that ictal events trigger a slowly propagating, stereotypical glial calcium wave, mediated by gap junctions, that is spatially and temporally independent of neuronal and hemodynamic activities. We introduce a novel ictally triggered propagating glial calcium wave calling into question the criticality of glial calcium wave in both ictal onset and neurovascular coupling.

Astrocyte disruption of neurovascular communication is linked to cortical damage in an animal model of multiple sclerosis

To elucidate mechanisms contributing to cortical pathology in multiple sclerosis (MS), we investigated neurovascular aberrations, in particular the association of astrocytes with cortical neurons and blood vessels, in mice induced with experimental autoimmune encephalomyelitis (EAE). Blood-brain barrier (BBB) dysfunction was evident by leakage of the tracer sodium fluorescein, along with reduced expression of claudin-5 by endothelial cells and desmin by pericytes. Immunohistological and ultrastructural analyses revealed detachment of the astroglial cell bodies from the blood vessels and loss of their connections with both the blood vessels and the neuronal synapses. Furthermore, examination of individual astrocytic processes at cortical layer IV, where well-defined neuronal columns (barrels) are linked to functional properties, revealed loss of astrocytic confinement to the functional neuronal boundaries. Thus, in contrast to the highly modulated patches of astrocyte processes in naïve mice overlapping the barrel cores, in EAE-mice process distribution was uniform ignoring the barrel boundaries. These aberrations are attributed to the surrounding inflammation, indicated by T-cells presence in the cortex as well as in the subcortical white matter and the meninges. Immunomodulatory treatment with glatiramer acetate partially abrogated the neurovascular damage. These combined findings indicate that under inflammatory conditions, activated perivascular astrocytes fail in neuro-hemodynamic coupling, resulting in obstructed cross-talk between the blood vessels and the neurons. We propose that loss of cortical astrocytic regulation and fine-tuning between the blood supply and the neuronal needs contributes to the neurological impairment and cognitive decline occurring in EAE/MS as well as to the disease progression.