Astrocytes produce nitric oxide via nitrite reduction in mitochondria to regulate cerebral blood flow during brain hypoxia

During hypoxia, increases in cerebral blood flow maintain brain oxygen delivery. Here, we describe a mechanism of brain oxygen sensing that mediates the dilation of intraparenchymal cerebral blood vessels in response to reductions in oxygen supply. In vitro and in vivo experiments conducted in rodent models show that during hypoxia, cortical astrocytes produce the potent vasodilator nitric oxide (NO) via nitrite reduction in mitochondria. Inhibition of mitochondrial respiration mimics, but also occludes, the effect of hypoxia on NO production in astrocytes. Astrocytes display high expression of the molybdenum-cofactor-containing mitochondrial enzyme sulfite oxidase, which can catalyze nitrite reduction in hypoxia. Replacement of molybdenum with tungsten or knockdown of sulfite oxidase expression in astrocytes blocks hypoxia-induced NO production by these glial cells and reduces the cerebrovascular response to hypoxia. These data identify astrocyte mitochondria as brain oxygen sensors that regulate cerebral blood flow during hypoxia via release of nitric oxide.

Mitochondrial malfunction and atrophy of astrocytes in the aged human cerebral cortex

How aging affects cells of the human brain active milieu remains largely unknown. Here, we analyze astrocytes and neurons in the neocortical tissue of younger (22-50 years) and older (51-72 years) adults. Aging decreases the amount of reduced mitochondrial cytochromes in astrocytes but not neurons. The protein-to-lipid ratio decreases in astrocytes and increases in neurons. Aged astrocytes show morphological atrophy quantified by the decreased length of branches, decreased volume fraction of leaflets, and shrinkage of the anatomical domain. Atrophy correlates with the loss of gap junction coupling between astrocytes and increased input resistance. Aging is accompanied by the upregulation of glial fibrillary acidic protein (GFAP) and downregulation of membrane-cytoskeleton linker ezrin associated with leaflets. No significant changes in neuronal excitability or spontaneous inhibitory postsynaptic signaling is observed. Thus, brain aging is associated with the impaired morphological presence and mitochondrial malfunction of cortical astrocytes, but not neurons.

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.

Dissociation Between Neuronal and Astrocytic Calcium Activity in Response to Locomotion in Mice

Locomotion triggers a coordinated response of both neurons and astrocytes in the brain. Here we performed calcium (Ca²⁺) imaging of these two cell types in the somatosensory cortex in head-fixed mice moving on the airlifted platform. Ca²⁺ activity in astrocytes significantly increased during locomotion from a low quiescence level. Ca²⁺ signals first appeared in the distal processes and then propagated to astrocytic somata, where it became significantly larger and exhibited oscillatory behaviour. Thus, astrocytic soma operates as both integrator and amplifier of Ca²⁺ signal. In neurons, Ca²⁺ activity was pronounced in quiescent periods and further increased during locomotion. Neuronal Ca²⁺ concentration ([Ca²⁺]_i) rose almost immediately following the onset of locomotion, whereas astrocytic Ca²⁺ signals lagged by several seconds. Such a long lag suggests that astrocytic [Ca²⁺]_i elevations are unlikely to be triggered by the activity of synapses among local neurons. Ca²⁺ responses to pairs of consecutive episodes of locomotion did not significantly differ in neurons, while were significantly diminished in response to the second locomotion in astrocytes. Such astrocytic refractoriness may arise from distinct mechanisms underlying Ca²⁺ signal generation. In neurons, the bulk of Ca²⁺ enters through the Ca²⁺ channels in the plasma membrane allowing for steady-level Ca²⁺ elevations in repetitive runs. Astrocytic Ca²⁺ responses originate from the intracellular stores, the depletion of which affects subsequent Ca²⁺ signals. Functionally, neuronal Ca²⁺ response reflects sensory input processed by neurons. Astrocytic Ca²⁺ dynamics is likely to provide metabolic and homeostatic support within the brain active milieu.

A high-fat diet changes astrocytic metabolism to promote synaptic plasticity and behavior

AIM

A high-fat diet (HFD) is generally considered to negatively influence the body, the brain, and cognition. Nonetheless, fat and fatty acids are essential for nourishing and constructing brain tissue. Astrocytes are central for lipolysis and fatty acids metabolism. We tested how HFD affects astrocyte metabolism, morphology, and physiology.

METHODS

We used Raman microspectroscopy to assess the redox state of mitochondria and lipid content in astrocytes and neurons in hippocampal slices of mice subjected to HFD. Astrocytes were loaded with fluorescent dye through patch pipette for morphological analysis. Whole-cell voltage-clamp recordings were performed to measure transporter and potassium currents. Western blot analysis quantified the expression of astrocyte-specific proteins. Field potential recordings measured the magnitude of long-term potentiation (LTP). Open filed test was performed to evaluate the effect of HFD on animal behavior.

RESULTS

We found that exposure of young mice to 1 month of HFD increases lipid content and relative amount of reduced cytochromes in astrocytes but not in neurons. Metabolic changes were paralleled with an enlargement of astrocytic territorial domains due to an increased outgrowth of branches and leaflets. Astrocyte remodeling was associated with an increase in expression of ezrin and with no changes in glial fibrillary acidic protein (GFAP), glutamate transporter-1 (GLT-1), and glutamine synthetase (GS). Such physiological (non-reactive) enlargement of astrocytes in the brain active milieu promoted glutamate clearance and LTP and translated into behavioral changes.

CONCLUSION

Dietary fat intake is not invariably harmful and might exert beneficial effects depending on the biological context.

Caloric restriction modifies spatiotemporal calcium dynamics in mouse hippocampal astrocytes

We analysed spatiotemporal properties of Ca²⁺ signals in protoplasmic astrocytes in the CA1 stratum radiatum of hippocampal slices from young (2-3 months old) mice housed in control conditions or exposed to a caloric restriction (CR) diet for one month. The astrocytic Ca²⁺ events became shorter in duration and smaller in size; they also demonstrated reduced velocity of expansion and shrinkage following CR. At the same time, Ca²⁺ signals in the astrocytes from the CR animals demonstrated higher amplitude and the faster rise and decay rates. These changes can be attributed to CR-induced morphological remodelling and uncoupling of astrocytes described in our previous study. CR-induced changes in the parameters of Ca²⁺ activity were partially reversed by inhibition of gap junctions/hemichannels with carbenoxolone (CBX). The effect of CBX on Ca²⁺ activity in CR-animals was unexpected because the diet already decreases gap junctional coupling in astrocytic syncytia. It may reflect the blockade of hemichannels also sensitive to this drug. Thus, CR-induced morphological remodelling of astrocytes is at least partly responsible for changes in the pattern of Ca²⁺ activity in the astrocytic network. How such changes in spatiotemporal Ca²⁺ landscape can translate into astrocytic physiology and neuron-glia interactions remains a matter for future studies.

Astrocyte dystrophy in ageing brain parallels impaired synaptic plasticity

Little is known about age-dependent changes in structure and function of astrocytes and of the impact of these on the cognitive decline in the senescent brain. The prevalent view on the age-dependent increase in reactive astrogliosis and astrocytic hypertrophy requires scrutiny and detailed analysis. Using two-photon microscopy in conjunction with 3D reconstruction, Sholl and volume fraction analysis, we demonstrate a significant reduction in the number and the length of astrocytic processes, in astrocytic territorial domains and in astrocyte-to-astrocyte coupling in the aged brain. Probing physiology of astrocytes with patch clamp, and Ca2+ imaging revealed deficits in K+ and glutamate clearance and spatiotemporal reorganisation of Ca2+ events in old astrocytes. These changes paralleled impaired synaptic long-term potentiation (LTP) in hippocampal CA1 in old mice. Our findings may explain the astroglial mechanisms of age-dependent decline in learning and memory.

Circadian Modulation of Neurons and Astrocytes Controls Synaptic Plasticity in Hippocampal Area CA1

Most animal species operate according to a 24-h period set by the suprachiasmatic nucleus (SCN) of the hypothalamus. The rhythmic activity of the SCN modulates hippocampal-dependent memory, but the molecular and cellular mechanisms that account for this effect remain largely unknown. Here, we identify cell-type-specific structural and functional changes that occur with circadian rhythmicity in neurons and astrocytes in hippocampal area CA1. Pyramidal neurons change the surface expression of NMDA receptors. Astrocytes change their proximity to synapses. Together, these phenomena alter glutamate clearance, receptor activation, and integration of temporally clustered excitatory synaptic inputs, ultimately shaping hippocampal-dependent learning in vivo. We identify corticosterone as a key contributor to changes in synaptic strength. These findings highlight important mechanisms through which neurons and astrocytes modify the molecular composition and structure of the synaptic environment, contribute to the local storage of information in the hippocampus, and alter the temporal dynamics of cognitive processing.

Astroglial asthenia and loss of function, rather than reactivity, contribute to the ageing of the brain

Astroglia represent a class of heterogeneous, in form and function, cells known as astrocytes, which provide for homoeostasis and defence of the central nervous system (CNS). Ageing is associated with morphological and functional remodelling of astrocytes with a prevalence of morphological atrophy and loss of function. In particular, ageing is associated with (i) decrease in astroglial synaptic coverage, (ii) deficits in glutamate and potassium clearance, (iii) reduced astroglial synthesis of synaptogenic factors such as cholesterol, (iv) decrease in aquaporin 4 channels in astroglial endfeet with subsequent decline in the glymphatic clearance, (v) decrease in astroglial metabolic support through the lactate shuttle, (vi) dwindling adult neurogenesis resulting from diminished proliferative capacity of radial stem astrocytes, (vii) decline in the astroglial-vascular coupling and deficient blood-brain barrier and (viii) decrease in astroglial ability to mount reactive astrogliosis. Decrease in reactive capabilities of astroglia are associated with rise of age-dependent neurodegenerative diseases. Astroglial morphology and function can be influenced and improved by lifestyle interventions such as intellectual engagement, social interactions, physical exercise, caloric restriction and healthy diet. These modifications of lifestyle are paramount for cognitive longevity.

Caloric restriction triggers morphofunctional remodeling of astrocytes and enhances synaptic plasticity in the mouse hippocampus

Calorie-restricted (CR) diet has multiple beneficial effects on brain function. Here we report morphological and functional changes in hippocampal astrocytes in 3-months-old mice subjected to 1 month of the diet. Whole-cell patch-clamp recordings were performed in the CA1 stratum (str.) radiatum astrocytes of hippocampal slices. The cells were also loaded with fluorescent dye through the patch pipette. CR did not affect the number of astrocytic branches but increased the volume fraction (VF) of distal perisynaptic astrocytic leaflets. The astrocyte growth did not lead to a decrease in the cell input resistance, which may be attributed to a decrease in astrocyte coupling through the gap junctions. Western blotting revealed a decrease in the expression of Cx43 but not Cx30. Immunocytochemical analysis demonstrated a decrease in the density and size of Cx43 clusters. Cx30 cluster density did not change, while their size increased in the vicinity of astrocytic soma. CR shortened K⁺ and glutamate transporter currents in astrocytes in response to 5 × 50 Hz Schaffer collateral stimulation. However, no change in the expression of astrocytic glutamate transporter 1 (GLT-1) was observed, while the level of glutamine synthetase (GS) decreased. These findings suggest that enhanced enwrapping of synapses by the astrocytic leaflets reduces glutamate and K⁺ spillover. Reduced spillover led to a decreased contribution of extrasynaptic N2B containing N-methyl-D-aspartate receptors (NMDARs) to the tail of burst-induced EPSCs. The magnitude of long-term potentiation (LTP) in the glutamatergic CA3–CA1 synapses was significantly enhanced after CR. This enhancement was abolished by N2B-NMDARs antagonist. Our findings suggest that astrocytic morphofunctional remodeling is responsible for enhanced synaptic plasticity, which provides a basis for improved learning and memory reported after CR.

Sensory Stimulation-Induced Astrocytic Calcium Signaling in Electrically Silent Ischemic Penumbra

Middle cerebral artery occlusion (MCAO) induces ischemia characterized by a densely ischemic focus, and a less densely ischemic penumbral zone in which neurons and astrocytes display age-dependent dynamic variations in spontaneous Ca²⁺ activities. However, it is unknown whether penumbral nerve cells respond to sensory stimulation early after stroke onset, which is critical for understanding stimulation-induced stroke therapy. In this study, we investigated the ischemic penumbra’s capacity to respond to somatosensory input. We examined adult (3- to 4-month-old) and old (18- to 24-month-old) male mice at 2–4 h after MCAO, using two-photon microscopy to record somatosensory stimulation-induced neuronal and astrocytic Ca²⁺ signals in the ischemic penumbra. In both adult and old mice, MCAO abolished spontaneous and stimulation-induced electrical activity in the penumbra, and strongly reduced stimulation-induced Ca²⁺ responses in neuronal somas (35–82%) and neuropil (92–100%) in the penumbra. In comparison, after stroke, stimulation-induced astrocytic Ca²⁺ responses in the penumbra were only moderately reduced (by 54–62%) in adult mice, and were even better preserved (reduced by 31–38%) in old mice. Our results suggest that somatosensory stimulation evokes astrocytic Ca²⁺ activity in the ischemic penumbra. We hypothesize that the relatively preserved excitability of astrocytes, most prominent in aged mice, may modulate protection from ischemic infarcts during early somatosensory activation of an ischemic cortical area. Future neuroprotective efforts in stroke may target spontaneous or stimulation-induced activity of astrocytes in the ischemic penumbra.

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.

Astrocytic Coverage of Dendritic Spines, Dendritic Shafts, and Axonal Boutons in Hippocampal Neuropil

Distal astrocytic processes have a complex morphology, reminiscent of branchlets and leaflets. Astrocytic branchlets are rod-like processes containing mitochondria and endoplasmic reticulum, capable of generating inositol-3-phosphate (IP₃)-dependent Ca²⁺ signals. Leaflets are small and flat processes that protrude from branchlets and fill the space between synapses. Here we use three-dimensional (3D) reconstructions from serial section electron microscopy (EM) of rat CA1 hippocampal neuropil to determine the astrocytic coverage of dendritic spines, shafts and axonal boutons. The distance to the maximum of the astrocyte volume fraction (VF) correlated with the size of the spine when calculated from the center of mass of the postsynaptic density (PSD) or from the edge of the PSD, but not from the spine surface. This suggests that the astrocytic coverage of small and larger spines is similar in hippocampal neuropil. Diffusion simulations showed that such synaptic microenvironment favors glutamate spillover and extrasynaptic receptor activation at smaller spines. We used complexity and entropy measures to characterize astrocytic branchlets and leaflets. The 2D projections of astrocytic branchlets had smaller spatial complexity and entropy than leaflets, consistent with the higher structural complexity and less organized distribution of leaflets. The VF of astrocytic leaflets was highest around dendritic spines, lower around axonal boutons and lowest around dendritic shafts. In contrast, the VF of astrocytic branchlets was similarly low around these three neuronal compartments. Taken together, these results suggest that astrocytic leaflets preferentially contact synapses as opposed to the dendritic shaft, an arrangement that might favor neurotransmitter spillover and extrasynaptic receptor activation along dendritic shafts.

Shearlet-based measures of entropy and complexity for two-dimensional patterns

New spatial entropy and complexity measures for two-dimensional patterns are proposed. The approach is based on the notion of disequilibrium and is built on statistics of directional multiscale coefficients of the fast finite shearlet transform. Shannon entropy and Jensen-Shannon divergence measures are employed. Both local and global spatial complexity and entropy estimates can be obtained, thus allowing for spatial mapping of complexity in inhomogeneous patterns. The algorithm is validated in numerical experiments with a gradually decaying periodic pattern and Ising surfaces near critical state. It is concluded that the proposed algorithm can be instrumental in describing a wide range of two-dimensional imaging data, textures, or surfaces, where an understanding of the level of order or randomness is desired.

Active role of capillary pericytes during stimulation-induced activity and spreading depolarization

Spreading depolarization is assumed to be the mechanism of migraine with aura, which is accompanied by an initial predominant hyperaemic response followed by persistent vasoconstriction. Cerebral blood flow responses are impaired in patients and in experimental animals after spreading depolarization. Understanding the regulation of cortical blood vessels during and after spreading depolarization could help patients with migraine attacks, but our knowledge of these vascular mechanisms is still incomplete. Recent findings show that control of cerebral blood flow does not only occur at the arteriole level but also at capillaries. Pericytes are vascular mural cells that can constrict or relax around capillaries, mediating local cerebral blood flow control. They participate in the constriction observed during brain ischaemia and might be involved the disruption of the microcirculation during spreading depolarization. To further understand the regulation of cerebral blood flow in spreading depolarization, we examined penetrating arterioles and capillaries with respect to vascular branching order, pericyte location and pericyte calcium responses during somatosensory stimulation and spreading depolarization. Mice expressing a red fluorescent indicator and intravenous injections of FITC-dextran were used to visualize pericytes and vessels, respectively, under two-photon microscopy. By engineering a genetically encoded calcium indicator we could record calcium changes in both pericytes around capillaries and vascular smooth muscle cells around arterioles. We show that somatosensory stimulation evoked a decrease in cytosolic calcium in pericytes located on dilating capillaries, up to the second order capillaries. Furthermore, we show that prolonged vasoconstriction following spreading depolarization is strongest in first order capillaries, with a persistent increase in pericyte calcium. We suggest that the persistence of the 'spreading cortical oligaemia' in migraine could be caused by this constriction of cortical capillaries. After spreading depolarization, somatosensory stimulation no longer evoked changes in capillary diameter and pericyte calcium. Thus, calcium changes in pericytes located on first order capillaries may be a key determinant in local blood flow control and a novel vascular mechanism in migraine. We suggest that prevention or treatment of capillary constriction in migraine with aura, which is an independent risk factor for stroke, may be clinically useful.

Astrocytic Atrophy Following Status Epilepticus Parallels Reduced Ca2+ Activity and Impaired Synaptic Plasticity in the Rat Hippocampus

Epilepsy is a group of neurological disorders commonly associated with the neuronal malfunction leading to generation of seizures. Recent reports point to a possible contribution of astrocytes into this pathology. We used the lithium-pilocarpine model of status epilepticus (SE) in rats to monitor changes in astrocytes. Experiments were performed in acute hippocampal slices 2-4 weeks after SE induction. Nissl staining revealed significant neurodegeneration in the pyramidal cell layers of hippocampal CA1, CA3 areas, and the hilus, but not in the granular cell layer of the dentate gyrus. A significant increase in the density of astrocytes stained with an astrocyte-specific marker, sulforhodamine 101, was observed in CA1 stratum (str.) radiatum. Astrocytes in this area were also whole-cell loaded with a morphological tracer, Alexa Fluor 594, for two-photon excitation imaging. Sholl analyses showed no changes in the size of the astrocytic domain or in the number of primary astrocytic branches, but a significant reduction in the number of distal branches that are resolved with diffraction-limited light microscopy (and are thought to contain Ca2+ stores, such as mitochondria and endoplasmic reticulum). The atrophy of astrocytic branches correlated with the reduced size, but not overall frequency of Ca2+ events. The volume tissue fraction of nanoscopic (beyond the diffraction limit) astrocytic leaflets showed no difference between control and SE animals. The results of spatial entropy-complexity spectrum analysis were also consistent with changes in ratio of astrocytic branches vs. leaflets. In addition, we observed uncoupling of astrocytes through the gap-junctions, which was suggested as a mechanism for reduced K+ buffering. However, no significant difference in time-course of synaptically induced K+ currents in patch-clamped astrocytes argued against possible alterations in K+ clearance by astrocytes. The magnitude of long-term-potentiation (LTP) was reduced after SE. Exogenous D-serine, a co-agonist of NMDA receptors, has rescued the initial phase of LTP. This suggests that the reduced Ca2+-dependent release of D-serine by astrocytes impairs initiation of synaptic plasticity. However, it does not explain the failure of LTP maintenance which may be responsible for cognitive decline associated with epilepsy.

Multiscale vision model for event detection and reconstruction in two-photon imaging data

Reliable detection of calcium waves in multiphoton imaging data is challenging because of the low signal-to-noise ratio and because of the unpredictability of the time and location of these spontaneous events. This paper describes our approach to calcium wave detection and reconstruction based on a modified multiscale vision model, an object detection framework based on the thresholding of wavelet coefficients and hierarchical trees of significant coefficients followed by nonlinear iterative partial object reconstruction, for the analysis of two-photon calcium imaging data. The framework is discussed in the context of detection and reconstruction of intercellular glial calcium waves. We extend the framework by a different decomposition algorithm and iterative reconstruction of the detected objects. Comparison with several popular state-of-the-art image denoising methods shows that performance of the multiscale vision model is similar in the denoising, but provides a better segmenation of the image into meaningful objects, whereas other methods need to be combined with dedicated thresholding and segmentation utilities.

GABAA Receptor-Mediated Bidirectional Control of Synaptic Activity, Intracellular Ca2+, Cerebral Blood Flow, and Oxygen Consumption in Mouse Somatosensory Cortex In Vivo

Neural activity regulates local increases in cerebral blood flow (ΔCBF) and the cortical metabolic rate of oxygen (ΔCMRO2) that constitutes the basis of BOLD functional neuroimaging signals. Glutamate signaling plays a key role in brain vascular and metabolic control; however, the modulatory effect of GABA is incompletely understood. Here we performed in vivo studies in mice to investigate how THIP (which tonically activates extrasynaptic GABAARs) and Zolpidem (a positive allosteric modulator of synaptic GABAARs) impact stimulation-induced ΔCBF, ΔCMRO2, local field potentials (LFPs), and fluorescent cytosolic Ca(2+) transients in neurons and astrocytes. Low concentrations of THIP increased ΔCBF and ΔCMRO2 at low stimulation frequencies. These responses were coupled to increased synaptic activity as indicated by LFP responses, and to Ca(2+) activities in neurons and astrocytes. Intermediate and high concentrations of THIP suppressed ΔCBF and ΔCMRO2 at high stimulation frequencies. Zolpidem had similar but less-pronounced effects, with similar dependence on drug concentration and stimulation frequency. Our present findings suggest that slight increases in both synaptic and extrasynaptic GABAAR activity might selectively gate and amplify transient low-frequency somatosensory inputs, filter out high-frequency inputs, and enhance vascular and metabolic responses that are likely to be reflected in BOLD functional neuroimaging signals.

Spontaneous calcium waves in Bergman glia increase with age and hypoxia and may reduce tissue oxygen

Glial calcium (Ca(2+)) waves constitute a means to spread signals between glial cells and to neighboring neurons and blood vessels. These waves occur spontaneously in Bergmann glia (BG) of the mouse cerebellar cortex in vivo. Here, we tested three hypotheses: (1) aging and reduced blood oxygen saturation alters wave activity; (2) glial Ca(2+) waves change cerebral oxygen metabolism; and (3) neuronal and glial wave activity is correlated. We used two-photon microscopy in the cerebellar cortexes of adult (8- to 15-week-old) and aging (48- to 80-week-old) ketamine-anesthetized mice after bolus loading with OGB-1/AM and SR101. We report that the occurrence of spontaneous waves is 20 times more frequent in the cerebellar cortex of aging as compared with adult mice, which correlated with a reduction in resting brain oxygen tension. In adult mice, spontaneous glial wave activity increased on reducing resting brain oxygen tension, and ATP-evoked glial waves reduced the tissue O(2) tension. Finally, although spontaneous Purkinje cell (PC) activity was not associated with increased glia wave activity, spontaneous glial waves did affect intracellular Ca(2+) activity in PCs. The increased wave activity during aging, as well as low resting brain oxygen tension, suggests a relationship between glial waves, brain energy homeostasis, and pathology.

Multiscale vision model highlights spontaneous glial calcium waves recorded by 2-photon imaging in brain tissue

Intercellular glial calcium waves (GCW) constitute a signaling pathway which can be visualized by fluorescence imaging of cytosolic Ca(2+) changes. Reliable detection of calcium waves in multiphoton imaging data is challenging because of low signal-to-noise ratio. We modified the multiscale vision model (MVM), originally employed to detect faint objects in astronomy data to process stacks of fluorescent images. We demonstrate that the MVM identified and characterized GCWs with much higher sensitivity and detail than pixel thresholding. Origins of GCWs were often associated with prolonged secondary Ca(2+) elevations. The GCWs had variable shapes, and secondary GCWs were observed to bud from the primary, larger GCW. GCWs evaded areas shortly before occupied by a preceding GCW instead circulating around the refractory area. Blood vessels uniquely reshaped GCWs and were associated with secondary GCW events. We conclude that the MVM provides unique possibilities to study spatiotemporally correlated Ca(2+) signaling in brain tissue.

Multinephron dynamics on the renal vascular network

Tubuloglomerular feedback (TGF) and the myogenic mechanism combine in each nephron to regulate blood flow and glomerular filtration rate. Both mechanisms are nonlinear, generate self-sustained oscillations, and interact as their signals converge on arteriolar smooth muscle, forming a regulatory ensemble. Ensembles may synchronize. Smooth muscle cells in the ensemble depolarize periodically, generating electrical signals that propagate along the vascular network. We developed a mathematical model of a nephron-vascular network, with 16 versions of a single nephron model containing representations of both mechanisms in the regulatory ensemble, to examine the effects of network structure on nephron synchronization. Symmetry, as a property of a network, facilitates synchronization. Nephrons received blood from a symmetric electrically conductive vascular tree. Symmetry was created by using identical nephron models at each of the 16 sites and symmetry breaking by varying nephron length. The symmetric model achieved synchronization of all elements in the network. As little as 1% variation in nephron length caused extensive desynchronization, although synchronization was maintained in small nephron clusters. In-phase synchronization predominated among nephrons separated by one or three vascular nodes and antiphase synchronization for five or seven nodes of separation. Nephron dynamics were irregular and contained low-frequency fluctuations. Results are consistent with simultaneous blood flow measurements in multiple nephrons. An interaction between electrical signals propagated through the network to cause synchronization; variation in vascular pressure at vessel bifurcations was a principal cause of desynchronization. The results suggest that the vasculature supplies blood to nephrons but also engages in robust information transfer.