Astroglial Ca2 Research Articles

Cannabidiol Exerts a Neuroprotective and Glia-Balancing Effect in the Subacute Phase of Stroke.

Pharmacological agents limiting secondary tissue loss and improving functional outcomes after stroke are still limited. Cannabidiol (CBD), the major non-psychoactive component of Cannabis sativa, has been proposed as a neuroprotective agent against experimental cerebral ischemia. The effects of CBD mostly relate to the modulation of neuroinflammation, including glial activation. To investigate the effects of CBD on glial cells after focal ischemia in vivo, we performed time-lapse imaging of microglia and astroglial Ca2+ signaling in the somatosensory cortex in the subacute phase of stroke by in vivo two-photon laser-scanning microscopy using transgenic mice with microglial EGFP expression and astrocyte-specific expression of the genetically encoded Ca2+ sensor GCaMP3. CBD (10 mg/kg, intraperitoneally) prevented ischemia-induced neurological impairment, reducing the neurological deficit score from 2.0 ± 1.2 to 0.8 ± 0.8, and protected against neurodegeneration, as shown by the reduction (more than 70%) in Fluoro-Jade C staining (18.8 ± 7.5 to 5.3 ± 0.3). CBD reduced ischemia-induced microglial activation assessed by changes in soma area and total branch length, and exerted a balancing effect on astroglial Ca2+ signals. Our findings indicate that the neuroprotective effects of CBD may occur in the subacute phase of ischemia, and reinforce its strong anti-inflammatory property. Nevertheless, its mechanism of action on glial cells still requires further studies.

Astrocytes and Microglia Exhibit Cell-Specific Ca2+ Signaling Dynamics in the Murine Spinal Cord.

The spinal cord is the main pathway connecting brain and peripheral nervous system. Its functionality relies on the orchestrated activity of both neurons and glial cells. To date, most advancement in understanding the spinal cord inner mechanisms has been made either by in vivo exposure of its dorsal surface through laminectomy or by acute ex vivo slice preparation, likely affecting spinal cord physiology in virtue of the necessary extensive manipulation of the spinal cord tissue. This is especially true of cells immediately responding to alterations of the surrounding environment, such as microglia and astrocytes, reacting within seconds or minutes and for up to several days after the original insult. Ca2+ signaling is considered one of the most immediate, versatile, and yet elusive cellular responses of glia. Here, we induced the cell-specific expression of the genetically encoded Ca2+ indicator GCaMP3 to evaluate spontaneous intracellular Ca2+ signaling in astrocytes and microglia. Ca2+ signals were then characterized in acute ex vivo (both gray and white matter) as well as in chronic in vivo (white matter) preparations using MSparkles, a MATLAB-based software for automatic detection and analysis of fluorescence events. As a result, we were able to segregate distinct astroglial and microglial Ca2+ signaling patterns along with method-specific Ca2+ signaling alterations, which must be taken into consideration in the reliable evaluation of any result obtained in physiological as well as pathological conditions. Our study revealed a high degree of Ca2+ signaling diversity in glial cells of the murine spinal cord, thus adding to the current knowledge of the astonishing glial heterogeneity and cell-specific Ca2+ dynamics in non-neuronal networks.

Versatile Surface Electrodes for Combined Electrophysiology and Two-Photon Imaging of the Mouse Central Nervous System.

Understanding and modulating CNS function in physiological as well as pathophysiological contexts remains a significant ambition in research and clinical applications. The investigation of the multifaceted CNS cell types including their interactions and contributions to neural function requires a combination of the state-of-the-art in vivo electrophysiology and imaging techniques. We developed a novel type of liquid crystal polymer (LCP) surface micro-electrode manufactured in three customized designs with up to 16 channels for recording and stimulation of brain activity. All designs include spare central spaces for simultaneous 2P-imaging. Nanoporous platinum-plated contact sites ensure a low impedance and high current transfer. The epidural implantation of the LCP micro-electrodes could be combined with standard cranial window surgery. The epidurally positioned electrodes did not only display long-term biocompatibility, but we also observed an additional stabilization of the underlying CNS tissue. We demonstrate the electrode’s versatility in combination with in vivo 2P-imaging by monitoring anesthesia-awake cycles of transgenic mice with GCaMP3 expression in neurons or astrocytes. Cortical stimulation and simultaneous 2P Ca2+ imaging in neurons or astrocytes highlighted the astrocytes’ integrative character in neuronal activity processing. Furthermore, we confirmed that spontaneous astroglial Ca2+ signals are dampened under anesthesia, while evoked signals in neurons and astrocytes showed stronger dependency on stimulation intensity rather than on various levels of anesthesia. Finally, we show that the electrodes provide recordings of the electrocorticogram (ECoG) with a high signal-to noise ratio and spatial signal differences which help to decipher brain activity states during experimental procedures. Summarizing, the novel LCP surface micro-electrode is a versatile, convenient, and reliable tool to investigate brain function in vivo.

From Physiology to Pathology of Cortico-Thalamo-Cortical Oscillations: Astroglia as a Target for Further Research.

The electrographic hallmark of childhood absence epilepsy (CAE) and other idiopathic forms of epilepsy are 2.5–4 Hz spike and wave discharges (SWDs) originating from abnormal electrical oscillations of the cortico-thalamo-cortical network. SWDs are generally associated with sudden and brief non-convulsive epileptic events mostly generating impairment of consciousness and correlating with attention and learning as well as cognitive deficits. To date, SWDs are known to arise from locally restricted imbalances of excitation and inhibition in the deep layers of the primary somatosensory cortex. SWDs propagate to the mostly GABAergic nucleus reticularis thalami (NRT) and the somatosensory thalamic nuclei that project back to the cortex, leading to the typical generalized spike and wave oscillations. Given their shared anatomical basis, SWDs have been originally considered the pathological transition of 11–16 Hz bursts of neural oscillatory activity (the so-called sleep spindles) occurring during Non-Rapid Eye Movement (NREM) sleep, but more recent research revealed fundamental functional differences between sleep spindles and SWDs, suggesting the latter could be more closely related to the slow (<1 Hz) oscillations alternating active (Up) and silent (Down) cortical activity and concomitantly occurring during NREM. Indeed, several lines of evidence support the fact that SWDs impair sleep architecture as well as sleep/wake cycles and sleep pressure, which, in turn, affect seizure circadian frequency and distribution. Given the accumulating evidence on the role of astroglia in the field of epilepsy in the modulation of excitation and inhibition in the brain as well as on the development of aberrant synchronous network activity, we aim at pointing at putative contributions of astrocytes to the physiology of slow-wave sleep and to the pathology of SWDs. Particularly, we will address the astroglial functions known to be involved in the control of network excitability and synchronicity and so far mainly addressed in the context of convulsive seizures, namely (i) interstitial fluid homeostasis, (ii) K+ clearance and neurotransmitter uptake from the extracellular space and the synaptic cleft, (iii) gap junction mechanical and functional coupling as well as hemichannel function, (iv) gliotransmission, (v) astroglial Ca2+ signaling and downstream effectors, (vi) reactive astrogliosis and cytokine release.

Ca2+ as the prime trigger of aerobic glycolysis in astrocytes

Astroglial aerobic glycolysis, a process during which d-glucose is converted to l-lactate, a brain fuel and signal, is regulated by the plasmalemmal receptors, including adrenergic receptors (ARs) and purinergic receptors (PRs), modulating intracellular Ca2+ and cAMP signals. However, the extent to which the two signals regulate astroglial aerobic glycolysis is poorly understood. By using agonists to stimulate intracellular α1-/β-AR-mediated Ca2+/cAMP signals, β-AR-mediated cAMP and P2R-mediated Ca2+ signals and genetically encoded fluorescence resonance energy transfer-based glucose and lactate nanosensors in combination with real-time microscopy, we show that intracellular Ca2+, but not cAMP, initiates a robust increase in the concentration of intracellular free d-glucose ([glc]i) and l-lactate ([lac]i), both depending on extracellular d-glucose, suggesting Ca2+-triggered glucose uptake and aerobic glycolysis in astrocytes. When the glycogen shunt, a process of glycogen remodelling, was inhibited, the α1-/β-AR-mediated increases in [glc]i and [lac]i were reduced by ∼65 % and ∼30 %, respectively, indicating that at least ∼30 % of the utilization of d-glucose is linked to glycogen remodelling and aerobic glycolysis. Additional activation of β-AR/cAMP signals aided to α1-/β-AR-triggered [lac]i increase, whereas the [glc]i increase was unaltered. Taken together, an increase in intracellular Ca2+ is the prime mechanism of augmented aerobic glycolysis in astrocytes, while cAMP has only a moderate role. The results provide novel information on the signals regulating brain metabolism and open new avenues to explore whether astroglial Ca2+ signals are dysregulated and contribute to neuropathologies with impaired brain metabolism.

The Paradox of Astroglial Ca2 + Signals at the Interface of Excitation and Inhibition.

Astroglial networks constitute a non-neuronal communication system in the brain and are acknowledged modulators of synaptic plasticity. A sophisticated set of transmitter receptors in combination with distinct secretion mechanisms enables astrocytes to sense and modulate synaptic transmission. This integrative function evolved around intracellular Ca2+ signals, by and large considered as the main indicator of astrocyte activity. Regular brain physiology meticulously relies on the constant reciprocity of excitation and inhibition (E/I). Astrocytes are metabolically, physically, and functionally associated to the E/I convergence. Metabolically, astrocytes provide glutamine, the precursor of both major neurotransmitters governing E/I in the central nervous system (CNS): glutamate and γ-aminobutyric acid (GABA). Perisynaptic astroglial processes are structurally and functionally associated with the respective circuits throughout the CNS. Astonishingly, in astrocytes, glutamatergic as well as GABAergic inputs elicit similar rises in intracellular Ca2+ that in turn can trigger the release of glutamate and GABA as well. Paradoxically, as gliotransmitters, these two molecules can thus strengthen, weaken or even reverse the input signal. Therefore, the net impact on neuronal network function is often convoluted and cannot be simply predicted by the nature of the stimulus itself. In this review, we highlight the ambiguity of astrocytes on discriminating and affecting synaptic activity in physiological and pathological state. Indeed, aberrant astroglial Ca2+ signaling is a key aspect of pathological conditions exhibiting compromised network excitability, such as epilepsy. Here, we gather recent evidence on the complexity of astroglial Ca2+ signals in health and disease, challenging the traditional, neuro-centric concept of segregating E/I, in favor of a non-binary, mutually dependent perspective on glutamatergic and GABAergic transmission.

Astroglial Calcium Signaling Encodes Sleep Need in Drosophila

Sleep is under homeostatic control, whereby increasing wakefulness generates sleep need and triggers sleep drive. However, the molecular and cellular pathways by which sleep need is encoded are poorly understood. In addition, the mechanisms underlying both how and when sleep need is transformed to sleep drive are unknown. Here, using exvivo and invivo imaging, we show in Drosophila that astroglial Ca2+ signaling increases with sleep need. We demonstrate that this signaling is dependent on a specific L-type Ca2+ channel and is necessary for homeostatic sleep rebound. Thermogenetically increasing Ca2+ in astrocytes induces persistent sleep behavior, and we exploit this phenotype to conduct a genetic screen for genes required for the homeostatic regulation of sleep. From this large-scale screen, we identify TyrRII, a monoaminergic receptor required in astrocytes for sleep homeostasis. TyrRII levels rise following sleep deprivation in a Ca2+-dependent manner, promoting further increases in astrocytic Ca2+ and resulting in a positive-feedback loop. Moreover, our findings suggest that astrocytes then transmit this sleep need to a sleep drive circuit by upregulating and releasing the interleukin-1 analog Spätzle, which then acts on Toll receptors on R5 neurons. These findings define astroglial Ca2+ signaling mechanisms encoding sleep need and reveal dynamic properties of the sleep homeostatic control system.

P2Y1 receptor inhibition rescues impaired synaptic plasticity and astroglial Ca2+-dependent activity in the epileptic hippocampus

Epilepsy is characterized by a progressive predisposition to suffer seizures due to neuronal hyperexcitability, and one of its most common co-morbidities is cognitive decline. In animal models of chronic epilepsy, such as kindling, electrically induced seizures impair long-term potentiation (LTP), deteriorating learning and memory performance. Astrocytes are known to actively modulate synaptic plasticity and neuronal excitability through Ca2+-dependent gliotransmitter release. It is unclear, however, if astroglial Ca2+ signaling could contribute to the development of synaptic plasticity alterations in the epileptic hippocampus. By employing electrophysiological tools and Ca2+ imaging, we found that glutamatergic CA3-CA1 synapses from kindled rats exhibit an impairment in theta burst (TBS) and high frequency stimulation (HFS)-induced LTP, which is accompanied by an increased probability of neurotransmitter release (Pr) and an abnormal pattern of astroglial Ca2+-dependent transients. Both the impairment in LTP and the Pr were reversed by inhibiting purinergic P2Y1 receptors (P2Y1R) with the specific antagonist MRS2179, which also restored the spontaneous and TBS-induced pattern of astroglial Ca2+-dependent signals. Two consecutive, spaced TBS protocols also failed to induce LTP in the kindled group, however, this impairment was reversed and a strong LTP was induced when the second TBS was applied in the presence of MRS2179, suggesting that the mechanisms underlying the alterations in TBS-induced LTP are likely associated with an aberrant modulation of the induction threshold for LTP. Altogether, these results indicate that P2Y1R inhibition rescues both the pattern of astroglial Ca2+-activity and the plastic properties of CA3-CA1 synapses in the epileptic hippocampus, suggesting that astrocytes might take part in the mechanisms that deteriorate synaptic plasticity and thus cause cognitive decline in epileptic patients.

Local Resting Ca2+ Controls the Scale of Astroglial Ca2+ Signals

SummaryAstroglia regulate neurovascular coupling while engaging in signal exchange with neurons. The underlying cellular machinery is thought to rely on astrocytic Ca2+ signals, but what controls their amplitude and waveform is poorly understood. Here, we employ time-resolved two-photon excitation fluorescence imaging in acute hippocampal slices and in cortex in vivo to find that resting [Ca2+] predicts the scale (amplitude) and the maximum (peak) of astroglial Ca2+ elevations. We bidirectionally manipulate resting [Ca2+] by uncaging intracellular Ca2+ or Ca2+ buffers and use ratiometric imaging of a genetically encoded Ca2+ indicator to establish that alterations in resting [Ca2+] change co-directionally the peak level and anti-directionally the amplitude of local Ca2+ transients. This relationship holds for spontaneous and for induced (for instance by locomotion) Ca2+ signals. Our findings uncover a basic generic rule of Ca2+ signal formation in astrocytes, thus also associating the resting Ca2+ level with the physiological “excitability” state of astroglia.

Astroglial Isopotentiality and Calcium-Associated Biomagnetic Field Effects on Cortical Neuronal Coupling.

Synaptic neurotransmission is necessary but does not sufficiently explain superior cognitive faculties. Growing evidence has shown that neuron–astroglial chemical crosstalk plays a critical role in the processing of information, computation, and memory. In addition to chemical and electrical communication among neurons and between neurons and astrocytes, other nonsynaptic mechanisms called ephaptic interactions can contribute to the neuronal synchronization from different brain regions involved in the processing of information. New research on brain astrocytes has clearly shown that the membrane potential of these cells remains very stable among neighboring and distant astrocytes due to the marked bioelectric coupling between them through gap junctions. This finding raises the possibility that the neocortical astroglial network exerts a guiding template modulating the excitability and synchronization of trillions of neurons by astroglial Ca2+-associated bioelectromagnetic interactions. We propose that bioelectric and biomagnetic fields of the astroglial network equalize extracellular local field potentials (LFPs) and associated local magnetic field potentials (LMFPs) in the cortical layers of the brain areas involved in the processing of information, contributing to the adequate and coherent integration of external and internal signals. This article reviews the current knowledge of ephaptic interactions in the cerebral cortex and proposes that the isopotentiality of cortical astrocytes is a prerequisite for the maintenance of the bioelectromagnetic crosstalk between neurons and astrocytes in the neocortex.

Association of Cerebrospinal Fluid S100B Protein with Core Biomarkers and Cognitive Deficits in Prodromal and Mild Alzheimer's Disease.

Increased expression of the astroglial Ca2+-binding protein S100B has been observed in various neurodegenerative diseases and also seems to play a role in the unfolding of pathophysiological events at early stages of Alzheimer's disease (AD). To examine the association of cerebrospinal fluid (CSF) levels of S100B with 1) established CSF core biomarkers total tau (tau), hyperphosphorylated tau (p-tau), and amyloid β1-42 (Aβ1-42) as well as neuron-specific enolase (NSE) CSF levels and 2) cognition in early AD and mild cognitive impairment (MCI) due to AD (MCI-AD). Retrospective study assessing 49 pooled charts of Memory Clinic and inpatients diagnosed with AD (N = 26) and MCI-AD (N = 23) according to the National Institute of Aging and Alzheimer's Disease Association (NIA-AA) criteria. Neuropsychological testing was performed with the Consortium to Establish a Registry for AD (CERAD)-Plus battery. CSF levels of S100B correlated with NSE, but not the other CSF parameters. Stepwise multiple linear regression, adjusted for age, sex, and educational level, revealed that only increased CSF S100B was independently associated with lower CERAD-Plus total and Mini-Mental Status Examination scores together with poorer performance in wordlist learning (delayed recall and overall performance). We found no independent associations with other CSF biomarkers or cognitive domains. Our data suggest that CSF S100B may have a diagnostic value particularly at early stages of AD reflecting the significance of neuroinflammatory/astroglial processes. Thus, CSF S100B may complement the established array of available AD biomarkers to improve early stage diagnosis.

Astroglial Ca2+ signals trigger pathological behaviour in optogenetic mouse

Astroglial Ca2+ signals trigger pathological behaviour in optogenetic mouse

Astroglial Calcium Signaling in Aging and Alzheimer's Disease.

Astrocytes are the homeostatic and protective cells of the central nervous system (CNS). In neurological diseases, astrocytes undergo complex changes, which are subclassified into (1) reactive astrogliosis, an evolutionary conserved defensive rearrangement of cellular phenotype aimed at neuroprotection; (2) pathological remodeling, when astrocytes acquire new features driving pathology; and (3) astrodegeneration, which is manifested by astroglial atrophy and loss of homeostatic functions. In aging brains as well as in the brains affected by Alzheimer's disease (AD), astrocytes acquire both atrophic and reactive phenotypes in a region- and disease-stage-dependent manner. Prevalence of atrophy overreactivity, observed in certain brain regions and in terminal stages of the disease, arguably facilitates the development of neurological deficits. Astrocytes exhibit ionic excitability mediated by changes in intracellular concentration of ions, most importantly of Ca2+ and Na+, with intracellular ion dynamics triggered by the activity of neural networks. AD astrocytes associated with senile plaques demonstrate Ca2+ hyperactivity in the form of aberrant Ca2+ oscillations and pathological long-range Ca2+ waves. Astroglial Ca2+ signaling originating from Ca2+ release from the endoplasmic reticulum is a key factor in initiating astrogliotic response; deficient Ca2+ signaling toolkits observed in entorhinal and prefrontal cortices of AD model animals may account for vulnerability of these regions to the pathology.

Calcium Microdomain Formation at the Perisynaptic Cradle Due to NCX Reversal: A Computational Study.

It has recently been proposed using a multi-compartmental mathematical model that negatively fixed charged membrane-associated sites constrain the flow of cations in perisynaptic astroglial processes. This restricted movement of ions between the perisynaptic cradle (PsC), principal astroglial processes and the astrocyte soma gives rise to potassium (K+) and sodium (Na+) microdomains at the PsC. The present paper extends the above model to demonstrate that the formation of an Na+ microdomain can reverse the Na+/Ca2+ exchanger (NCX) thus providing an additional source of calcium (Ca2+) at the PsC. Results presented clearly show that reversal of the Na+/Ca2+ exchanger is instigated by a glutamate transporter coupled increase in concentration of cytoplasmic [Na+]i at the PsC, which and instigates Ca2+ influx through the NCX. As the flow of Ca2+ along the astrocyte process and away from the PsC is also constrained by Ca2+ binding proteins, then a Ca2+ microdomain forms at the PsC. The paper also serves to demonstrate that the EAAT, NKA, and NCX represent the minimal requirement necessary and sufficient for the development of a Ca2+ microdomain and that these mechanisms directly link neuronal activity and glutamate release to the formation of localized Na+ and Ca2+ microdomains signals at the PsC. This local source of Ca2+ can provide a previously underexplored form of astroglial Ca2+ signaling.

Gi/o protein-coupled receptors inhibit neurons but activate astrocytes and stimulate gliotransmission.

G protein-coupled receptors (GPCRs) play key roles in intercellular signaling in the brain. Their effects on cellular function have been largely studied in neurons, but their functional consequences on astrocytes are less known. Using both endogenous and chemogenetic approaches with DREADDs, we have investigated the effects of Gq and Gi/o GPCR activation on astroglial Ca2+ -based activity, gliotransmitter release, and the functional consequences on neuronal electrical activity. We found that while Gq GPCR activation led to cellular activation in both neurons and astrocytes, Gi/o GPCR activation led to cellular inhibition in neurons and cellular activation in astrocytes. Astroglial activation by either Gq or Gi/o protein-mediated signaling stimulated gliotransmitter release, which increased neuronal excitability. Additionally, activation of Gq and Gi/o DREADDs in vivo increased astrocyte Ca2+ activity and modified neuronal network electrical activity. Present results reveal additional complexity of the signaling consequences of excitatory and inhibitory neurotransmitters in astroglia-neuron network operation and brain function.

Astroglial Ca2+-Dependent Hyperexcitability Requires P2Y1 Purinergic Receptors and Pannexin-1 Channel Activation in a Chronic Model of Epilepsy.

Astrocytes from the hippocampus of chronic epileptic rats exhibit an abnormal pattern of intracellular calcium oscillations, characterized by an augmented frequency of long lasting spontaneous Ca2+ transients, which are sensitive to purinergic receptor antagonists but resistant to tetrodotoxin. The above suggests that alterations in astroglial Ca2+-dependent excitability observed in the epileptic tissue could arise from changes in astrocyte-to-astrocyte signaling, which is mainly mediated by purines in physiological and pathological conditions. In spite of that, how purinergic signaling contributes to astrocyte dysfunction in epilepsy remains unclear. Here, we assessed the possible contribution of P2Y1R as well as pannexin1 and connexin43 hemichannels—both candidates for non-vesicular ATP-release—by performing astroglial Ca2+ imaging and dye uptake experiments in hippocampal slices from control and fully kindled rats. P2Y1R blockade with MRS2179 decreased the mean duration of astroglial Ca2+ oscillations by reducing the frequency of slow Ca2+ transients, and thereby restoring the balance between slow (ST) and fast transients (FT) in the kindled group. The potential contribution of astroglial pannexin1 and connexin43 hemichannels as pathways for purine release (e.g., ATP) was assessed through dye uptake experiments. Astrocytes from kindled hippocampi exhibit three-fold more EtBr uptake than controls, whereby pannexin1 hemichannels (Panx1 HCs) accounts for almost all dye uptake with only a slight contribution from connexin43 hemichannels (Cx43 HCs). Confirming its functional involvement, Panx1 HCs inhibition decreased the mean duration of astroglial Ca2+ transients and the frequency of slow oscillations in kindled slices, but had no noticeable effects on the control group. As expected, Cx43 HCs blockade did not have any effects over the mean duration of astroglial Ca2+ oscillations. These findings suggest that P2Y1R and Panx1 HCs play a pivotal role in astroglial pathophysiology, which would explain the upregulation of glutamatergic neurotransmission in the epileptic brain and thus represents a new potential pharmacological target for the treatment of drug-refractory epilepsy.

Disentangling astroglial physiology with a realistic cell model in silico

Electrically non-excitable astroglia take up neurotransmitters, buffer extracellular K+ and generate Ca2+ signals that release molecular regulators of neural circuitry. The underlying machinery remains enigmatic, mainly because the sponge-like astrocyte morphology has been difficult to access experimentally or explore theoretically. Here, we systematically incorporate multi-scale, tri-dimensional astroglial architecture into a realistic multi-compartmental cell model, which we constrain by empirical tests and integrate into the NEURON computational biophysical environment. This approach is implemented as a flexible astrocyte-model builder ASTRO. As a proof-of-concept, we explore an in silico astrocyte to evaluate basic cell physiology features inaccessible experimentally. Our simulations suggest that currents generated by glutamate transporters or K+ channels have negligible distant effects on membrane voltage and that individual astrocytes can successfully handle extracellular K+ hotspots. We show how intracellular Ca2+ buffers affect Ca2+ waves and why the classical Ca2+ sparks-and-puffs mechanism is theoretically compatible with common readouts of astroglial Ca2+ imaging.

Diversity of Astroglial Effects on Aging- and Experience-Related Cortical Metaplasticity

Activity-dependent regulation of synaptic plasticity, or metaplasticity, plays a key role in the adaptation of neuronal networks to physiological and biochemical changes in aging brain. There is a growing evidence that experience-related alterations in the mechanisms of synaptic plasticity can underlie beneficial effects of physical exercise and caloric restriction (CR) on brain health and cognition. Astrocytes, which form neuro-vascular interface and can modulate synaptic plasticity by release of gliotransmitters, attract an increasing attention as important element of brain metaplasticity. We investigated the age- and experience-related alterations in astroglial calcium signaling and stimulus-dependence of long-term synaptic plasticity in the neocortex of mice exposed to the mild CR and environmental enrichment (EE) which included ad libitum physical exercise. We found out that astrocytic Ca2+-signaling underwent considerable age-related decline but EE and CR enhanced astroglial signaling, in particular mediated by noradrenaline (NA) and endocannabinoid receptors. The release of ATP and D-Serine from astrocytes followed the same trends of age-related declined and EE-induced increase. Our data also showed that astrocyte-derived ATP and D-Serine can have diverse effects on the threshold and magnitude of long-term changes in the strength of neocortical synapses; these effects were age-dependent. The CR- and EE-induced enhancement of astroglial Ca2+-signaling had more stronger effect on synaptic plasticity in the old (14–18 months) than in the young (2–5 months) wild-type (WT) mice. The effects of CR and EE on synaptic plasticity were significantly altered in both young and aged dnSNARE mice. Combined, our data suggest astrocyte-neuron interactions are important for dynamic regulation of cortical synaptic plasticity. This interaction can significantly decline with aging and thus contributes to the age-related cognitive impairment. On another hand, experience-related increase in the astroglial Ca2+-signaling can ameliorate the age-related decline.

Astroglial CB1 Receptors Determine Synaptic D-Serine Availability to Enable Recognition Memory

Bidirectional communication between neurons and astrocytes shapes synaptic plasticity and behavior. D-serine is a necessary co-agonist of synaptic N-methyl-D-aspartate receptors (NMDARs), but the physiological factors regulating its impact on memory processes are scantly known. We show that astroglial CB1 receptors are key determinants of object recognition memory by determining the availability of D-serine at hippocampal synapses. Mutant mice lacking CB1 receptors from astroglial cells (GFAP-CB1-KO) displayed impaired object recognition memory and decreased invivo andinvitro long-term potentiation (LTP) at CA3-CA1hippocampal synapses. Activation of CB1 receptors increased intracellular astroglial Ca2+ levels and extracellular levels of D-serine in hippocampal slices. Accordingly, GFAP-CB1-KO displayed lower occupancy of the co-agonist binding site of synaptic hippocampal NMDARs. Finally,elevation of D-serine levels fully rescued LTP and memory impairments of GFAP-CB1-KO mice. These data reveal a novel mechanism of invivo astroglial control of memory and synaptic plasticity via the D-serine-dependent control of NMDARs.

Astroglial Ca2+ signaling is generated by the coordination of IP3R and store-operated Ca2+ channels

Astrocytes play key roles in the central nervous system and regulate local blood flow and synaptic transmission via intracellular calcium (Ca2+) signaling. Astrocytic Ca2+ signals are generated by multiple pathways: Ca2+ release from the endoplasmic reticulum (ER) via the inositol 1, 4, 5-trisphosphate receptor (IP3R) and Ca2+ influx through various Ca2+ channels on the plasma membrane. However, the Ca2+ channels involved in astrocytic Ca2+ homeostasis or signaling have not been fully characterized. Here, we demonstrate that spontaneous astrocytic Ca2+ transients in cultured hippocampal astrocytes were induced by cooperation between the Ca2+ release from the ER and the Ca2+ influx through store-operated calcium channels (SOCCs) on the plasma membrane. Ca2+ imaging with plasma membrane targeted GCaMP6f revealed that spontaneous astroglial Ca2+ transients were impaired by pharmacological blockade of not only Ca2+ release through IP3Rs, but also Ca2+ influx through SOCCs. Loss of SOCC activity resulted in the depletion of ER Ca2+, suggesting that SOCCs are activated without store depletion in hippocampal astrocytes. Our findings indicate that sustained SOCC activity, together with that of the sarco-endoplasmic reticulum Ca2+-ATPase, contribute to the maintenance of astrocytic Ca2+ store levels, ultimately enabling astrocytic Ca2+ signaling.