Selective stimulation of astrocyte calcium in situ does not affect neuronal excitatory synaptic activity

Heterogeneity of astrocytic form and function

Astrocytes participate in all essential CNS functions, including blood flow regulation, energy metabolism, ion and water homeostasis, immune defence, neurotransmission, and adult neurogenesis. It is thus not surprising that astrocytic morphology and function differ between regions, and that different subclasses of astrocytes exist within the same brain region. Recent lines of work also show that the complexity of protoplasmic astrocytes increases during evolution. Human astrocytes are structurally more complex, larger, and propagate calcium signals significantly faster than rodent astrocytes. In this chapter, we review the diversity of astrocytic form and function, while considering the markedly expanded roles of astrocytes with phylogenetic evolution. We also define major challenges for the future, which include determining how astrocytic functions are locally specified, defining the molecular controls upon astrocytic fate and physiology and establishing how evolutionary changes in astrocytes contribute to higher cognitive functions.

Calcium signaling in cerebral vasoregulation

The tight coupling of regional neurometabolic activity with synaptic activity and regional cerebral blood perfusion constitutes a single functional unit, described generally as a neurovascular unit. This is central to any discussion of haemodynamic response linked to any neuronal activation. In normal as well as in pathologic conditions, neurons, astrocytes and endothelial cells of the vasculature interact to generate the complex activity-induced cerebral haemodynamic responses, with astrocytes not only partaking in the signaling but actually controlling it in many cases. Neurons and astrocytes have highly integrated signaling mechanisms, yet they form two separate networks. Bidirectional neuron-astrocyte interactions are crucial for the function and survival of the central nervous system. The primary purpose of such regulation is the homeostasis of the brain's microenvironment. In the maintenance of such homeostasis, astrocytic calcium response is a crucial variable in determining neurovascular control. Future work will be directed towards resolving the nature and extent of astrocytic calcium-mediated mechanisms for gene transcription, in modelling neurovascular control, and in determining calcium sensitive imaging assays that can capture disease variables.

Bidirectional coupling between astrocytes and neurons mediates learning and dynamic coordination in the brain: a multiple modeling approach

In recent years research suggests that astrocyte networks, in addition to nutrient and waste processing functions, regulate both structural and synaptic plasticity. To understand the biological mechanisms that underpin such plasticity requires the development of cell level models that capture the mutual interaction between astrocytes and neurons. This paper presents a detailed model of bidirectional signaling between astrocytes and neurons (the astrocyte-neuron model or AN model) which yields new insights into the computational role of astrocyte-neuronal coupling. From a set of modeling studies we demonstrate two significant findings. Firstly, that spatial signaling via astrocytes can relay a "learning signal" to remote synaptic sites. Results show that slow inward currents cause synchronized postsynaptic activity in remote neurons and subsequently allow Spike-Timing-Dependent Plasticity based learning to occur at the associated synapses. Secondly, that bidirectional communication between neurons and astrocytes underpins dynamic coordination between neuron clusters. Although our composite AN model is presently applied to simplified neural structures and limited to coordination between localized neurons, the principle (which embodies structural, functional and dynamic complexity), and the modeling strategy may be extended to coordination among remote neuron clusters.

Activation of astroglial calcium signaling by endogenous metabolites succinate and gamma-hydroxybutyrate in the nucleus accumbens

Accumulating evidence suggests that different energy metabolites play a role not only in neuronal but also in glial signaling. Recently, astroglial Ca²⁺ transients evoked by the major citric acid cycle metabolite succinate (SUC) and gamma-hydroxybutyrate (GHB) that enters the citric acid cycle via SUC have been described in the brain reward area, the nucleus accumbens (NAc). Cells responding to SUC by Ca²⁺ transient constitute a subset of ATP-responsive astrocytes that are activated in a neuron-independent way. In this study we show that GHB-evoked Ca²⁺ transients were also found to constitute a subset of ATP-responsive astrocytes in the NAc. Repetitive Ca²⁺ dynamics evoked by GHB suggested that Ca²⁺ was released from internal stores. Similarly to SUC, the GHB response was also characterized by an effective concentration of 50 μM. We observed that the number of ATP-responsive cells decreased with increasing concentration of either SUC or GHB. Moreover, the concentration dependence of the number of ATP-responsive cells were highly identical as a function of both [SUC] and [GHB], suggesting a mutual receptor for SUC and GHB, therefore implying the existence of a distinct GHB-recognizing astroglial SUC receptor in the brain. The SUC-evoked Ca²⁺ signal remained in mice lacking GABA_B receptor type 1 subunit in the presence and absence of the N-Methyl-d-Aspartate (NMDA) receptor antagonist (2R)-amino-5-phosphonovaleric acid (APV), indicating action mechanisms independent of the GABA_B or NMDA receptor subtypes. By molecular docking calculations we found that residues R99, H103, R252, and R281 of the binding crevice of the kidney SUC-responsive membrane receptor SUCNR1 (GPCR91) also predict interaction with GHB, further implying similar GHB and SUC action mechanisms. We conclude that the astroglial action of SUC and GHB may represent a link between brain energy states and Ca²⁺ signaling in astrocytic networks.

A tale of two stories: astrocyte regulation of synaptic depression and facilitation

Short-term presynaptic plasticity designates variations of the amplitude of synaptic information transfer whereby the amount of neurotransmitter released upon presynaptic stimulation changes over seconds as a function of the neuronal firing activity. While a consensus has emerged that the resulting decrease (depression) and/or increase (facilitation) of the synapse strength are crucial to neuronal computations, their modes of expression in vivo remain unclear. Recent experimental studies have reported that glial cells, particularly astrocytes in the hippocampus, are able to modulate short-term plasticity but the mechanism of such a modulation is poorly understood. Here, we investigate the characteristics of short-term plasticity modulation by astrocytes using a biophysically realistic computational model. Mean-field analysis of the model, supported by intensive numerical simulations, unravels that astrocytes may mediate counterintuitive effects. Depending on the expressed presynaptic signaling pathways, astrocytes may globally inhibit or potentiate the synapse: the amount of released neurotransmitter in the presence of the astrocyte is transiently smaller or larger than in its absence. But this global effect usually coexists with the opposite local effect on paired pulses: with release-decreasing astrocytes most paired pulses become facilitated, namely the amount of neurotransmitter released upon spike i+1 is larger than that at spike i, while paired-pulse depression becomes prominent under release-increasing astrocytes. Moreover, we show that the frequency of astrocytic intracellular Ca(2+) oscillations controls the effects of the astrocyte on short-term synaptic plasticity. Our model explains several experimental observations yet unsolved, and uncovers astrocytic gliotransmission as a possible transient switch between short-term paired-pulse depression and facilitation. This possibility has deep implications on the processing of neuronal spikes and resulting information transfer at synapses.

Gap junction-mediated calcium waves define communication networks among murine postnatal neural progenitor cells

In the postnatal neurogenic niche, two populations of astrocyte-like cells (B cells) persist, one acting as neural progenitor cells (NPCs, B1 cells) and one forming a structural boundary between the neurogenic niche and the striatum (B2 cells, niche astrocytes). Despite being viewed as two distinct entities, we found that B1 and B2 cells express the gap junction protein connexin 43 and display functional coupling involving 50-60 cells. Using neonatal electroporation to label slowly cycling radial glia-derived B1 cells, which send a basal process onto blood vessels, we further confirmed dye coupling between NPCs. To assess the functionality of the coupling, we used calcium imaging in a preparation preserving the three-dimensional architecture of the subventricular zone. Intercellular calcium waves were observed among B cells. These waves travelled bidirectionally between B1 and B2 cells and propagated on blood vessels. Inter-B-cell calcium waves were absent in the presence of a gap junction blocker but persisted with purinergic receptor blockers. These findings show that privileged microdomains of communication networks exist among NPCs and niche astrocytes. Such functional coupling between these two cell types suggests that niche astrocytes do not merely have a structural role, but may play an active role in shaping the behavior of NPCs.

Role of astrocytes in neurovascular coupling

Neural activity is intimately tied to blood flow in the brain. This coupling is specific enough in space and time that modern imaging methods use local hemodynamics as a measure of brain activity. In this review, we discuss recent evidence indicating that neuronal activity is coupled to local blood flow changes through an intermediary, the astrocyte. We highlight unresolved issues regarding the role of astrocytes and propose ways to address them using novel techniques. Our focus is on cellular level analysis in vivo, but we also relate mechanistic insights gained from ex vivo experiments to native tissue. We also review some strategies to harness advances in optical and genetic methods to study neurovascular coupling in the intact brain.

Calcium signals in the nucleus accumbens: activation of astrocytes by ATP and succinate

Background

Accumulating evidence suggests that glial signalling is activated by different brain functions. However, knowledge regarding molecular mechanisms of activation or their relation to neuronal activity is limited. The purpose of the present study is to identify the characteristics of ATP-evoked glial signalling in the brain reward area, the nucleus accumbens (NAc), and thereby to explore the action of citric acid cycle intermediate succinate (SUC).

Results

We described the burst-like propagation of Ca²⁺ transients evoked by ATP in acute NAc slices from rat brain. Co-localization of the ATP-evoked Ca²⁺ signalling with immunoreactivities of the astroglia-specific gap junction forming channel protein connexin43 (Cx43) and the glial fibrillary acidic protein (GFAP) indicated that the responsive cells were a subpopulation of Cx43 and GFAP immunoreactive astrocytes. The ATP-evoked Ca²⁺ transients were present under the blockade of neuronal activity, but were inhibited by Ca²⁺ store depletion and antagonism of the G protein coupled purinergic P2Y₁ receptor subtype-specific antagonist MRS2179. Similarly, Ca²⁺ transients evoked by the P2Y₁ receptor subtype-specific agonist 2-(Methylthio)adenosine 5'-diphosphate were also blocked by MRS2179. These characteristics implied that intercellular Ca²⁺ signalling originated from the release of Ca²⁺ from internal stores, triggered by the activation of P2Y₁ receptors. Inhibition by the gap junction blockers carbenoxolone and flufenamic acid and by an antibody raised against the gating-associated segment of Cx43 suggested that intercellular Ca²⁺ signalling proceeded through gap junctions. We demonstrated for the first time that extracellular SUC also evoked Ca²⁺ transients (EC₅₀ = 50-60 μM) in about 15% of the ATP-responsive NAc astrocytes. By contrast to glial cells, electrophysiologically identified NAc neurons surrounded by ATP-responsive astrocytes were not activated simultaneously.

Conclusions

We concluded, therefore, that ATP- and SUC-sensitive Ca²⁺ transients appear to represent a signalling layer independent of NAc neurons. This previously unrecognised glial action of SUC, a major cellular energy metabolite, may play a role in linking metabolism to Ca²⁺ signalling in astrocytic networks under physiological and pathological conditions such as exercise and metabolic diseases.

Astrocytes as regulators of synaptic function: a quest for the Ca2+ master key

The emerging role of astrocytes in neural communication represents a conceptual challenge. In striking contrast to the rapid and highly space- and time-constrained machinery of neuronal spike propagation and synaptic release, astroglia appear slow and imprecise. Although a large body of independent experiments documents active signal exchange between astrocytes and neurons, some genetic models have raised doubts about the major Ca2+ -dependent molecular mechanism routinely associated with release of "gliotransmitters." A limited understanding of astrocytic Ca2+ signaling and the imperfect compatibility between physiology and experimental manipulations seem to have contributed to this conceptual bottleneck. Experimental approaches providing mechanistic insights into the diverse mechanisms of intra-astrocyte Ca2+ signaling on the nanoscale are needed to understand Ca2+ -dependent astrocytic function in vivo. This review highlights limitations and potential advantages of such approaches from the current methodological perspective.

Immunogold detection of L-glutamate and D-serine in small synaptic-like microvesicles in adult hippocampal astrocytes

Glutamate and the N-methyl-D-aspartate receptor ligand D-serine are putative gliotransmitters. Here, we show by immunogold cytochemistry of the adult hippocampus that glutamate and D-serine accumulate in synaptic-like microvesicles (SLMVs) in the perisynaptic processes of astrocytes. The estimated concentration of fixed glutamate in the astrocytic SLMVs is comparable to that in synaptic vesicles of excitatory nerve terminals (≈ 45 and ≈ 55 mM, respectively), whereas the D-serine level is about 6 mM. The vesicles are organized in small spaced clusters located near the astrocytic plasma membrane. Endoplasmic reticulum is regularly found in close vicinity to SLMVs, suggesting that astrocytes contain functional nanodomains, where a local Ca(2+) increase can trigger release of glutamate and/or D-serine.

Local Ca2+ detection and modulation of synaptic release by astrocytes

Astrocytes communicate with synapses by means of intracellular calcium ([Ca(2+)](i)) elevations, but local calcium dynamics in astrocytic processes have never been thoroughly investigated. By taking advantage of high-resolution two-photon microscopy, we identify the characteristics of local astrocyte calcium activity in the adult mouse hippocampus. Astrocytic processes showed intense activity, triggered by physiological transmission at neighboring synapses. They encoded synchronous synaptic events generated by sparse action potentials into robust regional (∼12 μm) [Ca(2+)](i) elevations. Unexpectedly, they also sensed spontaneous synaptic events, producing highly confined (∼4 μm), fast (millisecond-scale) miniature Ca(2+) responses. This Ca(2+) activity in astrocytic processes is generated through GTP- and inositol-1,4,5-trisphosphate-dependent signaling and is relevant for basal synaptic function. Thus, buffering astrocyte [Ca(2+)](i) or blocking a receptor mediating local astrocyte Ca(2+) signals decreased synaptic transmission reliability in minimal stimulation experiments. These data provide direct evidence that astrocytes are integrated in local synaptic functioning in adult brain.

Astrocytes display complex and localized calcium responses to single-neuron stimulation in the hippocampus

Astrocytes show a complex structural and physiological interplay with neurons and respond to neuronal activation in vitro and in vivo with intracellular calcium elevations. These calcium changes enable astrocytes to modulate synaptic transmission and plasticity through various mechanisms. However, the response pattern of astrocytes to single neuronal depolarization events still remains unresolved. This information is critical for fully understanding the coordinated network of neuron-glial signaling in the brain. To address this, we developed a system to map astrocyte calcium responses along apical dendrites of CA1 pyramidal neurons in hippocampal slices using single-neuron stimulation with channelrhodopsin-2. This technique allowed selective neuronal depolarization without invasive manipulations known to alter calcium levels in astrocytes. Light-evoked neuronal depolarization was elicited and calcium events in surrounding astrocytes were monitored using the calcium-sensitive dye Calcium Orange. Stimulation of single neurons caused calcium responses in populations of astrocytes along the apical axis of CA1 cell dendrites. Calcium responses included single events that were synchronized with neuronal stimulation and poststimulus changes in calcium event frequency, both of which were modulated by glutamatergic and purinergic signaling. Individual astrocytes near CA1 cells showed low ability to respond to repeated neuronal depolarization events. However, the response of the surrounding astrocyte population was remarkably accurate. Interestingly, the reliability of responses was graded with respect to astrocyte location along the CA1 cell dendrite, with astrocytes residing in the primary dendrite subregion being most responsive. This study provides a new perspective on the dynamic response property of astrocyte ensembles to neuronal activity.

Implications of gliotransmission for the pharmacotherapy of CNS disorders

The seminal discovery that glial cells, particularly astrocytes, can release a number of gliotransmitters that serve as signalling molecules for the cross-talk with neighbouring cellular populations has recently changed our perception of brain functioning, as well as our view of the pathogenesis of several disorders of the CNS. Since glutamate was one of the first gliotransmitters to be identified and characterized, we tackle the mechanisms that underlie its release from astrocytes, including the Ca2+ signals underlying its efflux from astroglia, and we discuss the involvement of these events in a number of relevant physiological processes, from the modulatory control of neighbouring synapses to the regulation of blood supply to cerebral tissues. The relevance of these mechanisms strongly indicates that the contribution of glial cells and gliotransmission to the activities of the brain cannot be overlooked, and any study of CNS physiopathology needs to consider glial biology to have a comprehensive overview of brain function and dysfunction. Abnormalites in the signalling that controls the astrocytic release of glutamate are described in several experimental models of neurological disorders, for example, AIDS dementia complex, Alzheimer's disease and cerebral ischaemia. While the modalities of glutamate release from astrocytes remain poorly understood, and this represents a major impediment to the definition of novel therapeutic strategies targeting this process at the molecular level, some key mediators deputed to the control of the glial release of this excitatory amino acid have been identified. Among these, we can mention, for instance, proinflammatory cytokines, such as tumour necrosis factor-α, and prostaglandins. Agents that are able to block the major steps of tumour necrosis factor-α and prostaglandin production and/or signalling can be proposed as novel therapeutic targets for the treatment of these disorders.

Indirect modulation of neuronal excitability and synaptic transmission in the hippocampus by activation of proteinase-activated receptor-2

BACKGROUND AND PURPOSE

Proteinase-activated receptor-2 (PAR2) is widely expressed in the CNS under normal physiological conditions. However, its potential role in modulating neuronal excitability and synaptic transmission remains to be determined. Here, we have investigated whether PAR2 activation modulates synaptic activity in the hippocampus.

EXPERIMENTAL APPROACH

PAR2 activation and its effect on the hippocampus were examined in rat primary cultures and acute slices using whole cell patch clamp and standard extracellular recordings, respectively.

KEY RESULTS

PAR2 activation leads to a depolarization of hippocampal neurones and a paradoxical reduction in the occurrence of synaptically driven spontaneous action potentials (APs). PAR2-induced neuronal depolarization was abolished following either the inhibition of astrocytic function or antagonism of ionotropic glutamate receptors whilst the PAR2-induced decrease in AP frequency was also reduced when astrocytic function was inhibited. Furthermore, when examined in acute hippocampal slices, PAR2 activation induced a profound long-term depression of synaptic transmission that was dependent on NMDA receptor activation and was sensitive to disruption of astrocytic function.

CONCLUSIONS AND IMPLICATIONS

These novel findings show that PAR2 activation indirectly inhibits hippocampal synaptic activity and indicate that these receptors may play an active role in modulating normal physiological CNS function, in addition to their role in pathophysiological disorders.

Bulk loading of calcium indicator dyes to study astrocyte physiology: key limitations and improvements using morphological maps

Calcium signaling has been studied in astrocyte cell bodies using bulk loading of calcium indicator dyes, and astrocytes are known to display intracellular calcium transients. An assumption in recent data on the neuronal impact of somatic astrocyte calcium transients has been that bulk loading reflects signaling in relevant astrocyte compartments such as processes. We assessed bulk loading using Sholl analysis (Sholl, 1953) of astrocytes loaded with common calcium indicator dyes and compared these data with Sholl analysis of astrocyte morphology. In the CA1 region of the hippocampus from rats, we found that bulk loading of calcium indicator dyes only reports on calcium signals within the soma and in the most proximal processes, leaving ∼90% of the area of an astrocyte and its extensive processes unsampled. By using morphological reconstructions as "maps" after the imaging session, we present simple procedures that remedy these shortfalls and permit reliable detection of calcium transients in distal astrocyte processes. The data thus reveal limitations in the interpretation of astrocyte calcium imaging data gathered with bulk loading and provide refinements to minimize these shortcomings.

Sustained neuronal activity generated by glial plasticity

Astrocytes release gliotransmitters, notably glutamate, that can affect neuronal and synaptic activity. In particular, astrocytic glutamate release results in the generation of NMDA receptor (NMDA-R)-mediated slow inward currents (SICs) in neurons. However, factors underlying the emergence of SICs and their physiological roles are essentially unknown. Here we show that, in acute slices of rat somatosensory thalamus, stimulation of lemniscal or cortical afferents results in a sustained increase of SICs in thalamocortical (TC) neurons that outlasts the duration of the stimulus by 1 h. This long-term enhancement of astrocytic glutamate release is induced by group I metabotropic glutamate receptors and is dependent on astrocytic intracellular calcium. Neuronal SICs are mediated by extrasynaptic NR2B subunit-containing NMDA-Rs and are capable of eliciting bursts. These are distinct from T-type Ca(2+) channel-dependent bursts of action potentials and are synchronized in neighboring TC neurons. These findings describe a previously unrecognized form of excitatory, nonsynaptic plasticity in the CNS that feeds forward to generate local neuronal firing long after stimulus termination.

TNFα controls glutamatergic gliotransmission in the hippocampal dentate gyrus

VIDEO ABSTRACT

Glutamatergic gliotransmission provides a stimulatory input to excitatory synapses in the hippocampal dentate gyrus. Here, we show that tumor necrosis factor-alpha (TNFα) critically controls this process. With constitutive TNFα present, activation of astrocyte P2Y1 receptors induces localized [Ca(2+)](i) elevations followed by glutamate release and presynaptic NMDA receptor-dependent synaptic potentiation. In preparations lacking TNFα, astrocytes respond with identical [Ca(2+)](i) elevations but fail to induce neuromodulation. We find that TNFα specifically controls the glutamate release step of gliotransmission. In cultured astrocytes lacking TNFα glutamate exocytosis is dramatically slowed down due to altered vesicle docking. Addition of low picomolar TNFα promptly reconstitutes both normal exocytosis in culture and gliotransmission in situ. Alternatively, gliotransmission can be re-established without adding TNFα, by limiting glutamate uptake, which compensates slower release. These findings demonstrate that gliotransmission and its synaptic effects are controlled not only by astrocyte [Ca(2+)](i) elevations but also by permissive/homeostatic factors like TNFα.

Ionotropic receptors in neuronal-astroglial signalling: what is the role of "excitable" molecules in non-excitable cells

Astroglial cells were long considered to serve merely as the structural and metabolic supporting cast and scenery against which the shining neurones perform their illustrious duties. Relatively recent evidence, however, indicates that astrocytes are intimately involved in many of the brain's functions. Astrocytes possess a diverse assortment of ionotropic transmitter receptors, which enable these glial cells to respond to many of the same signals that act on neurones. Ionotropic receptors mediate neurone-driven signals to astroglial cells in various brain areas including neocortex, hippocampus and cerebellum. Activation of ionotropic receptors trigger rapid signalling events in astroglia; these events, represented by local Ca(2+) or Na(+) signals provide the mechanism for fast neuronal-glial signalling at the synaptic level. Since astrocytes can detect chemical transmitters that are released from neurones and can release their own extracellular signals, gliotransmitters, they are intricately involved in homocellular and heterocellular signalling mechanisms in the nervous system. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

ATP: a ubiquitous gliotransmitter integrating neuron-glial networks

Astrocytes are ideally situated to integrate glial and neuronal functions and neurovascular coupling by way of their multiple contacts with neurons, glia and blood vessels. There is a high degree of specialisation of astroglial membranes at the different sites of contact, including the expression of neurotransmitter receptors, ion channels, transporters and gap junctional proteins. An apparently universal property of astrocytes throughout the CNS is their responsiveness to ATP acting via metabotropic P2Y receptors, with a prominent role for the P2Y1 receptor subtype. Activation of astroglial P2Y receptors triggers a rise in intracellular calcium, which is the substrate for astroglial excitability and intercellular communication. In addition, astrocytes have a number of mechanisms for the release of ATP, which can be considered a 'gliotransmitter'. Astrocytes may be the most widespread source of ATP release in the CNS, and astroglial ATP and its metabolite adenosine activate purine receptors on neurons, microglia, oligodendrocytes and blood vessels. There is compelling evidence that astroglial ATP and adenosine regulate neuronal synaptic strength, although the physiological significance of this astrocyte-to-neuron signalling is questioned. A less appreciated aspect of astrocyte signalling is that they also release neurotransmitters onto other glia. Notably, both ATP and adenosine control microglial behaviour and regulate oligodendrocyte differentiation and myelination. P2 receptors also mediate injury responses in all glial cell types, with a prominent role for the P2X7 receptor subtype. In addition, ATP is a potent vasoconstrictor and astrocytes provide a route for coupling blood flow to neuronal activity by way of their synaptic and perivascular connections. Thus, astrocytes are the fulcrum of neuron-glial-vascular networks and purinergic signalling is the primary mechanism by which astrocytes can integrate the functions of these diverse elements.

Large-scale calcium waves traveling through astrocytic networks in vivo

Macroscopic changes in cerebral blood flow, such as those captured by functional imaging of the brain, require highly organized, large-scale dynamics of astrocytes, glial cells that interact with both neuronal and cerebrovascular networks. However, astrocyte activity has been studied mainly at the level of individual cells, and information regarding their collective behavior is lacking. In this work, we monitored calcium activity simultaneously from hundreds of mouse hippocampal astrocytes in vivo and found that almost all astrocytes participated en masse in regenerative waves that propagated from cell to cell (referred to here as "glissandi"). Glissandi emerged depending on the neuronal activity and accompanied a reduction in infraslow fluctuations of local field potentials and a decrease in the flow of red blood cells. This novel phenomenon was heretofore overlooked, probably because of the high vulnerability of astrocytes to light damage; glissandi occurred only when observed at much lower laser intensities than previously used.

Intercellular calcium waves in glial cells with bistable dynamics

A two-dimensional model is proposed for intercellular calcium (Ca(2 +)) waves with Ca(2 +)-induced IP(3) regeneration and the diffusion of IP(3) through gap junctions. Many experimental observations in glial cells, i.e. responding to local mechanical stimulation, glutamate application, mechanical stimulation followed by ACh application, and glutamate followed by mechanical stimulation, are reproduced and classified by the model. We show that a glial cell model with bistable dynamics, i.e. a Ca(2 +) oscillation state coexisting with a fixed point, can cause a prolonged plateau of Ca(2 +) signals in the cells nearby the stimulated cell when the cell network responds to the local mechanical stimulation.

Astrocytes control GABAergic inhibition of neurons in the mouse barrel cortex

Astrocytes in the barrel cortex respond with a transient Ca2+ increase to neuronal stimulation and this response is restricted to the stimulated barrel field. In the present study we suppressed the astrocyte response by dialysing these cells with the Ca2+ chelator BAPTA. Electrical stimulation triggered a depolarization in stellate or pyramidal ‘regular spiking' neurons from cortex layer 4 and 2/3 and this response was augmented in amplitude and duration after astrocytes were dialysed with BAPTA. Combined blockade of GABAA and GABAB receptors mimicked the effect of BAPTA dialysis, while glutamate receptor blockers had no effect. Moreover, the frequency of spontaneous postsynaptic currents was increased after BAPTA dialysis. Outside the range of BAPTA dialysis astrocytes responded with a Ca2+ increase, but in contrast to control, the response was no longer restricted to one barrel field. Our findings indicate that astrocytes control neuronal inhibition in the barrel cortex.

Treatment with ammonia induces apoptosis and necrosis of rat astrocytes in vitro

AIM: To investigate whether treatment with ammonia induces apoptosis and necrosis of rat astrocytes in vitro and to analyze the pathogenesis of hepatic encephalopathy.

METHODS: Primary astrocytes isolated from the cerebral cortex of newborn rats were cultured in vitro and identified by immunocytochemistry (positive for glial fibrillary acidic protein). Cells were then randomly assigned to low-concentration ammonia group (treated with 2.5 mmol/L NH₄Cl), high-concentration ammonia group (treated with 5 mmol/L NH₄Cl), and control group (untreated). After culture for 24, 48, and 72 h, cell apoptosis and necrosis were detected by flow cytometry.

RESULTS: Treatment with ammonia at a concentration of 2.5 mmol/L could induce astrocyte apoptosis and necrosis, and apoptosis- and necrosis-inducing activity of ammonia was concentration- and time-dependent. Compared with the control group, the apoptosis and necrosis rates of astrocytes in the high-concentration ammonia group were significantly higher (24 h: 12.5% ± 4.0% vs 7.7% ± 1.9%, 9.3% ± 1.6% vs 6.3% ± 0.7%; 48 h: 17.7% ± 4.2% vs 8.5% ± 1.3%, 10.5% ± 2.8% vs 7.2% ± 1.1%; 72 h: 23.9% ± 4.1% vs 9.6% ± 1.9%, 11.4% ± 2.5% vs 7.9% ± 1.5%, all P < 0.05).

CONCLUSION: Treatment with ammonia could induce astrocyte apoptosis and necrosis in vitro in a time- and concentration-dependent manner.

Opposing actions of endothelin-1 on glutamatergic transmission onto vasopressin and oxytocin neurons in the supraoptic nucleus

Endothelin (ET-1) given centrally has many reported actions on hormonal and autonomic outputs from the CNS. However, it is unclear whether these effects are due to local ischemia via its vasoconstrictor properties or to a direct neuromodulatory action. ET-1 stimulates the release of oxytocin (OT) and vasopressin (VP) from supraoptic magnocellular (MNCs) neurons in vivo; therefore, we asked whether ET-1 modulates the excitatory inputs onto MNCs that are critical in sculpting the activity of these neurons. To investigate whether ET-1 modulates excitatory synaptic transmission, we obtained whole-cell recordings and analyzed quantal glutamate release onto MNCs in the supraoptic nucleus (SON). Neurons identified as VP-containing neurosecretory cells displayed a decrease in quantal frequency in response to ET-1 (10-100 pm). This decrease was mediated by ET(A) receptor activation and production of a retrograde messenger that targets presynaptic cannabinoid-1 receptors. In contrast, neurons identified as OT-containing MNCs displayed a transient increase in quantal glutamate release in response to ET-1 application via ET(B) receptor activation. Application of TTX to block action potential-dependent glutamate release inhibited the excitatory action of ET-1 in OT neurons. There were no changes in quantal amplitude in either MNC type, suggesting that the effects of ET-1 were via presynaptic mechanisms. A gliotransmitter does not appear to be involved as ET-1 failed to elevate astrocytic calcium in the SON. Our results demonstrate that ET-1 differentially modulates glutamate release onto VP- versus OT-containing MNCs, thus implicating it in the selective regulation of neuroendocrine output from the SON.

Specific disruption of astrocytic Ca2+ signaling pathway in vivo by adeno-associated viral transduction

Astrocytes are the predominant glial-cell type in the CNS and they are known to play an active role in modulating neuronal function. Since many of the same molecules including G-protein coupled receptors (GPCRs) are expressed in both neurons and astrocytes, in vivo pharmacological manipulations to target astrocytes lack specificity. In this study, we investigated the effect of Pleckstrin Homology (PH) domain of Phospholipase C (PLC)-like protein p130 (p130PH) on Ca(2+) signaling in astrocytes in vivo. We used the serotype 2/5 recombinant adeno-associated virus (rAAV2/5) vectors to introduce p130PH fused with a tagged protein monomer red fluorescent protein at the N-terminal (i.e., transgene mRFP-p130PH). In order to selectively disrupt the Ca(2+) signaling pathway in astrocytes, the transgene was driven by a novel astrocyte-specific promoter gfaABC(1)D. Our results show that mRFP-p130PH is exclusively expressed in astrocytes with a high efficiency and a stable expression level. In vivo imaging using two-photon microscopy demonstrated reduced Ca(2+) signal in transduced astrocytes in response to ATP stimulation. As Ca(2+) signaling is a characteristic form of cellular excitability in astrocytes that can mediate chemical transmitter release and contribute to neuronal excitotoxicity, the current study provides an in vivo approach to better understand Ca(2+)-dependent gliotransmission and its involvement in glia-related diseases.

Glial cells in neuronal network function

Numerous evidence demonstrates that astrocytes, a type of glial cell, are integral functional elements of the synapses, responding to neuronal activity and regulating synaptic transmission and plasticity. Consequently, they are actively involved in the processing, transfer and storage of information by the nervous system, which challenges the accepted paradigm that brain function results exclusively from neuronal network activity, and suggests that nervous system function actually arises from the activity of neuron-glia networks. Most of our knowledge of the properties and physiological consequences of the bidirectional communication between astrocytes and neurons resides at cellular and molecular levels. In contrast, much less is known at higher level of complexity, i.e. networks of cells, and the actual impact of astrocytes in the neuronal network function remains largely unexplored. In the present article, we summarize the current evidence that supports the notion that astrocytes are integral components of nervous system networks and we discuss some functional properties of intercellular signalling in neuron-glia networks.

Astrocytes and epilepsy

Astrocytes form a significant constituent of seizure foci in the human brain. For a long time it was believed that astrocytes play a significant role in the causation of seizures. With the increase in our understanding of the unique biology of these cells, their precise role in seizure foci is receiving renewed attention. This article reviews the information now available on the role of astrocytes in the hippocampal seizure focus in patients with temporal lobe epilepsy with hippocampal sclerosis. Our intent is to try to integrate the available data. Astrocytes at seizure foci seem to not be a homogeneous population of cells, and in addition to typical glial fibrillary acidic protein, positive reactive astrocytes also include a population of neuron glia-2-like cells The astrocytes in sclerotic hippocampi differ from those in nonsclerotic hippocampi in their membrane physiology, having elevated Na+ channels and reduced inwardly rectifying potassium ion channels, and some having the capacity to generate action potentials. They also have reduced glutamine synthetase and increased glutamate dehydrogenase activity. The molecular interface between the astrocyte and microvasculature is also changed. The astrocytes are also associated with increased expression of many molecules normally concerned with immune and inflammatory functions. A speculative mechanism postulates that neuron glia-2-like cells may be involved in creating a high glutamate environment, whereas the function of more typical reactive astrocytes contribute to maintain high extracellular K+ levels; both factors contributing to the hyperexcitability of subicular neurons to generate epileptiform activity. The functions of the astrocyte vascular interface may be more critical to the processes involved in epileptogenesis.

Ionotropic NMDA and P2X1/5 receptors mediate synaptically induced Ca2+ signalling in cortical astrocytes

Local, global and propagating calcium (Ca(2+)) signals provide the substrate for glial excitability. Here we analyse Ca(2+) permeability of NMDA and P2X(1/5) receptors expressed in cortical astrocytes and provide evidence that activation of these receptors trigger astroglial Ca(2+) signals when stimulated by either endogenous agonists or by synaptic release of neurotransmitters. The Ca(2+) permeability of the ionotropic receptors was determined by reversal potential shift analysis; the permeability ratio P(Ca)/P(K) was 3.1 for NMDA receptors and 2.2 for P2X(1/5) receptors. Selective stimulation of ionotropic receptors (with NMDA and α,β-methyleneATP) in freshly isolated cortical astrocytes induced ion currents associated with transient increases in cytosolic Ca(2+) concentration ([Ca(2+)](i)). Stimulation of neuronal afferents in cortical slices triggered glial synaptic currents and [Ca(2+)](i) responses, which were partially blocked by selective antagonists of NMDA (D-AP5 and UBP141) and P2X(1/5) (NF449) receptors. We conclude that ionotropic receptors contribute to astroglial Ca(2+) signalling and may provide a specific mechanism for fast neuronal-glial signalling at the synaptic level.

Monitoring astrocyte calcium microdomains with improved membrane targeted GCaMP reporters

Astrocytes are involved in synaptic and cerebrovascular regulation in the brain. These functions are regulated by intracellular calcium signalling that is thought to reflect a form of astrocyte excitability. In a recent study, we reported modification of the genetically encoded calcium indicator (GECI) GCaMP2 with a membrane-tethering domain, Lck, to generate Lck-GCaMP2. This GECI allowed us to detect novel microdomain calcium signals. The microdomains were random and 'spotty' in nature. In order to detect such signals more reliably, in the present study we further modified Lck-GCaMP2 to carry three mutations in the GCaMP2 moiety (M153K, T203V within EGFP and N60D in the CaM domain) to generate Lck-GCaMP3. We directly compared Lck-GCaMP2 and Lck-GCaMP3 by assessing their ability to monitor several types of astrocyte calcium signals with a focus on spotty microdomains. Our data show that Lck-GCaMP3 is between two- and four-times better than Lck-GCaMP2 in terms of its basal fluorescence intensity, signal-to-noise and its ability to detect microdomains. The use of Lck-GCaMP3 thus represents a significantly improved way to monitor astrocyte calcium signals, including microdomains, and will facilitate detailed exploration of their molecular mechanisms and physiological roles.

Synaptic plasticity and Ca2+ signalling in astrocytes

There is a growing body of evidence suggesting a functional relationship between Ca2+ signals generated in astroglia and the functioning of nearby excitatory synapses. Interference with endogenous Ca2+ homeostasis inside individual astrocytes has been shown to affect synaptic transmission and its use-dependent changes. However, establishing the causal link between source-specific, physiologically relevant intracellular Ca2+ signals, the astrocytic release machinery and the consequent effects on synaptic transmission has proved difficult. Improved methods of Ca2+ monitoring in situ will be essential for resolving the ambiguity in understanding the underlying Ca2+ signalling cascades.

Astrocyte heterogeneity: an underappreciated topic in neurobiology

Astrocytes, one of the most numerous types of cells in the central nervous system, are crucial for potassium homeostasis, neurotransmitter uptake, synapse formation, regulation of blood-brain-barrier, and the development of the nervous system. Historically, astrocytes have been studied as a homogeneous group of cells. However, evidence has accumulated that suggests heterogeneity of astrocytes across brain regions as well as within the same brain regions. Astrocytes differ in their morphology, developmental origin, gene expression profile, physiological properties, function, and response to injury and disease. A better understanding of the heterogeneity of astrocytes will greatly aid investigation of the function of astrocytes in normal brain as well as the roles of astrocytes in neurological disorders.

Imaging and recording subventricular zone progenitor cells in live tissue of postnatal mice

The subventricular zone (SVZ) is one of two regions where neurogenesis persists in the postnatal brain. The SVZ, located along the lateral ventricle, is the largest neurogenic zone in the brain that contains multiple cell populations including astrocyte-like cells and neuroblasts. Neuroblasts migrate in chains to the olfactory bulb where they differentiate into interneurons. Here, we discuss the experimental approaches to record the electrophysiology of these cells and image their migration and calcium activity in acute slices. Although these techniques were in place for studying glial cells and neurons in mature networks, the SVZ raises new challenges due to the unique properties of SVZ cells, the cellular diversity, and the architecture of the region. We emphasize different methods, such as the use of transgenic mice and in vivo electroporation that permit identification of the different SVZ cell populations for patch clamp recording or imaging. Electroporation also permits genetic labeling of cells using fluorescent reporter mice and modification of the system using either RNA interference technology or floxed mice. In this review, we aim to provide conceptual and technical details of the approaches to perform electrophysiological and imaging studies of SVZ cells.

Functions of mature mammalian astrocytes: a current view

Before the roles of normal, mature astrocytes in the mammalian CNS can be discussed, we first need to define these cells. A definition proposed here is that such a class is best defined as consisting of the protoplasmic and fibrous astrocytes of the gray and white matter, respectively, the Bergmann glia of the molecular layer of the cerebellum, and the Muller cells of the retina. It is concluded that the established properties and functions of these mature astrocytes are essential support for neuronal activity, in the sense of Claude Bernard's principle of maintaining "la fixité du milieu intérieur." This milieu would be the extracellular space common to astrocytes and neurons. More specialized roles, such as the recently described "light guides" for retinal Muller cells can also be viewed as support and facilitation. The ECS is also, of course, common to all other neural cells, but here, I limit the discussion to perturbations of the ECS caused only by neuronal activities and the resolution of these perturbations by astrocytes, such as control of increases in extracellular K(+), uptake of excitatory amino acids, and alterations in blood vessel diameter and therefore blood flow. It is also proposed how this fits into the current morphological picture for the protoplasmic astrocytes as having small cell bodies with up to 100,000 process endings that occupy separate territories on which the processes of neighboring astrocytes scarcely intrude.

A genetically targeted optical sensor to monitor calcium signals in astrocyte processes

Calcium signaling is studied as a potential form of astrocyte excitability that may control astrocyte involvement in synaptic and cerebrovascular regulation. Fundamental questions remain unanswered about astrocyte calcium signaling, as current methods can not resolve calcium in small volume compartments, such as near the cell membrane and in distal cell processes. We modified the genetically encoded calcium sensor GCaMP2 with a membrane-tethering domain, Lck, increasing the level of Lck-GCaMP2 near the plasma membrane tenfold as compared with conventional GCaMP2. Using Lck-GCaMP2 in rat hippocampal astrocyte-neuron cocultures, we measured near-membrane calcium signals that were evoked pharmacologically or by single action potential-mediated neurotransmitter release. Moreover, we identified highly localized and frequent spontaneous calcium signals in astrocyte somata and processes that conventional GCaMP2 failed to detect. Lck-GCaMP2 acts as a genetically targeted calcium sensor for monitoring calcium signals in previously inaccessible parts of astrocytes, including fine processes.

Neuron-astrocyte communication and synaptic plasticity

By forming close contacts with synapses, astrocytes secrete neuroactive substances and remove neurotransmitters, thus influencing the processing of information by the nervous system. Here, we review recent work on astrocytes and their roles in regulating neuronal function and synaptic plasticity. Astrocytes are organized as networks and communicate with each other, thereby affecting larger neural circuits. They also provide a link between neurons and the vasculature, potentially changing the cerebral microcirculation. Recent work has provided insights into the relative contributions of specific astrocytic cues and transporters to synaptic transmission, plasticity, and animal behavior.

Neurotransmitter signaling in postnatal neurogenesis: The first leg

Like the liver or other peripheral organs, two regions of the adult brain possess the ability of self-renewal through a process called neurogenesis. This raises tremendous hope for repairing the damaged brain, and it has stimulated research on identifying signals controlling neurogenesis. Neurogenesis involves several stages from fate determination to synaptic integration via proliferation, migration, and maturation. While fate determination primarily depends on a genetic signature, other stages are controlled by the interplay between genes and microenvironmental signals. Here, we propose that neurotransmitters are master regulators of the different stages of neurogenesis. In favor of this idea, a description of selective neurotransmitter signaling and their functions in the largest neurogenic zone, the subventricular zone (SVZ), is provided. In particular, we emphasize the interactions between neuroblasts and astrocyte-like cells that release gamma-aminobutyric acid (GABA) and glutamate, respectively. However, we also raise several limitations to our knowledge on neurotransmitters in neurogenesis. The function of neurotransmitters in vivo remains largely unexplored. Neurotransmitter signaling has been viewed as uniform, which dramatically contrasts with the cellular and molecular mosaic nature of the SVZ. How neurotransmitters are integrated with other well-conserved molecules, such as sonic hedgehog, is poorly understood. In an effort to reconcile these differences, we discuss how specificity of neurotransmitter functions can be provided through their multitude of receptors and intracellular pathways in different cell types and their possible interactions with sonic hedgehog.

GABA(A) Increases Calcium in Subventricular Zone Astrocyte-Like Cells Through L- and T-Type Voltage-Gated Calcium Channels

In the adult neurogenic subventricular zone (SVZ), the behavior of astrocyte-like cells and some of their functions depend on changes in intracellular Ca²⁺ levels and tonic GABA_A receptor activation. However, it is unknown whether, and if so how, GABA_A receptor activity regulates intracellular Ca²⁺ dynamics in SVZ astrocytes. To monitor Ca²⁺ activity selectively in astrocyte-like cells, we used two lines of transgenic mice expressing either GFP fused to a Gq-coupled receptor or DsRed under the human glial fibrillary acidic protein (hGFAP) promoter. GABA_A receptor activation induced Ca²⁺ increases in 40–50% of SVZ astrocytes. GABA_A-induced Ca²⁺ increases were prevented with nifedipine and mibefradil, blockers of L- and T-type voltage-gated calcium channels (VGCC). The L-type Ca²⁺ channel activator BayK 8644 increased the percentage of GABA_A-responding astrocyte-like cells to 75%, suggesting that the majority of SVZ astrocytes express functional VGCCs. SVZ astrocytes also displayed spontaneous Ca²⁺ activity, the frequency of which was regulated by tonic GABA_A receptor activation. These data support a role for ambient GABA in tonically regulating intracellular Ca²⁺ dynamics through GABA_A receptors and VGCC in a subpopulation of astrocyte-like cells in the postnatal SVZ.

Do astrocytes really exocytose neurotransmitters?

In the past 20 years, an extra layer of information processing, in addition to that provided by neurons, has been proposed for the CNS. Neuronally evoked increases of the intracellular calcium concentration in astrocytes have been suggested to trigger exocytotic release of the 'gliotransmitters' glutamate, ATP and D-serine. These are proposed to modulate neuronal excitability and transmitter release, and to have a role in diseases as diverse as stroke, epilepsy, schizophrenia, Alzheimer's disease and HIV infection. However, there is intense controversy about whether astrocytes can exocytose transmitters in vivo. Resolving this issue would considerably advance our understanding of brain function.

Glia as transmitter sources and sensors in health and disease

Glial cells express a bewildering array of neurotransmitter receptors. To illustrate the complexity of expression, we have assayed non-glutamatergic neurotransmitter receptor mRNA in isolated rat optic nerve, a preparation devoid of neurons and neuronal synapses and from which relatively pure "glial" RNA can be isolated. Of the 44 receptor subunits examined which span the GABA-A, nicotinic, adreno- and glycine receptor families, over three quarters were robustly expressed in this mixed population of white matter glial cells, with several expressed at higher levels than found in control whole brain RNA. In addition to the complexity of glial receptor expression, numerous neurotransmitter release mechanisms have been identified. We have focused on glutamate release from astrocytes, which can occur via at least seven distinct pathways and which is implicated in excitotoxic injury and are neurons and glia. Recent findings suggest that non-glutamatergic receptors can also mediate acute glial injury are also discussed.

Astrocytic activation and an inhibition of MAP kinases are required for proteinase-activated receptor-2-mediated protection from neurotoxicity

Proteinase-activated receptor-2 (PAR-2) expression levels are altered in several CNS disorders with these changes being proposed to either exacerbate or diminish the disease state depending on the cell type in which this occurs. Here we present data investigating the consequence of PAR-2 activation on kainate (KA)-induced neurotoxicity in organotypic hippocampal slices cultures (OHSC). Exposure of OHSC to the PAR-2 activators trypsin or Ser-Leu-Ile-Gly-Arg-Leu (SLIGRL) induced no neurotoxicity when applied alone but was neuroprotective against KA-induced neurotoxicity. SLIGRL-mediated neuroprotection involved astrocytic activation as the neuroprotective effect was abolished following OHSC pre-treatment with fluoroacetate. Moreover, co-application of either reparixin or LY341495, antagonists of the CXCR2 chemokine receptor and metabotropic glutamate receptors respectively, inhibited the SLIGRL-mediated neuroprotection. SLIGRL application inhibited both p38 MAPK and ERK activity in OHSC, but not the JNK 1/2 signalling pathway. Accordingly, the co-application of the p38 MAPK and ERK inhibitors SB203580 and UO126 reduced KA-induced cell death, mimicking PAR-2-mediated neuroprotection. These data indicate that PAR-2 activation is neuroprotective and involves astrocytic activation, gliotransmitter release, and the subsequent inhibition of MAPK signalling cascades, providing further evidence for PAR-2 as an interesting therapeutic target in certain CNS disorders.

Dynamical Mean Field Model of a Neural-Glial Mass

Our goal is to model the behavior of an ensemble of interacting neurons and astrocytes (the neural-glial mass). For this, a model describing N tripartite synapses is proposed. Each tripartite synapse consists of presynaptic and postsynaptic nerve terminals, as well as the synaptically associated astrocytic microdomain, and is described by a system of 13 stochastic differential equations. Then, by applying the dynamical mean field approximation (DMA) (Hasegawa, 2003a , 2003b ) the system of 13N equations is reduced to 13(13 + 2) = 195 deterministic differential equations for the means and the second-order moments of local and global variables. Simulations are carried out for studying the response of the neural-glial mass to external inputs applied to either the presynaptic terminals or the astrocytes. Three cases were considered: the astrocytes influence only the presynaptic terminal, only the postsynaptic terminal, or both the presynaptic and postsynaptic terminals. As a result, a wide range of responses varying from singles spikes to train of spikes was evoked on presynaptic and postsynaptic terminals. The experimentally observed phenomenon of spontaneous activity in astrocytes was replicated on the neural-glial mass. The model predicts that astrocytes can have a strong and activity-dependent influence on synaptic transmission. Finally, simulations show that the dynamics of astrocytes influences the synchronization ratio between neurons, predicting a peak in the synchronization for specific values of the astrocytes’ parameters.

NMDA receptors activated by subventricular zone astrocytic glutamate are critical for neuroblast survival prior to entering a synaptic network

Even before integrating into existing circuitry, adult-born neurons express receptors for neurotransmitters, but the intercellular mechanisms and their impact on neurogenesis remain largely unexplored. Here, we show that neuroblasts born in the postnatal subventricular zone (SVZ) acquire NMDA receptors (NMDARs) during their migration to the olfactory bulb. Along their route, neuroblasts are ensheathed by astrocyte-like cells expressing vesicular glutamate release machinery. Increasing calcium in these specialized astrocytes induced NMDAR activity in neuroblasts, and blocking astrocytic vesicular release eliminated spontaneous NMDAR activity. Single-cell knockout of NMDARs using neonatal electroporation resulted in neuroblast apoptosis at the time of NMDAR acquisition. This cumulated in a 40% loss of neuroblasts along their migratory route, demonstrating that NMDAR acquisition is critical for neuroblast survival prior to entering a synaptic network. In addition, our findings suggest an unexpected mechanism wherein SVZ astrocytes use glutamate signaling through NMDARs to control the number of adult-born neurons reaching their final destination.

Hippocampal short- and long-term plasticity are not modulated by astrocyte Ca2+ signaling

The concept that astrocytes release neuroactive molecules (gliotransmitters) to affect synaptic transmission has been a paradigm shift in neuroscience research over the past decade. This concept suggests that astrocytes, together with pre- and postsynaptic neuronal elements, make up a functional synapse. Astrocyte release of gliotransmitters (for example, glutamate and adenosine triphosphate) is generally accepted to be a Ca2+-dependent process. We used two mouse lines to either selectively increase or obliterate astrocytic Gq G protein-coupled receptor Ca2+ signaling to further test the hypothesis that astrocytes release gliotransmitters in a Ca2+-dependent manner to affect synaptic transmission. Neither increasing nor obliterating astrocytic Ca2+ fluxes affects spontaneous and evoked excitatory synaptic transmission or synaptic plasticity. Our findings suggest that, at least in the hippocampus, the mechanisms of gliotransmission need to be reconsidered.