Intracellular astrocyte calcium waves in situ increase the frequency of spontaneous AMPA receptor currents in CA1 pyramidal neurons

Sonic hedgehog regulates discrete populations of astrocytes in the adult mouse forebrain

Astrocytes are an essential component of the CNS, and recent evidence points to an increasing diversity of their functions. Identifying molecular pathways that mediate distinct astrocyte functions, is key to understanding how the nervous system operates in the intact and pathological states. In this study, we demonstrate that the Hedgehog (Hh) pathway, well known for its roles in the developing CNS, is active in astrocytes of the mature mouse forebrain in vivo. Using multiple genetic approaches, we show that regionally distinct subsets of astrocytes receive Hh signaling, indicating a molecular diversity between specific astrocyte populations. Furthermore, we identified neurons as a source of Sonic hedgehog (Shh) in the adult forebrain, suggesting that Shh signaling is involved in neuron-astrocyte communication. Attenuation of Shh signaling in postnatal astrocytes by targeted removal of Smoothened, an obligate Shh coreceptor, resulted in upregulation of GFAP and cellular hypertrophy specifically in astrocyte populations regulated by Shh signaling. Collectively, our findings demonstrate a role for neuron-derived Shh in regulating specific populations of differentiated astrocytes.

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.

Effect of resveratrol on gliotransmitter levels and p38 activities in cultured astrocytes

Accumulating evidence suggests that resveratrol may have beneficial effects against traumatic brain injury. However, its effect on the regulation of extracellular levels of gliotransmitter and on the activation of p38 MAPK in astrocytes is still unknown. We have examined whether resveratrol regulates extracellular levels of gliotransmitter as well as the activation of p38 MAPK in cultured astrocytes before and after stretch injury. The extracellular levels of glutamate, D-/L-serine and D-serine were apparently reduced by 100 μM resveratrol in control astrocyte cultures. The dramatic increase of glutamate and D-serine release induced by stretch injury was also clearly inhibited by resveratrol. Resveratrol mediates this response by reduction of release through inhibition of extracellular calcium influx and increment of gliotransmitter uptake through enhancement of amino acid transporter expressed in the membrane of astrocyte. In addition, resveratrol definitely reduced the activation of p38 MAPK in cultured astrocytes following stretch injury. AMPA receptor is involved in the activation of p38 following injury. Conversely, the levels of glutamine and glycine were not obviously affected by resveratrol before and after injury. Intracellular levels of glutamate and D-serine are not apparently changed by stretch injury. Collectively, our data suggest that resveratrol might play an important role in protection of the nervous system after injury by decreasing the extracellular levels of gliotransmitter and inhibiting activation of p38 MAPK following injury.

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.

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.

Integrated brain circuits: astrocytic networks modulate neuronal activity and behavior

The past decade has seen an explosion of research on roles of neuron-astrocyte interactions in the control of brain function. We highlight recent studies performed on the tripartite synapse, the structure consisting of pre- and postsynaptic elements of the synapse and an associated astrocytic process. Astrocytes respond to neuronal activity and neurotransmitters, through the activation of metabotropic receptors, and can release the gliotransmitters ATP, d-serine, and glutamate, which act on neurons. Astrocyte-derived ATP modulates synaptic transmission, either directly or through its metabolic product adenosine. d-serine modulates NMDA receptor function, whereas glia-derived glutamate can play important roles in relapse following withdrawal from drugs of abuse. Cell type-specific molecular genetics has allowed a new level of examination of the function of astrocytes in brain function and has revealed an important role of these glial cells that is mediated by adenosine accumulation in the control of sleep and in cognitive impairments that follow sleep deprivation.

An excitatory loop with astrocytes contributes to drive neurons to seizure threshold

Studies in rodent brain slices suggest that seizures in focal epilepsies are sustained and propagated by the reciprocal interaction between neurons and astroglial cells

Seizures in focal epilepsies are sustained by a highly synchronous neuronal discharge that arises at restricted brain sites and subsequently spreads to large portions of the brain. Despite intense experimental research in this field, the earlier cellular events that initiate and sustain a focal seizure are still not well defined. Their identification is central to understand the pathophysiology of focal epilepsies and to develop new pharmacological therapies for drug-resistant forms of epilepsy. The prominent involvement of astrocytes in ictogenesis was recently proposed. We test here whether a cooperation between astrocytes and neurons is a prerequisite to support ictal (seizure-like) and interictal epileptiform events. Simultaneous patch-clamp recording and Ca²⁺ imaging techniques were performed in a new in vitro model of focal seizures induced by local applications of N-methyl-D-aspartic acid (NMDA) in rat entorhinal cortex slices. We found that a Ca²⁺ elevation in astrocytes correlates with both the initial development and the maintenance of a focal, seizure-like discharge. A delayed astrocyte activation during ictal discharges was also observed in other models (including the whole in vitro isolated guinea pig brain) in which the site of generation of seizure activity cannot be precisely monitored. In contrast, interictal discharges were not associated with Ca²⁺ changes in astrocytes. Selective inhibition or stimulation of astrocyte Ca²⁺ signalling blocked or enhanced, respectively, ictal discharges, but did not affect interictal discharge generation. Our data reveal that neurons engage astrocytes in a recurrent excitatory loop (possibly involving gliotransmission) that promotes seizure ignition and sustains the ictal discharge. This neuron–astrocyte interaction may represent a novel target to develop effective therapeutic strategies to control seizures.

In focal epilepsy, seizures are generated by a localized, synchronous neuronal electrical discharge that may spread to large portions of the brain. Despite intense experimental research in this field, a key question relevant to the human epilepsy condition remains completely unanswered: what are the cellular events that lead to the onset of a seizure in the first place? In various in vitro models of seizures using rodent brain slices, we simultaneously recorded neuronal firing and Ca²⁺ signals both from neurons and from astrocytes, the principal population of glial cells in the brain. We found that activation of astrocytes by neuronal activity and signalling from astrocytes back to neurons contribute to the initiation of a focal seizure. This reciprocal excitatory loop between neurons and astrocytes represents a new mechanism in the pathophysiology of epilepsy that should be considered by those aiming to develop more effective therapies for epilepsies that are not controlled by currently available treatments.

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.

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.

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.

Glutamate-mediated astrocyte-to-neuron signalling in the rat dorsal horn

By releasing neuroactive agents, including proinflammatory cytokines, prostaglandins and neurotrophins, microglia and astrocytes are proposed to be involved in nociceptive transmission, especially in conditions of persistent, pathological pain. The specific action on dorsal horn neurons of agents released from astrocytes, such as glutamate, has been, however, poorly investigated. By using patch-clamp and confocal microscope calcium imaging techniques in rat spinal cord slices, we monitored the activity of dorsal horn lamina II neurons following astrocyte activation. Results obtained revealed that stimuli that triggered Ca(2+) elevations in astrocytes, such as the purinergic receptor agonist BzATP and low extracellular Ca(2+), induce in lamina II neurons slow inward currents (SICs). Similarly to SICs triggered by astrocytic glutamate in neurons from other central nervous system regions, these currents (i) are insensitive to tetrodotoxin (TTX), (ii) are blocked by the NMDA receptor (NMDAR) antagonist d-AP5, (iii) lack an AMPA component, and (iv) have slow rise and decay times. Ca(2+) imaging also revealed that astrocytic glutamate evokes NMDAR-mediated episodes of synchronous activity in groups of substantia gelatinosa neurons. Importantly, in a model of peripheral inflammation, the development of thermal hyperalgesia and mechanical allodynia was accompanied by a significant increase of spontaneous SICs in dorsal horn neurons. The NMDAR-mediated astrocyte-to-neuron signalling thus represents a novel pathway that may contribute to the control of central sensitization in pathological pain.

Astrocyte-mediated distributed plasticity at hypothalamic glutamate synapses

Afferent activity can induce fast, feed-forward changes in synaptic efficacy that are synapse specific. Using combined electrophysiology, caged molecule photolysis, and Ca(2+) imaging, we describe a plasticity in which the recruitment of astrocytes in response to afferent activity causes a fast and feed-forward, yet distributed increase in the amplitude of quantal synaptic currents at multiple glutamate synapses on magnocellular neurosecretory cells in the hypothalamic paraventricular nucleus. The plasticity is largely multiplicative, consistent with a proportional increase or "scaling" in the strength of all synapses on the neuron. This effect requires a metabotropic glutamate receptor-mediated rise in Ca(2+) in the astrocyte processes surrounding the neuron and the release of the gliotransmitter ATP, which acts on postsynaptic purinergic receptors. These data provide evidence for a form of distributed synaptic plasticity that is feed-forward, expressed quickly, and mediated by the synaptic activation of neighboring astrocytes.

GLIA modulates synaptic transmission

The classical view of glial cells as simple supportive cells for neurons is being replaced by a new vision in which glial cells are active elements involved in the physiology of the nervous system. This new vision is based on the fact that astrocytes, a subtype of glial cells in the CNS, are stimulated by synaptically released neurotransmitters, which increase the astrocyte Ca(2+) levels and stimulate the release of gliotransmitters that regulate synaptic efficacy and plasticity. Consequently, our understanding of synaptic function, previously thought to exclusively result from signaling between neurons, has also changed to include the bidirectional signaling between neurons and astrocytes. Hence, astrocytes have been revealed as integral elements involved in the synaptic physiology, therefore contributing to the processing, transfer and storage of information by the nervous system. Reciprocal communication between astrocytes and neurons is therefore part of the intercellular signaling processes involved in brain function.

Spatiotemporal characteristics of calcium dynamics in astrocytes

Although Ca(i)(2+) waves in networks of astrocytes in vivo are well documented, propagation in vivo is much more complex than in culture, and there is no consensus concerning the dominant roles of intercellular and extracellular messengers [inositol 1,4,5-trisphosphate (IP(3)) and adenosine-5'-triphosphate (ATP)] that mediate Ca(i)(2+) waves. Moreover, to date only simplified models that take very little account of the geometrical struture of the networks have been studied. Our aim in this paper is to develop a mathematical model based on realistic cellular morphology and network connectivity, and a computational framework for simulating the model, in order to address these issues. In the model, Ca(i) (2+) wave propagation through a network of astrocytes is driven by IP(3) diffusion between cells and ATP transport in the extracellular space. Numerical simulations of the model show that different kinetic and geometric assumptions give rise to differences in Ca(i)(2+) wave propagation patterns, as characterized by the velocity, propagation distance, time delay in propagation from one cell to another, and the evolution of Ca(2+) response patterns. The temporal Ca(i)(2+) response patterns in cells are different from one cell to another, and the Ca(i)(2+) response patterns evolve from one type to another as a Ca(i)(2+) wave propagates. In addition, the spatial patterns of Ca(i)(2+) wave propagation depend on whether IP(3), ATP, or both are mediating messengers. Finally, two different geometries that reflect the in vivo and in vitro configuration of astrocytic networks also yield distinct intracellular and extracellular kinetic patterns. The simulation results as well as the linear stability analysis of the model lead to the conclusion that Ca(i)(2+) waves in astrocyte networks are probably mediated by both intercellular IP(3) transport and nonregenerative (only the glutamate-stimulated cell releases ATP) or partially regenerative extracellular ATP signaling.

Mechanically induced reactive gliosis causes ATP-mediated alterations in astrocyte stiffness

Reactive gliosis is a process triggered in astrocytes after traumatic injury, yet the exact consequences of gliosis on cellular survival and neural regenerative processes in the injured brain remain only partly understood. One recently discovered feature influencing neuronal growth and differentiation is the physical stiffness of the environment surrounding pioneering neurites. In this study, the mechanical properties of cultured cortical astrocytes are measured following a mechanical stretch injury that induces reactive gliosis. In mechanically injured cultures, there was a significant increase in glial fibrillary acidic protein (GFAP) immunoreactivity 24 h following a rapid, transient 15% strain. In these same cultures, astrocytes in the surrounding region--the "mechanical penumbra"--also exhibited increased GFAP immunoreactivity compared to naive cultures. Correlated with these changes in GFAP was a general softening of the non-nuclear regions of the astrocytes, both in the injured and penumbra cells, as measured by atomic force microscopy (AFM). The elastic modulus in naive cultures was observed to be 57.7+/-5.8 kPa in non-nuclear regions of naive cultures, while 24 h after injury the modulus was observed to be 26.4+/-4.9 kPa in the same region of injured cells. In the penumbra of injured cultures, the modulus was 23.7+/-3.6 kPa. Alterations in astrocyte stiffness in the area of injury and mechanical penumbra were ameliorated by pretreating cultures with a nonselective P2 receptor antagonist (PPADS). Since neuronal cells generally prefer softer substrates for growth and neurite extension, these findings may indicate that the mechanical characteristics of reactive astrocytes are favorable for neuronal recovery after traumatic brain injury.

Differential expression of utrophin-A and -B promoters in the central nervous system (CNS) of normal and dystrophic mdx mice

Utrophin (Utrn) is the autosomal homolog of dystrophin, the Duchene Muscular Dystrophy (DMD) locus product and of therapeutic interest, as its overexpression can compensate dystrophin's absence. Utrn is transcribed by Utrn-A and -B promoters with mRNAs differing at their 5' ends. However, previous central nervous system (CNS) studies used C-terminal antibodies recognizing both isoforms. As this distinction may impact upregulation strategies, we generated Utrn-A and -B promoter-specific antibodies, Taqman Polymerase chain reaction (PCR)-based absolute copy number assays, and luciferase-reporter constructs to study CNS of normal and dystrophic mdx mice. Differential expression of Utrn-A and -B was noted in microdissected and capillary-enriched fractions. At the protein level, Utrn-B was predominantly expressed in vasculature and ependymal lining, whereas Utrn-A was expressed in neurons, astrocytes, choroid plexus and pia mater. mRNA quantification demonstrated matching patterns of differential expression; however, transcription-translation mismatch was noted for Utrn-B in caudal brain regions. Utrn-A and Utrn-B proteins were significantly upregulated in olfactory bulb and cerebellum of mdx brain. Differential promoter activity, mRNA and protein expressions were studied in cultured C2C12, bEnd3, neurons and astrocytes. Promoter activity ranking for Utrn-A and -B was neurons > astrocytes > C2C12 > bEnd3 and bEnd3 > astrocytes > neurons > C2C12, respectively. Our results identify promoter usage patterns for therapeutic targeting and define promoter-specific differential distribution of Utrn isoforms in normal and dystrophic CNS.

Transgenic mice expressing a cameleon fluorescent Ca2+ indicator in astrocytes and Schwann cells allow study of glial cell Ca2+ signals in situ and in vivo

Glial cell Ca2+ signals play a key role in glial-neuronal and glial-glial network communication. Numerous studies have thus far utilized cell-permeant and injected Ca2+ indicator dyes to investigate glial Ca2+ signals in vitro and in situ. Genetically encoded fluorescent Ca2+ indicators have emerged as novel probes for investigating cellular Ca2+ signals. We have expressed one such indicator protein, the YC 3.60 cameleon, under the control of the S100beta promoter and directed its expression predominantly in astrocytes and Schwann cells. Expression of YC 3.60 extended into the entire cellular cytoplasmic compartment and the fine terminal processes of protoplasmic astrocytes and Schwann cell Cajal bands. In the brain, all the cells known to express S100beta in the adult or during development, expressed YC 3.60. While expression was most extensive in astrocytes, other glial cell types that express S100beta, such as NG2 and CNP-positive oligodendrocyte progenitor cells (OP cells), microglia, and some of the large motor neurons in the brain stem, also contained YC 3.60 fluorescence. Using a variety of known in situ and in vivo assays, we found that stimuli known to elicit Ca2+ signals in astrocytes caused substantial and rapid Ca2+ signals in the YC 3.60-expressing astrocytes. In addition, forepaw stimulation while imaging astrocytes through a cranial window in the somatosensory cortex in live mice, revealed robust evoked and spontaneous Ca2+ signals. These results, for the first time, show that genetically encoded reporter is capable of recording activity-dependent Ca2+ signals in the astrocyte processes, and networks.

gamma-Hydroxybutyrate (GHB) induces GABA(B) receptor independent intracellular Ca2+ transients in astrocytes, but has no effect on GHB or GABA(B) receptors of medium spiny neurons in the nucleus accumbens

We report on cellular actions of the illicit recreational drug gamma-hydroxybutyrate (GHB) in the brain reward area nucleus accumbens. First, we compared the effects of GHB and the GABA(B) receptor agonist baclofen. Neither of them affected the membrane currents of medium spiny neurons in rat nucleus accumbens slices. GABAergic and glutamatergic synaptic potentials of medium spiny neurons, however, were reduced by baclofen but not GHB. These results indicate the lack of GHB as well as postsynaptic GABA(B) receptors, and the presence of GHB insensitive presynaptic GABA(B) receptors in medium spiny neurons. In astrocytes GHB induced intracellular Ca(2+) transients, preserved in slices from GABA(B) receptor type 1 subunit knockout mice. The effects of tetrodotoxin, zero added Ca(2+) with/without intracellular Ca(2+) store depletor cyclopiazonic acid or vacuolar H-ATPase inhibitor bafilomycin A1 indicate that GHB-evoked Ca(2+) transients depend on external Ca(2+) and intracellular Ca(2+) stores, but not on vesicular transmitter release. GHB-induced astrocytic Ca(2+) transients were not affected by the GHB receptor-specific antagonist NCS-382, suggesting the presence of a novel NCS-382-insensitive target for GHB in astrocytes. The activation of astrocytes by GHB implies their involvement in physiological actions of GHB. Our findings disclose a novel profile of GHB action in the nucleus accumbens. Here, unlike in other brain areas, GHB does not act on GABA(B) receptors, but activates an NCS-382 insensitive GHB-specific target in a subpopulation of astrocytes. The lack of either post- or presynaptic effects on medium spiny neurons in the nucleus accumbens distinguishes GHB from many drugs and natural rewards with addictive properties and might explain why GHB has only a weak reinforcing capacity.

Sorting out astrocyte physiology from pharmacology

A number of exciting findings have been made in astrocytes during the past 15 years that have led many researchers to redefine how the brain works. Astrocytes are now widely regarded as cells that propagate Ca(2+) over long distances in response to stimulation, and, similar to neurons, release transmitters (called gliotransmitters) in a Ca(2+)-dependent manner to modulate a host of important brain functions. Although these discoveries have been very exciting, it is essential to place them in the proper context of the approaches used to obtain them to determine their relevance to brain physiology. This review revisits the key observations made in astrocytes that greatly impact how they are thought to regulate brain function, including the existence of widespread propagating intercellular Ca(2+) waves, data suggesting that astrocytes signal to neurons through Ca(2+)-dependent release of glutamate, and evidence for the presence of vesicular machinery for the regulated exocytosis of gliotransmitters.

Purinergic neuron-to-glia signaling in the enteric nervous system

BACKGROUND & AIMS

Enteric glia are intimately associated with neurons in the enteric nervous system (ENS) and display morphologic and molecular similarities to central nervous system (CNS) astrocytes. Enteric glia express neurotransmitter receptors, suggesting that, like astrocytes, they are active participants in neuronal communication. In the ENS, the purine adenosine triphosphate (ATP) is co-released with the neurotransmitters noradrenaline and acetylcholine. Enteric glia express purinergic receptors and respond to ATP in vitro, suggesting that enteric glia participate in functional gastrointestinal responses to nerve signaling. We investigated whether enteric glia are activated by ATP released from enteric neurons.

METHODS

Synaptic activity was elicited in enteric neurons by electrically stimulating interganglionic connectives in the myenteric plexus of the guinea pig colon. Activity in enteric glial cells was detected by imaging intracellular calcium in situ.

RESULTS

Neuronal stimulation elicited increases in intracellular calcium in enteric glial cells that were blocked by tetrodotoxin, the nonselective purinergic receptor antagonist pyridoxal phosphate-6-azo(benzene-2,4-disulfonic acid) tetrasodium salt hydrate (PPADS), and the phospholipase C inhibitor U73122. Furthermore, enteric glia responded robustly to exogenously applied ATP in situ, and the ATP response was blocked by PPADS and U73122. Data from pharmacologic profiling and immunohistochemical analyses support the hypothesis that P2Y4 is the major functional receptor underlying the ATP response in enteric glia.

CONCLUSIONS

Our results provide direct evidence for functional purinergic neuron-glia communication in the enteric nervous system, raising the possibility that ATP released with neurotransmitters during enteric synaptic transmission functions to signal to enteric glia.

Astrocytes process synaptic information

Astrocytes were classically considered as simple supportive cells for neurons without a significant role in information processing by the nervous system. However, considerable amounts of evidence obtained by several groups during the past years demonstrated the existence of a bidirectional communication between astrocytes and neurons, which prompted a re-examination of the role of astrocytes in the physiology of the nervous system. While neurons base their excitability on electrical signals generated across the membrane, astrocytes base their cellular excitability on variations of the Ca2+ concentration in the cytosol. This article discusses our current knowledge of the properties of the synaptically evoked astrocyte Ca2+ signal, which reveals that astrocytes display integrative properties for synaptic information processing. Astrocytes respond selectively to different axon pathways, discriminate between the activity of different synapses and their Ca2+ signal is non-linearly modulated by the simultaneous activity of different synaptic inputs. Furthermore, this Ca2+ signal modulation depends on astrocyte cellular intrinsic properties and is bidirectionally regulated by the level of synaptic activity. Finally, astrocyte Ca2+ elevations can trigger the release of gliotransmitters, which modulate neuronal activity as well as synaptic transmission and plasticity, hence granting the bidirectional communication with neurons. Consequently, astrocytes can be considered as cellular elements involved in information processing by the nervous system.

Communication between neurons and astrocytes: relevance to the modulation of synaptic and network activity

Neuromodulation is a fundamental process in the brain that regulates synaptic transmission, neuronal network activity and behavior. Emerging evidence demonstrates that astrocytes, a major population of glial cells in the brain, play previously unrecognized functions in neuronal modulation. Astrocytes can detect the level of neuronal activity and release chemical transmitters to influence neuronal function. For example, recent findings show that astrocytes play crucial roles in the control of Hebbian plasticity, the regulation of neuronal excitability and the induction of homeostatic plasticity. This review discusses the importance of astrocyte-to-neuron signaling in different aspects of neuronal function from the activity of single synapses to that of neuronal networks.

Photostimulation of astrocytes with femtosecond laser pulses

The involvement of astrocytes in brain functions rather than support has been identified and widely concerned. However the lack of an effective stimulation of astrocytes hampers our understanding of their essential roles. Here, we employed 800-nm near infrared (NIR) femtosecond laser to induce Ca2+ wave in astrocytes. It was demonstrated that photostimulation of astrocytes with femtosecond laser pulses is efficient with the advantages of non-contact, non-disruptiveness, reproducibility, and high spatiotemporal precision. Photostimulation of astrocytes would facilitate investigations on information processing in neuronal circuits by providing effective way to excite astrocytes.

Implication of activated astrocytes in the development of drug dependence: differences between methamphetamine and morphine

Astrocytes are a subpopulation of glial cells that directly affect neuronal function. This review focuses on the potential functional roles of astrocytes in the development of behavioral sensitization and rewarding effects induced by chronic treatment with drugs of abuse. In vitro treatment of cortical neuron/glia cocultures with either methamphetamine or morphine caused activation of astrocytes via protein kinase C (PKC). Purified cortical astrocytes were markedly activated by methamphetamine, whereas morphine had no such effect. Methamphetamine, but not morphine, caused a long-lasting astrocytic activation in cortical neuron/glia cocultures. Morphine-induced behavioral sensitization, assessed as hyperlocomotion, was reversed by 2 months of withdrawal from intermittent morphine administration, whereas behavioral sensitization to methamphetamine-induced hyperlocomotion was maintained even after 2 months of withdrawal. In vivo treatment with methamphetamine, which was associated with behavioral sensitization, caused PKC-dependent astrocytic activation in the mouse cingulate cortex and nucleus accumbens. Furthermore, the glial modulator propentofylline dramatically diminished the activation of astrocytes and the rewarding effect induced by methamphetamine and morphine. On the other hand, intra-nucleus accumbens and intra-cingulate cortex administration of astrocyte-conditioned medium aggravated the development of rewarding effects induced by methamphetamine and morphine. Furthermore, astrocyte-conditioned medium, but not methamphetamine itself, clearly induced differentiation of neural stem cells into astrocytes. These findings provide direct evidence that astrocytes may, at least in part, contribute to the development of the rewarding effects induced by drugs of abuse in the nucleus accumbens and cingulate cortex.

Astrocytic Ca(2+) waves guide CNS growth cones to remote regions of neuronal activity

Activity plays a critical role in network formation during developmental, experience-dependent, and injury related remodeling. Here we report a mechanism by which axon trajectory can be altered in response to remote neuronal activity. Using photoconductive stimulation to trigger high frequency action potentials in rat hippocampal neurons in vitro, we find that activity functions as an attractive cue for growth cones in the local environment. The underlying guidance mechanism involves astrocyte Ca(2+) waves, as the connexin-43 antagonist carbenoxolone abolishes the attraction when activity is initiated at a distance greater than 120 microm. The asymmetric growth cone filopodia extension that precedes turning can be blocked with CNQX (10 microM), but not with the ATP and adenosine receptor antagonists suramin (100 microM) and alloxazine (4 microM), suggesting non-NMDA glutamate receptors on the growth cone mediate the interaction with astrocytes. These results define a potential long-range signalling pathway for activity-dependent axon guidance in which growth cones turn towards directional, temporally coordinated astrocyte Ca(2+) waves that are triggered by neuronal activity. To assess the viability of the guidance effect in an injury paradigm, we performed the assay in the presence of conditioned media from lipopolysaccharide (LPS) activated purified microglial cultures, as well as directly activating the glia present in our co-cultures. Growth cone attraction was not inhibited under these conditions, suggesting this mechanism could be used to guide regeneration following axonal injury.

Nitric oxide-mediated modulation of synaptic activity by astrocytic P2Y receptors

We investigated the mechanism of synaptic suppression by P2Y receptors in mixed hippocampal cultures wherein networked neurons exhibit synchronized Ca²⁺ oscillations (SCO) due to spontaneous glutamatergic synaptic transmission. Pharmacological studies suggested that SCO suppression was mediated by P2Y2/P2Y4 receptors. Immunostaining studies and characterization of ATP/UTP-stimulated Ca²⁺ responses in solitary neurons and astrocytes revealed that the SCO attenuation was effectuated by astrocytes. We demonstrate that nitric oxide released from activated astrocytes causes synaptic suppression by inhibiting neurotransmitter release. Physiological concentrations of ATP and UTP evoked NO production in astrocytes. SCO suppression was considerably diminished by removal of extracellular NO by membrane-impermeable scavenger c-PTIO or by pretreatment of cells with nitric oxide synthase inhibitor L-NAME. The nitric oxide donor DETA/NO effectively suppressed the SCO. ATP/UTP inhibited KCl-induced exocytosis at presynaptic terminals in an NO-dependent manner. In the absence of exogenously added ATP/UTP, both the NO scavenger and NOS inhibitor enhanced the frequency of SCO, implying that astrocytes release NO during spontaneous synaptic activity and exert a suppressive effect. We report for the first time that under physiological conditions astrocytes use NO as a messenger molecule to modulate the synaptic strength in the networked neurons.

What is the role of astrocyte calcium in neurophysiology?

Astrocytes comprise approximately half of the volume of the adult mammalian brain and are the primary neuronal structural and trophic supportive elements. Astrocytes are organized into distinct nonoverlapping domains and extend elaborate and dense fine processes that interact intimately with synapses and cerebrovasculature. The recognition in the mid 1990s that astrocytes undergo elevations in intracellular calcium concentration following activation of G protein-coupled receptors by synaptically released neurotransmitters demonstrated not only that astrocytes display a form of excitability but also that astrocytes may be active participants in brain information processing. The roles that astrocytic calcium elevations play in neurophysiology and especially in modulation of neuronal activity have been intensely researched in recent years. This review will summarize the current understanding of the function of astrocytic calcium signaling in neurophysiological processes and discuss areas where the role of astrocytes remains controversial and will therefore benefit from further study.

The astrocyte odyssey

Neurons have long held the spotlight as the central players of the nervous system, but we must remember that we have equal numbers of astrocytes and neurons in the brain. Are these cells only filling up the space and passively nurturing the neurons, or do they also contribute to information transfer and processing? After several years of intense research since the pioneer discovery of astrocytic calcium waves and glutamate release onto neurons in vitro, the neuronal-glial studies have answered many questions thanks to technological advances. However, the definitive in vivo role of astrocytes remains to be addressed. In addition, it is becoming clear that diverse populations of astrocytes coexist with different molecular identities and specialized functions adjusted to their microenvironment, but do they all belong to the umbrella family of astrocytes? One population of astrocytes takes on a new function by displaying both support cell and stem cell characteristics in the neurogenic niches. Here, we define characteristics that classify a cell as an astrocyte under physiological conditions. We will also discuss the well-established and emerging functions of astrocytes with an emphasis on their roles on neuronal activity and as neural stem cells in adult neurogenic zones.

Astrocytic control of synaptic transmission and plasticity: a target for drugs of abuse?

It is well recognized that drugs of abuse lead to plastic changes in synapses and that these long-term modifications have the potential to underlie adaptive changes of the brain that lead to substance abuse. However the variety of molecular mechanisms involved in these responses are not completely defined. We are just beginning to understand some of the roles of glial cells that are associated with synapses. At many synapses an astrocyte process is associated with pre- and postsynaptic neuron processes leading to the naming of this synaptic structure as the Tripartite Synapse. Therefore, these glial cells are positioned so that they influence synaptic transmission and thus could potentially regulate the actions of some drugs of abuse. In mammalian systems there are correlations between long-term structural changes in astrocytes and responses to drugs of abuse. However, whether such changes in glia impact brain function and subsequent behaviors associated with addiction is poorly understood. Studies using Drosophila show important roles of fly glia in mediating responses to cocaine pointing to the potential for the involvement of mammalian glia in the brain's responses to this as well as other drugs. In agreement with this possibility three receptor systems known to be important in substance abuse, mGluR5, GABA(B) and CB-1 receptors, are all expressed by astrocytes and the activation of these glial receptors is now known to impact neuronal excitability and synaptic transmission. Given our new knowledge about the presence of reciprocal signaling between astrocytes and synapses we are now at a time when it becomes appropriate to determine how glial cells respond to drugs of abuse and whether they contribute to the changes in brain function underlying substance abuse.

Astrocyte and neuron intone through glutamate

The unexpected finding of astrocytes to release glutamate as gliotransmitter challenges the traditional concepts on astrocyte being "passive" in CNS communications. Glutamate is the major excitatory transmitter in transferring information between neurons, but is now also known to activate astrocyte through transporters and receptors. Together with the sensitive swelling response, astrocytes could respond directly to glutamate and neuronal activity. Other new functions of astrocytes include modulation of synaptic plasticity and cerebral blood flow (CBF). The classic glutamate deplenishment through glutamine synthesis and CO(2) production does not account for the total glutamate internalized into astrocytes. This leads us to speculate there are many hidden functions of glutamate in neurons and astrocytes waiting to be discovered. In this review, we attempted to reexamine some of these new and older functions of glutamate and to reevaluate the roles of glutamate intoning these two cell types.

Loss of IP3 receptor-dependent Ca2+ increases in hippocampal astrocytes does not affect baseline CA1 pyramidal neuron synaptic activity

Astrocytes in the hippocampus release calcium (Ca(2+)) from intracellular stores intrinsically and in response to activation of G(q)-linked G-protein-coupled receptors (GPCRs) through the binding of inositol 1,4,5-trisphosphate (IP(3)) to its receptor (IP(3)R). Astrocyte Ca(2+) has been deemed necessary and sufficient to trigger the release of gliotransmitters, such as ATP and glutamate, from astrocytes to modulate neuronal activity. Several lines of evidence suggest that IP(3)R type 2 (IP(3)R2) is the primary IP(3)R expressed by astrocytes. To determine whether IP(3)R2 is the primary functional IP(3)R responsible for astrocytic Ca(2+) increases, we conducted experiments using an IP(3)R2 knock-out mouse model (IP(3)R2 KO). We show, for the first time, that lack of IP(3)R2 blocks both spontaneous and G(q)-linked GPCR-mediated increases in astrocyte Ca(2+). Furthermore, neuronal G(q)-linked GPCR Ca(2+) increases remain intact, suggesting that IP(3)R2 does not play a major functional role in neuronal calcium store release or may not be expressed in neurons. Additionally, we show that lack of IP(3)R2 in the hippocampus does not affect baseline excitatory neuronal synaptic activity as measured by spontaneous EPSC recordings from CA1 pyramidal neurons. Whole-cell recordings of the tonic NMDA receptor-mediated current indicates that ambient glutamate levels are also unaffected in the IP(3)R2 KO. These data show that IP(3)R2 is the key functional IP(3)R driving G(q)-linked GPCR-mediated Ca(2+) increases in hippocampal astrocytes and that removal of astrocyte Ca(2+) increases does not significantly affect excitatory neuronal synaptic activity or ambient glutamate levels.

Astrocytes optimize the synaptic transmission of information

Chemical synapses transmit information via the release of neurotransmitter-filled vesicles from the presynaptic terminal. Using computational modeling, we predict that the limited availability of neurotransmitter resources in combination with the spontaneous release of vesicles limits the maximum degree of enhancement of synaptic transmission. This gives rise to an optimal tuning that depends on the number of active zones. There is strong experimental evidence that astrocytes that enwrap synapses can modulate the probabilities of vesicle release through bidirectional signaling and hence regulate synaptic transmission. For low-fidelity hippocampal synapses, which typically have only one or two active zones, the predicted optimal values lie close to those determined by experimentally measured astrocytic feedback, suggesting that astrocytes optimize synaptic transmission of information.

Release of chemical (neurotransmitter)-filled vesicles at neuronal junctions called synapses leads to transmission of information between neurons. In a successful synaptic transmission, a voltage spike (action potential) generated by a presynaptic neuron initiates neurotransmitter vesicle release and leads to a small current in the postsynaptic neuron. For many synapses in the central nervous system, the probability that a neurotransmitter vesicle is released in response to an action potential is conspicuously small, raising the question whether transmission failures can in any way prove advantageous. Apart from “induced vesicle release” (in response to an action potential), vesicles are also released asynchronously (in absence of an action potential). An induced release probability that is too small samples the information poorly, as many of the incoming action potentials do not result in a postsynaptic current response. Maximizing induced release in order to maximize information transmission at a synapse is accompanied by the exceptionable outcome of increased asynchronous release; in addition, both these releases draw from the same neurotransmitter resource pool. A large release rate thus comprising both induced as well as asynchronous release of vesicles can suppress synaptic transmission via either depletion of neurotransmitter resources or desensitization of postsynaptic receptors. In this paper, we propose that the competing dynamics of induced and asynchronous vesicle release gives rise to an optimal release probability. Further, by comparing experimental data of astrocyte-enhanced synaptic transmission with simulations, we argue that synapses enwrapped by astrocytes operate close to our predicted optimum. This optimality is achieved through a closed-loop control circuitry that involves the presynaptic neuron and the synaptic astrocyte.

Astrocytes in the epileptic brain

The roles that astrocytes play in the evolution of abnormal network excitability in chronic neurological disorders involving brain injury, such as acquired epilepsy, are receiving renewed attention due to improved understanding of the molecular events underpinning the physiological functions of astrocytes. In epileptic tissue, evidence is pointing to enhanced chemical signaling and disrupted linkage between water and potassium balance by reactive astrocytes, which together conspire to enhance local synchrony in hippocampal microcircuits. Reactive astrocytes in epileptic tissue both promote and oppose seizure development through a variety of specific mechanisms; the new findings suggest several novel astrocyte-related targets for drug development.

Plasmin potentiates synaptic N-methyl-D-aspartate receptor function in hippocampal neurons through activation of protease-activated receptor-1

Protease-activated receptor-1 (PAR1) is activated by a number of serine proteases, including plasmin. Both PAR1 and plasminogen, the precursor of plasmin, are expressed in the central nervous system. In this study we examined the effects of plasmin in astrocyte and neuronal cultures as well as in hippocampal slices. We find that plasmin evokes an increase in both phosphoinositide hydrolysis (EC(50) 64 nm) and Fura-2/AM fluorescence (195 +/- 6.7% above base line, EC(50) 65 nm) in cortical cultured murine astrocytes. Plasmin also activates extracellular signal-regulated kinase (ERK1/2) within cultured astrocytes. The plasmin-induced rise in intracellular Ca(2+) concentration ([Ca(2+)](i)) and the increase in phospho-ERK1/2 levels were diminished in PAR1(-/-) astrocytes and were blocked by 1 microm BMS-200261, a selective PAR1 antagonist. However, plasmin had no detectable effect on ERK1/2 or [Ca(2+)](i) signaling in primary cultured hippocampal neurons or in CA1 pyramidal cells in hippocampal slices. Plasmin (100-200 nm) application potentiated the N-methyl-D-aspartate (NMDA) receptor-dependent component of miniature excitatory postsynaptic currents recorded from CA1 pyramidal neurons but had no effect on alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate- or gamma-aminobutyric acid receptor-mediated synaptic currents. Plasmin also increased NMDA-induced whole cell receptor currents recorded from CA1 pyramidal cells (2.5 +/- 0.3-fold potentiation over control). This effect was blocked by BMS-200261 (1 microm; 1.02 +/- 0.09-fold potentiation over control). These data suggest that plasmin may serve as an endogenous PAR1 activator that can increase [Ca(2+)](i) in astrocytes and potentiate NMDA receptor synaptic currents in CA1 pyramidal neurons.

Presynaptic glutamate receptors: physiological functions and mechanisms of action

Glutamate acts on postsynaptic glutamate receptors to mediate excitatory communication between neurons. The discovery that additional presynaptic glutamate receptors can modulate neurotransmitter release has added complexity to the way we view glutamatergic synaptic transmission. Here we review evidence of a physiological role for presynaptic glutamate receptors in neurotransmitter release. We compare the physiological roles of ionotropic and metabotropic glutamate receptors in short- and long-term regulation of synaptic transmission. Furthermore, we discuss the physiological conditions that are necessary for their activation, the source of the glutamate that activates them, their mechanisms of action and their involvement in higher brain function.

Synaptic modulation by astrocytes via Ca2+-dependent glutamate release

In the past 15 years the classical view that astrocytes play a relatively passive role in brain function has been overturned and it has become increasingly clear that signaling between neurons and astrocytes may play a crucial role in the information processing that the brain carries out. This new view stems from two seminal observations made in the early 1990s: 1. astrocytes respond to neurotransmitters released during synaptic activity with elevation of their intracellular Ca2+ concentration ([Ca2+]i); 2. astrocytes release chemical transmitters, including glutamate, in response to [Ca2+]i elevations. The simultaneous recognition that astrocytes sense neuronal activity and release neuroactive agents has been instrumental for understanding previously unknown roles of these cells in the control of synapse formation, function and plasticity. These findings open a conceptual revolution, leading to rethink how brain communication works, as they imply that information travels (and is processed) not just in the neuronal circuitry but in an expanded neuron-glia network. In this review we critically discuss the available information concerning: 1. the characteristics of the astrocytic Ca2+ responses to synaptic activity; 2. the basis of Ca2+-dependent glutamate exocytosis from astrocytes; 3. the modes of action of astrocytic glutamate on synaptic function.

Astrocytic function and its alteration in the epileptic brain

Currently available anticonvulsant drugs and complementary therapies are insufficient to control seizures in about a third of epileptic patients. Thus, there is an urgent need for new treatments that prevent the development of epilepsy and control it better in patients already afflicted with the disease. A prerequisite to reach this goal is a deeper understanding of the cellular basis of hyperexcitability and synchronization in the affected tissue. Epilepsy is often accompanied by massive reactive gliosis. Although the significance of this alteration is poorly understood, recent findings suggest that modified astroglial function may have a role in the generation and spread of seizure activity. Here we summarize properties of astrocytes as well as their changes that can be associated with epileptic tissue. The goal is to provide an understanding of the current knowledge of these cells with the long-term view of providing a foundation for the development of novel hypotheses about the role of glia in epilepsy.

A review of the role of glial cells in understanding retinal disease

Retinal vascular diseases such as diabetic retinopathy and retinopathy of prematurity are major causes of visual loss. Although the focus of a great deal of research has been on the aetiology of vascular growth, it is now emerging that anomalies in other retinal cell types, especially glial cells, occur very early in the course of the disease. Glial cells have major roles in every stage of disease, from the earliest subtle variations in neural function, to the development of epi-retinal membranes and tractional detachment. Therefore, having a firm understanding of the function of retinal glia is important in our understanding of retinal disease and is crucial for the development of new treatment strategies.

Spinal astrocyte glutamate receptor 1 overexpression after ischemic insult facilitates behavioral signs of spasticity and rigidity

Using a rat model of ischemic paraplegia, we examined the expression of spinal AMPA receptors and their role in mediating spasticity and rigidity. Spinal ischemia was induced by transient occlusion of the descending aorta combined with systemic hypotension. Spasticity/rigidity were identified by simultaneous measurements of peripheral muscle resistance (PMR) and electromyography (EMG) before and during ankle flexion. In addition, Hoffman reflex (H-reflex) and motor evoked potentials (MEPs) were recorded from the gastrocnemius muscle. Animals were implanted with intrathecal catheters for drug delivery and injected with the AMPA receptor antagonist NGX424 (tezampanel), glutamate receptor 1 (GluR1) antisense, or vehicle. Where intrathecal vehicle had no effect, intrathecal NGX424 produced a dose-dependent suppression of PMR [ED50 of 0.44 microg (0.33-0.58)], as well as tonic and ankle flexion-evoked EMG activity. Similar suppression of MEP and H-reflex were also seen. Western blot analyses of lumbar spinal cord tissue from spastic animals showed a significant increase in GluR1 but decreased GluR2 and GluR4 proteins. Confocal and electron microscopic analyses of spinal cord sections from spastic animals revealed increased GluR1 immunoreactivity in reactive astrocytes. Selective GluR1 knockdown by intrathecal antisense treatment resulted in a potent reduction of spasticiy and rigidity and concurrent downregulation of neuronal/astrocytic GluR1 in the lumbar spinal cord. Treatment of rat astrocyte cultures with AMPA led to dose-dependent glutamate release, an effect blocked by NGX424. These data suggest that an AMPA/kainate receptor antagonist can represent a novel therapy in modulating spasticity/rigidity of spinal origin and that astrocytes may be a potential target for such treatment.

Conditional knock-out of Kir4.1 leads to glial membrane depolarization, inhibition of potassium and glutamate uptake, and enhanced short-term synaptic potentiation

During neuronal activity, extracellular potassium concentration ([K+]out) becomes elevated and, if uncorrected, causes neuronal depolarization, hyperexcitability, and seizures. Clearance of K+ from the extracellular space, termed K+ spatial buffering, is considered to be an important function of astrocytes. Results from a number of studies suggest that maintenance of [K+]out by astrocytes is mediated by K+ uptake through the inward-rectifying Kir4.1 channels. To study the role of this channel in astrocyte physiology and neuronal excitability, we generated a conditional knock-out (cKO) of Kir4.1 directed to astrocytes via the human glial fibrillary acidic protein promoter gfa2. Kir4.1 cKO mice die prematurely and display severe ataxia and stress-induced seizures. Electrophysiological recordings revealed severe depolarization of both passive astrocytes and complex glia in Kir4.1 cKO hippocampal slices. Complex cell depolarization appears to be a direct consequence of Kir4.1 removal, whereas passive astrocyte depolarization seems to arise from an indirect developmental process. Furthermore, we observed a significant loss of complex glia, suggestive of a role for Kir4.1 in astrocyte development. Kir4.1 cKO passive astrocytes displayed a marked impairment of both K+ and glutamate uptake. Surprisingly, membrane and action potential properties of CA1 pyramidal neurons, as well as basal synaptic transmission in the CA1 stratum radiatum appeared unaffected, whereas spontaneous neuronal activity was reduced in the Kir4.1 cKO. However, high-frequency stimulation revealed greatly elevated posttetanic potentiation and short-term potentiation in Kir4.1 cKO hippocampus. Our findings implicate a role for glial Kir4.1 channel subunit in the modulation of synaptic strength.

The role of nucleotides in the neuron--glia communication responsible for the brain functions

Accumulating findings indicate that nucleotides play an important role in cell-to-cell communication through P2 purinoceptors, even though ATP is recognized primarily to be a source of free energy and nucleotides are key molecules in cells. P2 purinoceptors are divided into two families, ionotropic receptors (P2X) and metabotropic receptors (P2Y). P2X receptors (7 types; P2X(1)-P2X(7)) contain intrinsic pores that open by binding with ATP. P2Y (8 types; P2Y(1, 2, 4, 6, 11, 12, 13,) and (14)) are activated by nucleotides and couple to intracellular second-messenger systems through heteromeric G-proteins. Nucleotides are released or leaked from non-excitable cells as well as neurons in physiological and pathophysiological conditions. One of the most exciting cells in non-excitable cells is the glia cells, which are classified into astrocytes, oligodendrocytes, and microglia. Astrocytes express many types of P2 purinoceptors and release the 'gliotransmitter' ATP to communicate with neurons, microglia and the vascular walls of capillaries. Microglia also express many types of P2 purinoceptors and are known as resident macrophages in the CNS. ATP and other nucleotides work as 'warning molecules' especially through activating microglia in pathophysiological conditions. Microglia play a key role in neuropathic pain and show phagocytosis through nucleotide-evoked activation of P2X(4) and P2Y(6) receptors, respectively. Such strong molecular, cellular and system-level evidence for extracellular nucleotide signaling places nucleotides in the central stage of cell communications in glia/CNS.

Vesicular release of glutamate mediates bidirectional signaling between astrocytes and neurons

The major excitatory neurotransmitter in the CNS, glutamate, can be released exocytotically by neurons and astrocytes. Glutamate released from neurons can affect adjacent astrocytes by changing their intracellular Ca(2+) dynamics and, vice versa, glutamate released from astrocytes can cause a variety of responses in neurons such as: an elevation of [Ca(2+)](i), a slow inward current, an increase of excitability, modulation of synaptic transmission, synchronization of synaptic events, or some combination of these. This astrocyte-neuron signaling pathway might be a widespread phenomenon throughout the brain with astrocytes possessing the means to be active participants in many functions of the CNS. Thus, it appears that the vesicular release of glutamate can serve as a common denominator for two of the major cellular components of the CNS, astrocytes and neurons, in brain function.