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

Emerging role of astroglia in pain hypersensitivity

Recent studies suggest that astroglia, a major non-neuronal cell type in the central nervous system, actively participate in synaptic activity and potentially contribute to neurological disorders including chronic pain. Astroglia exhibit a hyperactive phenotype, also referred to as reactive astrocytosis, in response to peripheral injury. This process is often referred to as astroglial activation. By immunostaining against glial fibrillary acidic proteins, an intermediate cytoskeleton filament protein selectively localized to matured astrocytes, hypertrophy of astrocytes are clearly visible in the spinal cord dorsal horn and spinal trigeminal nucleus following a variety of injuries. Injury-related astroglial activation correlates with behavioral hyperalgesia and conversely, astroglial inhibition attenuates pain hypersensitivity. Astrocytes have a close anatomical relationship with neurons. Interactions between astrocytes and neurons contribute to the mechanisms of chronic pain. Astroglial activation is accompanied by initiation of cellular signal transduction pathways that lead to transcriptional gene regulation and release of a variety of chemical mediators or gliotransmitters, down-regulation of glutamate transporter activity that directly affects synaptic transmission, changes in gap junction proteins by which calcium waves spread, and alterations of the blood brain barrier. These coordinated changes in astroglial functions in turn modulate neuronal activity and facilitate pain transmission.

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.

Long-term potentiation depends on release of D-serine from astrocytes

Long-term potentiation (LTP) of synaptic transmission provides an experimental model for studying mechanisms of memory. The classical form of LTP relies on N-methyl-D-aspartate receptors (NMDARs), and it has been shown that astroglia can regulate their activation through Ca(2+)-dependent release of the NMDAR co-agonist D-serine. Release of D-serine from glia enables LTP in cultures and explains a correlation between glial coverage of synapses and LTP in the supraoptic nucleus. However, increases in Ca(2+) concentration in astroglia can also release other signalling molecules, most prominently glutamate, ATP and tumour necrosis factor-alpha, whereas neurons themselves can synthesize and supply D-serine. Furthermore, loading an astrocyte with exogenous Ca(2+) buffers does not suppress LTP in hippocampal area CA1 (refs 14-16), and the physiological relevance of experiments in cultures or strong exogenous stimuli applied to astrocytes has been questioned. The involvement of glia in LTP induction therefore remains controversial. Here we show that clamping internal Ca(2+) in individual CA1 astrocytes blocks LTP induction at nearby excitatory synapses by decreasing the occupancy of the NMDAR co-agonist sites. This LTP blockade can be reversed by exogenous D-serine or glycine, whereas depletion of D-serine or disruption of exocytosis in an individual astrocyte blocks local LTP. We therefore demonstrate that Ca(2+)-dependent release of D-serine from an astrocyte controls NMDAR-dependent plasticity in many thousands of excitatory synapses nearby.

Calcium homeostasis of acutely denervated and lesioned dentate gyrus in organotypic entorhino-hippocampal co-cultures

Denervation of neurons, e.g. upon traumatic injury or neuronal degeneration, induces transneuronal degenerative events, such as spine loss, dendritic pruning, and even cell loss. We studied one possible mechanism proposed to trigger such events, i.e. excess glutamate release from severed axons conveyed transsynaptically via postsynaptic calcium influx. Using 2-photon microscopical calcium imaging in organotypic entorhino-hippocampal co-cultures, we show that acute transection of the perforant path elicits two independent effects on calcium homeostasis in the dentate gyrus: a brief, short-latency elevation of postsynaptic calcium levels in denervated granule cells, which can be blocked by preincubation with tetrodotoxin, and a long-latency astroglial calcium wave, not blocked by tetrodotoxin and propagating slowly through the hippocampus. While neuronal calcium elevations upon axonal transection placed remote from the target area were similar to those elicited by brief trains of electrical stimulation of the perforant path, large-scale calcium signals were observed upon lesions placed close to or within the dendritic field of granule cells. Concordantly, induction of c-fos in denervated neurons coincided spatially with cell populations showing prolonged calcium elevations upon concomitant dendritic damage. Since denervation of dentate granule cells by remote transection of the perforant path induces transsynaptic dendritic reorganization in the utilized organotypic cultures, a generalized breakdown of the cellular calcium homeostasis is unlikely to underlie these transneuronal changes.

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.

Micropatterned substrates for studying astrocytes in culture

Recent studies of the physiological roles of astrocytes have ignited renewed interest in the functional significance of these glial cells in the central nervous system. Many of the newly discovered astrocytic functions were initially demonstrated and characterized in cell culture systems. We discuss the use of microculture techniques and micropatterning of cell-adhesive substrates in studies of astrocytic Ca²⁺ excitability and bidirectional neuron-astrocyte signaling. This culturing approach aims to reduce the level of complexity of the system by limiting the interacting partners and by controlling the localization of cells. It provides tight control over experimental conditions allowing detailed characterization of cellular functions and intercellular communication. Although such a reductionist approach yields some difference in observations between astrocytic properties in culture and in situ, general phenomena discovered in cell culture systems, however, have also been found in vivo.

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.

Pharmacological and genetic approaches to study connexin-mediated channels in glial cells of the central nervous system

This review gives an overview of connexin expression in glial cells of the central nervous system, the different modes of connexin action, including gap junctional channels and hemichannels, as well as the available methodologies to measure their activity. We summarize the strengths and limitations of current pharmacological and genetic approaches to interfere with connexin channel functions. We outline new avenues not only to study specific mechanisms by which connexins exert these functions but also to selectively investigate well-defined coupling compartments among glial networks.

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.

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.

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.

Astrocytes going live: advances and challenges

Astrocytes are one of the most numerous cell types in the CNS. They have emerged as sophisticated cells participating in a large and diverse variety of functions vital for normal brain development, adult physiology and pathology. Recent in vivo studies have provided exciting new insight into astrocyte physiology in the intact healthy brain. This review will summarize some of their most intriguing findings, discuss some of their implications, and look ahead at some of the challenges we face in studying astrocyte function in vivo.

Limited intravascular coupling in the rodent brainstem and retina supports a role for glia in regional blood flow

Regional synaptic activity induces local increases in perfusion that are coupled to upstream vasodilation and improved blood flow. In the cerebral circulation, it has been proposed that astrocytes mediate the link between the initiating stimulus and local vasodilation through propagated intracellular calcium waves. In the systemic circulation the mechanism by which local vasodilation triggers upstream alterations in blood flow involves electrotonic propagation of hyperpolarization via endothelial gap junctions, although less is known concerning the cerebral circulation. The present study aimed to investigate the extent of coupling in microvessels of the rodent brainstem and retina and the subtypes of intracellular calcium stores that might mediate astrocytic signaling. Within the brainstem, connexins (Cxs) 37 and 40 were restricted to the endothelium of pial vessels and larger penetrating arterioles, whereas astrocytic Cxs30 and 43 were found closely associated with pre- and postsynaptic neurons and nearby microvessels. Within the rat retina, Cxs37 and 40 were expressed in large radiating arterioles, but were not found in smaller vessels on the retinal surface or in the deeper retinal layers. These Cxs were absent from all retinal vessels in mice. Astrocytes, expressing Cxs30 and 43 in the rat, but only Cx43 in the mouse, were found closely associated with superficial, but not deeper blood vessels. Inositol-trisphosphate receptors (IP(3)R) 1 and 2 were expressed within brainstem astrocytes, whereas IP(3)R1 and 3 were expressed within retinal astrocytes. Limited intravascular coupling and the proximity of astrocytic networks to blood vessels supports a role for glia in activity-dependent alterations in central blood flow.

Developmental regulation of TREM2 and DAP12 expression in the murine CNS: implications for Nasu-Hakola disease

Trem2 is an orphan, DAP12 associated receptor constitutively expressed in vivo by subsets of microglia in the healthy adult murine CNS and in vitro by subsets of oligodendrocytes in neonatal mixed glial cultures. Loss of a functional Trem2 signaling pathway is the genetic cause of Nasu-Hakola disease. Whether the early onset cognitive dementia and myelin-pallor associated with this disorder are due to deficits in functional Trem2 signaling in microglia and/or oligodendrocytes is still being debated. Here, we find that Trem2/DAP12 expression is detected in embryonic day 14 CNS mRNA. Using dual immunohistochemistry/in situ hybridization, we find that both Trem2 and DAP12 expression always co-localized with markers of microglia/macrophages. However, Trem2/DAP12 positive microglia are found in very close apposition with CNP+ oligodendrocytes prior to myelination (post-natal day 1). In addition, CNS expression of TREM2 and DAP12 are not detected in PU.1KO which lack microglia and macrophages. Our data provide continuing support for Nasu-Hakola disease being identified as a cognitive disorder caused by a primary dysfunction of CNS microglia.

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.

Neuron-glia crosstalk gets serious: role in pain hypersensitivity

PURPOSE OF REVIEW

Recent studies show that peripheral injury activates both neuronal and nonneuronal or glial components of the peripheral and central cellular circuitry. The subsequent neuron-glia interactions contribute to pain hypersensitivity. This review will briefly discuss novel findings that have shed light on the cellular mechanisms of neuron-glia interactions in persistent pain.

RECENT FINDINGS

Two fundamental questions related to neuron-glia interactions in pain mechanisms have been addressed: what are the signals that lead to central glial activation after injury and how do glial cells affect central nervous system neuronal activity and promote hyperalgesia?

SUMMARY

Evidence indicates that central glial activation depends on nerve inputs from the site of injury and release of chemical mediators. Hematogenous immune cells may migrate to/infiltrate the brain and circulating inflammatory mediators may penetrate the blood-brain barrier to participate in central glial responses to injury. Inflammatory cytokines such as interleukin-1beta released from glia may facilitate pain transmission through its coupling to neuronal glutamate receptors. This bidirectional neuron-glia signaling plays a key role in glial activation, cytokine production and the initiation and maintenance of hyperalgesia. Recognition of the contribution of the mutual neuron-glia interactions to central sensitization and hyperalgesia prompts new treatment for chronic pain.

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.

Lysosomes are the major vesicular compartment undergoing Ca2+-regulated exocytosis from cortical astrocytes

Although Ca(2+)-dependent exocytosis is considered to be a pathway for gliotransmitter release from astrocytes, the structural and functional bases of this process remain controversial. We studied the relationship between near-membrane Ca(2+) elevations and the dynamics of single astroglial vesicles with styryl (FM) dyes. We show that cultured astrocytes, unlike neurons, spontaneously internalize FM dyes, resulting in the labeling of the entire acidic vesicle population within minutes. Interestingly, metabotropic glutamate receptor activation did not affect the FM labeling. Most FM-stained vesicles expressed sialin, CD63/LAMP3, and VAMP7, three markers for lysosomes and late endosomes. A subset of lysosomes underwent asynchronous exocytosis that required both Ca(2+) mobilization from intracellular stores and Ca(2+) influx across the plasma membrane. Lysosomal fusion occurred within seconds and was complete with no evidence for kiss and run. Our experiments suggest that astroglial Ca(2+)-regulated exocytosis is carried by lysosomes and operates on a timescale orders of magnitude slower than synaptic transmission.

Activity-Dependent Structural and Functional Plasticity of Astrocyte-Neuron Interactions

Observations from different brain areas have established that the adult nervous system can undergo significant experience-related structural changes throughout life. Less familiar is the notion that morphological plasticity affects not only neurons but glial cells as well. Yet there is abundant evidence showing that astrocytes, the most numerous cells in the mammalian brain, are highly mobile. Under physiological conditions as different as reproduction, sensory stimulation, and learning, they display a remarkable structural plasticity, particularly conspicuous at the level of their lamellate distal processes that normally ensheath all portions of neurons. Distal astrocytic processes can undergo morphological changes in a matter of minutes, a remodeling that modifies the geometry and diffusion properties of the extracellular space and relationships with adjacent neuronal elements, especially synapses. Astrocytes respond to neuronal activity via ion channels, neurotransmitter receptors, and transporters on their processes; they transmit information via release of neuroactive substances. Where astrocytic processes are mobile then, astrocytic-neuronal interactions become highly dynamic, a plasticity that has important functional consequences since it modifies extracellular ionic homeostasis, neurotransmission, gliotransmission, and ultimately neuronal function at the cellular and system levels. Although a complete picture of intervening cellular mechanisms is lacking, some have been identified, notably certain permissive molecular factors common to systems capable of remodeling (cell surface and extracellular matrix adhesion molecules, cytoskeletal proteins) and molecules that appear specific to each system (neuropeptides, neurotransmitters, steroids, growth factors) that trigger or reverse the morphological changes.

Two forms of astrocyte calcium excitability have distinct effects on NMDA receptor-mediated slow inward currents in pyramidal neurons

Astrocytes display excitability in the form of intracellular calcium concentration ([Ca(2+)](i)) increases, but the signaling impact of these for neurons remains debated and controversial. A key unresolved issue is whether astrocyte [Ca(2+)](i) elevations impact neurons or not. Here we report that in the CA1 region of the hippocampus, agonists of native P2Y(1) and PAR-1 receptors, which are preferentially expressed in astrocytes, equally elevated [Ca(2+)](i) levels without affecting the passive membrane properties of pyramidal neurons. However, under conditions chosen to isolate NMDA receptor responses, we found that activation of PAR-1 receptors led to the appearance of NMDA receptor-mediated slow inward currents (SICs) in pyramidal neurons. In stark contrast, activation of P2Y(1) receptors was ineffective in this regard. The PAR-1 receptor-mediated increased SICs were abolished by several strategies that selectively impaired astrocyte [Ca(2+)](i) excitability and function. Our studies therefore indicate that evoked astrocyte [Ca(2+)](i) transients are not a binary signal for interactions with neurons, and that astrocytes result in neuronal NMDA receptor-mediated SICs only when appropriately excited. The data thus provide a basis to rationalize recent contradictory data on astrocyte-neuron interactions.

Cortical layer 1 and layer 2/3 astrocytes exhibit distinct calcium dynamics in vivo

Cumulative evidence supports bidirectional interactions between astrocytes and neurons, suggesting glial involvement of neuronal information processing in the brain. Cytosolic calcium (Ca(2+)) concentration is important for astrocytes as Ca(2+) surges co-occur with gliotransmission and neurotransmitter reception. Cerebral cortex is organized in layers which are characterized by distinct cytoarchitecture. We asked if astrocyte-dominant layer 1 (L1) of the somatosensory cortex was different from layer 2/3 (L2/3) in spontaneous astrocytic Ca(2+) activity and if it was influenced by background neural activity. Using a two-photon laser scanning microscope, we compared spontaneous Ca(2+) activity of astrocytic somata and processes in L1 and L2/3 of anesthetized mature rat somatosensory cortex. We also assessed the contribution of background neural activity to the spontaneous astrocytic Ca(2+) dynamics by investigating two distinct EEG states ("synchronized" vs. "de-synchronized" states). We found that astrocytes in L1 had nearly twice higher Ca(2+) activity than L2/3. Furthermore, Ca(2+) fluctuations of processes within an astrocyte were independent in L1 while those in L2/3 were synchronous. Pharmacological blockades of metabotropic receptors for glutamate, ATP, and acetylcholine, as well as suppression of action potentials did not have a significant effect on the spontaneous somatic Ca(2+) activity. These results suggest that spontaneous astrocytic Ca(2+) surges occurred in large part intrinsically, rather than neural activity-driven. Our findings propose a new functional segregation of layer 1 and 2/3 that is defined by autonomous astrocytic activity.

ATP signalling in epilepsy

This paper focuses on a role for ATP neurotransmission and gliotransmission in the pathophysiology of epileptic seizures. ATP along with gap junctions propagates the glial calcium wave, which is an extraneuronal signalling pathway in the central nervous system. Recently astrocyte intercellular calcium waves have been shown to underlie seizures, and conventional antiepileptic drugs have been shown to attenuate these calcium waves. Blocking ATP-mediated gliotransmission, therefore, represents a potential target for antiepileptic drugs. Furthermore, while knowledge of an antiepileptic role for adenosine is not new, a recent study showed that adenosine accumulates from the hydrolysis of accumulated ATP released by astrocytes and is believed to inhibit distant synapses by acting on adenosine receptors. Such a mechanism is consistent with a surround-inhibitory mechanism whose failure would predispose to seizures. Other potential roles for ATP signalling in the initiation and spread of epileptiform discharges may involve synaptic plasticity and coordination of synaptic networks. We conclude by making speculations about future developments.

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.

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.

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.

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.

Can diffuse extrasynaptic signaling form a guiding template?

Brain functions such as information processing, learning and memory are commonly associated with changes in synaptic strength, the synaptic plasticity. Extrasynaptic diffusion of transmitters thought to mediate only a modulatory effect. Here I suggest a hypothesis that concentration profile of signaling molecules in the extracellular space can form a "diffuse guiding template" for signal propagation through neuronal network. Such template can be potentially involved in information processing and storage. This hypothesis requires further experimental investigation and, thus, provides a framework for future studies in the field of non-synaptic transmission in the brain.

Plasticity of neuron-glial interactions mediated by astrocytic EphARs

Ephrin (Eph) signaling via Eph receptors affects neuronal structure and function. We report here that exogenous ephrinAs (EphAs) induce outgrowth of filopodial processes from astrocytes within minutes in rat hippocampal slice cultures. Identical effects were induced by release of endogenous ephrinAs by cleavage of their glycosylphosphatidylinositol anchor. Reverse transcription-PCR and immunocytochemistry revealed the expression of multiple EphA receptors (EphARs) in astrocytes. Exogenous and endogenous ephrins did not induce process outgrowth from astrocytes transfected with a kinase-dead EphAR construct, indicating that the critical EphARs were located on glia. Concomitant with these morphological changes, ephrinA reduced the frequency of (S)-3,5-dihydroxyphenylglycine-evoked NMDA receptor-mediated inward currents in CA1 pyramidal cells, elicited by release of glutamate from glial cells. The sensitivity of CA1 cell synaptic or extrasynaptic NMDA receptors was unaffected by ephrinA, indicating that this effect was mediated by inhibition of glutamate release from glial cells. Finally, ephrinA application decreased the frequency and increased the duration of spontaneous oscillations of the intracellular [Ca2+] in astrocytes. We conclude that ephrinA-EphA signaling is a pluripotent regulator of neuron-astrocyte interactions mediating rapid structural and functional plasticity.

Developmental profile and properties of sulforhodamine 101--Labeled glial cells in acute brain slices of rat hippocampus

The reliable identification of astrocytes for physiological measurements was always time-consuming and difficult. Recently, the fluorescent dye sulforhodamine 101 (SR101) was reported to label cortical glial cells in vivo [Nimmerjahn A, Kirchhoff F, Kerr JN, Helmchen F. Sulforhodamine 101 as a specific marker of astroglia in the neocortex in vivo. Nat Methods 2004;1:31-7]. We adapted this technique for use in acute rat hippocampal slices at early postnatal stages (P3, 7, 15) and in young adults (P24-27) and describe a procedure for double-labeling of SR101 and ion-selective dyes. Using whole-cell patch-clamp, imaging, and immunohistochemistry, we characterized the properties of SR101-positive versus SR101-negative cells in the stratum radiatum. Our data show that SR101, in contrast to Fura-2 or SBFI, only stains a subset of glial cells. Throughout development, SR101-positive and SR101-negative cells differ in their basic membrane properties. Furthermore, SR101-positive cells undergo a developmental switch from variably rectifying to passive between P3 and P15 and lack voltage-gated Na+ currents. At P15, the majority of SR101-positive cells is positive for GFAP. Thus, our data demonstrate that SR101 selectively labels a subpopulation of glial cells in early juvenile hippocampi that shows the typical developmental changes and characteristics of classical astrocytes. Owing to its reliability and uncomplicated handling, we expect that this technique will be helpful in future investigations studying astrocytes in the developing brain.

Store-operated Ca2+ entry in astrocytes: different spatial arrangement of endoplasmic reticulum explains functional diversity in vitro and in situ

Ca(2+) signaling is the astrocyte form of excitability and the endoplasmic reticulum (ER) plays an important role as an intracellular Ca(2+) store. Since the subcellular distribution of the ER influences Ca(2+) signaling, we compared the arrangement of ER in astrocytes of hippocampus tissue and astrocytes in cell culture by electron microscopy. While the ER was usually located in close apposition to the plasma membrane in astrocytes in situ, the ER in cultured astrocytes was close to the nuclear membrane. Activation of metabotropic receptors linked to release of Ca(2+) from ER stores triggered distinct responses in cultured and in situ astrocytes. In culture, Ca(2+) signals were commonly first recorded close to the nucleus and with a delay at peripheral regions of the cells. Store-operated Ca(2+) entry (SOC) as a route to refill the Ca(2+) stores could be easily identified in cultured astrocytes as the Zn(2+)-sensitive component of the Ca(2+) signal. In contrast, such a Zn(2+)-sensitive component was not recorded in astrocytes from hippocampal slices despite of evidence for SOC. Our data indicate that both, astrocytes in situ and in vitro express SOC necessary to refill stores, but that a SOC-related signal is not recorded in the cytoplasm of astrocytes in situ since the stores are close to the plasma membrane and the refill does not affect cytoplasmic Ca(2+) levels.

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.

Astrocytes play a critical role in transient heterosynaptic depression in the rat hippocampal CA1 region

Active synapses can reduce the probability of transmitter release at neighbouring synapses. Depending on whether such heterosynaptic depression is mediated by intersynaptic diffusion of transmitter or by release of gliotransmitters, astrocytes should either hinder or promote the heterosynaptic depression. In the present study we have examined the developmental profile and astrocytic involvement in a transient heterosynaptic depression (tHeSD) in the CA1 region of the rat hippocampal slice preparation. A short stimulus burst (3 impulses at 50 Hz) to one group of synapses elicited a depression of the field EPSP evoked in another group of synapses that amounted to about 25% 0.5 s after the conditioning burst. This tHeSD was associated with an increase in the paired-pulse ratio of about 30%. The tHeSD was not present in slices from rats younger than 10 postnatal days and developed towards the adult magnitude between postnatal days 10 and 20. The tHeSD was totally prevented by the glia-specific toxin fluoroacetate (FAC), by carbenoxolone, a general blocker of connexin-based channels, and by endothelin, an endogenous peptide that has been shown to block astrocytic connexin-based channels. Antagonists to GABA(B) receptors and group II/III metabotropic glutamate receptors (mGluRs) abolished the tHeSD whereas antagonists to NMDA- and adenosine A1 receptors, and to group I mGluRs, did not affect the tHeSD. These results suggest that the tHeSD relies on GABA(B) receptors, group II/III mGluRs and on gliotransmitter release from functionally mature astrocytes.

Extracellular glutamate concentration in hippocampal slice

Synaptic glutamate transients resulting from vesicular exocytosis are superimposed on a low baseline concentration of glutamate in the extracellular space. Reported values of baseline glutamate concentrations range up to 4 microM. If glutamate were present tonically at low micromolar concentrations, many receptors, especially the high-affinity NMDA receptors (NMDARs), would be activated or desensitized, altering neuronal excitability. Using NMDARs expressed by CA1 pyramidal cells in acute hippocampal slices to monitor extracellular glutamate, we find that its baseline concentration is much lower, near 25 nM. In addition, superfusion of low micromolar concentrations of glutamate had no effect on neurons, indicating that glutamate transport prevents access to receptors within the slice. However, equipotent concentrations of NMDA, a nontransported agonist, depolarized neurons dramatically. We suggest that ambient concentrations of glutamate in vivo are also in the nanomolar range and are too low to cause significant receptor activation.

Supportive or information-processing functions of the mature protoplasmic astrocyte in the mammalian CNS? A critical appraisal

It has been proposed that astrocytes should no longer be viewed purely as support cells for neurons, such as providing a constant environment and metabolic substrates, but that they should also be viewed as being involved in affecting synaptic activity in an active way and, therefore, an integral part of the information-processing properties of the brain. This essay discusses the possible differences between a support and an instructive role, and concludes that any distinction has to be blurred. In view of this, and a brief overview of the nature of the data, the new evidence seems insufficient to conclude that the physiological roles of mature astrocytes go beyond a general support role. I propose a model of mature protoplasmic astrocyte function that is drawn from the most recent data on their structure, the domain concept and their syncytial characteristics, of an independent rather than integrative functioning of the ends of each process where the activities that affect synaptic activity and blood vessel diameter will be concentrated.

Glutamate-induced exocytosis of glutamate from astrocytes

Recent studies indicate that astrocytes can play a much more active role in neuronal circuits than previously believed, by releasing neurotransmitters such as glutamate and ATP. Here we report that local application of glutamate or glutamine synthetase inhibitors induces astrocytic release of glutamate, which activates a slowly decaying transient inward current (SIC) in CA1 pyramidal neurons and a transient inward current in astrocytes in hippocampal slices. The occurrence of SICs was accompanied by an appearance of large vesicles around the puffing pipette. The frequency of SICs was positively correlated with [glutamate]o. EM imaging of anti-glial fibrillary acid protein-labeled astrocytes showed glutamate-induced large astrocytic vesicles. Imaging of FM 1-43 fluorescence using two-photon laser scanning microscopy detected glutamate-induced formation and fusion of large vesicles identified as FM 1-43-negative structures. Fusion of large vesicles, monitored by collapse of vesicles with a high intensity FM 1-43 stain in the vesicular membrane, coincided with SICs. Glutamate induced two types of large vesicles with high and low intravesicular [Ca2+]. The high [Ca2+] vesicle plays a major role in astrocytic release of glutamate. Vesicular fusion was blocked by infusing the Ca2+ chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, or the SNARE blocker, tetanus toxin, suggesting Ca2+- and SNARE-dependent fusion. Infusion of the vesicular glutamate transport inhibitor, Rose Bengal, reduced astrocytic glutamate release, suggesting the involvement of vesicular glutamate transports in vesicular transport of glutamate. Our results demonstrate that local [glutamate]o increases induce formation and exocytotic fusion of glutamate-containing large astrocytic vesicles. These large vesicles could play important roles in the feedback control of neuronal circuits and epileptic seizures.

Defining the Role of Astrocytes in Neuromodulation

Astrocytes undergo elevations in intracellular calcium following activation of metabotropic receptors, which may trigger glutamate secretion and excitation of surrounding neurons. In this issue of Neuron, Fiacco et al. use transgenic mice that express a foreign G(q)-coupled receptor in astrocytes to show that selective stimulation of astrocytes is not sufficient to induce the release of glutamate.