Glutamate-dependent astrocyte modulation of synaptic transmission between cultured hippocampal neurons

Neurobiology of HIV

The importance of HIV cognitive impairment, including HIV associated dementia (HAD) and minor cognitive/motor disorder, has continued in the era of highly active antiretroviral therapy (HAART). Despite the relative efficacy of HAART in controlling HIV disease, there is no treatment which specifically targets the cause of HAD nor promotes neuronal protection from the effects of the virus. Much work has been done to elucidate the complex signalling pathways, effects of virus and viral proteins, and dysregulation of endogenous targets which lead to HIV associated neurotoxicity, but the concise mechanism remains elusive. It is widely accepted that the majority of viral replication in the brain occurs in monocyte derived macrophages (MDM) and microglia, and immune activation of these cells, along with astrocytic cells, may be the most important cause of neurotoxicity in the central nervous system (CNS). Additional complications arise when co-factors such as drug use, age related neuropathology, and other viruses are present. Further exploration of the molecular mechanisms leading to HIV neurotoxicity and neurodegeneration may reveal targets for prophylactic neuroprotective or other CNS-specific drugs. Given the variable success of the current HAART drugs against virus in the CNS, such therapies would greatly benefit the HIV infected population as they live longer and more productive lives.

Computational modeling of paroxysmal depolarization shifts in neurons induced by the glutamate release from astrocytes

Recent experimental studies have shown that astrocytes respond to external stimuli with a transient increase of the intracellular calcium concentration or can exhibit self-sustained spontaneous activity. Both evoked and spontaneous astrocytic calcium oscillations are accompanied by exocytosis of glutamate caged in astrocytes leading to paroxysmal depolarization shifts (PDS) in neighboring neurons. Here, we present a simple mathematical model of the interaction between astrocytes and neurons that is able to numerically reproduce the experimental results concerning the initiation of the PDS. The timing of glutamate release from the astrocyte is studied by means of a combined modeling of a vesicle cycle and the dynamics of SNARE-proteins. The neuronal slow inward currents (SICs), induced by the astrocytic glutamate and leading to PDS, are modeled via the activation of presynaptic glutamate receptors. The dependence of the bidirectional communication between neurons and astrocytes on the concentration of glutamate transporters is analyzed, as well. Our numerical results are in line with experimental findings showing that astrocyte can induce synchronous PDSs in neighboring neurons, resulting in a transient synchronous spiking activity.

Acute treatment with 17beta-estradiol attenuates astrocyte-astrocyte and astrocyte-neuron communication

Astrocytes are now recognized as dynamic signaling elements in the brain. Bidirectional communication between neurons and astrocytes involves integration of neuronal inputs by astrocytes and release of gliotransmitters that modulate neuronal excitability and synaptic transmission. The ovarian steroid hormone, 17beta-estradiol, in addition to its rapid actions on neuronal electrical activity can rapidly alter astrocyte intracellular calcium concentration ([Ca2+]i) through a membrane-associated estrogen receptor. Using calcium imaging and electrophysiological techniques, we investigated the functional consequences of acute treatment with estradiol on astrocyte-astrocyte and astrocyte-neuron communication in mixed hippocampal cultures. Mechanical stimulation of an astrocyte evoked a [Ca2+]i rise in the stimulated astrocyte, which propagated to the surrounding astrocytes as a [Ca2+]i wave. Following acute treatment with estradiol, the amplitude of the [Ca2+]i elevation in astrocytes around the stimulated astrocyte was attenuated. Further, estradiol inhibited the [Ca2+]i rise in individual astrocytes in response to the metabotropic glutamate receptor agonist, trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid. Mechanical stimulation of astrocytes induced [Ca2+]i elevations and electrophysiological responses in adjacent neurons. Estradiol rapidly attenuated the astrocyte-evoked glutamate-mediated [Ca2+]i rise and slow inward current in neurons. Also, the incidence of astrocyte-induced increase in spontaneous postsynaptic current frequency was reduced in the presence of estradiol. The effects of estradiol were stereo-specific and reversible following washout. These findings may indicate that the regulation of neuronal excitability and synaptic transmission by astrocytes is sensitive to rapid estradiol-mediated hormonal control.

Enhanced astrocytic Ca2+ signals contribute to neuronal excitotoxicity after status epilepticus

Status epilepticus (SE), an unremitting seizure, is known to cause a variety of traumatic responses including delayed neuronal death and later cognitive decline. Although excitotoxicity has been implicated in this delayed process, the cellular mechanisms are unclear. Because our previous brain slice studies have shown that chemically induced epileptiform activity can lead to elevated astrocytic Ca2+ signaling and because these signals are able to induce the release of the excitotoxic transmitter glutamate from these glia, we asked whether astrocytes are activated during status epilepticus and whether they contribute to delayed neuronal death in vivo. Using two-photon microscopy in vivo, we show that status epilepticus enhances astrocytic Ca2+ signals for 3 d and that the period of elevated glial Ca2+ signaling is correlated with the period of delayed neuronal death. To ask whether astrocytes contribute to delayed neuronal death, we first administered antagonists which inhibit gliotransmission: MPEP [2-methyl-6-(phenylethynyl)pyridine], a metabotropic glutamate receptor 5 antagonist that blocks astrocytic Ca2+ signals in vivo, and ifenprodil, an NMDA receptor antagonist that reduces the actions of glial-derived glutamate. Administration of these antagonists after SE provided significant neuronal protection raising the potential for a glial contribution to neuronal death. To test this glial hypothesis directly, we loaded Ca2+ chelators selectively into astrocytes after status epilepticus. We demonstrate that the selective attenuation of glial Ca2+ signals leads to neuronal protection. These observations support neurotoxic roles for astrocytic gliotransmission in pathological conditions and identify this process as a novel therapeutic target.

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.

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.

The hemo-neural hypothesis: on the role of blood flow in information processing

Brain vasculature is a complex and interconnected network under tight regulatory control that exists in intimate communication with neurons and glia. Typically, hemodynamics are considered to exclusively serve as a metabolic support system. In contrast to this canonical view, we propose that hemodynamics also play a role in information processing through modulation of neural activity. Functional hyperemia, the basis of the functional MRI (fMRI) BOLD signal, is a localized influx of blood correlated with neural activity levels. Functional hyperemia is considered by many to be excessive from a metabolic standpoint, but may be appropriate if interpreted as having an activity-dependent neuro-modulatory function. Hemodynamics may impact neural activity through direct and indirect mechanisms. Direct mechanisms include delivery of diffusible blood-borne messengers and mechanical and thermal modulation of neural activity. Indirect mechanisms are proposed to act through hemodynamic modulation of astrocytes, which can in turn regulate neural activity. These hemo-neural mechanisms should alter the information processing capacity of active local neural networks. Here, we focus on analysis of neocortical sensory processing. We predict that hemodynamics alter the gain of local cortical circuits, modulating the detection and discrimination of sensory stimuli. This novel view of information processing-that includes hemodynamics as an active and significant participant-has implications for understanding neural representation and the construction of accurate brain models. There are also potential medical benefits of an improved understanding of the role of hemodynamics in neural processing, as it directly bears on interpretation of and potential treatment for stroke, dementia, and epilepsy.

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

Astrocytes are considered the third component of the synapse, responding to neurotransmitter release from synaptic terminals and releasing gliotransmitters--including glutamate--in a Ca(2+)-dependent manner to affect neuronal synaptic activity. Many studies reporting astrocyte-driven neuronal activity have evoked astrocyte Ca(2+) increases by application of endogenous ligands that directly activate neuronal receptors, making astrocyte contribution to neuronal effect(s) difficult to determine. We have made transgenic mice that express a Gq-coupled receptor only in astrocytes to evoke astrocyte Ca(2+) increases using an agonist that does not bind endogenous receptors in brain. By recording from CA1 pyramidal cells in acute hippocampal slices from these mice, we demonstrate that widespread Ca(2+) elevations in 80%-90% of stratum radiatum astrocytes do not increase neuronal Ca(2+), produce neuronal slow inward currents, or affect excitatory synaptic activity. Our findings call into question the developing consensus that Ca(2+)-dependent glutamate release by astrocytes directly affects neuronal synaptic activity in situ.

Reduced CXCL12/CXCR4 results in impaired learning and is downregulated in a mouse model of Alzheimer disease

Alzheimer disease (AD) is characterized by the presence of plaques and tangles in parallel with progressive cognitive decline. The underlying cause of the cognitive decline is unknown. The purpose of this study was to identify factors that could affect learning and memory using the Tg2576 mouse model of AD. Un-biased GeneChip analysis at the time-point coinciding with the onset of behavioral deficits but prior to plaque deposition revealed that Tg2576 show altered gene expression for a number of molecules including the chemokine CXCL12. We show that this chemokine's mRNA, protein and receptor are downregulated in this mouse model coinciding with cognitive deficits. Furthermore, we demonstrate that CXCL12 levels are decreased in AD patients as compared to controls. To determine if CXCL12 might be related to impaired learning and memory, we chronically treated young non-transgenic mice with an antagonist to the CXCL12 receptor to simulate the reduction seen in transgenic animals. Treated animals showed selectively impaired learning and memory suggesting a potential role for this chemokine in cognitive functioning.

Rapid astrocyte calcium signals correlate with neuronal activity and onset of the hemodynamic response in vivo

Elevation of intracellular Ca2+ in astrocytes can influence cerebral microcirculation and modulate synaptic transmission. Recently, in vivo imaging studies identified delayed, sensory-driven Ca2+ oscillations in cortical astrocytes; however, the long latencies of these Ca2+ signals raises questions in regards to their suitability for a role in short-latency modulation of cerebral microcirculation or rapid astrocyte-to-neuron communication. Here, using in vivo two-photon Ca2+ imaging, we demonstrate that approximately 5% of sulforhodamine 101-labeled astrocytes in the hindlimb area of the mouse primary somatosensory cortex exhibit short-latency (peak amplitude approximately 0.5 s after stimulus onset), contralateral hindlimb-selective sensory-evoked Ca2+ signals that operate on a time scale similar to neuronal activity and correlate with the onset of the hemodynamic response as measured by intrinsic signal imaging. The kinetics of astrocyte Ca2+ transients were similar in rise and decay times to postsynaptic neuronal transients, but decayed more slowly than neuropil Ca2+ transients that presumably reflect presynaptic transients. These in vivo findings suggest that astrocytes can respond to sensory activity in a selective manner and process information on a subsecond time scale, enabling them to potentially form an active partnership with neurons for rapid regulation of microvascular tone and neuron-astrocyte network properties.

Involvement of glia in central sensitization in trigeminal subnucleus caudalis (medullary dorsal horn)

Central sensitization is a crucial mechanism underlying the increased excitability of nociceptive pathways following peripheral tissue injury and inflammation. We have previously demonstrated that the small-fiber excitant and inflammatory irritant mustard oil (MO) applied to the tooth pulp produces glutamatergic- and purinergic-dependent central sensitization in brainstem nociceptive neurons of trigeminal subnucleus caudalis (Vc). Recent studies have implicated both astrocytes and microglia in spinal nociceptive mechanisms, showing, for example, that inhibition of spinal astroglial metabolism or spinal microglial p38MAPK activation can attenuate hyperalgesia in inflammatory pain models but have not tested effects of glial inhibitors on central sensitization in functionally identified spinal nociceptive neurons. The aim of the present study was to determine whether glial cells are involved in the MO-induced central sensitization in Vc nociceptive neurons, by examining the effects of intrathecally applied SB203580 (SB), an inhibitor of p38MAPK, and fluoroacetate (FA), an inhibitor of the astroglial metabolic enzyme aconitase. During continuous superfusion of phosphate-buffered saline over Vc, MO application to the pulp-induced central sensitization in Vc nociceptive neurons reflected in significant increases in cutaneous mechanoreceptive field (RF) size and responses to noxious mechanical stimuli and a decrease in mechanical activation threshold. The i.t. application of SB or FA markedly attenuated the MO-induced increases in pinch RF size and responses to noxious stimuli and the decrease in activation threshold. Neither SB nor FA application significantly affected the baseline (i.e., pre-MO application) RF and response properties. These results suggest that glial metabolic processes are important in the development of Vc central sensitization.

Mechanisms of glutamate release from astrocytes

Astrocytes can release the excitatory transmitter glutamate which is capable of modulating activity in nearby neurons. This astrocytic glutamate release can occur through six known mechanisms: (i) reversal of uptake by glutamate transporters (ii) anion channel opening induced by cell swelling, (iii) Ca2+-dependent exocytosis, (iv) glutamate exchange via the cystine-glutamate antiporter, (v) release through ionotropic purinergic receptors and (vi) functional unpaired connexons, "hemichannels", on the cell surface. Although these various pathways have been defined, it is not clear how often and to what extent astrocytes employ different mechanisms. It will be necessary to determine whether the same glutamate release mechanisms that operate under physiological conditions operate during pathological conditions or whether there are specific release mechanisms that operate under particular conditions.

A novel neurotransmitter-independent communication pathway between axons and glial cells

Recent studies have provided evidence that transmitters released by neurons can activate glial receptors and stimulate calcium signalling in glial cells. Glial calcium signalling, in turn, may affect neuronal performance such as long-term changes in synaptic efficacy. Olfactory ensheathing cells (OECs) are a special glial cell type in vertebrates and insects and promote axon growth in the developing and mature nervous system. Physiological properties of OECs, however, have not been studied so far in detail. We measured changes in the calcium concentration in OECs of the moth Manduca sexta, in situ and in vivo. Electrical stimulation of olfactory receptor neurons in pupae or odour stimulation of receptor neurons in adults resulted in calcium transients in OECs. Olfactory receptor axons release acetylcholine; however, application of acetylcholine or other transmitters such as glutamate, GABA or nitric oxide did not induce calcium transients in OECs. Upon nerve stimulation, extracellular potassium rose by several millimolar as measured with potassium-sensitive microelectrodes. When potassium in the perfusion saline was increased from 4 to 10 mM or higher, voltage-dependent calcium transients in OECs that resembled stimulation-induced calcium transients were evoked. Blocking neuronal potassium channels with TEA reduced both the stimulation-induced increases in extracellular potassium and the calcium transients in OECs, whereas calcium transients in receptor axons were augmented. Our results show for the first time that accumulation of potassium, released by electrically active axons, is sufficient to evoke voltage-dependent calcium influx into glial cells, whereas neurotransmitters appear not to be involved in this neuron-glia communication in Manduca.

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.

Glutamatergic functions of primary afferent neurons with special emphasis on vagal afferents

Glutamate has been identified as the main transmitter of primary afferent neurons. This was established based on biochemical, electrophysiological, and immunohistochemical data from studies on glutamatergic receptors and their agonists/antagonists. The availability of specific antibodies directed against glutamate and, more recently, vesicular glutamate transporters corroborated this and led to significant new discoveries. In particular, peripheral endings of various classes of afferents contain vesicular glutamate transporters, suggesting vesicular storage in and exocytotic release of glutamate from peripheral afferent endings. This suggests that autocrine mechanisms regulate sensory transduction processes. However, glutamate release from peripheral sensory terminals could also enable afferent neurons to influence various cells associated with them. This may be particularly relevant for vagal intraganglionic laminar endings, which could represent glutamatergic sensor-effector components of intramural reflex arcs in the gastrointestinal tract. Thus, morphological analysis of the relationships of putative glutamatergic primary afferents with associated tissues may direct forthcoming studies on their functions.

The astrocyte as a gatekeeper of synaptic information transfer

We present a simple biophysical model for the coupling between synaptic transmission and the local calcium concentration on an enveloping astrocytic domain. This interaction enables the astrocyte to modulate the information flow from presynaptic to postsynaptic cells in a manner dependent on previous activity at this and other nearby synapses. Our model suggests a novel, testable hypothesis for the spike timing statistics measured for rapidly firing cells in culture experiments.

Motor learning induces astrocytic hypertrophy in the cerebellar cortex

Motor skill learning, but not mere motor activity, is associated with an increase in both synapse number and glial cell volume within the cerebellar cortex. The increase in synapse number has been shown to persist for at least 4 weeks in the absence of continued training. The present experiment similarly examined how a prolonged interruption in training affects the training-induced increase in astrocytic volume. Adult female rats were randomly allocated to either an acrobatic motor learning condition (AC) or a motor control condition (MC). The AC animals were trained to traverse a complex series of obstacles and each AC animal was pair matched with an MC animal that traversed an obstacle-free runway. These groups were further assigned to one of three training conditions. Animals in the early condition were trained for 10 consecutive days, animals in the delay condition received the same 10 days of training followed by a 28-day period without training, and animals in the continuous condition were trained for the entire 38 days. Unbiased stereological techniques were used to determine that AC animals had a significantly greater volume of astrocytes per Purkinje cell in the cerebellar paramedian lobule than the MC animals, a difference which was reduced (and not statistically detectable) among animals in the delay condition. These findings demonstrate that learning triggers the hypertrophy of astrocytic processes and furthermore that, unlike learning-induced synaptogenesis, astrocytic growth is reduced in the absence of continued training.

Modeling synaptic transmission of the tripartite synapse

The tripartite synapse denotes the junction of a pre- and postsynaptic neuron modulated by a synaptic astrocyte. Enhanced transmission probability and frequency of the postsynaptic current-events are among the significant effects of the astrocyte on the synapse as experimentally characterized by several groups. In this paper we provide a mathematical framework for the relevant synaptic interactions between neurons and astrocytes that can account quantitatively for both the astrocytic effects on the synaptic transmission and the spontaneous postsynaptic events. Inferred from experiments, the model assumes that glutamate released by the astrocytes in response to synaptic activity regulates store-operated calcium in the presynaptic terminal. This source of calcium is distinct from voltage-gated calcium influx and accounts for the long timescale of facilitation at the synapse seen in correlation with calcium activity in the astrocytes. Our model predicts the inter-event interval distribution of spontaneous current activity mediated by a synaptic astrocyte and provides an additional insight into a novel mechanism for plasticity in which a low fidelity synapse gets transformed into a high fidelity synapse via astrocytic coupling.

Role of astrocytes in matching blood flow to neuronal activity

The brain is critically dependent on oxygen and glucose supply for normal function. Various neurovascular control mechanisms assure that the blood supply of the brain is adequate to meet the energy needs of its components. Emerging evidence shows that neuronal activity can control microcirculation using astrocytes as a mediator. Astrocytes can sense neuronal activity and are involved in signal transmission. Synaptic activity triggers an increase in the intracellular calcium concentration [Ca(2+)]i of adjacent astrocytes, stimulating the release of adenosine triphosphate (ATP) and glutamate. The released ATP mediates the propagation of Ca(2+) waves between neighboring astrocytes, thereby recruiting them to mediate adequate cerebrovascular response to neuronal activation. Simultaneously, sodium-dependent glutamate uptake in astrocytes generates Na(+) waves and subsequently increases glucose uptake and metabolism that leads to the formation of lactate, which is then delivered to neurons as an energy substrate. Further, astrocytic Ca(2+) elevations can lead to secretion of vasodilatory substances from perivascular endfeet, such as epoxyeicosatrienoic acid (EETs), adenosine, nitric oxide (NO), and cyclooxygenase-2 (COX-2) metabolites, resulting in increased local blood flow. Thus, astrocytes by releasing vasoactive molecules mediate the neuron-astrocyte-endothelial signaling pathway and play a profound role in coupling blood flow to neuronal activity.

Astrocyte calcium elevations: properties, propagation, and effects on brain signaling

The possibility that astrocytes are involved in brain signaling began to emerge in the late 1970s, when it was first shown that astroglia in vitro possess numerous receptors for neurotransmitters. It was later demonstrated that cultured astroglia and astrocytes in situ respond to neurotransmitters with increases in intracellular second messengers, including cyclic AMP and calcium. Astrocyte calcium responses have since been extensively studied both in culture and in intact tissue. We continue to gather information regarding the various compounds able to trigger astrocyte calcium increases, as well as the mechanisms involved in their initiation, propagation as a calcium wave within and between astrocytes, and effects on signaling within the brain. This review will focus on each of these aspects of astrocyte calcium regulation, and attempt to sort out which effects are more likely to occur in developmental, pathological, and physiological conditions. While we have come far in our understanding of the properties or potential of astrocytes' ability to signal to neurons using our array of pharmacological tools, we still understand very little regarding the level of involvement of astrocyte signaling in normal brain physiology.

Astrocyte calcium waves: what they are and what they do

Several lines of evidence indicate that the elaborated calcium signals and the occurrence of calcium waves in astrocytes provide these cells with a specific form of excitability. The identification of the cellular and molecular steps involved in the triggering and transmission of Ca(2+) waves between astrocytes resulted in the identification of two pathways mediating this form of intercellular communication. One of them involves the direct communication between the cytosols of two adjoining cells through gap junction channels, while the other depends upon the release of "gliotransmitters" that activates membrane receptors on neighboring cells. In this review we summarize evidence in favor of these two mechanisms of Ca(2+) wave transmission and we discuss that they may not be mutually exclusive, but are likely to work in conjunction to coordinate the activity of a group of cells. To address a key question regarding the functional consequences following the passage of a Ca(2+) wave, we list, in this review, some of the potential intracellular targets of these Ca(2+) transients in astrocytes, and discuss the functional consequences of the activation of these targets for the interactions that astrocytes maintain with themselves and with other cellular partners, including those at the glial/vasculature interface and at perisynaptic sites where astrocytic processes tightly interact with neurons.

Vesicular transmitter release from astrocytes

Astrocytes can release a variety of transmitters, including glutamate and ATP, in response to stimuli that induce increases in intracellular Ca(2+) levels. This release occurs via a regulated, exocytotic pathway. As evidence of this, astrocytes express protein components of the vesicular secretory apparatus, including synaptobrevin 2, syntaxin, and SNAP-23. Additionally, astrocytes possess vesicular organelles, the essential morphological elements required for regulated Ca(2+)-dependent transmitter release. The location of specific exocytotic sites on these cells, however, remains to be unequivocally determined.

Glutamate and GABA receptors and transporters in the basal ganglia: what does their subsynaptic localization reveal about their function?

GABA and glutamate, the main transmitters in the basal ganglia, exert their effects through ionotropic and metabotropic receptors. The dynamic activation of these receptors in response to released neurotransmitter depends, among other factors, on their precise localization in relation to corresponding synapses. The use of high resolution quantitative electron microscope immunocytochemical techniques has provided in-depth description of the subcellular and subsynaptic localization of these receptors in the CNS. In this article, we review recent findings on the ultrastructural localization of GABA and glutamate receptors and transporters in monkey and rat basal ganglia, at synaptic, extrasynaptic and presynaptic sites. The anatomical evidence supports numerous potential locations for receptor-neurotransmitter interactions, and raises important questions regarding mechanisms of activation and function of synaptic versus extrasynaptic receptors in the basal ganglia.

Neuronal-glial networks as substrate for CNS integration

Astrocytes have been considered, for a long time, as the support and house-keeping cells of the nervous system. Indeed, the astrocytes play very important metabolic roles in the brain, but the catalogue of nervous system functions or activities that involve directly glial participation has extended dramatically in the last decade. In addition to the further refining of the signalling capacity of the neuroglial networks and the detailed reassessment of the interactions between glia and vascular bed in the brain, one of the important salient features of the increased glioscience activity in the last few years was the morphological and functional demonstration that protoplasmic astrocytes occupy well defined spatial territories, with only limited areas of morphological overlapping, but still able to communicate with adjacent neighbours through intercellular junctions. All these features form the basis for a possible reassessment of the nature of integration of activity in the central nervous system that could raise glia to a role of central integrator.

Glutamate release from astrocytes is stimulated via the appearance of exocytosis during cyclic AMP-induced morphologic changes

Recent studies have shown that astrocytes release various transmitters including glutamate and thus directly affect synaptic neurotransmission. The mechanisms involved in the release of glutamate from astrocytes remain unclear, however. In the present study, we examined differences in 1) the amount of glutamate released, 2) the appearance of exocytosis, and 3) the expression of SNARE (soluble N-ethylmaleimide sensitive fusion protein attachment protein receptor) proteins between cyclic AMP-treated and non-treated astrocytes in culture. Extracellular glutamate was detected in the recording solution of cyclic AMP-treated astrocytes after stimulation with ATP by high-performance liquid chromatography and NADH imaging. Exocytosis, which was observed by FM1-43 imaging, appeared in cyclic AMP-treated astrocytes in a punctiform fashion, but not in non-treated cells, after stimulation with ATP and glutamate. Immunocytochemistry and Western blotting showed that the amount of SNARE proteins increased during cAMP-induced morphologic changes, and in particular, a v-SNARE, synaptobrevin, appeared as punctiform staining in the cytosol of cyclic AMP-treated astrocytes. These findings show that astrocytes acquire SNARE proteins during cyclic AMP-induced differentiation, and suggest that glutamate is released by exocytosis in cyclic AMP-treated astrocytes in response to ATP released from neighboring neurons and astrocytes.

Neuropathologic and neuroinflammatory activities of HIV-1-infected human astrocytes in murine brain

The balance between astrocyte and microglia neuroprotection and neurotoxicity defines the tempo of neuronal dysfunction during HIV-1-associated dementia (HAD). Astrocytes maintain brain homeostasis and respond actively to brain damage by providing functional and nutritive neuronal support. In HAD, low-level, continuous infection of astrocytes occurs, but the functional consequences of this infection are poorly understood. To this end, human fetal astrocytes (HFA) and monocyte-derived macrophages (MDM) were infected with HIV-1DJV and HIV-1NL4-3 (neurotropic and lymphotropic strains respectively) and a pseudotyped Vesicular Stomatitis Virus (VSV/HIV-1NL4-3) prior to intracranial injection into the basal ganglia of severe combined immunodeficient mice. Neuropathological and immunohistochemical comparisons for inflammatory and neurotoxic activities were performed amongst the infected cell types at 7 or 14 days. HIV-1-infected MDM induced significant increases in Mac-1, glial fibrillary acidic protein, ionized calcium-binding adapter molecule 1, and proinflammatory cytokine RNA and/or protein expression when compared with HSV/HIV-1- and HIV-1-infected HFA and sham-operated mice. Levels of neuron-specific nuclear protein, microtubule-associated protein 2, and neurofilament antigens were reduced significantly in the brain regions injected with human MDM infected with HIV-1DJV or VSV/HIV-1. We conclude that HIV-1 infection of astrocytes leads to limited neurodegeneration, underscoring the early and active role of macrophage-driven neurotoxicity in disease.

Oestradiol up-regulates glutamine synthetase mRNA and protein expression in the hypothalamus and hippocampus: implications for a role of hormonally responsive glia in amino acid neurotransmission

Rapidly emerging evidence suggests that glial cells in the central nervous system are sensitive to oestrogen actions. However, the functional consequences of the cellular mechanisms of these cells have proven difficult to study in vivo because of the intimate relationships between neurones and glia. Microarray technology offers the potential to uncover steroid hormone regulation of glial-specific genes that may play a role in hormone-dependent neuronal-glial interactions. Analysis of transcriptomes from the medial basal hypothalamus (MBH) of oestradiol and vehicle-treated adult ovariectomised mice revealed an up-regulation of several glial specific genes by oestradiol, including glutamine synthetase (GS), which facilitates the conversion of glutamate to glutamine and plays an integral role in amino acid neurotransmission. In situ hybridisation confirmed that oestradiol treatment resulted in an up-regulation of GS gene expression in the arcuate and ventromedial nuclei of the MBH, as well as the medial amygdala and hippocampus. Moreover, oestradiol increased protein expression of GS in both the MBH and hippocampus. Neurones are incapable of de novo net synthesis of glutamate from glucose and are dependent on glial-provided precursors such as glutamine to renew their amino acid transmitter pools. Thus, oestradiol induced expression of GS suggests a significant role for glial cells in hormonal modulation of glutamatergic neurotransmission important to female reproductive behaviours, neuroendocrine physiology and cognitive functions.

Astrocyte control of synaptic transmission and neurovascular coupling

From a structural perspective, the predominant glial cell of the central nervous system, the astrocyte, is positioned to regulate synaptic transmission and neurovascular coupling: the processes of one astrocyte contact tens of thousands of synapses, while other processes of the same cell form endfeet on capillaries and arterioles. The application of subcellular imaging of Ca2+ signaling to astrocytes now provides functional data to support this structural notion. Astrocytes express receptors for many neurotransmitters, and their activation leads to oscillations in internal Ca2+. These oscillations induce the accumulation of arachidonic acid and the release of the chemical transmitters glutamate, d-serine, and ATP. Ca2+ oscillations in astrocytic endfeet can control cerebral microcirculation through the arachidonic acid metabolites prostaglandin E2 and epoxyeicosatrienoic acids that induce arteriole dilation, and 20-HETE that induces arteriole constriction. In addition to actions on the vasculature, the release of chemical transmitters from astrocytes regulates neuronal function. Astrocyte-derived glutamate, which preferentially acts on extrasynaptic receptors, can promote neuronal synchrony, enhance neuronal excitability, and modulate synaptic transmission. Astrocyte-derived d-serine, by acting on the glycine-binding site of the N-methyl-d-aspartate receptor, can modulate synaptic plasticity. Astrocyte-derived ATP, which is hydrolyzed to adenosine in the extracellular space, has inhibitory actions and mediates synaptic cross-talk underlying heterosynaptic depression. Now that we appreciate this range of actions of astrocytic signaling, some of the immediate challenges are to determine how the astrocyte regulates neuronal integration and how both excitatory (glutamate) and inhibitory signals (adenosine) provided by the same glial cell act in concert to regulate neuronal function.

Glutamate release from astrocytes as a non-synaptic mechanism for neuronal synchronization in the hippocampus

Synchronization of activity of anatomically distributed groups of neurons represents a fundamental event in the processing of information in the brain. While this phenomenon is believed to result from dynamic interactions within the neuronal circuitry, how exactly populations of neurons become synchronized remains largely to be clarified. We propose that astrocytes are directly involved in the generation of neuronal synchrony in the hippocampus. By using a combination of experimental approaches in hippocampal slice preparations, including patch-clamp recordings and confocal microscopy calcium imaging, we studied the effect on CA1 pyramidal neurons of glutamate released from astrocytes upon various stimuli that trigger Ca2+ elevations in these glial cells, including Schaffer collateral stimulation. We found that astrocytic glutamate evokes synchronous, slow inward currents (SICs) and Ca2+ elevations in CA1 pyramidal neurons by acting preferentially, if not exclusively, on extrasynaptic NMDA receptors. Due to desensitization, AMPA receptors were not activated by astrocytic glutamate unless cyclothiazide was present. In the virtual absence of extracellular Mg2+, glutamate released from astrocytes was found to evoke, in paired recordings, highly synchronous SICs from two CA1 pyramidal neurons and, in Ca2+ imaging experiments, Ca2+ elevations that occurred synchronously in domains composed of 2-12 CA1 neurons. In the presence of extracellular Mg2+ (1 mM), synchronous SICs in two neurons as well as synchronous Ca2+ elevations in neuronal domains were still observed, although with a reduced frequency. Our results reveal a functional link between astrocytic glutamate and extrasynaptic NMDA receptors that contributes to the overall dynamics of neuronal synchrony. Our observations also raise a series of questions on possible roles of this astrocyte-to-neuron signaling in pathological changes in the hippocampus such as excitotoxic neuronal damage or the generation of epileptiform activity.

Patching the glia reveals the functional organisation of the brain

The neuroglia was initially conceived by Rudolf Virchow as a non-cellular connective tissue holding neurones together. In 1894, Carl Ludwig Schleich proposed a hypothesis of fully integrated and interconnected neuronal-glial circuits as a substrate for brain function. This hypothesis received direct experimental support only hundred years later, after several physiological techniques, and most notably the patch-clamp method, were applied to glial cells. These experiments have demonstrated the existence of active and bi-directional neuronal-glial communications, integrating neuronal networks and glial syncytium into one functional circuit. The data accumulated during last 15 years prompt rethinking of the neuronal doctrine towards more inclusive concept, which regards both neurones and glia as equally responsible for information processing in the brain.

GABAergic network activation of glial cells underlies hippocampal heterosynaptic depression

Tetanus-induced heterosynaptic depression in the hippocampus is a key cellular mechanism in neural networks implicated in learning and memory. A growing body of evidence indicates that glial cells are important modulators of synaptic functions, but very little is known about their role in heterosynaptic plasticity. We examined the role of glial cells in heterosynaptic depression, knowing that tetanization and NMDA application caused depression of synaptic field responses (fEPSPs) and induced Ca2+ rise in glial cells. Here we report that chelating Ca2+ in a glial syncytium interfered with heterosynaptic depression and NMDA-induced fEPSP depression, suggesting that Ca2+ activation of glial cells is necessary for heterosynaptic depression. The NMDA-induced Ca2+ rise in glial cells was sensitive to tetrodotoxin and reduced by the GABAB antagonist CGP55845. Both heterosynaptic depression and simultaneous Ca2+ activation of glial cells were prevented by CGP55845, suggesting an involvement of the GABAergic network in glial activation and heterosynaptic depression. Also, the GABAB agonist baclofen caused both a Ca2+ rise in glial cells and fEPSP depression. Heterosynaptic depression, as well as NMDA- and baclofen-induced depression, were attenuated by an A1 antagonist, cyclopentyl-theophylline, whereas glial cell activation was not, indicating a role of adenosine downstream of glial activation. Finally, heterosynaptic depression requires ATP degradation because ectonucleotidase inhibitors reduced this plasticity. Our work indicates that Ca2+ activation of glial cells is necessary for heterosynaptic depression, which involves the sequential interaction of Schaffer collaterals, the GABAergic network, and glia. Thus, glial and neuronal networks are functionally associated during the genesis of heterosynaptic plasticity at mammalian central excitatory synapses.

Astrocytes Coordinate Synaptic Networks: Balanced Excitation and Inhibition

Although neurons are essential for brain function, an emerging alternative view holds that astrocytes, the dominant glial cell type, coordinate synaptic networks. Through the release of glutamate, astrocytes locally excite neurons, and via adenosine, which accumulates due to the hydrolysis of released ATP, astrocytes suppress distant synapses.

Innervation of the mammalian esophagus

Understanding the innervation of the esophagus is a prerequisite for successful treatment of a variety of disorders, e.g., dysphagia, achalasia, gastroesophageal reflux disease (GERD) and non-cardiac chest pain. Although, at first glance, functions of the esophagus are relatively simple, their neuronal control is considerably complex. Vagal motor neurons of the nucleus ambiguus and preganglionic neurons of the dorsal motor nucleus innervate striated and smooth muscle, respectively. Myenteric neurons represent the interface between the dorsal motor nucleus and smooth muscle but they are also involved in striated muscle innervation. Intraganglionic laminar endings (IGLEs) represent mechanosensory vagal afferent terminals. They also establish intricate connections with enteric neurons. Afferent information is implemented by the swallowing central pattern generator in the brainstem, which generates and coordinates deglutitive activity in both striated and smooth esophageal muscle and orchestrates esophageal sphincters as well as gastric adaptive relaxation. Disturbed excitation/inhibition balance in the lower esophageal sphincter results in motility disorders, e.g., achalasia and GERD. Loss of mechanosensory afferents disrupts adaptation of deglutitive motor programs to bolus variables, eventually leading to megaesophagus. Both spinal and vagal afferents appear to contribute to painful sensations, e.g., non-cardiac chest pain. Extrinsic and intrinsic neurons may be involved in intramural reflexes using acetylcholine, nitric oxide, substance P, CGRP and glutamate as main transmitters. In addition, other molecules, e.g., ATP, GABA and probably also inflammatory cytokines, may modulate these neuronal functions.

Properties of synaptically evoked astrocyte calcium signal reveal synaptic information processing by astrocytes

The synaptic control of the astrocytic intracellular Ca2+ is crucial in the reciprocal astrocyte-neuron communication. Using electrophysiological and Ca2+ imaging techniques in rat hippocampal slices, we investigated the astrocytic Ca2+ signal modulation induced by synaptic terminals that use glutamate and acetylcholine. Ca2+ elevations were evoked by glutamate released from Schaffer collaterals and by acetylcholine, but not glutamate, released by alveus stimulation, indicating that astrocytes discriminate the activity of different synapses belonging to different axon pathways. The Ca2+ signal was modulated bidirectionally by simultaneous activation of both pathways, being depressed at high stimulation frequencies and enhanced at low frequencies. The Ca2+ modulation was attributable to astrocytic intrinsic properties, occurred at discrete regions of the processes, and controlled the intracellular expansion of the Ca2+ signal. In turn, astrocyte Ca2+ signal elicited NMDA receptor-mediated currents in pyramidal neurons. Therefore, because astrocytes discriminate and integrate synaptic information, we propose that they can be considered as cellular elements involved in the information processing by the nervous system.

alpha1-Adrenergic modulation of synaptic input to Purkinje neurons in rat cerebellar brain slices

The inhibitory activity in the cerebellar network, as investigated in acute brain slices from 14-20 days old rats, is modulated by alpha1-adrenergic stimulation. The specific alpha1-adrenoceptor agonist phenylephrine (PhE; 10 microM) or the alpha-adrenoceptor agonist 6-fluoronoradrenaline (10 microM) increases the frequency and the amplitude of spontaneous postsynaptic currents (sPSC) in Purkinje neurons. The effects are sensitive to the alpha1-adrenoceptor antagonists prazosin (30 microM) and phentolamine (10 microM). The PhE-induced augmentation is suppressed when phospholipase C is blocked by preincubation with U73122 (10 microM) but is not affected by inhibition of protein kinases with H7 (10 microM) or GF109203X (10 microM). Involvement of intracellular Ca(2+) stores was shown by a reduced PhE effect after blocking of SERCA pumps with cyclopiazonic acid (30 microM) and thapsigargin (1 microM). The persistence of the PhE effect on the frequency of miniature postsynaptic currents, as recorded in presence of tetrodotoxin, indicates a presynaptic localization of the alpha1-adrenoceptors. A block of voltage-gated Ca(2+) channels with nifedipine, verapamil, or omega-conotoxin MVIIC did not suppress the PhE-induced increase of the frequency and amplitude of sPSC. The results suggest that alpha1-adrenoceptors at presynaptic terminals mediate an increase of the spontaneous synaptic inhibition of Purkinje neurons in the cerebellar cortex via release of Ca(2+) from intracellular stores.

Glial cells in synaptic plasticity

Plasticity of synaptic transmission is believed to be the cellular basis for learning and memory, and depends upon different pre- and post-synaptic neuronal mechanisms. Recently, however, an increasing number of studies have implicated a third element in plasticity; the perisynaptic glial cell. Originally glial cells were thought to be important for metabolic maintenance and support of the nervous system. However, work in the past decade has clearly demonstrated active involvement of glia in stability and overall nervous system function as well as synaptic plasticity. Through specific modulation of glial cell function, a wide variety of roles for glia in synaptic plasticity have been uncovered. Furthermore, interesting circumstantial evidence suggests a glial involvement in multiple other types of plasticity. We will discuss recent advances in neuron-glial interactions that take place during synaptic plasticity and explore different plasticity phenomena in which glial cells may be involved.

Synaptic information processing by astrocytes

Glial cells were classically considered as supportive cells that do not contribute to information processing in the nervous system. However, considerable amount of evidence obtained by several groups during the last few years has demonstrated the existence of a bidirectional communication between astrocytes and neurons, which prompted a re-examination of the role of glial cells in the physiology of the nervous system. This review will discuss recent advances in the neuron-to-astrocyte communication, focusing on the recently reported properties of the synaptically evoked astrocyte Ca2+ signal that indicate that astrocytes show integrative properties for synaptic information processing. Indeed, we have recently shown that hippocampal astrocytes discriminate between the activity of different synapses, and respond selectively to different axon pathways. Furthermore, the astrocyte Ca2+ signal is modulated by the simultaneous activity of different synaptic inputs. This Ca2+ signal modulation depends on cellular intrinsic properties of the astrocytes, is bidirectionally regulated by the level of synaptic activity, and controls the spatial extension of the intracellular Ca2+ signal. Consequently, we propose that astrocytes can be considered as cellular elements involved in information processing by the nervous system.

Glia re-sealed particles freshly prepared from adult rat brain are competent for exocytotic release of glutamate

Glial subcellular re-sealed particles (referred to as gliosomes here) were purified from rat cerebral cortex and investigated for their ability to release glutamate. Confocal microscopy showed that the glia-specific proteins glial fibrillary acidic protein (GFAP) and S-100, but not the neuronal proteins 95-kDa postsynaptic density protein (PSD-95), microtubule-associated protein 2 (MAP-2) and beta-tubulin III, were enriched in purified gliosomes. Furthermore, gliosomes exhibited labelling neither for integrin-alphaM nor for myelin basic protein, which are specific for microglia and oligodendrocytes respectively. The Ca2+ ionophore ionomycin (0.1-5 microm) efficiently stimulated the release of tritium from gliosomes pre-labelled with [3H]d-aspartate and of endogenous glutamate in a Ca(2+)-dependent and bafilomycin A1-sensitive manner, suggesting the involvement of an exocytotic process. Accordingly, ionomycin was found to induce a Ca(2+)-dependent increase in the vesicular fusion rate, when exocytosis was monitored with acridine orange. ATP stimulated [3H]d-aspartate release in a concentration- (0.1-3 mm) and Ca(2+)-dependent manner. The gliosomal fraction contained proteins of the exocytotic machinery [syntaxin-1, vesicular-associated membrane protein type 2 (VAMP-2), 23-kDa synaptosome-associated protein (SNAP-23) and 25-kDa synaptosome-associated protein (SNAP-25)] co-existing with GFAP immunoreactivity. Moreover, GFAP or VAMP-2 co-expressed with the vesicular glutamate transporter type 1. Consistent with ultrastructural analysis, several approximately 30-nm non-clustered vesicles were present in the gliosome cytoplasm. It is concluded that gliosomes purified from adult brain contain glutamate-accumulating vesicles and can release the amino acid by a process resembling neuronal exocytosis.

A role for glutamate and glia in the fast network oscillations preceding spreading depression

The mechanism of the propagation of spreading depression is unclear. Classical theories proposed a self-maintained cycle fed by elevated potassium and/or glutamate in the extracellular space. Earlier we found in vivo a characteristic oscillatory field activity that is synchronous in a strip of tissue ahead of the oncoming wave of neuron depolarization and that occurs before the extracellular potassium level begins to rise [Herreras O, Largo C, Ibarz JM, Somjen GG, Marrín del Río R (1994) Role of neuronal synchronizing mechanisms in the propagation of spreading depression in the in vivo hippocampus. J Neurosci 14:7087-7098]. We investigated here the possible participation of glutamate and the role of glia in the prodromal field oscillations using extra and intracellular recordings and pharmacological manipulations in rat hippocampal slices. As earlier shown in vivo, field oscillations propagated ahead of the negative potential shift covering distances of up to 1 mm. The oscillatory prodromals were initially subthreshold but then each wave became crowned by a population spike. The frequency of the oscillatory prodromals was variable among slices (80-115 Hz), but constant in individual slices. The blockade of ionotropic glutamate receptors decreased the frequency of prodromal oscillations, retarded spreading depression propagation, and shortened the duration of depolarization. Blocking the glutamate membrane transport increased the oscillatory frequency. The selective metabolic poisoning of astrocytes led to gradual disorganization of prodromal oscillations whose frequency first increased and then decreased. Also, the amplitude of the population spikes within the burst diminished as individual cells fired fewer action potentials, although still phase-locked with population spikes. The effects of glial metabolic impairment were observed within the period when neuron electrical properties were still normal, and were blocked by glutamate receptor antagonists. These data suggest that glutamate released from glial cells and possibly also from neurons has a role in the generation of oscillations and neuron firing synchronization that precede the spreading depression-related depolarization, but additional mechanisms are required to fully explain the onset and propagation of spreading depression.

Localization of glutamate receptors in developing cortical neurons in culture and relationship to susceptibility to excitotoxicity

Overactivation of glutamate receptors leading to excitotoxicity has been implicated in the neurodegenerative alterations of a range of central nervous system (CNS) disorders. We have investigated the cell-type-specific changes in glutamate receptor localization in developing cortical neurons in culture, as well as the relationship between glutamate receptor subunit distribution with synapse formation and susceptibility to excitotoxicity. Glutamate receptor subunit clustering was present prior to the formation of synapses. However, different receptor types showed distinctive temporal patterns of subunit clustering, localization to spines, and apposition to presynaptic terminals. N-methyl-D-aspartate (NMDA) receptor subunit immunolabelling was present in puncta along dendrites prior to the formation of synapses, with relatively little localization to spines. Vulnerability to NMDA receptor-mediated excitotoxicity occurred before receptor subunits became localized in apposition to presynaptic terminals. Clustering of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors occurred concurrently with development of vulnerability to excitotoxicity and was related to localization of AMPA receptors at synapses and in spines. Different AMPA receptor subunits demonstrated cell-type-specific localization as well as distribution to spines, dendrites, and extrasynaptic subunit clusters. A subclass of neurons demonstrated substantial perineuronal synaptic innervation, and these neurons expressed relatively high levels of GluR1 and/or GluR4 at receptor puncta, indicating the presence of calcium-permeable AMPA receptors and suggesting alternative synaptic signalling mechanisms and vulnerability to excitotoxicity. These data demonstrate the relationship between glutamate receptor subunit expression and localization with synaptogenesis and development of neuronal susceptibility to excitotoxicity. These data also suggest that excitotoxicity can be mediated through extrasynaptic receptor subunit complexes along dendrites.

Calcium signaling in Schwann cells at synaptic and extra-synaptic sites: Active glial modulation of neuronal activity

Glial cells are widely dispersed in the central nervous system (CNS) as well as in the peripheral nervous system (PNS). In the PNS, perisynaptic Schwann cells (PSCs) are the glial cells associated with the pre- and postsynaptic elements of the neuromuscular junction (NMJ). They, as other glial cells of the CNS, respond to high-frequency motor nerve stimulation with an increase in intracellular Ca(2+). In addition to detecting and responding to neurotransmission, PSCs are involved in short-term plasticity events where they depress neurotransmission through G-protein-dependent mechanisms and potentiate synaptic activity via Ca(2+)-dependent mechanisms. In this review, we will discuss evidence that outlines the role of PSCs in short- and long-term modulation of synaptic activity. We will also emphasize present functional similarities and differences in PSC activation at different NMJs. The importance of glial-neural interactions along myelinating axons will also be discussed.

Glutamate-induced glutamate release: a proposed mechanism for calcium bursting in astrocytes

Here we present a new model for the generation of complex calcium-bursting patterns in astrocytes, a type of brain cell recently implicated in a variety of neural functions including memory formation. The model involves two positive feedback processes, in which the key feedback species are calcium ion and glutamate. The latter is the most abundant excitatory neurotransmitter in the brain and has been shown to be involved in bidirectional communication between astrocytes and nearby neurons. The glutamate feedback process considered here is shown to be critical for the generation of complex bursting oscillations in the astrocytes and to, perhaps, code for information which may be passed from neuron to neuron via the astrocyte. These processes may be involved in memory storage and formation as well as in mechanisms which lead to dynamical diseases such as epilepsy.

Epileptiform activity preferentially arises outside tumor invasion zone in glioma xenotransplants

Seizures occur commonly with brain tumors. The underlying mechanisms are not understood. We analyzed network and cellular excitability changes in tumor-invaded and sham neocortical tissue in vitro using a rat glioblastoma model. Rat C6 glioma cells were transplanted into rat neocortex, yielding diffusely invading gliomas resembling human glioblastomas. We hypothesized that network excitability would increase in regions neighboring the tumor, and that initiation of epileptic discharges might be correlated to a higher density of intrinsically bursting neurones. Voltage-sensitive dye imaging revealed epileptic activity to be initiated in paratumoral zones (1-2 mm from main tumor mass), in contrast to control tissue, where epileptic foci appeared randomly throughout the neocortex. Neuronal firing patterns revealed significantly more intrinsically bursting neurones within these initiation zones than in regions directly adjacent to the tumor or in control tissue. We conclude that gliomas are associated with a higher density of intrinsically bursting neurones, and that these may preferentially initiate epileptiform events.

Astrocytic purinergic signaling coordinates synaptic networks

To investigate the role of astrocytes in regulating synaptic transmission, we generated inducible transgenic mice that express a dominant-negative SNARE domain selectively in astrocytes to block the release of transmitters from these glial cells. By releasing adenosine triphosphate, which accumulates as adenosine, astrocytes tonically suppressed synaptic transmission, thereby enhancing the dynamic range for long-term potentiation and mediated activity-dependent, heterosynaptic depression. These results indicate that astrocytes are intricately linked in the regulation of synaptic strength and plasticity and provide a pathway for synaptic cross-talk.

Glial calcium signaling and neuron–glia communication

The existence of bidirectional signaling between astrocytes and neurons has revealed an important active role of astrocytes in the physiology of the nervous system. As a consequence, there is a new concept of the synaptic physiology-"the tripartite synapse", where astrocytes exchange information with the pre- and postsynaptic elements and participate as dynamic regulatory elements in neurotransmission. The control of the Ca2+ excitability in astrocytes is a key element in this loop of information exchange. The ability of astrocytes to respond to neuronal activity and discriminate between the activity of different synapses, the modulation of the astrocytic cellular excitability by the synaptic activity, and the expression of cellular intrinsic properties indicate that astrocytes are endowed with cellular computational characteristics that process synaptic information. Therefore, we propose that astrocytes can be considered as cellular elements involved in the information processing by the nervous system.