Glutamate receptor-linked changes in membrane potential and intracellular Ca2+ in primary rat astrocytes

The Water Transport System in Astrocytes–Aquaporins

Highlights

(AQPs) are transmembrane proteins responsible for fast water movement across cell membranes, including those of astrocytes.

The expression and subcellular localization of AQPs in astrocytes are highly dynamic under physiological and pathological conditions.

Besides their primary function in water homeostasis, AQPs participate in many ancillary functions including glutamate clearance in tripartite synapses and cell migration.

Abstract

Astrocytes have distinctive morphological and functional characteristics, and are found throughout the central nervous system. Astrocytes are now known to be far more than just housekeeping cells in the brain. Their functions include contributing to the formation of the blood–brain barrier, physically and metabolically supporting and communicating with neurons, regulating the formation and functions of synapses, and maintaining water homeostasis and the microenvironment in the brain. Aquaporins (AQPs) are transmembrane proteins responsible for fast water movement across cell membranes. Various subtypes of AQPs (AQP1, AQP3, AQP4, AQP5, AQP8 and AQP9) have been reported to be expressed in astrocytes, and the expressions and subcellular localizations of AQPs in astrocytes are highly correlated with both their physiological and pathophysiological functions. This review describes and summarizes the recent advances in our understanding of astrocytes and AQPs in regard to controlling water homeostasis in the brain. Findings regarding the features of different AQP subtypes, such as their expression, subcellular localization, physiological functions, and the pathophysiological roles of astrocytes are presented, with brain edema and glioma serving as two representative AQP-associated pathological conditions. The aim is to provide a better insight into the elaborate “water distribution” system in cells, exemplified by astrocytes, under normal and pathological conditions.

Astrocytes: The Housekeepers and Guardians of the CNS

Astroglia are a diverse group of cells in the central nervous system. They are of the ectodermal, neuroepithelial origin and vary in morphology and function, yet, they can be collectively defined as cells having principle function to maintain homeostasis of the central nervous system at all levels of organisation, including homeostasis of ions, pH and neurotransmitters; supplying neurones with metabolic substrates; supporting oligodendrocytes and axons; regulating synaptogenesis, neurogenesis, and formation and maintenance of the blood-brain barrier; contributing to operation of the glymphatic system; and regulation of systemic homeostasis being central chemosensors for oxygen, CO₂ and Na⁺. Their basic physiological features show a lack of electrical excitability (inapt to produce action potentials), but display instead a rather active excitability based on variations in cytosolic concentrations of Ca²⁺ and Na⁺. It is expression of neurotransmitter receptors, pumps and transporters at their plasmalemma, along with transports on the endoplasmic reticulum and mitochondria that exquisitely regulate the cytosolic levels of these ions, the fluctuation of which underlies most, if not all, astroglial homeostatic functions.

Caloric restriction modifies spatiotemporal calcium dynamics in mouse hippocampal astrocytes

We analysed spatiotemporal properties of Ca²⁺ signals in protoplasmic astrocytes in the CA1 stratum radiatum of hippocampal slices from young (2-3 months old) mice housed in control conditions or exposed to a caloric restriction (CR) diet for one month. The astrocytic Ca²⁺ events became shorter in duration and smaller in size; they also demonstrated reduced velocity of expansion and shrinkage following CR. At the same time, Ca²⁺ signals in the astrocytes from the CR animals demonstrated higher amplitude and the faster rise and decay rates. These changes can be attributed to CR-induced morphological remodelling and uncoupling of astrocytes described in our previous study. CR-induced changes in the parameters of Ca²⁺ activity were partially reversed by inhibition of gap junctions/hemichannels with carbenoxolone (CBX). The effect of CBX on Ca²⁺ activity in CR-animals was unexpected because the diet already decreases gap junctional coupling in astrocytic syncytia. It may reflect the blockade of hemichannels also sensitive to this drug. Thus, CR-induced morphological remodelling of astrocytes is at least partly responsible for changes in the pattern of Ca²⁺ activity in the astrocytic network. How such changes in spatiotemporal Ca²⁺ landscape can translate into astrocytic physiology and neuron-glia interactions remains a matter for future studies.

Physiology of Astroglial Excitability

Classic physiology divides all neural cells into excitable neurons and nonexcitable neuroglia. Neuroglial cells, chiefly responsible for homeostasis and defense of the nervous tissue, coordinate their complex homeostatic responses with neuronal activity. This coordination reflects a specific form of glial excitability mediated by complex changes in intracellular concentration of ions and second messengers organized in both space and time. Astrocytes are equipped with multiple molecular cascades, which are central for regulating homeostasis of neurotransmitters, ionostasis, synaptic connectivity, and metabolic support of the central nervous system. Astrocytes are further provisioned with multiple receptors for neurotransmitters and neurohormones, which upon activation trigger intracellular signals mediated by Ca²⁺, Na⁺, and cyclic AMP. Calcium signals have distinct organization and underlying mechanisms in different astrocytic compartments thus allowing complex spatiotemporal signaling. Signals mediated by fluctuations in cytosolic Na⁺ are instrumental for coordination of Na⁺ dependent astrocytic transporters with tissue state and homeostatic demands. Astroglial ionic excitability may also involve K⁺, H^+, and Cl^-. The cyclic AMP signalling system is, in comparison to ions, much slower in targeting astroglial effector mechanisms. This evidence review summarizes the concept of astroglial intracellular excitability.

Physiology of Astroglia

Astrocytes are principal cells responsible for maintaining the brain homeostasis. Additionally, these glial cells are also involved in homocellular (astrocyte-astrocyte) and heterocellular (astrocyte-other cell types) signalling and metabolism. These astroglial functions require an expression of the assortment of molecules, be that transporters or pumps, to maintain ion concentration gradients across the plasmalemma and the membrane of the endoplasmic reticulum. Astrocytes sense and balance their neurochemical environment via variety of transmitter receptors and transporters. As they are electrically non-excitable, astrocytes display intracellular calcium and sodium fluctuations, which are not only used for operative signalling but can also affect metabolism. In this chapter we discuss the molecules that achieve ionic gradients and underlie astrocyte signalling.

Astroglial Calcium Signaling in Aging and Alzheimer's Disease

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

Astroglia in Alzheimer's Disease

Alzheimer's disease is the most common cause of dementia. Cellular changes in the brains of the patients suffering from Alzheimer's disease occur well in advance of the clinical symptoms. At the cellular level, the most dramatic is a demise of neurones. As astroglial cells carry out homeostatic functions of the brain, it is certain that these cells are at least in part a cause of Alzheimer's disease. Historically, Alois Alzheimer himself has recognised this at the dawn of the disease description. However, the role of astroglia in this disease has been understudied. In this chapter, we summarise the various aspects of glial contribution to this disease and outline the potential of using these cells in prevention (exercise and environmental enrichment) and intervention of this devastating disease.

Astroglial Glutamate Signaling and Uptake in the Hippocampus

Astrocytes have long been regarded as essentially unexcitable cells that do not contribute to active signaling and information processing in the brain. Contrary to this classical view, it is now firmly established that astrocytes can specifically respond to glutamate released from neurons. Astrocyte glutamate signaling is initiated upon binding of glutamate to ionotropic and/or metabotropic receptors, which can result in calcium signaling, a major form of glial excitability. Release of so-called gliotransmitters like glutamate, ATP and D-serine from astrocytes in response to activation of glutamate receptors has been demonstrated to modulate various aspects of neuronal function in the hippocampus. In addition to receptors, glutamate binds to high-affinity, sodium-dependent transporters, which results in rapid buffering of synaptically-released glutamate, followed by its removal from the synaptic cleft through uptake into astrocytes. The degree to which astrocytes modulate and control extracellular glutamate levels through glutamate transporters depends on their expression levels and on the ionic driving forces that decrease with ongoing activity. Another major determinant of astrocytic control of glutamate levels could be the precise morphological arrangement of fine perisynaptic processes close to synapses, defining the diffusional distance for glutamate, and the spatial proximity of transporters in relation to the synaptic cleft. In this review, we will present an overview of the mechanisms and physiological role of glutamate-induced ion signaling in astrocytes in the hippocampus as mediated by receptors and transporters. Moreover, we will discuss the relevance of astroglial glutamate uptake for extracellular glutamate homeostasis, focusing on how activity-induced dynamic changes of perisynaptic processes could shape synaptic transmission at glutamatergic synapses.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

NMDA Receptors in Glia

The amino acid L-Glutamate acts as the most ubiquitous mediator of excitatory synaptic transmission in the central nervous system. Glutamatergic transmission is central for diverse brain functions, being particularly important for learning, memory, and cognition. In brain pathology, excessive release of glutamate triggers excitotoxic neural cell death through necrotic or apoptotic pathways. Glutamate effects are mediated by several classes of glutamate receptors, expressed in virtually all cells of neural origin. Specifically important for both physiological information processing and cell damage are glutamate receptors of NMDA ( N-methyl-D-aspartate) type, which, for a long time, were considered to be expressed exclusively in neurons. Recent studies have found functional NMDA receptors in brain macroglia, in astrocytes, and oligodendrocytes. Glial and neuronal NMDA receptors are functionally and structurally different; the glial receptors are weakly (if at all) sensitive to the extracellular magnesium block, which may indicate a predominant expression of the NR3 receptor subunit. In the cortex, astroglial NMDA receptors are activated upon physiological synaptic transmission. The physiological relevance of NMDA receptors in the white matter remains unknown; their activation upon ischemia triggers Ca2+-dependent damage of oligodendrocytes and myelin. The discovery of glial NMDA receptors further indicates the complex nature of intercellular signaling mechanisms in the brain, which involve all types of neural cells, connected through diverse types of chemical and electrical synapses.

Glutamatergic Transmission: A Matter of Three

Glutamatergic transmission in the vertebrate brain requires the involvement of glia cells, in a continuous molecular dialogue. Glial glutamate receptors and transporters are key molecules that sense synaptic activity and by these means modify their physiology in the short and long term. Posttranslational modifications that regulate protein-protein interactions and modulate transmitter removal are triggered in glial cells by neuronal released glutamate. Moreover, glutamate signaling cascades in these cells are linked to transcriptional and translational control and are critically involved in the control of the so-called glutamate/glutamine shuttle and by these means in glutamatergic neurotransmission. In this contribution, we summarize our current understanding of the biochemical consequences of glutamate synaptic activity in their surrounding partners and dissect the molecular mechanisms that allow neurons to take control of glia physiology to ensure proper glutamate-mediated neuronal communication.

Purinergic and glutamatergic receptors on astroglia

Astroglial cells express many neurotransmitter receptors; the receptors to glutamate and ATP being the most abundant. Here, we provide a concise overview on the expression and main properties of astroglial glutamate receptors (ionotropic receptors represented by AMPA and NMDA subtypes) and metabotropic (mainly mGluR5 and mGluR3 subtypes) and purinoceptors (adenosine receptors of A1, A2A, A2B, and A3 types, ionotropic P2X1/5 and P2X7 subtypes, and metabotropic P2Y purinoceptors). We also discuss the role of these receptors in glial physiology and pathophysiology.

Inhibitory effects of beta-amyloid on the nicotinic receptors which stimulate glutamate release in rat hippocampus: the glial contribution

We investigated on the neuronal nicotinic acetylcholine receptor subtypes involved in the cholinergic control of in vivo hippocampal glutamate (GLU), aspartate (ASP) and inhibitory γ-aminobutyric acid (GABA) overflow. We also investigated on the possible contribution of nicotinic acetylcholine receptors subtypes present on astrocytes in the regulation of the three neurotransmitter amino acids overflow using hippocampal gliosomes and on the effects of beta-amyloid (Aβ) 1-40 on the nicotinic control of amino acid neurotransmitter release. Nicotine was able to enhance the in vivo overflow of the three amino acids being more potent in stimulating GLU overflow. The α7 selective agonist PHA543613 induced an overflow very similar to that of nicotine. The α4β2 selective agonist 5IA85380 was significantly less potent in inducing GLU overflow while the overflow of ASP and GABA were almost inconsistent. Aβ1-40 inhibited the neurotransmitter overflow stimulated by PHA543613 but not the one evoked by 5IA85380. In hippocampal gliosomes nicotine elicited selectively GLU overflow which was also evoked by 5IA85380 and by the α7 selective agonist choline. Nicotine- and choline-induced glutamate overflow in gliosomes was inhibited by Aα1-40. In conclusion nicotine administration in vivo elicits hippocampal GLU release mostly through α7 nicotinic acetylcholine receptors likely present both on neurons and astrocytes. Aβ inhibitory effect on the nicotinic-control of GLU release seems to depend primarily to the inhibition of α7 nicotinic acetylcholine receptors functional responses.

Sleep-wake sensitive mechanisms of adenosine release in the basal forebrain of rodents: an in vitro study

Adenosine acting in the basal forebrain is a key mediator of sleep homeostasis. Extracellular adenosine concentrations increase during wakefulness, especially during prolonged wakefulness and lead to increased sleep pressure and subsequent rebound sleep. The release of endogenous adenosine during the sleep-wake cycle has mainly been studied in vivo with microdialysis techniques. The biochemical changes that accompany sleep-wake status may be preserved in vitro. We have therefore used adenosine-sensitive biosensors in slices of the basal forebrain (BFB) to study both depolarization-evoked adenosine release and the steady state adenosine tone in rats, mice and hamsters. Adenosine release was evoked by high K⁺, AMPA, NMDA and mGlu receptor agonists, but not by other transmitters associated with wakefulness such as orexin, histamine or neurotensin. Evoked and basal adenosine release in the BFB in vitro exhibited three key features: the magnitude of each varied systematically with the diurnal time at which the animal was sacrificed; sleep deprivation prior to sacrifice greatly increased both evoked adenosine release and the basal tone; and the enhancement of evoked adenosine release and basal tone resulting from sleep deprivation was reversed by the inducible nitric oxide synthase (iNOS) inhibitor, 1400 W. These data indicate that characteristics of adenosine release recorded in the BFB in vitro reflect those that have been linked in vivo to the homeostatic control of sleep. Our results provide methodologically independent support for a key role for induction of iNOS as a trigger for enhanced adenosine release following sleep deprivation and suggest that this induction may constitute a biochemical memory of this state.

Astrocytes in Alzheimer's disease

The circuitry of the human brain is formed by neuronal networks embedded into astroglial syncytia. The astrocytes perform numerous functions, providing for the overall brain homeostasis, assisting in neurogenesis, determining the micro-architecture of the grey matter, and defending the brain through evolutionary conserved astrogliosis programs. Astroglial cells are engaged in neurological diseases by determining the progression and outcome of neuropathological process. Astrocytes are specifically involved in various neurodegenerative diseases, including Alzheimer's disease, amyotrophic lateral sclerosis, Parkinson's disease, and various forms of dementia. Recent evidence suggest that early stages of neurodegenerative processes are associated with atrophy of astroglia, which causes disruptions in synaptic connectivity, disbalance in neurotransmitter homeostasis, and neuronal death through increased excitotoxicity. At the later stages, astrocytes become activated and contribute to the neuroinflammatory component of neurodegeneration.

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

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

Differential hippocampal protection when blocking intracellular sodium and calcium entry during traumatic brain injury in rats

This study investigated the contributions of the reverse mode of the sodium-calcium exchanger (NCX) and the type 1 sodium-proton antiporter (NHE-1) to acute astrocyte and neuronal pathology in the hippocampus following fluid percussion traumatic brain injury (TBI) in the rat. KB-R7943, EIPA, or amiloride, which respectively inhibit NCX, NHE-1, or NCX, NHE-1, and ASIC1a (acid-sensing ion channel type 1a), was infused intraventricularly over a 60-min period immediately prior to TBI. Astrocytes were immunostained for glial fibrillary acidic protein (GFAP), and degenerating neurons were identified by Fluoro-Jade staining at 24 h after injury. Stereological analysis of the CA2/3 sub-regions of the hippocampus demonstrated that higher doses of KB-R7943 (2 and 20 nmoles) significantly reduced astrocyte GFAP immunoreactivity compared to vehicle-treated animals. EIPA (2-200 nmoles) did not alter astrocyte GFAP immunoreactivity. Amiloride (100 nmoles) significantly attenuated the TBI-induced acute reduction in astrocyte GFAP immunoreactivity. Of the three compounds examined, only amiloride (100 nmoles) reduced hippocampal neuronal degeneration assessed with Fluoro-Jade. The results provide additional evidence of acute astrocyte pathology in the hippocampus following TBI, while suggesting that activation of NHE-1 and the reverse mode of NCX contribute to both astrocyte and neuronal pathology following experimental TBI.

Adult astroglia is competent for Na+/Ca2+ exchanger-operated exocytotic glutamate release triggered by mild depolarization

Glutamate release induced by mild depolarization was studied in astroglial preparations from the adult rat cerebral cortex, that is acutely isolated glial sub-cellular particles (gliosomes), cultured adult or neonatal astrocytes, and neuron-conditioned astrocytes. K+ (15, 35 mmol/L), 4-aminopyridine (0.1, 1 mmol/L) or veratrine (1, 10 micromol/L) increased endogenous glutamate or [3H]D-aspartate release from gliosomes. Neurotransmitter release was partly dependent on external Ca2+, suggesting the involvement of exocytotic-like processes, and partly because of the reversal of glutamate transporters. K+ increased gliosomal membrane potential, cytosolic Ca2+ concentration [Ca2+]i, and vesicle fusion rate. Ca2+ entry into gliosomes and glutamate release were independent from voltage-sensitive Ca2+ channel opening; they were instead abolished by 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiurea (KB-R7943), suggesting a role for the Na+/Ca2+ exchanger working in reverse mode. K+ (15, 35 mmol/L) elicited increase of [Ca2+]i and Ca2+-dependent endogenous glutamate release in adult, not in neonatal, astrocytes in culture. Glutamate release was even more marked in in vitro neuron-conditioned adult astrocytes. As seen for gliosomes, K+-induced Ca2+ influx and glutamate release were abolished by KB-R7943 also in cultured adult astrocytes. To conclude, depolarization triggers in vitro glutamate exocytosis from in situ matured adult astrocytes; an aptitude grounding on Ca2+ influx driven by the Na+/Ca2+ exchanger working in the reverse mode.

Glutamate-mediated neuronal-glial transmission

The brain is the most complex organ of the human body. It is composed of several highly specialized and heterogeneous populations of cells, represented by neurones (e.g. motoneurons, projection neurons or interneurons), and glia represented by astrocytes, oligodendrocytes and microglia. In recent years there have been numerous studies demonstrating close bidirectional communication of neurons and glia at structural and functional levels. In particular, the excitatory transmitter glutamate has been shown to evoke a variety of responses in astrocytes and oligodendrocytes in the healthy as well as the diseased brain. Here we overview the multitude of glutamate sensing molecules expressed in glia and describe some general experiments which have been performed to identify the glutamate-responsive molecules, i.e. the ionotropic and metabotropic glutamate receptors as well as the glutamate transporters. We also discuss a transgenic mouse model that permits detailed and specific investigations of the role of glial glutamate receptors.

Regulation of cell-to-cell communication mediated by astrocytic ATP in the CNS

It has become apparent that glial cells, especially astrocytes, not merely supportive but are integrative, being able to receive inputs, assimilate information and send instructive chemical signals to other neighboring cells including neurons. At first, the excitatory neurotransmitter glutamate was found to be a major extracellular messenger that mediates these communications because it can be released from astrocytes in a Ca(2+)-dependent manner, diffused, and can stimulate extra-synaptic glutamate receptors in adjacent neurons, leading to a dynamic modification of synaptic transmission. However, recently extracellular ATP has come into the limelight as an important extracellular messenger for these communications. Astrocytes express various neurotransmitter receptors including P2 receptors, release ATP in response to various stimuli and respond to extracellular ATP to cause various physiological responses. The intercellular communication "Ca(2+) wave" in astrocytes was found to be mainly mediated by the release of ATP and the activation of P2 receptors, suggesting that ATP is a dominant "gliotransmitter" between astrocytes. Because neurons also express various P2 receptors and synapses are surrounded by astrocytes, astrocytic ATP could affect neuronal activities and even dynamically regulate synaptic transmission in adjacent neurons as if forming a "tripartite synapse". In this review, we summarize the role of astrocytic ATP, as compared with glutamate, in gliotransmission and synaptic transmission in neighboring cells, mainly focusing on the hippocampus. Dynamic communication between astrocytes and neurons mediated by ATP would be a key event in the processing or integration of information in the CNS.

Characterization of l-2-Amino-4-Phosphonobutanoate Action Following Sensitization by Quisqualate in Rat Hippocampal Slice Cultures

An excitatory action of l-2-amino-4-phosphonobutanoate (l-AP4), a glutamate analogue, is observed following pre-exposure of tissue to quisqualate. We have studied the mechanism of sensitization of l-AP4 responses by quisqualate in voltage-clamped CA3 pyramidal cells in rat hippocampal slice cultures in the presence of tetrodotoxin. Prior to quisqualate addition, CA3 cells did not respond to l-AP4 (50 - 1000 microM). Following brief application of quisqualate (500 nM for 30 s), l-AP4 (50 - 200 microM) induced a complex excitatory response which could be obtained for >1 h. l-AP4 caused an ionotropic inward current associated with a conductance increase. This response was in part sensitive to 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX) and in part sensitive to d-2-amino-5-phosphonovalerate (d-AP5) and Mg2+ ions. At depolarizing potentials, in the presence of CNQX and d-AP5, l-AP4 caused excitation by depressing K+ currents, mimicking the metabotropic action of glutamate. This indicates that the action of l-AP4 is mediated by three different receptor types: N-methyl-d-aspartate (NMDA) receptors, alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) receptors, and glutamatergic metabotropic receptors. The l-AP4 response persisted in solutions containing low Ca2+ and high Mg2+ concentrations or 100 - 200 microM Cd2+, suggesting that it is independent of extracellular Ca2+. We were unable to identify any substance other than quisqualate capable of sensitizing the l-AP4 action. This effect also occurred when quisqualate was applied in Ca2+-free solution or in solutions containing low concentrations of Na+ or Cl-. Sensitization of l-AP4 responses by quisqualate was not observed in acutely dissociated pyramidal cells recorded by means of the whole-cell recording mode, although ionotropic quisqualate responses were present. Sensitization was readily reversed by short applications of the endogenous excitatory amino acids glutamate, aspartate and homocysteate at concentrations of 10 - 100 microM. Our data are consistent with the hypothesis that the excitatory action of l-AP4 results from a Ca2+-independent release of endogenous excitatory amino acids from some presynaptic neuronal or glial site.

Quantitative immunochemistry on neuronal loss, reactive gliosis and BBB damage in cortex/striatum and hippocampus/amygdala after systemic kainic acid administration

Cell specific markers were quantified in the hippocampus, the amygdala/pyriform cortex, the frontal cerebral cortex and the striatum of the rat brain after systemic administration of kainic acid. Neuron specific enolase (NSE) reflects loss of neurons, glial fibrillary acidic protein (GFAP) reflects reactive gliosis, and brain levels of serum proteins measures blood-brain-barrier permeability. While the concentration of NSE remained unaffected in the frontal cerebral cortex and the striatum, their GFAP content increased during the first three days. In the hippocampus and amygdala, NSE levels decreased significantly. GFAP levels in the hippocampus were unaffected after one day and decreased in the amygdala/pyriform cortex. After that, GFAP increased strikingly until day 9 or, in the case of amygdala/pyriform cortex, even longer. This biphasic time course for GFAP was accompanied by a decrease of S-100 during days 1-9 followed by a significant increase at day 27 above the initial level. The regional differences in GFAP and S-100 could result from the degree of neuronal degeneration, the astrocytic receptor set-up and/or effects on the blood-brain barrier.

Ion channels in glial cells

Functional and molecular analysis of glial voltage- and ligand-gated ion channels underwent tremendous boost over the last 15 years. The traditional image of the glial cell as a passive, structural element of the nervous system was transformed into the concept of a plastic cell, capable of expressing a large variety of ion channels and neurotransmitter receptors. These molecules might enable glial cells to sense neuronal activity and to integrate it within glial networks, e.g., by means of spreading calcium waves. In this review we shall give a comprehensive summary of the main functional properties of ion channels and ionotropic receptors expressed by macroglial cells, i.e., by astrocytes, oligodendrocytes and Schwann cells. In particular we will discuss in detail glial sodium, potassium and anion channels, as well as glutamate, GABA and ATP activated ionotropic receptors. A majority of available data was obtained from primary cell culture, these results have been compared with corresponding studies that used acute tissue slices or freshly isolated cells. In view of these data, an active glial participation in information processing seems increasingly likely and a physiological role for some of the glial channels and receptors is gradually emerging.

Recording of intracellular Ca2+, Cl-, pH and membrane potential in cultured astrocytes using a fluorescence plate reader

A multi-well fluorescence plate reader was used to determine changes in intracellular ionic concentrations and changes in transmembrane voltage in primary cortical or hippocampal astrocytes. Recordings were compared to those obtained using a traditional microscope-based imaging setup that utilizes a monochromatic light source for excitation and an intensified CCD camera for detection. Measurement of pHi with the ratiometric dye BCECF provided resolution similar to that of a microscopic approach. We also demonstrate using Fluo-3 that the measurement of glutamate-induced [Ca2+]i fluctuations are comparable to recordings on the microscope-based system when 25 cells were averaged. This is expected because the plate reader averages responses from many cells. Voltage changes induced by changes in K+(o) from 3 to 5 mM were readily resolvable with the plate reader using the potentiometric dye bis-oxynol, and overall sensitivity was similar to that of microscopically-determined voltage changes. We also found that the plate reader was capable of resolving GABA-induced Cl-(i) fluctuations using the Cl(-)-sensitive indicator MEQ. From these experiments we conclude that multi-well fluorescence plate readers can be used to effectively record changes in intracellular ion concentrations and transmembrane voltage of populations of cells affording time and amplitude resolution approaching that of conventional fluorescence imaging methods. In addition, plate reader-based fluorescence studies demonstrate the added capability to rapidly screen large numbers of samples.

5-Hydroxytryptamine and glutamate modulate velocity and extent of intercellular calcium signalling in hippocampal astroglial cells in primary cultures

The effects of 5-hydroxytryptamine or glutamate treatment on mechanically induced intercellular calcium waves were studied in gap junction-coupled astroglial cells using rat astroglial-neuronal primary cultures from hippocampus. Imaging software was developed to study amplitude, velocity and extent of wave propagation. Velocity software was designed to find the cell contours automatically and to calculate travelled distance and time-delay of the calcium wave as it propagates from the stimulated cell to all other cells. Propagation analyses were performed to calculate the area of wave propagation. Mechanical stimulation of a single astroglial cell induced an intercellular calcium wave spreading from cell to cell in the astroglial syncytium. When registering the appearances of calcium signals in individual cells along the wave path upon re-stimulation of the same cell, 44.7% of the cells responded with similar calcium signal appearances the second time as the first time. A second wave from the opposite direction resulted in similar calcium signal appearances in 27.3% of the studied cells. Both amplitude and velocity of the calcium signal decreased most prominently in the first part and showed a later flattening out. Treatment with 5-hydroxytryptamine or glutamate for 20-30 s before mechanical stimulation increased the velocity of the calcium waves. 5-Hydroxytryptamine treatment for varying times decreased the propagation area of the calcium waves. In contrast, glutamate treatment increased the propagation area.

Calcium signalling in glial cells

Calcium signals are the universal way of glial responses to the various types of stimulation. Glial cells express numerous receptors and ion channels linked to the generation of complex cytoplasmic calcium responses. The glial calcium signals are able to propagate within glial cells and to create a spreading intercellular Ca2+ wave which allow information exchange within the glial networks. These propagating Ca2+ waves are primarily mediated by intracellular excitable media formed by intracellular calcium storage organelles. The glial calcium signals could be evoked by neuronal activity and vice versa they may initiate electrical and Ca2+ responses in adjacent neurones. Thus glial calcium signals could integrate glial and neuronal compartments being therefore involved in the information processing in the brain.

Selective coupling of mu-calpain activation with the NMDA receptor is independent of translocation and autolysis in primary cortical neurons

Excessive mu-calpain activation has been linked to several cellular pathologies including excitotoxicity and ischemia. In erythrocytes and other non-central nervous system (CNS) cells, calpain activation is thought to occur following a Ca2+-induced translocation of inactive cytosolic enzyme to membranes and subsequent autolysis. In the present report, we show that transiently exposing primary rat cortical neurons to lethal (50 microM) N-methyl-D-aspartic acid (NMDA) caused protracted calpain activation, measured as increased spectrin hydrolysis, but this was independent of translocation or autolysis of the protease. An anti-mu-calpain antibody showed that calpain was largely membrane associated in cortical neurons, and, consequently, neither translocation nor autolysis of the protease was observed following ionomycin or lethal NMDA treatment. By contrast, in rat erythrocytes, calpain was largely cytosolic and underwent rapid translocation and autolysis in response to ionomycin. Calpain-mediated spectrin hydrolysis was specifically coupled to Ca2+ entry through the NMDA receptor because nonspecific Ca2+ influx via ionomycin or KCl-mediated depolarization failed to activate the enzyme. Thus, calpain appears selectively linked to glutamate receptors in cortical neurons and regulated by mechanisms distinct from that occurring in many non-CNS cells. The data suggest that intracellular signals coupled to the NMDA receptor are responsible for activating calpain already associated with cellular membranes in cortical cells.

Glial calcium: homeostasis and signaling function

Glial cells respond to various electrical, mechanical, and chemical stimuli, including neurotransmitters, neuromodulators, and hormones, with an increase in intracellular Ca2+ concentration ([Ca2+]i). The increases exhibit a variety of temporal and spatial patterns. These [Ca2+]i responses result from the coordinated activity of a number of molecular cascades responsible for Ca2+ movement into or out of the cytoplasm either by way of the extracellular space or intracellular stores. Transplasmalemmal Ca2+ movements may be controlled by several types of voltage- and ligand-gated Ca(2+)-permeable channels as well as Ca2+ pumps and a Na+/Ca2+ exchanger. In addition, glial cells express various metabotropic receptors coupled to intracellular Ca2+ stores through the intracellular messenger inositol 1,4,5-triphosphate. The interplay of different molecular cascades enables the development of agonist-specific patterns of Ca2+ responses. Such agonist specificity may provide a means for intracellular and intercellular information coding. Calcium signals can traverse gap junctions between glial cells without decrement. These waves can serve as a substrate for integration of glial activity. By controlling gap junction conductance, Ca2+ waves may define the limits of functional glial networks. Neuronal activity can trigger [Ca2+]i signals in apposed glial cells, and moreover, there is some evidence that glial [Ca2+]i waves can affect neurons. Glial Ca2+ signaling can be regarded as a form of glial excitability.

Neuronal nitric oxide synthase expression is induced in neocortical astrocytes after spreading depression

Spreading depression (SD) confers either increased susceptibility to ischemic injury or a delayed protection. Because nitric oxide modulates ischemic injury, we investigated if altered expression of nitric oxide synthase (NOS) by SD could account for the effect of SD on ischemia. Furthermore, the identity of cells expressing NOS after SD is important, since SD results in heterogeneous, cell type-specific changes in intracellular environment, which can control NOS activity. Immunohistochemical, computer-based image analyses and Western blotting show that the number of neuronal NOS (nNOS)-positive cells in the somatosensory cortex was significantly increased at 6 hours and 3 days after SD (P < 0.05 and 0.01, respectively), whereas inducible NOS expression remained unchanged. Double-labeling of nNOS and glial fibrillary acidic protein identified these nNOS-positive cells as astrocytes. The effect of altered NO production on induced nNOS expression was examined by treating rats with sodium nitroprusside or NA-nitro-L-arginine methyl ester (LNAM) during SD. Increased nNOS expression was prevented by sodium nitroprusside and phenylephrine or phenylephrine alone, but not LNAM. Because SD increased astrocytic nNOS expression at time points correlating with both ischemic hypersensitivity and ischemic tolerance, the ability of SD to modulate ischemic injury must be complex, perhaps involving NOS but other factors as well.

Intracellular calcium transients and potassium current oscillations evoked by glutamate in cultured rat astrocytes

Glutamate responses in cultured rat astrocytes from cerebella of neonatal rats were investigated using the perforated-patch configuration to record membrane currents without rundown of intracellular messenger cascades, and microfluorometric measurements to measure the intracellular Ca2+ concentration ([Ca2+]i) and intracellular pH (pHi) with fura-2 AM and 2',7'-bis-(2-carboxyethyl)-5,6-carboxyfluorescein acetoxy methylester respectively. In the perforated-patch mode, glutamate evoked single or multiple outward current transients in 82% of the cells, which disappeared when the recording technique was converted into a conventional whole-cell mode. The outward current transients were accompanied by [Ca2+]i transients, whereas pHi fell monophasically, without any sign of oscillation. Pharmacological analysis of the glutamate-induced responses indicated that ionotropic receptor activation evoked an inward current but no outward current transients, and metabotropic receptor activation (of the mGluR1/5 type) elicited outward current transients but no inward current. The outward current transients were reduced in frequency, or even abolished, after depletion of the intracellular Ca2+-stores by the Ca2+-ATPase inhibitor cyclopiaconic acid (10 microM). They reversed near -85 mV and were reduced by tetraethylammonium (10 mM), suggesting that they were caused by K+ channel activation. It is concluded that glutamate evoked these K+ outward current transients by oscillatory Ca2+ release mediated by mGluR activation. The corresponding membrane potential waves across the astroglial syncytium could provide spatial and temporal dynamics to the glial K+ uptake capacity and other voltage-dependent processes.

Metabotropic glutamate receptor expression in cultured rat astrocytes and human gliomas

In order to confirm the existence of metabotropic glutamate receptors in astroglial cultures and to provide information on different receptor subtypes, the expression of different mGluRs was analysed in cultures highly enriched in rat astroglial cells. mRNA levels for mGluR1, 2, 3, 4, 7 were undetectable by Northern blot analysis in primary type-1 astroglial cultures derived from total cerebral hemispheres, cerebral cortex and striatum. Interestingly, these cultures expressed a low, but detectable, level of mGluR5 mRNA. The more sensitive technique Reverse Transcription-Polymerase Chain Reaction (RT-PCR) confirmed the presence of mGluR5 transcript in cultured astrocytes and, in addition, revealed the presence of mGluR3 mRNA. The lack of expression of mGluR5 in CG-4 cells, a rat cell line able to differentiate in type-2 astrocytes or oligodendrocytes depending on the culture conditions, suggested that the presence of mGluR5 was not a general feature of cells of glial origin. Moreover, all the examined mGluR transcript were undetectable by RT-PCR in CG4 cells. In order to confirm the possible expression of mGluR5 in cell of glial origin we examined the mRNA levels for this receptor in tissue samples from human gliomas obtained after surgical resection of the tumors: only 1 sample (grade II astrocytoma), out of 8 examined, showed the presence of mGluR5 mRNA. In conclusion our data show that the only cloned metabotropic receptor linked to phosphoinositide hydrolysis, whose expression is detectable in cultured type-1 astrocytes, in mGluR5. It remains to be established if the low level of expression of mGluR3 could be responsible for the group II metabotropic glutamate receptor activity previously observed in cultured astroglial cells.

Intracellular acidification and Ca2+ transients in cultured rat cerebellar astrocytes evoked by glutamate agonists and noradrenaline

The effect of different neurotransmitters on the intracellular pH (pHi) and intracellular calcium (Ca2+i) was studied in cultured astrocytes from neonatal rat cerebellum, using the fluorescent dyes 2,7'-bis(carboxyethyl)-5,6-carboxy-fluorescein (BCECF) and Fura-2. Application of glutamate or kainate (100 microM) in a HEPES-buffered, CO2/HCO3(-) -free saline induced a decrease in pHi and an increase in Ca2+i. Amplitude and time course of the pHi and Ca2+i transients were different. Glutamate and kainate evoked a mean acidification of 0.22 +/- 0.05 (n = 29) and 0.20 +/- 0.09 (n = 12) pH units, respectively. The changes in pHi and Ca2+i induced by kainate, but not by glutamate, were inhibited by 6-cyano-7-dinitroquinozalin-2,3-dion (CNQX; 50 microM). In order to elucidate the mechanism of the agonist-induced acidification, whether the pHi changes were secondary to the Ca2+ rises was tested. In the absence of extracellular Ca2+, the kainate-induced Ca2+i transient was suppressed, while the intracellular acidification was only reduced by 13%. Removal of extracellular Ca2+ reduced the glutamate-induced pHi change by 8%, while the second component of the Ca2+i transient was abolished. Application of trans-( +/- )-1-amino-(1S,3R)-cyclopentadicarboxylic acid (t-ACPD, 100 microM), a metabotropic glutamate receptor agonist, and of noradrenaline (20 microM) evoked a Ca2+i increase, but no change of pHi. D-aspartate, which has a low affinity to glutamate receptors, but is known to be transported by the glutamate uptake system in some astrocytes, evoked an intracellular acidification, similar to that induced by glutamate, but no Ca2+i transient. The results suggest that the kainate-induced acidification is only partly due to the concomitant Ca2+i rise, while the glutamate/aspartate-induced acidification is mainly due to the activation of the glutamate uptake system.

Age-dependent changes in the regulation by external calcium ions of the phosphorylation of glial fibrillary acidic protein in slices of rat hippocampus

We studied the effect of external Ca2+ on the incorporation of [32P]phosphate into the astrocytic marker protein, glial fibrillary acidic protein (GFAP), in hippocampal slices from rats in the postnatal age range 12-16 days to +60 days (P12-P16 to +P60). At age P12-P16 the presence of Ca2+ in the incubation medium inhibited the incorporation of 32P into GFAP; this inhibition declined to near zero by P21 and subsequently 32P-incorporation became progressively more dependent on Ca2+ until by P60 no GFAP phosphorylation was observed in the absence of Ca2+. With tissue from immature rats inhibition of 32P-incorporation into GFAP started at a medium concentration of 7.5 microM Ca2+, reached 50% at 100 microM and then remained constant up to 1 mM; with adults maximal phosphorylation required 1 mM Ca2+ in the medium. The inorganic Ca(2+)-channel blockers, Co2+ and Ni2+, and a high concentration of the L-type blocker, nifedipine, reversed the effects of external Ca2+ on GFAP phosphorylation. The results suggest a late developmental change in the compartmental disposition of Ca2+ in astrocytes.

Intercellular signaling in neuronal-glial networks

Glial cells have recently been found to exhibit electrophysiological and metabolic responses to many neurotransmitters and neuromodulators. These findings have focused attention on the possibility that active signaling between neurons and glia could represent an important form of intercellular communication within the brain. Since glial and neuronal networks are both physically and metabolically interlinked, such intercellular signaling may represent a mechanism for inducing collective changes in the cellular physiology of neuronal and glial cell populations. Within the nervous tissue of both vertebrate and invertebrate organisms, glial cells are known to secrete extracellular signal molecules, modulate carbohydrate metabolism, and control the volume and ionic composition of extracellular space. In this paper, the roles that cytoplasmic [Ca2+] transients may play in regulating these glial cell functions are reviewed. Mechanisms by which intracellular Ca oscillations and intercellular Ca waves may be generated in neurotransmitter-stimulated glial cells are also discussed. In addition, it is proposed that rhythmic glial cell contractions and shape changes, which have been observed for many decades, are linked to Ca-induced secretion of ions, water, and neuroactive compounds. These activities represent mechanisms by which Ca-induced changes in glial cell physiology could potentially alter the excitability of neuronal networks.

Glutamate stimulates the phosphorylation of glial fibrillary acidic protein in slices of immature rat hippocampus via a metabotropic receptor

Phosphorylation of the astrocyte cell marker glial fibrillary acidic protein (GFAP) in hippocampal slices from immature rats (10-16 days postnatal) was strongly stimulated by glutamate in the presence of Ca2+. This effect apparently occurred via a metabotropic receptor since the specific agonist of metabotropic glutamate receptors, 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), stimulated GFAP phosphorylation by 173% whilst the mixed agonists, ibotenate and quisqualate, stimulated to a lesser extent. Ionotropic agonists were mainly ineffective. The action of 1S,3R-ACPD was blocked by L(+)-2-amino-3-phosphonopropionic acid (L-AP3) a specific antagonist of the metabotropic glutamate receptor coupled to the hydrolysis of phosphoinositides and was reduced by 70% by preincubation of the slices with pertussis toxin. In contrast to these results with immature animals glutamate had little or no effect on the phosphorylation of GFAP in hippocampal slices from adult rats.

Glutamate agonists and [3H]GABA release from rat hippocampal slices: involvement of metabotropic glutamate receptors in the quisqualate-evoked release

The effects of glutamate agonists and their selective antagonists on the Ca(2+)-dependent and independent releases of [3H]GABA from rat coronal hippocampal slices were studied in a superfusion system. The Ca(2+)-dependent release evoked by glutamate, kainate and N-methyl-D-aspartate (NMDA) gradually declined with time despite the continuous presence of the agonists. Quisqualate (QA) caused a sustained release which exhibited no tendency to decline within the 20-min period of stimulation. This release was enhanced in Ca(2+)-free medium. The release evoked by QA in Ca(2+)-containing medium was significantly inhibited by (+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohept-5,10-imine hydrogen maleate (MK-801) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), showing that QA activates NMDA receptors directly or indirectly through (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors. The inhibition of MK-801 was slightly diminished and that of CNQX totally abolished in Ca(2+)-free medium. Verapamil inhibited the QA-activated release in both Ca(2+)-containing and Ca(2+)-free media. The effect of QA but not that of AMPA was blocked in Ca(2+)-free medium by L(+)-2-amino-3-phosphonopropionate (L-AP3), a selective antagonist of the metabotropic glutamate receptor. It is suggested that the sustained release of GABA is also mediated partly by activation of metabotropic receptors and mobilization of Ca2+ form intracellular stores.

Cytosolic calcium transient increase through the AMPA/kainate receptor in cultured Müller cells

The intracellular concentration of free calcium ions ([Ca2+]i) following administration of glutamate agonist was monitored for retinal Müller cells cultured from adult rabbits using a fluorescence microscope equipped with a video camera system. The calcium concentration was imaged with fura-2. The transient increase of [Ca2+]i was observed following the administration of L-glutamate (3 mM), kainate (0.07-7 mM) and L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA, 0.07-7 mM), but not N-methyl-L-aspartate (NMDA, 0.7-7 mM) in Mg(2+)-free medium. The AMPA/kainate-induced increase of [Ca2+]i was blocked by the non-NMDA glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or low concentrations of external calcium. High K+ solution induced a slight but definite increase in [Ca2+]i which was blocked by nifedipine, a voltage-dependent calcium channel blocker, at 100 microM, suggesting that L-type calcium channels are present in cultured Müller cells. The AMPA-induced transient increase of [Ca2+]i was not blocked at the same concentration of nifedipine. There must be an influx of calcium ions through non-NMDA AMPA-kainate receptors in Müller cells. In the retina, glutamate receptor-linked events are no longer considered as specific to neurons.

Glutamate blocks astroglial stellation: effect of glutamate uptake and volume changes

Neurotransmitters which increase intracellular cAMP levels can cause cultured astroglia to change from a flat, polygonal shape to a stellate morphology. Little is known about how glial stellation can be regulated by other transmitters. In the present study, we demonstrated that L-glutamate blocked isoproterenol (ISO) or dibutyryl-cAMP induced stellation in astroglia. The glutamate inhibition was concentration dependent, with its maximal effect on > 90% of cells at 500 microM. Glutamate also reversed glial stellation within a short period (< 30 min). Glutamate uptake analogues, D-glutamate and D-aspartate, rather than receptor agonists, kainate and quisqualate, mimicked the glutamate effect. Likewise, the glutamate uptake blocker, D-thero-beta-hydroxyaspartate, blocked the glutamate effect. The glutamate inhibition was not a result of inhibition of cAMP formation, since norepinephrine, which inhibited 80% of ISO-stimulated cAMP, also caused glial stellation. Increases in extracellular K+ to 50 mM also reduced glial stellation, whereas 25 mM K+ had little effect. Since 25 mM K+ caused much greater depolarization than 400 microM glutamate, it was unlikely that the effects of both glutamate and high [K+] on glial stellation were due to membrane depolarization. Hypotonic treatment (120 mOsm) enhanced, whereas hypertonic treatment (520 mOsm) prevented, the glutamate reversal of glial stellation. Thus, glial swelling appeared to be a primary mechanism for the inhibitory effect of glutamate and high [K+] on glial stellation. This mechanism could also explain the observation that glutamate inhibited stellation induced by PMA, a PKC activator. Our data suggest that glutamate released from neurons during neuronal activity or pathology can be taken up by astrocytes and alter their morphology. Changes in glial morphology may in turn affect the volume and composition of the extracellular space and, as a result, neuronal activity.

Properties of GABA and glutamate responses in identified glial cells of the mouse hippocampal slice

In this study, the patch-clamp technique was applied to brain slices to test for the presence of GABAA and glutamate receptors in glial cells of an intact tissue preparation, the hippocampus from 9-12 day old mice. Two types of glial cells were studied in the CA1 stratum pyramidale, termed passive and complex cells, which were distinct by their characteristic pattern of voltage-dependent currents. Both cell types were previously identified as glial by combining electrophysiology with ultrastructural inspection (Steinhüser et al., 1992, Eur J Neurosci 4:472-484). A subpopulation of passive cells was positive, all complex cells were negative for immunocytochemical staining against glial fibrillary acidic protein, a marker of mature astrocytes. In both cell types, GABA activated currents compatible with GABAA-receptor mediated responses. The glutamate response in complex and in most of the passive cells was mediated by a ligand-gated ion channel and closely matched the pharmacology of the kainate receptor. Activation of glutamate receptors led to a transient decrease of the resting K+ conductance in complex cells and to an irreversible decrease in the passive cells. In three passive cells, glutamate-activated currents were most likely dominated by an electrogenic uptake. In a small group of passive cells NMDA-activated currents were observed. This study provides evidence that glial cells from an intact tissue express receptors for the most abundant transmitters in the central nervous system, glutamate, and GABA.

Pharmacological characterization of glutamate binding sites in cultured cerebellar granule cells and cortical astrocytes

Membranes prepared from cerebellar granule cells and cortical astrocytes exhibited specific, saturable binding of L-[3H]glutamate. The apparent binding constant KD was 135 nM and 393 nM and the maximal binding capacity Bmax 42 and 34 mumol/kg in granule cells and astrocytes, respectively. In granule cells the binding was strongly inhibited by the glutamate receptor agonists kainate, quisqualate, N-methyl-D-aspartate (NMDA), L-homocysteate and ibotenate, and the antagonist DL-5-aminophosphonovalerate. In astrocytes, only quisqualate among these was effective. L-Aspartate, L-cysteate, L-cysteinesulphinate and gamma-D-glutamylglycine were inhibitors in both cell types. The binding was totally displaced in both cell types by L-cysteinesulphinate with IC50 in the micromolar range. In astrocytes the binding was also totally displaced by quisqualate, but in granule cells only partially by NMDA, kainate and quisqualate in turn. It is concluded from the relative potencies of agonists and antagonists in [3H]glutamate binding that cerebellar granule cells express the NMDA, kainate and quisqualate types of the glutamate receptor, while only the quisqualate-sensitive binding seems to be present in cortical astrocytes.

AMPA-selective glutamate receptor subunits in astroglial cultures

We analysed AMPA ionotropic receptor subunits at the mRNA level (GluR-1 to -4) and at the protein level (GluR-1 and GluR-2/3/4c) in "primary astroglial cultures" (non-neuronal cell cultures highly enriched in glial fibrillary acidic protein [GFAP] positive cells) prepared from newborn rat cerebral hemispheres, cerebral cortex, hippocampus, and striatum and in "brain non-neuronal cell cultures" (low percentage of GFAP positive cells) prepared from cerebellum, brainstem, mesencephalon, and hypothalamus. For comparison, we also determined AMPA subunit mRNA and protein levels in different brain regions. By Northern blot analysis mRNAs for the AMPA receptor subunits (GluR-1,-2,-3,-4) were detected in primary rat cerebral hemispheres astroglial cultures. Immunoblotting analysis with anti-GluR-1 and anti-GluR-2/3/4c polyclonal antibodies confirmed the presence of low level of immunoreactive proteins of the same size of those identified in vivo as GluR subunits. Expression of GluR genes varied depending on the brain area used as starting material for the preparation of the cultures: GluR-1, -2, and -3 were mainly expressed in cortical cultures, while GluR-4 expression predominated in brainstem derived cultures. Interestingly this pattern of expression correlates with that observed in the intact brain, where high levels of GluR-4 mRNA and low levels of the other GluR subunits were found in the brainstem. In conclusion our results confirm the existence of glutamate ionotropic receptors of the AMPA type in primary astroglial cultures and suggest that GluR-4 is the main AMPA receptor subunit expressed in non-neuronal cells of the central nervous system.

Glial calcium

This review summarizes current knowledge relating intracellular calcium and glial function. During steady state, glia maintain a low cytosolic calcium level by pumping calcium into intracellular stores and by extruding calcium across the plasma membrane. Glial Ca2+ increases in response to a variety of physiological stimuli. Some stimuli open membrane calcium channels, others release calcium from intracellular stores, and some do both. The temporal and spatial complexity of glial cytosolic calcium changes suggest that these responses may form the basis of an intracellular or intercellular signaling system. Cytosolic calcium rises effect changes in glial structure and function through protein kinases, phospholipases, and direct interaction with lipid and protein constituents. Ultimately, calcium signaling influence glial gene expression, development, metabolism, and regulation of the extracellular milieu. Disturbances in glial calcium homeostasis may have a role in certain pathological conditions. The discovery of complex calcium-based glial signaling systems, capable of sensing and influencing neural activity, suggest a more integrated neuro-glial model of information processing in the central nervous system.