Glutamate-triggered calcium signalling in mouse bergmann glial cells in situ: role of inositol-1,4,5-trisphosphate-mediated intracellular calcium release

Endocannabinoid signaling in astrocytes

The study of the astrocytic contribution to brain functions has been growing in popularity in the neuroscience field. In the last years, and especially since the demonstration of the involvement of astrocytes in synaptic functions, the astrocyte field has revealed multiple functions of these cells that seemed inconceivable not long ago. In parallel, cannabinoid investigation has also identified different ways by which cannabinoids are able to interact with these cells, modify their functions, alter their communication with neurons and impact behavior. In this review, we will describe the expression of different endocannabinoid system members in astrocytes. Moreover, we will relate the latest findings regarding cannabinoid modulation of some of the most relevant astroglial functions, namely calcium (Ca²⁺ ) dynamics, gliotransmission, metabolism, and inflammation.

Role of ammonia for brain abnormal protein glycosylation during the development of hepatitis B virus-related liver diseases

Background

Ammonia is the most typical neurotoxin in hepatic encephalopathy (HE), but the underlying pathophysiology between ammonia and aberrant glycosylation in HE remains unknown.

Results

Here, we used HBV transgenic mice and astrocytes to present a systems-based study of glycosylation changes and corresponding enzymes associated with the key factors of ammonia in HE. We surveyed protein glycosylation changes associated with the brain of HBV transgenic mice by lectin microarrays. Upregulation of Galβ1-3GalNAc mediated by core 1 β1,3-galactosyltransferase (C1GALT1) was identified as a result of ammonia stimulation. Using in vitro assays, we validated that upregulation of C1GALT1 is a driver of deregulates calcium (Ca²⁺) homeostasis by overexpression of inositol 1,4,5-trisphosphate receptor type 1 (IP3R1) in astrocytes.

Conclusions

We demonstrated that silencing C1GALT1 could depress the IP3R1 expression, an effective strategy to inhibit the ammonia-induced upregulation of Ca²⁺ activity, thereby C1GALT1 and IP3R1 may serve as therapeutic targets in hyperammonemia of HE.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-022-00751-4.

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.

Calcium signaling in neuroglia

Glial cells exploit calcium (Ca²⁺) signals to perceive the information about the activity of the nervous tissue and the tissue environment to translate this information into an array of homeostatic, signaling and defensive reactions. Astrocytes, the best studied glial cells, use several Ca²⁺ signaling generation pathways that include Ca²⁺ entry through plasma membrane, release from endoplasmic reticulum (ER) and from mitochondria. Activation of metabotropic receptors on the plasma membrane of glial cells is coupled to an enzymatic cascade in which a second messenger, InsP₃ is generated thus activating intracellular Ca²⁺ release channels in the ER endomembrane. Astrocytes also possess store-operated Ca²⁺ entry and express several ligand-gated Ca²⁺ channels. In vivo astrocytes generate heterogeneous Ca²⁺ signals, which are short and frequent in distal processes, but large and relatively rare in soma. In response to neuronal activity intracellular and inter-cellular astrocytic Ca²⁺ waves can be produced. Astrocytic Ca²⁺ signals are involved in secretion, they regulate ion transport across cell membranes, and are contributing to cell morphological plasticity. Therefore, astrocytic Ca²⁺ signals are linked to fundamental functions of the central nervous system ranging from synaptic transmission to behavior. In oligodendrocytes, Ca²⁺ signals are generated by plasmalemmal Ca²⁺ influx, or by release from intracellular stores, or by combination of both. Microglial cells exploit Ca²⁺ permeable ionotropic purinergic receptors and transient receptor potential channels as well as ER Ca²⁺ release. In this contribution, basic morphology of glial cells, glial Ca²⁺ signaling toolkit, intracellular Ca²⁺ signals and Ca²⁺-regulated functions are discussed with focus on astrocytes.

Restoring glutamate homeostasis in the nucleus accumbens via endocannabinoid-mimetic drug prevents relapse to cocaine seeking behavior in rats

Impaired glutamate homeostasis is a key characteristic of the neurobiology of drug addiction in rodent models and contributes to the vulnerability to relapse to drug seeking. Although disrupted astrocytic and presynaptic regulation of glutamate release has been considered to constitute with impaired glutamate homeostasis in rodent model of drug relapse, the involvement of endocannabinoids (eCBs) in this neurobiological process has remained largely unknown. Here, using cocaine self-administration in rats, we investigated the role of endocannabinoids in impaired glutamate homeostasis in the core of nucleus accumbens (NAcore), which was indicated by augmentation of spontaneous synaptic glutamate release, downregulation of metabotropic glutamate receptor 2/3 (mGluR2/3), and mGluR5-mediated astrocytic glutamate release. We found that the endocannabinoid, anandamide (AEA), rather than 2-arachidonoylglycerol elicited glutamate release through presynaptic transient receptor potential vanilloid 1 (TRPV1) and astrocytic cannabinoid type-1 receptors (CB1Rs) in the NAcore of saline-yoked rats. In rats with a history of cocaine self-administration and extinction training, AEA failed to alter synaptic glutamate release in the NAcore, whereas CB1R-mediated astrocytic glutamate release by AEA remained functional. In order to induce increased astrocytic glutamate release via exogenous AEA, (R)-methanandamide (methAEA, a metabolically stable form of AEA) was chronically infused in the NAcore via osmotic pumps during extinction training. Restoration of mGluR2/3 function and mGluR5-mediated astrocytic glutamate release was observed after chronic methAEA infusion. Additionally, priming or cue-induced reinstatement of cocaine seeking was inhibited in methAEA-infused rats. These results demonstrate that enhancing endocannabinoid signaling is a potential pathway to restore glutamate homeostasis and may represent a promising therapeutic strategy for preventing cocaine relapse.

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.

The neurotoxin diethyl dithiophosphate impairs glutamate transport in cultured Bergmann glia cells

Glutamate, the main excitatory neurotransmitter in the vertebrate Central Nervous System, is involved in almost every aspect of brain physiology, and its signaling properties are severely affected in most neurodegenerative diseases. This neurotransmitter has to be efficiently removed from the synaptic cleft in order to prevent an over-stimulation of glutamate receptors that leads to neuronal death. Specific sodium-dependent membrane transporters, highly enriched in glial cells, elicit the clearance of glutamate. Once internalized, it is metabolized to glutamine by the glia-enriched enzyme Glutamine synthetase. Accumulated glutamine is released into the extracellular space for its uptake into pre-synaptic neurons and its conversion to glutamate that is packed into synaptic vesicles completing the glutamate/glutamine cycle. Diverse chemical compounds, like organophosphates, directly affect brain chemistry by altering levels of neurotransmitters in the synaptic cleft. Organophosphate compounds are widely used as pesticides, and all living organisms are continuously exposed to these substances, either in a direct or indirect manner. Its metabolites, like the diethyl dithiophosphate, are capable of causing brain damage through diverse mechanisms including perturbation of neuronal-glial cell interactions and have been associated with attention-deficit disorders and other mental illness. In order to characterize the neurotoxic mechanisms of diethyl dithiophosphate, we took advantage of the well characterized model of chick cerebellar Bergmann glia cultures. A significant impairment of [³H] d-Aspartate transport was found upon exposure to the metabolite. These results indicate that glia cells are targets of neurotoxic substances such as pesticides and that these cells might be critically involved in the associated neuronal death.

Crosslink between calcium and sodium signalling

NEW FINDINGS

What is the topic of this review? This paper overviews the links between Ca2+ and Na+ signalling in various types of cells. What advances does it highlight? This paper highlights the general importance of ionic signalling and overviews the molecular mechanisms linking Na+ and Ca2+ dynamics. In particular, the narrative focuses on the molecular physiology of plasmalemmal and mitochondrial Na+ -Ca2+ exchangers and plasmalemmal transient receptor potential channels. Functional consequences of Ca2+ and Na+ signalling for co-ordination of neuronal activity with astroglial homeostatic pathways fundamental for synaptic transmission are discussed.

ABSTRACT

Transmembrane ionic gradients, which are an indispensable feature of life, are used for generation of cytosolic ionic signals that regulate a host of cellular functions. Intracellular signalling mediated by Ca2+ and Na+ is tightly linked through several molecular pathways that generate Ca2+ and Na+ fluxes and are in turn regulated by both ions. Transient receptor potential (TRP) channels bridge endoplasmic reticulum Ca2+ release with generation of Na+ and Ca2+ currents. The plasmalemmal Na+ -Ca2+ exchanger (NCX) flickers between forward and reverse mode to co-ordinate the influx and efflux of both ions with membrane polarization and cytosolic ion concentrations. The mitochondrial calcium uniporter channel (MCU) and mitochondrial Na+ -Ca2+ exchanger (NCLX) mediate Ca2+ entry into and release from this organelle and couple cytosolic Ca2+ and Na+ fluctuations with cellular energetics. Cellular Ca2+ and Na+ signalling controls numerous functional responses and, in the CNS, provides for fast regulation of astroglial homeostatic cascades that are crucial for maintenance of synaptic transmission.

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.

The homeostatic astroglia emerges from evolutionary specialization of neural cells

Evolution of the nervous system progressed through cellular diversification and specialization of functions. Conceptually, the nervous system is composed from electrically excitable neuronal networks connected with chemical synapses and non-excitable glial cells that provide for homeostasis and defence. Astrocytes are integrated into neural networks through multipartite synapses; astroglial perisynaptic processes closely enwrap synaptic contacts and control homeostasis of the synaptic cleft, supply neurons with glutamate and GABA obligatory precursor glutamine and contribute to synaptic plasticity, learning and memory. In neuropathology, astrocytes may undergo reactive remodelling or degeneration; to a large extent, astroglial reactions define progression of the pathology and neurological outcome.This article is part of the themed issue 'Evolution brings Ca(2+) and ATP together to control life and death'.

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.

Glutamate and ATP at the Interface Between Signaling and Metabolism in Astroglia: Examples from Pathology

Glutamate is the main excitatory transmitter in the brain, while ATP represents the most important energy currency in any living cell. Yet, these chemicals play an important role in both processes, enabling them with dual-acting functions in metabolic and intercellular signaling pathways. Glutamate can fuel ATP production, while ATP can act as a transmitter in intercellular signaling. We discuss the interface between glutamate and ATP in signaling and metabolism of astrocytes. Not only do glutamate and ATP cross each other's paths in physiology of the brain, but they also do so in its pathology. We present the fabric of this process in (patho)physiology through the discussion of synthesis and metabolism of ATP and glutamate in astrocytes as well as by providing a general description of astroglial receptors for these molecules along with the downstream signaling pathways that may be activated. It is astroglial receptors for these dual-acting molecules that could hold a key for medical intervention in pathological conditions. We focus on two examples disclosing the role of activation of astroglial ATP and glutamate receptors in pathology of two kinds of brain tissue, gray matter and white matter, respectively. Interventions at the interface of metabolism and signaling show promise for translational medicine.

Astrocytes as secretory cells of the central nervous system: idiosyncrasies of vesicular secretion

Astrocytes are housekeepers of the central nervous system (CNS) and are important for CNS development, homeostasis and defence. They communicate with neurones and other glial cells through the release of signalling molecules. Astrocytes secrete a wide array of classic neurotransmitters, neuromodulators and hormones, as well as metabolic, trophic and plastic factors, all of which contribute to the gliocrine system. The release of neuroactive substances from astrocytes occurs through several distinct pathways that include diffusion through plasmalemmal channels, translocation by multiple transporters and regulated exocytosis. As in other eukaryotic cells, exocytotic secretion from astrocytes involves divergent secretory organelles (synaptic-like microvesicles, dense-core vesicles, lysosomes, exosomes and ectosomes), which differ in size, origin, cargo, membrane composition, dynamics and functions. In this review, we summarize the features and functions of secretory organelles in astrocytes. We focus on the biogenesis and trafficking of secretory organelles and on the regulation of the exocytotic secretory system in the context of healthy and diseased astrocytes.

Neuron-glia signaling in developing retina mediated by neurotransmitter spillover

Neuron-glia interactions play a critical role in the maturation of neural circuits; however, little is known about the pathways that mediate their communication in the developing CNS. We investigated neuron-glia signaling in the developing retina, where we demonstrate that retinal waves reliably induce calcium transients in Müller glial cells (MCs). During cholinergic waves, MC calcium transients were blocked by muscarinic acetylcholine receptor antagonists, whereas during glutamatergic waves, MC calcium transients were inhibited by ionotropic glutamate receptor antagonists, indicating that the responsiveness of MCs changes to match the neurotransmitter used to support retinal waves. Using an optical glutamate sensor we show that the decline in MC calcium transients is caused by a reduction in the amount of glutamate reaching MCs. Together, these studies indicate that neurons and MCs exhibit correlated activity during a critical period of retinal maturation that is enabled by neurotransmitter spillover from retinal 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.

The role of the tripartite glutamatergic synapse in the pathophysiology of Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia in individuals over 65 years of age and is characterized by accumulation of beta-amyloid (Aβ) and tau. Both Aβ and tau alter synaptic plasticity, leading to synapse loss, neural network dysfunction, and eventually neuron loss. However, the exact mechanism by which these proteins cause neurodegeneration is still not clear. A growing body of evidence suggests perturbations in the glutamatergic tripartite synapse, comprised of a presynaptic terminal, a postsynaptic spine, and an astrocytic process, may underlie the pathogenic mechanisms of AD. Glutamate is the primary excitatory neurotransmitter in the brain and plays an important role in learning and memory, but alterations in glutamatergic signaling can lead to excitotoxicity. This review discusses the ways in which both beta-amyloid (Aβ) and tau act alone and in concert to perturb synaptic functioning of the tripartite synapse, including alterations in glutamate release, astrocytic uptake, and receptor signaling. Particular emphasis is given to the role of N-methyl-D-aspartate (NMDA) as a possible convergence point for Aβ and tau toxicity.

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.

TRP channels coordinate ion signalling in astroglia

Astroglial excitability is based on highly spatio-temporally coordinated fluctuations of intracellular ion concentrations, among which changes in Ca(2+) and Na(+) take the leading role. Intracellular signals mediated by Ca(2+) and Na(+) target numerous molecular cascades that control gene expression, energy production and numerous homeostatic functions of astrocytes. Initiation of Ca(2+) and Na(+) signals relies upon plasmalemmal and intracellular channels that allow fluxes of respective ions down their concentration gradients. Astrocytes express several types of TRP channels of which TRPA1 channels are linked to regulation of functional expression of GABA transporters, whereas TRPV4 channels are activated following osmotic challenges and are up-regulated in ischaemic conditions. Astrocytes also ubiquitously express several isoforms of TRPC channels of which heteromers assembled from TRPC1, 4 and/or 5 subunits that likely act as stretch-activated channels and are linked to store-operated Ca(2+) entry. The TRPC channels mediate large Na(+) fluxes that are associated with the endoplasmic reticulum Ca(2+) signalling machinery and hence coordinate Na(+) and Ca(2+) signalling in astroglia.

Neurotransmitters and integration in neuronal-astroglial networks

Two major neural cell types, glia, astrocytes in particular, and neurones can release chemical transmitters that act as soluble signalling compounds for intercellular communication. Exocytosis, a process which depends on an increase in cytosolic Ca(2+) levels, represents a common denominator for release of neurotransmitters, stored in secretory vesicles, from these neural cells. While neurones rely predominately on the immediate entry of Ca(2+) from the extracellular space to the cytosol in this process, astrocytes support their cytosolic Ca(2+) increases by appropriating this ion from the intracellular endoplasmic reticulum store and extracellular space. Additionally, astrocytes can release neurotransmitters using a variety of non-vesicular pathways which are mediated by an assortment of plasmalemmal channels and transporters. Once a neuronal and/or astrocytic neurotransmitter is released into the extracellular space, it can activate plasma membrane neurotransmitter receptors on neural cells, causing autocrine and/or paracrine signalling. Moreover, chemical transmission is essential not only for homocellular, but also for heterocellular bi-directional communication in the brain. Further detailed understanding of chemical transmission will aid our comprehension of the brain (dys)function in heath and disease.

Glial cells in (patho)physiology

Neuroglial cells define brain homeostasis and mount defense against pathological insults. Astroglia regulate neurogenesis and development of brain circuits. In the adult brain, astrocytes enter into intimate dynamic relationship with neurons, especially at synaptic sites where they functionally form the tripartite synapse. At these sites, astrocytes regulate ion and neurotransmitter homeostasis, metabolically support neurons and monitor synaptic activity; one of the readouts of the latter manifests in astrocytic intracellular Ca(2+) signals. This form of astrocytic excitability can lead to release of chemical transmitters via Ca(2+) -dependent exocytosis. Once in the extracellular space, gliotransmitters can modulate synaptic plasticity and cause changes in behavior. Besides these physiological tasks, astrocytes are fundamental for progression and outcome of neurological diseases. In Alzheimer's disease, for example, astrocytes may contribute to the etiology of this disorder. Highly lethal glial-derived tumors use signaling trickery to coerce normal brain cells to assist tumor invasiveness. This review not only sheds new light on the brain operation in health and disease, but also points to many unknowns.

Homeostatic function of astrocytes: Ca(2+) and Na(+) signalling

The name astroglia unifies many non-excitable neural cells that act as primary homeostatic cells in the nervous system. Neuronal activity triggers multiple homeostatic responses of astroglia that include increase in metabolic activity and synthesis of neuronal preferred energy substrate lactate, clearance of neurotransmitters and buffering of extracellular K(+) ions to name but a few. Many (if not all) of astroglial homeostatic responses are controlled by dynamic changes in the cytoplasmic concentration of two cations, Ca(2+) and Na(+). Intracellular concentration of these ions is tightly controlled by several transporters and can be rapidly affected by the activation of respective fluxes through ionic channels or ion exchangers. Here, we provide a comprehensive review of astroglial Ca(2+) and Na(+) signalling.

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

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

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.

Where the thoughts dwell: the physiology of neuronal-glial "diffuse neural net"

The mechanisms underlying the production of thoughts by exceedingly complex cellular networks that construct the human brain constitute the most challenging problem of natural sciences. Our understanding of the brain function is very much shaped by the neuronal doctrine that assumes that neuronal networks represent the only substrate for cognition. These neuronal networks however are embedded into much larger and probably more complex network formed by neuroglia. The latter, although being electrically silent, employ many different mechanisms for intercellular signalling. It appears that astrocytes can control synaptic networks and in such a capacity they may represent an integral component of the computational power of the brain rather than being just brain "connective tissue". The fundamental question of whether neuroglia is involved in cognition and information processing remains, however, open. Indeed, a remarkable increase in the number of glial cells that distinguishes the human brain can be simply a result of exceedingly high specialisation of the neuronal networks, which delegated all matters of survival and maintenance to the neuroglia. At the same time potential power of analogue processing offered by internally connected glial networks may represent the alternative mechanism involved in cognition.

Neuronismo y reticulismo: neuronal-glial circuits unify the reticular and neuronal theories of brain organization

The neuronal doctrine, which shaped the development of neuroscience, was born from a long-lasting struggle between reticularists, who assumed internal continuity of neural networks and neuronists, who defined the brain as a network of physically separated cellular entities, defined as neurones. Modern views regard the brain as a complex of constantly interacting cellular circuits, represented by neuronal networks embedded into internally connected astroglial syncytium. The neuronal-glial circuits endowed with distinct signalling cascades form a 'diffuse nervous net' suggested by Golgi, where millions of synapses belonging to very different neurones are integrated first into neuronal-glial-vascular units and then into more complex structures connected through glial syncytium. These many levels of integration, both morphological and functional, presented by neuronal-glial circuitry ensure the spatial and temporal multiplication of brain cognitive power.

Sodium signals in cerebellar Purkinje neurons and Bergmann glial cells evoked by glutamatergic synaptic transmission

Glial cells express specific high-affinity transporters for glutamate that play a central role in glutamate clearance at excitatory synapses in the brain. These transporters are electrogenic and are mainly energized by the electrochemical gradient for sodium. In the present study, we combined somatic whole-cell patch-clamp recordings with quantitative Na+ imaging in fine cellular branches of cerebellar Bergmann glial cells and in dendrites of Purkinje neurons to analyze intracellular Na+ signals close to activated synapses. We demonstrate that pressure application of glutamate and glutamate agonists causes local Na+ signals in the mM range. Furthermore, we analyzed the pharmacological profile, as well as the time course and spatial distribution of Na+ signals following short synaptic burst stimulation of parallel or climbing fibers. While parallel fibers stimulation resulted in local sodium transients that were largest in processes close to the stimulation pipette, climbing fibers stimulation elicited global sodium transients throughout the entire cell. Glial sodium signals amounted to several mM, were mainly caused by sodium influx following inward transport of glutamate and persisted for tens of seconds. Sodium transients in dendrites of Purkinje neurons, in contrast, were mainly caused by activation of AMPA receptors and had much faster kinetics. By reducing the driving force for sodium-dependent glutamate uptake, intracellular sodium accumulation in glial cells upon repetitive activity might provide a negative feedback mechanism, promoting the diffusion of glutamate and the activation of extrasynaptic glutamate receptors at active synapses in the cerebellum.

Discrimination of cell types in mixed cortical culture using calcium imaging: a comparison to immunocytochemical labeling

Neuronal-glial interactions in the central nervous system are important for both normal function and response to pathological states. Differences in calcium processing between these cell types may be exploited to allow dynamic differentiation using calcium-imaging protocols without the need to fix and immunostain the study population. Mixed rat primary cortical cultures were grown on coverslips, incubated for 30 min in 2 microM fluo-3 AM and mounted in a devised, low volume imaging chamber. Calcium influx was measured over the duration of a 50s exposure to 30 microM glutamate in all cells. Cells were then fixed in situ, and immunostained for NeuN and GFAP. Direct comparison between live calcium dynamics and cell type markers were made. Over the duration of the glutamate exposure, those cells that subsequently stained for NeuN exhibited a sustained increase in intracellular calcium, whereas GFAP positive and non-staining cells exhibited a decline over the duration of the glutamate exposure. We found that examining the average calcium fluorescence over the last 10s of glutamate exposure allowed the identification of cells as neuronal if the average was >85% of the maximal calcium change, or non-neuronal if the average was <85% of the maximal calcium change. This technique compares very favourably to the established technique of immunocytochemical labeling for the identification of cell types; both techniques agreed in their classification of cells as neuronal or non-neuronal 96.83% of the time. However, this technique cannot reliably distinguish between non-neuronal cell types.

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.

Glutamatergic and purinergic receptor-mediated calcium transients in Bergmann glial cells

Astrocytes respond to neuronal activity with [Ca2+]i increases after activation of specific receptors. Bergmann glial cells (BGs), astrocytes of the cerebellar molecular layer (ML), express various receptors that can mobilize internal Ca2+. BGs also express Ca2+ permeable AMPA receptors that may be important for maintaining the extensive coverage of Purkinje cell (PC) excitatory synapses by BG processes. Here, we examined Ca2+ signals in single BGs evoked by synaptic activity in cerebellar slices. Short bursts of high-frequency stimulation of the ML elicited Ca2+ transients composed of a small-amplitude fast rising phase, followed by a larger and slower rising phase. The first phase resulted from Ca2+ influx through AMPA receptors, whereas the second phase required release of Ca2+ from internal stores initiated by P2 purinergic receptor activation. We found that such Ca2+ responses could be evoked by direct activation of neurons releasing ATP onto BGs or after activation of metabotropic glutamate receptor 1 on these neurons. Moreover, examination of BG and PC responses to various synaptic stimulation protocols suggested that ML interneurons are likely the cellular source of ATP.

Dynamic inositol trisphosphate-mediated calcium signals within astrocytic endfeet underlie vasodilation of cerebral arterioles

Active neurons communicate to intracerebral arterioles in part through an elevation of cytosolic Ca²⁺ concentration ([Ca²⁺]_i) in astrocytes, leading to the generation of vasoactive signals involved in neurovascular coupling. In particular, [Ca²⁺]_i increases in astrocytic processes (“endfeet”), which encase cerebral arterioles, have been shown to result in vasodilation of arterioles in vivo. However, the spatial and temporal properties of endfoot [Ca²⁺]_i signals have not been characterized, and information regarding the mechanism by which these signals arise is lacking. [Ca²⁺]_i signaling in astrocytic endfeet was measured with high spatiotemporal resolution in cortical brain slices, using a fluorescent Ca²⁺ indicator and confocal microscopy. Increases in endfoot [Ca²⁺]_i preceded vasodilation of arterioles within cortical slices, as detected by simultaneous measurement of endfoot [Ca²⁺]_i and vascular diameter. Neuronal activity–evoked elevation of endfoot [Ca²⁺]_i was reduced by inhibition of inositol 1,4,5-trisphosphate (InsP₃) receptor Ca²⁺ release channels and almost completely abolished by inhibition of endoplasmic reticulum Ca²⁺ uptake. To probe the Ca²⁺ release mechanisms present within endfeet, spatially restricted flash photolysis of caged InsP₃ was utilized to liberate InsP₃ directly within endfeet. This maneuver generated large amplitude [Ca²⁺]_i increases within endfeet that were spatially restricted to this region of the astrocyte. These InsP₃-induced [Ca²⁺]_i increases were sensitive to depletion of the intracellular Ca²⁺ store, but not to ryanodine, suggesting that Ca²⁺-induced Ca²⁺ release from ryanodine receptors does not contribute to the generation of endfoot [Ca²⁺]_i signals. Neuronally evoked increases in astrocytic [Ca²⁺]_i propagated through perivascular astrocytic processes and endfeet as multiple, distinct [Ca²⁺]_i waves and exhibited a high degree of spatial heterogeneity. Regenerative Ca²⁺ release processes within the endfeet were evident, as were localized regions of Ca²⁺ release, and treatment of slices with the vasoactive neuropeptides somatostatin and vasoactive intestinal peptide was capable of inducing endfoot [Ca²⁺]_i increases, suggesting the potential for signaling between local interneurons and astrocytic endfeet in the cortex. Furthermore, photorelease of InsP₃ within individual endfeet resulted in a local vasodilation of adjacent arterioles, supporting the concept that astrocytic endfeet function as local “vasoregulatory units” by translating information from active neurons into complex InsP₃-mediated Ca²⁺ release signals that modulate arteriolar diameter.

Membrane currents and cytoplasmic sodium transients generated by glutamate transport in Bergmann glial cells

Effects of glutamate and kainate (KA) on Bergmann glial cells were investigated in mouse cerebellar slices using the whole-cell configuration of the patch-clamp technique combined with SBFI-based Na(+) microfluorimetry. L-glutamate (1 mM) and KA (100 microM) induced inward currents in Bergmann glial cells voltage-clamped at -70 mV. These currents were accompanied by an increase in intracellular Na+ concentration ([Na+](i)) from the average resting level of 5.2 +/- 0.5 mM to 26 +/- 5 mM and 33 +/- 7 mM, respectively. KA-evoked signals (1) were completely blocked in the presence of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM), an antagonist of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/KA ionotropic glutamate receptors; (2) reversed at 0 mV, and (3) disappeared in Na+ -free, N-methyl-D-glucamine (NMDG+)-containing solution, but remained almost unchanged in Na+ -free, Li+ -containing solution. Conversely, L-glutamate-induced signals (1) were marginally CNQX sensitive (approximately 10% inhibition), (2) did not reverse at a holding potential of +20 mV, (3) were markedly suppressed by Na+ substitution with both NMDG+ and Li+, and (4) were inhibited by D,L-threo-beta-benzyloxyaspartate. Further, D-glutamate, L -, and D-aspartate were also able to induce Na+ -dependent inward current. Stimulation of parallel fibres triggered inward currents and [Na+](i) transients that were insensitive to CNQX and MK-801; hence, we suggested that synaptically released glutamate activates glutamate/Na+ transporter in Bergmann glial cells, which produces a substantial increase in intracellular Na+ concentration.

Calcium signaling in specialized glial cells

This article reviews calcium signaling in three specialized types of glial cells: Müller cells of the retina, Bergmann glial cells of the cerebellum, and radial glial cells of the developing cortex. Müller cells generate spontaneous and neuronal activity-evoked increases in Ca(2+). Neuron to Müller cell signaling is mediated by neuronal release of ATP and activation of glial P2Y receptors. Müller cells, in turn, modulate neuronal excitability and mediate vasomotor responses. Bergmann glial cells also generate spontaneous and activity-evoked Ca(2+) increases. Neuron to Bergmann glia signaling is mediated by neuronal release of nitric oxide, noradrenaline, and glutamate. In Bergmann glia, Ca(2+) increases control the structural and functional interactions between these cells and Purkinje cell synapses. In the ventricular zone of the developing cortex, radial glial cells generate spontaneous Ca(2+) increases that propagate as Ca(2+) waves through clusters of neighboring glial cells. These Ca(2+) increases control cell proliferation and neurogenesis.

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.

Calcium microdomains in regulated exocytosis

Katz and co-workers showed that Ca(2+) triggers exocytosis. The existence of sub-micrometer domains of greater than 100 microM [Ca(2+)](i) was postulated on theoretical grounds. Using a modified, low-affinity aequorin, Llinas et al. were the first to demonstrate the existence of Ca(2+) 'microdomains' in squid presynaptic terminals. Over the past several years, it has become clear that individual Ca(2+) nano- and microdomains forming around the mouth of voltage-gated Ca(2+) channels ascertain the tight coupling of fast synaptic vesicle release to membrane depolarization by action potentials. Recent work has established different geometric arrangements of vesicles and Ca(2+) channels at different central synapses and pointed out the role of Ca(2+) syntillas - localized, store operated Ca(2+) signals - in facilitation and spontaneous release. The coupling between Ca(2+) increase and evoked exocytosis is more sluggish in peripheral terminals and neuroendocrine cells, where channels are less clustered and Ca(2+) comes from different sources, including Ca(2+) influx via the plasma membrane and the mobilization of Ca(2+) from intracellular stores. Finally, also non- (electrically) excitable cells display highly localized Ca(2+) signaling domains. We discuss in particular the organization of structural microdomains of Bergmann glia, specialized astrocytes of the cerebellum that have only recently been considered as secretory cells. Glial microdomains are the spatial substrate for functionally segregated Ca(2+) signals upon metabotropic activation. Our review emphasizes the large diversity of different geometric arrangements of vesicles and Ca(2+) sources, leading to a wide spectrum of Ca(2+) signals triggering release.

Neuronal dysfunction in Down syndrome: contribution of neuronal models in cell culture

Down syndrome (DS) in humans, or trisomy of autosome 21, represents the hyperdiploidy that most frequently survives gestation, reaching an incidence of 1 in 700 live births. The condition is associated with multisystemic anomalies, including those affecting the central nervous system (CNS), determining a characteristic mental retardation. At a neuronal level, our group and others have shown that the condition determines marked alterations of action potential and ionic current kinetics, which may underlie abnormal processing of information by the CNS. Since the use of human tissue presents both practical and ethical problems, animal models of the human condition have been sought. Murine trisomy 16 (Ts16) is a model of the human condition, due to the great homology between human autosome 21 and murine 16. Both conditions share the same alterations of electrical membrane properties. However, the murine Ts16 condition is unviable (animals die in utero), thus limiting the quantity of tissue procurable. To overcome this obstacle, we have established immortal cell lines from normal and Ts16 mice with a method developed by our group that allows the stable in vitro immortalization of mammalian tissue, yielding cell lines which retain the characteristics of the originating cells. Cell lines derived from cerebral cortex, hippocampus, spinal cord and dorsal root ganglion of Ts16 animals show alterations of intracellular Ca2+ signals in response to several neurotransmitters (glutamate, acetylcholine, and GABA). Gene overdose most likely underlies these alterations in cell function, and the identification of the relative contribution of DS associated genes on such specific neuronal dysfunction should be investigated. This could enlighten our understanding on the contribution of these genes in DS, and identify new therapeutic targets.

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.

Interactions between Purkinje neurones and Bergmann glia

Throughout the development of the cerebellar cortex, Purkinje neurones interact closely with Bergmann glial cells, a specialized form of astrocyte. This review summarizes the intimate developmental, anatomical and functional relationships between these two cell types, with particular emphasis on recent discoveries regarding glutamate release from climbing and parallel fibres as a pathway for signalling synaptic activity to Bergmann glia.

NMDA receptors mediate neuron-to-glia signaling in mouse cortical astrocytes

Chemical transmission between neurons and glial cells is an important element of integration in the CNS. Here, we describe currents activated by NMDA in cortical astrocytes, identified in transgenic mice that express enhanced green fluorescent protein under control of the human glial fibrillary acidic protein promoter. Astrocytes were studied by whole-cell voltage clamp either in slices or after gentle nonenzymatic mechanical dissociation. Acutely isolated astrocytes showed a three-component response to glutamate. The initial rapid component was blocked by 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide (NBQX), which is an antagonist of AMPA receptors (IC50, 2 microM), and the NMDA receptor antagonist D-AP-5 blocked the later sustained component (IC50, 0.6 microM). The third component of glutamate application response was sensitive to D,L-threo-beta-benzyloxyaspartate, a glutamate transporter blocker. Fast application of NMDA evoked concentration-dependent inward currents (EC50, 0.3 microM); these showed use-dependent block by (+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate (MK-801). These NMDA-evoked currents were linearly dependent on membrane potential and were not affected by extracellular magnesium at concentrations up to 10 mM. Electrical stimulation of axons in layer IV-VI induced a complex inward current in astrocytes situated in the cortical layer II, part of which was sensitive to MK-801 at holding potential -80 mV and was not affected by the AMPA glutamate receptor antagonist NBQX. The fast miniature spontaneous currents were observed in cortical astrocytes in slices as well. These currents exhibited both AMPA and NMDA receptor-mediated components. We conclude that cortical astrocytes express functional NMDA receptors that are devoid of Mg2+ block, and these receptors are involved in neuronal-glial signal transmission.

Characterization of AMPA receptors targeted by the climbing fiber transmitter mediating presynaptic inhibition of GABAergic transmission at cerebellar interneuron-Purkinje cell synapses

The climbing fiber (CF) neurotransmitter not only excites the postsynaptic Purkinje cell (PC) but also suppresses GABA release from inhibitory interneurons converging onto the same PC depending on AMPA-type glutamate receptor (AMPAR) activation. Although the CF-/AMPAR-mediated inhibition of GABA release provides a likely mechanism boosting the CF input-derived excitation, how the CF transmitter reaches target AMPARs to elicit this action remains unknown. Here, we report that the CF transmitter diffused from its release sites directly targets GluR2/GluR3 AMPARs on interneuron terminals to inhibit GABA release. A weak GluR3-AMPAR agonist, bromohomoibotenic acid, produced excitatory currents in the postsynaptic PCs without presynaptic inhibitory effect on GABAergic transmission. Conversely, a specific inhibitor of the GluR2-lacking/Ca2+-permeable AMPARs, philanthotoxin-433, did not affect the CF-induced inhibition but suppressed AMPAR-mediated currents in Bergmann glia. A low-affinity GluR antagonist, gamma-D-glutamylglycine, or retardation of neurotransmitter diffusion by dextran reduced the inhibitory action of CF-stimulation, whereas blockade of glutamate transporters enhanced the CF-induced inhibition. The results suggest that the CF transmitter released after repeated stimulation overwhelms local glutamate uptake and thereby diffuses from the release site to reach GluR2/GluR3 AMPARs on nearby interneuron terminals. Double immunostaining showed that GluR2/3 subunits and glutamate decarboxylase or synaptophysin are colocalized at the perisomatic GABAergic processes surrounding PCs. Finally, electron microscopy detected specific immunoreactivity for GluR2/3 at the presynaptic terminals of symmetric axosomatic synapses on the PC. These findings demonstrate that the CF transmitter directly inhibits GABA release from interneurons to the PC, relying on extrasynaptic diffusion and local heterogeneity in AMPAR subunit compositions.

Short-term plasticity of Bergmann glial cell extrasynaptic currents during parallel fiber stimulation in rat cerebellum

Bergmann glial cells (BGC) enclose the synapses of Purkinje neurons (PN) and interneurons in the molecular layer of the cerebellar cortex. During synaptic transmission, glutamate evokes inward currents in the glia by activation of Ca2+-permeable aminohydroxymethylisoxazole propionic acid receptors (AMPAR) and electrogenic transporters. We describe the plasticity of BGC currents during paired-pulse and repetitive stimulation of parallel fibers in cerebellar slices. Paired-pulse facilitation (PPF) of BGC AMPAR currents was 4-fold, twice that of PN PPF. Experiments with a low-affinity AMPAR antagonist showed an increase in extrasynaptic glutamate concentration during the second pulse of the pair. PPF of glial transporter currents was 1.8-fold, similar to synaptic PPF. Tetanic stimulation revealed that facilitation of BGC AMPAR currents is not sustained during high-frequency stimulation, and substantial depression is observed after a few pulses. Consequently, Ca2+ influx through glial AMPARs would initially be facilitated but subsequently depressed, generating a transient Ca2+ influx in response to a sustained tetanus. This pattern of plasticity may be important in enabling Bergmann glial cell processes to detect and support synapses with high-frequency input. Finally, a new current was observed in BGC during repetitive stimulation. It was blocked by NBQX and intracellular GDP-beta-S, increased by glutamate uptake inhibition, had PPF similar to synaptic PPF, and was unaffected by an inhibitor of fast glial AMPAR currents. The evidence suggests that activation of neuronal AMPARs causes the release of a paracrine messenger to activate a G-protein coupled receptor in the BGC.