Multiple conductance channels in type-2 cerebellar astrocytes activated by excitatory amino acids

Photonics tools begin to clarify astrocyte calcium transients

Astrocytes integrate information from neurons and the microvasculature to coordinate brain activity and metabolism. Using a variety of calcium-dependent cellular mechanisms, these cells impact numerous aspects of neurophysiology in health and disease. Astrocyte calcium signaling is highly diverse, with complex spatiotemporal features. Here, we review astrocyte calcium dynamics and the optical imaging tools used to measure and analyze these events. We briefly cover historical calcium measurements, followed by our current understanding of how calcium transients relate to the structure of astrocytes. We then explore newer photonics tools including super-resolution techniques and genetically encoded calcium indicators targeted to specific cellular compartments and how these have been applied to astrocyte biology. Finally, we provide a brief overview of analysis software used to accurately quantify the data and ultimately aid in our interpretation of the various functions of astrocyte calcium transients.

Single-channel mechanisms underlying the function, diversity and plasticity of AMPA receptors

The functional properties of AMPA receptors shape many of the essential features of excitatory synaptic signalling in the brain, including high-fidelity point-to-point transmission and long-term plasticity. Understanding the behaviour and regulation of single AMPAR channels is fundamental in unravelling how central synapses carry, process and store information. There is now an abundance of data on the importance of alternative splicing, RNA editing, and phosphorylation of AMPAR subunits in determining central synaptic diversity. Furthermore, auxiliary subunits have emerged as pivotal players that regulate AMPAR channel properties and add further diversity. Single-channel studies have helped reveal a fascinating picture of the unique behaviour of AMPAR channels - their concentration-dependent single-channel conductance, the basis of their multiple-conductance states, and the influence of auxiliary proteins in controlling many of their gating and conductance properties. Here we summarize basic hallmarks of AMPAR single-channels, in relation to function, diversity and plasticity. We also present data that reveal an unexpected feature of AMPAR sublevel behaviour. This article is part of the special Issue on 'Glutamate Receptors - AMPA receptors'.

NMDA receptors elicit flux-independent intracellular Ca2+ signals via metabotropic glutamate receptors and flux-dependent nitric oxide release in human brain microvascular endothelial cells

The excitatory neurotransmitter glutamate gates post-synaptic N-methyl-d-aspartate (NMDA) receptors (NMDARs) to mediate extracellular Ca²⁺ entry and stimulate neuronal nitric oxide (NO) synthase to release NO and trigger neurovascular coupling (NVC). Neuronal and glial NMDARs may also operate in a flux-independent manner, although it is unclear whether their non-ionotropic mode of action is involved in NVC. Recently, endothelial NMDARs were found to trigger Ca²⁺-dependent NO production and induce NVC, but the underlying mode of signaling remains elusive. Herein, we report that GluN1 protein, as well as GluN2C and GluN3B transcripts and proteins, were expressed and that NMDA did not elicit inward currents, but induced a dose-dependent increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) in the human brain microvascular endothelial cell line, hCMEC/D3. A multidisciplinary approach, including live cell imaging, whole-cell patch-clamp recordings, pharmacological manipulation and gene targeting, revealed that NMDARs increase the [Ca²⁺]_i in a flux-independent manner in hCMEC/D3 cells. The Ca²⁺ response to NMDA was triggered by endogenous Ca²⁺ release from the endoplasmic reticulum and the lysosomal Ca²⁺ stores and sustained by store-operated Ca²⁺ entry. Unexpectedly, pharmacological and genetic blockade of mGluR1 and mGluR5 dramatically impaired NMDARs-mediated Ca²⁺ signals. These findings indicate that NMDARs may increase the endothelial [Ca²⁺]_i in a flux-independent manner via group 1 mGluRs. However, imaging of DAF-FM fluorescence revealed that NMDARs may also induce Ca²⁺-dependent NO release by signaling in a flux-dependent manner. These findings, therefore, shed novel light on the mechanisms whereby brain microvascular endothelium decodes glutamatergic signaling and regulates NVC.

Glutamate Activates AMPA Receptor Conductance in the Developing Schwann Cells of the Mammalian Peripheral Nerves

Schwann cells (SCs) are myelinating cells of the PNS. Although SCs are known to express different channels and receptors on their surface, little is known about the activation and function of these proteins. Ionotropic glutamate receptors are thought to play an essential role during development of SC lineage and during peripheral nerve injury, so we sought to study their functional properties. We established a novel preparation of living peripheral nerve slices with preserved cellular architecture and used a patch-clamp technique to study AMPA-receptor (AMPAR)-mediated currents in SCs for the first time. We found that the majority of SCs in the nerves dissected from embryonic and neonatal mice of both sexes respond to the application of glutamate with inward current mediated by Ca²⁺-permeable AMPARs. Using stationary fluctuation analysis (SFA), we demonstrate that single-channel conductance of AMPARs in SCs is 8-11 pS, which is comparable to that in neurons. We further show that, when SCs become myelinating, they downregulate functional AMPARs. This study is the first to demonstrate AMPAR-mediated conductance in SCs of vertebrates, to investigate elementary properties of AMPARs in these cells, and to provide detailed electrophysiological and morphological characterization of SCs at different stages of development.SIGNIFICANCE STATEMENT We provide several important conceptual and technical advances in research on the PNS. We pioneer the first description of AMPA receptor (AMPAR)-mediated currents in the PNS glia of vertebrates and provide new insights into the properties of AMPAR channels in peripheral glia; for example, their Ca²⁺ permeability and single-channel conductance. We describe for the first time the electrophysiological and morphological properties of Schwann cells (SCs) at different stages of development and show that functional AMPARs are expressed only in developing, not mature, SCs. Finally, we introduce a preparation of peripheral nerve slices for patch-clamp recordings. This preparation opens new possibilities for studying the physiology of SCs in animal models and in surgical human samples.

Astrocyte-Dependent Slow Inward Currents (SICs) Participate in Neuromodulatory Mechanisms in the Pedunculopontine Nucleus (PPN)

Slow inward currents (SICs) are known as excitatory events of neurons caused by astrocytic glutamate release and consequential activation of neuronal extrasynaptic NMDA receptors. In the present article we investigate the role of these astrocyte-dependent excitatory events on a cholinergic nucleus of the reticular activating system (RAS), the pedunculopontine nucleus (PPN). It is well known about this and other elements of the RAS, that they do not only give rise to neuromodulatory innervation of several areas, but also targets neuromodulatory actions from other members of the RAS or factors providing the homeostatic drive for sleep. Using slice electrophysiology, optogenetics and morphological reconstruction, we revealed that SICs are present in a population of PPN neurons. The frequency of SICs recorded on PPN neurons was higher when the soma of the given neuron was close to an astrocytic soma. SICs do not appear simultaneously on neighboring neurons, thus it is unlikely that they synchronize neuronal activity in this structure. Occurrence of SICs is regulated by cannabinoid, muscarinic and serotonergic neuromodulatory mechanisms. In most cases, SICs occurred independently from tonic neuronal currents. SICs were affected by different neuromodulatory agents in a rather uniform way: if control SIC activity was low, the applied drugs increased it, but if SIC activity was increased in control, the same drugs lowered it. SICs of PPN neurons possibly represent a mechanism which elicits network-independent spikes on certain PPN neurons; forming an alternative, astrocyte-dependent pathway of neuromodulatory mechanisms.

Granular Layer Neurons Control Cerebellar Neurovascular Coupling Through an NMDA Receptor/NO-Dependent System

Neurovascular coupling (NVC) is the process whereby neuronal activity controls blood vessel diameter. In the cerebellum, the molecular layer is regarded as the main NVC determinant. However, the granular layer is a region with variable metabolic demand caused by large activity fluctuations that shows a prominent expression of NMDA receptors (NMDARs) and nitric oxide synthase (NOS) and is therefore much more suitable for effective NVC. Here, we show, in the granular layer of acute rat cerebellar slices, that capillary diameter changes rapidly after mossy fiber stimulation. Vasodilation required neuronal NMDARs and NOS stimulation and subsequent guanylyl cyclase activation that probably occurred in pericytes. Vasoconstriction required metabotropic glutamate receptors and CYP ω-hydroxylase, the enzyme regulating 20-hydroxyeicosatetraenoic acid production. Therefore, granular layer capillaries are controlled by the balance between vasodilating and vasoconstricting systems that could finely tune local blood flow depending on neuronal activity changes at the cerebellar input stage.

SIGNIFICANCE STATEMENT

The neuronal circuitry and the biochemical pathways that control local blood flow supply in the cerebellum are unclear. This is surprising given the emerging role played by this brain structure, not only in motor behavior, but also in cognitive functions. Although previous studies focused on the molecular layer, here, we shift attention onto the mossy fiber granule cell (GrC) relay. We demonstrate that GrC activity causes a robust vasodilation in nearby capillaries via the NMDA receptors-neuronal nitric oxide synthase signaling pathway. At the same time, metabotropic glutamate receptors mediate 20-hydroxyeicosatetraenoic acid-dependent vasoconstriction. These results reveal a complex signaling network that hints for the first time at the granular layer as a major determinant of cerebellar blood-oxygen-level-dependent signals.

Aquaporins: Multiple Roles in the Central Nervous System

Aquaporins (AQPs) represent a diverse family of membrane proteins found in prokaryotes and eukaryotes. The primary aquaporins expressed in the mammalian brain are AQP1, which is densely packed in choroid plexus cells lining the ventricles, and AQP4, which is abundant in astrocytes and concentrated especially in the end-feet structures that surround capillaries throughout the brain and are present in glia limitans structures, notably in osmosensory areas such the supraoptic nucleus. Water movement in brain tissues is carefully regulated from the micro- to macroscopic levels, with aquaporins serving key roles as multifunctional elements of complex signaling assemblies. Intriguing possibilities suggest links for AQP1 in Alzheimer's disease, AQP4 as a target for therapy in brain edema, and a possible contribution of AQP9 in Parkinson's disease. For all the aquaporins, new contributions to physiological functions are likely to continue to be discovered with ongoing work in this rapidly expanding field of research. NEUROSCIENTIST 13(5):470—485, 2007.

Hemichannels: new roles in astroglial function

The role of astrocytes in brain function has evolved over the last decade, from support cells to active participants in the neuronal synapse through the release of "gliotransmitters."Astrocytes express receptors for most neurotransmitters and respond to them through Ca(2+) intracellular oscillations and propagation of intercellular Ca(2+) waves. While such waves are able to propagate among neighboring astrocytes through gap junctions, thereby activating several astrocytes simultaneously, they can also trigger the release of gliotransmitters, including glutamate, d-serine, glycine, ATP, adenosine, or GABA. There are several mechanisms by which gliotransmitter release occurs, including functional hemichannels. These gliotransmitters can activate neighboring astrocytes and participate in the propagation of intercellular Ca(2+) waves, or activate pre- and post-synaptic receptors, including NMDA, AMPA, and purinergic receptors. In consequence, hemichannels could play a pivotal role in astrocyte-to-astrocyte communication and astrocyte-to-neuron cross-talk. Recent evidence suggests that astroglial hemichannels are involved in higher brain functions including memory and glucose sensing. The present review will focus on the role of hemichannels in astrocyte-to-astrocyte and astrocyte-to neuron communication and in brain physiology.

NMDA Receptors in Glial Cells: Pending Questions

Glutamate receptors of the N-methyl-D-aspartate (NMDA) type are involved in many cognitive processes, including behavior, learning and synaptic plasticity. For a long time NMDA receptors were thought to be the privileged domain of neurons; however, discoveries of the last 25 years have demonstrated their active role in glial cells as well. Despite the large number of studies in the field, there are many unresolved questions connected with NMDA receptors in glia that are still a matter of debate. The main objective of this review is to shed light on these controversies by summarizing results from all relevant works concerning astrocytes, oligodendrocytes and polydendrocytes (also known as NG2 glial cells) in experimental animals, further extended by studies performed on human glia. The results are divided according to the study approach to enable a better comparison of how findings obtained at the mRNA level correspond with protein expression or functionality. Furthermore, special attention is focused on the NMDA receptor subunits present in the particular glial cell types, which give them special characteristics different from those of neurons – for example, the absence of Mg²⁺ block and decreased Ca²⁺ permeability. Since glial cells are implicated in important physiological and pathophysiological roles in the central nervous system (CNS), the last part of this review provides an overview of glial NMDA receptors with respect to ischemic brain injury.

Modes of glutamate receptor gating

The time course of excitatory synaptic currents, the major means of fast communication between neurons of the central nervous system, is encoded in the dynamic behaviour of post-synaptic glutamate-activated channels. First-pass attempts to explain the glutamate-elicited currents with mathematical models produced reaction mechanisms that included only the most basic functionally defined states: resting vs. liganded, closed vs. open, responsive vs. desensitized. In contrast, single-molecule observations afforded by the patch-clamp technique revealed an unanticipated kinetic multiplicity of transitions: from microseconds-lasting flickers to minutes-long modes. How these kinetically defined events impact the shape of the synaptic response, how they relate to rearrangements in receptor structure, and whether and how they are physiologically controlled represent currently active research directions. Modal gating, which refers to the slowest, least frequently observed ion-channel transitions, has been demonstrated for representatives of all ion channel families. However, reaction schemes have been largely confined to the short- and medium-range time scales. For glutamate receptors as well, modal gating has only recently come under rigorous scrutiny. This article reviews the evidence for modal gating of glutamate receptors and the still developing hypotheses about the mechanism(s) by which modal shifts occur and the ways in which they may impact the time course of synaptic transmission.

Fate of neuron-glia synapses during proliferation and differentiation of NG2 cells

Progenitor cells expressing proteoglycan NG2 (also known as oligodendrocyte precursor cells or polydendrocytes) are widespread in the grey and white matter of the CNS; they comprise 8-9% of the total cell population in adult white matter, and 2-3% of total cells in adult grey matter. NG2 cells have a complex stellate morphology, with highly branched processes that may extend more than 100 μm around the cell body. NG2 cells express a complex set of voltage-gated channels, AMPA/kainate and/or γ-aminobutyric acid (GABA)(A) receptors, and receive glutamatergic and/or GABAergic synaptic input from neurons. In every region of the brain NG2 cells are found as proliferative cells, and the fraction of actively cycling NG2 cells is quite high in young as well as in adult animals. During development NG2 cells either differentiate into myelinating oligodendrocytes (and possibly also few astrocytes and neurons) or persist in the brain parenchyma as NG2 cells. This review highlights new findings related to the morphological and electrophysiological changes of NG2 cells, and the fate of synaptic input between neurons and NG2 cells during proliferation and differentiation of these cells in the neonatal and adult nervous system of rodents.

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.

Organic electronics for precise delivery of neurotransmitters to modulate mammalian sensory function

Significant advances have been made in the understanding of the pathophysiology, molecular targets and therapies for the treatment of a variety of nervous-system disorders. Particular therapies involve electrical sensing and stimulation of neural activity, and significant effort has therefore been devoted to the refinement of neural electrodes. However, direct electrical interfacing suffers from some inherent problems, such as the inability to discriminate amongst cell types. Thus, there is a need for novel devices to specifically interface nerve cells. Here, we demonstrate an organic electronic device capable of precisely delivering neurotransmitters in vitro and in vivo. In converting electronic addressing into delivery of neurotransmitters, the device mimics the nerve synapse. Using the peripheral auditory system, we show that out of a diverse population of cells, the device can selectively stimulate nerve cells responding to a specific neurotransmitter. This is achieved by precise electronic control of electrophoretic migration through a polymer film. This mechanism provides several sought-after features for regulation of cell signalling: exact dosage determination through electrochemical relationships, minimally disruptive delivery due to lack of fluid flow, and on-off switching. This technology has great potential as a therapeutic platform and could help accelerate the development of therapeutic strategies for nervous-system disorders.

Diffusion modeling of ATP signaling suggests a partially regenerative mechanism underlies astrocyte intercellular calcium waves

Network signaling through astrocyte syncytiums putatively contribute to the regulation of a number of both physiological and pathophysiological processes in the mammalian central nervous system. As such, an understanding of the underlying mechanisms is critical to determining any roles played by signaling through astrocyte networks. Astrocyte signaling is primarily mediated by the propagation of intercellular calcium waves (ICW) in the sense that paracrine signaling results in measurable intracellular calcium transients. Although the molecular mechanisms are relatively well known, there is conflicting data regarding the mechanism by which the signal propagates through the network. Experimentally there is evidence for both a point source signaling model in which adenosine triphosphate (ATP) is released by an initially activated astrocyte only, and a regenerative signaling model in which downstream astrocytes release ATP. We modeled both conditions as a simple lumped parameter phenomenological diffusion model and show that the only possible mechanism that can accurately reproduce experimentally measured results is a dual signaling mechanism that incorporates elements of both proposed signaling models. Specifically, we were able to accurately simulate experimentally measured in vitroICW dynamics by assuming a point source signaling model with a downstream regenerative component. These results suggest that seemingly conflicting data in the literature are actually complimentary, and represents a highly efficient and robustly engineered signaling mechanism.

The triggering of astrocytic calcium waves by NMDA-induced neuronal activation

It has been well established that astrocytes possess functional receptors for the excitatory neurotransmitter glutamate and respond to physiological concentrations of this substance with oscillations in cytoplasmic Ca2+ concentrations and spatially propagating Ca2+ waves. These findings strongly suggest that glutamate released during synaptic transmission triggers such phenomena within the perisynaptic astrocyte in situ. We test this hypothesis in two preparations, the organotypic hippocampal slice and hippocampal neuron-astrocyte co-cultures, using the Ca2+ indicator fluo-3 and confocal laser microscopy. An agonist for the N-methyl-D-aspartate (NMDA)-preferring glutamate receptor is employed to stimulate neuronal populations specifically, leaving the astrocytic population unaffected as these cells appear to lack this glutamate receptor subtype. Such pharmacological stimulation initially elicits large Ca2+ transients within the neuronal populations, followed by Ca2+ spikes in surrounding astrocytes, presumably as the result of neuronal glutamate release. During continuous neuronal stimulation, the astrocyte's Ca2+ response becomes oscillatory, with a period averaging 33 s and ranging from 15 to 50 s at 21 degrees C. These findings establish another form of communication within the brain, that between neurons and astrocytes, which perhaps acts to couple astrocytic regulatory responses to neuronal activity.

Astrocyte calcium waves: what they are and what they do

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

Transient versus prolonged hyperlocomotion following lateral fluid percussion injury in mongolian gerbils

Posttraumatic hyperactivity is a neurobehavioral symptom commonly seen in patients after traumatic brain injury (TBI). No useful animal model has yet been established for evaluation of this important symptom. We induced either mild (MILD, 0.7-0.9 atm) or moderate (MOD, 1.3-1.6 atm) lateral fluid percussion injury (LFPI) in Mongolian gerbils. Open-field and T-maze tests were used during a 7-day period after the trauma. All animals were perfusion fixed for histopathological examinations. Transient locomotor hyperactivity was found with a peak at 6 hr after injury in the MILD animals, whereas MOD animals showed prolonged and severe hyperlocomotion throughout the 7-day posttrauma period (P < 0.0001). Interestingly, the temporal profile of the posttraumatic hyperactivity was similar to that of the working memory deficit in both injury groups. Histological examination revealed significant neural tissue damages, including cortical necrosis, white matter rarefaction, and neuronal loss in the hippocampus in the ipsilateral hemisphere of the MOD animals, vs. only negligible changes in the MILD animals. Correlation analysis revealed that the volume of white matter lesions was significantly correlated with both posttraumatic hyperactivity (r = 0.591, P < 0.01) and working memory deficit (r = -0.859, P < 0.0001). Taken together, our findings confirm the successful reproduction of posttraumatic hyperactivity following experimental TBI. The posttraumatic hyperlocomotion probably shared pathomechanisms common to those of cognitive dysfunction caused by LFPI, supporting the speculation from previous studies that some neurobehavioral abnormities intimately correlate with TBI-induced cognitive dysfunction. Histopathologically, significant involvement of white matter damage in the posttraumatic functional deficits was indicated.

The role of glial membrane ion channels in seizures and epileptogenesis

Epilepsy is one of the most common neurological disorders, but the cellular basis of human epilepsy remains largely a mystery, and about 30% of all epilepsies remain uncontrolled. The vast bulk of epilepsy research has focused on neuronal and synaptic mechanisms, but the hypersynchronous firing that is the hallmark of epilepsy could also result from the abnormal function of glial cells by virtue of their critical role in the homeostasis of the brain's extracellular milieu. Therefore, increasing our understanding of glial pro-epileptic and epileptogenic mechanisms holds promise for the development of improved pharmacological treatments for epilepsy. Reactive astrocytes, a prominent feature of the human epileptic brain, undergo changes in their membrane properties and electrophysiology, in particular in the expression of membrane K(+) and Na(+) channels, which result in pro-epileptic changes in their homeostatic control of the extracellular space. Nonetheless, a causal role for reactive astrocytosis in epilepsy has been difficult to determine because glial reactivity can be induced by a wide range of central nervous system insults, including epileptic seizures themselves. A complicating factor is that different insults to the central nervous system result in reactive astrocytes with different membrane properties. Therefore, most animal models of epilepsy preselect the properties of the reactive glia studied. Finally, a causal role for reactive glia in epilepsy cannot be firmly established by examining human epileptic tissue because of its chronic and pharmacoresistant pathological condition that warranted the surgical intervention. Therefore, the development of clinically relevant models of reactive astrocytosis, and of symptomatic epileptogenesis, is needed to investigate the issue. A recently developed model of post-traumatic epileptogenesis in the rat, where chronic spontaneous recurrent seizures develop after a single event of a clinically relevant form of closed head injury, the fluid percussion injury, offers hope to help understand the role of reactive glia in seizures and epileptogenesis and lead to the development of improved therapies.

Astrocyte apoptosis: implications for neuroprotection

Astrocytes, the most abundant glial cell types in the brain, provide metabolic and trophic support to neurons and modulate synaptic activity. Accordingly, impairment in these astrocyte functions can critically influence neuronal survival. Recent studies show that astrocyte apoptosis may contribute to pathogenesis of many acute and chronic neurodegenerative disorders, such as cerebral ischemia, Alzheimer's disease and Parkinson's disease. We found that incubation of cultured rat astrocytes in a Ca(2+)-containing medium after exposure to a Ca(2+)-free medium causes an increase in intracellular Ca(2+) concentration followed by apoptosis, and that NF-kappa B, reactive oxygen species, and enzymes such as calpain, xanthine oxidase, calcineurin and caspase-3 are involved in reperfusion-induced apoptosis. Furthermore, we demonstrated that heat shock protein, mitogen-activated protein/extracellular signal-regulated kinase, phosphatidylinositol-3 kinase and cyclic GMP phosphodiesterase are target molecules for anti-apoptotic drugs. This review summarizes (1) astrocytic functions in neuroprotection, (2) current evidence of astrocyte apoptosis in both in vitro and in vivo studies including its molecular pathways such as Ca(2+) overload, oxidative stress, NF-kappa B activation, mitochondrial dysfunction, endoplasmic reticulum stress, and protease activation, and (3) several drugs preventing astrocyte apoptosis. As a whole, this article provides new insights into the potential role of astrocytes as targets for neuroprotection. In addition, the advance in the knowledge of molecular mechanisms of astrocyte apoptosis may lead to the development of novel therapeutic strategies for neurodegenerative disorders.

Electrical and pharmacological properties of the slow depolarization induced by Schaffer collateral stimulation in astrocytes of the rat hippocampal CA1 region

Intracellular recordings were made from astrocytes of the hippocampal CA1 region in rat slice preparations. A single stimulus at the Schaffer collaterals with high intensities (>/=10 V), but not low intensities (<10 V), induced a slow depolarization similar to the evoked postsynaptic potentials observed in hippocampal CA1 neurons. The slow depolarization was almost abolished in the presence of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (20 microM) and N-methyl-D-aspartate receptor antagonist DL-2-amino-5-phosphonopentanoic acid (100 microM). Tetrodotoxin (0.5 microM) or Co(2+) (2 mM) also abolished this slow depolarization. The slow depolarization reversed its polarity at -8 mV in the presence of tetraethylammonium (20 mM). These results suggest that the synaptically released glutamate (Glu) induces depolarization, which is mainly mediated by ionotropic Glu receptors, in astrocytes in the hippocampal CA1 region.

Both MK801 and NBQX reduce the neuronal damage after impact-acceleration brain injury

To understand the pathogenesis of diffuse axonal injury, we investigated the temporal and spatial profiles of neuronal degeneration in impact-acceleration injury in rats using Fluoro-Jade (FJ) staining. Impact-acceleration injury was produced in Wistar rats by the method described by Marmarou et al. with some modifications. Animals were sacrificed 1, 2, 7, 14, or 28 days after injury. Paraffin-embedded coronal sections were stained with HE or FJ, or analyzed immunohistochemically for GFAP or amyloid precursor protein (APP). FJ-positive degenerative neurons were found primarily in the dorsal brainstem and thalamus from 1 to 2 days following injury and these were associated with GFAP expression. However, FJ-positive cells were rarely found after 7 days. In all rats, significant expression of APP was observed primarily in the cingulum, cerebral peduncle and pontomedullary junction. FJ also stained these injured axons. Intrathecal administration of both NMDA and AMPA/kinate glutamate receptor antagonists MK-801 and NBQX, respectively, reduced the neuronal injury. NBQX showed more significant effects on axonal injury than MK-801. These observations indicate that not only axonal damage, but also primary neuronal damage occurs in this impact-acceleration injury model. It is also suggested that NBQX can act both directly on neuronal cells and white matter and that NMDA could have a significant protective effect against not only neuronal, but also axonal injury.

Heterogeneity in the Membrane Current Pattern of Identified Glial Cells in the Hippocampal Slice

Glial cells, acutely isolated or in tissue culture, have previously been shown to express a variety of voltage-gated channels. To resolve the question whether such channels are also expressed by glial cells in their normal cellular environment, we have applied the patch-clamp technique to study glial cells in hippocampal slices of 10 - 12-day-old mice. Based on the membrane current pattern, we distinguished four glial cell types. One was characterized by passive, symmetrical K+ currents activated in depolarizing and hyperpolarizing directions. A second population showed a similar current pattern, but with a marked decay of the current during the 50-ms voltage jumps. In a third population, the decaying passive currents were superimposed with a delayed rectifier outward current and, in some cases, with a slow inward current activated by depolarization. The fourth population expressed delayed rectifying outward currents, an inward rectifier K+ current and fast inward currents activated by depolarization. To unequivocally identify the glial cells we combined electrophysiological and ultrastructural characterizations. Therefore, cells were filled with the fluorescent dye lucifer yellow during characterization of their membrane currents, the fluorescence of the dye was used to convert diaminobenzidine to an electron-dense material, and subsequently slices were inspected in the electron microscope. Recordings were obtained from cells in the stratum radiatum and were identified as glial by their size, the characteristic chromatin distribution, and the lack of synaptic membrane specializations.

Expression and Developmental Regulation of a GABAA Receptor in Cultured Murine Cells of the Oligodendrocyte Lineage

The inhibitory neurotransmitter GABA activated Cl- currents in oligodendrocytes and their precursor cells. Most of the pharmacological features of these GABA-evoked currents matched those described for the neuronal GABAA/benzodiazepine receptor complex, such as the blockade by picrotoxin and bicuculline and the enhancement by barbiturates and benzodiazepines. In contrast to the astrocytic GABA receptor, but similar to the neuronal GABAA receptor, the inverse benzodiazepine agonist DMCM decreased GABA-induced current responses. A further similarity to the neuronal receptor is the strong run-down of the current in the absence of ATP in the pipette. A difference between oligodendroglial receptors and receptors expressed on neurons and astrocytes was revealed by the dose - response curve, which indicated only one binding site for GABA or weak allosterical interactions between two putative binding sites. Thus, GABAA receptors of precursor cells and oligodendrocytes might represent a third class of GABAA receptors, in addition to those expressed by neurons and astrocytes. The density of these receptors in the membrane, as calculated on the basis of whole cell currents and membrane capacitance, decreased by a factor of 100 when cells matured along the oligodendrocyte lineage, indicating a developmental regulation of the expression of the GABA receptor.

Reciprocal communication systems between astrocytes and neurones

Over the past decade, a growing body of evidence has emerged on the existence in the brain of a close bidirectional communication system between neurones and astrocytes. This article reviews recent advances in understanding the rules governing these interactions and describes putative, novel functions attributable to astrocytes in neuronal transmission. Astrocytes can respond to the neurotransmitter released from active synaptic terminals, with cytosolic Ca(2+) oscillations whose frequency is under the dynamic control of neuronal activity. In response to these neuronal signals, astrocytes can signal back to neurones by releasing various neurone active compounds, such as the excitatory neurotransmitter glutamate. Interestingly, there is accumulating evidence that glutamate is released via a Ca(2+)-dependent mechanism which may share common properties with neurotransmitter exocytosis in neurones. This bidirectional communication system between neurones and astrocytes may lead to profound changes in neuronal excitability and synaptic transmission. While there clearly is an enormous amount of experimental and theoretical work yet to figure out, a coherent view is now emerging which incorporates the astrocyte, with the presynaptic terminal and the postsynaptic target neurone, as a possible third functional element of the synapse.

The suprachiasmatic nucleus and the circadian time-keeping system revisited

Many physiological and behavioral processes show circadian rhythms which are generated by an internal time-keeping system, the biological clock. In rodents, evidence from a variety of studies has shown the suprachiasmatic nucleus (SCN) to be the site of the master pacemaker controlling circadian rhythms. The clock of the SCN oscillates with a near 24-h period but is entrained to solar day/night rhythm by light. Much progress has been made recently in understanding the mechanisms of the circadian system of the SCN, its inputs for entrainment and its outputs for transfer of the rhythm to the rest of the brain. The present review summarizes these new developments concerning the properties of the SCN and the mechanisms of circadian time-keeping. First, we will summarize data concerning the anatomical and physiological organization of the SCN, including the roles of SCN neuropeptide/neurotransmitter systems, and our current knowledge of SCN input and output pathways. Second, we will discuss SCN transplantation studies and how they have contributed to knowledge of the intrinsic properties of the SCN, communication between the SCN and its targets, and age-related changes in the circadian system. Third, recent findings concerning the genes and molecules involved in the intrinsic pacemaker mechanisms of insect and mammalian clocks will be reviewed. Finally, we will discuss exciting new possibilities concerning the use of viral vector-mediated gene transfer as an approach to investigate mechanisms of circadian time-keeping.

Neomycin blocks substance P-induced calcium entry in cultured rat spinal cord neurons

Substance P (SP) plays an important role in sensitization of spinal cord neurons. In this study, we investigated SP-induced calcium activities in cultured rat spinal cord neurons with confocal laser scanning microscopy. Our results showed that SP increased [Ca(2+)](i) by calcium entry rather than release from intracellular calcium stores. Neomycin (100 microM), an antagonist of phosphatidylinositol 4,5-bisphosphate (PIP(2)), blocked SP-induced calcium entry. Ca(2+)-free medium induced capacitative entry, which was significantly potentiated by SP. As activation of SP receptor (NK-1) leads to production of inositol 1,4,5-trisphosphate (IP(3)) by PIP(2) turnover, the results indicates that SP-induced calcium entry and SP-potentiated capacitative calcium entry might be mediated or regulated by IP(3)/diacylglycerol (DG) pathway.

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.

AMPA-kainate subtypes of glutamate receptor in rat cerebral microglia

Microglial cells were isolated from rat cerebral cortex, and kainate (KA)-induced inward current was measured at a holding potential of -40 or -60 mV. 6-Cyano-7-nitroquinoxaline-2, 3-dione-sensitive KA-induced currents increased with increasing KA concentration. The half-activation concentration and Hill coefficient were 3.3 x 10(-4) M and 1.4, respectively. Although glutamate (Glu) and AMPA-induced currents were much smaller than that induced by KA, all KA-, Glu-, and AMPA-induced currents were greatly and consistently enhanced in the presence of cyclothiazide (CTZ). On the other hand, KA-induced currents were much less sensitive to potentiation by concanavain A, suggesting that the KA-induced response in rat microglia is predominantly mediated by AMPA-preferring receptors (subunits GluR1-GluR4). The current-voltage relationships of KA- and AMPA-CTZ-induced currents were almost linear or slightly outward rectifying. The reversal potential of KA-induced current shifted to negative potentials (from +4 to -40 mV) on switching from high Na(+) to high Ca(2+) external solution, indicating the low Ca(2+) permeability through the AMPA-KA receptor channel complexes. AMPA-KA receptor expression was studied with immunohistochemistry and reverse transcription-PCR, from which GluR2, GluR3, GluR4, and GluR5 were identified. Lower levels of mRNAs for GluR7 and KA-1-KA-2 were also indicated. Finally, activation of these receptors with KA or Glu significantly enhanced the production of tumor necrosis factor-alpha. These results suggest that primary cultured rat microglia possesses functional Glu receptor, which may mediate neuron to microglia communication in the physiological and pathological states.

Expression of neuropeptide Y by glutamatergic stimulation in rat C6 glioma cells

We have investigated the expression of neuropeptide Y (NPY) in C6 glioma cells after the glutamatergic stimulation by the in situ RT-PCR and immunocytochemical techniques. The expression of NPY mRNA correlated well with immunocytological findings in each series of experiments. NPY protein expression was enhanced by glutamate (1, 10, 50, 100 microM, and 1 mM) dose-dependently, and its expression was slightly increased by N-methyl-D-aspartate (NMDA; 1, 10, 100, 500 microM, and 1 mM) and kainic acid (1, 10, 100, 300 microM, and 1 mM). We pretreated the cells with dopamine, haloperidol, pentylenetetrazol, and muscimol before each stimulation. The pentylenetetrazol and muscimol did not significantly alter the patterns of NPY expression induced by the glutamatergic stimulation. On the other hand, the dopamine and haloperidol pretreatment significantly elevated the levels of NPY expression that were induced by NMDA and kainic acid. Our results indicate that NPY release is closely related to glutamatergic stimulation, and it could be dynamically mediated by GABAergic and dopaminergic costimulation.

ATP-stimulated Ca2+ influx and phospholipase D activities of a rat brain-derived type-2 astrocyte cell line, RBA-2, are mediated through P2X7 receptors

This study characterizes and examines the P2 receptor-mediated signal transduction pathway of a rat brain-derived type 2 astrocyte cell line, RBA-2. ATP induced Ca2+ influx and activated phospholipase D (PLD). The ATP-stimulated Ca2+ influx was inhibited by pretreating cells with P2 receptor antagonist, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), in a concentration-dependent manner. The agonist 2'- and 3'-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate (BzATP) stimulated the largest increases in intracellular Ca2+ concentrations ([Ca2+]i); ATP, 2-methylthioadenosine triphosphate tetrasodium, and ATPgammaS were much less effective, whereas UTP, ADP, alpha,beta-methylene-ATP, and beta,gamma-methylene-ATP were ineffective. Furthermore, removal of extracellular Mg2+ enhanced the ATP- and BzATP-stimulated increases in [Ca2+]i. BzATP stimulated PLD in a concentration- and time-dependent manner that could be abolished by removal of extracellular Ca2+ and was inhibited by suramin, PPADS, and oxidized ATP. In addition, PLD activities were activated by the Ca2+ mobilization agent, ionomycin, in an extracellular Ca2+ concentration-dependent manner. Both staurosporine and prolonged phorbol ester treatment inhibited BzATP-stimulated PLD activity. Taken together, these data indicate that activation of the P2X7 receptors induces Ca2+ influx and stimulates a Ca2+-dependent PLD in RBA-2 astrocytes. Furthermore, protein kinase C regulates this PLD.

Role of group I metabotropic glutamate receptors in traumatic spinal cord white matter injury

Metabotropic glutamate receptors (mGluRs) participate in glutamate neural transmission, but their role in the pathophysiology of spinal cord injury (SCI) has not been explored. Accordingly, we examined the role of group I mGluRs, which are linked to phospholipase C, in mediating SCI using an in vitro model. A dorsal column segment was isolated from the spinal cord of adult rats, maintained in vitro, and injured by compression for 15 sec with a clip having a 2 g closing force. Under control conditions after SCI, the compound action potential (CAP) amplitude was reduced to 69.1 +/- 5.4% of baseline. Blockade of group I mGluR receptors with MCPG, 4CPG, or AIDA resulted in improved recovery of CAP amplitude (82.2 +/- 2.0%, 86.2 +/- 3.9%, and 86.0 +/- 2.5% of baseline, respectively). The group I/II agonist trans-ACPD and selective group I agonist DHPG exacerbated the posttraumatic reduction of CAP amplitude. The phospholipase C inhibitor U-73122 improved recovery of CAP amplitude after traumatic spinal cord axonal injury. Western blotting and immunocytochemistry demonstrated the presence of mGluR1alpha-immunopositive astrocytes and the absence of mGluR5 in spinal cord white matter. These studies are consistent with the hypothesis that activation of group I mGluR receptors after SCI exacerbates posttraumatic axonal injury through a phospholipase C dependent mechanism. The presence of mGluR1alpha labeling on astrocytes suggests a role for these cells in the pathophysiology of SCI. Additional studies in vivo, are required to further clarify the role of mGluRs in acute traumatic SCI.

N-acetylaspartylglutamate activates cyclic AMP-coupled metabotropic glutamate receptors in cerebellar astrocytes

N-Acetylaspartylglutamate (NAAG) is the most prevalent peptide in the mammalian nervous system. NAAG meets the traditional criteria of a neurotransmitter, including localization in synaptic vesicles, depolarization-induced release, low potency activation of some N-methyl-D-aspartate receptors, and highly selective activation of a cAMP-coupled metabotropic glutamate receptor (mGluR) with potency approaching that of glutamate. The peptide is present in cultured cortical glia in high concentration and is hydrolyzed by cell surface peptidase activity. In the present work, we tested the hypothesis that NAAG selectively activates a subclass of metabotropic receptors on cultured rat cerebellar glia, primarily astrocytes. These glial cells express mRNA for mGluR subtypes 1, 3, 4, 5, 6, 7, and 8. We were not able to detect message for mGluR2 in these cells using the reverse transcriptase-polymerase chain reaction. Cerebellar glia responded to NAAG, glutamate, and trans-ACPD with a decrease in forskolin-stimulated cAMP formation. AP4, an agonist of the group III receptors mGluR4, mGluR6, mGluR7, and mGluR8, had little or no effect on stimulated cAMP levels. Treatment with low micromolar NAAG significantly increased uptake of radioactive thymidine by cultured astrocytes through activation of metabotropic glutamate receptors. Antagonists of group II mGluRs prevented the decrease in cAMP and the increase in uptake of thymidine by NAAG. Cultured cerebellar astrocytes expressed 20 pmol NAAG per mg protein, a value that is at least 30-fold lower than that expressed by mixed glial cultures prepared from mouse cortex. We conclude that cerebellar astrocytes respond to NAAG via the mGluR3 receptor and that the peptide may selectively activate this receptor subtype in astrocytes following release from neurons or glia.

A role of adhesion molecules in neuroglial plasticity

Glial cells are exquisitely sensitive to changes in neuronal activity, and their capacity for structural plasticity including migration is critical for remodeling an repair of nervous tissue. Our in vitro studies suggest that isoforms of the neural cell adhesion molecule (NCAM) carrying an unconventional carbohydrate polymer, polysialic acid (PSA), are involved in these events. We have demonstrated that neurohypophyseal explants from newborn rats generate cellular outgrowth of immature astrocytes displaying the characteristics of oligodendrocyte-type 2 astrocyte (O-2A) progenitor cells previously identified in the optic nerve. Treatment of O-2A cells with the enzyme Endo N, which specifically removes PSA from the cells surface, produced a complete blockade of the dispersion of the O-2A cell population from the explant. Identical effects of Endo N were observed in migration assays using cortical O-2A cells. Neurohypophyseal O-2A cells express functional NMDA class of glutamate receptors and the pharmacological blockade of these receptors inhibit PSA-NCAM biosynthesis and dramatically diminish O-2A cell migration from neurohypophyseal explants. This suggests a potential mechanism through which neuronal activity via glutamate release may regulate PSA-NCAM expression on immature glial cells, which in turn is critical for their migration.

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.

Expression and heteromeric interactions of non-N-methyl-D-aspartate glutamate receptor subunits in the developing and adult cerebellum

The localization and expression of ionotropic non-N-methyl-D-aspartate glutamate receptors (GluR) were investigated in the developing and adult rat cerebellum using subunit-specific polyclonal antibodies for immunocytochemical, immunoblot and immunoprecipitation studies. In P7 animals, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor immunoreactivity was detected in all layers of the cerebellar cortex with the exception of the external granule cell layer. Antibodies against the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor subunits GluR1 and GluR4 gave strong immunoreactive staining of Bergmann glia in both young and adult animals, and both antibodies showed prominent staining of the molecular layer in the adult cerebellum. Dense immunoreactive staining of Purkinje cell somata and dendrites was obtained with anti-GluR2/3/4c in both the developing and adult cerebellum. Whereas each of the three alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor antibodies stained the internal, but not the external, granule cell layer, immunostaining for the kainate-type subunits GluR6/7 and KA2 was detected in both the external and internal granule cell layers. as well as in the molecular layer in both P7 and adult cerebellum. Immunoblot analysis of total cerebellar protein indicated that the level of GluR4 expression increased 15-fold from P1 to P18, whereas the expression of the KA2 subunit protein was nine-fold lower in adult cerebellum than it was at P1. The expression of GluR1 increased moderately (two-fold) from P1 to adult. Subunit interactions between GluR1 and GluR4, as well as between GluR6/7 and KA2, were demonstrated in immunoprecipitation experiments; and the GluR4 and KA2 subunits appear to be present exclusively in heteromeric assemblies with GluR1 and GluR6/7, respectively. The results show that the various alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate- and kainate-type subunits are differentially expressed during cerebellar development and further define the possible subunit composition of non-N-methyl-D-aspartate receptors in the major cerebellar cell types.

A presynaptic N-methyl-D-aspartate autoreceptor in rat hippocampus modulating amino acid release from a cytoplasmic pool

A possible role of the N-methyl-D-aspartate receptor (NMDA-R) as a presynaptic autoreceptor was investigated using Percoll-purified hippocampus nerve terminals (synaptosomes). This preparation contained only a neglectable amount of postsynaptic structures. Two main effects of NMDA were observed. First, NMDA dose-dependently (10-100 microM) and in the absence of Mg2+, stimulated basal release of aspartate and glutamate, but not of GABA. MK801 (10 microM), an open NMDA-R-channel blocker, reduced this effect even below control levels, indicating endogenous NMDA-R activation. By superfusing synaptosomes, which prevents a tonic receptor occupation, also basal GABA release was stimulated by NMDA. The NMDA-induced potentiation of amino acid superfusate levels was blocked both by MK801 and Mg2+ (1 mM), was slow in onset and returned to baseline after NMDA-removal. The NMDA-effect was also found in the absence of extracellular Ca2+, suggesting that amino acids were released from a non-vesicular (cytoplasmic) pool. Secondly, in KCl-depolarized synaptosomes exposed to 1 mM Mg2+, NMDA did not affect the release of the amino acids. MK801, however, reduced the KCl-evoked Ca2+-independent release of aspartate and glutamate, but not of GABA. L-trans-PDC, the selective inhibitor of the glutamate/aspartate transporter, prevented this MK801-effect, suggesting a coupling between NMDA-Rs and these transporters. These data provide evidence for a presynaptic NMDA autoreceptor in rat hippocampus. We speculate on the role of this NMDA-R to depolarize the presynaptic membrane by Na+-entry, which may induce reversal of amino acid transporters and thereby releasing amino acids from a cytoplasmic pool.

Glutamate-induced release of the nitric oxide precursor, arginine, from glial cells

Arginine, the nitric oxide precursor, is predominantly localized in glial cells, whereas the constitutive nitric oxide synthase is mainly found in neurons. Therefore, a transfer of arginine from glial cells to neurons is needed to replenish the neuronal precursor pool. This is further supported by the finding that arginine is released upon selective pathway stimulation both in vitro and in vivo. We investigated the mechanism underlying this glial-neuronal interaction by analysing the effect of glutamate receptor agonists on the extracellular [3H]arginine level in cerebellar and cortical slices and in cultures of either cortical astroglial cells or neurons. We present data indicating that arginine is released from cerebellar and cortical slices and astroglial cell cultures upon activation of ionotropic non-NMDA glutamate receptors. Glutamate had no effect on the extracellular [3H]arginine level in neuronal cultures. Moreover, the effect of glutamate in cerebellar slices was tetrodotoxin-insensitive, and the calcium ionophore A23187 evoked the release of [3H]arginine from astroglial cell cultures. Thus, nitric oxide synthesis and nitric oxide transmission may be based on the glial-neuronal transfer of arginine which is induced by activation of excitatory amino acid receptors on glial cells.

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.

Changes in AMPA glutamate and dopamine D2 receptors in hypoxic-ischemic basal ganglia necrosis

Changes in the AMPA glutamate receptor subunits (GluR1, 2-3, and 4) and dopamine D2 receptor (D2R) were investigated in 16 cases of hypoxic-ischemic basal ganglia necrosis (BGN) by immunohistochemistry. Immunoreactivity to GluR1, 2-3, and 4 was observed in the cytoplasm and dendrites of small and large neurons in the basal ganglia. Neuronal immunoreactivity to GluR1, 2-3, and 4 was decreased in cases with acute BGN as compared with that in age-matched controls, the areas of decreased immunoreactivity corresponding to the damaged regions observed on hematoxylin and eosin staining. Glia in the basal ganglia exhibited immunoreactivity to GluR4 in 4 patients with acute BGN, 3 of the 4 surviving for 12 to 35 days. In addition, neuronal immunoreactivity to D2R was also decreased in cases of acute BGN, the decrease being similar to that of GluR1, 2-3, and 4. Our results suggest that excitotoxicity mediated by GluR1, 2-3, and 4 is involved in the pathogenesis of hypoxic-ischemic neuronal damage, and that GluR4 expressed in glia of the BG in the late stage of hypoxic-ischemic injury may participate in the delayed and long-term response of the glia to injury. The decrease in neuronal D2R may be related to downregulated synthesis of the D2R protein induced by the decrease in GluR1-4 in the basal ganglia.