AMPA receptor subunits expressed by single astrocytes in the juvenile mouse hippocampus

Impact of Developmental Changes of GABAA Receptors on Interneuron-NG2 Glia Transmission in the Hippocampus

NG2 glia receive synaptic input from neurons, but the functional impact of this glial innervation is not well understood. In the developing cerebellum and somatosensory cortex the GABAergic input might regulate NG2 glia differentiation and myelination, and a switch from synaptic to extrasynaptic neuron-glia signaling was reported in the latter region. Myelination in the hippocampus is sparse, and most NG2 glia retain their phenotype throughout adulthood, raising the question of the properties and function of neuron-NG2 glia synapses in that brain region. Here, we compared spontaneous and evoked GABAA receptor-mediated currents of NG2 glia in juvenile and adult hippocampi of mice of either sex and assessed the mode of interneuron-glial signaling changes during development. With patch-clamp and pharmacological analyses, we found a decrease in innervation of hippocampal NG2 glia between postnatal days 10 and 60. At the adult stage, enhanced activation of extrasynaptic receptors occurred, indicating a spillover of GABA. This switch from synaptic to extrasynaptic receptor activation was accompanied by downregulation of γ2 and upregulation of the α5 subunit. Molecular analyses and high-resolution expansion microscopy revealed mechanisms of glial GABAA receptor trafficking and clustering. We found that gephyrin and radixin are organized in separate clusters along glial processes. Surprisingly, the developmental loss of γ2 and postsynaptic receptors were not accompanied by altered glial expression of scaffolding proteins, auxiliary receptor subunits or postsynaptic interaction proteins. The GABAergic input to NG2 glia might contribute to the release of neurotrophic factors from these cells and influence neuronal synaptic plasticity.

Revealing the contribution of astrocytes to glutamatergic neuronal transmission

Research on glutamatergic neurotransmission has focused mainly on the function of presynaptic and postsynaptic neurons, leaving astrocytes with a secondary role only to ensure successful neurotransmission. However, recent evidence indicates that astrocytes contribute actively and even regulate neuronal transmission at different levels. This review establishes a framework by comparing glutamatergic components between neurons and astrocytes to examine how astrocytes modulate or otherwise influence neuronal transmission. We have included the most recent findings about the role of astrocytes in neurotransmission, allowing us to understand the complex network of neuron-astrocyte interactions. However, despite the knowledge of synaptic modulation by astrocytes, their contribution to specific physiological and pathological conditions remains to be elucidated. A full understanding of the astrocyte's role in neuronal processing could open fruitful new frontiers in the development of therapeutic applications.

Auxiliary Subunits Control Function and Subcellular Distribution of AMPA Receptor Complexes in NG2 Glia of the Developing Hippocampus

Synaptic and axonal glutamatergic signaling to NG2 glia in white matter is critical for the cells' differentiation and activity dependent myelination. However, in gray matter the impact of neuron-to-NG2 glia signaling is still elusive, because most of these cells keep their non-myelinating phenotype throughout live. Early in postnatal development, hippocampal NG2 glia express AMPA receptors with a significant Ca²⁺ permeability allowing for plasticity of the neuron-glia synapses, but whether this property changes by adulthood is not known. Moreover, it is unclear whether NG2 glia express auxiliary transmembrane AMPA receptor related proteins (TARPs), which modify AMPA receptor properties, including their Ca²⁺ permeability. Through combined molecular and functional analyses, here we show that hippocampal NG2 glia abundantly express TARPs γ4, γ7, and γ8 as well as cornichon (CNIH)-2. TARP γ8 undergoes profound downregulation during development. Receptors of adult NG2 glia showed an increased sensitivity to blockers of Ca²⁺ permeable AMPA receptors, but this increase mainly concerned receptors located close to the soma. Evoked synaptic currents of NG2 glia were also sensitive to blockers of Ca²⁺ permeable AMPA receptors. The presence of AMPA receptors with varying Ca²⁺ permeability during postnatal maturation may be important for the cells' ability to sense and respond to local glutamatergic activity and for regulating process motility, differentiation, and proliferation.

Glial Cell AMPA Receptors in Nervous System Health, Injury and Disease

Glia form a central component of the nervous system whose varied activities sustain an environment that is optimised for healthy development and neuronal function. Alpha-amino-3-hydroxy-5-methyl-4-isoxazole (AMPA)-type glutamate receptors (AMPAR) are a central mediator of glutamatergic excitatory synaptic transmission, yet they are also expressed in a wide range of glial cells where they influence a variety of important cellular functions. AMPAR enable glial cells to sense the activity of neighbouring axons and synapses, and as such many aspects of glial cell development and function are influenced by the activity of neural circuits. However, these AMPAR also render glia sensitive to elevations of the extracellular concentration of glutamate, which are associated with a broad range of pathological conditions. Excessive activation of AMPAR under these conditions may induce excitotoxic injury in glial cells, and trigger pathophysiological responses threatening other neural cells and amplifying ongoing disease processes. The aim of this review is to gather information on AMPAR function from across the broad diversity of glial cells, identify their contribution to pathophysiological processes, and highlight new areas of research whose progress may increase our understanding of nervous system dysfunction and disease.

Heterogeneity of Cell Surface Glutamate and GABA Receptor Expression in Shank and CNTN4 Autism Mouse Models

Autism spectrum disorder (ASD) refers to a large set of neurodevelopmental disorders, which have in common both repetitive behavior and abnormalities in social interactions and communication. Interestingly, most forms of ASD have a strong genetic contribution. However, the molecular underpinnings of this disorder remain elusive. The SHANK3 gene (and to a lesser degree SHANK2) which encode for the postsynaptic density (PSD) proteins SHANK3/SHANK2 and the CONTACTIN 4 gene which encodes for the neuronal glycoprotein CONTACTIN4 (CNTN4) exhibit mutated variants which are associated with ASD. Like many of the other genes associated with ASD, both SHANKs and CNTN4 affect synapse formation and function and are therefore related to the proper development and signaling capability of excitatory and inhibitory neuronal networks in the adult mammal brain. In this study, we used mutant/knock-out mice of Shank2 (Shank2^−/−), Shank3 (Shank3αβ^−/−), and Cntn4 (Cntn4^−/−) as ASD-models to explore whether these mice share a molecular signature in glutamatergic and GABAergic synaptic transmission in ASD-related brain regions. Using a biotinylation assay and subsequent western blotting we focused our analysis on cell surface expression of several ionotropic glutamate and GABA receptor subunits: GluA1, GluA2, and GluN1 were analyzed for excitatory synaptic transmission, and the α1 subunit of the GABA_A receptor was analyzed for inhibitory synaptic transmission. We found that both Shank2^−/− and Shank3αβ^−/− mice exhibit reduced levels of several cell surface glutamate receptors in the analyzed brain regions—especially in the striatum and thalamus—when compared to wildtype controls. Interestingly, even though Cntn4^−/− mice also show reduced levels of some cell surface glutamate receptors in the cortex and hippocampus, increased levels of cell surface glutamate receptors were found in the striatum. Moreover, Cntn4^−/− mice do not only show brain region-specific alterations in cell surface glutamate receptors but also a downregulation of cell surface GABA receptors in several of the analyzed brain regions. The results of this study suggest that even though mutations in defined genes can be associated with ASD this does not necessarily result in a common molecular phenotype in surface expression of glutamatergic and GABAergic receptor subunits in defined brain regions.

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.

Ca2+-permeable AMPA receptors in mouse olfactory bulb astrocytes

Ca²⁺ signaling in astrocytes is considered to be mainly mediated by metabotropic receptors linked to intracellular Ca²⁺ release. However, recent studies demonstrate a significant contribution of Ca²⁺ influx to spontaneous and evoked Ca²⁺ signaling in astrocytes, suggesting that Ca²⁺ influx might account for astrocytic Ca²⁺ signaling to a greater extent than previously thought. Here, we investigated AMPA-evoked Ca²⁺ influx into olfactory bulb astrocytes in mouse brain slices using Fluo-4 and GCaMP6s, respectively. Bath application of AMPA evoked Ca²⁺ transients in periglomerular astrocytes that persisted after neuronal transmitter release was inhibited by tetrodotoxin and bafilomycin A1. Withdrawal of external Ca²⁺ suppressed AMPA-evoked Ca²⁺ transients, whereas depletion of Ca²⁺ stores had no effect. Both Ca²⁺ transients and inward currents induced by AMPA receptor activation were partly reduced by Naspm, a blocker of Ca²⁺-permeable AMPA receptors lacking the GluA2 subunit. Antibody staining revealed a strong expression of GluA1 and GluA4 and a weak expression of GluA2 in periglomerular astrocytes. Our results indicate that Naspm-sensitive, Ca²⁺-permeable AMPA receptors contribute to Ca²⁺ signaling in periglomerular astrocytes in the olfactory bulb.

Immunolocalization of AMPA receptor subunits within the enteric nervous system of the mouse colon and the effect of their activation on spontaneous colonic contractions

BACKGROUND

The appropriate expression of specific neurotransmitter receptors within the cellular networks that compose the enteric nervous system (ENS) is central to the regulation of gastrointestinal (GI) functions. While the ENS expression patterns of the neurotransmitter glutamate have been well documented, the localization of its receptors on ENS neurons remains to be fully characterized. We investigated the expression patterns of glutamate receptor AMPA subunits within ENS neurons of the mouse colon and the consequences of their pharmacological activation on spontaneous colonic contractility.

METHODS

RT-PCR was used to detect individual AMPA receptor (GluR 1-4) subunit expression at the mRNA level in mouse colon tissue. Immunohistochemistry and confocal microscopy was used to localize the expression of the GluR1 and 4 subunits in colon tissue. Brain tissue was used as a positive control. Organ bath preparations were used to determine the effect of AMPA receptors activation on the force and frequency of colonic longitudinal smooth muscle spontaneous contractions.

KEY RESULTS

GluR1, 3, 4 mRNA was detected in the mouse colon. Immunoreactivity for GluR1 and 4 subunits was detected on the somatic and dendritic surfaces of subpopulations of neurochemically defined ENS neurons. The pharmacological activation of AMPA receptors increased the force but not frequency of spontaneous colonic contractions.

CONCLUSIONS & INFERENCES

Molecularly distinct AMPA receptor subtypes are differentially expressed within the neural networks of the mouse colon and have a direct role in motility. These data provide the rationale for the development of AMPA-selective ligands for the therapeutic delivery to the GIT in motility disorders.

Electrophysiological properties of NG2(+) cells: Matching physiological studies with gene expression profiles

NG2(+) glial cells are a dynamic population of non-neuronal cells that give rise to myelinating oligodendrocytes in the central nervous system. These cells express numerous ion channels and neurotransmitter receptors, which endow them with a complex electrophysiological profile that is unique among glial cells. Despite extensive analysis of the electrophysiological properties of these cells, relatively little was known about the molecular identity of the channels and receptors that they express. The generation of new RNA-Seq datasets for NG2(+) cells has provided the means to explore how distinct genes contribute to the physiological properties of these progenitors. In this review, we systematically compare the results obtained through RNA-Seq transcriptional analysis of purified NG2(+) cells to previous physiological and molecular studies of these cells to define the complement of ion channels and neurotransmitter receptors expressed by NG2(+) cells in the mammalian brain and discuss the potential significance of the unique physiological properties of these cells. This article is part of a Special Issue entitled SI:NG2-glia(Invited only).

Oligodendrocyte Development and Plasticity

Oligodendrocyte precursor cells (OPCs) originate in the ventricular zones (VZs) of the brain and spinal cord and migrate throughout the developing central nervous system (CNS) before differentiating into myelinating oligodendrocytes (OLs). It is not known whether OPCs or OLs from different parts of the VZ are functionally distinct. OPCs persist in the postnatal CNS, where they continue to divide and generate myelinating OLs at a decreasing rate throughout adult life in rodents. Adult OPCs respond to injury or disease by accelerating their cell cycle and increasing production of OLs to replace lost myelin. They also form synapses with unmyelinated axons and respond to electrical activity in those axons by generating more OLs and myelin locally. This experience-dependent "adaptive" myelination is important in some forms of plasticity and learning, for example, motor learning. We review the control of OL lineage development, including OL population dynamics and adaptive myelination in the adult CNS.

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.

Contributions of astrocytes to epileptogenesis following status epilepticus: opportunities for preventive therapy?

Status epilepticus (SE) is a life threatening condition that often precedes the development of epilepsy. Traditional treatments for epilepsy have been focused on targeting neuronal mechanisms contributing to hyperexcitability, however, approximately 30% of patients with epilepsy do not respond to existing neurocentric pharmacotherapies. A growing body of evidence has demonstrated that profound changes in the morphology and function of astrocytes accompany SE and persist in epilepsy. Astrocytes are increasingly recognized for their diverse roles in modulating neuronal activity, and understanding the changes in astrocytes following SE could provide important clues about the mechanisms underlying seizure generation and termination. By understanding the contributions of astrocytes to the network changes underlying epileptogenesis and the development of epilepsy, we will gain a greater appreciation of the contributions of astrocytes to dynamic circuit changes, which will enable us to develop more successful therapies to prevent and treat epilepsy. This review summarizes changes in astrocytes following SE in animal models and human temporal lobe epilepsy and addresses the functional consequences of those changes that may provide clues to the process of epileptogenesis.

Methodological limitations in determining astrocytic gene expression

Traditionally, astrocytic mRNA and protein expression are studied by in situ hybridization (ISH) and immunohistochemically. This led to the concept that astrocytes lack aralar, a component of the malate-aspartate-shuttle. At least similar aralar mRNA and protein expression in astrocytes and neurons isolated by fluorescence-assisted cell sorting (FACS) reversed this opinion. Demonstration of expression of other astrocytic genes may also be erroneous. Literature data based on morphological methods were therefore compared with mRNA expression in cells obtained by recently developed methods for determination of cell-specific gene expression. All Na,K-ATPase-α subunits were demonstrated by immunohistochemistry (IHC), but there are problems with the cotransporter NKCC1. Glutamate and GABA transporter gene expression was well determined immunohistochemically. The same applies to expression of many genes of glucose metabolism, whereas a single study based on findings in bacterial artificial chromosome (BAC) transgenic animals showed very low astrocytic expression of hexokinase. Gene expression of the equilibrative nucleoside transporters ENT1 and ENT2 was recognized by ISH, but ENT3 was not. The same applies to the concentrative transporters CNT2 and CNT3. All were clearly expressed in FACS-isolated cells, followed by biochemical analysis. ENT3 was enriched in astrocytes. Expression of many nucleoside transporter genes were shown by microarray analysis, whereas other important genes were not. Results in cultured astrocytes resembled those obtained by FACS. These findings call for reappraisal of cellular nucleoside transporter expression. FACS cell yield is small. Further development of cell separation methods to render methods more easily available and less animal and cost consuming and parallel studies of astrocytic mRNA and protein expression by ISH/IHC and other methods are necessary, but new methods also need to be thoroughly checked.

Expression of the AMPA Receptor Subunits GluR1 and GluR2 is Associated with Granule Cell Maturation in the Dentate Gyrus

The dentate gyrus produces new granule neurons throughout adulthood in mammals from rodents to humans. During granule cell maturation, defined markers are expressed in a highly regulated sequential process, which is necessary for directed neuronal differentiation. In the present study, we show that α-amino-3-hydroxy-5-methy-4-isoxazole propionate (AMPA) receptor subunits GluR1 and GluR2 are expressed in differentiated granule cells, but not in stem cells, in neonatal, and adult dentate gyrus. Using markers for neural progenitors, immature and mature granule cells, we found that GluR1 and GluR2 were expressed mainly in mature cells and in some immature cells. A time-course analysis of 5-bromo-2′-deoxyuridine staining revealed that granule cells express GluR1 around 3 weeks after being generated. In mice heterozygous for the alpha-isoform of calcium/calmodulin-dependent protein kinase II, a putative animal model of schizophrenia and bipolar disorder in which dentate gyrus granule cells fail to mature normally, GluR1 and GluR2 immunoreactivities were substantially downregulated in the dentate gyrus granule cells. In the granule cells of mutant mice, the expression of both presynaptic and postsynaptic markers was decreased, suggesting that GluR1 and GluR2 are also associated with synaptic maturation. Moreover, GluR1 and GluR2 were also expressed in mature granule cells of the neonatal dentate gyrus. Taken together, these findings indicate that GluR1 and GluR2 expression closely correlates with the neuronal maturation state, and that GluR1 and GluR2 are useful markers for mature granule cells in the dentate gyrus.

Gray matter NG2 cells display multiple Ca2+-signaling pathways and highly motile processes

NG2 cells, the fourth type of glia in the mammalian CNS, receive synaptic input from neurons. The function of this innervation is unknown yet. Postsynaptic changes in intracellular Ca(2+)-concentration ([Ca(2+)](i)) might be a possible consequence. We employed transgenic mice with fluorescently labeled NG2 cells to address this issue. To identify Ca(2+)-signaling pathways we combined patch-clamp recordings, Ca(2+)-imaging, mRNA-transcript analysis and focal pressure-application of various substances to identified NG2-cells in acute hippocampal slices. We show that activation of voltage-gated Ca(2+)-channels, Ca(2+)-permeable AMPA-receptors, and group I metabotropic glutamate-receptors provoke [Ca(2+)](i)-elevations in NG2 cells. The Ca(2+)-influx is amplified by Ca(2+)-induced Ca(2+)-release. Minimal electrical stimulation of presynaptic neurons caused postsynaptic currents but no somatic [Ca(2+)](i) elevations, suggesting that [Ca(2+)](i) elevations in NG2 cells might be restricted to their processes. Local Ca(2+)-signaling might provoke transmitter release or changes in cell motility. To identify structural prerequisites for such a scenario, we used electron microscopy, immunostaining, mRNA-transcript analysis, and time lapse imaging. We found that NG2 cells form symmetric and asymmetric synapses with presynaptic neurons and show immunoreactivity for vesicular glutamate transporter 1. The processes are actin-based, contain ezrin but not glial filaments, microtubules or endoplasmic reticulum. Furthermore, we demonstrate that NG2 cell processes in situ are highly motile. Our findings demonstrate that gray matter NG2 cells are endowed with the cellular machinery for two-way communication with neighboring cells.

Neuron-glia synapses in the brain

The ability to investigate the electrophysiological properties of individual cells in acute brain tissue led to the discovery that many glial cells have the capacity to respond rapidly to neuronal activity. In particular, a distinct class of neuroglial cells known as NG2 cells, which exhibit many of the properties that have been described for glial subtypes such as complex cells, polydendrocytes, synantocytes and GluR cells, express ionotropic receptors for glutamate and GABA. In both gray and white matter, NG2 cells form direct synaptic junctions with axons, which enable transient activation of these receptors. Electrophysiological analyses have shown that these neuron-glia synapses exhibit all the hallmarks of 'classical' neuron-neuron synapses, including rapid activation, quantized responses, facilitation and depression, and presynaptic inhibition. Electron microscopy indicates that axons form morphologically distinct junctions at discrete sites along processes of NG2 cells, suggesting that NG2 cells are an overt target of axonal projections. AMPA receptors expressed by NG2 cells exhibit varying degrees of Ca(2+) permeability, depending on the brain region and stage of development, and in white matter NG2 cells have also been shown to express functional NMDA receptors. Ca(2+) influx through AMPA receptors following repetitive stimulation can trigger long term potentiation of synaptic currents in NG2 cells. The expression of receptors with significant Ca(2+) permeability may increase the susceptibility of NG2 cells to excitotoxic injury. Future studies using transgenic mice in which expression of receptors can be manipulated selectively in NG2 cells have to define the functions of this enigmatic neuron-glia signaling in the normal and diseased CNS.

Kainate receptors and signal integration by NG2 glial cells

It is well established that NG2 cells throughout the young and adult brain consistently detect the release of single vesicles filled with glutamate from nearby axons. The released neurotransmitter glutamate electrically excites NG2 cells via non-NMDA (N-methyl-D-aspartic acid) glutamate receptors but the individual contribution of AMPA and kainate receptors to neuron-NG2 cell signalling, is not well understood. Here we pharmacologically block AMPA-type glutamate receptors and investigate whether hippocampal NG2 cells also express the kainate subtype of glutamate receptors and what may be their contribution to synaptic connectivity. It has been shown previously that vesicular glutamate release does not lead to a detectable activation of kainate receptors on NG2 cells. Here we report that while bath application of 250 nM-1 muM kainate does not have a major effect on NG2 cells it consistently induces a small and persistent depolarising current. This current was not mimicked by ATPA, suggesting that this current is carried by non-GluR5 containing kainate receptors. In addition to this inward current, nanomolar concentrations of kainate also produced a dramatic increase in the frequency of spontaneous GABA-A receptor-mediated synaptic currents (IPSCs) in NG2 cells. This increase in spontaneous IPSC frequency was even more pronounced on application of the GluR5-specific agonist ATPA (approximately 15-fold increase in frequency). In contrast, mono-synaptic stimulated IPSCs recorded in NG2 cells were unaffected by kainate receptor activation. Those and further experiments show that the occurrence of the high frequency of IPSCs is due to action potential firing of hippocampal interneurons caused by activation of GluR5 receptors on the somatodendritic membrane of the interneurons. Our data suggest that hippocampal kainate receptors are not only important for communication between neurons but may also play a dual and subtype-specific role for neuron-glia signalling: Firstly, extra-synaptic non-GluR5 kainate receptors in the membrane of NG2 cells are ideally suited to instruct NG2 cells on the population activity of local excitatory neurons via ambient glutamate. Secondly, based on the known importance of GluR5 receptors on hippocampal interneurons for the generation of network rhythms and based on our finding that these interneurons heavily project onto NG2 cells, it appears that synaptic activation of interneuronal GluR5 receptors triggers signalling to NG2 cells which transmits the phase and frequency of ongoing network oscillations in the developing hippocampus.

Analysis of astroglial K+ channel expression in the developing hippocampus reveals a predominant role of the Kir4.1 subunit

Astrocytes in different brain regions display variable functional properties. In the hippocampus, astrocytes predominantly express time- and voltage-independent currents, but the underlying ion channels are not well defined. This ignorance is partly attributable to abundant intercellular coupling of these cells through gap junctions, impeding quantitative analyses of intrinsic membrane properties. Moreover, distinct types of cells with astroglial properties coexist in a given brain area, a finding that confused previous analyses. In the present study, we investigated expression of inwardly rectifying (Kir) and two-pore-domain (K2P) K+ channels in astrocytes, which are thought to be instrumental in the regulation of K+ homeostasis. Freshly isolated astrocytes were used to improve space-clamp conditions and allow for quantitative assessment of functional parameters. Patch-clamp recordings were combined with immunocytochemistry, Western blot analysis, and semiquantitative transcript analysis. Comparative measurements were performed in different CA1 subregions of astrocyte-targeted transgenic mice. While confirming weak Ba2+ sensitivity in situ, our data demonstrate that in freshly isolated astrocytes, the main proportion of membrane currents is sensitive to micromolar Ba2+ concentrations. Upregulation of Kir4.1 transcripts and protein during the first 10 postnatal days was accompanied by a fourfold increase in astrocyte inward current density. Hippocampal astrocytes from Kir4.1-/- mice lacked Ba2+-sensitive currents. In addition, we report functional expression of K2P channels of the TREK subfamily (TREK1, TREK2), which mediate astroglial outward currents. Together, our findings demonstrate that Kir4.1 constitutes the pivotal K+ channel subunit and that superposition of currents through Kir4.1 and TREK channels underlies the "passive" current pattern of hippocampal astrocytes.

Glial-neuronal interactions--implications for plasticity and drug addiction

Among neuroscientists, astrocytes have for long played Cinderella to their neuron stepsisters. While the importance of glia in regulating brain activity was predicted by Ramon y Cajal more than a century ago (Garcia-Marin et al., Trends. Neurosci. 30:479-787, 2007), these cells, until recently, have been thought to play mainly a passive part in synaptic signaling. Results obtained over the last decade have begun to suggest otherwise. Experiments carried out in a number of labs have shown that glial cells, especially astrocytes, directly participate in synaptic signaling and potentially regulate synaptic plasticity and network excitability. The presence of signaling pathways on astrocytes that are analogous to those at presynaptic terminals suggests a role for these cells in network plasticity. Findings that the same signaling pathways can be activated by receptors for drugs of abuse present on astrocytes suggest a role for these cells in the addictive process. In this review, we summarize current understanding of astrocytic role in synaptic signaling and suggest that a complete understanding of the process of addiction requires a better understanding of the functional role of these cells.

Lamina-specific abnormalities of NMDA receptor-associated postsynaptic protein transcripts in the prefrontal cortex in schizophrenia and bipolar disorder

The hypothesis of N-methyl-D-aspartate (NMDA) receptor hypofunction in schizophrenia was initially based on observations that blockade of the NMDA subtype of glutamate receptor by noncompetitive antagonists, such as phencyclidine and ketamine, can lead to clinical symptoms similar to those present in schizophrenia. Recently, glutamate has also been implicated in the pathophysiology of the mood disorders. As impaired NMDA receptor activity may be the result of a primary defect in the NMDA receptors themselves, or secondary to dysfunction in the protein complexes that mediate their signaling, we measured expression of both NMDA subunits and associated postsynaptic density (PSD) proteins (PSD95, neurofilament-light (NF-L), and SAP102) transcripts in the dorsolateral prefrontal cortex in subjects with schizophrenia, bipolar disorder, major depression, and a comparison group using tissue from the Stanley Foundation Neuropathology Consortium. We found decreased NR1 expression in all three illnesses, decreased NR2A in schizophrenia and major depression, and decreased NR2C in schizophrenia. We found no changes of NR2B or NR2D. Receptor autoradiography revealed no alterations in receptor binding in any of the illnesses, indicating no change in total receptor number, but taken with the subunit data suggests abnormal receptor stoichiometry. In the same subjects, PSD95 was unchanged in all three illnesses, while reduced NF-L expression was found in schizophrenia, especially in large cells of layer V. SAP102 expression was reduced in bipolar disorder restricted to small cells of layer II and large cells of layer III in bipolar disorder. These alterations likely reflect altered signaling cascades associated with glutamate-mediated neurotransmission within specific cortical circuits in these psychiatric illnesses.

Neural pathways underlying lactate-induced panic

Panic disorder is a severe anxiety disorder characterized by susceptibility to induction of panic attacks by subthreshold interoceptive stimuli such as 0.5 M sodium lactate infusions. Although studied for four decades, the mechanism of lactate sensitivity in panic disorder has not been understood. The dorsomedial hypothalamus/perifornical region (DMH/PeF) coordinates rapid mobilization of behavioral, autonomic, respiratory and endocrine responses to stress, and rats with disrupted GABA inhibition in the DMH/PeF exhibit panic-like responses to lactate, similar to panic disorder patients. Utilizing a variety of anatomical and pharmacological methods, we provide evidence that lactate, via osmosensitive periventricular pathways, activates neurons in the compromised DMH/PeF, which relays this signal to forebrain limbic structures such as the bed nucleus of the stria terminalis to mediate anxiety responses, and specific brainstem sympathetic and parasympathetic pathways to mediate the respiratory and cardiovascular components of the panic-like response. Acutely restoring local GABAergic tone in the DMH/PeF blocked lactate-induced panic-like responses. Autonomic panic-like responses appear to be a result of DMH/PeF-mediated mobilization of sympathetic responses (verified with atenolol) and resetting of the parasympathetically mediated baroreflex. Based on our findings, DMH/PeF efferent targets such as the C1 adrenergic neurons, paraventricular hypothalamus, and the central amygdala are implicated in sympathetic mobilization; the nucleus of the solitary tract is implicated in baroreflex resetting; and the parabrachial nucleus is implicated in respiratory responses. These results elucidate neural circuits underlying lactate-induced panic-like responses and the involvement of both sympathetic and parasympathetic systems.

G protein coupling profile of mGluR6 and expression of Gα proteins in retinal ON bipolar cells

Metabotropic glutamate receptor 6 (mGluR6) is a group III, pertussis toxin (PTX)-sensitive G protein coupled mGluR that plays a specialized role in the retina. Retinal ON bipolar cells, which receive direct glutamatergic input from photoreceptor cells, express mGluR6 as their primary postsynaptic glutamate receptor. Activation of mGluR6 in these cells initiates an intracellular signaling cascade ultimately leading to inhibition of a cation channel and cell hyperpolarization. The primary mediator of this pathwayin vivois Gαo, but the potential roles of other G proteins from the Gαi/ofamily in the regulation of this or other signaling pathways in ON bipolar cells are unclear. To determine which specific G proteins from the Gαi/ofamily are able to couple to mGluR6, a Gα reconstitution system was employed using PTX-insensitive Gα mutants expressed with mGluR6 in PTX-treated sympathetic neurons from the rat superior cervical ganglion (SCG). The efficiency of coupling to mGluR6 was Goa> Gob, Gi1> Gi2, Gi3, whereas no coupling was observed with Gαz, nor with the retinal Gα proteins, rod (GNAT2) or cone (GNAT1) transducin (GαTr-R, GαTr-C). Finally, the expression of Gα proteins determined to couple with mGluR6 was examined in rat ON bipolar cells using single cell RT-PCR. Co-expression of mGluR6 message was used to distinguish ON from OFF bipolar cells. Expression of Gαowas detected in every ON bipolar cell examined. Message for Gαi1, which coupled moderately to mGluR6, was not detected in ON bipolar cells, nor was Gαi3, which coupled to mGluR6 in only a few cells but on average did not exhibit statistically significant coupling. Finally, though Gαi2was detectable in ON bipolar cells, its coupling to mGluR6 in the SCG system was not significant. Together, these data indicate that signaling through mGluR6 in mammalian ON bipolar cells is highly focused, apparently acting through a single Gα protein subtype.

Functional changes in astroglial cells in epilepsy

Epilepsy comprises a group of disorders characterized by the periodic occurrence of seizures, and pathologic specimens from patients with temporal lobe epilepsy demonstrate marked reactive gliosis. Since recent studies have implicated glial cells in novel physiological roles in the CNS, such as modulation of synaptic transmission, it is plausible that glial cells may have a functional role in the hyperexcitability characteristic of epilepsy. Indeed, alterations in distinct astrocyte membrane channels, receptors and transporters have all been associated with the epileptic state. This review integrates the current evidence regarding astroglial dysfunction in epilepsy and the potential underlying mechanisms of hyperexcitability. Functional understanding of the cellular and molecular alterations of astroglia-dependent hyperexcitability will help to clarify the physiological role of astrocytes in neural function as well as lead to the identification of novel therapeutic targets.

Classification of projection neurons and interneurons in the rat lateral amygdala based upon cluster analysis

Neurons in the rat lateral amygdala in situ were classified based upon electrophysiological and molecular parameters, as studied by patch-clamp, single-cell RT-PCR and unsupervised cluster analyses. Projection neurons (class I) were characterized by low firing rates, frequency adaptation and expression of the vesicular glutamate transporter (VGLUT1). Two classes were distinguished based upon electrotonic properties and the presence (IB) or absence (IA) of vasointestinal peptide (VIP). Four classes of glutamate decarboxylase (GAD67) containing interneurons were encountered. Class III reflected "classical" interneurons, generating fast spikes with no frequency adaptation. Class II neurons generated fast spikes with early frequency adaptation and differed from class III by the presence of VIP and the relatively rare presence of neuropeptide Y (NPY) and somatostatin (SOM). Class IV and V were not clearly separated by molecular markers, but by membrane potential values and spike patterns. Morphologically, projection neurons were large, spiny cells, whereas the other neuronal classes displayed smaller somata and spine-sparse dendrites.

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.

Transplantation of embryonic neuroectodermal progenitor cells into the site of a photochemical lesion: Immunohistochemical and electrophysiological analysis

GFP labeled/NE-4C neural progenitor cells cloned from primary neuroectodermal cultures of p53- mouse embryos give rise to neurons when exposed to retinoic acid in vitro. To study their survival and differentiation in vivo, cells were transplanted into the cortex of 6-week-old rats, 1 week after the induction of a photochemical lesion or into noninjured cortex. The electrophysiological properties of GFP/NE-4C cells were studied in vitro (8-10 days after differentiation induction) and 4 weeks after transplantation using the whole-cell patch-clamp technique, and immunohistochemical analyses were carried out. After transplantation into a photochemical lesion, a large number of cells survived, some of which expressed the astrocytic marker GFAP. GFP/GFAP-positive cells, with an average resting membrane potential (Vrest) of -71.9 mV, displayed passive time- and voltage-independent K+ currents and, additionally, voltage-dependent A-type K+ currents (KA) and/or delayed outwardly rectifying K+ currents (KDR). Numerous GFP-positive cells expressed NeuN, betaIII-tubulin, or 68 kD neurofilaments. GFP/betaIII-tubulin-positive cells, with an average Vrest of -61.6 mV, were characterized by the expression of KA and KDR currents and tetrodotoxin-sensitive Na+ currents. GFP/NE-4C cells also gave rise to oligodendrocytes, based on the detection of oligodendrocyte-specific markers. Our results indicate that GFP/NE-4C neural progenitors transplanted into the site of a photochemical lesion give rise to neurons and astrocytes with membrane properties comparable to those transplanted into noninjured cortex. Therefore, GFP/NE-4C cells provide a suitable model for studying neuro- and gliogenesis in vivo. Further, our results suggest that embryonic neuroectodermal progenitor cells may hold considerable promise for the repair of ischemic brain lesions.

Topiramate reduces AMPA-induced Ca(2+) transients and inhibits GluR1 subunit phosphorylation in astrocytes from primary cultures

Topiramate (TPM) is a structurally novel broad spectrum anticonvulsant known to have a negative modulatory effect on the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/kainate subtypes of glutamate receptors and some types of voltage-gated Na(+) and Ca(2+) channels, and a positive modulatory effect on some types of gamma-aminobutyric acid(A) (GABA(A)) receptors and at least one type of K(+) channels in neurons. In an earlier work, we showed that the negative modulatory effect of TPM (100 mum) on AMPA/kainate receptors in neurons is dependent on TPM modulation of the phosphorylation state of these receptors. In this work, we investigated the effect of TPM on AMPA-induced intracellular calcium ([Ca(2+)](i)) responses in cultured rat cortical astrocytes, with special interest in intracellular mechanisms. Here, we report that the ability of TPM (1-100 mum) to inhibit AMPA-induced accumulation of Ca(2+) in astrocytes is inversely related to the level of protein kinase A (PKA) -mediated phosphorylation of channels activated by AMPA. The level of receptor phosphorylation was further determined with western blot using phosphorylation specific antibodies that recognize the glutamate receptor 1 (GluR1) subunit phosphorylated on Ser845. These results demonstrated that, even in cultured cortical astrocytes, TPM significantly reduced the phophorylation level of GluR1 subunits. Furthermore, it was shown that TPM binds to AMPA receptors in the dephosphorylated state and thereby exerts an allosteric modulatory effect on the ion channel.

Glutamate receptor 1-immunopositive neurons in the gliotic CA1 area of the mouse hippocampus after pilocarpine-induced status epilepticus

Significant reduction in glutamate receptor 1 (GluR1)- and GluR2/3-immunopositive neurons was demonstrated in the hilus of the dentate gyrus in mice killed on days 1, 7 and 60 after pilocarpine-induced status epilepticus (PISE). In addition, GluR1 and GluR2/3 immunostaining in the strata oriens, radiatum and lacunosum moleculare of areas CA1-3 decreased drastically on days 7 and 60 after PISE. Neuronal loss observed in the above regions may account, at least in part, for a decrease in GluR immunoreactivity. By contrast, many GluR1-immunopositive neurons were observed in the gliotic area of CA1. Of these, about 42.8% were immunopositive for markers for hippocampal interneurons, namely calretinin (7.6%), calbindin (12.8%) and parvalbumin (22.4%). GluR1 or GluR2/3 and BrdU double-labelling showed that the GluR1- and GluR2/3-immunopositive neurons at 60 days after PISE were neurons that had survived rather than newly generated neurons. Furthermore, anterograde tracer and double-labelling studies performed on animals at 60 days after PISE indicated a projection from the hilus of the dentate gyrus to gliotic areas in both CA3 and CA1, where the projecting fibres apparently established connections with GluR1-immunopositive neurons. The projection to CA1 was unexpected. These novel findings suggest that the intrinsic hippocampal neuronal network is altered after PISE. We speculate that GluR1-immunopositive neurons in gliotic CA1 act as a bridge between dentate gyrus and subiculum contributing towards epileptogenesis.

Molecular biology and ontogeny of glutamate receptors in the mammalian central nervous system

Glutamate is the principal excitatory neurotransmitter in the mammalian central nervous system. After release from presynaptic terminals, glutamate binds to both ionotropic and metabotropic receptors to mediate fast, slow, and persistent effects on synaptic transmission and integrity. There are three types of ionotropic glutamate receptors. N-Methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA), and kainate receptors are principally activated by the agonist bearing its name and are permeable to cationic flux; hence, their activation results in membrane depolarization. All ionotropic glutamate receptors are believed to be composed of four distinct subunits, each of which is topologically arranged with three transmembrane-spanning and one pore-lining (hairpin loop) domain. In contrast, metabotropic glutamate receptors are G protein (guanine nucleotide-binding protein) -coupled receptors linked to second-messenger systems. Group I metabotropic glutamate receptors are linked to phospholipase C, which results in phosphoinositide hydrolysis and release of calcium from intracellular stores. Group II and group III metabotropic glutamate receptors are negatively linked to adenylate cyclase, which catalyzes the production of cyclic adenosine monophosphate. Each metabotropic glutamate receptor is composed of seven transmembrane-spanning domains, similar to other members of the superfamily of metabotropic receptors, which includes noradrenergic, muscarinic acetylcholinergic, dopaminergic, serotonergic (except type 3 receptors), and gamma-aminobutyric acid (GABA) type B receptors. This review summarizes the relevant molecular biology and ontogeny of glutamate receptors in the central nervous system and highlights some of the roles that they can play during brain development and in certain disease states.

Enhanced relative expression of glutamate receptor 1 flip AMPA receptor subunits in hippocampal astrocytes of epilepsy patients with Ammon's horn sclerosis

Astrocytes express ionotropic glutamate receptors (GluRs), and recent evidence suggests that these receptors contribute to direct signaling between neurons and glial cells in vivo. Here, we have used functional and molecular analyses to investigate receptor properties in astrocytes of human hippocampus resected from patients with pharmacoresistant temporal lobe epilepsy (TLE). Histopathological analysis allowed us to distinguish two forms of epilepsy: Ammon's horn sclerosis (AHS) and lesion-associated TLE. Human hippocampal astrocytes selectively expressed the AMPA subtype of ionotropic glutamate receptors. Single-cell RT-PCR found preferential expression of the subunits GluR1 and GluR2 in human astrocytes, and the expression patterns were similar in patients with AHS and lesion-associated epilepsy. The AMPA receptor-specific modulators, cyclothiazide (CTZ) and 4-[2-(phenylsulfonylamino)ethylthio]-2,6-difluoro-phenoxyacetamide (PEPA), were used to investigate splice variant expression. Astrocytes of sclerotic specimens displayed a slower dissociation of CTZ from the receptor and a lower ratio of current potentiation by PEPA to potentiation by CTZ, suggesting enhanced expression of flip receptor variants in AHS versus lesion-associated epilepsy. Real-time PCR and restriction analysis substantiated this presumption by identifying elevated flip-to-flop mRNA ratios of GluR1 in single astrocytes of AHS specimens. These findings imply that in AHS, glutamate may lead to prolonged depolarization of astrocytes, thereby facilitating the generation or spread of seizure activity.

Electrophysiologically "complex" glial cells freshly isolated from the hippocampus are immunopositive for the chondroitin sulfate proteoglycan NG2

We have recently described a subgroup of isolated glial fibrillary acidic protein-positive (GFAP+) hippocampal astrocytes that predominantly express outwardly rectifying currents (which we term "ORAs" for outwardly rectifying astrocytes), which are similar to the currents already described for hippocampal GFAP- "complex glia." We now report that post-recording staining of cells that were first selected as "complex" by morphology and then confirmed by their electrophysiological characteristics were NG2+ approximately 90% of the time. Also, the morphology of freshly isolated NG2+ cells differs from that of isolated GFAP+ ORAs in having a smaller and round cell body with thinner processes, which usually are collapsed back onto the soma. Upon detailed examination, NG2+ cells were found to differ quantitatively in some electrophysiological characteristics from GFAP+ ORAs. The outward, transient K+ currents (IKa) in the NG2+ cells showed a slower decay than the IKa in ORAs, and their density decreased in NG2+ cells from older animals. The other two major cation currents, the voltage-activated Na+ and outwardly delayed rectifier K+ currents, were similar in NG2+ cells and ORAs. To further distinguish isolated complex cells from outwardly rectifying GFAP+ astrocytes, we performed immunocytochemistry for glial markers in fixed, freshly isolated rat hippocampal glia. NG2+ cells were negative for GFAP and also for the astrocytic glutamate transporters GLT-1 and GLAST. Thus the isolated hippocampal NG2+ glial cells, though having an electrophysiological phenotype similar to that of ORAs, are an immunologically and morphologically distinct glial cell population and most likely represent NG2+ cells in situ.

Segregated expression of AMPA-type glutamate receptors and glutamate transporters defines distinct astrocyte populations in the mouse hippocampus

Recent data have suggested the existence of direct signaling pathways between glial cells and neurons. Here we report the coexistence of distinct types of cells expressing astrocyte-specific markers within the hippocampus that display diverse morphological, molecular, and functional profiles. Usage of transgenic mice with GFAP promoter-controlled enhanced green fluorescent protein (EGFP) expression allowed the identification of astroglial cells after fresh isolation or in brain slices. Combining patch-clamp recordings and single-cell reverse transcription-PCR, we distinguished two morphologically distinct types of EGFP-positive cells, one expressing glutamate transporters and the other expressing ionotropic glutamate receptors. None of the EGFP-positive cells coexpressed glutamate receptors and transporters. Subpopulations of glutamate receptor-bearing EGFP-positive cells expressed AN2, the mouse homolog of the rat NG2 proteoglycan or transcripts for excitatory amino acid carrier 1, a neuronal glutamate transporter. Our data demonstrate the presence of distinct, independent populations of cells with astroglial properties in the developing hippocampus that can differently modulate neuronal signaling pathways. The observed heterogeneity of cells with GFAP promoter-regulated EGFP expression and S100beta/GFAP immunoreactivity challenges the hitherto accepted definition of astrocytes.

The proteoglycan NG2 is complexed with alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors by the PDZ glutamate receptor interaction protein (GRIP) in glial progenitor cells. Implications for glial-neuronal signaling

The proteoglycan NG2 is expressed by immature glial cells in the developing and adult central nervous system. Using the COOH-terminal region of NG2 as bait in a yeast two-hybrid screen, we identified the glutamate receptor interaction protein GRIP1, a multi-PDZ domain protein, as an interacting partner. NG2 exhibits a PDZ binding motif at the extreme COOH terminus which binds to the seventh PDZ domain of GRIP1. In addition to the published expression in neurons, GRIP1 is expressed by immature glial cells. GRIP1 is known to bind to the GluRB subunit of the AMPA glutamate receptor expressed by subpopulations of neurons and immature glial cells. In cultures of primary oligodendrocytes, cells coexpress GluRB and NG2. A complex of NG2, GRIP1, and GluRB can be precipitated from transfected mammalian cells and from cultures of primary oligodendrocytes. Furthermore, NG2 and GRIP can be coprecipitated from developing brain tissue. These data suggest that GRIP1 acts as a scaffolding molecule clustering NG2 and AMPA receptors in immature glia. In view of the presence of synaptic contacts between neurons and NG2-positive glial cells in the hippocampus and the close association of NG2-expressing glial cells with axons, we suggest a role for the NG2.AMPA receptor complex in glial-neuronal recognition and signaling.

Changes in splice variant expression and subunit assembly of AMPA receptors during maturation of hippocampal astrocytes

Astrocytes in the hippocampus express glutamate receptors of the AMPA subtype. An increasing body of evidence suggests a contribution of astroglial AMPA receptors to a direct signaling between neurons and glial cells in vivo. Here, we have combined functional analysis with singlecell RT-PCR to investigate whether hippocampal astrocytes express Ca(2+)-permeable AMPA receptors. We show that by postnatal day 5, a mosaic of Ca(2+)-permeable and less Ca(2+)-permeable AMPA receptors coexists in individual astrocytes, while receptors with a more uniform, low divalent permeability dominate in older cells. Moreover, we report an upregulation of the flip form of the GluR2 subunit during maturation, while the splicing status of GluR1 and GluR4 remains unchanged. Due to its specific properties, Ca(2+)-permeable AMPA receptors in astrocytes might strengthen neuron-to-glia signaling and enable proper formation of structural and functional connections between glial cells and glutamatergic synapses in the developing hippocampus.

Type-2 astrocyte-like cells are more resistant than oligodendrocyte-like cells against non-N-methyl-D-aspartate glutamate receptor-mediated excitotoxicity

Glutamate causes excitotoxicity via non-N-methyl-D-aspartate (NMDA) glutamate receptors (GluR) in oligodendrocytes. Because both oligodendrocytes and type 2 astrocytes are differentiated from oligodendrocyte-type 2 astrocyte (O-2A) progenitor cells, we investigated whether astrocytes are also vulnerable to non-NMDA GluR-mediated excitotoxicity. For these studies, oligodendrocyte-like cells (OLC) and type 2 astrocyte-like cells (2ALC) were derived from CG-4 cells, an immortalized rat O-2A progenitor cell line. About 50% of 2ALC were positive for glial fibrillary acidic protein and 90% were positive for A2B5, verifying that these cells have an type 2 astrocytic phenotype. A 24-hr exposure of OLC to 2 mM kainate, an activator of non-NMDA GluR, caused cell damage as shown by the release of lactate dehydrogenase. The extent of kainate-induced OLC damage was increased by cyclothiazide. In contrast, exposure of 2ALC to 2 mM kainate alone did not induce injury, though mild 2ALC injury was elicited by exposure to 2 mM kainate plus 100 microM cyclothiazide. Furthermore, we found that the kainate induced Ca(2+) uptake by 2ALC was 27.5% of that induced by kainate in OLC. Finally, both OLC and 2ALC expressed non-NMDA GluR subunit mRNAs, including GluR2, GluR3, GluR4, GluR6, GluR7, KA1, and KA2, but quantitative Western blot analysis revealed higher immunodetectable GluR2 and lower immunodetectable GluR3 and GluR4 in 2ALC than in OLC. Together, these results suggest that astrocytes are relatively resistant to non-NMDA GluR-mediated excitotoxicity because they have a higher expression of GluR2 and lower expression of GluR3 and GluR4.

Changes in flip/flop splicing of astroglial AMPA receptors in human temporal lobe epilepsy

PURPOSE

Recent data suggested a role for glial cells in epilepsy. This study sought to identify and functionally characterize AMPA receptors expressed by astrocytes in human hippocampal tissue resected from patients with intractable temporal lobe epilepsy.

METHODS

Patch-clamp and fast application methods were combined to investigate astrocytes in situ and after fresh isolation from the stratum radiatum of the hippocampal CA1 subfield. Relying on presurgical and histopathologic analysis, we divided human specimens into two groups, Ammon's horn sclerosis (AHS) and lesion-associated epilepsy.

RESULTS

Fast application of glutamate and kainate evoked receptor currents in all cells studied. Reversal-potential analysis revealed an intermediate Ca2+ permeability of the receptor channels that did not vary between the two groups of patients. However, preapplication of the AMPA receptor-specific modulator, cyclothiazide, disclosed differences in flip-flop splicing. This treatment considerably enhanced the receptor conductance, with potentiation being significantly stronger in cells from AHS specimens compared with lesion-associated cells, suggesting upregulation of AMPA receptor flip splice variants in astrocytes of the sclerotic tissue.

CONCLUSIONS

Compelling evidence has been accumulated showing direct and rapid signaling between neurons and glial cells. Our data suggest that in AHS patients, neuronally released glutamate will lead to an enhanced and prolonged depolarization of astrocytes, which might be involved in seizure generation and spread in this particular condition of human temporal lobe epilepsy.

Beyond the role of glutamate as a neurotransmitter

Glutamate is the principal excitatory neurotransmitter of the central nervous system, but many studies have expanded its functional repertoire by showing that glutamate receptors are present in a variety of non-excitable cells. How does glutamate receptor activation modulate their activity? Do non-excitable cells release glutamate, and, if so, how? These questions remain enigmatic. Here, we review the current knowledge on glutamatergic signalling in non-neuronal cells, with a special emphasis on astrocytes.

AMPA receptor-mediated modulation of inward rectifier K+ channels in astrocytes of mouse hippocampus

Astrocytes and neurons are tightly associated and recent data suggest a direct signaling between neuronal and glial cells in vivo. To further analyze these interactions, the patch-clamp technique was combined with single-cell RT-PCR in acute hippocampal brain slices. Subsequent to functional analysis, the cytoplasm of the same cell was harvested to perform transcript analysis and identify subunits that underlie inwardly rectifying K+ currents (I(Kir)) in astrocytes of the CA1 stratum radiatum. Transcripts encoding Kir2.1, Kir2.2, or Kir2.3, were encountered in a majority of cells, while Kir4.1 was less frequent. Further investigation revealed that glial Kir channels are rapidly inhibited upon activation of AMPA-type glutamate receptors, most probably due a receptor-mediated influx of Na+, which plugs the channels from the intracellular side. A transient inhibition of I(Kir) in astrocytes in response to neuronal glutamate release and glial AMPA receptor activation represents a further, so far undetected mechanism to balance neuronal excitability.

Functional and molecular properties of human astrocytes in acute hippocampal slices obtained from patients with temporal lobe epilepsy

PURPOSE

The specific role of glial cells in epilepsy is still elusive. In this study, functional properties of astrocytes were investigated in acute hippocampal brain slices obtained from surgical specimens of patients with drug-resistant temporal lobe epilepsy (TLE).

METHODS

The patch-clamp technique together with a single-cell reverse transcription-polymerase chain reaction approach were used to combine functional and molecular analysis in the same individual cell in situ.

RESULTS

In patients with Ammon's horn sclerosis, the glial current patterns resembled properties of immature astrocytes in rodent hippocampus. Depolarizing voltage steps activated delayed rectifier and transient K+ currents as well as tetrodotoxin-sensitive Na+ currents. Hyperpolarizing voltages elicited inward rectifier K+ currents. Comparative recordings were made in astrocytes from patients with lesion-associated TLE that lacked significant histopathological hippocampal alterations. The inward rectifier K+ current density was significantly smaller in astrocytes from the sclerotic group compared with lesion-associated TLE patients.

CONCLUSIONS

During normal development of rodent brain, astroglial inward rectification gradually increases. It thus appears that astrocytes in human sclerotic tissue reexpress an immature current pattern. Reduced astroglial inward rectification in conjunction with seizure-induced shrinkage of the extracellular space may lead to impaired spatial K+ buffering. This will result in stronger and prolonged depolarization of glial cells and neurons in response to activity-dependent K+ release and may thus contribute to seizure generation and spread in this particular condition of human TLE.

Astrocytes in the hippocampus of patients with temporal lobe epilepsy display changes in potassium conductances

Functional properties of astrocytes were investigated with the patch-clamp technique in acute hippocampal brain slices obtained from surgical specimens of patients suffering from pharmaco-resistant temporal lobe epilepsy (TLE). In patients with significant neuronal cell loss, i.e. Ammon's horn sclerosis, the glial current patterns resembled properties characteristic of immature astrocytes in the murine or rat hippocampus. Depolarizing voltage steps activated delayed rectifier and transient K+ currents as well as tetrodotoxin-sensitive Na+ currents in all astrocytes analysed in the sclerotic human tissue. Hyperpolarizing voltages elicited inward rectifier currents that inactivated at membrane potentials negative to -130 mV. Comparative recordings were performed in astrocytes from patients with lesion-associated TLE that lacked significant histopathological hippocampal alterations. These cells displayed stronger inward rectification. To obtain a quantitative measure, current densities were calculated and the ratio of inward to outward K+ conductances was determined. Both values were significantly smaller in astrocytes from the sclerotic group compared with lesion-associated TLE. During normal development of rodent brain, astroglial inward rectification gradually increases. It thus appears reasonable to suggest that astrocytes in human sclerotic tissue return to an immature current pattern. Reduced astroglial inward rectification in conjunction with seizure-induced shrinkage of the extracellular space may lead to impaired spatial K+ buffering. This will result in stronger and prolonged depolarization of glial cells and neurons in response to activity-dependent K+ release, and may thus contribute to seizure generation in this particular condition of human TLE.

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.

Acutely isolated astrocytes as models to probe astrocyte functions

Neuroscientists have become increasingly aware and accepting of the concept that astrocytes likely have many important functions in the CNS. One limitation in establishing these functions is the usual problem of what constitutes suitable experimental approaches. A major experimental step for functional studies of astrocytes has been the widespread use of primary astrocyte cultures, an approach that Leif Hertz pioneered. However, it is now becoming clear that, building on this work, an experimental paradigm shift is now needed. Namely, to increasingly study preparations corresponding to in situ conditions, such as slices. An alternative experimental system where the cells have some of the technical advantages of primary astrocyte cultures is freshly isolated astrocytes. Recent experiments from our laboratory have shown metabotropic glutamate receptor expression by such cells. Examples are given of how functional receptor studies and channel activity measured by patch clamp electrophysiology can be combined with single cell RT-PCR to define further the receptor or channel type are described to illustrate the uses of such preparations.

GFAP mRNA positive glia acutely isolated from rat hippocampus predominantly show complex current patterns

Electrophysiologically complex glial cells have been widely identified from different regions of the central nervous system and constitute a dominant glial type in juvenile mice or rats. As these cells express several types of ion channels and neurotransmitter channels that were thought to be only present in neurons, this glial cell type has attracted considerable attention. However, the actual classification of these electrophysiologically complex glial cells remains unclear. They have been speculated to be an immature astrocyte because, although these cells show positive staining for the predominantly astrocytic marker S 100beta, it has not been possible to show staining for the commonly accepted mature astrocytic marker, glial fibrillary acidic protein (GFAP). To address the question of whether these cells might express GFAP at the transcript level, we combined patch-clamp electrophysiological recording with single cell RT-PCR for GFAP mRNA in glial cells acutely isolated from 4 to 12 postnatal day rats. In fresh cell suspensions from the CA1 region, complex glial cells were found to be the dominant cell type (65% total cells). We found that the majority of these electrophysiologically complex cells (74%) were positive for GFAP mRNA. We also showed that the complex cells responded to AMPA and GABA application, and these were also GFAP mRNA positive. We also fixed and stained the preparations for GFAP without electrophysiological recording to better preserve GFAP immunoreactively. In agreement with other studies, only 1.5% of these presumed electrophysiologically complex cells, based on morphology, showed immunoreactivity for GFAP. The expression of GFAP at the transcript level indicates GFAP (-)/GFAP mRNA (+) glial cells have an astrocytic identity. As single cell RT-PCR is able to detect both GFAP (-)/GFAP mRNA (+) and GFAP (+)/GFAP mRNA (+) astrocytic subtypes, the present study also suggests it is a feasible approach for astrocytic lineage studies.

Developmental changes in the expression of Shaker- and Shab-related K(+) channels in neurons of the rat trigeminal ganglion

We have investigated properties of voltage-gated K(+) channels in neurons of the pre- and postnatal rat trigeminal ganglion (TG). To correlate functional data with information on gene expression of Shaker- and Shab-related channels in these pseudo-unipolar neurons, the patch-clamp technique was combined with the single-cell reverse transcription-polymerase chain reaction (RT-PCR). A majority (80%) of prenatal TG neurons possessed only sustained delayed rectifier currents with half-maximal current inactivation at -30 mV. In the postnatal cells, steady-state inactivation of sustained currents occurred at more negative voltages (half-maximal inactivation at -58 mV). About 65% of the postnatal cells displayed a transient outward component in addition to the sustained currents. With increasing age, the sensitivity of sustained currents to 4-aminopyridine (4-AP) decreased significantly. The Shaker channel toxins, alpha-dendrotoxin and agitoxin-2 (50 and 10 nM), were much less effective. Discrimination between both stages with tetraethylammonium chloride (5 mM) was not possible since the currents were reduced generally by about 50%. After recording, the cell content was harvested and single-cell RT-PCR was performed to compare K(+) current properties and mRNA expression within the same cell. Most cells simultaneously expressed several different Shaker- and Shab-like transcripts. At postnatal day 14, the frequency of cells carrying transcripts encoding Kv1.1 decreased. Detailed analysis revealed a higher 4-AP sensitivity of TG neurons expressing Kv1.1 transcripts.