Properties of voltage-activated Na+ and K+ currents in mouse hippocampal glial cells in situ and after acute isolation from tissue slices

Membrane properties and coupling of macroglia in the optic nerve

We established a longitudinal acute slice preparation of transgenic mouse optic nerve to characterize membrane properties and coupling of glial cells by patch-clamp and dye-filling, complemented by immunohistochemistry. Unlike in cortex or hippocampus, the majority of EGFP + cells in optic nerve of the hGFAP-EGFP transgenic mouse, a tool to identify astrocytes, were characterized by time and voltage dependent K+-currents including A-type K+-currents, properties previously described for NG2 glia. Indeed, the majority of transgene expressing cells in optic nerve were immunopositive for NG2 proteoglycan, whereas only a minority show GFAP immunoreactivity. Similar physiological properties were seen in YFP + cells from NG2-YFP transgenic mice, indicating that in optic nerve the transgene of hGFAP-EGFP animals is expressed by NG2 glia instead of astrocytes. Using Cx43kiECFP transgenic mice as another astrocyte-indicator revealed that astrocytes had passive membrane currents. Dye-filling showed that hGFAP-EGFP+ cells in optic nerve were coupled to none or few neighboring cells while hGFAP-EGFP+ cells in the cortex form large networks. Similarly, dye-filling of NG2-YFP+ and Cx43-CFP+ cells in optic nerve revealed small networks. Our work shows that identification of astrocytes in optic nerve requires distinct approaches, that the cells express membrane current patterns distinct from cortex and that they form small networks.

Novel Toolboxes for the Investigation of Activity-Dependent Myelination in the Central Nervous System

Myelination is essential for signal processing within neural networks. Emerging data suggest that neuronal activity positively instructs myelin development and myelin adaptation during adulthood. However, the underlying mechanisms controlling activity-dependent myelination have not been fully elucidated. Myelination is a multi-step process that involves the proliferation and differentiation of oligodendrocyte precursor cells followed by the initial contact and ensheathment of axons by mature oligodendrocytes. Conventional end-point studies rarely capture the dynamic interaction between neurons and oligodendrocyte lineage cells spanning such a long temporal window. Given that such interactions and downstream signaling cascades are likely to occur within fine cellular processes of oligodendrocytes and their precursor cells, overcoming spatial resolution limitations represents another technical hurdle in the field. In this mini-review, we discuss how advanced genetic, cutting-edge imaging, and electrophysiological approaches enable us to investigate neuron-oligodendrocyte lineage cell interaction and myelination with both temporal and spatial precision.

On the electrical passivity of astrocyte potassium conductance

Predominant expression of leak-type K⁺ channels provides astrocytes a high membrane permeability to K⁺ ions and a hyperpolarized membrane potential that are crucial for astrocyte function in brain homeostasis. In functionally mature astrocytes, the expression of leak K⁺ channels creates a unique membrane K⁺ conductance that lacks voltage-dependent rectification. Accordingly, the conductance is named ohmic or passive K⁺ conductance. Several inwardly rectifying and two-pore domain K⁺ channels have been investigated for their contributions to passive conductance. Meanwhile, gap junctional coupling has been postulated to underlie the passive behavior of membrane conductance. It is now clear that the intrinsic properties of K⁺ channels and gap junctional coupling can each act alone or together to bring about a passive behavior of astrocyte conductance. Additionally, while the passive conductance can generally be viewed as a K⁺ conductance, the actual representation of this conductance is a combined expression of multiple known and unknown K⁺ channels, which has been further modified by the intricate morphology of individual astrocytes and syncytial gap junctional coupling. The expression of the inwardly rectifying K⁺ channels explains the inward-going component of passive conductance disobeying Goldman-Hodgkin-Katz constant field outward rectification. However, the K⁺ channels encoding the outward-going passive currents remain to be determined in the future. Here, we review our current understanding of ion channels and biophysical mechanisms engaged in the passive astrocyte K⁺ conductance, propose new studies to resolve this long-standing puzzle in astrocyte physiology, and discuss the functional implication(s) of passive behavior of K⁺ conductance on astrocyte physiology.

Heterogeneity and function of hippocampal macroglia

The contribution of glial cells to normal and impaired hippocampal function is increasingly being recognized, although important questions as to the mechanisms that these cells use for their crosstalk with neurons and capillaries are still unanswered or lead to controversy. Astrocytes in the hippocampus are morphologically variable and a single cell contacts with its processes more than 100,000 synapses. They predominantly express inward rectifier K⁺ channels and transporters serving homeostatic function but may also release gliotransmitters to modify neuronal signaling and brain circulation. Intracellular Ca²⁺ transients are key events in the interaction of astrocytes with neurons and the vasculature. Hippocampal NG2 glia represent a population of cells with proliferative capacity throughout adulthood. Intriguingly, they receive direct synaptic input from pyramidal neurons and interneurons and express a multitude of ion channels and receptors. Despite in-depth knowledge about the features of these transmembrane proteins, the physiological impact of NG2 glial cells and their synaptic input remain nebulous. Because of the low abundance of oligodendrocytes in the hippocampus, limited information is available about their specific properties. Given the multitude of signaling molecules expressed by the various types of hippocampal glial cells (and because of space constraints), we focus, in this review, on those properties that are considered key for the interaction of the respective cell type with its neighborhood.

NG2 glial cells integrate synaptic input in global and dendritic calcium signals

Synaptic signaling to NG2-expressing oligodendrocyte precursor cells (NG2 cells) could be key to rendering myelination of axons dependent on neuronal activity, but it has remained unclear whether NG2 glial cells integrate and respond to synaptic input. Here we show that NG2 cells perform linear integration of glutamatergic synaptic inputs and respond with increasing dendritic calcium elevations. Synaptic activity induces rapid Ca²⁺ signals mediated by low-voltage activated Ca²⁺ channels under strict inhibitory control of voltage-gated A-type K⁺ channels. Ca²⁺ signals can be global and originate throughout the cell. However, voltage-gated channels are also found in thin dendrites which act as compartmentalized processing units and generate local calcium transients. Taken together, the activity-dependent control of Ca²⁺ signals by A-type channels and the global versus local signaling domains make intracellular Ca²⁺ in NG2 cells a prime signaling molecule to transform neurotransmitter release into activity-dependent myelination.

DOI:http://dx.doi.org/10.7554/eLife.16262.001

Changes in the proliferative capacity of NG2 cell subpopulations during postnatal development of the mouse hippocampus

Besides astrocytes and oligodendrocytes, NG2 proteoglycan-expressing cells (NG2 glia) represent a third subtype of macroglia in the brain. Originally described as oligodendrocyte precursor cells, they feature several characteristics not expected from mere progenitor cells, including synaptic connections with neurons. There is accumulating evidence that the properties of NG2 glia differ between different brain regions and developmental stages. To further analyze this proposed heterogeneity, we studied electrophysiological properties, transcript and protein expression, distribution and proliferative capacity of NG2 glia during postnatal development, focusing on the hippocampus and corpus callosum. All NG2 glia displayed a 'complex' current pattern consisting of voltage- and time-dependent in- and outward currents. In juvenile mice, Kir current densities and rectification index were highly variable and on average significantly lower than in adult animals. Single cell RT-PCR analyses of electrophysiologically characterized cells demonstrated that different glial genes were expressed at variable extent, independent of developmental stage and genetic background. In the hippocampus proper and the corpus callosum, the density of NG2 glia was highest at postnatal days (P)10-12, decreased by ~50 % at P25-35 and then remained stable in adults (P80-90). Interestingly, co-expression of NG2 and S100β, a marker for mature astrocytes, increased from 7 % at P10-12 to 27 % at P25-35 in the hippocampus proper, and then dropped again in the stratum radiatum at P80-90. In the dentate gyrus and corpus callosum, co-expression of NG2 and S100β was very low (3 %) and constant throughout development. Age-related differences were also observed with Ki-67, a proliferation marker. In NG2 glia of the CA1 region, its expression decreased from 16 % at P10-12 to 9 % (P25-35) and then 3 % (P80-90). Triple-stainings revealed that Ki-67 was also expressed in 2-3 % of NG2/S100β-positive cells in the juvenile and mature stratum radiatum, indicating that the latter, in contrast to S100β-positive astrocytes, still host proliferative potential. Taken together, we found significant differences in transcript and protein expression, electrophysiological properties and proliferative capacity of NG2 glia in the mouse forebrain, suggesting the co-existence of several subpopulations of NG2 glia. Our data thus support the idea of a substantial regional and developmental heterogeneity in this subtype of macroglia.

Astrocytic glutamate transport regulates a Drosophila CNS synapse that lacks astrocyte ensheathment

Anatomical, molecular, and physiological interactions between astrocytes and neuronal synapses regulate information processing in the brain. The fruit fly Drosophila melanogaster has become a valuable experimental system for genetic manipulation of the nervous system and has enormous potential for elucidating mechanisms that mediate neuron-glia interactions. Here, we show the first electrophysiological recordings from Drosophila astrocytes and characterize their spatial and physiological relationship with particular synapses. Astrocyte intrinsic properties were found to be strongly analogous to those of vertebrate astrocytes, including a passive current-voltage relationship, low membrane resistance, high capacitance, and dye-coupling to local astrocytes. Responses to optogenetic stimulation of glutamatergic premotor neurons were correlated directly with anatomy using serial electron microscopy reconstructions of homologous identified neurons and surrounding astrocytic processes. Robust bidirectional communication was present: neuronal activation triggered astrocytic glutamate transport via excitatory amino acid transporter 1 (Eaat1), and blocking Eaat1 extended glutamatergic interneuron-evoked inhibitory postsynaptic currents in motor neurons. The neuronal synapses were always located within 1 μm of an astrocytic process, but none were ensheathed by those processes. Thus, fly astrocytes can modulate fast synaptic transmission via neurotransmitter transport within these anatomical parameters. J. Comp. Neurol. 524:1979-1998, 2016. © 2016 Wiley Periodicals, Inc.

Electrophysiological behavior of neonatal astrocytes in hippocampal stratum radiatum

BACKGROUND

Neonatal astrocytes are diverse in origin, and undergo dramatic change in gene expression, morphological differentiation and  syncytial networking throughout development. Neonatal astrocytes also play multifaceted roles in neuronal circuitry establishment. However, the extent to which neonatal astrocytes differ from their counterparts in the adult brain remains unknown.

RESULTS

Based on ALDH1L1-eGFP expression or sulforhodamine 101 staining, neonatal astrocytes at postnatal day 1-3 can be reliably identified in hippocampal stratum radiatum. They exhibit a more negative resting membrane potential (V M), -85 mV, than mature astrocytes, -80 mV and a variably rectifying whole-cell current profile due to complex expression of voltage-gated outward transient K(+) (IKa), delayed rectifying K(+) (IKd) and inward K(+) (IKin) conductances. Differing from NG2 glia, depolarization-induced inward Na(+) currents (INa) could not be detected in neonatal astrocytes. A quasi-physiological V M of -69 mV was retained when inwardly rectifying Kir4.1 was inhibited by 100 μM Ba(2+) in both wild type and TWIK-1/TREK-1 double gene knockout astrocytes, indicating expression of additional leak K(+) channels yet unknown. In dual patch recording, electrical coupling was detected in 74 % (14/19 pairs) of neonatal astrocytes with largely variable coupling coefficients. The increasing gap junction coupling progressively masked the rectifying K(+) conductances to account for an increasing number of linear voltage-to-current relationship passive astrocytes (PAs). Gap junction inhibition, by 100 μM meclofenamic acid, substantially reduced membrane conductance and converted all the neonatal PAs to variably rectifying astrocytes. The low density expression of leak K(+) conductance in neonatal astrocytes corresponded  to a ~50 % less K(+) uptake capacity compared to adult astrocytes.

CONCLUSIONS

Neonatal astrocytes predominantly express a variety of rectifying K(+) conductances, form discrete cell-to-cell gap junction coupling and are deficient in K(+) homeostatic capacity.

The NG2 Protein Is Not Required for Glutamatergic Neuron-NG2 Cell Synaptic Signaling

NG2 glial cells (as from now NG2 cells) are unique in receiving synaptic input from neurons. However, the components regulating formation and maintenance of these neuron-glia synapses remain elusive. The transmembrane protein NG2 has been considered a potential mediator of synapse formation and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) clustering, because it contains 2 extracellular Laminin G/Neurexin/Sex Hormone-Binding Globulin domains, which in neurons are crucial for formation of transsynaptic neuroligin-neurexin complexes. NG2 is connected via Glutamate Receptor-Interacting Protein with GluA2/3-containing AMPARs, thereby possibly mediating receptor clustering in glial postsynaptic density. To elucidate the role of NG2 in neuron-glia communication, we investigated glutamatergic synaptic transmission in juvenile and aged hippocampal NG2 cells of heterozygous and homozygous NG2 knockout mice. Neuron-NG2 cell synapses readily formed in the absence of NG2. Short-term plasticity, synaptic connectivity, postsynaptic AMPAR current kinetics, and density were not affected by NG2 deletion. During development, an NG2-independent acceleration of AMPAR current kinetics and decreased synaptic connectivity were observed. Our results indicate that the lack of NG2 does not interfere with genesis and basic properties of neuron-glia synapses. In addition, we demonstrate frequent expression of neuroligins 1-3 in juvenile and aged NG2 cells, suggesting a role of these molecules in synapse formation between NG2 glia and neurons.

Synaptic integration by NG2 cells

NG2 expressing oligodendrocyte precursor cells stand out from other types of glial cells by receiving classical synaptic contacts from many neurons. This unconventional form of signaling between neurons and glial cells enables NG2 cells to receive information about the activity of presynaptic neurons with high temporal and spatial precision and has been postulated to be involved in activity-dependent myelination. While this still unproven concept is generally compelling, how NG2 cells may integrate synaptic input has hardly been addressed to date. Here we review the biophysical characteristics of synaptic currents and membrane properties of NG2 cells and discuss their capabilities to perform complex temporal and spatial signal integration and how this may be important for activity-dependent myelination.

Studying subcellular detail in fixed astrocytes: dissociation of morphologically intact glial cells (DIMIGs)

Studying the distribution of astrocytic antigens is particularly hard when they are localized in their fine, peripheral astrocyte processes (PAPs), since these processes often have a diameter comparable to vesicles and small organelles. The most appropriate technique is immunoelectron microscopy, which is, however, a time-consuming procedure. Even in high resolution light microscopy, antigen localization is difficult to detect due to the small dimensions of these processes, and overlay from antigen in surrounding non-glial cells. Yet, PAPs frequently display antigens related to motility and glia-synaptic interaction. Here, we describe the dissociation of morphologically intact glial cells (DIMIGs), permitting unambiguous antigen localization using epifluorescence microscopy. Astrocytes are dissociated from juvenile (p13-15) mouse cortex by applying papain treatment and cytospin centrifugation to attach the cells to a slide. The cells and their complete processes including the PAPs is thus projected in 2D. The entire procedure takes 2.5-3 h. We show by morphometry that the diameter of DIMIGs, including the PAPs is similar to that of astrocytes in situ. In contrast to cell culture, results derived from this procedure allow for direct conclusions relating to (1) the presence of an antigen in cortical astrocytes, (2) subcellular antigen distribution, in particular when localized in the PAPs. The detailed resolution is shown in an exemplary study of the organization of the astrocytic cytoskeleton components actin, ezrin, tubulin, and GFAP. The distribution of connexin 43 in relation to a single astrocyte's process tree is also investigated.

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.

Sensory and cortical activation of distinct glial cell subtypes in the somatosensory thalamus of young rats

The rodent ventrobasal (VB) thalamus receives sensory inputs from the whiskers and projects to the cortex, from which it receives reciprocal excitatory afferents. Much is known about the properties and functional roles of these glutamatergic inputs to thalamocortical neurons in the VB, but no data are available on how these afferents can affect thalamic glial cells. In this study, we used combined electrophysiological recordings and intracellular calcium ([Ca²⁺]_i) imaging to investigate glial cell responses to synaptic afferent stimulation. VB thalamus glial cells can be divided into two groups based on their [Ca²⁺]_i and electrophysiological responses to sensory and corticothalamic stimulation. One group consists of astrocytes, which stain positively for S100B and preferentially load with SR101, have linear current–voltage relations and low input resistance, show no voltage-dependent [Ca²⁺]_i responses, but express mGluR5-dependent [Ca²⁺]_i transients following stimulation of the sensory and/or corticothalamic excitatory afferent pathways. Cells of the other glial group, by contrast, stain positively for NG2, and are characterized by high input resistance, the presence of voltage-dependent [Ca²⁺]_i elevations and voltage-gated inward currents. There were no synaptically induced [Ca²⁺]_i elevations in these cells under control conditions. These results show that thalamic glial cell responses to synaptic input exhibit different properties to those of thalamocortical neurons. As VB astrocytes can respond to synaptic stimulation and signal to neighbouring neurons, this glial cell organization may have functional implications for the processing of somatosensory information and modulation of behavioural state-dependent thalamocortical network activities.

Anti-inflammatory effects of the anticonvulsant drug levetiracetam on electrophysiological properties of astroglia are mediated via TGFβ1 regulation

BACKGROUND AND PURPOSE

The involvement of astrocytes as immune-competent players in inflammation and the pathogenesis of epilepsy and seizure-induced brain damage has recently been recognized. In clinical trials and practice, levetiracetam (LEV) has proven to be an effective antiepileptic drug (AED) in various forms of epileptic seizures, when applied as mono- or added therapy. Little is known about the mechanism(s) of action of LEV. Evidence so far suggests a mode of action different from that of classical AEDs. We have shown that LEV restored functional gap junction coupling and basic membrane properties in an astrocytic inflammatory model in vitro.

EXPERIMENTAL APPROACH

Here, we used neonatal rat astrocytes co-cultured with high proportions (30%) of activated microglia or treated with the pro-inflammatory cytokine interleukin-1β to provoke inflammatory responses. Effects of LEV (50 µg·mL⁻¹) on electrophysiological properties of astrocytes (by whole cell patch clamp) and on secretion of TGFβ1 (by (ELISA)) were studied in these co-cultures.

KEY RESULTS

LEV restored impaired astrocyte membrane resting potentials via modification of inward and outward rectifier currents, and promoted TGFβ1 expression in inflammatory and control co-cultures. Furthermore, LEV and TGFβ1 exhibited similar facilitating effects on the generation of astrocyte voltage-gated currents in inflammatory co-cultures and the effects of LEV were prevented by antibody to TGFβ1.

CONCLUSIONS AND IMPLICATIONS

Our data suggest that LEV is likely to reduce the harmful spread of excitation elicited by seizure events within the astro-glial functional syncytium, with stabilizing consequences for neuronal-glial interactions.

Increased coupling and altered glutamate transport currents in astrocytes following kainic-acid-induced status epilepticus

Profound astrogliosis coincident with neuronal cell loss is universally described in human and animal models of temporal lobe epilepsy (TLE). In the kainic acid-induced status epilepticus (SE) model of TLE, astrocytes in the hippocampus become reactive soon after SE and before the onset of spontaneous seizures. To determine if astrocytes in the hippocampus exhibit changes in function soon after SE, we recorded from SR101-labeled astrocytes using the whole-cell patch technique in hippocampal brain slices prepared from control and kainic-acid-treated rats. Glutamate transporter-dependent currents were found to have significantly faster decay time kinetics and in addition, dye coupling between astrocytes was substantially increased. Consistent with an increase in dye coupling in reactive astrocytes, immunoblot experiments demonstrated a significant increase in both glial fibrillary acidic protein (GFAP) and connexin 43, a major gap junction protein expressed by astrocytes. In contrast to what has been observed in resected tissue from patients with refractory epilepsy, changes in potassium currents were not observed shortly after KA-induced SE. While many changes in neuronal function have been identified during the initial period of low seizure probability in this model of TLE, the present study contributes to the growing body of literature suggesting a role for astrocytes in the process of epileptogenesis.

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

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

Lack of P2X receptor mediated currents in astrocytes and GluR type glial cells of the hippocampal CA1 region

Purinergic signalling plays a major role in intercellular communication between neurons and glial cells. Glial cells express metabotropic receptors for ATP and adenosine, the latter being activated after ATP cleavage through extracellular ecto-ATPase activity. Ionotropic receptors for extracellular ATP, so called P2X receptors, might contribute to neuron-glia signalling. However, experimental evidence for the presence of these receptors in glial cells is less convincing so far. In a previous study, immunohistochemistry was used to identify P2X(1-4,6,7) receptor protein in S100beta-positive hippocampal glial cells. Applying patch clamp and fast application techniques, here we challenged the question of the functional expression of these receptors. Time correlated membrane currents served as test criterion for receptor function, since P2X receptor activation leads to the opening of unspecific cation channels in a millisecond time scale. Agonists were applied via short pressure puffs, with a fast concentration clamp method and through UV flash triggered photolysis of caged ATP. Two types of murine hippocampal macroglial cells, both labelled by the expression of green fluorescence protein driven by the human glial fibrillary acid protein promoter, were analysed in acute brain slices and in freshly dissociated cell suspensions. Surprisingly, ATP or related agonists completely failed to activate currents. Additionally, changes in spontaneously occurring glial postsynaptic currents were never observed. These results have been verified using rat and human hippocampal tissue as well as investigating cells from P2X7 knock out mice. It is concluded that in acute preparations, astroglial cells of the hippocampal CA1 subfield do not express functional P2X receptors.

Developmental profile and properties of sulforhodamine 101--Labeled glial cells in acute brain slices of rat hippocampus

The reliable identification of astrocytes for physiological measurements was always time-consuming and difficult. Recently, the fluorescent dye sulforhodamine 101 (SR101) was reported to label cortical glial cells in vivo [Nimmerjahn A, Kirchhoff F, Kerr JN, Helmchen F. Sulforhodamine 101 as a specific marker of astroglia in the neocortex in vivo. Nat Methods 2004;1:31-7]. We adapted this technique for use in acute rat hippocampal slices at early postnatal stages (P3, 7, 15) and in young adults (P24-27) and describe a procedure for double-labeling of SR101 and ion-selective dyes. Using whole-cell patch-clamp, imaging, and immunohistochemistry, we characterized the properties of SR101-positive versus SR101-negative cells in the stratum radiatum. Our data show that SR101, in contrast to Fura-2 or SBFI, only stains a subset of glial cells. Throughout development, SR101-positive and SR101-negative cells differ in their basic membrane properties. Furthermore, SR101-positive cells undergo a developmental switch from variably rectifying to passive between P3 and P15 and lack voltage-gated Na+ currents. At P15, the majority of SR101-positive cells is positive for GFAP. Thus, our data demonstrate that SR101 selectively labels a subpopulation of glial cells in early juvenile hippocampi that shows the typical developmental changes and characteristics of classical astrocytes. Owing to its reliability and uncomplicated handling, we expect that this technique will be helpful in future investigations studying astrocytes in the developing brain.

Sodium channel currents in rat hippocampal NG2 glia: characterization and contribution to resting membrane potential

We have recently reported that most of NG2 glycoprotein expressing glial cells, or NG2 glia, in rat hippocampus persistently express sodium channel currents (I(Na)) during development, but little is known about its function. We report here that hippocampal NG2 glia recorded in either acute slices or freshly isolated preparations from postnatal days (P) 7-21 rats express low density I(Na) (9.5-15.7 pA/pF) that is characterized by a fast activation and rapid inactivation kinetics with a tetrodotoxin (TTX) IC(50) value of 39.3 nM. The I(Na) expression correlated with a approximately 25 mV more depolarized resting membrane potential (RMP) as compared with non-I(Na)-expressing GLAST(+) astrocytes in situ at the same age. In the presence of the sodium channel blocker TTX (0.1 microM), these depolarized RMPs were negatively shifted by an average of 19 mV and 16 mV for I(Na)-expressing glia recordings from in situ and freshly isolated preparations, respectively. The I(Na) expressing glia actually showed a positive RMP (+12 mV) in the absence of potassium conductance that was inhibited to 0 mV by 0.1 microM TTX. Analysis of the I(Na) activation/inactivation curves yields an I(Na) "window current" at -40+/-20 mV, implying a persistent I(Na) component being active around the NG2 glia RMP of approximately -45 mV. According to the constant-field equation analysis, this active I(Na) component leads to a pNa/pK ratio of 0.14 at RMP which is approximately threefold higher than astrocytes (0.05). These results indicate that a TTX sensitive I(Na) component in NG2 glia contributes significantly to the depolarized NG2 glia RMP in the developing brain.

Expression and functional characterization of transient receptor potential vanilloid-related channel 4 (TRPV4) in rat cortical astrocytes

Cell-cell communication in astroglial syncytia is mediated by intracellular Ca(2+) ([Ca(2+)](i)) responses elicited by extracellular signaling molecules as well as by diverse physical and chemical stimuli. Despite the evidence that astrocytic swelling promotes [Ca(2+)](i) elevation through Ca(2+) influx, the molecular identity of the channel protein underlying this response is still elusive. Here we report that primary cultured cortical astrocytes express the transient receptor potential vanilloid-related channel 4 (TRPV 4), a Ca(2+)-permeable cation channel gated by a variety of stimuli, including cell swelling. Immunoblot and confocal microscopy analyses confirmed the presence of the channel protein and its localization in the plasma membrane. TRPV4 was functional because the selective TRPV4 agonist 4-alpha-phorbol 12,13-didecanoate (4alphaPDD) activated an outwardly rectifying cation current with biophysical and pharmacological properties that overlapped those of recombinant human TRPV4 expressed in COS cells. Moreover, 4alphaPDD and hypotonic challenge promoted [Ca(2+)](i) elevation mediated by influx of extracellular Ca(2+). This effect was abolished by low micromolar concentration of the TRPV4 inhibitor Ruthenium Red. Immunofluorescence and immunogold electron microscopy of rat brain revealed that TRPV4 was enriched in astrocytic processes of the superficial layers of the neocortex and in astrocyte end feet facing pia and blood vessels. Collectively, these data indicate that cultured cortical astroglia express functional TRPV4 channels. They also demonstrate that TRPV4 is particularly abundant in astrocytic membranes at the interface between brain and extracerebral liquid spaces. Consistent with its roles in other tissues, these results support the view that TRPV4 might participate in astroglial osmosensation and thus play a key role in brain volume homeostasis.

Local origin and activity-dependent generation of nestin-expressing protoplasmic astrocytes in CA1

Since reports that precursor cells in the adult subventricular zone (SVZ) contribute to regenerative neuro- and gliogenesis in CA1, we wondered whether a similar route of migration might also exist under physiological conditions. Permanent labeling of SVZ precursor cells with a lentiviral vector for green fluorescent protein did not reveal any migration from the SVZ into CA1 in the intact murine brain. However, in a nestin-GFP reporter mouse we found proliferating cells within the corpus callosum/alveus region expressing nestin and glial fibrillary acidic protein similar to precursor cells in the neighboring neurogenic region of the adult dentate gyrus. Within 3 weeks of BrdU administration, BrdU-positive nestin-GFP-expressing protoplasmic astrocytes emerged in CA1. Similar to precursor cells isolated from the dentate gyrus and the SVZ, nestin-GFP-expressing cells from corpus callosum/alveus were self-renewing and multipotent in vitro, whereas cells isolated from CA1 were not. Nestin-GFP-expressing cells in CA1 differentiated into postmitotic astrocytes characterized by S100beta expression. No new neurons were found in CA1. The number of nestin-GFP-expressing astrocytes in CA1 was increased by environmental enrichment. We conclude that astrogenesis in CA1 is influenced by environmental conditions. However, SVZ precursor cells do not contribute to physiological cellular plasticity in CA1.

Polydendrocytes: NG2 cells with many roles in development and repair of the CNS

NG2 cells, or polydendrocytes, are defined as glial cells that express the NG2 proteoglycan and represent a fourth major glial cell population in the mammalian central nervous system. They are morphologically, antigenically, and functionally distinct from mature astrocytes, oligodendrocytes, and microglia. Although they are most often equated with oligodendrocyte progenitor cells, they exhibit some properties that are not commonly associated with those of progenitor cells that generate myelinating cells. This review discusses recent observations and unanswered issues related to their lineage and their role in remyelination, neural signaling, and axonal growth.

Guanosine promotes the up-regulation of inward rectifier potassium current mediated by Kir4.1 in cultured rat cortical astrocytes

Guanosine (Guo) is an endogenous neuroprotective molecule of the CNS, which has various acute and long-term effects on both neurones and astroglial cells. Whether Guo also modulates the activity/expression of ion channels involved in homeostatic control of extracellular potassium by the astrocytic syncytium is still unknown. Here we provide electrophysiological evidence that chronic exposure (48 h) to Guo (500 microm) promotes the functional expression of an inward rectifier K+ (Kir) conductance in primary cultured rat cortical astrocytes. Molecular screening indicated that Guo promotes the up-regulation of the Kir4.1 channel, the major component of the Kir current in astroglia in vivo. Furthermore, the properties of astrocytic Kir current overlapped those of the recombinant Kir4.1 channel expressed in a heterologous system, strongly suggesting that the Guo-induced Kir conductance is mainly gated by Kir4.1. In contrast, the expression levels of two other Kir channel proteins were either unchanged (Kir2.1) or decreased (Kir5.1). Finally, we showed that inhibition of translational process, but not depression of transcription, prevents the Guo-induced up-regulation of Kir4.1, indicating that this nucleoside acts through de novo protein synthesis. Because accumulating data indicate that down-regulation of astroglial Kir current contributes to the pathogenesis of neurodegenerative diseases associated with dysregulation of extracellular K+ homeostasis, these results support the notion that Guo might be a molecule of therapeutic interest for counteracting the detrimental effect of K+-buffering impairment of the astroglial syncytium that occurs in pathological conditions.

Direct evidence for mutual interactions between perineuronal astrocytes and interneurons in the CA1 region of the rat hippocampus

Recent studies have demonstrated that astrocytes express a variety of ion channels and neurotransmitter receptors and can modulate the activity of neurons. Since a single astrocyte makes tight contacts with many neighboring neuronal cells, they can provide efficient and wide modulation of neuronal networks. Here, we provide direct evidence for mutual interactions between perineuronal astrocytes and interneurons in the stratum radiatum of the rat hippocampus. Direct depolarization of a perineuronal astrocyte suppressed the excitatory postsynaptic currents in an adjacent interneuron and increased the paired-pulse ratio, indicating that perineuronal astrocytes have a suppressive effect on presynaptic elements. Moreover, perineuronal astrocyte activation modulated the directly induced firing pattern of the interneuron, with initial facilitation and subsequent suppression. Conversely, direct firing of the interneuron depolarized the membrane potential and reduced the input resistance of the perineuronal astrocyte. These results directly demonstrate the existence of bidirectional interactions between neurons and perineuronal astrocytes.

Distinct electrophysiological alterations in dentate gyrus versus CA1 glial cells from epileptic humans with temporal lobe sclerosis

Previous studies have characterized the electrophysiological properties of astrocytes in the CA1 region of hippocampi resected from patients with intractable temporal lobe epilepsy (TLE). However, the properties of hilar astrocytes from such patients have not been studied although astrocytes display regional heterogeneity and a non-uniform response to injury. Thus, we performed patch-clamp recordings of putative astrocytes in hilar and CA1 regions of surgically removed epileptic hippocampi with and without sclerosis (mesial TLE, MTLE patients, and paradoxical TLE, PTLE patients, respectively), and non-epileptic, non-sclerotic hippocampi (tumor patients). Our data show that the current profile of hilar astrocytes undergoes significant changes in MTLE but not in PTLE or tumor hippocampi. In particular, inwardly rectifying K(+) (K(IR)) and outwardly rectifying K(+) currents were reduced, inward Na(+) currents and membrane resistances were increased in putative astrocytes from MLTE cases compared to PTLE and tumor cases. Because the conductance of K(IR) channels in cell-attached patches (approximately 34pS) from MTLE tissue was not altered, a reduction in the number of K(IR) channels likely accounts for the decrease in whole-cell K(IR) conductance. Presumed astrocytes in the CA1 region from each patient group displayed intercellular coupling and a passive current profile; these characteristics were never observed in hilar glial cells. No apparent changes in the current profile of coupled CA1 glial cells could be detected between MTLE, PTLE and tumor tissues. Additionally, CA1 glial cells expressed a high density of 34pS K(IR) channels. These data suggest that K(+) buffering via K(IR) channels may be functionally compromised in hilar astrocytes of epileptic and sclerotic (MTLE) human hippocampi. By contrast, CA1 astrocytes retained their intercellular coupling and K(IR) channel expression necessary for K(+) buffering.

Functional network integration of embryonic stem cell-derived astrocytes in hippocampal slice cultures

Embryonic stem (ES) cells provide attractive prospects for neural transplantation. So far, grafting strategies in the CNS have focused mainly on neuronal replacement. Employing a slice culture model, we found that ES cell-derived glial precursors (ESGPs) possess a remarkable capacity to integrate into the host glial network. Following deposition on the surface of hippocampal slices, ESGPs actively migrate into the recipient tissue and establish extensive cell-cell contacts with recipient glia. Gap junction-mediated coupling between donor and host astrocytes permits widespread delivery of dye from single donor cells. During maturation,engrafted donor cells display morphological, immunochemical and electrophysiological properties that are characteristic of differentiating native glia. Our findings provide the first evidence of functional integration of grafted astrocytes, and depict glial network integration as a potential route for widespread transcellular delivery of small molecules to the CNS.

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.

Astrocytes from mouse brain slices express ClC-2-mediated Cl- currents regulated during development and after injury

Chloride channels are important for astrocytic volume regulation and K+ buffering. We demonstrate functional expression of a hyperpolarization-activated Cl- current in a subpopulation of astrocytes in acute slices or after fresh isolation from adult brain of GFAP/EGFP transgenic animals in which astrocytes are selectively labeled. When Na+ and K+ were substituted with NMDG+ and Cs+ in extra- and intracellular solutions, an inward current was observed at negative membrane potentials. The current displayed features as described for a Cl- current characterized in cultured astrocytes: it activated time dependently at potentials negative to -40 mV, displayed no inactivation within 1 s, and was inhibited reversibly by submicromolar concentrations of Cd2+. The current was not detectable in astrocytes from ClC-2 knockout mice, indicating that the ClC-2 chloride channel generated the conductance. Current density was significantly lower in a corresponding population of astrocytes isolated from immature brain and in reactive astrocytes within a lesion site.

Connexin 47 (Cx47)-deficient mice with enhanced green fluorescent protein reporter gene reveal predominant oligodendrocytic expression of Cx47 and display vacuolized myelin in the CNS

To further characterize the recently described gap junction gene connexin 47 (Cx47), we generated Cx47-null mice by replacing the Cx47 coding DNA with an enhanced green fluorescent protein (EGFP) reporter gene, which was thus placed under control of the endogenous Cx47 promoter. Homozygous mutant mice were fertile and showed no obvious morphological or behavioral abnormalities. Colocalization of EGFP fluorescence and immunofluorescence of cell marker proteins revealed that Cx47 was mainly expressed in oligodendrocytes in highly myelinated CNS tissues and in few calcium-binding protein S100beta subunit-positive cells but not in neurons or peripheral sciatic nerve. This corrects our previous conclusion that Cx47 mRNA is expressed in brain and spinal cord neurons (Teubner et al., 2001). Cx47 protein was detected by Western blot analysis after immunoprecipitation in CNS tissues of wild-type mice but not in heart or Cx47-deficient tissues. Electron microscopic analysis of CNS white matter in Cx47-deficient mice revealed a conspicuous vacuolation of nerve fibers, particularly at the site of the optic nerve where axons are first contacted by oligodendrocytes and myelination starts. Initial analyses of Cx32/Cx47-double-deficient mice showed that these mice developed an action tremor and died on average at 51 d after birth. The central white matter of these double-deficient mice exhibited much more abundant vacuolation in nerve fibers than mice deficient only in Cx47.

Subpopulation of nestin-expressing progenitor cells in the adult murine hippocampus shows electrophysiological and morphological characteristics of astrocytes

Based on the expression of glial fibrillary acidic protein (GFAP), a recent hypothesis considered stem or progenitor cells in the adult hippocampus to be a type of astrocyte. In a complementary approach, we used transgenic mice expressing green fluorescent protein (GFP) under the promoter for nestin, an intermediate filament present in progenitor cells, to demonstrate astrocytic features in nestin-GFP-positive cells. Morphologically, two subpopulations of nestin-GFP-positive cells were distinguishable; one had an elaborate tree of processes in the granule cell layer and expression of GFAP (but not of S100beta, another astrocytic marker). Electron microscopy revealed vascular end feet of nestin-positive cells, further supporting astrocytic differentiation. Electrophysiological examination of nestin-GFP-positive cells on acutely isolated hippocampal slices showed passive current characteristics of astrocytes in one subset of cells. Among the nestin-GFP-expressing cells with lacking astrocytic features, two cell types could be identified electrophysiologically: cells with delayed-rectifying potassium currents and a very small number of cells with sodium currents, potentially representing signs of the earliest steps of neuronal differentiation.

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.

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.

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.

Freshly isolated astrocytes from rat hippocampus show two distinct current patterns and different [K(+)](o) uptake capabilities

Whether astrocytes predominantly express ohmic K(+) channels in vivo, and how expression of different K(+) channels affects [K(+)](o) homeostasis in the CNS have been long-standing questions for how astrocytes function. In the present study, we have addressed some of these questions in glial fibrillary acidic protein [GFAP(+)], freshly isolated astrocytes (FIAs) from CA1 and CA3 regions of P7-15 rat hippocampus. As isolated, these astrocytes were uncoupled allowing a higher resolution of electrophysiological study. FIAs showed two distinct ion current profiles, with neither showing a purely linear I-V relationship. One population of astrocytes had a combined expression of outward potassium currents (I(Ka), I(Kd)) and inward sodium currents (I(Na)). We term these outwardly rectifying astrocytes (ORA). Another population of astrocytes is characterized by a relatively symmetric potassium current pattern, comprising outward I(Kdr), I(Ka), and abundant inward potassium currents (I(Kin)), and a larger membrane capacitance (C(m)) and more negative resting membrane potential (RMP) than ORAs. We term these variably rectifying astrocytes (VRA). The I(Kin) in 70% of the VRAs was essentially insensitive to Cs(+), while I(Kin) in the remaining 30% of VRAs was sensitive. The I(Ka) of VRAs was most sensitive to 4-aminopyridine (4-AP), while I(Kdr) of ORAs was more sensitive to tetraethylammonium (TEA). ORAs and VRAs occurred approximately equally in FIAs isolated from the CA1 region (52% ORAs versus 48% VRAs), but ORAs were enriched in FIAs isolated from the CA3 region (71% ORAs versus 29% VRAs), suggesting an anatomical segregation of these two types of astrocytes within the hippocampus. VRAs, but not ORAs, showed robust inward currents in response to an increase in extracellular K(+) from 5 to 10 mM. As VRAs showed a similar current pattern and other passive membrane properties (e.g., RMP, R(in)) to "passive astrocytes"in situ (i.e., these showing linear I-V curves), such passive astrocytes possibly represent VRAs influenced by extensive gap-junction coupling in situ. Thus, our data suggest that, at least in CA1 and CA3 regions from P7-15 rats, there are two classes of GFAP(+) astrocytes which possess different K(+) currents. Only VRAs seem suited to uptake of extracellular K(+) via I(Kin) channels at physiological membrane potentials and increases of [K(+)](o). ORAs show abundant outward potassium currents with more depolarized RMP. Thus VRAs and ORAs may cooperate in vivo for uptake and release of K(+), respectively.

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.

Excitable properties in astrocytes derived from human embryonic CNS stem cells

Although it is widely believed that astrocytes lack excitability in adult tissue, primitive action potential-like responses have been elicited from holding potentials negative to -80 mV, in cultured and injury-induced gliotic rodent astrocytes and in human glia under pathological conditions such as glioblastomas and temporal lobe epilepsy. The present study was designed to investigate the properties of astrocytes (identified by immunoreactivity for glial fibrillary acidic protein) derived from multipotent human embryonic CNS stem cells and cultured for 12-25 days in differentiating conditions. We describe here for the first time that brief (1 ms) current pulses elicit spikes from a resting potential (VREST) of approximately -37 mV and, more interestingly, that spontaneous firing can be occasionally recorded in human astrocytes. A voltage-clamp study revealed that in these cells: (i) the half-inactivation of the tetrodotoxin (TTX)-sensitive Na+ channels is around VREST; (ii) the delayed rectifier K+ current is very small; (iii) the ever-present transient outward A-type K+ channels are paradoxically capable of inhibiting the action potentials elicited from a negative membrane potential (-55 to -60 mV); and (iv) inwardly rectifying currents are not present. The responses predicted from a simulation model are in agreement with the experiments. As suggested by recent studies, the decrease of Na+ channel expression and the changes of the electrophysiological properties during the postnatal maturation of the CNS seem to exclude the possibility that astrocytes may play an excitable role in adult tissue. Our data show that excitability and firing should be considered an intrinsic attribute of human astrocytes during CNS development. This is likely to have physiological importance because the role of astrocytes during development is different from the [K+]o-buffering role played in adult CNS, namely the glutamate release and/or the guiding of migrating neurons.

Controversy surrounding the existence of discrete functional classes of astrocytes in adult gray matter

Since 1992, it has been possible to record ionic currents from identified astrocytes in situ, using brain slice technology. Brain slice recordings confirm previous in vitro findings that expression of voltage-gated K(+) and Na(+) channels are a feature of this cell type. In contrast to cultured astrocytes, most investigators found that astrocytes in situ did not contain detectable, or at very best only low, levels of glial fibrillary acidic protein (GFAP). Structural and immunocytochemical investigations determined that these cells are different from oligodendrocyte precursors. In addition to cells with this current pattern, many but not all investigators found a second pool of astrocytes with no voltage-gated ion channels and high GFAP content. These two subpopulations of cells were termed complex and passive astrocytes. The existence of passive astrocytes has been questioned because of possible problems with space clamp conditions and spillage of EGTA-buffered pipette solution around the cells before recordings. Another problem is the fact there is a discrepancy regarding the GFAP content of complex astrocytes. It is of interest that recent immunocytochemical studies suggest the existence of two pools of astrocytes, one with a high GFAP content and one with nondetectable GFAP. Given this, it is tempting to correlate the two (controversial) electrophysiological patterns with immunochemical differences (GFAP) in order to demonstrate two functionally discrete classes of astrocytes in adult gray matter. However, despite evidence that some of the K(+) channels may be involved in proliferation, the role of voltage-gated ion channels in this nonexcitable cell type remains unknown. This is despite the fact that astrocytic Na(+) channels show dramatic changes after pathological events, re-enforcing the notion that the expression of this channel is under tight neuronal control. Several studies suggest that there is a great degree of flexibility and that astrocytes can undergo rapid changes in expression of both membrane ion currents and GFAP. Although it is likely that astrocytes exhibit different structural and membrane properties, this heterogeneity might be a reflection of the flexible plasticity of one astrocyte type under influence of environmental factors rather than of the existence of two distinct and permanent subtypes.

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.

Neuronal-glial interactions and behaviour

Both neurons and glia interact dynamically to enable information processing and behaviour. They have had increasingly intimate, numerous and differentiated associations during brain evolution. Radial glia form a scaffold for neuronal developmental migration and astrocytes enable later synapse elimination. Functionally syncytial glial cells are depolarised by elevated potassium to generate slow potential shifts that are quantitatively related to arousal, levels of motivation and accompany learning. Potassium stimulates astrocytic glycogenolysis and neuronal oxidative metabolism, the former of which is necessary for passive avoidance learning in chicks. Neurons oxidatively metabolise lactate/pyruvate derived from astrocytic glycolysis as their major energy source, stimulated by elevated glutamate. In astrocytes, noradrenaline activates both glycogenolysis and oxidative metabolism. Neuronal glutamate depends crucially on the supply of astrocytically derived glutamine. Released glutamate depolarises astrocytes and their handling of potassium and induces waves of elevated intracellular calcium. Serotonin causes astrocytic hyperpolarisation. Astrocytes alter their physical relationships with neurons to regulate neuronal communication in the hypothalamus during lactation, parturition and dehydration and in response to steroid hormones. There is also structural plasticity of astrocytes during learning in cortex and cerebellum.

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.

Ion channel expression by astrocytes in situ: comparison of different CNS regions

Patch-clamp recordings were obtained in brain slices from 283 rat astrocytes. The expression of voltage-activated whole-cell currents was compared in four different CNS regions (hippocampus, cerebral cortex, spinal cord, and cerebellum). Our data show that CNS astrocytes do not show significant regional differences in their ion channel complement. With the exception of cerebellar Bergmann glial cells, essentially all astrocytes express a combination of delayed rectifying outward K(+) currents, transient A-type K(+) currents, and small Na(+) currents. Developmentally, an increasing percentage of astrocytes and Bergmann glial cells express inwardly rectifying K(+) currents. We did not observe cells that were passive, i.e., lacking voltage-activated currents. A few cells that appeared "passive" in initial recordings showed voltage-activated K(+) currents after off-line leak subtraction. The heterogeneity observed in the ion channel complement was found to be identical when cell-to-cell variations observed within a given CNS region and between various CNS regions were compared, suggesting a common and fairly stereotypical complement of ion channels in CNS astrocytes. Ion channel expression in Bergmann glial cells differed from that of all other CNS regions studied. These cells typically showed very low input resistances attributable to a significant time- and voltage-independent resting K(+) conductance. However, as with electrophysiologically "passive"-appearing astrocytes, Bergmann glial cells showed expression of delayed rectifying K(+) currents after off-line leak subtraction. Inwardly rectifying K(+) currents were observed in Bergmann glial cells after postnatal day 17. Collectively, our data suggest that all astrocytes contain voltage-gated ion channels that display a common pattern of expression during development.

Differential inhibition of glial K(+) currents by 4-AP

Spinal cord astrocytes express four biophysically and pharmacologically distinct voltage-activated potassium (K(+)) channel types. The K(+) channel blocker 4-aminopyridine (4-AP) exhibited differential and concentration-dependent block of all of these currents. Specifically, 100 microM 4-AP selectively inhibited a slowly inactivating outward current (K(SI)) that was insensitive to dendrototoxin (< or = 10 microM) and that activated at -50 mV. At 2 mM, 4-AP inhibited fast-inactivating, low-threshold (-70 mV) A-type currents (K(A)) and sustained, TEA-sensitive noninactivating delayed-rectifier-type currents (K(DR)). At an even higher concentration (8 mM), 4-AP additionally blocked inwardly rectifying, Cs(+)- and Ba(2+)-sensitive K(+) currents (K(IR)). Current injection into current-clamped astrocytes in culture or in acute spinal cord slices induced an overshooting voltage response reminiscent of slow neuronal action potentials. Increasing concentrations of 4-AP selectively modulated different phases in the repolarization of these glial spikes, suggesting that all four K(+) currents serve different roles in stabilization and repolarization of the astrocytic membrane potential. Our data suggest that 4-AP is an useful, dose-dependent inhibitor of all four astrocytic K(+) channels. We show that the slowly inactivating astrocytic K(+) currents, which had not been described as separate current entities in astrocytes, contribute to the resting K(+) conductance and may thus be involved in K(+) homeostatic functions of astrocytes. The high sensitivity of these currents to micromolar 4-AP suggests that application of 4-AP to inhibit neuronal A-currents or to induce epileptiform discharges in brain slices also may influence astrocytic K(+) buffering.

Immunocytochemical evidence for a distinct GFAP-negative subpopulation of astrocytes in the adult rat hippocampus

In order to establish the relative distribution of a GFAP-negative population of astrocytes, and its change in gliotic tissue, sections of the stratum radiatum of the CA1 hippocampal layer of male, adult, Wistar rats were analyzed by immunocytochemical methods. Ten micrometer-thick sections were triple-stained to detect nuclei, glial fibrillary acidic protein (GFAP) and glutamine synthetase (GS). In another set of experiments, the rats received a one-time intraperitoneal injection of kainic acid that caused epileptic seizures. With the use of a behavioral protocol, animals with substantial neuronal loss in the pyramidal layer were selected. Five days after the injection these rats were analyzed similarly to control rats. We find that GFAP-positive cells are a subpopulation of GS-positive cells and that the GFAP-negative subpopulation is quite large (40%). After gliosis the density of GFAP-negative, GS-positive cells stays stable, whereas the GFAP-positive population triples. These experiments confirm electrophysiological experiments showing a distinct, GFAP-negative subset of astrocytes that remains consistent even after injury-induced gliosis and accompanying up-regulation of GFAP.

Receptor-activated Ca2+ increases in vibrodissociated cortical astrocytes: a nonenzymatic method for acute isolation of astrocytes

A new nonenzymatic method for the acute isolation of astrocytes from rat cerebral cortex is described. A vibratory device was used to dissociate the cells from thin brain slices, and the method yielded fresh and relatively well-preserved astrocytes without previous enzyme incubation. These cells were examined in a microspectrofluorometric system for measurement of changes in intracellular free calcium concentrations ([Ca2+]i), and their expression of various neurotransmitter receptors was determined. Acutely isolated glial fibrillary acidic protein (GFAP)-positive astrocytes (p7-p18) were seen to respond to the metabotropic glutamate receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD, 10(-4) M) with increases in [Ca2+]i, and this response was blocked by (RS)-1-aminoindan-1,5 dicarboxylic acid (AIDA, 10(-3) M), an antagonist to group 1 metabotropic glutamate receptors. The delta-opioid receptor agonist D-Pen2, D-Pen5-enkephalin (DPDPE, 10(-6) M) evoked [Ca2+]i increases that were blocked by the delta-opioid antagonist ICI 174.388 (10(-5) M). The astrocytes failed to respond to 5-hydroxytryptamine (5-HT, 10(-5) M), although the same cells subsequently were found to respond to other agonists. Furthermore, [Ca2+]i responses evoked by phenylephrine (10(-5) M) were blocked by prazosin (0.2x10(-6) M), suggesting the expression of alpha1-adrenergic receptors on the acutely isolated astrocytes. The cells were also shown to react with [Ca2+]i increases in response to depolarization with high extracellular potassium concentrations (50x10(-3) M). The signals induced by depolarization were not seen in Ca2+-free buffer, indicating the presence of voltage-activated calcium channels in these cells. Thus, the present study confirms some of the results earlier obtained in cell cultures, suggesting that cortical astrocytes in vivo express glutamate, opiate, and adrenergic receptors, coupled to increases in [Ca2+]i, whereas no receptors for 5-HT could be detected.