Glutamate-dependent astrocyte modulation of synaptic transmission between cultured hippocampal neurons

Optical characterization of glutamate dehydrogenase monolayers chemisorbed on SiO2

This paper describes the formation of glutamate dehydrogenase monolayers on silicon dioxide, and their characterization by means of physical techniques, i.e., fluorescence spectroscopy and Fourier-transform infrared spectroscopy. Detailed investigations of the intrinsic stability of native proteins in solution were carried out to elucidate the occurrence of conformational changes induced by the immobilization procedure. The enzyme monolayers were deposited on SiO2 after preexposing silicon surfaces to 3-aminopropyltriethoxysilane and reacting the silylated surfaces with glutaric dialdehyde. The optical characterization demonstrates that the immobilization does not interfere with the fold pattern of the native enzyme. In addition, fluorescence spectroscopy, thermal denaturation, and quenching studies performed on the enzyme in solution well describe the folding and unfolding properties of glutamate dehydrogenase. The photophysical studies reported here are relevant for nanobioelectronics applications requiring protein immobilization on a chip.

Glial cell inhibition of neurons by release of ATP

ATP is released by neurons and functions as a neurotransmitter and modulator in the CNS. Here I show that ATP released from glial cells can also serve as a potent neuromodulator, inhibiting neurons in the retina of the rat. Activation of glial cells by focal ejection of ATP, ATPgammaS, dopamine, thrombin, or lysophosphatidic acid or by mechanical stimulation evoked hyperpolarizing responses and outward currents in a subset of retinal ganglion cells by increasing a Ba(2+)-sensitive K(+) conductance in the neurons. This glia-evoked inhibition reduced the firing rate of those neurons that displayed spontaneous spike activity. The inhibition was abolished by the A(1) adenosine receptor antagonist DPCPX (8-cyclopentyl-1,3-dipropylxanthine) (10 nm) and was reduced by the ecto-ATPase inhibitor ARL-67156 (6-N,N-diethyl-D-beta,gamma-dibromomethyleneATP) (50 microm) and by the ectonucleotidase inhibitor AOPCP [adenosine-5'-O-(alpha,beta-methylene)-diphosphonate] (250 microm). Selective activation of retinal glial cells demonstrated that Müller cells, but not astrocytes, mediate the inhibition. ATP release from Müller cells into the inner plexiform layer of the retina was shown using the luciferin-luciferase chemiluminescence assay. These findings demonstrate that activated glial cells can inhibit neurons in the retina by the release of ATP, which is converted to adenosine by ectoenzymes and subsequently activates neuronal adenosine receptors. The results lend support to the hypothesis that glial cells play an active role in information processing in the CNS.

P2X7 receptor-mediated release of excitatory amino acids from astrocytes

Astrocyte glutamate release can modulate synaptic activity and participate in brain intercellular signaling. P2X7 receptors form large ion channels when activated by ATP or other ligands. Here we show that P2X7 receptors provide a route for excitatory amino acid release from astrocytes. Studies were performed using murine cortical astrocyte cultures. ATP produced an inward current in patch-clamped astrocytes with properties characteristic of P2X7 receptor activation: the current was amplified in low divalent cation medium, blocked by pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), and more potently activated by 3'-O-(4-benzoyl)benzoyl ATP (BzATP) than by ATP itself. Measurement of current reversal potentials showed the relative BzATP-induced permeabilities to different substrates to be Na+, 1 > Cl-, 0.34 > N-methyl-D-glucamine, 0.27 > L-glutamate, 0.15 approximately D-aspartate, 0.16. Astrocytes exposed to BzATP also became permeable to Lucifer yellow, indicating a large channel opening. Release of L-glutamate and D-aspartate through P2X7 channels was confirmed using radiolabeled tracers. As with the inward current, release of glutamate and D-aspartate was induced by BzATP more potently than ATP, amplified in Ca2+/Mg2+-free medium, and blocked by PPADS or oxidized ATP. Efflux through P2X7 channels is a previously unrecognized route of ligand-stimulated, nonvesicular astrocyte glutamate release.

Accelerated hippocampal spreading depression and enhanced locomotory activity in mice with astrocyte-directed inactivation of connexin43

Using a human glial fibrillary acidic protein (hGFAP) promoter-driven cre transgene, we have achieved efficient inactivation of a floxed connexin43 (Cx43) gene in astrocytes of adult mice. The loss of Cx43 expression was monitored in a cell-autonomous manner via conditional replacement of the Cx43-coding region by a lacZ reporter gene. In this way, we bypassed the early postnatal lethality previously reported for Cx43 null mice and characterized the phenotypic consequences of Cx43 deficiency in the CNS. Mice lacking Cx43 in astrocytes were viable and showed no evidence of either neurodegeneration or astrogliosis. Spreading depression (SD) is a pathophysiological phenomenon observed in the CNS that is characterized by a propagating wave of depolarization followed by neuronal inactivation. Inhibitors of gap junctional communication have previously been shown to block initiation and propagation of SD. In contrast, we observed an increase in the velocity of hippocampal SD in the stratum radiatum of mice lacking Cx43 in astrocytes. In the same brain subregion, dye-coupling experiments revealed a reduction in overall astrocytic intercellular communication by approximately 50%. This strongly suggests separate and different neuronal and glial contributions of gap junctional intercellular communication to SD. Concomitant with increased velocity of spreading depression, we observed enhanced locomotory activity in mice lacking Cx43 in astrocytes.

Astrocytes and stroke: networking for survival?

Astrocytes are now known to be involved in the most integrated functions of the central nervous system. These functions are not only necessary for the normally working brain but are also critically involved in many pathological conditions, including stroke. Astrocytes may contribute to damage by propagating spreading depression or by sending proapoptotic signals to otherwise healthy tissue via gap junction channels. Astrocytes may also inhibit regeneration by participating in formation of the glial scar. On the other hand, astrocytes are important in neuronal antioxidant defense and secrete growth factors, which probably provide neuroprotection in the acute phase, as well as promoting neurogenesis and regeneration in the chronic phase after injury. A detailed understanding of the astrocytic response, as well as the timing and location of the changes, is necessary to develop effective treatment strategies for stroke patients.

Glial glutamate transporters mediate a functional metabolic crosstalk between neurons and astrocytes in the mouse developing cortex

Neuron-glia interactions are essential for synaptic function, and glial glutamate (re)uptake plays a key role at glutamatergic synapses. In knockout mice, for either glial glutamate transporters, GLAST or GLT-1, a classical metabolic response to synaptic activation (i.e., enhancement of glucose utilization) is decreased at an early functional stage in the somatosensory barrel cortex following activation of whiskers. Investigation in vitro demonstrates that glial glutamate transport represents a critical step for triggering enhanced glucose utilization, but also lactate release from astrocytes through a mechanism involving changes in intracellular Na(+) concentration. These data suggest that a metabolic crosstalk takes place between neurons and astrocytes in the developing cortex, which would be regulated by synaptic activity and mediated by glial glutamate transporters.

Astrocytes selectively enhance N-type calcium current in hippocampal neurons

Astrocytes influence neuronal development, synapse formation, and synaptic transmission, partly through affecting neuronal calcium signals. In order to elucidate the extent to which astrocytes modulate neuronal voltage-gated calcium currents, we performed a whole-cell patch clamp analysis of neurons in astrocyte-deplete and astrocyte-enriched conditions. We demonstrate that hippocampal neurons in an astrocyte-enriched environment show augmentation of voltage-gated calcium current at 1-3 days in vitro. Further study in pairs of adjacent neurons showed that the augmentation in calcium current was dependent on direct contact with the astrocyte. Pharmacological analysis demonstrated the augmentation is selective for the N-type calcium current, although immunochemical labeling of the alpha1(B) subunit of the N-type calcium channel was unchanged. These findings show that astrocytes regulate neuronal voltage-gated calcium currents in a contact-dependent manner. The specificity of the effect for the N-type calcium current at early days in culture has special significance regarding the role of astrocytes in hippocampal synaptogenesis.

Experience effects on brain development: possible contributions to psychopathology

Researchers and clinicians are increasingly recognizing that psychological and psychiatric disorders are often developmentally progressive, and that diagnosis often represents a point along that progression that is defined largely by our abilities to detect symptoms. As a result, strategies that guide our searches for the root causes and etiologies of these disorders are beginning to change. This review describes interactions between genetics and experience that influence the development of psychopathologies. Following a discussion of normal brain development that highlights how specific cellular processes may be targeted by genetic or environmental factors, we focus on four disorders whose origins range from genetic (fragile X syndrome) to environmental (fetal alcohol syndrome) or a mixture of both factors (depression and schizophrenia). C.H. Waddington's canalization model (slightly modified) is used as a tool to conceptualize the interactive influences of genetics and experience in the development of these psychopathologies. Although this model was originally proposed to describe the 'canalizing' role of genetics in promoting normative development, it serves here to help visualize, for example, the effects of adverse (stressful) experience in the kindling model of depression, and the multiple etiologies that may underlie the development of schizophrenia. Waddington's model is also useful in understanding the canalizing influence of experience-based therapeutic approaches, which also likely bring about 'organic' changes in the brain. Finally, in light of increased evidence for the role of experience in the development and treatment of psychopathologies, we suggest that future strategies for identifying the underlying causes of these disorders be based less on the mechanisms of action of effective pharmacological treatments, and more on increased knowledge of the brain's cellular mechanisms of plastic change.

Novel neuronal and astrocytic mechanisms in thalamocortical loop dynamics

In this review, we summarize three sets of findings that have recently been observed in thalamic astrocytes and neurons, and discuss their significance for thalamocortical loop dynamics. (i) A physiologically relevant 'window' component of the low-voltage-activated, T-type Ca(2+) current (I(Twindow)) plays an essential part in the slow (less than 1 Hz) sleep oscillation in adult thalamocortical (TC) neurons, indicating that the expression of this fundamental sleep rhythm in these neurons is not a simple reflection of cortical network activity. It is also likely that I(Twindow) underlies one of the cellular mechanisms enabling TC neurons to produce burst firing in response to novel sensory stimuli. (ii) Both electrophysiological and dye-injection experiments support the existence of gap junction-mediated coupling among young and adult TC neurons. This finding indicates that electrical coupling-mediated synchronization might be implicated in the high and low frequency oscillatory activities expressed by this type of thalamic neuron. (iii) Spontaneous intracellular Ca(2+) ([Ca(2+)](i)) waves propagating among thalamic astrocytes are able to elicit large and long-lasting N-methyl-D-aspartate-mediated currents in TC neurons. The peculiar developmental profile within the first two postnatal weeks of these astrocytic [Ca(2+)](i) transients and the selective activation of these glutamate receptors point to a role for this astrocyte-to-neuron signalling mechanism in the topographic wiring of the thalamocortical loop. As some of these novel cellular and intracellular properties are not restricted to thalamic astrocytes and neurons, their significance may well apply to (patho)physiological functions of glial and neuronal elements in other brain areas.

A brain adaptation view of plasticity: is synaptic plasticity an overly limited concept?

A view that is emerging is that the brain has multiple forms of plasticity that must be governed, at least in part, by independent mechanisms. This view is illustrated by: (1) the apparent separate governance of some non-neural changes by activity, in contrast to synaptic changes driven by learning; (2) the apparent independence of different kinds of synaptic changes that occur in response to the learning aspects of training; (3) the occurrence of separate patterns of synaptic plasticity in the same system in response to different task demands; and (4) apparent dissociations between behaviorally induced synaptogenesis and LTP. The historical focus of research and theory in areas ranging from learning and memory to experiential modulation of brain development has been heavily upon synaptic plasticity since shortly after the discovery of the synapse. Based upon available data, it could be argued that: (1) synaptic, and even neuronal, plasticity is but a small fraction of the range of changes that occur in response to experience; and (2) we are just beginning to understand the importance of these other forms of brain plasticity. Appreciation of this aspect of the brain's adaptive process may allow us to better understand the capacity of the brain to tailor a particular set of changes to the demands of the specific experiences that generated them.

Ontogeny of sexually dimorphic astrocytes in the neonatal rat arcuate

The arcuate nucleus of the hypothalamus is one of several sexually dimorphic hypothalamic nuclei. We have previously demonstrated that astrocytes in the neonatal arcuate nucleus exhibit a marked sexually dimorphic morphology as a result of differential exposure to gonadal steroids by postnatal day (PN) 2, with males having complex stellate cells compared to the simple bipolar ones found in females. Here, we present data demonstrating that arcuate astrocytes are sexually dimorphic by the day of birth and continue as such throughout postnatal development (PN0-PN15), and persist into adulthood. These data suggest that early gonadal steroid exposure permanently organizes arcuate astrocyte morphology. The male versus female difference in astrocyte morphology may contribute to the sexually dimorphic regulation of neuroendrocrine secretions from the pituitary in the adult.

Calcium rise in cultured neurons from medial septum elicits calcium waves in surrounding glial cells

One prerequisite for understanding the physiological relevance of intercellular calcium waves in glia is the examination of mechanisms that trigger these waves. Here, we show that stimulation of cultured septal neurons to produce a large and sustained calcium rise in the soma can initiate calcium waves in surrounding glial cells. The initiation of calcium waves is dependent on calcium influx through voltage-gated calcium channels on the neuron. The waves are not due to direct stimulation of the glial cells or to loss of neuronal membrane integrity. Mechanism of wave initiation is distinct from that involved in wave propagation and does not involve glutamate or acetylcholine release. Communication via gap junctions, or nitric oxide production, is not involved in the initial signaling between a stimulated neuron and the surrounding astrocytes. Suramin, a blocker of P2 receptors blocked the waves but failed to abolish the responses in glial cells immediately surrounding the stimulated neuron. Our results suggest that patterns of calcium rises on neurons, like those seen in glutamate cytotoxicity, can cause calcium waves in surrounding glial cells.

Long-lasting modulation of synaptic input to Purkinje neurons by Bergmann glia stimulation in rat brain slices

Information processing in the nervous system is achieved primarily at chemical synapses between neurons. Recent evidence suggests that glia-neuron interactions contribute in multiple ways to the synaptic process. In the present study we used the frequency of spontaneous postsynaptic currents (sPSC) in Purkinje neurons in acute cerebellar brain slices from juvenile rats (13-19 days old) as a measure of synaptic activity. Following 50 depolarizing pulses to an adjacent Bergmann glial cell (50 mV; duration 0.5 s; 1 Hz) the sPSC frequency of the Purkinje neuron was reduced to 65 +/- 7 % of control values within 10 min after glial stimulation and remained depressed for at least 40 min. Depolarizing pulses to 0 mV had a comparable effect (70 +/- 5 % of control). The frequency of miniature PSCs, as recorded in 300 nM TTX, was not modulated after glial stimulation. Blockade of ionotropic glutamate receptors (iGluRs) with kynurenic acid (1 mM) or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 5 microM) suppressed the reduction of neuronal activity induced by glial depolarization, whereas the glial modulation of synaptic activity was not inhibited by a block of N-methyl-D-aspartate iGluRs, metabotropic glutamate receptors, cannabinoid receptors or GABA(B) receptors. Fluorometric measurements of the intraglial Ca(2+) concentration revealed no glial Ca(2+) transients during the depolarization series, and glial cell stimulation reduced the neuronal sPSC frequency even after loading the glial cell with 20 mM of the Ca(2+) chelator BAPTA. Our results indicate a glia-induced long-lasting depression of neuronal communication mediated by iGluRs.

Neuronal activity regulates correlated network properties of spontaneous calcium transients in astrocytes in situ

Spontaneous neuronal activity is essential to neural development. Until recently, neurons were believed to be the only excitable cells to display spontaneous activity. However, cultured astrocytes and, more recently, astrocytes in situ are now known to exhibit spontaneous Ca2+ transients. Here we used Ca2+ imaging of astrocytes from transgenic mice for the simultaneous monitoring of [Ca2+]i changes in large numbers of astrocytes. We found that spontaneous activity is a common property of most brain astrocytes that is lost in response to a lesion. These spontaneous [Ca2+]i oscillations require extracellular and intracellular Ca2+. Moreover, network analysis revealed that most astrocytes formed correlated networks of dozens of these cells, which were synchronous with both astrocytes and neurons. We found that decreasing spontaneous [Ca2+]i transients in neurons by TTX does not alter the number of active astrocytes, although it impairs their synchronous network activity. Conversely, bicuculline-induced epileptic patterns of [Ca2+]i transients in neurons cause an increase in the number of active astrocytes and in their network synchrony. Furthermore, activation of non-NMDA and NMDA ionotropic glutamate receptors is required to correlate astrocytic networks. These results show that spontaneous activity in astrocytes and neurons is patterned into correlated neuronal/astrocytic networks in which neuronal activity regulates the network properties of astrocytes. This network activity may be essential for neural development and synaptic plasticity.

Vesicular release mechanisms in astrocytic signalling

Astrocytes play an important role in chemical signalling, acting as receptive as well as secretory elements. They can express receptors for essentially all classical neurotransmitter substances and for a large variety of peptides. Recent evidence indicates that astrocytes are involved in the information processing within the nervous system. Astrocytes respond to various neurotransmitters with elevations in intracellular calcium which can either be long-duration Ca(2+) spikes or oscillations in Ca(2+) levels. Astrocytic excitation can be propagated to adjacent astrocytes in the form of Ca(2+) waves. Due to their intimate spatial relationship with synaptic contacts, astrocytes can directly respond to synaptically released messengers and communicate, via signalling substances, with neurons in a reciprocal manner. Cultured astrocytes and astroglioma cells express synaptic vesicle proteins and members of the synaptic SNARE complex. Astrocytes can release a variety of messenger substances via receptor-mediated mechanisms implicating their potential for regulated exocytosis and the participation of proteins of the SNARE complex.

Neuroendocrine plasticity in GnRH release during rat estrous cycle: correlation with molecular markers of synaptic remodeling

Morphological changes in the gonadotropin releasing hormone (GnRH) neurons in the preoptic area (POA) and their terminals in the median eminence-arcuate (ME-ARC) region are reported to occur during ovarian cycle that may be involved in the GnRH release into the portal blood during preovulatory surge. However, the neuronal substrates participating in altered GnRH neuronal plasticity are poorly understood. The present study was designed to determine whether morphological changes occurring in the GnRH neuron cell bodies in the POA and their terminals in the ME-ARC region of hypothalamus with pulsatile GnRH release in cycling female rats are associated with expression of intrinsic determinants of neuronal plasticity. The plasticity markers studied are polysialylated neural cell adhesion molecule (PSA-NCAM), high molecular weight isoforms of NCAM, growth associated protein (GAP-43), glial fibrillary acidic protein (GFAP) and synaptophysin. Regularly cycling female rats were sacrificed at diestrous, i.e., when GnRH release is low, and at proestrous, i.e., when preovulatory GnRH surge occurs, using perfusion fixation method for immunohistochemical staining of GnRH cells. GnRH cell bodies and their terminals from the POA and ME-ARC region respectively, were localized using immunohistochemical technique in proestrous and diestrous phase of estrous cycle and our results showed a marked increase in the GnRH nerve terminals length and immunoreactivity in the ME-ARC region from proestrous phase rats as compared to diestrous rats. Immunoblot analyses of the POA and ME-ARC region of the hypothalamus revealed a significant increase in the content of PSA-NCAM, NCAM-180, NCAM-140, GAP-43 and synaptophysin from proestrous phase rats as compared to diestrous phase rats. The ME-ARC region showed more pronounced increase in the protein expression of these markers of neuronal plasticity as compared to the POA, whereas, hippocampal region did not show any significant change in the content of these markers showing specificity of the changes to the GnRH system. GFAP content was significantly decreased in the POA with a marginal increase in the GFAP level from the ME-ARC region. These results demonstrate the involvement of synaptic proteins in the dynamic plasticity of the ME-ARC region of hypothalamus, allowing GnRH nerve terminals to release the neurohormone into the pituitary portal blood on the day of proestrous.

Micropatterned substrates: approach to probing intercellular communication pathways

Intercellular signaling is critical for the normal development and physiology of the central nervous system (CNS). To study such signaling, it is vital to control where and when the cells make contact with one another. It is also important to determine whether the process used for cell localization has an impact on signaling. This paper describes a technique that controls the location for cell growth in vitro and demonstrates that the technique has minimal (if any) impact on intercellular signaling. By using photolithographic methods, poly(dimethylsiloxane) molds were fabricated to function as templates for micrometer-level patterning of a nonadhesive agar (agarose) onto glass coverslips coated with a cell adhesive film (poly(L-lysine)). This process yields a surface composed of well-defined adhesive and nonadhesive microdomains. When endothelia or astrocytes are plated onto these substrates, confluent domains of endothelia or astrocytes grow on the poly(L-lysine) domains. Cocultures of astrocytes and neurons can also successfully be used to form interwoven networks on the adhesive domains. Moreover, studies of calcium signaling revealed that astrocytes grown on such patterns retain their native physiological activity. This conclusion is based on the observed propagation rate for calcium waves within individual astrocyte domains and across neighboring, but spatially disconnected, astrocyte domains. The potential to apply these micropatterned substrates as platforms for interrogating communication pathways in key components of the CNS is discussed.

Zinc-induced inhibition of protein synthesis and reduction of connexin-43 expression and intercellular communication in mouse cortical astrocytes

Zinc released from a subpopulation of glutamatergic synapses, mainly localized in the cerebral cortex and the hippocampus, facilitates or reduces glutamatergic transmission by acting on neuronal AMPA and NMDA receptors, respectively. However, neurons are not the only targets of zinc. In the present study, we provide evidence that zinc inhibits protein synthesis in cultured astrocytes from the cerebral cortex of embryonic mice. This inhibition, which reached 85% in the presence of 100 micro m zinc, was partially and slowly reversible and resulted from the successive inhibition of the elongation and the initiation steps of the protein translation process. This was assessed by measuring the phosphorylation level of the elongation factor eEF-2 and of the alpha subunit of the initiation factor eIF-2. Due to the rapid turnover of connexin-43 that forms junction channels in cultured astrocytes, the zinc-induced decrease of protein synthesis led to a partial disappearance of connexin-43, which was associated with an inhibition of the cellular coupling in the astrocytic syncitium. In conclusion, zinc not only inhibits protein synthesis in neurons, as previously demonstrated, but also in astrocytes. The resulting decrease in the intercellular communication between astrocytes should alter the function of surrounding neurons as well as their survival.

Kappa-Opioid and NMDA glutamate receptors are differentially targeted within rat medial prefrontal cortex

Activation of kappa-opioid receptors (KOR) in the medial prefrontal cortex (mPFC) modulates excitatory transmission, which may involve interactions with N-methyl-D-aspartate (NMDA) glutamate receptors. We investigated possible anatomical correlates of this modulation by using dual labeling electron microscopy to examine the cellular distributions of antibodies raised against KOR and the R1 subunit of the NMDA receptor (NR1). KOR immunoreactivity primarily was localized to plasma and vesicular membranes of axons and axon terminals that were morphologically heterogeneous. A small proportion of KOR immunoreactivity was associated with cytosolic compartments of dendrites and membranes of glial processes. NR1 labeling was mainly postsynaptic, associated most often with membranes of cytoplasmic organelles in cell bodies and large dendrites and plasmalemmal surfaces of distal dendrites. The remaining NR1-labeled profiles were axonal profiles and glial processes. Of all cellular associations between labeled profiles, the majority were KOR-labeled axons that contacted NR1-immunoreactive dendrites or cell bodies. Occasionally the two antigens were colocalized in axon terminals that formed either asymmetric synapses or displayed varicose morphology. KOR and NR1 also were colocalized within dendrites, and rarely were observed in the same cell bodies. Occasionally glial processes coursing adjacent to axo-spinous appositions expressed both KOR and NR1 immunoreactivity. These results indicate that ligand activation of KOR or NMDA receptors differentially modulates excitatory transmission in the mPFC through pre- and postsynaptic mechanisms, respectively. The data also suggest more minor roles for colocalized KOR and NMDA receptors in shared regulation of presynaptic transmitter release, postsynaptic responsivity, and glial function.

Stimulation of chemokine CXC receptor 4 induces synaptic depression of evoked parallel fibers inputs onto Purkinje neurons in mouse cerebellum

In the present work, we studied the effects of the stimulation of the chemokine CXC receptor 4 (CXCR4) by the stromal-derived cell growth factor-1alpha (SDF-1alpha) on the evoked excitatory postsynaptic current. This was generated in Purkinje neurons (PN) from mouse cerebellar slices by the stimulation of parallel fibers. It was found that the amplitude of EPSC was reversibly reduced by SDF-1alpha application. This effect was dose-dependent (IC(50)=0.34 nM) and was abolished by the anti-CXCR4 monoclonal antibody (mAb) 12G5. This SDF-1alpha-induced synaptic depression was caused by a decrease of evoked glutamate release, rather than a decrease in the postsynaptic glutamate receptor (GluR) sensitivity, as the mean amplitude of the spontaneous EPSCs was not influenced by chemokine application. Moreover, NMDA receptors (NMDARs) are involved in EPSC depression being inhibited by the NMDAR blocker 2-amino-5-phosphonopentanoic acid (AP-5). The mechanisms by which SDF-1alpha modulates neurotransmission in the cerebellar cortex are discussed.

Short-term synaptic plasticity

Synaptic transmission is a dynamic process. Postsynaptic responses wax and wane as presynaptic activity evolves. This prominent characteristic of chemical synaptic transmission is a crucial determinant of the response properties of synapses and, in turn, of the stimulus properties selected by neural networks and of the patterns of activity generated by those networks. This review focuses on synaptic changes that result from prior activity in the synapse under study, and is restricted to short-term effects that last for at most a few minutes. Forms of synaptic enhancement, such as facilitation, augmentation, and post-tetanic potentiation, are usually attributed to effects of a residual elevation in presynaptic [Ca(2+)]i, acting on one or more molecular targets that appear to be distinct from the secretory trigger responsible for fast exocytosis and phasic release of transmitter to single action potentials. We discuss the evidence for this hypothesis, and the origins of the different kinetic phases of synaptic enhancement, as well as the interpretation of statistical changes in transmitter release and roles played by other factors such as alterations in presynaptic Ca(2+) influx or postsynaptic levels of [Ca(2+)]i. Synaptic depression dominates enhancement at many synapses. Depression is usually attributed to depletion of some pool of readily releasable vesicles, and various forms of the depletion model are discussed. Depression can also arise from feedback activation of presynaptic receptors and from postsynaptic processes such as receptor desensitization. In addition, glial-neuronal interactions can contribute to short-term synaptic plasticity. Finally, we summarize the recent literature on putative molecular players in synaptic plasticity and the effects of genetic manipulations and other modulatory influences.

Neural stem cells and the regulation of neurogenesis in the adult hippocampus

Neurogenesis continues in the hippocampal dentate gyrus of adult rodents and primates including humans. Neurons are born in the underlying subgranular layer (SGL) and move into the granule cell layer (GCL) to become mature granule neurons. Recent work indicates that the primary precursors for these new neurons correspond to radial astrocytes whose cell body is in the SGL and their processes traverse the GCL. These astrocytes divide to give rise to intermediate precursors, D cells that likely become mature granule neurons. Here we propose that the anatomy of radial astrocytes may allow for signals within the GCL to regulate neurogenesis in the SGL. Levels of neuronal activity within the granule cell layer may regulate the proliferation rates of radial astrocytes and determine the number of new neurons produced in the dentate gyrus.

The expression of the glutamate re-uptake transporter excitatory amino acid transporter 1 (EAAT1) in the normal human CNS and in motor neurone disease: an immunohistochemical study

A monoclonal antibody to excitatory amino acid transporter 1 (EAAT1) has been generated which robustly stains paraffin-embedded, formaldehyde-fixed as well as snap-frozen human post-mortem brain tissue. We have used this antibody to map the distribution of EAAT1 throughout normal human CNS tissue. In addition this antibody has been used to perform a semi-quantitative immunohistochemical analysis of the expression of EAAT1 in motor cortex and cervical cord tissue taken from motor neurone disease cases (n=17) and neurologically normal controls (n=12). By comparing the relative optical density measurements of identical regions of motor cortex and cervical spinal cord an increase in the expression levels of EAAT1 was observed in motor neurone disease tissue compared to the control tissue and in both motor cortex and cervical spinal cord (9-17% and 13-33% increases respectively). EAAT1 was observed to be the most abundant transporter in more "caudal" brain regions such as the diencephalon and brainstem and its expression in other regions was frequently more uniform than that of EAAT2. In the motor cortex, EAAT1 immunoreactivity was present in all grey matter laminae, with some staining of individual astrocytes in the white matter. In spinal cord, EAAT1 immunoreactivity was strongest in the substantia gelatinosa. In the ventral horn, motor neurones were surrounded with a dense rim of perisomatic EAAT1 immunoreactivity, and the neuropil showed diffuse staining. Additional studies using double-labelling immunocytochemistry demonstrated that astrocytic co-localisation of EAAT1 and EAAT2 may occasionally be seen, but was not widespread in the human CNS and that in general astrocytes were positive for either EAAT1 or EAAT2. These results demonstrate that the EAAT1 has a widespread abundance throughout all regions of the human CNS examined and that there exist discrete populations of astrocytes that are positive solely for either EAAT1 or EAAT2. Furthermore, there is evidence to suggest that altered EAAT1 expression in motor neurone disease follows a different pattern to the reported changes of EAAT2 expression in this condition, indicating that the role of glutamate transporters in the pathogenesis of motor neurone disease appears more complex than previously appreciated.

Synaptically released acetylcholine evokes Ca2+ elevations in astrocytes in hippocampal slices

Recent results have demonstrated the existence of bidirectional communication between glial cells and neurons. We investigated in brain slices whether rat hippocampal astrocytes respond to acetylcholine synaptically released by an extrinsic pathway. We stimulated the stratum oriens/alveus, which contains cholinergic afferents from the septum and diagonal band of Broca, and recorded whole-cell membrane currents and intracellular Ca2+ levels of astrocytes located in the hippocampal stratum oriens. Nerve-fiber stimulation evoked a long-lasting inward current and increased the Ca2+ levels in astrocytes. Both astrocytic responses were abolished by tetrodotoxin or Cd2+ and were increased by 4-aminopyridine, indicating that the responses were attributable to synaptically released neurotransmitter. The inward current was inhibited by glutamate transporter antagonists, indicating that it was attributable to the electrogenic glutamate transporter activity. The synaptically evoked intracellular Ca2+ elevations were not affected by glutamate receptor antagonists but were abolished by atropine, indicating that they were mediated by muscarinic cholinergic receptors. Thapsigargin prevented the Ca2+ elevation but did not modify the inward current, indicating that the Ca2+ signal was attributable to intracellular Ca2+ mobilization. These results indicate that hippocampal astrocytes respond to acetylcholine released by synaptic terminals. The synaptically released acetylcholine acts on muscarinic receptors, mobilizing Ca2+ from the intracellular stores. Different regions in the recorded astrocytes showed independent stimulus-induced Ca2+ variations, suggesting the existence of subcellular domains in the astrocytic responses evoked by the synaptic cholinergic activity. Therefore, our results show the existence of cholinergic neuron-astrocyte signaling and suggest that astrocytes are a target of axonal inputs from different brain areas.

Communication between astrocytes and neurons: a complex language

In recent years, accumulating evidence suggests the existence of bidirectional communication between astrocytes and neurons, indicating an important active role of astrocytes in the physiology of the nervous system. As a consequence of this evidence, a new concept of the synaptic physiology--"the tripartite synapse"--has been proposed, in which the synapse is formed by three functional elements, i.e. the pre- and postsynaptic elements and the surrounding astrocytes. In the present article we review and discuss the current knowledge on the cellular mechanisms and physiological properties of this communication that displays highly complex characteristics. We are beginning to realize that the communication between astrocytes and neurons uses a quite complex language.

Astrocytes, spontaneity, and the developing thalamus

Recent studies in the ventrobasal (VB) thalamus have shown that astrocytes display spontaneous intracellular calcium [Ca(2+)](i) oscillations early postnatally. [Ca(2+)](i) oscillations are correlated in groups of up to five astrocytes, and propagate between cells. NMDA receptor-mediated, long lasting inward currents in thalamocortical (TC) neurons of the VB complex are correlated to [Ca(2+)](i) increases in neighbouring astrocytes, and stimulation of astrocytic [Ca(2+)](i) increases also lead to inward currents in neurons. These findings suggest that astrocytes are spontaneously active and can induce neuronal activity, a reversal of the previously held view of neuron-glia interactions in the central nervous system. This activity occurs at an important period in the development of the thalamus and therefore suggests a potential functional role in a variety of processes. Along with data on the neurotransmitter receptor repertoire of thalamic astrocytes these findings enlarge the body of knowledge on astrocytes in the thalamus, and further contribute to the emerging field of astrocyte-neuron and neuron-astrocyte interactions in the central nervous system.

Calcium oscillations encoding neuron-to-astrocyte communication

The observation that the excitatory neurotransmitter glutamate released from presynaptic terminals can activate, beside the post-synaptic neuron, the glial cell astrocyte, stimulated glial cell research like no other event since the recognition in the 1980s that astrocytes can express on their membrane many receptors for classical neurotransmitters. The properties and the functional role(s) of such a neuron-to-astrocyte signaling have now become the focus of intense research in neurobiology. Indeed, a growing body of evidence has recently highlighted the ability of astrocytes to work as sophisticated detectors of synaptic activity: by changing the frequency of [Ca(2+)](i) oscillations evoked by the synaptic release of glutamate, these cells display the remarkable capacity to discriminate between different levels and patterns of synaptic activity. Furthermore, the observation that astrocytes increase the frequency of [Ca(2+)](i) oscillations in response to repetitive episodes of high neuronal activity challenges the common concept that memory function in the brain is an exclusive property of neuronal cells. Glutamate-mediated [Ca(2+)](i) elevations can also trigger in astrocytes the release of glutamate that can ultimately affect neuronal transmission. Given the wide role played by glutamate in brain physiology, our view on how the brain operates needs now to be revised taking into account the bi-directional, glutamatergic communication between neurons and astrocytes.

Intercellular calcium signaling in astrocytes via ATP release through connexin hemichannels

Astrocytes are capable of widespread intercellular communication via propagated increases in intracellular Ca(2+) concentration. We have used patch clamp, dye flux, ATP assay, and Ca(2+) imaging techniques to show that one mechanism for this intercellular Ca(2+) signaling in astrocytes is the release of ATP through connexin channels ("hemichannels") in individual cells. Astrocytes showed low Ca(2+)-activated whole-cell currents consistent with connexin hemichannel currents that were inhibited by the connexin channel inhibitor flufenamic acid (FFA). Astrocytes also showed molecular weight-specific influx and release of dyes, consistent with flux through connexin hemichannels. Transmembrane dye flux evoked by mechanical stimulation was potentiated by low Ca(2+) and was inhibited by FFA and Gd(3+). Mechanical stimulation also evoked release of ATP that was potentiated by low Ca(2+) and inhibited by FFA and Gd(3+). Similar whole-cell currents, transmembrane dye flux, and ATP release were observed in C6 glioma cells expressing connexin43 but were not observed in parent C6 cells. The connexin hemichannel activator quinine evoked ATP release and Ca(2+) signaling in astrocytes and in C6 cells expressing connexin43. The propagation of intercellular Ca(2+) waves in astrocytes was also potentiated by quinine and inhibited by FFA and Gd(3+). Release of ATP through connexin hemichannels represents a novel signaling pathway for intercellular communication in astrocytes and other non-excitable cells.

Astrocytic and microglia cells reactivity induced by neonatal administration of glutamate in cerebral cortex of the adult rats

Recent studies confirm that astrocytes and neurons are associated with the synaptic transmission, particularly with the regulation of glutamate (Glu) levels. Therefore, they have the capacity to modulate the Glu released from neurons into the extracellular space. It has also been demonstrated an intense astrocytic and microglia response to physical or chemical lesions of the central nervous system. However, the persistence of the response of the glial cells in adult brain had not been previously reported, after the excitotoxic damage caused by neonatal dosage of monosodium glutamate (MSG) to newborn rats. In this study, 4 mg/g body weight of MSG were administered to newborn rats at 1, 3, 5, and 7 days after birth, at the age of 60 days the astrocytes and the microglia cells were analyzed with immunohistochemical methods in the fronto-parietal cortex. Double labeling to glial fibrillary acidic protein (GFAP) and BrdU, or isolectin-B(4) and BrdU identified astrocytes or microglia cells that proliferated; immunoblotting and immunoreactivity to vimentin served for assess immaturity of astrocytic intermediate filaments. The results show that the neonatal administration of MSG-induced reactivity of astrocytes and microglia cells in the fronto-parietal cortex, which was characterized by hyperplasia; an increased number of astrocytes and microglia cells that proliferated, hypertrophy; increased complexity of the cytoplasm extension of both glial cells and expression of RNAm to vimentin, with the presence of vimentin-positive astrocytes. This glial response to neuroexcitotoxic stimulus of Glu on the immature brain, which persisted to adulthood, suggests that the neurotransmitter Glu could trigger neuro-degenerative illnesses.

Subcellular distribution of high-voltage-activated calcium channel subtypes in rat globus pallidus neurons

Globus pallidus (GP) neurons receive dense inhibitory synaptic inputs interspersed with sparse excitatory inputs distributed across the entire extent of their somata and dendrites. Yet, despite this predominance of inhibitory influence, GP neurons fire at a high tonic rate, suggesting that intrinsic properties play an important role in determining the physiological characteristics of these neurons. High-voltage-activated (HVA) calcium channels represent an important class of conductances that plays roles in controlling neurotransmitter release, postsynaptic excitability, and intracellular calcium signaling. To better understand the intrinsic properties of GP neurons, we examined the subcellular localization of HVA calcium channels by using immunocytochemistry at the electron microscopic level. Peroxidase labeling with antibodies against P/Q-, N-, and R-type HVA calcium channels demonstrated the presence of these channels in both proximal and distal dendrites of GP neurons. P/Q-, N-, and R-type channels were also found in presynaptic terminals, whereas L-type channels were found exclusively postsynaptically in neuronal elements. Immunogold labeling demonstrated that, although the density of intracellular L-type calcium channel labeling remains constant throughout the proximal-distal extent of the dendritic tree of GP neurons, the density of plasma membrane-bound channels is greater in distal dendrites. The finding of HVA calcium channels distributed throughout the whole dendritic tree of GP neurons indicates that these channels may interact with synaptic inputs to allow rich processing possibilities for GP neuron dendrites. Furthermore, the finding of a greater density of plasma membrane-bound L-type channels in distal dendrites expands the view that L-type channels are important only in somatic and proximal locations.

Disturbance of hippocampal long-term potentiation after transient ischemia in GFAP deficient mice

GFAP (glial fibrillary acidic protein) is an intermediate filament protein found exclusively in the astrocytes of the central nervous system. We studied the role of GFAP in the neuronal degeneration in the hippocampus after transient ischemia using knockout mice. Wild-type C57 Black/6 (GFAP(+/+)) mice and mutant (GFAP(-/-)) mice were subjected to occlusion of both carotid arteries for 5-15 min. Hippocampal slices were prepared 3 days after reperfusion and the field excitatory postsynaptic potentials (fEPSP) in the CA1 were recorded. High frequency stimulation induced robust long-term potentiation (LTP) in GFAP(-/-), as in GFAP(+/+) mice. After ischemia, however, the LTP in GFAP(-/-) was significantly depressed. Similarly, paired pulse facilitation (PPF) displayed little difference between GFAP(+/+) and GFAP(-/-), but after ischemia, the PPF in GFAP(-/-) showed a depression. Histological study revealed that loss of CA1 and CA3 pyramidal neurons after ischemia was marked in GFAP(-/-). MAP2 (dendritic) immunostaining in the post-ischemic hippocampus showed little difference but NF200 (axonal) immunoreactivity was reduced in GFAP(-/-). S100beta (glial) immunoreactivity was similar in the post-ischemic hippocampus of the GFAP(+/+) and GFAP(-/-), indicating that reactive astrocytosis did not require GFAP. Our results suggest that GFAP has an important role in astrocyte-neural interactions and that ischemic insult impairs LTP and accelerates neuronal death.

Hippocampal astrocytes in situ exhibit calcium oscillations that occur independent of neuronal activity

Results presented in this study indicate that a large subpopulation (approximately 65%) of hippocampal astrocytes in situ exhibit calcium oscillations in the absence of neuronal activity. Further, the spontaneous oscillations observed within individual hippocampal astrocytes generally developed asynchronously throughout the astrocyte's fine processes and occasionally spread through a portion of that astrocyte as a calcium wave but do not appear to spread among astrocytes as an intercellular calcium wave. Bath application of cyclopiazonic acid and injection of individual astrocytes with heparin blocked astrocytic calcium oscillations. Application of tetrodotoxin or incubation of slices with bafilomycin A1 had no effect on astrocytic calcium oscillations but did block evoked and spontaneous postsynaptic currents measured in CA1 pyramidal neurons. Application of a cocktail of antagonists for metabotropic glutamate receptors and purinergic receptors had no effect on the astrocytic calcium oscillations but blocked the ability of purinergic and metabotropic glutamatergic agonists to increase astrocytic calcium levels. These results indicate that the spontaneous calcium oscillations observed in hippocampal astrocytes in situ are mediated by IP3 receptor activation, are not dependent on neuronal activity, and do not depend on activation of metabotropic glutamate receptors or purinergic receptors. To our knowledge, this is the first demonstration that astrocytes in situ exhibit intrinsic signaling. This finding supports the hypothesis that astrocytes, independent of neuronal input, may act as pacemakers to modulate neuronal activity in situ.

Alpha 1-adrenergic modulation of metabotropic glutamate receptor-induced calcium oscillations and glutamate release in astrocytes

Astrocytic responses to activation of metabotropic glutamate receptors group I (mGluRs I) and alpha(1)-adrenoreceptors in cultured cells have been assessed using spectral analyzes and calcium imaging. Concentration-dependent changes were observed after stimulation with the mGluR I agonist (S)-3,5-dihydroxyphenylglycine (DHPG). These responses changed from a regular low frequency signal with sharp peaks at 1 microm to a pronounced stage of irregularity at 10 microm. After stimulation with 100 microm the signal was again homogenous in shape and regularity but occurred at a higher frequency. In contrast, the spectral properties after stimulation with the alpha(1)-adrenoreceptor agonist phenylephrine, exhibited considerable variation for all investigated concentrations. DHPG-induced increases in [Ca(2+)](i) were also associated with astroglial glutamate release, whereas no release was observed after noradrenergic stimulation. Both DHPG-mediated calcium signaling and glutamate release were inhibited by preincubation with 10 or 100 microm phenylephrine. Collectively, the present investigation provides new information about the spatial-temporal characteristics of astroglial intracellular calcium responses and demonstrates distinct differences between noradrenergic and glutamatergic receptors regarding intracellular calcium signaling and coupling to glutamate release. The noradrenergic modulation of DHPG-induced responses indicates that intracellular astroglial processes can be regulated in a bi-directional feedback loop between closely connected astrocytes and neurons in the central nervous system.

Kir6.1 is the principal pore-forming subunit of astrocyte but not neuronal plasma membrane K-ATP channels

ATP-sensitive potassium channels (K-ATP channels) directly couple the energy state of a cell to its excitability, are activated by hypoxia, and have been suggested to protect neurons during disturbances of energy metabolism such as transient ischemic attacks or stroke. Molecular studies have demonstrated that functional K-ATP channels are octameric protein complexes, consisting of four sulfonylurea receptor proteins and four pore-forming subunits which are members of the Kir6 family of inwardly rectifying potassium channels. Here we show, using specific antibodies against the two known pore-forming subunits (Kir6.1 and Kir6.2) of K-ATP channels, that only Kir6.1 and not Kir6.2 subunits are expressed in astrocytes. In addition to a minority of neurons, Kir6.1 protein is present on hippocampal, cortical, and cerebellar astrocytes, tanycytes, and Bergmann glial cells. We also provide ultrastructural evidence that Kir6.1 immunoreactivity is primarily localized to distal perisynaptic and peridendritic astrocyte plasma membrane processes, and we confirm the presence of functional K-ATP channels in Bergmann glial cells by slice-patch-clamp experiments. The identification of Kir6.1 as the principal pore-forming subunit of plasma membrane K-ATP channels in astrocytes suggests that these glial K-ATP channels act in synergy with neuronal Kir6.2-mediated K-ATP channels during metabolic challenges in the brain.

Glutamate on demand: astrocytes as a ready source

The past decade of studies has changed our view of the integrative capacities and roles of glia. A picture is emerging in which neurons and astrocytes, a subtype of glial cell, are in a continuous regulatory dialogue. Initial studies demonstrated that chemical transmitters, which are released from neurons, induce elevations of astrocytic calcium. Furthermore, stimulation of neuronal afferents at modest frequencies induces a calcium response in astrocytes that is graded with stimulation frequency. The consequence of this astrocytic calcium response is now beginning to be appreciated in that changes in calcium level can induce the release of the chemical transmitter glutamate from this nonneuronal cell. During the past few years, it has been shown that by releasing glutamate, astrocytes can regulate synaptic transmission and contribute to certain forms of synaptic plasticity. The roles played in information processing by this glial feedback loop remain to be determined. However, it is likely that the results of these recent studies will signal a new way of thinking about the nervous system, in which the glial cell comes to the forefront of our attention.

NR2B-containing NMDA autoreceptors at synapses on entorhinal cortical neurons

We have previously shown that presynaptic N-methyl-D-aspartate receptors (NMDARs) can facilitate glutamate release onto principal neurons in the entorhinal cortex (EC). In the present study, we have investigated the subunit composition of these presynaptic NMDARs. We recorded miniature alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated excitatory postsynaptic currents (mEPSCs), from visually identified neurons in layers II and V of the EC in vitro. In both layers, bath application of the NR2A/B subunit-selective agonist, homoquinolinic acid (HQA), resulted in a marked facilitation of mEPSC frequency. Blockade of presynaptic Ca(2+) entry through either NMDARs or voltage-gated Ca(2+) channels with Co(2+) prevented the effects of HQA, confirming that Ca(2+) entry to the terminal was required for facilitation. When the NR2B-selective antagonist, ifenprodil, was applied prior to HQA, the increase in mEPSC frequency was greatly reduced. In addition, we found that an NMDAR antagonist blocked frequency-dependent facilitation of evoked release and reduced mEPSC frequency in layer V. Thus we have demonstrated that NMDA autoreceptors in layer V of the EC bear the NR2B subunit, and that NMDARs are also present at terminals onto superficial neurons.

Indirect actions of bradykinin on neonatal rat dorsal root ganglion neurones: a role for non-neuronal cells as nociceptors

1. In this study we have investigated the action of bradykinin (Bk) on cultured neonatal rat dorsal root ganglion (DRG) cells, with the aim of elucidating whether the neuronal response to Bk is influenced by association with non-neuronal satellite cells. 2. Bradykinin (100 nM) evoked an inward current (I(Bk)) in 51 of 58 voltage clamped DRG neurones (holding potential (V(h)) = -80 mV) that were in contact with non-neuronal satellite cells. 3. Bradykinin failed to evoke an inward current in isolated DRG neurones (V(h) = -80 mV) that were not in contact with non-neuronal satellite cells (n = 41). 4. The lack of neuronal response to Bk was not influenced by time in culture. Bradykinin failed to evoke a response in isolated neurones through 1-5 days in culture. By contrast neurones in contact with satellite cells responded to Bk throughout the same time period. 5. Failure of isolated neurones to respond to Bk was not due to the replating procedure or to selective subcellular distribution of receptors/ion channels to the processes rather than the somata of neurones. 6. Using Indo-1 AM microfluorimetry Bk (100 nM) was demonstrated to evoke an intracellular Ca(2+) increase (Ca(Bk)) in DRG neurones in contact with non-neuronal satellite cells and in isolated neurones. 7. These data suggest that the inward current response to Bk requires contact between DRG neurones and non-neuronal satellite cells. This implies an indirect mechanism of action for Bk via the non-neuronal cells, which may perform a nociceptive role. However, Bk can also act directly on the neurones, since it evokes Ca(Bk) in isolated neurones. The relationship between Ca(Bk) and the Bk-induced inward current is unknown at present.

Differential mechanisms of Ca2+ responses in glial cells evoked by exogenous and endogenous glutamate in rat hippocampus

The mechanisms of Ca2+ responses evoked in hippocampal glial cells in situ, by local application of glutamate and by synaptic activation, were studied in slices from juvenile rats using the membrane permeant fluorescent Ca2+ indicator fluo-3AM and confocal microscopy. Ca2+ responses induced by local application of glutamate were unaffected by the sodium channel blocker tetrodotoxin and were therefore due to direct actions on glial cells. Glutamate-evoked responses were significantly reduced by the L-type Ca2+ channel blocker nimodipine, the group I/II metabotropic glutamate receptor antagonist (S)-alpha-methyl-4-carboxyphenylglycine (MCPG), and the N-methyl-D-aspartate (NMDA) receptor antagonist (+/-)2-amino-5-phosphonopentanoic acid (APV). However, glutamate-induced Ca2+ responses were not significantly reduced by the non-NMDA receptor antagonist 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX). These results indicate that local application of glutamate increases intracellular Ca2+ levels in glial cells via the activation of L-type Ca2+ channels, NMDA receptors, and metabotropic glutamate receptors. Brief (1 s) tetanization of Schaffer collaterals produced increases in intracellular Ca2+ levels in glial cells that were dependent on the frequency of stimulation (> or =50 Hz) and on synaptic transmission (abolished by tetrodotoxin). These Ca2+ responses were also antagonized by the L-type Ca2+ channel blocker nimodipine and the metabotropic glutamate receptor antagonist MCPG. However, the non-NMDA receptor antagonist CNQX significantly reduced the Schaffer collateral-evoked Ca2+ responses, while the NMDA antagonist APV did not. Thus, these synaptically mediated Ca2+ responses in glial cells involve the activation of L-type Ca2+ channels, group I/II metabotropic glutamate receptors, and non-NMDA receptors. These findings indicate that increases in intracellular Ca2+ levels induced in glial cells by local glutamate application and by synaptic activity share similar mechanisms (activation of L-type Ca2+ channels and group I/II metabotropic glutamate receptors) but also have distinct components (NMDA vs. non-NMDA receptor activation, respectively). Therefore, neuron-glia interactions in rat hippocampus in situ involve multiple, complex Ca2+-mediated processes that may not be mimicked by local glutamate application.

Spontaneous astrocytic Ca2+ oscillations in situ drive NMDAR-mediated neuronal excitation

Astrocytes respond to chemical, electrical and mechanical stimuli with transient increases in intracellular calcium concentration ([Ca2+]i). We now show that astrocytes in situ display intrinsic [Ca2+]i oscillations that are not driven by neuronal activity. These spontaneous astrocytic oscillations can propagate as waves to neighboring astrocytes and trigger slowly decaying NMDA receptor-mediated inward currents in neurons located along the wave path. These findings show that astrocytes in situ can act as a primary source for generating neuronal activity in the mammalian central nervous system.