Expression of synaptobrevin II, cellubrevin and syntaxin but not SNAP-25 in cultured astrocytes

Extracellular high K+ stimulates vesicular glutamate release from astrocytes by activating voltage-dependent calcium channels

Extracellular high K(+) (75 mM) increased intracellular Ca(2+) concentrations in cultured rat hippocampal astrocytes, and the Ca(2+) rise was abolished by deleting extracellular Ca(2+) or cadmium, a non-selective inhibitor of voltage-dependent calcium channels (VDCCs). In the reverse transcription-polymerase chain reaction analysis, cultured astrocytes expressed mRNAs for L type-VDCC subunits such as alpha1B, alpha1C, alpha1D, and alpha1E. Extracellular high K(+) (75 mM) stimulated glutamate release from astrocytes. The glutamate release was not prevented by the glutamate transporter inhibitor, L-trans-pyrrolidine-2,4-dicarboxylic acid (PDC), or deleting extracellular Na(+), but otherwise it was clearly inhibited by deleting extracellular Ca(2+), cadmium, vesicular transport inhibitors such as brefeldin A, bafilomycin A1, and latrunculin B, or botulinum toxin-A, an exocytosis inhibitor. Extracellular high K(+) (75 mM) bleached fluorescent signals of FM1-43, taken up into the vesicular membrane in astrocytes, that was also inhibited by deleting extracellular Ca(2+), cadmium, brefeldin A, bafilomycin A1, latrunculin B, or botulinum toxin-A, but not by PDC. Taken together, the results of the present study indicate that extracellular high K(+)-evoked depolarization activates VDCCs expressed in astrocytes, causing an increase in intracellular Ca(2+) concentrations through VDCCs, which triggers vesicular glutamate release from astrocytes, independently of reverse transport through glutamate transporters.

Do astrocytes really exocytose neurotransmitters?

In the past 20 years, an extra layer of information processing, in addition to that provided by neurons, has been proposed for the CNS. Neuronally evoked increases of the intracellular calcium concentration in astrocytes have been suggested to trigger exocytotic release of the 'gliotransmitters' glutamate, ATP and D-serine. These are proposed to modulate neuronal excitability and transmitter release, and to have a role in diseases as diverse as stroke, epilepsy, schizophrenia, Alzheimer's disease and HIV infection. However, there is intense controversy about whether astrocytes can exocytose transmitters in vivo. Resolving this issue would considerably advance our understanding of brain function.

Immunocytochemical evidence for SNARE protein-dependent transmitter release from guinea pig horizontal cells

Horizontal cells are lateral interneurons that participate in visual processing in the outer retina but the cellular mechanisms underlying transmitter release from these cells are not fully understood. In non-mammalian horizontal cells, GABA release has been shown to occur by a non-vesicular mechanism. However, recent evidence in mammalian horizontal cells favors a vesicular mechanism as they lack plasmalemmal GABA transporters and some soluble NSF attachment protein receptor (SNARE) core proteins have been identified in rodent horizontal cells. Moreover, immunoreactivity for GABA and the molecular machinery to synthesize GABA have been found in guinea pig horizontal cells, suggesting that if components of the SNARE complex are expressed they could contribute to the vesicular release of GABA. In this study we investigated whether these vesicular and synaptic proteins are expressed by guinea pig horizontal cells using immunohistochemistry with well-characterized antibodies to evaluate their cellular distribution. Components of synaptic vesicles including vesicular GABA transporter, synapsin I and synaptic vesicle protein 2A were localized to horizontal cell processes and endings, along with the SNARE core complex proteins, syntaxin-1a, syntaxin-4 and synaptosomal-associated protein 25 (SNAP-25). Complexin I/II, a cytosolic protein that stabilizes the activated SNARE fusion core, strongly immunostained horizontal cell soma and processes. In addition, the vesicular Ca(2+)-sensor, synaptotagmin-2, which is essential for Ca(2+)-mediated vesicular release, was also localized to horizontal cell processes and somata. These morphological findings from guinea pig horizontal cells suggest that mammalian horizontal cells have the capacity to utilize a regulated Ca(2+)-dependent vesicular pathway to release neurotransmitter, and that this mechanism may be shared among many mammalian species.

Regulated exocytosis in astrocytic signal integration

Astrocytes can be considered as signal integrators in central nervous system activity. These glial cells can respond to signals from the heterocellular milieu of the brain and subsequently release various molecules to signal to themselves and/or other neighboring neural cells. An important functional module that enables signal integration in astrocytes is exocytosis, a Ca(2+)-dependent process consisting of vesicular fusion to the plasma membrane. Astrocytes utilize regulated exocytosis to release various signaling molecules stored in the vesicular lumen. Here we review the properties of exocytotic release of three classes of gliotransmitters: (i) amino acids, (ii) nucleotides and (iii) peptides. Vesicles may carry not only lumenal cargo, but also membrane-associated molecules. Therefore, we also discuss exocytosis as a delivery mechanism for transporters and receptors to the plasma membrane, where these proteins are involved in astrocytic intercellular signaling.

Gliotransmission: Exocytotic release from astrocytes

Gliotransmitters are chemicals released from glial cells fulfilling a following set of criteria: (i) they are synthesized by and/or stored in glia; (ii) their regulated release is triggered by physiological and/or pathological stimuli; (iii) they activate rapid (milliseconds to seconds) responses in neighboring cells; and (iv) they play a role in (patho)physiological processes. Astrocytes can release a variety of gliotransmitters into the extracellular space using several different mechanisms. In this review, we focus on exocytotic mechanism(s) underlying the release of three classes of gliotransmitters: (i) amino acids, such as, glutamate and d-serine; (ii) nucleotides, like adenosine 5'-triphosphate; and (iii) peptides, such as, atrial natriuretic peptide and brain-derived neurotrophic factor. It is becoming clear that astrocytes are endowed with elements that qualify them as cells communicating with neurons and other cells within the central nervous system by employing regulated exocytosis.

Subchronic alpha-linolenic acid treatment enhances brain plasticity and exerts an antidepressant effect: a versatile potential therapy for stroke

Omega-3 polyunsaturated fatty acids are known to have therapeutic potential in several neurological and psychiatric disorders. However, the molecular mechanisms of action underlying these effects are not well elucidated. We previously showed that alpha-linolenic acid (ALA) reduced ischemic brain damage after a single treatment. To follow-up this finding, we investigated whether subchronic ALA treatment promoted neuronal plasticity. Three sequential injections with a neuroprotective dose of ALA increased neurogenesis and expression of key proteins involved in synaptic functions, namely, synaptophysin-1, VAMP-2, and SNAP-25, as well as proteins supporting glutamatergic neurotransmission, namely, V-GLUT1 and V-GLUT2. These effects were correlated with an increase in brain-derived neurotrophic factor (BDNF) protein levels, both in vitro using neural stem cells and hippocampal cultures and in vivo, after subchronic ALA treatment. Given that BDNF has antidepressant activity, this led us to test whether subchronic ALA treatment could produce antidepressant-like behavior. ALA-treated mice had significantly reduced measures of depressive-like behavior compared with vehicle-treated animals, suggesting another aspect of ALA treatment that could stimulate functional stroke recovery by potentially combining acute neuroprotection with long-term repair/compensatory plasticity. Indeed, three sequential injections of ALA enhanced protection, either as a pretreatment, wherein it reduced post-ischemic infarct volume 24 h after a 1-hour occlusion of the middle cerebral artery or as post-treatment therapy, wherein it augmented animal survival rates by threefold 10 days after ischemia.

Trafficking of astrocytic vesicles in hippocampal slices

The increasingly appreciated role of astrocytes in neurophysiology dictates a thorough understanding of the mechanisms underlying the communication between astrocytes and neurons. In particular, the uptake and release of signaling substances into/from astrocytes is considered as crucial. The release of different gliotransmitters involves regulated exocytosis, consisting of the fusion between the vesicle and the plasma membranes. After fusion with the plasma membrane vesicles may be retrieved into the cytoplasm and may continue to recycle. To study the mobility implicated in the retrieval of secretory vesicles, these structures have been previously efficiently and specifically labeled in cultured astrocytes, by exposing live cells to primary and secondary antibodies. Since the vesicle labeling and the vesicle mobility properties may be an artifact of cell culture conditions, we here asked whether the retrieving exocytotic vesicles can be labeled in brain tissue slices and whether their mobility differs to that observed in cell cultures. We labeled astrocytic vesicles and recorded their mobility with two-photon microscopy in hippocampal slices from transgenic mice with fluorescently tagged astrocytes (GFP mice) and in wild-type mice with astrocytes labeled by Fluo4 fluorescence indicator. Glutamatergic vesicles and peptidergic granules were labeled by the anti-vesicular glutamate transporter 1 (vGlut1) and anti-atrial natriuretic peptide (ANP) antibodies, respectively. We report that the vesicle mobility parameters (velocity, maximal displacement and track length) recorded in astrocytes from tissue slices are similar to those reported previously in cultured astrocytes.

Regulation of exocytotic protein expression and Ca2+-dependent peptide secretion in astrocytes

Vesicular transmitter release from astrocytes influences neuronal development, function and plasticity. However, secretory pathways and the involved molecular mechanisms in astroglial cells are poorly known. In this study, we show that a variety of SNARE and Munc18 isoforms are expressed by cultured astrocytes, with syntaxin-4, Munc18c, SNAP-23 and VAMP-3 being the most abundant variants. Exocytotic protein expression was differentially regulated by activating and differentiating agents. Specifically, proteins controlling Ca(2+)-dependent secretion in neuroendocrine cells were up-regulated after long-term 8Br-cAMP administration in astrocytes, but not by proinflammatory cytokines. Moreover, 8Br-cAMP treatment greatly increased the cellular content of the peptidic vesicle marker secretogranin-2. Release assays performed on cAMP-treated astrocytes showed that basal and stimulated secretogranin-2 secretion are dependent on [Ca(2+)](i). As shown release of the chimeric hormone ANP.emd from transfected cells, cAMP-induced differentiation in astrocytes enhances Ca(2+)-regulated peptide secretion. We conclude that astroglial cells display distinctive molecular components for exocytosis. Moreover, the regulation of both exocytotic protein expression and Ca(2+)-dependent peptide secretion in astrocytes by differentiating and activating agents suggest that glial secretory pathways are adjusted in different physiological states.

Dynamics of a minimal neural model consisting of an astrocyte, a neuron, and an interneuron

In this paper, a biophysical neural network model consisting of a pyramidal neuron, an interneuron, and the astrocyte is studied. The corresponding dynamical properties are mainly investigated by using numerical simulations. The results show that the presence of the adenosine triphosphate and of the interneuron impacts the overall neural activity. It is shown that the fluxes of calcium through the cellular membrane strongly affect the modulation of the neural activity arising from the astrocyte.

Regulated exocytosis and vesicle trafficking in astrocytes

Astrocytes are increasingly viewed as crucial cells supporting and integrating brain functions. It is thought that the release of gliotransmitters into the extracellular space by regulated exocytosis supports a significant part of communication between astrocytes and neurons. Prior to exocytosis, the membrane-bound vesicles are transported through the astrocyte cytoplasm. Our recent studies have revealed new insights into vesicle trafficking in the cytoplasm of astrocytes and are reviewed in this article. The prefusion mobility of fluorescently labeled peptidergic vesicles was studied in cultured rat and mouse astrocytes. Vesicle delivery to the plasma membrane involved an interaction with the cytoskeleton, in particular with microtubules and actin filaments. Interestingly, vesicle mobility in mouse astrocytes deficient in intermediate filaments show impaired directionality of peptidergic vesicle mobility. To explore whether stimuli that increase the concentration of free calcium ions in the cytoplasm triggered vesicular ATP release from astrocytes, human embryonic kidney-293T cells transfected with a P2X(3) receptor were used as sniffers to detect ATP release. Glutamate stimulation of astrocytes was followed by an increase in the incidence of small, transient, inward currents in sniffer cells, reminiscent of postsynaptic quantal events observed at synapses. Some of the membrane-bound vesicles are retrieved from the plasma membrane to be recycled back into the cytosol. Trafficking velocity of postfusion (recycling) atrial natriuretic peptide vesicles was one order of magnitude slower in comparison to the mobility of prefusion vesicles. However, transport of all vesicle types studied required an intact cytoskeleton.

Bradykinin-induced astrocyte-neuron signalling: glutamate release is mediated by ROS-activated volume-sensitive outwardly rectifying anion channels

Glial cells release gliotransmitters which signal to adjacent neurons and glial cells. Previous studies showed that in response to stimulation with bradykinin, glutamate is released from rat astrocytes and causes NMDA receptor-mediated elevation of intracellular Ca(2+) in adjacent neurons. Here, we investigate how bradykinin-induced glutamate release from mouse astrocytes signals to neighbouring neurons in co-cultures. Astrocyte-to-neuron signalling and bradykinin-induced glutamate release from mouse astrocytes were both inhibited by the anion channel blocker 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) and phloretin. Glutamate release was also sensitive to 4-(2-Butyl-6,7-dichlor-2-cyclopentylindan-1-on-5-yl) oxybutyric acid (DCPIB), a specific blocker of the volume-sensitive outwardly rectifying anion channel (VSOR). Astrocytes, but not neurons, responded to bradykinin with activation of whole-cell Cl- currents. Although astrocytes stimulated with bradykinin did not undergo cell swelling, the bradykinin-activated current exhibited properties typical of VSOR: outward rectification, inhibition by osmotic shrinkage, sensitivity to DIDS, phloretin and DCPIB, dependence on intracellular ATP, and permeability to glutamate. Bradykinin increased intracellular reactive oxygen species (ROS) in mouse astrocytes. Pretreatment of mouse astrocytes with either a ROS scavenger or an NAD(P)H oxidase inhibitor blocked bradykinin-induced activation of VSOR, glutamate release and astrocyte-to-neuron signalling. By contrast, pretreatment with BAPTA-AM or tetanus neurotoxin A failed to suppress bradykinin-induced glutamate release. Thus, VSOR activated by ROS in mouse astrocytes in response to stimulation with bradykinin, serves as the pathway for glutamate release to mediate astrocyte-to-neuron signalling. Since bradykinin is an initial mediator of inflammation, VSOR might play a role in glia-neuron communication in the brain during inflammation.

Regulated exocytosis from astrocytes physiological and pathological related aspects

Astrocytes have traditionally been considered ancillary, satellite cells of the nervous system. However, it is a very recent acquisition that glial cells generate signaling loops which are integral to the brain circuitry and participate, interactively with neuronal networks, in the processing of information. Such a conceptual breakthrough makes this field of investigation one of the hottest in neuroscience, as it calls for a revision of past theories of brain function as well as for new strategies of experimental exploration of brain function. Glial cells are electrically not excitable, and it was only the use of optical recording techniques together with calcium sensitive dyes, that allowed the chemical excitability of glial cells to become apparent. Studies using these new techniques have shown for the first time that glial cells are activated by surrounding synaptic activity and translate neuronal signals into their own calcium code. Intracellular calcium concentration([Ca2+]i) elevations in glial cells have then shown to underlie spatial transfer of information in the glial network, accompanied by release of chemical transmitters (gliotransmitters) such as glutamate and back-signaling to neurons. As a consequence, optical imaging techniques applied to cell cultures or intact tissue have become a state-of-the-art technology for studying glial cell signaling. The molecular mechanisms leading to release of "gliotransmitters," especially glutamate, from glia are under debate. Accumulating evidence clearly indicates that astrocytes secrete numerous transmitters by Ca(2+)-dependent exocytosis. This review will discuss the mechanisms underlying the release of chemical transmitters from astrocytes with a particular emphasis to the regulated exocytosis processes.

Confocal imaging and tracking of the exocytotic routes for D-serine-mediated gliotransmission

D-Serine is an astrocyte-derived regulator for N-methyl-D-aspartate receptors, but the intracellular routes of its trafficking are still largely unknown. Here, we combined confocal microscopy with colocalization quantification to track the astrocytic organelles that store D-serine. We report that D-serine colocalizes with the transfected eGFP-synaptobrevin/VAMP2 and eGFP-cellubrevin/VAMP3, two v-SNAREs of the regulated secretory pathway. No significant colocalization was found with markers of the endosomal sorting and recycling system: EEA1, eGFP-endobrevin/VAMP8, eGFP-TI-VAMP/VAMP7, LAMP1, and CD63. Blockade of vesicular budding with colchicine shows that secretory vesicles import D-serine downstream to the Golgi apparatus. Finally, treatment of astrocytes with the Ca2+-ionophore A23187, glutamate agonists, or bradykinin trigger translocation of synaptobrevin/VAMP2 to the plasma membrane with a concomitant disappearance of D-serine from the regulated secretory pathway. Our results provide morphological evidence for a vesicular storage of D-serine in the regulated secretory pathway and the possible recruitment of these stores by Ca2+ mobilization to release D-serine.

Functional evidence for presynaptic P2X7 receptors in adult rat cerebrocortical nerve terminals

The presynaptic P2X7 receptor (P2X7R) plays an important role in the modulation of transmitter release. We recently demonstrated that, in nerve terminals of the adult rat cerebral cortex, P2X7R activation induced Ca2+-dependent vesicular glutamate release and significant Ca2+-independent glutamate efflux through the P2X7R itself. In the present study, we investigated the effect of the new selective P2X(7)R competitive antagonist 3-(5-(2,3-dichlorophenyl)-1H-tetrazol-1-yl)methyl pyridine (A-438079) on cerebrocortical terminal intracellular calcium (intrasynaptosomal calcium concentration;[Ca2+](i) signals and glutamate release, and evaluated whether P2X7R immunoreactivity was consistent with these functional tests. A-438079 inhibited functional responses. P2X7R immunoreactivity was found in about 45% of cerebrocortical terminals, including glutamatergic and non-glutamatergic terminals. This percentage was similar to that of synaptosomes showing P2X7R-mediated [Ca2+]i signals. These findings provide compelling evidence of functional presynaptic P2X7R in cortical nerve terminals.

The role of the t-SNARE SNAP-25 in action potential-dependent calcium signaling and expression in GABAergic and glutamatergic neurons

BACKGROUND

The soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, comprised of SNAP-25, syntaxin 1A, and VAMP-2, has been shown to be responsible for action potential (AP)-dependent, calcium-triggered release of several neurotransmitters. However, this basic fusogenic protein complex may be further specialized to suit the requirements for different neurotransmitter systems, as exemplified by neurons and neuroendocrine cells. In this study, we investigate the effects of SNAP-25 ablation on spontaneous neuronal activity and the expression of functionally distinct isoforms of this t-SNARE in GABAergic and glutamatergic neurons of the adult brain.

RESULTS

We found that neurons cultured from Snap25 homozygous null mutant (Snap25-/-) mice failed to develop synchronous network activity seen as spontaneous AP-dependent calcium oscillations and were unable to trigger glial transients following depolarization. Voltage-gated calcium channel (VGCC) mediated calcium transients evoked by depolarization, nevertheless, did not differ between soma of SNAP-25 deficient and control neurons. Furthermore, we observed that although the expression of SNAP-25 RNA transcripts varied among neuronal populations in adult brain, the relative ratio of the transcripts encoding alternatively spliced SNAP-25 variant isoforms was not different in GABAergic and glutamatergic neurons.

CONCLUSION

We propose that the SNAP-25b isoform is predominantly expressed by both mature glutamatergic and GABAergic neurons and serves as a fundamental component of SNARE complex used for fast synaptic communication in excitatory and inhibitory circuits required for brain function. Moreover, SNAP-25 is required for neurons to establish AP-evoked synchronous network activity, as measured by calcium transients, whereas the loss of this t-SNARE does not affect voltage-dependent calcium entry.

Mitochondria modulate Ca2+-dependent glutamate release from rat cortical astrocytes

Vesicular glutamate release from astrocytes depends on mobilization of free Ca(2+) from the endoplasmic reticulum (ER), and extracellular space to elevate cytosolic Ca(2+) (Ca(2+)(cyt)). Although mitochondria in neurons, and other secretory cells, have been shown to sequester free Ca(2+) and have been implicated in the modulation of Ca(2+)-dependent transmitter release, the role of mitochondria in Ca(2+)-dependent glutamate release from astrocytes is not known. A pharmacological approach was taken to manipulate Ca(2+) accumulation in mitochondria and thereby affect Ca(2+)(cyt) of solitary astrocytes in response to mechanical stimuli. Ca(2+)(cyt) responses and levels of glutamate release were measured optically in parallel experiments using a fluorescent Ca(2+) indicator and an enzyme-linked assay, respectively. It was observed that inhibiting mitochondrial Ca(2+) accumulation is correlated to increased Ca(2+)(cyt) and glutamate release, whereas enhancing mitochondrial Ca(2+) accumulation is correlated to decreased Ca(2+)(cyt) and glutamate release. These observations suggest that, in addition to the activity of ER and plasma membrane ion channels, mitochondria modulate Ca(2+)(cyt) dynamics in astrocytes and play a role in Ca(2+)-dependent glutamate release from astrocytes.

Lysosomes are the major vesicular compartment undergoing Ca2+-regulated exocytosis from cortical astrocytes

Although Ca(2+)-dependent exocytosis is considered to be a pathway for gliotransmitter release from astrocytes, the structural and functional bases of this process remain controversial. We studied the relationship between near-membrane Ca(2+) elevations and the dynamics of single astroglial vesicles with styryl (FM) dyes. We show that cultured astrocytes, unlike neurons, spontaneously internalize FM dyes, resulting in the labeling of the entire acidic vesicle population within minutes. Interestingly, metabotropic glutamate receptor activation did not affect the FM labeling. Most FM-stained vesicles expressed sialin, CD63/LAMP3, and VAMP7, three markers for lysosomes and late endosomes. A subset of lysosomes underwent asynchronous exocytosis that required both Ca(2+) mobilization from intracellular stores and Ca(2+) influx across the plasma membrane. Lysosomal fusion occurred within seconds and was complete with no evidence for kiss and run. Our experiments suggest that astroglial Ca(2+)-regulated exocytosis is carried by lysosomes and operates on a timescale orders of magnitude slower than synaptic transmission.

EAAT2 density at the astrocyte plasma membrane and Ca(2 + )-regulated exocytosis

We studied whether regulated exocytosis affects the glutamate transporter density in cultured astrocytes, in which the expression of a fluorescently labeled excitatory amino acid transporter 2 (EAAT2-EGFP) predominantly labeled the plasma membrane. The addition of ionomycin that elevates cytosolic Ca(2+) strongly increased the fluorescence of FM 4-64 membrane area dye, confirming the presence of regulated exocytosis in transfected astrocytes. However, concomitant with Ca(2+)-dependent FM 4-64 fluorescence increase, ionomycin induced a significant steady-state decrease in EAAT2-EGFP fluorescence. This is likely due to a secondary inner filter effect since,(i) in the absence of FM 4-64, ionomycin stimulation was ineffective in changing the EAAT2-EGFP fluorescence, and (ii) fluorescence changes in FM 4-64 and EAAT2-EGFP were inversely correlated. To test whether subcellular EAAT2-EGFP structures are translocated from the cytoplasm to the plasma membrane during ionomycin stimulation, EAAT2-EGFP fluorescence was monitored locally at the plasma membrane and a few microns away in the adjacent cytoplasm. Measurements revealed sites with an increase in EAAT2-EGFP plasma membrane fluorescence correlated with a fluorescence decrease beneath the plasma membrane, and sites with plasma membrane fluorescence decrease correlated with fluorescence increase within the adjacent cytoplasm. The sites of rapid translocation/retrieval of EAAT2-EGFP structures to/from the plasma membrane appeared to be distributed in a punctuate pattern around the cell perimeter. The density of EAAT2-EGFP was regulated in a Ca(2+)-dependent manner, since in the absence of extracellular Ca(2+) local translocation/retrieval events were absent, revealing rapid surface density regulation of EAAT2 in astrocytes by regulated exo/endocytosis.

Ca2+ entry through TRPC1 channels contributes to intracellular Ca2+ dynamics and consequent glutamate release from rat astrocytes

Astrocytes can respond to a variety of stimuli by elevating their cytoplasmic Ca2+ concentration and can in turn release glutamate to signal adjacent neurons. The majority of this Ca2+ is derived from internal stores while a portion also comes from outside of the cell. Astrocytes use Ca2+ entry through store-operated Ca2+ channels to refill their internal stores. Therefore, we investigated what role this store-operated Ca2+ entry plays in astrocytic Ca2+ responses and subsequent glutamate release. Astrocytes express canonical transient receptor potential (TRPC) channels that have been implicated in mediating store-operated Ca2+ entry. Here, we show that astrocytes in culture and freshly isolated astrocytes from visual cortex express TRPC1, TRPC4, and TRPC5. Indirect immunocytochemistry reveals that these proteins are present throughout the cell; the predominant expression of functionally tested TRPC1, however, is on the plasma membrane. Labeling in freshly isolated astrocytes reveals changes in TRPC expression throughout development. Using an antibody against TRPC1 we were able to block the function of TRPC1 channels and determine their involvement in mechanically and agonist-evoked Ca2+ entry in cultured astrocytes. Blocking TRPC1 was also found to reduce mechanically induced Ca2+-dependent glutamate release. These data indicate that Ca2+ entry through TRPC1 channels contributes to Ca2+ signaling in astrocytes and the consequent glutamate release from these cells.

Morphological evidence for vesicular glutamate release from astrocytes

There is now growing evidence that astrocytes, like neurons, can release transmitters. One transmitter that in a vast number of studies has been shown to be released from astrocytes is glutamate. Although asytrocytic glutamate may be released by several mechanisms, the evidence in favor of exocytosis is most compelling. Astrocytes may respond to neuronal activity by such exocytotic release of glutamate. The astrocyte derived glutamate can in turn activate neuronal glutamate receptors, in particular N-methyl-D-aspartate (NMDA) receptors. Here we review the morphological data supporting that astrocytes possess the machinery for exocytosis of glutamate. We describe the presence of small synaptic-like microvesicles, SNARE proteins and vesicular glutamate transporters in astrocytes, as well as NMDA receptors situated in vicinity of the astrocytic vesicles.

Stimulation inhibits the mobility of recycling peptidergic vesicles in astrocytes

Astrocytes are increasingly viewed as playing many roles in the integration of brain function. These cells store among other gliotransmitters also neuroactive peptides in membrane bound vesicles, the trafficking and release of which, may be changed in altered conditions, therefore affecting the physiological status of neurons. In general, peptidergic membrane-bound secretory vesicles fuse with the plasma membrane in the process of exocytosis. Some of them are retrieved from the plasma membrane to be recycled back into the cytosol. The mobility of retrieving vesicles in astrocytes was not studied yet, however, understanding the mechanisms of such trafficking would highlight the communication paths between astrocytes and neurons. We labeled vesicles with antibodies against the vesicle atrial natriuretic peptide (ANP), which is stored inside secretory vesicles. ANP-vesicles in astrocytes have been proposed to enter Ca2+-dependent secretion and here we show that they are associated with synaptotagmin IV (SytIV), a regulator of exocytosis in astrocytes. Moreover, the results show that recycling ANP-vesicles are to a significant extent acidic. Their velocity (0.06+/-0.001 microm/s) is one order of magnitude lower than the velocity of vesicles trafficking to the plasma membrane (Potokar et al. (2005) Biochem Biophys Res Commun 329:678-683; Potokar et al. (2007) Traffic 8:12-20). Interestingly, ionomycin or ATP application further attenuated ANP-vesicle mobility to 0.02+/-0.002 and to 0.03+/-0.001 microm/s, respectively. In summary, the mobility of recycling peptidergic vesicles appears to be slower than the vesicle traffic to the plasma membrane and it requires an intact cytoskeleton. Physiological implications of attenuated traffic of ANP-vesicles are considered in the discussion.

Sub-micromolar increase in [Ca(2+)](i) triggers delayed exocytosis of ATP in cultured astrocytes

Astrocytes release a variety of transmitter molecules, which mediate communication between glial cells in the brain and modulate synaptic transmission. ATP is a major glia-derived transmitter, but the mechanisms and kinetics of ATP release from astrocytes remain largely unknown. Here, we combined epifluorescence and total internal reflection fluorescence microscopy to monitor individual quinacrine-loaded ATP-containing vesicles undergoing exocytosis in cultured astrocytes. In resting cells, vesicles exhibited three-dimensional motility, spontaneous docking and release at low rate. Extracellular ATP application induced a Ca(2+)-dependent increase in the rate of exocytosis, which persisted for several minutes. Using UV flash photolysis of caged Ca(2+), the threshold [Ca(2+)](i) for ATP exocytosis was found to be approximately 350 nM. Subthreshold [Ca(2+)](i) transients predominantly induced vesicle docking at plasma membrane without subsequent release. ATP exocytosis triggered either by purinergic stimulation or by Ca(2+) uncaging occurred after a substantial delay ranging from tens to hundreds of seconds, with only approximately 4% of release occurring during the first 30 s. The time course of the cargo release from vesicles had two peaks centered on <or=10 s and 60 s. These results demonstrate that: (1) [Ca(2+)](i) elevations in cultured astrocytes trigger docking and release of ATP-containing vesicles; (2) vesicle docking and release have different Ca(2+) thresholds; (3) ATP exocytosis is delayed by several minutes and highly asynchronous; (4) two populations of ATP-containing vesicles with distinct (fast and slow) time course of cargo release exist in cultured astrocytes.

Mechanisms of glutamate release from astrocytes

Astrocytes can release the excitatory transmitter glutamate which is capable of modulating activity in nearby neurons. This astrocytic glutamate release can occur through six known mechanisms: (i) reversal of uptake by glutamate transporters (ii) anion channel opening induced by cell swelling, (iii) Ca2+-dependent exocytosis, (iv) glutamate exchange via the cystine-glutamate antiporter, (v) release through ionotropic purinergic receptors and (vi) functional unpaired connexons, "hemichannels", on the cell surface. Although these various pathways have been defined, it is not clear how often and to what extent astrocytes employ different mechanisms. It will be necessary to determine whether the same glutamate release mechanisms that operate under physiological conditions operate during pathological conditions or whether there are specific release mechanisms that operate under particular conditions.

Vesicular release of glutamate mediates bidirectional signaling between astrocytes and neurons

The major excitatory neurotransmitter in the CNS, glutamate, can be released exocytotically by neurons and astrocytes. Glutamate released from neurons can affect adjacent astrocytes by changing their intracellular Ca(2+) dynamics and, vice versa, glutamate released from astrocytes can cause a variety of responses in neurons such as: an elevation of [Ca(2+)](i), a slow inward current, an increase of excitability, modulation of synaptic transmission, synchronization of synaptic events, or some combination of these. This astrocyte-neuron signaling pathway might be a widespread phenomenon throughout the brain with astrocytes possessing the means to be active participants in many functions of the CNS. Thus, it appears that the vesicular release of glutamate can serve as a common denominator for two of the major cellular components of the CNS, astrocytes and neurons, in brain function.

Systematic colocalization errors between acridine orange and EGFP in astrocyte vesicular organelles

Dual-color imaging of acridine orange (AO) and EGFP fused to a vesicular glutamate transporter or the vesicle-associated membrane proteins 2 or 3 has been used to visualize a supposedly well-defined subpopulation of glutamatergic astrocytic secretory vesicles undergoing regulated exocytosis. However, AO metachromasy results in the concomitant emission of green and red fluorescence from AO-stained tissue. Therefore, the question arises whether AO and EGFP fluorescence can be distinguished reliably. We used evanescent-field imaging with spectral fluorescence detection as well as fluorescence lifetime imaging microscopy to demonstrate that green fluorescent AO monomers inevitably coexist with red fluorescing AO dimers, at the level of single astroglial vesicles. The green monomer emission spectrally overlaps with that of EGFP and produces a false apparent colocalization on dual-color images. On fluorophore abundance maps calculated from spectrally resolved and unmixed single-vesicle spectral image stacks, EGFP is obscured by the strong green monomer fluorescence, precluding the detection of EGFP. Hence, extreme caution is required when deriving quantitative colocalization information from images of dim fluorescing EGFP-tagged organelles colabeled with bright and broadly emitting dyes like AO. We finally introduce FM4-64/EGFP dual-color imaging as a remedy for imaging a distinct population of astroglial fusion-competent secretory vesicles.

Exocytotic release of ATP from cultured astrocytes

Astrocytes appear to communicate with each other as well as with neurons via ATP. However, the mechanisms of ATP release are controversial. To explore whether stimuli that increase [Ca(2+)](i) also trigger vesicular ATP release from astrocytes, we labeled ATP-containing vesicles with the fluorescent dye quinacrine, which exhibited a significant co-localization with atrial natriuretic peptide. The confocal microscopy study revealed that quinacrine-loaded vesicles displayed mainly non-directional spontaneous mobility with relatively short track lengths and small maximal displacements, whereas 4% of vesicles exhibited directional mobility. After ionomycin stimulation only non-directional vesicle mobility could be observed, indicating that an increase in [Ca(2+)](i) attenuated vesicle mobility. Total internal reflection fluorescence (TIRF) imaging in combination with epifluorescence showed that a high percentage of fluorescently labeled vesicles underwent fusion with the plasma membrane after stimulation with glutamate or ionomycin and that this event was Ca(2+)-dependent. This was confirmed by patch-clamp studies on HEK-293T cells transfected with P2X(3) receptor, used as sniffers for ATP release from astrocytes. Glutamate stimulation of astrocytes was followed by an increase in the incidence of small transient inward currents in sniffers, reminiscent of postsynaptic quantal events observed at synapses. Their incidence was highly dependent on extracellular Ca(2+). Collectively, these findings indicate that glutamate-stimulated ATP release from astrocytes was most likely exocytotic and that after stimulation the fraction of quinacrine-loaded vesicles, spontaneously exhibiting directional mobility, disappeared.

Exocytosis of ATP from astrocyte progenitors modulates spontaneous Ca2+ oscillations and cell migration

In the mature central nervous system (CNS) regulated secretion of ATP from astrocytes is thought to play a significant role in cell signaling. Whether such a mechanism is also operative in the developing nervous system and, if so, during which stage of development, has not been investigated. We have tackled this question using cells derived from reconstituted neurospheres, as well as brain explants of embryonic mice. Here, we show that in both models of neural cell development, astrocyte progenitors are competent for the regulated secretion of ATP-containing vesicles. We further document that this secretion is dependent on cytosolic Ca(2+) and the v-SNARE system, and takes place by exocytosis. Interference with ATP secretion alters spontaneous Ca(2+) oscillations and migration of neural progenitors. These data indicate that astrocyte progenitors acquire early in development the competence for regulated secretion of ATP, and that this event is implicated in the regulation of at least two cell functions, which are critical for the proper morphogenesis and functional maturation of the CNS.

Two forms of single-vesicle astrocyte exocytosis imaged with total internal reflection fluorescence microscopy

Transmitters such as glutamate and ATP are released from brain astrocytes. Several pathways for their release have been proposed, including exocytosis. In the present study we sought to measure exocytosis from astrocytes with single vesicle imaging methods using synaptopHlourin (SpH) as an optical reporter. We imaged single SpH-laden vesicles with total internal reflection fluorescence (TIRF) microscopy. We observed spontaneous, as well as evoked, single-vesicle exocytosis events. Analysis of the kinetics and spatial spread associated with these events indicated two discernible forms of single vesicle exocytosis. One form, constituting approximately 40% of the spontaneous events, was akin to kiss-and-run vesicle fusion and captured a mobile proton buffer from the extracellular medium. The other form seems to represent full vesicle fusion, constitutes approximately 60% of the spontaneous events, and is associated with complete mixing of the vesicle and plasma membranes. Activation of calcium-mobilizing receptors on the astrocyte surface selected between the different forms of exocytosis. These data provide evidence for two forms of simultaneously occurring single-vesicle exocytosis events in astrocytes, and also suggest that SpH imaging and TIRF microscopy is useful to study the mechanisms of astrocyte transmitter release.

Astrocyte calcium elevations: properties, propagation, and effects on brain signaling

The possibility that astrocytes are involved in brain signaling began to emerge in the late 1970s, when it was first shown that astroglia in vitro possess numerous receptors for neurotransmitters. It was later demonstrated that cultured astroglia and astrocytes in situ respond to neurotransmitters with increases in intracellular second messengers, including cyclic AMP and calcium. Astrocyte calcium responses have since been extensively studied both in culture and in intact tissue. We continue to gather information regarding the various compounds able to trigger astrocyte calcium increases, as well as the mechanisms involved in their initiation, propagation as a calcium wave within and between astrocytes, and effects on signaling within the brain. This review will focus on each of these aspects of astrocyte calcium regulation, and attempt to sort out which effects are more likely to occur in developmental, pathological, and physiological conditions. While we have come far in our understanding of the properties or potential of astrocytes' ability to signal to neurons using our array of pharmacological tools, we still understand very little regarding the level of involvement of astrocyte signaling in normal brain physiology.

Vesicular transmitter release from astrocytes

Astrocytes can release a variety of transmitters, including glutamate and ATP, in response to stimuli that induce increases in intracellular Ca(2+) levels. This release occurs via a regulated, exocytotic pathway. As evidence of this, astrocytes express protein components of the vesicular secretory apparatus, including synaptobrevin 2, syntaxin, and SNAP-23. Additionally, astrocytes possess vesicular organelles, the essential morphological elements required for regulated Ca(2+)-dependent transmitter release. The location of specific exocytotic sites on these cells, however, remains to be unequivocally determined.

Astrocyte calcium waves: what they are and what they do

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

Glutamate release from astrocytes is stimulated via the appearance of exocytosis during cyclic AMP-induced morphologic changes

Recent studies have shown that astrocytes release various transmitters including glutamate and thus directly affect synaptic neurotransmission. The mechanisms involved in the release of glutamate from astrocytes remain unclear, however. In the present study, we examined differences in 1) the amount of glutamate released, 2) the appearance of exocytosis, and 3) the expression of SNARE (soluble N-ethylmaleimide sensitive fusion protein attachment protein receptor) proteins between cyclic AMP-treated and non-treated astrocytes in culture. Extracellular glutamate was detected in the recording solution of cyclic AMP-treated astrocytes after stimulation with ATP by high-performance liquid chromatography and NADH imaging. Exocytosis, which was observed by FM1-43 imaging, appeared in cyclic AMP-treated astrocytes in a punctiform fashion, but not in non-treated cells, after stimulation with ATP and glutamate. Immunocytochemistry and Western blotting showed that the amount of SNARE proteins increased during cAMP-induced morphologic changes, and in particular, a v-SNARE, synaptobrevin, appeared as punctiform staining in the cytosol of cyclic AMP-treated astrocytes. These findings show that astrocytes acquire SNARE proteins during cyclic AMP-induced differentiation, and suggest that glutamate is released by exocytosis in cyclic AMP-treated astrocytes in response to ATP released from neighboring neurons and astrocytes.

Astrocyte control of synaptic transmission and neurovascular coupling

From a structural perspective, the predominant glial cell of the central nervous system, the astrocyte, is positioned to regulate synaptic transmission and neurovascular coupling: the processes of one astrocyte contact tens of thousands of synapses, while other processes of the same cell form endfeet on capillaries and arterioles. The application of subcellular imaging of Ca2+ signaling to astrocytes now provides functional data to support this structural notion. Astrocytes express receptors for many neurotransmitters, and their activation leads to oscillations in internal Ca2+. These oscillations induce the accumulation of arachidonic acid and the release of the chemical transmitters glutamate, d-serine, and ATP. Ca2+ oscillations in astrocytic endfeet can control cerebral microcirculation through the arachidonic acid metabolites prostaglandin E2 and epoxyeicosatrienoic acids that induce arteriole dilation, and 20-HETE that induces arteriole constriction. In addition to actions on the vasculature, the release of chemical transmitters from astrocytes regulates neuronal function. Astrocyte-derived glutamate, which preferentially acts on extrasynaptic receptors, can promote neuronal synchrony, enhance neuronal excitability, and modulate synaptic transmission. Astrocyte-derived d-serine, by acting on the glycine-binding site of the N-methyl-d-aspartate receptor, can modulate synaptic plasticity. Astrocyte-derived ATP, which is hydrolyzed to adenosine in the extracellular space, has inhibitory actions and mediates synaptic cross-talk underlying heterosynaptic depression. Now that we appreciate this range of actions of astrocytic signaling, some of the immediate challenges are to determine how the astrocyte regulates neuronal integration and how both excitatory (glutamate) and inhibitory signals (adenosine) provided by the same glial cell act in concert to regulate neuronal function.

Immunohistochemical analysis of subcortical white matter astroglia of infant and adult primate brains, with a note on resident neurons

An immunohistochemical analysis of brain subcortical white matter astroglia from human (infant, adult) and adult monkey (Cebus apella, Macaca nemestrina) cases without any known neurological disease, is described. Expression of synaptic vesicle-associated proteins, excitatory amino acid transporters (EAAT1 and EAAT2) and GABAA Ralpha2 receptor produced coarse punctate labeling in human adult white matter astrocytes. A finer, generalized, punctate labeling was observed in human infants and adult C. apella monkeys. Labeling of neuronal somata and processes with microtubule-associated proteins (MAP2a-c) and neuron nuclear (NeuN) antibodies, was also observed in subcortical white matter of humans and monkeys. Results suggest competence of subcortical white matter astroglia of the primate brain to participate in various transmitter regulatory pathways. It is also proposed that, collectively with resident neurons, they may exert some role in affecting the transfer of information that takes place through the various associational and projecting fiber systems coursing through this brain compartment.

Single molecule mechanical probing of the SNARE protein interactions

Exocytotic release of neurotransmitters is mediated by the ternary soluble N-ethyl maleimide-sensitive fusion protein attachment protein receptors (SNAREs) complex, comprised of syntaxin (Sx), synaptosome-associated protein of 25 kDa (SNAP25), and synaptobrevin 2 (Sb2). Since exocytosis involves the nonequilibrium process of association and dissociation of bonds between molecules of the SNARE complex, dynamic measurements at the single molecule level are necessary for a detailed understanding of these interactions. To address this issue, we used the atomic force microscope in force spectroscopy mode to show from single molecule investigations of the SNARE complex, that Sx1A and Sb2 are zippered throughout their entire SNARE domains without the involvement of SNAP25. When SNAP25B is present in the complex, it creates a local interaction at the 0 (ionic) layer by cuffing Sx1A and Sb2. Force loading rate studies indicate that the ternary complex interaction is more stable than the Sx1A-Sb2 interaction.

Vesicular glutamate transporter expression in supraoptic neurones suggests a glutamatergic phenotype

Magnocellular neuroendocrine cells of the supraoptic nucleus (SON) release the peptides oxytocin (OT) and vasopressin (VP) from their dendrites and terminals. In addition to peptide-containing large dense-core vesicles, axon terminals from these cells contain clear microvesicles that have been shown to contain glutamate. Using multilabelling confocal microscopy, we investigated the presence of vesicular glutamate transporters (VGLUTs) in astrocytes as well as VP and OT neurones of the SON. Simultaneous probing of the SON with antibodies against VGLUT isoforms 1-3, OT, VP and glial fibrillary acidic protein (GFAP) revealed the presence of VGLUT-2 in somata and dendrites of SON neurones. Immunoreactivity (-ir) for VGLUT-3 was also detected in both OT and VP neurones as well as in GFAP-ir astrocytes and other cells of the ventral glial lamina. Colocalisation of VGLUT-2 and VGLUT-3 in individual SON neurones was also examined and VGLUT-ir with both antibodies could be detected in both types of SON neurones. Although VGLUT-1-ir was strong lateral to the SON, only sparse labelling was apparent within the nucleus, and no colocalisation with either SON neurones or astrocytes was observed. The SON or the SON plus its surrounding perinuclear zone was probed using the reverse transcriptase-polymerase chain reaction and the presence of mRNA for all three VGLUT isoforms was detected. These results suggest that similar arrangements of transmitters exist in SON neuronal dendrites and their neurohypophysial terminals and that magnocellular neuroendocrine somata and dendrites may be capable of glutamatergic transmission.

Differential mechanisms of glutamate receptor regulation of SynGAP in cortical neurones

One prime candidate linking N-methyl-D-aspartate (NMDA) receptors to the regulation of the MAP kinase cascade is SynGAP, a negative regulator of Ras. In order to assess how a physiological stimulus can alter SynGAP activity, an appropriate whole cell system must be used and SynGAP must be specifically extracted from membranes whilst preserving the catalytic activity of the protein. Here, we have achieved this and studied the effect of NMDA/alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and kainate receptor stimulations on SynGAP activity in cortical neurones. Furthermore, we have examined the role of extracellular Ca2+, CaM kinase II and the PSD-95-NR2B subunit interaction in SynGAP activity regulation and propose a novel convergence of signalling between AMPA, kainate and NMDA receptors.

Glia re-sealed particles freshly prepared from adult rat brain are competent for exocytotic release of glutamate

Glial subcellular re-sealed particles (referred to as gliosomes here) were purified from rat cerebral cortex and investigated for their ability to release glutamate. Confocal microscopy showed that the glia-specific proteins glial fibrillary acidic protein (GFAP) and S-100, but not the neuronal proteins 95-kDa postsynaptic density protein (PSD-95), microtubule-associated protein 2 (MAP-2) and beta-tubulin III, were enriched in purified gliosomes. Furthermore, gliosomes exhibited labelling neither for integrin-alphaM nor for myelin basic protein, which are specific for microglia and oligodendrocytes respectively. The Ca2+ ionophore ionomycin (0.1-5 microm) efficiently stimulated the release of tritium from gliosomes pre-labelled with [3H]d-aspartate and of endogenous glutamate in a Ca(2+)-dependent and bafilomycin A1-sensitive manner, suggesting the involvement of an exocytotic process. Accordingly, ionomycin was found to induce a Ca(2+)-dependent increase in the vesicular fusion rate, when exocytosis was monitored with acridine orange. ATP stimulated [3H]d-aspartate release in a concentration- (0.1-3 mm) and Ca(2+)-dependent manner. The gliosomal fraction contained proteins of the exocytotic machinery [syntaxin-1, vesicular-associated membrane protein type 2 (VAMP-2), 23-kDa synaptosome-associated protein (SNAP-23) and 25-kDa synaptosome-associated protein (SNAP-25)] co-existing with GFAP immunoreactivity. Moreover, GFAP or VAMP-2 co-expressed with the vesicular glutamate transporter type 1. Consistent with ultrastructural analysis, several approximately 30-nm non-clustered vesicles were present in the gliosome cytoplasm. It is concluded that gliosomes purified from adult brain contain glutamate-accumulating vesicles and can release the amino acid by a process resembling neuronal exocytosis.

Synaptobrevin2-expressing vesicles in rat astrocytes: insights into molecular characterization, dynamics and exocytosis

The SNARE-dependent exocytosis of glutamate-containing vesicles in astrocytes is increasingly viewed as an important signal at the basis of the astrocyte-to-neurone communication system in the brain. Here we provide further insights into the molecular features and dynamics of vesicles in cultured astrocytes. We found that immunoisolated synaptobrevin2 vesicles are clear vesicles quite heterogenous in size and contain the vesicular glutamate transporter v-Glut-2. Moreover, they are immunopositive for synaptotagmin IV, for AMPA receptor subunits GluR2,3 and, to a lesser extent, for GluR1. We also provide direct evidence for the functional expression of v-Glut-2 in astrocytes and demonstrate that synaptobrevin2-positive vesicles can specifically take up (3H)L-glutamate via a bafilomycin-sensitive mechanism. Finally, by time lapse confocal microscopy, we show that a subpopulation of vesicles (tagged with a synaptobrevin2-EGFP chimera) is highly mobile and can fuse with the plasma membrane, preferentially at the level of the astrocyte processes, in a Ca2+-dependent manner. These latter observations, together with the evidence reported here for the expression of functional v-Glut-2 in synaptobrevin2-positive vesicles, provide a molecular basis for regulated exocytosis in astrocyte.

"Kiss-and-run" glutamate secretion in cultured and freshly isolated rat hippocampal astrocytes

Under physiological conditions, astrocytes not only passively support and nourish adjacent neurons, but also actively modulate neuronal transmission by releasing "glial transmitters," such as glutamate, ATP, and D-serine. Unlike the case for neurons, the mechanisms by which glia release transmitters are essentially unknown. Here, by using electrochemical amperometry and frequency-modulated single-vesicle imaging, we discovered that hippocampal astrocytes exhibit two modes of exocytosis of glutamate in response to various stimuli. After physiological stimulation, a glial vesicle releases a quantal content that is only 10% of that induced by nonphysiological, mechanical stimulation. The small release event arises from a brief (approximately 2 ms) opening of the fusion pore. We conclude that, after physiological stimulation, astrocytes release glutamate via a vesicular "kiss-and-run" mechanism.

Frequent coexpression of the vesicular glutamate transporter 1 and 2 genes, as well as coexpression with genes for choline acetyltransferase or glutamic acid decarboxylase in neurons of rat brain

It is widely believed that expression of the vesicular glutamate transporter genes VGLUT1 and VGLUT2 is restricted to glutamatergic neurons and that the two transporters segregate in different sets of neurons. Using single-cell multiplex RT-PCR (sc-RT-mPCR), we show that VGLUT1 and VGLUT2 mRNAs were coexpressed in most of the sampled neurons from the rat hippocampus, cortex, and cerebellum at postnatal Day (P)14 but not P60. In accordance, changes in VGLUT1 and VGLUT2 mRNA concentrations were found to occur in these and other brain areas between P14 and P60, as revealed by semiquantitative RT-PCR and quantitated by ribonuclease protection assay. VGLUT1 and -2 coexpression in the hippocampal formation is supported further by in situ hybridization data showing that virtually all cells in the CA1-CA3 pyramidal and granule cell layers were highly positive for both transcripts until P14. It was revealed using sc-RT-mPCR that transcripts for VGLUT1 and VGLUT2 were also present in neurons of the cerebellum, striatum, and septum that expressed markers for gamma-aminobutyric acid (GABA)ergic or cholinergic phenotypes, as well as in hippocampal cells containing transcripts for the glial fibrillary acidic protein. Our study suggests that VGLUT1 and VGLUT2 proteins may often transport glutamate into vesicles within the same neuron, especially during early postnatal development, and that they are expressed widely in presumed glutamatergic, GABAergic, and cholinergic neurons, as well as in astrocytes. Furthermore, our study shows that such coexpressing neurons remain in the adult brain and identifies several areas that contain them in both young and adult rats.

Vesicle mobility studied in cultured astrocytes

Astrocytes release many neuroactive substances, which are stored in membrane bound vesicles and may play a role in synapse modulation and in the coupling between neuronal activity and the local blood flow. However, the mobility of these vesicles in astrocytes has not been studied yet. We here used a fluorescently tagged proatrial natriuretic peptide to label single vesicles and dynamic microscopy to monitor their mobility. To track and analyze labeled vesicles, we employed a computer software. We found two modes of vesicle mobility, directional and non-directional. The mobility of non-directional vesicles is likely determined mainly by free diffusion. Only directional vesicles displayed a straight-line motion. The relationship of mean square displacement with time in directional vesicles resembled a quadratic function, indicating that in addition to free diffusion other mechanisms may contribute to vesicle movements in astrocytes, the biophysical properties of which are similar to those of neurons.

Enhanced release of synaptic glutamate underlies the potentiation of oxygen-glucose deprivation-induced neuronal injury after induction of NOS-2

Reactive nitrogen oxide species (RNOS) may contribute to the progression/enhancement of ischemic injury by augmentation of glutamate release, reduction of glutamate uptake, or a combination of both. Consistent with this, induction of nitric oxide synthase (NOS-2) in murine neocortical cell cultures potentiated neuronal cell death caused by combined oxygen-glucose deprivation in association with a net increase in extracellular glutamate accumulation. However, uptake of glutamate via high affinity, sodium-dependent glutamate transporters was unimpaired by induction of NOS-2 under either aerobic or anaerobic conditions. Further, blocking possible routes of extra-synaptic glutamate release with NPPB [5-nitro-2-(3-phenylpropylamino)-benzoic acid], a volume-sensitive organic anion channel blocker, or TBOA (d,l-threo-beta-benzyloxyaspartate), an inhibitor of glutamate transport, exacerbated rather than ameliorated injury. Finally, treatment with riluzole or tetanus toxin attenuated the enhancement in both glutamate accumulation and oxygen-glucose deprivation-induced neuronal injury supporting the idea that increased synaptic release of glutamate underlies, at least in part, the potentiation of neuronal injury by RNOS after NOS-2 induction.

ATP- and adenosine-mediated signaling in the central nervous system: adenosine stimulates glutamate release from astrocytes via A2a adenosine receptors

Adenosine enhanced intracellular Ca(2+) concentrations in astrocytes via A(2a) adenosine receptors involving protein kinase A (PKA) activation. The Ca(2+) rise is inhibited by brefeldin A, an inhibitor of vesicular transport; but not by neomycin and U73122, phospholipase C inhibitors; xestospongin, an IP(3)-receptor inhibitor; ryanodine, a ryanodine-receptor inhibitor; TMB-8, an endoplasmic reticulum calcium-release blocker; octanol, a gap-junction inhibitor; or cadmium, a non-selective, calcium-channel blocker. Adenosine stimulates astrocytic glutamate release via an A(2a) adenosine receptors/PKA pathway, and the release is inhibited by the vesicular transport inhibitors brefeldin A and bafilomycin A1. A(2a) adenosine receptors and the ensuing PKA events, thus, are endowed with vesicular Ca(2+) release from an unknown intracellular calcium store and vesicular glutamate release from astrocytes.

Mechanisms of glutamate release from astrocytes: gap junction "hemichannels", purinergic receptors and exocytotic release

Neuronal exocytotic release of glutamate at synapses involves a highly specialized vesicular apparatus, consisting of a variety of proteins connected to the vesicles or required for vesicular fusion to the presynaptic membrane. Astrocytes also release glutamate, and recent evidence indicates that this release can modify neuronal function. Several mechanisms have been proposed for astrocytic release of glutamate under pathological conditions, such as reversal of glutamate transporters and opening of volume sensitive ion channels. In this review we limit our discussion to findings supporting the exocytotic release of glutamate, as well as two new pathways implicated in this release, the ionotropic (P2X) purinergic receptors and gap junction hemichannels.

Neurone-to-astrocyte signalling in the brain represents a distinct multifunctional unit

Astrocytes can respond to neurotransmitters released at the synapse by generating elevations in intracellular Ca(2+) concentration ([Ca(2+)](i)) and releasing glutamate that signals back to neurones. This discovery opens new perspectives for the possible participation of these glial cells in actual information processing by the brain and raises the hypothesis that astrocyte activation by neuronal signals plays a key role in distinct, functional events. Depending on the level of neuronal activity, the [Ca(2+)](i) response that is activated by neurotransmitters can either remain restricted to an astrocytic process or it can propagate as an intracellular [Ca(2+)](i) wave to other astrocytic processes in contact with different neurones, astrocytes, microglia or endothelial cells of cerebral arterioles. Glutamate release triggered by the [Ca(2+)](i) rise at the astrocytic process represents a feedback, short-distance signal that affects synaptic transmission locally. The release of glutamate as well as of other compounds far away from the site of initial activation represents a feedforward, long-distance signal that can be involved in the regulation of distinct processes. For instance, through the release of vasoactive molecules from the astrocytic processes in contact with cerebral arterioles, the neurone-astrocyte-endothelial cell signalling pathway plays a pivotal role in the neuronal control of vascular tone. In this article we will review recent results that should persuade us to reshape our current thinking on the roles of astroglial cells in the brain. We propose that neurones and astrocytes represent an integral unit that has a distinctive role in different fundamental events in brain function. Furthermore, while recent findings provide important evidences for the vesicular hypothesis of glutamate release, we discuss also the proposals for a possible physiological role of hemichannels and purinergic P2X(7) receptors in glutamate release from astrocytes. A full clarification of the functional significance of the bidirectional communication that astrocytes establish with neurones as well as with other brain cells represents one of the most intriguing challenges in neurobiological research at the moment and should fuel stimulating debates in years to come.

Oriented astroglial cell growth on micropatterned polystyrene substrates

In an effort to develop a permissive environment for neural stem cell differentiation, directional growth of astrocytes has been achieved on polymer substrates in vitro. Manipulating a combination of physical and chemical cues, astrocyte adhesion and alignment in vitro were examined. To provide physical guidance, micropatterned polymer substrates of polystyrene (PS) were fabricated. Laminin was selectively adsorbed onto the grooves of the patterned surface. Rat type-1 astrocytes were seeded onto the micropatterned PS substrates, and the effects of substrate topography and the adsorption of laminin to the PS substrates on the behavior and morphology of the astrocytes were explored. The astrocytes were found to align parallel to the micropatterned grooves at initial seeding densities of approximately 7500, 13,000, and 20,000 cells/cm(2) due to the effects of the physical and chemical guidance mechanisms. Adsorbing laminin in the microgrooves of the micropatterned PS substrates improved cell adhesion and spreading of cytoskeletal filaments significantly. At these initial seeding densities, over 85% astrocyte alignment in the direction of the grooves was achieved on the micropatterned PS substrates with laminin adsorbed in the grooves. This combination of guidance cues has the potential to provide a permissive substrate for in vivo regeneration within the central nervous system.