Long-term potentiation depends on release of D-serine from astrocytes

Methylphenidate administration determines enduring changes in neuroglial network in rats

Repeated exposure to psychostimulant drugs induces complex molecular and structural modifications in discrete brain regions of the meso-cortico-limbic system. This structural remodeling is thought to underlie neurobehavioral adaptive responses. Administration to adolescent rats of methylphenidate (MPH), commonly used in attention deficit and hyperactivity disorder (ADHD), triggers alterations of reward-based behavior paralleled by persistent and plastic synaptic changes of neuronal and glial markers within key areas of the reward circuits. By immunohistochemistry, we observe a marked increase of glial fibrillary acidic protein (GFAP) and neuronal nitric oxide synthase (nNOS) expression and a down-regulation of glial glutamate transporter GLAST in dorso-lateral and ventro-medial striatum. Using electron microscopy, we find in the prefrontal cortex a significant reduction of the synaptic active zone length, paralleled by an increase of dendritic spines. We demonstrate that in limbic areas the MPH-induced reactive astrocytosis affects the glial glutamatergic uptake system that in turn could determine glutamate receptor sensitization. These processes could be sustained by NO production and synaptic rearrangement and contribute to MPH neuroglial induced rewiring.

Bidirectional coupling between astrocytes and neurons mediates learning and dynamic coordination in the brain: a multiple modeling approach

In recent years research suggests that astrocyte networks, in addition to nutrient and waste processing functions, regulate both structural and synaptic plasticity. To understand the biological mechanisms that underpin such plasticity requires the development of cell level models that capture the mutual interaction between astrocytes and neurons. This paper presents a detailed model of bidirectional signaling between astrocytes and neurons (the astrocyte-neuron model or AN model) which yields new insights into the computational role of astrocyte-neuronal coupling. From a set of modeling studies we demonstrate two significant findings. Firstly, that spatial signaling via astrocytes can relay a "learning signal" to remote synaptic sites. Results show that slow inward currents cause synchronized postsynaptic activity in remote neurons and subsequently allow Spike-Timing-Dependent Plasticity based learning to occur at the associated synapses. Secondly, that bidirectional communication between neurons and astrocytes underpins dynamic coordination between neuron clusters. Although our composite AN model is presently applied to simplified neural structures and limited to coordination between localized neurons, the principle (which embodies structural, functional and dynamic complexity), and the modeling strategy may be extended to coordination among remote neuron clusters.

Activity-induced remodeling of olfactory bulb microcircuits revealed by monosynaptic tracing

The continued addition of new neurons to mature olfactory circuits represents a remarkable mode of cellular and structural brain plasticity. However, the anatomical configuration of newly established circuits, the types and numbers of neurons that form new synaptic connections, and the effect of sensory experience on synaptic connectivity in the olfactory bulb remain poorly understood. Using in vivo electroporation and monosynaptic tracing, we show that postnatal-born granule cells form synaptic connections with centrifugal inputs and mitral/tufted cells in the mouse olfactory bulb. In addition, newly born granule cells receive extensive input from local inhibitory short axon cells, a poorly understood cell population. The connectivity of short axon cells shows clustered organization, and their synaptic input onto newborn granule cells dramatically and selectively expands with odor stimulation. Our findings suggest that sensory experience promotes the synaptic integration of new neurons into cell type-specific olfactory circuits.

Early astrocytic atrophy in the entorhinal cortex of a triple transgenic animal model of Alzheimer's disease

The EC (entorhinal cortex) is fundamental for cognitive and mnesic functions. Thus damage to this area appears as a key element in the progression of AD (Alzheimer's disease), resulting in memory deficits arising from neuronal and synaptic alterations as well as glial malfunction. In this paper, we have performed an in-depth analysis of astroglial morphology in the EC by measuring the surface and volume of the GFAP (glial fibrillary acidic protein) profiles in a triple transgenic mouse model of AD [3xTg-AD (triple transgenic mice of AD)]. We found significant reduction in both the surface and volume of GFAP-labelled profiles in 3xTg-AD animals from very early ages (1 month) when compared with non-Tg (non-transgenic) controls (48 and 54%, reduction respectively), which was sustained for up to 12 months (33 and 45% reduction respectively). The appearance of Aβ (amyloid β-peptide) depositions at 12 months of age did not trigger astroglial hypertrophy; nor did it result in the close association of astrocytes with senile plaques. Our results suggest that the AD progressive cognitive deterioration can be associated with an early reduction of astrocytic arborization and shrinkage of the astroglial domain, which may affect synaptic connectivity within the EC and between the EC and other brain regions. In addition, the EC seems to be particularly vulnerable to AD pathology because of the absence of evident astrogliosis in response to Aβ accumulation. Thus we can consider that targeting astroglial atrophy may represent a therapeutic strategy which might slow down the progression of AD.

Microglia activation triggers astrocyte-mediated modulation of excitatory neurotransmission

Fine control of neuronal activity is crucial to rapidly adjust to subtle changes of the environment. This fine tuning was thought to be purely neuronal until the discovery that astrocytes are active players of synaptic transmission. In the adult hippocampus, microglia are the other major glial cell type. Microglia are highly dynamic and closely associated with neurons and astrocytes. They react rapidly to modifications of their environment and are able to release molecules known to control neuronal function and synaptic transmission. Therefore, microglia display functional features of synaptic partners, but their involvement in the regulation of synaptic transmission has not yet been addressed. We have used a combination of pharmacological approaches with electrophysiological analysis on acute hippocampal slices and ATP assays in purified cell cultures to show that activation of microglia induces a rapid increase of spontaneous excitatory postsynaptic currents. We found that this modulation is mediated by binding of ATP to P2Y1R located on astrocytes and is independent of TNFα or NOS2. Our data indicate that, on activation, microglia cells rapidly release small amounts of ATP, and astrocytes, in turn, amplified this release. Finally, P2Y1 stimulation of astrocytes increased excitatory postsynaptic current frequency through a metabotropic glutamate receptor 5-dependent mechanism. These results indicate that microglia are genuine regulators of neurotransmission and place microglia as upstream partners of astrocytes. Because pathological activation of microglia and alteration of neurotransmission are two early symptoms of most brain diseases, our work also provides a basis for understanding synaptic dysfunction in neuronal diseases.

Lactate produced by glycogenolysis in astrocytes regulates memory processing

When administered either systemically or centrally, glucose is a potent enhancer of memory processes. Measures of glucose levels in extracellular fluid in the rat hippocampus during memory tests reveal that these levels are dynamic, decreasing in response to memory tasks and loads; exogenous glucose blocks these decreases and enhances memory. The present experiments test the hypothesis that glucose enhancement of memory is mediated by glycogen storage and then metabolism to lactate in astrocytes, which provide lactate to neurons as an energy substrate. Sensitive bioprobes were used to measure brain glucose and lactate levels in 1-sec samples. Extracellular glucose decreased and lactate increased while rats performed a spatial working memory task. Intrahippocampal infusions of lactate enhanced memory in this task. In addition, pharmacological inhibition of astrocytic glycogenolysis impaired memory and this impairment was reversed by administration of lactate or glucose, both of which can provide lactate to neurons in the absence of glycogenolysis. Pharmacological block of the monocarboxylate transporter responsible for lactate uptake into neurons also impaired memory and this impairment was not reversed by either glucose or lactate. These findings support the view that astrocytes regulate memory formation by controlling the provision of lactate to support neuronal functions.

TRPA1 channels regulate astrocyte resting calcium and inhibitory synapse efficacy through GAT-3

Astrocytes contribute to the formation and function of synapses and are found throughout the brain, where they show intracellular store-mediated Ca(2+) signals. Here, using a membrane-tethered, genetically encoded calcium indicator (Lck-GCaMP3), we report the serendipitous discovery of a new type of Ca(2+) signal in rat hippocampal astrocyte-neuron cocultures. We found that Ca(2+) fluxes mediated by transient receptor potential A1 (TRPA1) channels gave rise to frequent and highly localized 'spotty' Ca(2+) microdomains near the membrane that contributed appreciably to resting Ca(2+) in astrocytes. Mechanistic evaluations in brain slices showed that decreases in astrocyte resting Ca(2+) concentrations mediated by TRPA1 channels decreased interneuron inhibitory synapse efficacy by reducing GABA transport by GAT-3, thus elevating extracellular GABA. Our data show how a transmembrane Ca(2+) source (TRPA1) targets a transporter (GAT-3) in astrocytes to regulate inhibitory synapses.

Secreted amyloid precursor proteins promote proliferation and glial differentiation of adult hippocampal neural progenitor cells

Amyloid precursor protein (APP) is an integral membrane glycoprotein present at high levels in nerve cells. Two soluble secreted forms, sAPPα and sAPPβ, are processed from APP by two mutually exclusive proteolytic pathways. sAPPα shows a range of neuroprotective and growth factor properties, including reduction of neuronal injury and improvement in memory performance, in contrast to the generally less potent sAPPβ. In addition, sAPPα has been shown to increase the proliferation of both embryonic neural stem cells and neural progenitor cells (NPCs) derived from the subventricular zone (SVZ) of the adult brain. However, an effect of sAPPα (or sAPPβ) on adult hippocampal progenitor cell proliferation and differentiation has not previously been observed. In this study, we examined the effect of both the α- and β-cleaved ectodomains of sAPP on adult NPCs isolated from the subgranular zone (SGZ) of the rat hippocampus in the presence or absence of depolarizing conditions. Assays were performed to examine the effect of sAPPα and sAPPβ on SGZ-derived adult NPC proliferation in parallel with SVZ-derived cells and on differentiation with SGZ-derived cells. We observed both sAPPα and sAPPβ increased the proliferation of SGZ-derived NPCs in vitro. Further, treatment of SGZ-derived NPCs with either sAPPα or sAPPβ increased the number of cells expressing the astrocytic marker GFAP and promoted cell survival. The effect on differential fate was observed in both the presence and absence of depolarizing conditions. Thus, both sAPPα and sAPPβ exert a complex range of effects on SGZ-derived adult NPCs, including increasing NPC proliferation, maintaining cell viability, yet promoting glial over neuronal differentiation. These findings provide the first direct support for the secreted forms of APP regulating SGZ-derived NPCs, and raise the possibility some or all of the effects may have therapeutic benefit in models of neurological disease.

Effects of low-dose D-serine on recognition and working memory in mice

RATIONALE

D -Serine is an endogenous co-agonist of the N-methyl-D: -aspartate (NMDA) receptor and has been suggested to improve cognitive deficits in schizophrenia.

OBJECTIVES

The present study investigates the effects of treatment with D -serine in mice on tasks that require recognition learning and working memory, two cognitive domains that are impaired in schizophrenia.

METHODS

We studied the effects of various regimens of systemic administration of D -serine (50 mg/kg/day) on BALB/c mice performing object recognition, T-maze alternation, and open-field exploration tasks. For the object recognition task, we also contrasted the effects of D -serine and D -cycloserine and investigated whether D -serine could reverse alterations induced by subchronic injections of the NMDA antagonist MK-801. D -Serine levels after injections were measured by high-performance liquid chromatography.

RESULTS

In the object recognition task, pre-training treatment with D -serine or D -cycloserine significantly enhanced recognition memory 24 h after training. A single administration of D -serine 30 min (but not 6 h) after training produced similar enhancement, suggesting an effect on memory consolidation. Daily treatment with D: -serine enhanced both object recognition and T-maze performance over multiple days and improved short-term memory in MK-801-treated mice. D -Serine treatment did not alter open-field exploration. Behavioral effects were accompanied by increased levels of D -serine in the hippocampus of treated animals.

CONCLUSIONS

Our results show that treatment with D -serine can improve performance in tasks related to recognition learning and working memory, suggesting that this agent can be useful for the treatment of disorders involving declines in these cognitive domains.

A tale of two stories: astrocyte regulation of synaptic depression and facilitation

Short-term presynaptic plasticity designates variations of the amplitude of synaptic information transfer whereby the amount of neurotransmitter released upon presynaptic stimulation changes over seconds as a function of the neuronal firing activity. While a consensus has emerged that the resulting decrease (depression) and/or increase (facilitation) of the synapse strength are crucial to neuronal computations, their modes of expression in vivo remain unclear. Recent experimental studies have reported that glial cells, particularly astrocytes in the hippocampus, are able to modulate short-term plasticity but the mechanism of such a modulation is poorly understood. Here, we investigate the characteristics of short-term plasticity modulation by astrocytes using a biophysically realistic computational model. Mean-field analysis of the model, supported by intensive numerical simulations, unravels that astrocytes may mediate counterintuitive effects. Depending on the expressed presynaptic signaling pathways, astrocytes may globally inhibit or potentiate the synapse: the amount of released neurotransmitter in the presence of the astrocyte is transiently smaller or larger than in its absence. But this global effect usually coexists with the opposite local effect on paired pulses: with release-decreasing astrocytes most paired pulses become facilitated, namely the amount of neurotransmitter released upon spike i+1 is larger than that at spike i, while paired-pulse depression becomes prominent under release-increasing astrocytes. Moreover, we show that the frequency of astrocytic intracellular Ca(2+) oscillations controls the effects of the astrocyte on short-term synaptic plasticity. Our model explains several experimental observations yet unsolved, and uncovers astrocytic gliotransmission as a possible transient switch between short-term paired-pulse depression and facilitation. This possibility has deep implications on the processing of neuronal spikes and resulting information transfer at synapses.

Eph receptors at synapses: implications in neurodegenerative diseases

Precise regulation of synapse formation, maintenance and plasticity is crucial for normal cognitive function, and synaptic failure has been suggested as one of the hallmarks of neurodegenerative diseases. In this review, we describe the recent progress in our understanding of how the receptor tyrosine kinase Ephs and their ligands ephrins regulate dendritic spine morphogenesis, synapse formation and maturation, as well as synaptic plasticity. In particular, we discuss the emerging evidence implicating that deregulation of Eph/ephrin signaling contributes to the aberrant synaptic functions associated with cognitive impairment in Alzheimer's disease. Understanding how Eph/ephrin regulates synaptic function may therefore provide new insights into the development of therapeutic agents against neurodegenerative diseases.

Astrocytes are endogenous regulators of basal transmission at central synapses

Basal synaptic transmission involves the release of neurotransmitters at individual synapses in response to a single action potential. Recent discoveries show that astrocytes modulate the activity of neuronal networks upon sustained and intense synaptic activity. However, their ability to regulate basal synaptic transmission remains ill defined and controversial. Here, we show that astrocytes in the hippocampal CA1 region detect synaptic activity induced by single-synaptic stimulation. Astrocyte activation occurs at functional compartments found along astrocytic processes and involves metabotropic glutamate subtype 5 receptors. In response, astrocytes increase basal synaptic transmission, as revealed by the blockade of their activity with a Ca(2+) chelator. Astrocytic modulation of basal synaptic transmission is mediated by the release of purines and the activation of presynaptic A(2A) receptors by adenosine. Our work uncovers an essential role for astrocytes in the regulation of elementary synaptic communication and provides insight into fundamental aspects of brain function.

Serine racemase deletion abolishes light-evoked NMDA receptor currents in retinal ganglion cells

Glycine and/or D-serine are obligatory coagonists of the N-methyl-D-aspartate receptor (NMDAR). Serine racemase, the D-serine-synthesizing enzyme, is expressed by astrocytes and Müller cells of the retina, but little is known about its role in retinal signalling. In this study, we utilize a serine racemase knockout (SRKO) mouse to explore the contribution of D-serine to inner-retinal function. Retinal tissue levels of D-serine in SRKO mice are reduced by 85%. Whole-cell recordings from SRKO retinal ganglion cells showed markedly reduced coagonist occupancy of NMDARs and consequently a dramatic reduction in the NMDAR component of light-evoked responses. NMDAR currents in SRKOs could be rescued by applying exogenous coagonist, but SRKO ganglion cells still displayed lower NMDA/AMPA receptor ratios than wild-type (WT) controls when the coagonist site was saturated. Despite having abnormalities in synaptic glutamatergic transmission, SRKO mice displayed no obvious signs of visual impairment in behavioural testing. These findings raise interesting questions about the role of D-serine in inner-retinal function and development.

Basal synaptic transmission: astrocytes rule!

In this issue, Panatier et al. (2011) show that astrocytes detect synaptic activity induced by single action potentials and upregulate basal synaptic transmission through calcium-dependent mechanisms and purinergic signaling. These results demonstrate the relevance of astrocyte calcium in neurophysiology and confirm that astrocytes are actively involved in synaptic function.

Determinants of functional coupling between astrocytes and respiratory neurons in the pre-Bötzinger complex

Respiratory neuronal network activity is thought to require efficient functioning of astrocytes. Here, we analyzed neuron-astrocyte communication in the pre-Bötzinger Complex (preBötC) of rhythmic slice preparations from neonatal mice. In astrocytes that exhibited rhythmic potassium fluxes and glutamate transporter currents, we did not find a translation of respiratory neuronal activity into phase-locked astroglial calcium signals. In up to 20% of astrocytes, 2-photon calcium imaging revealed spontaneous calcium fluctuations, although with no correlation to neuronal activity. Calcium signals could be elicited in preBötC astrocytes by metabotropic glutamate receptor activation or after inhibition of glial glutamate uptake. In the latter case, astrocyte calcium elevation preceded a surge of respiratory neuron discharge activity followed by network failure. We conclude that astrocytes do not exhibit respiratory-rhythmic calcium fluctuations when they are able to prevent synaptic glutamate accumulation. Calcium signaling is, however, observed when glutamate transport processes in astrocytes are suppressed or neuronal discharge activity is excessive.

Calcium signals in the nucleus accumbens: activation of astrocytes by ATP and succinate

Background

Accumulating evidence suggests that glial signalling is activated by different brain functions. However, knowledge regarding molecular mechanisms of activation or their relation to neuronal activity is limited. The purpose of the present study is to identify the characteristics of ATP-evoked glial signalling in the brain reward area, the nucleus accumbens (NAc), and thereby to explore the action of citric acid cycle intermediate succinate (SUC).

Results

We described the burst-like propagation of Ca²⁺ transients evoked by ATP in acute NAc slices from rat brain. Co-localization of the ATP-evoked Ca²⁺ signalling with immunoreactivities of the astroglia-specific gap junction forming channel protein connexin43 (Cx43) and the glial fibrillary acidic protein (GFAP) indicated that the responsive cells were a subpopulation of Cx43 and GFAP immunoreactive astrocytes. The ATP-evoked Ca²⁺ transients were present under the blockade of neuronal activity, but were inhibited by Ca²⁺ store depletion and antagonism of the G protein coupled purinergic P2Y₁ receptor subtype-specific antagonist MRS2179. Similarly, Ca²⁺ transients evoked by the P2Y₁ receptor subtype-specific agonist 2-(Methylthio)adenosine 5'-diphosphate were also blocked by MRS2179. These characteristics implied that intercellular Ca²⁺ signalling originated from the release of Ca²⁺ from internal stores, triggered by the activation of P2Y₁ receptors. Inhibition by the gap junction blockers carbenoxolone and flufenamic acid and by an antibody raised against the gating-associated segment of Cx43 suggested that intercellular Ca²⁺ signalling proceeded through gap junctions. We demonstrated for the first time that extracellular SUC also evoked Ca²⁺ transients (EC₅₀ = 50-60 μM) in about 15% of the ATP-responsive NAc astrocytes. By contrast to glial cells, electrophysiologically identified NAc neurons surrounded by ATP-responsive astrocytes were not activated simultaneously.

Conclusions

We concluded, therefore, that ATP- and SUC-sensitive Ca²⁺ transients appear to represent a signalling layer independent of NAc neurons. This previously unrecognised glial action of SUC, a major cellular energy metabolite, may play a role in linking metabolism to Ca²⁺ signalling in astrocytic networks under physiological and pathological conditions such as exercise and metabolic diseases.

Astrocytes as regulators of synaptic function: a quest for the Ca2+ master key

The emerging role of astrocytes in neural communication represents a conceptual challenge. In striking contrast to the rapid and highly space- and time-constrained machinery of neuronal spike propagation and synaptic release, astroglia appear slow and imprecise. Although a large body of independent experiments documents active signal exchange between astrocytes and neurons, some genetic models have raised doubts about the major Ca2+ -dependent molecular mechanism routinely associated with release of "gliotransmitters." A limited understanding of astrocytic Ca2+ signaling and the imperfect compatibility between physiology and experimental manipulations seem to have contributed to this conceptual bottleneck. Experimental approaches providing mechanistic insights into the diverse mechanisms of intra-astrocyte Ca2+ signaling on the nanoscale are needed to understand Ca2+ -dependent astrocytic function in vivo. This review highlights limitations and potential advantages of such approaches from the current methodological perspective.

Vagal afferent stimulation activates astrocytes in the nucleus of the solitary tract via AMPA receptors: evidence of an atypical neural-glial interaction in the brainstem

The nucleus of the solitary tract (NST), located in the dorsomedial medulla, is the site of visceral sensory modulation of a variety of homeostatic reflexes. Given recent advancements in the understanding of active regulation of synaptic information flow by astrocytes, we sought to determine whether afferent sensory inputs to NST neurons also activates NST astrocytes. Using confocal, live-cell calcium imaging of brainstem slices, we investigated the possibility that stimulation of vagal sensory afferents, the major sensory input into the NST, activated NST astrocytes, as indicated by increases in astrocytic intracellular calcium concentrations ([Ca²⁺](i)). Astrocytes and neurons were preloaded with the calcium reporter dye Calcium Green, and astrocytes were selectively stained by sulforhodamine 101. Electrical stimulation of vagal afferent axons produced rapid increases in [Ca²⁺](i) in NST astrocytes as well as neurons. Surprisingly, this effect on astrocytes was blocked by the AMPA receptor antagonist NBQX and was unaffected by antagonism of NMDA and metabotropic glutamate receptors. Bath application of AMPA also activated astrocytes. This activation was dependent on extracellular Ca²⁺ influx through both typical AMPA receptors and calcium-permeable AMPA receptors. This AMPA-mediated Ca²⁺ influx was further amplified by actions of the ryanodine receptor by way of calcium-induced calcium release. Our immunohistochemical staining of NST cells further verified the presence of the AMPAR subunit GluR1 on astrocytes. These observations suggest that NST astrocytes may be active participants in the regulation of autonomic reflexes even in the normal, healthy state.

A protein synthesis and nitric oxide-dependent presynaptic enhancement in persistent forms of long-term potentiation

Long-term potentiation (LTP) is an important process underlying learning and memory in the brain. At CA3-CA1 synapses in the hippocampus, three discrete forms of LTP (LTP1, 2, and 3) can be differentiated on the basis of maintenance and induction mechanisms. However, the relative roles of pre- and post-synaptic expression mechanisms in LTP1, 2, and 3 are unknown. Neurotransmitter release in the expression of LTP1, 2, and 3 was measured via FM 1-43 destaining from CA3 terminals in hippocampal slices from male Wistar rats (7-8 wk). No difference in vesicle turnover rate was observed for LTP1 up to 160 min following induction by one train of theta-burst stimulation (1TBS). A presynaptic enhancement was found for LTP2 at 160 min after induction by 4TBS, and for LTP3 at both 80 and 160 min after induction by 8TBS. Inhibition of nitric oxide (NO) signaling blocked both LTP2 and LTP3 maintenance and the associated enhanced release. LTP2 maintenance and its presynaptic expression were dependent on protein synthesis, but not gene transcription. LTP3 maintenance was dependent on both translation and transcription, but like LTP2, the enhanced release only required translation. These data considerably strengthen the mechanistic separation of LTP1, 2, and 3, supporting a model of multiple, discrete forms of LTP at CA3-CA1 synapses rather than different temporal phases.

Calcium triggers exocytosis from two types of organelles in a single astrocyte

Astrocytes release a variety of signaling molecules including glutamate, D-serine, and ATP in a regulated manner. Although the functions of these molecules, from regulating synaptic transmission to controlling specific behavior, are well documented, the identity of their cellular compartment(s) is still unclear. Here we set out to study vesicular exocytosis and glutamate release in mouse hippocampal astrocytes. We found that small vesicles and lysosomes coexisted in the same freshly isolated or cultured astrocytes. Both small vesicles and lysosome fused with the plasma membrane in the same astrocytes in a Ca(2+)-regulated manner, although small vesicles were exocytosed more efficiently than lysosomes. Blockade of the vesicle glutamate transporter or cleavage of synaptobrevin 2 and cellubrevin (both are vesicle-associated membrane proteins) with a clostridial toxin greatly inhibited glutamate release from astrocytes, while lysosome exocytosis remained intact. Thus, both small vesicles and lysosomes contribute to Ca(2+)-dependent vesicular exocytosis, and small vesicles support glutamate release from astrocytes.

Presynaptic ionotropic receptors controlling and modulating the rules for spike timing-dependent plasticity

Throughout life, activity-dependent changes in neuronal connection strength enable the brain to refine neural circuits and learn based on experience. In line with predictions made by Hebb, synapse strength can be modified depending on the millisecond timing of action potential firing (STDP). The sign of synaptic plasticity depends on the spike order of presynaptic and postsynaptic neurons. Ionotropic neurotransmitter receptors, such as NMDA receptors and nicotinic acetylcholine receptors, are intimately involved in setting the rules for synaptic strengthening and weakening. In addition, timing rules for STDP within synapses are not fixed. They can be altered by activation of ionotropic receptors located at, or close to, synapses. Here, we will highlight studies that uncovered how network actions control and modulate timing rules for STDP by activating presynaptic ionotropic receptors. Furthermore, we will discuss how interaction between different types of ionotropic receptors may create "timing" windows during which particular timing rules lead to synaptic changes.

Rescue of synaptic plasticity and spatial learning deficits in the hippocampus of Homer1 knockout mice by recombinant Adeno-associated viral gene delivery of Homer1c

Homer1 belongs to a family of scaffolding proteins that interact with various post-synaptic density proteins including group I metabotropic glutamate receptors (mGluR1/5). Previous research in our laboratory implicates the Homer1c isoform in spatial learning. Homer1 knockout mice (H1-KO) display cognitive impairments, but their synaptic plasticity properties have not been described. Here, we investigated the role of Homer1 in long-term potentiation (LTP) in the hippocampal CA1 region of H1-KO mice in vitro. We found that late-phase LTP elicited by high frequency stimulation (HFS) was impaired, and that the induction and maintenance of theta burst stimulation (TBS) LTP were reduced in H1-KO. To test the hypothesis that Homer1c was sufficient to rescue these LTP deficits, we delivered Homer1c to the hippocampus of H1-KO using recombinant adeno-associated virus (rAAV). We found that rAAV-Homer1c rescued HFS and TBS-LTP in H1-KO animals. Next, we tested whether the LTP rescue by Homer1c was occurring via mGluR1/5. A selective mGluR5 antagonist, but not an mGluR1 antagonist, blocked the Homer1c-induced recovery of late-LTP, suggesting that Homer1c mediates functional effects on plasticity via mGluR5. To investigate the role of Homer1c in spatial learning, we injected rAAV-Homer1c to the hippocampus of H1-KO. We found that rAAV-Homer1c significantly improved H1-KO performance in the Radial Arm Water Maze. These results point to a significant role for Homer1c in synaptic plasticity and learning.

Immunogold detection of L-glutamate and D-serine in small synaptic-like microvesicles in adult hippocampal astrocytes

Glutamate and the N-methyl-D-aspartate receptor ligand D-serine are putative gliotransmitters. Here, we show by immunogold cytochemistry of the adult hippocampus that glutamate and D-serine accumulate in synaptic-like microvesicles (SLMVs) in the perisynaptic processes of astrocytes. The estimated concentration of fixed glutamate in the astrocytic SLMVs is comparable to that in synaptic vesicles of excitatory nerve terminals (≈ 45 and ≈ 55 mM, respectively), whereas the D-serine level is about 6 mM. The vesicles are organized in small spaced clusters located near the astrocytic plasma membrane. Endoplasmic reticulum is regularly found in close vicinity to SLMVs, suggesting that astrocytes contain functional nanodomains, where a local Ca(2+) increase can trigger release of glutamate and/or D-serine.

Radial glia: progenitor, pathway, and partner

Radial glia (RG) are a glial cell type that can be found from the earliest stages of CNS development. They are clearly identifiable by their unique morphology, having a periventricular cell soma and a long process extending all the way to the opposite pial surface. Due to this striking morphology, RG have long been thought of as a transient substrate for neuron migration in the developing brain. In fact, RG cells, far from exclusively serving as a passive scaffold for cell migration, have a remarkably diverse range of critical functions in CNS development and function. These include serving as progenitors of neurons and glia both during development as well as in response to injury, helping to direct axonal and dendritic process outgrowth, and regulating synaptic development and function. RG also engage in extensive bidirectional signaling both with neurons and one another. This review describes the diversity of RG cell types in the CNS and discusses their many important activities.

Local Ca2+ detection and modulation of synaptic release by astrocytes

Astrocytes communicate with synapses by means of intracellular calcium ([Ca(2+)](i)) elevations, but local calcium dynamics in astrocytic processes have never been thoroughly investigated. By taking advantage of high-resolution two-photon microscopy, we identify the characteristics of local astrocyte calcium activity in the adult mouse hippocampus. Astrocytic processes showed intense activity, triggered by physiological transmission at neighboring synapses. They encoded synchronous synaptic events generated by sparse action potentials into robust regional (∼12 μm) [Ca(2+)](i) elevations. Unexpectedly, they also sensed spontaneous synaptic events, producing highly confined (∼4 μm), fast (millisecond-scale) miniature Ca(2+) responses. This Ca(2+) activity in astrocytic processes is generated through GTP- and inositol-1,4,5-trisphosphate-dependent signaling and is relevant for basal synaptic function. Thus, buffering astrocyte [Ca(2+)](i) or blocking a receptor mediating local astrocyte Ca(2+) signals decreased synaptic transmission reliability in minimal stimulation experiments. These data provide direct evidence that astrocytes are integrated in local synaptic functioning in adult brain.

Astrocytes promote peripheral nerve injury-induced reactive synaptogenesis in the neonatal CNS

Neonatal damage to the trigeminal nerve leads to "reactive synaptogenesis" in the brain stem sensory trigeminal nuclei. In vitro models of brain injury-induced synaptogenesis have implicated an important role for astrocytes. In this study we tested the role of astrocyte function in reactive synaptogenesis in the trigeminal principal nucleus (PrV) of neonatal rats following unilateral transection of the infraorbital (IO) branch of the trigeminal nerve. We used electrophysiological multiple input index analysis (MII) to estimate the number of central trigeminal afferent fibers that converge onto single barrelette neurons. In the developing PrV, about 30% of afferent connections are eliminated within 2 postnatal weeks. After neonatal IO nerve damage, multiple trigeminal inputs (2.7 times that of the normal inputs) converge on single barrelette cells within 3-5 days; they remain stable up to the second postnatal week. Astrocyte proliferation and upregulation of astrocyte-specific proteins (GFAP and ALDH1L1) accompany reactive synaptogenesis in the IO nerve projection zone of the PrV. Pharmacological blockade of astrocyte function, purinergic receptors, and thrombospondins significantly reduced or eliminated reactive synaptogenesis without changing the MII in the intact PrV. GFAP immunohistochemistry further supported these electrophysiological results. We conclude that immature astrocytes, purinergic receptors, and thrombospondins play an important role in reactive synaptogenesis in the peripherally deafferented neonatal PrV.

Prostaglandin E2 release from astrocytes triggers gonadotropin-releasing hormone (GnRH) neuron firing via EP2 receptor activation

Astrocytes in the hypothalamus release prostaglandin E(2) (PGE(2)) in response to cell-cell signaling initiated by neurons and glial cells. Upon release, PGE(2) stimulates the secretion of gonadotropin-releasing hormone (GnRH), the neuropeptide that controls reproduction, from hypothalamic neuroendocrine neurons. Whether this effect on GnRH secretion is accompanied by changes in the firing behavior of these neurons is unknown. Using patch-clamp recording we demonstrate that PGE(2) exerts a dose-dependent postsynaptic excitatory effect on GnRH neurons. These effects are mimicked by an EP2 receptor agonist and attenuated by protein kinase A (PKA) inhibitors. The acute blockade of prostaglandin synthesis by indomethacin (INDO) or the selective inhibition of astrocyte metabolism by fluoroacetate (FA) suppresses the spontaneous firing activity of GnRH neurons in brain slices. Similarly, GnRH neuronal activity is reduced in mice with impaired astrocytic PGE(2) release due to defective erbB signaling in astrocytes. These results indicate that astrocyte-to-neuron communication in the hypothalamus is essential for the activity of GnRH neurons and suggest that PGE(2) acts as a gliotransmitter within the GnRH neurosecretory system.

Astrocytes display complex and localized calcium responses to single-neuron stimulation in the hippocampus

Astrocytes show a complex structural and physiological interplay with neurons and respond to neuronal activation in vitro and in vivo with intracellular calcium elevations. These calcium changes enable astrocytes to modulate synaptic transmission and plasticity through various mechanisms. However, the response pattern of astrocytes to single neuronal depolarization events still remains unresolved. This information is critical for fully understanding the coordinated network of neuron-glial signaling in the brain. To address this, we developed a system to map astrocyte calcium responses along apical dendrites of CA1 pyramidal neurons in hippocampal slices using single-neuron stimulation with channelrhodopsin-2. This technique allowed selective neuronal depolarization without invasive manipulations known to alter calcium levels in astrocytes. Light-evoked neuronal depolarization was elicited and calcium events in surrounding astrocytes were monitored using the calcium-sensitive dye Calcium Orange. Stimulation of single neurons caused calcium responses in populations of astrocytes along the apical axis of CA1 cell dendrites. Calcium responses included single events that were synchronized with neuronal stimulation and poststimulus changes in calcium event frequency, both of which were modulated by glutamatergic and purinergic signaling. Individual astrocytes near CA1 cells showed low ability to respond to repeated neuronal depolarization events. However, the response of the surrounding astrocyte population was remarkably accurate. Interestingly, the reliability of responses was graded with respect to astrocyte location along the CA1 cell dendrite, with astrocytes residing in the primary dendrite subregion being most responsive. This study provides a new perspective on the dynamic response property of astrocyte ensembles to neuronal activity.

Sustained neuronal activity generated by glial plasticity

Astrocytes release gliotransmitters, notably glutamate, that can affect neuronal and synaptic activity. In particular, astrocytic glutamate release results in the generation of NMDA receptor (NMDA-R)-mediated slow inward currents (SICs) in neurons. However, factors underlying the emergence of SICs and their physiological roles are essentially unknown. Here we show that, in acute slices of rat somatosensory thalamus, stimulation of lemniscal or cortical afferents results in a sustained increase of SICs in thalamocortical (TC) neurons that outlasts the duration of the stimulus by 1 h. This long-term enhancement of astrocytic glutamate release is induced by group I metabotropic glutamate receptors and is dependent on astrocytic intracellular calcium. Neuronal SICs are mediated by extrasynaptic NR2B subunit-containing NMDA-Rs and are capable of eliciting bursts. These are distinct from T-type Ca(2+) channel-dependent bursts of action potentials and are synchronized in neighboring TC neurons. These findings describe a previously unrecognized form of excitatory, nonsynaptic plasticity in the CNS that feeds forward to generate local neuronal firing long after stimulus termination.

A sensitive and simple ultra-high-performance-liquid chromatography-tandem mass spectrometry based method for the quantification of D-amino acids in body fluids

D-Amino acids are increasingly being recognized as important signaling molecules in mammals, including humans. D-Serine and D-aspartate are believed to act as signaling molecules in the central nervous system. Interestingly, several other D-amino acids also occur in human plasma, but very little is currently known regarding their function and origin. Abnormal levels of D-amino acids have been implicated in the pathogenesis of different diseases, including schizophrenia and amyotrophic lateral sclerosis (ALS), indicating that D-amino acid levels hold potential as diagnostic markers. Research into the biological functions of D-amino acids is hindered, however, by the lack of sufficiently sensitive, high-throughput analytical methods. In particular, the interference of large amounts of L-amino acids in biological samples and the low concentrations of D-amino acids are challenging. In this paper, we compared 7 different chiral derivatization agents for the analysis of D-amino acids and show that the chiral reagent (S)-NIFE offers outstanding performance in terms of sensitivity and enantioselectivity. An UPLC-MS/MS based method for the quantification of D-amino acids human biological fluids was then developed using (S)-NIFE. Baseline separation (R(s)>2.45) was achieved for the isomers of all 19 chiral proteinogenic amino acids. The limit of detection was <1 nM for all amino acids except d-alanine (1.98 nM), d-methionine (1.18 nM) and d-asparagine (5.15 nM). For measurements in human plasma, cerebrospinal fluid and urine, the accuracy ranged between 85% and 107%. The intra-assay and inter-assay were both <16% RSD for these three different matrices. Importantly, the method does not suffer from spontaneous racemization during sample preparation and derivatization. Using the described method, D-amino acid levels in human cerebrospinal fluid, plasma and urine were measured.

Implications of gliotransmission for the pharmacotherapy of CNS disorders

The seminal discovery that glial cells, particularly astrocytes, can release a number of gliotransmitters that serve as signalling molecules for the cross-talk with neighbouring cellular populations has recently changed our perception of brain functioning, as well as our view of the pathogenesis of several disorders of the CNS. Since glutamate was one of the first gliotransmitters to be identified and characterized, we tackle the mechanisms that underlie its release from astrocytes, including the Ca2+ signals underlying its efflux from astroglia, and we discuss the involvement of these events in a number of relevant physiological processes, from the modulatory control of neighbouring synapses to the regulation of blood supply to cerebral tissues. The relevance of these mechanisms strongly indicates that the contribution of glial cells and gliotransmission to the activities of the brain cannot be overlooked, and any study of CNS physiopathology needs to consider glial biology to have a comprehensive overview of brain function and dysfunction. Abnormalites in the signalling that controls the astrocytic release of glutamate are described in several experimental models of neurological disorders, for example, AIDS dementia complex, Alzheimer's disease and cerebral ischaemia. While the modalities of glutamate release from astrocytes remain poorly understood, and this represents a major impediment to the definition of novel therapeutic strategies targeting this process at the molecular level, some key mediators deputed to the control of the glial release of this excitatory amino acid have been identified. Among these, we can mention, for instance, proinflammatory cytokines, such as tumour necrosis factor-α, and prostaglandins. Agents that are able to block the major steps of tumour necrosis factor-α and prostaglandin production and/or signalling can be proposed as novel therapeutic targets for the treatment of these disorders.

Astrocytes support hippocampal-dependent memory and long-term potentiation via interleukin-1 signaling

Recent studies indicate that astrocytes play an integral role in neural and synaptic functioning. To examine the implications of these findings for neurobehavioral plasticity we investigated the involvement of astrocytes in memory and long-term potentiation (LTP), using a mouse model of impaired learning and synaptic plasticity caused by genetic deletion of the interleukin-1 receptor type I (IL-1RI). Neural precursor cells (NPCs), derived from either wild type (WT) or IL-1 receptor knockout (IL-1rKO) neonatal mice, were labeled with bromodeoxyuridine (BrdU) and transplanted into the hippocampus of either IL-1rKO or WT adult host mice. Transplanted NPCs survived and differentiated into astrocytes (expressing GFAP and S100β), but not to neurons or oligodendrocytes. The NPCs-derived astrocytes from WT but not IL-1rKO mice displayed co-localization of GFAP with the IL-1RI. Four to twelve weeks post-transplantation, memory functioning was examined in the fear-conditioning and the water maze paradigms and LTP of perforant path-dentate gyrus synapses was assessed in anesthetized mice. As expected, IL-1rKO mice transplanted with IL-1rKO cells or sham operated displayed severe memory disturbances in both paradigms as well as a marked impairment in LTP. In contrast, IL-1rKO mice transplanted with WT NPCs displayed a complete rescue of the impaired memory functioning as well as partial restoration of LTP. These findings indicate that astrocytes play a critical role in memory functioning and LTP, and specifically implicate astrocytic IL-1 signaling in these processes. The results suggest novel conceptualization and therapeutic targets for neuropsychiatric disorders characterized by impaired astrocytic functioning concomitantly with disturbed memory and synaptic plasticity.

Indirect modulation of neuronal excitability and synaptic transmission in the hippocampus by activation of proteinase-activated receptor-2

BACKGROUND AND PURPOSE

Proteinase-activated receptor-2 (PAR2) is widely expressed in the CNS under normal physiological conditions. However, its potential role in modulating neuronal excitability and synaptic transmission remains to be determined. Here, we have investigated whether PAR2 activation modulates synaptic activity in the hippocampus.

EXPERIMENTAL APPROACH

PAR2 activation and its effect on the hippocampus were examined in rat primary cultures and acute slices using whole cell patch clamp and standard extracellular recordings, respectively.

KEY RESULTS

PAR2 activation leads to a depolarization of hippocampal neurones and a paradoxical reduction in the occurrence of synaptically driven spontaneous action potentials (APs). PAR2-induced neuronal depolarization was abolished following either the inhibition of astrocytic function or antagonism of ionotropic glutamate receptors whilst the PAR2-induced decrease in AP frequency was also reduced when astrocytic function was inhibited. Furthermore, when examined in acute hippocampal slices, PAR2 activation induced a profound long-term depression of synaptic transmission that was dependent on NMDA receptor activation and was sensitive to disruption of astrocytic function.

CONCLUSIONS AND IMPLICATIONS

These novel findings show that PAR2 activation indirectly inhibits hippocampal synaptic activity and indicate that these receptors may play an active role in modulating normal physiological CNS function, in addition to their role in pathophysiological disorders.

Effects of glial release and somatic receptors on bursting in synchronized neuronal networks

A model is constructed to study the phenomenon of bursting in cultured neuronal networks by considering the effects of glial release and the extrasynaptic receptors on neurons. In the frequently observed situations of synchronized bursting, the whole neuronal network can be described by a mean-field model. In this model, the dynamics of the synchronized network in the presence of glia is represented by an effective two-compartment neuron with stimulations on both the dendrite and soma. Numerical simulations of this model show that most of the experimental observations in bursting, in particular the high plateau and the slow repolarization, can be reproduced. Our findings suggest that the effects of glia release and extrasynaptic receptors, which are usually neglected in neuronal models, can become important in intense network activities. Furthermore, simulations of the model are also performed for the case of glia-suppressed cultures to compare with recent experimental results.

Spatio-temporal credit assignment in neuronal population learning

In learning from trial and error, animals need to relate behavioral decisions to environmental reinforcement even though it may be difficult to assign credit to a particular decision when outcomes are uncertain or subject to delays. When considering the biophysical basis of learning, the credit-assignment problem is compounded because the behavioral decisions themselves result from the spatio-temporal aggregation of many synaptic releases. We present a model of plasticity induction for reinforcement learning in a population of leaky integrate and fire neurons which is based on a cascade of synaptic memory traces. Each synaptic cascade correlates presynaptic input first with postsynaptic events, next with the behavioral decisions and finally with external reinforcement. For operant conditioning, learning succeeds even when reinforcement is delivered with a delay so large that temporal contiguity between decision and pertinent reward is lost due to intervening decisions which are themselves subject to delayed reinforcement. This shows that the model provides a viable mechanism for temporal credit assignment. Further, learning speeds up with increasing population size, so the plasticity cascade simultaneously addresses the spatial problem of assigning credit to synapses in different population neurons. Simulations on other tasks, such as sequential decision making, serve to contrast the performance of the proposed scheme to that of temporal difference-based learning. We argue that, due to their comparative robustness, synaptic plasticity cascades are attractive basic models of reinforcement learning in the brain.

The key mechanisms supporting memory and learning in the brain rely on changing the strength of synapses which control the transmission of information between neurons. But how are appropriate changes determined when animals learn from trial and error? Information on success or failure is likely signaled to synapses by neurotransmitters like dopamine. But interpreting this reward signal is difficult because the number of synaptic transmissions occurring during behavioral decision making is huge and each transmission may have contributed differently to the decision, or perhaps not at all. Extrapolating from experimental evidence on synaptic plasticity, we suggest a computational model where each synapse collects information about its contributions to the decision process by means of a cascade of transient memory traces. The final trace then remodulates the reward signal when the persistent change of the synaptic strength is triggered. Simulation results show that with the suggested synaptic plasticity rule a simple neural network can learn even difficult tasks by trial and error, e.g., when the decision - reward sequence is scrambled due to large delays in reward delivery.

Insulin promotes glycogen storage and cell proliferation in primary human astrocytes

INTRODUCTION

In the human brain, there are at least as many astrocytes as neurons. Astrocytes are known to modulate neuronal function in several ways. Thus, they may also contribute to cerebral insulin actions. Therefore, we examined whether primary human astrocytes are insulin-responsive and whether their metabolic functions are affected by the hormone.

METHODS

Commercially available Normal Human Astrocytes were grown in the recommended medium. Major players in the insulin signaling pathway were detected by real-time RT-PCR and Western blotting. Phosphorylation events were detected by phospho-specific antibodies. Glucose uptake and glycogen synthesis were assessed using radio-labeled glucose. Glycogen content was assessed by histochemistry. Lactate levels were measured enzymatically. Cell proliferation was assessed by WST-1 assay.

RESULTS

We detected expression of key proteins for insulin signaling, such as insulin receptor β-subunit, insulin receptor substrat-1, Akt/protein kinase B and glycogen synthase kinase 3, in human astrocytes. Akt was phosphorylated and PI-3 kinase activity increased following insulin stimulation in a dose-dependent manner. Neither increased glucose uptake nor lactate secretion after insulin stimulation could be evidenced in this cell type. However, we found increased insulin-dependent glucose incorporation into glycogen. Furthermore, cell numbers increased dose-dependently upon insulin treatment.

DISCUSSION

This study demonstrated that human astrocytes are insulin-responsive at the molecular level. We identified glycogen synthesis and cell proliferation as biological responses of insulin signaling in these brain cells. Hence, this cell type may contribute to the effects of insulin in the human brain.

Serine proteases, serine protease inhibitors, and protease-activated receptors: roles in synaptic function and behavior

Serine proteases, serine protease inhibitors, and protease-activated receptors have been intensively investigated in the periphery and their roles in a wide range of processes-coagulation, inflammation, and digestion, for example-have been well characterized (see Coughlin, 2000; Macfarlane et al., 2001; Molinari et al., 2003; Wang et al., 2008; Di Cera, 2009 for reviews). A growing number of studies demonstrate that these protein systems are widely expressed in many cell types and regions in mammalian brains. Accumulating lines of evidence suggest that the brain has co-opted the activities of these interesting proteins to regulate various processes underlying synaptic activity and behavior. In this review, we discuss emerging roles for serine proteases in the regulation of mechanisms underlying synaptic plasticity and memory formation.

Glial D-serine gates NMDA receptors at excitatory synapses in prefrontal cortex

N-methyl-D-aspartate receptors (NMDARs) subserve numerous neurophysiological and neuropathological processes in the cerebral cortex. Their activation requires the binding of glutamate and also of a coagonist. Whereas glycine and D-serine (D-ser) are candidates for such a role at central synapses, the nature of the coagonist in cerebral cortex remains unknown. We first show that the glycine-binding site of NMDARs is not saturated in acute slices preparations of medial prefrontal cortex (mPFC). Using enzymes that selectively degrade either D-ser or glycine, we demonstrate that under the present conditions, D-ser is the principle endogenous coagonist of synaptic NMDARs at mature excitatory synapses in layers V/VI of mPFC where it is essential for long-term potentiation (LTP) induction. Furthermore, blocking the activity of glia with the metabolic inhibitor, fluoroacetate, impairs NMDAR-mediated synaptic transmission and prevents LTP induction by reducing the extracellular levels of D-serine. Such deficits can be restored by exogenous D-ser, indicating that the D-amino acid mainly originates from glia in the mPFC, as further confirmed by double-immunostaining studies for D-ser and anti-glial fibrillary acidic protein. Our findings suggest that D-ser modulates neuronal networks in the cerebral cortex by gating the activity of NMDARs and that altering its levels is relevant to the induction and potentially treatment of psychiatric and neurological disorders.

Astrocytes control glutamate receptor levels at developing synapses through SPARC-beta-integrin interactions

Neurons recruit numerous mechanisms to facilitate the development of synaptic connections. However, little is known about activity-dependent mechanisms that control the timing and fidelity of this process. Here we describe a novel pathway used by neurons to regulate glutamate receptors at maturing central synapses. This pathway relies on communication between neurons and astrocytes and the ability of astrocytes to release the factor SPARC (secreted protein, acidic and rich in cysteine). SPARC expression is dynamically regulated and plays a critical role in determining the level of synaptic AMPARs. SPARC ablation in mice increases excitatory synapse function, causes an abnormal accumulation of surface AMPARs at synapses, and impairs synaptic plasticity during development. We further demonstrate that SPARC inhibits the properties of neuronal β3-integrin complexes, which are intimately coupled to AMPAR stabilization at synapses. Thus neuron-glial signals control glutamate receptor levels at developing synapses to enable activity-driven modifications of synaptic strength.

Diacylglycerol signaling underlies astrocytic ATP release

Astrocytes have the ability to modulate neuronal excitability and synaptic transmission by the release of gliotransmitters. The importance of ATP released downstream of the activation of Gq-coupled receptors has been well established, but the mechanisms by which this release is regulated are unclear. The current work reveals that the elevation of diacylglycerol (DAG) in astrocytes induces vesicular ATP release. Unexpectedly, DAG-induced ATP release was found to be independent of PKC activation, but dependent upon activation of a C1 domain-containing protein. Astrocytes express the C1 domain-containing protein Munc13-1, which has been implicated in neuronal transmitter release, and RNAi-targeted downregulation of Munc13-1 inhibits astrocytic ATP release. These studies demonstrate that elevations of DAG induce the exocytotic release of ATP in astrocytes, likely via a Munc13-1-dependent mechanism.

SNARE protein expression in synaptic terminals and astrocytes in the adult hippocampus: a comparative analysis

Several evidences suggest that astrocytes release small transmitter molecules, peptides, and protein factors via regulated exocytosis, implying that they function as specialized neurosecretory cells. However, very little is known about the molecular and functional properties of regulated secretion in astrocytes in the adult brain. Establishing these properties is central to the understanding of the communication mode(s) of these cells and their role(s) in the control of synaptic functions and of cerebral blood flow. In this study, we have set-up a high-resolution confocal microscopy approach to distinguish protein expression in astrocytic structures and neighboring synaptic terminals in adult brain tissue. This approach was applied to investigate the expression pattern of core SNARE proteins for vesicle fusion in the dentate gyrus and CA1 regions of the mouse hippocampus. Our comparative analysis shows that astrocytes abundantly express, in their cell body and main processes, all three protein partners necessary to form an operational SNARE complex but not in the same isoforms expressed in neighbouring synaptic terminals. Thus, SNAP25 and VAMP2 are absent from astrocytic processes and typically concentrated in terminals, while SNAP23 and VAMP3 have the opposite expression pattern. Syntaxin 1 is present in both synaptic terminals and astrocytes. These data support the view that astrocytes in the adult hippocampus can communicate via regulated exocytosis and also indicates that astrocytic exocytosis may differ in its properties from action potential-dependent exocytosis at neuronal synapses, as it relies on a distinctive set of SNARE proteins.

Blood-brain barrier dysfunction in epileptogenesis of the temporal lobe

Epilepsy of the temporal lobe (TLE) is the most common form of focal epilepsy, and in adults, it most frequently develops after injury. However, the mechanisms by which a normal functioning brain turns into an epileptic one still remain obscure. Recent studies point to vascular involvement and particularly blood-brain barrier (BBB) dysfunction in the development of epilepsy. The BBB is a specialized structure which functions to control the neuronal extracellular milieu. BBB dysfunction is found in many diseases of the central nervous system, including stroke, traumatic injuries, tumors and infections. Interestingly, all these insults may initiate an epileptogenic process which eventually leads to spontaneous, recurrent seizures. This epileptogenic time frame usually lasts weeks, months, or even years in man, and days to weeks in rodents and may serve as a "window of opportunity" for the prevention of epilepsy. However, no prevention strategy exists, stressing the importance of research into the mechanisms of epileptogenesis. Here, we will underscore recent experiments suggesting that BBB dysfunction directly induces epileptogenesis. We will provide new evidence to support the hypothesis that BBB breakdown and specifically exposure of temporal lobe structures to the most common serum protein, albumin, is sufficient to induce epileptogenesis.

Ethanol reduces kainate-evoked glutamate secretion in rat hippocampal astrocytes

In this study we have used rat hippocampal astrocytes in culture to investigate the effect of ethanol on kainate-induced glutamate secretion. Our results show that kainate (10 μM to 500 μM) stimulated glutamate release from astrocytes. Preincubation of astrocytes in the presence of ethanol induced a concentration-dependent (1mM-50mM) inhibition of glutamate release caused by stimulation of cells with 100 μM kainate. Inhibition of alcohol-dehydrogenase, by preincubation of astrocytes in the presence of 4-methylpyrazole (1mM), abolished ethanol-induced inhibition of glutamate release in response to kainate. On the other hand, preincubation of astrocytes in the presence of the antioxidant cinnamtannin B-1 (10 μM) also blocked ethanol inhibitory action on glutamate release in response to kainate. Ethanol (50mM) reduced Ca(2+) mobilization in response to kainate, whereas cinnamtannin B-1 reversed the inhibitory action of ethanol on Ca(2+) mobilization by kainate. Our results are consistent with an inhibitory action of ethanol on glutamate secretion from hippocampal astrocytes. The inhibitory effects of ethanol are probably due to its oxidative metabolization, involves reactive oxygen species production, and a lower Ca(2+) mobilization by kainate. Taking into account the pivotal role that astrocytes play within the central nervous system, especially in relation to neurons, the negative effects of ethanol on the release of glutamate might affect neuron-glia communication in the hippocampus, which might lead to functional defects in the brain.

D-serine: the right or wrong isoform?

Only recently, d-amino acids have been identified in mammals. Of these, d-serine has been most extensively studied. d-Serine was found to play an important role as a neurotransmitter in the human central nervous system (CNS) by binding to the N-methyl-d-aspartate receptor (NMDAr), similar to glycine. Therefore, d-serine may well play a role in all physiological and pathological processes in which NMDArs have been implied. In this review, we discuss the findings implying an important role for d-serine in human physiology (CNS development and memory and learning) and pathology (excitotoxicity, perinatal asphyxia, amyotrophic lateral sclerosis (ALS), Alzheimer's disease, epilepsy, schizophrenia and bipolar disorder). We will debate on the relative contribution of d-serine versus glycine and conclude with clinical applications derived from these results and future directions to progress in this field. In general, adequate concentrations of d-serine are required for normal CNS development and function, while both decreased and increased concentrations can lead to CNS pathology. Therefore, d-serine appears to be the right isoform when present in the right concentrations.

BB14, a Nerve Growth Factor (NGF)-like peptide shown to be effective in reducing reactive astrogliosis and restoring synaptic homeostasis in a rat model of peripheral nerve injury

Peptidomimetics hold a great promise as therapeutic agents for neurodegenerative disorders. We previously described a Nerve Growth Factor (NGF)-like peptide, now named BB14, which was found to act as a strong TrkA agonist and to be effective in the sciatic nerve injury model of neuropathic pain. In this report we present the effects of BB14 in reducing reactive astrocytosis and reverting neuroplastic changes of the glutamate/GABAergic circuitry in the lumbar spinal cord following spared nerve injury (SNI) of the sciatic nerve. Immunohistochemical analysis of spinal cord sections revealed that SNI was associated with increased microglial (Iba1) and astrocytic (GFAP) responses, indicative of reactive gliosis. These changes were paralleled by (i) decreased glial aminoacid transporters (GLT1 and GlyT1) and increased levels of (ii) neuronal glutamate transporter EAAC1, (iii) neuronal vesicular GABA transporter (vGAT) and (iv) the GABAergic neuron marker GAD65/67. A remarkable increase of the Glutamate/GABA ratio and the reduction of glutathione (GSH) levels were also indicative of modifications of glial function in neuroprotection. All these molecular changes were found to be linked to an alteration of endogenous NGF metabolism, as demonstrated by decreased levels of mature NGF, increase of proNGF and increased activity of NGF-degrading methallo-proteinases (MMPs). Biochemical alterations and SNI-related neuropathic behavior, characterized by allodynia and hyperalgesia, were reversed by 7-days i.t. administration of the NGF-like peptide BB14, as well as by increasing endogenous NGF levels by i.t. infusion of GM6001, a MMPs inhibitor. All together, while confirming the correlation between reactive astrogliosis and perturbation of synaptic circuitry in the SNI model of peripheral nerve injury, these data strongly support the beneficial effect of BB14 in reducing reactive astrogliosis and restoring synaptic homeostasis under pathological conditions linked to alteration of NGF availability and signaling, thereby suggesting a potential role of BB14 as a therapeutic agent.

TNFα controls glutamatergic gliotransmission in the hippocampal dentate gyrus

VIDEO ABSTRACT

Glutamatergic gliotransmission provides a stimulatory input to excitatory synapses in the hippocampal dentate gyrus. Here, we show that tumor necrosis factor-alpha (TNFα) critically controls this process. With constitutive TNFα present, activation of astrocyte P2Y1 receptors induces localized [Ca(2+)](i) elevations followed by glutamate release and presynaptic NMDA receptor-dependent synaptic potentiation. In preparations lacking TNFα, astrocytes respond with identical [Ca(2+)](i) elevations but fail to induce neuromodulation. We find that TNFα specifically controls the glutamate release step of gliotransmission. In cultured astrocytes lacking TNFα glutamate exocytosis is dramatically slowed down due to altered vesicle docking. Addition of low picomolar TNFα promptly reconstitutes both normal exocytosis in culture and gliotransmission in situ. Alternatively, gliotransmission can be re-established without adding TNFα, by limiting glutamate uptake, which compensates slower release. These findings demonstrate that gliotransmission and its synaptic effects are controlled not only by astrocyte [Ca(2+)](i) elevations but also by permissive/homeostatic factors like TNFα.

Astrocytic activation of A1 receptors regulates the surface expression of NMDA receptors through a Src kinase dependent pathway

Chemical transmitters released from astrocytes, termed gliotransmitters, modulate synaptic transmission and neuronal function. Using astrocyte-specific inducible transgenicmice (dnSNARE mice), we have demonstrated that inhibiting gliotransmission leads to reduced activation of adenosine A1 receptors (A1R) and impaired sleep homeostasis (Halassa et al. (2009) Neuron 61:213-219); Pascual et al. (2005) Science 310:113-116). Additionally, synaptic N-methyl-D-aspartate receptor (NMDAR) currents are reduced in these astrocyte-specific transgenic animals (Fellin et al. (2009) Proc Natl Acad Sci USA 106:15037-15042). Because of the importance of adenosine and NMDA receptors to sleep processes we asked whether there is a causal linkage between changes in A1R activation and synaptic NMDA receptors. We show that astrocytic dnSNARE expression leads to reduced tyrosine phosphorylation of Srckinase and NR2 subunits concomitant with the decreased surface expression of the NR2 subunits. To test the role of A1R signaling in mediating these actions, we show that incubation of wildtype (WT) slices with an A1R antagonist reduces tyrosine phosphorylation of Src kinase and NR2B, decreases the surface expression of the NR2B subunits and leads to smaller NMDA component of miniature EPSCs. In dnSNARE mice we could rescue WT phenotype by incubation in an A1R agonist:activation of A1 receptor led to increased tyrosine phosphorylation of Src kinase and NR2B subunits as well as increased the surface expression of the NR2B subunit and increased NMDA component of the synaptic mEPSC. These results provide the first demonstration that astrocytes can affect neuronal excitability on a long time scale by regulating the surface expression of NMDA receptors through the activation of specific intracellular signaling pathways.

Differential acute and chronic effects of leptin on hypothalamic astrocyte morphology and synaptic protein levels

Astrocytes participate in neuroendocrine functions partially through modulation of synaptic input density in the hypothalamus. Indeed, glial ensheathing of neurons is modified by specific hormones, thus determining the availability of neuronal membrane space for synaptic inputs, with the loss of this plasticity possibly being involved in pathological processes. Leptin modulates synaptic inputs in the hypothalamus, but whether astrocytes participate in this action is unknown. Here we report that astrocyte structural proteins, such as glial fibrillary acidic protein (GFAP) and vimentin, are induced and astrocyte morphology modified by chronic leptin administration (intracerebroventricular, 2 wk), with these changes being inversely related to modifications in synaptic protein densities. Similar changes in glial structural proteins were observed in adult male rats that had increased body weight and circulating leptin levels due to neonatal overnutrition (overnutrition: four pups/litter vs. control: 12 pups/litter). However, acute leptin treatment reduced hypothalamic GFAP levels and induced synaptic protein levels 1 h after administration, with no effect on vimentin. In primary hypothalamic astrocyte cultures leptin also reduced GFAP levels at 1 h, with an induction at 24 h, indicating a possible direct effect of leptin. Hence, one mechanism by which leptin may affect metabolism is by modifying hypothalamic astrocyte morphology, which in turn could alter synaptic inputs to hypothalamic neurons. Furthermore, the responses to acute and chronic leptin exposure are inverse, raising the possibility that increased glial activation in response to chronic leptin exposure could be involved in central leptin resistance.