A distinct set of synaptic-like microvesicles in atroglial cells contain VGLUT3

A New Player in the Hippocampus: A Review on VGLUT3+ Neurons and Their Role in the Regulation of Hippocampal Activity and Behaviour

Glutamate is the most abundant excitatory amino acid in the central nervous system. Neurons using glutamate as a neurotransmitter can be characterised by vesicular glutamate transporters (VGLUTs). Among the three subtypes, VGLUT3 is unique, co-localising with other "classical" neurotransmitters, such as the inhibitory GABA. Glutamate, manipulated by VGLUT3, can modulate the packaging as well as the release of other neurotransmitters and serve as a retrograde signal through its release from the somata and dendrites. Its contribution to sensory processes (including seeing, hearing, and mechanosensation) is well characterised. However, its involvement in learning and memory can only be assumed based on its prominent hippocampal presence. Although VGLUT3-expressing neurons are detectable in the hippocampus, most of the hippocampal VGLUT3 positivity can be found on nerve terminals, presumably coming from the median raphe. This hippocampal glutamatergic network plays a pivotal role in several important processes (e.g., learning and memory, emotions, epilepsy, cardiovascular regulation). Indirect information from anatomical studies and KO mice strains suggests the contribution of local VGLUT3-positive hippocampal neurons as well as afferentations in these events. However, further studies making use of more specific tools (e.g., Cre-mice, opto- and chemogenetics) are needed to confirm these assumptions.

Exocytosis in Astrocytes

Until recently, astrocytes were thought to be a part of a simple "brain glue" providing only a supporting role for neurons. However, the discoveries of the last two decades have proven astrocytes to be dynamic partners participating in brain metabolism and actively influencing communication between neurons. The means of astrocyte-neuron communication are diverse, although regulated exocytosis has received the most attention but also caused the most debate. Similar to most of eukaryotic cells, astrocytes have a complex range of vesicular organelles which can undergo exocytosis as well as intricate molecular mechanisms that regulate this process. In this review, we focus on the components needed for regulated exocytosis to occur and summarise the knowledge about experimental evidence showing its presence in astrocytes.

Relationships and Interactions between Ionotropic Glutamate Receptors and Nicotinic Receptors in the CNS

Although ionotropic glutamate receptors and nicotinic receptors for acetylcholine (ACh) have usually been studied separately, they are often co-localized and functionally inter-dependent. The objective of this review is to survey the evidence for interactions between the two receptor families and the mechanisms underlying them. These include the mutual regulation of subunit expression, which change the NMDA:AMPA response balance, and the existence of multi-functional receptor complexes which make it difficult to distinguish between individual receptor sites, especially in vivo. This is followed by analysis of the functional relationships between the receptors from work on transmitter release, cellular electrophysiology and aspects of behavior where these can contribute to understanding receptor interactions. It is clear that nicotinic receptors (nAChRs) on axonal terminals directly regulate the release of glutamate and other neurotransmitters, α7-nAChRs generally promoting release. Hence, α7-nAChR responses will be prevented not only by a nicotinic antagonist, but also by compounds blocking the indirectly activated glutamate receptors. This accounts for the apparent anticholinergic activity of some glutamate antagonists, including the endogenous antagonist kynurenic acid. The activation of presynaptic nAChRs is by the ambient levels of ACh released from pre-terminal synapses, varicosities and glial cells, acting as a 'volume neurotransmitter' on synaptic and extrasynaptic sites. In addition, ACh and glutamate are released as CNS co-transmitters, including 'cholinergic' synapses onto spinal Renshaw cells. It is concluded that ACh should be viewed primarily as a modulator of glutamatergic neurotransmission by regulating the release of glutamate presynaptically, and the location, subunit composition, subtype balance and sensitivity of glutamate receptors, and not primarily as a classical fast neurotransmitter. These conclusions and caveats should aid clarification of the sites of action of glutamate and nicotinic receptor ligands in the search for new centrally-acting drugs.

GABA, but Not Bestrophin-1, Is Localized in Astroglial Processes in the Mouse Hippocampus and the Cerebellum

GABA is proposed to act as a gliotransmitter in the brain. Differences in GABA release from astroglia are thought to underlie differences in tonic inhibition between the cerebellum and the CA1 hippocampus. Here we used quantitative immunogold cytochemistry to localize and compare the levels of GABA in astroglia in these brain regions. We found that the density of GABA immunogold particles was similar in delicate processes of Bergman glia in the cerebellum and astrocytes in the CA1 hippocampus. The astrocytic GABA release is proposed to be mediated by, among others, the Ca²⁺ activated Cl^- channel bestrophin-1. The bestrophin-1 antibodies did not show any significant bestrophin-1 signal in the brain of wt mice, nor in bestrophin-1 knockout mice. The bestrophin-1 signal was low both on Western blots and immunofluorescence laser scanning microscopic images. These results suggest that GABA is localized in astroglia, but in similar concentrations in the cerebellum and CA1 hippocampus, and thus cannot account for differences in tonic inhibition between these brain regions. Furthermore, our data seem to suggest that the GABA release from astroglia previously observed in the hippocampus and cerebellum occurs via mechanisms other than bestrophin-1.

Reports of L-Norvaline Toxicity in Humans May Be Greatly Overstated

Recently, a study published in “Toxicology In Vitro” (Kate Samardzic and Kenneth J. Rodgers) was entitled: “Cytotoxicity and Mitochondrial Dysfunction Caused by the Dietary Supplement L-Norvaline”. The title may be greatly overstated, and here we provide several arguments showing that norvaline is not as toxic as reported.

Gliotransmitters and cytokines in the control of blood-brain barrier permeability

The contribution of astrocytes and microglia to the regulation of neuroplasticity or neurovascular unit (NVU) is based on the coordinated secretion of gliotransmitters and cytokines and the release and uptake of metabolites. Blood-brain barrier (BBB) integrity and angiogenesis are influenced by perivascular cells contacting with the abluminal side of brain microvessel endothelial cells (pericytes, astrocytes) or by immune cells existing (microglia) or invading the NVU (macrophages) under pathologic conditions. The release of gliotransmitters or cytokines by activated astroglial and microglial cells is provided by distinct mechanisms, affects intercellular communication, and results in the establishment of microenvironment controlling BBB permeability and neuroinflammation. Glial glutamate transporters and connexin and pannexin hemichannels working in the tight functional coupling with the purinergic system serve as promising molecular targets for manipulating the intercellular communications that control BBB permeability in brain pathologies associated with excessive angiogenesis, cerebrovascular remodeling, and BBB-mediated neuroinflammation. Substantial progress in deciphering the molecular mechanisms underlying the (patho)physiology of perivascular glia provides promising approaches to novel clinically relevant therapies for brain disorders. The present review summarizes the current understandings on the secretory machinery expressed in glial cells (glutamate transporters, connexin and pannexin hemichannels, exocytosis mechanisms, membrane-derived microvesicles, and inflammasomes) and the role of secreted gliotransmitters and cytokines in the regulation of NVU and BBB permeability in (patho)physiologic conditions.

Characterizing a novel vGlut3-P2A-iCreER knockin mouse strain in cochlea

Precise mouse genetic studies rely on specific tools that can label specific cell types. In mouse cochlea, previous studies suggest that vesicular glutamate transporter 3 (vGlut3), also known as Slc17a8, is specifically expressed in inner hair cells (IHCs) and loss of vGlut3 causes deafness. To take advantage of its unique expression pattern, here we generate a novel vGlut3-P2A-iCreER knockin mouse strain. The P2A-iCreER cassette is precisely inserted before stop codon of vGlut3, by which the endogenous vGlut3 is intact and paired with iCreER as well. Approximately, 10.7%, 85.6% and 41.8% of IHCs are tdtomato + when tamoxifen is given to vGlut3-P2A-iCreER/+; Rosa26-LSL-tdtomato/+ reporter strain at P2/P3, P10/P11 and P30/P31, respectively. Tdtomato + OHCs are never observed. Interestingly, besides IHCs, glia cells, but not spiral ganglion neurons (SGNs), are tdtomato+, which is further evidenced by the presence of Sox10+/tdtomato+ and tdtomato+/Prox1(Gata3 or Tuj1)-negative cells in SGN region. We further independently validate vGlut3 expression in SGN region by vGlut3 in situ hybridization and antibody staining. Moreover, total number of tdtomato + glia cells decreased gradually when tamoxifen is given from P2/P3 to P30/P31. Taken together, vGlut3-P2A-iCreER is an efficient genetic tool to specifically target IHCs for gene manipulation, which is complimentary to Prestin-CreER strain exclusively labelling cochlear outer hair cells (OHCs).

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

The structural and functional evidence for vesicular release from astrocytes in situ

The concept of the tripartite synapse states that bi-directional signalling between perisynaptic astrocyte processes, presynaptic axonal boutons and postsynaptic neuronal structures defines the properties of synaptic information processing. Ca²⁺-dependent vesicular release from astrocytes, as one of the mechanisms of astrocyte-neuron communication, has attracted particular attention but has also been the subject of intense debate. In neurons, regulated vesicular release is a strongly coordinated process. It requires a complex release machinery comprised of many individual components ranging from vesicular neurotransmitter transporters and soluble NSF attachment protein receptors (SNARE) proteins to Ca²⁺-sensors and the proteins that spatially and temporally control exocytosis of synaptic vesicles. If astrocytes employ similar mechanisms to release neurotransmitters is less well understood. The aim of this review is therefore to discuss recent experimental evidence that sheds light on the central structural components responsible for vesicular release from astrocytes in situ.

Astrocytes as secretory cells of the central nervous system: idiosyncrasies of vesicular secretion

Astrocytes are housekeepers of the central nervous system (CNS) and are important for CNS development, homeostasis and defence. They communicate with neurones and other glial cells through the release of signalling molecules. Astrocytes secrete a wide array of classic neurotransmitters, neuromodulators and hormones, as well as metabolic, trophic and plastic factors, all of which contribute to the gliocrine system. The release of neuroactive substances from astrocytes occurs through several distinct pathways that include diffusion through plasmalemmal channels, translocation by multiple transporters and regulated exocytosis. As in other eukaryotic cells, exocytotic secretion from astrocytes involves divergent secretory organelles (synaptic-like microvesicles, dense-core vesicles, lysosomes, exosomes and ectosomes), which differ in size, origin, cargo, membrane composition, dynamics and functions. In this review, we summarize the features and functions of secretory organelles in astrocytes. We focus on the biogenesis and trafficking of secretory organelles and on the regulation of the exocytotic secretory system in the context of healthy and diseased astrocytes.

What do we know about gliotransmitter release from astrocytes?

Astrocytes participate in information processing by actively modulating synaptic properties via gliotransmitter release. Various mechanisms of astrocytic release have been reported, including release from storage organelles via exocytosis and release from the cytosol via plasma membrane ion channels and pumps. It is still not fully clear which mechanisms operate under which conditions, but some of them, being Ca²⁺-regulated, may be physiologically relevant. The properties of Ca²⁺-dependent transmitter release via exocytosis or via ion channels are different and expected to produce different extracellular transmitter concentrations over time and to have distinct functional consequences. The molecular aspects of these two release pathways are still under active investigation. Here, we discuss the existing morphological and functional evidence in support of either of them. Transgenic mouse models, specific antagonists and localization studies have provided insight into regulated exocytosis, albeit not in a systematic fashion. Even more remains to be uncovered about the details of channel-mediated release. Better functional tools and improved ultrastructural approaches are needed in order fully to define specific modalities and effects of astrocytic gliotransmitter release pathways.

Neuroglial Transmission

Neuroglia, the "glue" that fills the space between neurons in the central nervous system, takes active part in nerve cell signaling. Neuroglial cells, astroglia, oligodendroglia, and microglia, are together about as numerous as neurons in the brain as a whole, and in the cerebral cortex grey matter, but the proportion varies widely among brain regions. Glial volume, however, is less than one-fifth of the tissue volume in grey matter. When stimulated by neurons or other cells, neuroglial cells release gliotransmitters by exocytosis, similar to neurotransmitter release from nerve endings, or by carrier-mediated transport or channel flux through the plasma membrane. Gliotransmitters include the common neurotransmitters glutamate and GABA, the nonstandard amino acid d-serine, the high-energy phosphate ATP, and l-lactate. The latter molecule is a "buffer" between glycolytic and oxidative metabolism as well as a signaling substance recently shown to act on specific lactate receptors in the brain. Complementing neurotransmission at a synapse, neuroglial transmission often implies diffusion of the transmitter over a longer distance and concurs with the concept of volume transmission. Transmission from glia modulates synaptic neurotransmission based on energetic and other local conditions in a volume of tissue surrounding the individual synapse. Neuroglial transmission appears to contribute significantly to brain functions such as memory, as well as to prevalent neuropathologies.

Calcium-permeable AMPA receptors trigger vesicular glutamate release from Bergmann gliosomes

The Bergmann glia is equipped with Ca2+-permeable AMPA receptors for glutamate, indispensable for structural and functional relations between the Bergmann glia and parallel/climbing fibers-Purkinje cell synapses. To better understand roles for the Bergmann AMPA receptors, herein we investigate on gliotransmitter release and Ca2+ signals in isolated Bergmann glia processes obtained from adult rat cerebellum. We found that: 1) the rat cerebellar purified astrocyte processes (gliosomes) expressed astrocytic and Bergmann markers and exhibited negligible contamination by nerve terminals, microglia, or oligodendrocytes; 2) activation of Ca2+-permeable AMPA receptors caused Ca2+ signals in the processes, and the release of glutamate from the processes; 3) effectiveness of rose bengal, trypan blue or bafilomycin A1, indicated that activation of the AMPA receptors evoked vesicular glutamate release. Cerebellar purified nerve terminals appeared devoid of glutamate-releasing Ca2+-permeable AMPA receptors, indicating that neuronal contamination may not be the source of the signals detected. Ultrastructural analysis indicated the presence of vesicles in the cytoplasm of the processes; confocal imaging confirmed the presence of vesicular glutamate transporters in Bergmann glia processes. We conclude that: a vesicular mechanism for release of the gliotransmitter glutamate is present in mature Bergmann processes; entry of Ca2+ through the AMPA receptors located on Bergmann processes is coupled with vesicular glutamate release. The findings would add a new role for a well-known Bergmann target for glutamate (the Ca2+-permeable AMPA receptors) and a new actor (the gliotransmitter glutamate) at the cerebellar excitatory synapses onto Purkinje cells.

Immunogold characteristics of VGLUT3-positive GABAergic nerve terminals suggest corelease of glutamate

There is compelling evidence that glutamate can act as a cotransmitter in the mammalian brain. Interestingly, the third vesicular glutamate transporter (VGLUT3) is primarily found in neurons that were anticipated to be nonglutamatergic. Whereas the function of VGLUT3 in acetylcholinergic and serotoninergic neurons has been elucidated, the role of VGLUT3 in neurons releasing gamma-aminobutyric acid (GABA) is not settled. We have previously shown that VGLUT3 is found together with the vesicular GABA transporter (VIAAT) on synaptic vesicle membranes in the hippocampus. Now we provide novel electron microscopic data from the rat hippocampus suggesting that glutamate is enriched in inhibitory nerve terminals containing VGLUT3 compared to those lacking VGLUT3. The opposite was found for GABA; VGLUT3-positive inhibitory terminals contained lower density of GABA labeling compared to VGLUT3-negative inhibitory terminals. In addition, semiquantitative confocal immunofluorescence showed that N-methyl-D-aspartate (NMDA)-receptor labeling was present more frequently in VGLUT3-positive/VIAAT-positive synapses versus in VGLUT3-negative/VIAAT-positive synapses. Electron microscopic immunogold data further suggest that NMDA receptors are enriched in VGLUT3 containing inhibitory terminals. Our data reveal new chemical characteristics of a subset of GABAergic interneurons in the hippocampus. The analyses suggest that glutamate is coreleased with GABA from hippocampal basket cell-synapses to act on NMDA receptors.

Perisynaptic astroglial processes: dynamic processors of neuronal information

Neuroglial interactions are now recognized as essential to brain functions. Extensive research has sought to understand the modalities of such dialog by focusing on astrocytes, the most abundant glial cell type of the central nervous system. Neuron-astrocyte exchanges occur at multiple levels, at different cellular locations. With regard to information processing, regulations occurring around synapses are of particular interest as synaptic networks are thought to underlie higher brain functions. Astrocytes morphology is tremendously complex in that their processes exceedingly branch out to eventually form multitudinous fine leaflets. The latter extremities have been shown to surround many synapses, forming perisynaptic astrocytic processes, which although recognized as essential to synaptic functioning, are poorly defined elements due to their tiny size. The current review sums up the current knowledge on their molecular and structural properties as well as the functional characteristics making them good candidates for information processing units.

Glutamate Release

Our aim was to review the processes of glutamate release from both biochemical and neurophysiological points of view. A large body of evidence now indicates that glutamate is specifically accumulated into synaptic vesicles, which provides strong support for the concept that glutamate is released from synaptic vesicles and is the major excitatory neurotransmitter. Evidence suggests the notion that synaptic vesicles, in order to sustain the neurotransmitter pool of glutamate, are endowed with an efficient mechanism for vesicular filling of glutamate. Glutamate-loaded vesicles undergo removal of Synapsin I by CaM kinase II-mediated phosphorylation, transforming to the release-ready pool. Vesicle docking to and fusion with the presynaptic plasma membrane are thought to be mediated by the SNARE complex. The Ca(2+)-dependent step in exocytosis is proposed to be mediated by synaptotagmin.

Altered vesicular glutamate transporter expression in human temporal lobe epilepsy with hippocampal sclerosis

Vesicular glutamate transporters (VGLUTs) are responsible for loading glutamate into synaptic vesicles. Altered VGLUT protein expression has been suggested to affect quantal size and glutamate release under both physiological and pathological conditions. In this study, we investigated mRNA and protein expression levels of the three VGLUT subtypes in hippocampal tissue of patients suffering from temporal lobe epilepsy (TLE) with hippocampal sclerosis (HS), International League Against Epilepsy type 1 (ILAE type 1) compared to autopsy controls, using quantitative polymerase chain reaction and semi-quantitative western blotting. mRNA expression levels of the VGLUTs are unaffected in hippocampal epileptic tissue compared to autopsy controls. At the protein level, VGLUT1 expression remains unaltered, while VGLUT2 is significantly decreased and VGLUT3 protein is significantly increased in hippocampal biopsies from TLE patients compared to controls. Our findings at the protein level can be explained by previously described histopathological changes observed in HS. Although VGLUTs have been repeatedly investigated in distinct rodent epilepsy models, their expression levels were hitherto not fully unraveled in the most difficult-to-treat form of epilepsy: TLE with HS ILAE type 1. We here, demonstrate for the first time that VGLUT2 protein expression is significantly decreased and VGLUT3 protein is significantly increased in the hippocampus of patients suffering from TLE with HS ILAE type 1 compared to autopsy controls.

Astrocytes contribute to gamma oscillations and recognition memory

Glial cells are an integral part of functional communication in the brain. Here we show that astrocytes contribute to the fast dynamics of neural circuits that underlie normal cognitive behaviors. In particular, we found that the selective expression of tetanus neurotoxin (TeNT) in astrocytes significantly reduced the duration of carbachol-induced gamma oscillations in hippocampal slices. These data prompted us to develop a novel transgenic mouse model, specifically with inducible tetanus toxin expression in astrocytes. In this in vivo model, we found evidence of a marked decrease in electroencephalographic (EEG) power in the gamma frequency range in awake-behaving mice, whereas neuronal synaptic activity remained intact. The reduction in cortical gamma oscillations was accompanied by impaired behavioral performance in the novel object recognition test, whereas other forms of memory, including working memory and fear conditioning, remained unchanged. These results support a key role for gamma oscillations in recognition memory. Both EEG alterations and behavioral deficits in novel object recognition were reversed by suppression of tetanus toxin expression. These data reveal an unexpected role for astrocytes as essential contributors to information processing and cognitive behavior.

Localisation of N-acetylaspartate in oligodendrocytes/myelin

The role of N-acetylaspartate in the brain is unclear. Here we used specific antibodies against N-acetylaspartate and immunocytochemistry of carbodiimide-fixed adult rodent brain to show that, besides staining of neuronal cell bodies in the grey matter, N-acetylaspartate labelling was present in oligodendrocytes/myelin in white matter tracts. Immunoelectron microscopy of the rat hippocampus showed that N-acetylaspartate was concentrated in the myelin. Also neuronal cell bodies and axons contained significant amounts of N-acetylaspartate, while synaptic elements and astrocytes were low in N-acetylaspartate. Mitochondria in axons and neuronal cell bodies contained higher levels of N-acetylaspartate compared to the cytosol, compatible with synthesis of N-acetylaspartate in mitochondria. In aspartoacylase knockout mice, in which catabolism of N-acetylaspartate is blocked, the levels of N-acetylaspartate were largely increased in oligodendrocytes/myelin. In these mice, the highest myelin concentration of N-acetylaspartate was found in the cerebellum, a region showing overt dysmyelination. In organotypic cortical slice cultures there was no evidence for N-acetylaspartate-induced myelin toxicity, supporting the notion that myelin damage is induced by the lack of N-acetylaspartate for lipid production. Our findings also implicate that N-acetylaspartate signals on magnetic resonance spectroscopy reflect not only vital neurons but also vital oligodendrocytes/myelin.

Are vesicular neurotransmitter transporters potential treatment targets for temporal lobe epilepsy?

The vesicular neurotransmitter transporters (VNTs) are small proteins responsible for packing synaptic vesicles with neurotransmitters thereby determining the amount of neurotransmitter released per vesicle through fusion in both neurons and glial cells. Each transporter subtype was classically seen as a specific neuronal marker of the respective nerve cells containing that particular neurotransmitter or structurally related neurotransmitters. More recently, however, it has become apparent that common neurotransmitters can also act as co-transmitters, adding complexity to neurotransmitter release and suggesting intriguing roles for VNTs therein. We will first describe the current knowledge on vesicular glutamate transporters (VGLUT1/2/3), the vesicular excitatory amino acid transporter (VEAT), the vesicular nucleotide transporter (VNUT), vesicular monoamine transporters (VMAT1/2), the vesicular acetylcholine transporter (VAChT) and the vesicular γ-aminobutyric acid (GABA) transporter (VGAT) in the brain. We will focus on evidence regarding transgenic mice with disruptions in VNTs in different models of seizures and epilepsy. We will also describe the known alterations and reorganizations in the expression levels of these VNTs in rodent models for temporal lobe epilepsy (TLE) and in human tissue resected for epilepsy surgery. Finally, we will discuss perspectives on opportunities and challenges for VNTs as targets for possible future epilepsy therapies.

Lack of evidence for vesicular glutamate transporter expression in mouse astrocytes

The concept of a tripartite synapse including a presynaptic terminal, a postsynaptic spine, and an astrocytic process that responds to neuronal activity by fast gliotransmitter release, confers to the electrically silent astrocytes an active role in information processing. However, the mechanisms of gliotransmitter release are still highly controversial. The reported expression of all three vesicular glutamate transporters (VGLUT1-3) by astrocytes suggests that astrocytes, like neurons, may release glutamate by exocytosis. However, the demonstration of astrocytic VGLUT expression is largely based on immunostaining, and the possibility of nonspecific labeling needs to be systematically addressed. We therefore examined the expression of VGLUT1-3 in astrocytes, both in culture and in situ. We used Western blots and single-vesicle imaging by total internal reflection fluorescence microscopy in live cultured astrocytes, and confocal microscopy, at the cellular level in cortical, hippocampal, and cerebellar brain slices, combined with quantitative image analysis. Control experiments were systematically performed in cultured astrocytes using wild-type, VGLUT1-3 knock-out, VGLUT1(Venus) knock-in, and VGLUT2-EGFP transgenic mice. In fixed brain slices, we quantified the degree of overlap between VGLUT1-3 and neuronal or astrocytic markers, both in an object-based manner using fluorescence line profiles, and in a pixel-based manner using dual-color scatter plots followed by the calculation of Pearson's correlation coefficient over all pixels with intensities significantly different from background. Our data provide no evidence in favor of the expression of any of the three VGLUTs by gray matter protoplasmic astrocytes of the primary somatosensory cortex, the thalamic ventrobasal nucleus, the hippocampus, and the cerebellum.

Rare contacts between synapses and microglial processes containing high levels of Iba1 and actin--a postembedding immunogold study in the healthy rat brain

Although microglia is recognised as the cell-mediating innate immunity in the brain, emerging evidence suggests a role of microglia in synaptic communication and modulation. The ability of microglia to move in the neuropil and contact synapses is crucial for such a function. However, the frequency of microglial contact with synapses is not known. Microglia motility is regulated by actin polymerisation and its interaction with ionising calcium-binding adaptor protein 1 (Iba1). In order to move and make contact with synapses, delicate microglial processes should contain high levels of actin and Iba1. To study this we refined an electron microscopic postembedding immunogold method enabling us to identify and quantitatively study different microglial constituents in intact brain tissue. We show that Iba1 and actin were colocalised at high densities in delicate processes in the rat frontal cortex, and that these delicate processes of microglia contact synaptic elements. About 3.5% of the synapses received direct contact from microglia. There was a marked inverse correlation between the densities of Iba1/actin gold particles and the area of the microglial processes, suggesting that the most delicate processes possess the machinery to provide movement in the neuropil. The low frequency of microglia interaction with synaptic elements suggests that microglia have a limited role in overall regulation of synaptic activity.