Glutamatergic Chemical Transmission: Look! Here, There, and Anywhere

Rett and Rett-related disorders: Common mechanisms for shared symptoms?

Rett syndrome is a neurodevelopmental disorder caused by loss-of-function mutations in the methyl-CpG binding protein-2 (MeCP2) gene that is characterized by epilepsy, intellectual disability, autistic features, speech deficits, and sleep and breathing abnormalities. Neurologically, patients with all three disorders display microcephaly, aberrant dendritic morphology, reduced spine density, and an imbalance of excitatory/inhibitory signaling. Loss-of-function mutations in the cyclin-dependent kinase-like 5 (CDKL5) and FOXG1 genes also cause similar behavioral and neurobiological defects and were referred to as congenital or variant Rett syndrome. The relatively recent realization that CDKL5 deficiency disorder (CDD), FOXG1 syndrome, and Rett syndrome are distinct neurodevelopmental disorders with some distinctive features have resulted in separate focus being placed on each disorder with the assumption that distinct molecular mechanisms underlie their pathogenesis. However, given that many of the core symptoms and neurological features are shared, it is likely that the disorders share some critical molecular underpinnings. This review discusses the possibility that deregulation of common molecules in neurons and astrocytes plays a central role in key behavioral and neurological abnormalities in all three disorders. These include KCC2, a chloride transporter, vGlut1, a vesicular glutamate transporter, GluD1, an orphan-glutamate receptor subunit, and PSD-95, a postsynaptic scaffolding protein. We propose that reduced expression or activity of KCC2, vGlut1, PSD-95, and AKT, along with increased expression of GluD1, is involved in the excitatory/inhibitory that represents a key aspect in all three disorders. In addition, astrocyte-derived brain-derived neurotrophic factor (BDNF), insulin-like growth factor 1 (IGF-1), and inflammatory cytokines likely affect the expression and functioning of these molecules resulting in disease-associated abnormalities.

Frost tolerance in apricot (Prunus armeniaca L.) receptacle and pistil organs: how is the relationship among amino acids, minerals, and cell death points?

To the better management of spring frost problem in the apricot cultivars, evaluation of biochemical changes in flower and/or flower organs during bud break could be one of the key factors. In this study, the relationship between the biochemical metabolites such as amino acids and minerals in the receptacle and pistil organs of two different apricot cultivars (frost-sensitive and frost-tolerant) and their relative effects on the frost tolerance of the cultivars and their organs were investigated during full blooming stage. In both apricot cultivars, it was found that the cell death points (CDP) of flower receptacle (- 6.3 to - 8.4 °C) were at higher temperatures than the CDP of flower pistil organs (- 13.1 to - 14.5 °C). Receptacle organs in flower, therefore, had less tolerance to spring frost damage. In addition, significant differences in mineral and amino acid contents were detected both between apricot cultivars and between the receptacle and pistil organs of the cultivars. Amino acid and mineral contents were lower both in the freezing-sensitive apricot cultivar ("Mihralibey") and the freezing-sensitive organ (receptacle) in comparison with the freezing-tolerant apricot cultivar ("Iğdır Şalak") and the freezing-tolerant organ (pistil). A significant negative correlation was also observed between the mean CDP values and both amino acid and mineral contents in the receptacle and pistil organs of both apricot cultivars. A negative correlation was found between CDP values and glutamate from amino acids and N, K, and Mg from minerals, and also these were determined that they had positive effects on frost tolerance increase. An important finding from our work revealed that the amount of each mineral and amino acid allocated differently to the receptacle and pistil organs of the apricot cultivars. The understanding of the amino acids and the mineral dynamics may contribute to improving the tolerance of flowers of apricot or other deciduous species to frost damage during spring. In the future, we may conclude that protection strategies such as increasing amino acids and mineral content in the receptacle organ of flowers would be necessary to cope with the negative effects of spring frost.

Structural Plastic Changes of Cortical Gray Matter Revealed by Voxel-Based Morphometry and Histological Analyses in a Monkey Model of Central Post-Stroke Pain

Central post-stroke pain (CPSP) is a chronic pain caused by stroke lesions of somatosensory pathways. Several brain imaging studies among patients with CPSP demonstrate that the pathophysiological mechanism underlying this condition is the maladaptive plasticity of pain-related brain regions. However, the temporal profile of the regional plastic changes, as suggested by brain imaging of CPSP patients, as well as their cellular basis, is unknown. To investigate these issues, we performed voxel-based morphometry (VBM) using T1-weighted magnetic resonance imaging and immunohistochemical analysis with our established CPSP monkey model. From 8 weeks after a hemorrhagic lesion to the unilateral ventral posterolateral nucleus of the thalamus, the monkeys exhibited significant behavioral changes that were interpreted as reflecting allodynia. The present VBM results revealed a decrease in gray matter volume in the pain-related areas after several weeks following the lesion. Furthermore, immunohistochemical staining in the ipsilesional posterior insular cortex (ipsi-PIC) and secondary somatosensory cortex (ipsi-SII), where the significant reduction in gray matter volume was observed in the VBM result, displayed a significant reduction in both excitatory and inhibitory synaptic terminals compared to intact monkeys. Our results suggest that progressive changes in neuronal morphology, including synaptic loss in the ipsi-PIC/SII, are involved in theCPSP.

Inhibition of the Vesicular Glutamate Transporter (VGLUT) with Congo Red Analogs: New Binding Insights

The vesicular glutamate transporter (VGLUT) facilitates the uptake of glutamate (Glu) into neuronal vesicles. VGLUT has not yet been fully characterized pharmacologically but a body of work established that certain azo-dyes bearing two Glu isosteres via a linker were potent inhibitors. However, the distance between the isostere groups that convey potent inhibition has not been delineated. This report describes the synthesis and pharmacologic assessment of Congo Red analogs that contain one or two glutamate isostere or mimic groups; the latter varied in the interatomic distance and spacer properties to probe strategic binding interactions within VGLUT. The more potent inhibitors had two glutamate isosteres symmetrically linked to a central aromatic group and showed IC₅₀ values ~ 0.3-2.0 μM at VGLUT. These compounds contained phenyl, diphenyl ether (PhOPh) or 1,2-diphenylethane as the linker connecting 4-aminonaphthalene sulfonic acid groups. A homology model for VGLUT2 using D-galactonate transporter (DgoT) to dock and identify R88, H199 and F219 as key protein interactions with Trypan Blue, Congo Red and selected potent analogs prepared and tested in this report.

VGLUT substrates and inhibitors: A computational viewpoint

The vesicular glutamate transporters (VGLUTs) bind and move glutamate (Glu) from the cytosol into the lumen of synaptic vesicles using a H⁺-electrochemical gradient (ΔpH and Δψ) generated by the vesicular H⁺-ATPase. VGLUTs show very low Glu binding and to date, no three-dimensional structure has been elucidated. Prior studies have attempted to identify the key residues involved in binding VGLUT substrates and inhibitors using homology models and docking experiments. Recently, the inward and outward oriented crystal structures of d-galactonate transporter (DgoT) emerged as possible structure templates for VGLUT. In this review, a new homology model for VGLUT2 based on DgoT has been developed and used to conduct docking experiments to identify and differentiate residues and binding orientations involved in ligand interactions. This review describes small molecule-ligand interactions including docking using a VGLUT2 homology model derived from DgoT.

Methamphetamine promotes habitual action and alters the density of striatal glutamate receptor and vesicular proteins in dorsal striatum

Goal-directed actions are controlled by the value of the consequences they produce and so increase when what they produce is valuable and decrease when it is not. With continued invariant practice, however, goal-directed actions can become habits, controlled not by their consequences but by antecedent, reward-related states and stimuli. Here, we show that pre-exposure to methamphetamine (METH) caused abnormally rapid development of habitual control. Furthermore, these drug-induced habits differed strikingly from conventional habits; we found that they were insensitive both to changes in reward value and to the effects of negative feedback. In addition to these behavioral changes, METH exposure produced bidirectional changes to synaptic proteins in the dorsal striatum. In the dorsomedial striatum, a structure critical for goal-directed action, METH exposure was associated with a reduction in glutamate receptor and glutamate vesicular proteins, whereas in the dorsolateral striatum, a region that has previously been implicated in habit learning, there was an increase in these proteins. Together, these results indicate that METH exposure promotes habitual control of action that appears to be the result of bidirectional changes in glutamatergic transmission in the circuits underlying goal-directed and habit-based learning.

Glutamate signaling through the NMDA receptor reduces the expression of scleraxis in plantaris tendon derived cells

Background

A body of evidence demonstrating changes to the glutaminergic system in tendinopathy has recently emerged. This hypothesis was further tested by studying the effects of glutamate on the tenocyte phenotype, and the impact of loading and exposure to glucocorticoids on the glutamate signaling machinery.

Methods

Plantaris tendon tissue and cultured plantaris tendon derived cells were immunohisto-/cytochemically stained for glutamate, N-Methyl-D-Aspartate receptor 1 (NMDAR1) and vesicular glutamate transporter 2 (VGluT2). Primary cells were exposed to glutamate or receptor agonist NMDA. Cell death/viability was measured via LDH/MTS assays, and Western blot for cleaved caspase 3 (c-caspase 3) and cleaved poly (ADP-ribose) polymerase (c-PARP). Scleraxis mRNA (Scx)/protein(SCX) were analyzed by qPCR and Western blot, respectively. A FlexCell system was used to apply cyclic strain. The effect of glucocorticoids was studies by adding dexamethasone (Dex). The mRNA of the glutamate synthesizing enzymes Got1 and Gls, and NMDAR1 protein were measured. Levels of free glutamate were determined by a colorimetric assay.

Results

Immunoreactions for glutamate, VGluT2, and NMDAR1 were found in tenocytes and peritendinous cells in tissue sections and in cultured cells. Cell death was induced by high concentrations of glutamate but not by NMDA. Scleraxis mRNA/protein was down-regulated in response to NMDA/glutamate stimulation. Cyclic strain increased, and Dex decreased, Gls and Got1 mRNA expression. Free glutamate levels were lower after Dex exposure.

Conclusions

In conclusion, NMDA receptor stimulation leads to a reduction of scleraxis expression that may be involved in a change of phenotype in tendon cells. Glutamate synthesis is increased in tendon cells in response to strain and decreased by glucocorticoid stimulation. This implies that locally produced glutamate could be involved in the tissue changes observed in tendinopathy.

Function and expression of a splicing variant of vesicular glutamate transporter 1

Vesicular glutamate transporter (VGLUT) is an active transporter responsible for vesicular storage of glutamate in synaptic vesicles and plays an essential role in glutamatergic neurotransmission. VGLUT consists of three isoforms, VGLUT1, VGLUT2, and VGLUT3. The VGLUT1 variant, VGLUT1v, with an additional 75-base pair sequence derived from a second intron between exons 2 and 3, which corresponds to 25 amino acid residues in the 1st loop of VGLUT1, is the only splicing variant among VGLUTs, although whether VGLUT1v protein is actually translated at the protein level remains unknown. In the present study, VGLUT1v was expressed in insect cells, solubilized, purified to near homogeneity, and its transport activity was examined. Proteoliposomes containing purified VGLUT1v were shown to accumulate glutamate upon imposition of an inside-positive membrane potential (Δψ). The Δψ-driven glutamate uptake activity requires Cl^- and its pharmacological profile and kinetics are comparable to those of other VGLUTs. The retinal membrane contained two VGLUT1 moieties with apparent molecular masses of 65 and 57kDa. VGLUT1v-specific antibodies against an inserted 25-amino acid residue sequence identified a 65-kDa immunoreactive polypeptide. Immunohistochemical analysis indicated that VGLUT1v immunoreactivity is present in photoreceptor cells and is associated with synaptic vesicles. VGLUT1v immunoreactivity is also present in pinealocytes, but not in other areas, including the brain. These results indicated that VGLUT1v exists in a functional state in rat photosensitive cells and is involved in glutamatergic chemical transmission.

[The cancer paradigm in pulmonary arterial hypertension: towards anti-remodeling therapies targeting metabolic dysfunction?]

Pulmonary arterial hypertension (PAH) is a rare, complex and multifactorial disease in which pulmonary vascular remodeling plays a major role ending in right heart failure and death. Current specific therapies of PAH that mainly target the vasoconstriction/vasodilatation imbalance are not curative. Bi-pulmonary transplantation remains the only option in patients resistant to current therapies. It is thus crucial to identify novel vascular anti-remodeling therapeutic targets. This remodeling displays several properties of cancer cells, especially overproliferation and apoptosis resistance of pulmonary vascular cells, hallmarks of cancer related to the metabolic shift known as the "Warburg effect". The latter is characterized by a shift of ATP production, from oxidative phosphorylation to low rate aerobic glycolysis. In compensation, the cancer cells exhibit exacerbated glutaminolysis thus resulting in glutamine addiction, necessary to their overproliferation. Glutamine intake results in glutamate production, a molecule at the crossroads of energy metabolism and cancer cell communication, thus contributing to cell proliferation. Accordingly, therapeutic strategies targeting glutamate production, its release into the extracellular space and its membrane receptors have been suggested to treat different types of cancers, not only in the central nervous system but also in the periphery. We propose that similar strategies targeting glutamatergic signaling may be considered in PAH, especially as they could affect not only the vascular remodeling but also the right heart hypertrophy known to involve the glutaminolysis pathway. Ongoing studies aim to characterize the involvement of the glutamate pathway and its receptors in vascular remodeling, and the therapeutic potential of specific molecules targeting this pathway.

Structure, Function, and Drug Interactions of Neurotransmitter Transporters in the Postgenomic Era

Vesicular neurotransmitter transporters are responsible for the accumulation of neurotransmitters in secretory vesicles and play essential roles in chemical transmission. The SLC17 family contributes to sequestration of anionic neurotransmitters such as glutamate, aspartate, and nucleotides. Identification and subsequent cellular and molecular biological studies of SLC17 transporters unveiled the principles underlying the actions of these transporters. Recent progress in reconstitution methods in combination with postgenomic approaches has advanced studies on neurotransmitter transporters. This review summarizes the molecular properties of SLC17-type transporters and recent findings regarding the novel SLC18 transporter.

[Vacuolar H(+)-ATPase and the Secondary Transporters: Their Identification, Mechanism, Function and Physiological Relevance]

Eukaryotic cells possess acidic compartments with pH ranging from 0.1-6.5. Studies in the last couple of decades indicated that the acid pool is established by vacuolar proton ATPase, and is essential for various physiological and pathological processes. I have identified the vacuolar ATPase as well as several secondary transporters which are energetically coupled with vacuolar ATPase. I have also established the protocol for analysis of the structure and function of transporter proteins applicable to essentially all known transporters. In this article, I am going to summarize this study and describe some topics.

Sigma receptors [σRs]: biology in normal and diseased states

This review compares the biological and physiological function of Sigma receptors [σRs] and their potential therapeutic roles. Sigma receptors are widespread in the central nervous system and across multiple peripheral tissues. σRs consist of sigma receptor one (σ₁R) and sigma receptor two (σ₂R) and are expressed in numerous regions of the brain. The sigma receptor was originally proposed as a subtype of opioid receptors and was suggested to contribute to the delusions and psychoses induced by benzomorphans such as SKF-10047 and pentazocine. Later studies confirmed that σRs are non-opioid receptors (not an µ opioid receptor) and play a more diverse role in intracellular signaling, apoptosis and metabolic regulation. σ₁Rs are intracellular receptors acting as chaperone proteins that modulate Ca²⁺ signaling through the IP₃ receptor. They dynamically translocate inside cells, hence are transmembrane proteins. The σ₁R receptor, at the mitochondrial-associated endoplasmic reticulum membrane, is responsible for mitochondrial metabolic regulation and promotes mitochondrial energy depletion and apoptosis. Studies have demonstrated that they play a role as a modulator of ion channels (K⁺ channels; N-methyl-d-aspartate receptors [NMDAR]; inositol 1,3,5 triphosphate receptors) and regulate lipid transport and metabolism, neuritogenesis, cellular differentiation and myelination in the brain. σ₁R modulation of Ca²⁺ release, modulation of cardiac myocyte contractility and may have links to G-proteins. It has been proposed that σ₁Rs are intracellular signal transduction amplifiers. This review of the literature examines the mechanism of action of the σRs, their interaction with neurotransmitters, pharmacology, location and adverse effects mediated through them.

Identification of potential biomarkers of P-glycoprotein substrate neurotoxicity in transgenic mice expressing the mutated canine ABCB1 gene

OBJECTIVE

To identify biomarkers of P-glycoprotein (P-gp) substrate neurotoxicity in transgenic mice expressing the mutant canine ABCB1 gene (ABCB1-1Δ).

ANIMALS

8 ABCB1 knock-in and knock-out transgenic mice expressing the ABCB1-1Δ gene and 8 control mice expressing the wild-type canine ABCB1 gene (ABCB1-WT).

PROCEDURES

Groups including 2 ABCB1-1Δ mutant mice and 2 ABCB1-WT mice were administered the P-gp substrates ivermectin (10 mg/kg, SC), doramectin (10 mg/kg, SC), moxidectin (10 mg/kg, PO), or digoxin (1.53 mg/kg, SC). A toxicogenomic approach based on DNA microarrays was used to examine whole-genome expression changes in mice administered P-gp substrates.

RESULTS

Compared with control ABCB1-WT mice, ABCB1-1Δ mutant mice developed neurotoxic signs including ataxia, lethargy, and tremors similar to those reported for dogs with the ABCB1-1Δ mutation. Microarray analysis revealed that gene expression was altered in ABCB1-1Δ mutant mice, compared with findings for ABCB1-WT mice, following administration of the same P-gp substrates. Gene pathway analysis revealed that genes with a ≥ 2-fold gene expression change were associated with behavior and nervous system development and function. Moreover, 34 genes were altered in the ABCB1-1Δ mutant mice in all 4 drug treatment groups. These genes were also associated with behavior, which was identified as the top-ranked gene network.

CONCLUSIONS AND CLINICAL RELEVANCE

These study data have facilitated understanding of the molecular mechanisms of neurotoxicosis in ABCB1-1Δ mutant mice following exposure to various P-gp substrates. Some genes appear to be potential biomarkers of P-gp substrate neurotoxicity that might be used to predict the safety of those drugs in dogs with the ABCB1-1Δ mutation.

Glutamatergic phenotype of glucagon-like peptide 1 neurons in the caudal nucleus of the solitary tract in rats

The expression of a vesicular glutamate transporter (VGLUT) suffices to assign a glutamatergic phenotype to neurons and other secretory cells. For example, intestinal L cells express VGLUT2 and secrete glutamate along with glucagon-like peptide 1 (GLP1). We hypothesized that GLP1-positive neurons within the caudal (visceral) nucleus of the solitary tract (cNST) also are glutamatergic. To test this, the axonal projections of GLP1 and other neurons within the cNST were labeled in rats via iontophoretic delivery of anterograde tracer. Dual immunofluorescence and confocal microscopy was used to visualize tracer-, GLP1-, and VGLUT2-positive fibers within brainstem, hypothalamic, and limbic forebrain nuclei that receive input from the cNST. Electron microscopy was used to confirm GLP1 and VGLUT2 immunolabeling within the same axon varicosities, and fluorescent in situ hybridization was used to examine VGLUT2 mRNA expression by GLP1-positive neurons. Most anterograde tracer-labeled fibers displayed VGLUT2-positive varicosities, providing new evidence that ascending axonal projections from the cNST are primarily glutamatergic. Virtually all GLP1-positive varicosities also were VGLUT2-positive. Electron microscopy confirmed the colocalization of GLP1 and VGLUT2 immunolabeling in axon terminals that formed asymmetric (excitatory-type) synapses with unlabeled dendrites in the hypothalamus. Finally, in situ hybridization confirmed that GLP1-positive cNST neurons express VGLUT2 mRNA. Thus, hindbrain GLP1 neurons in rats are equipped to store glutamate in synaptic vesicles, and likely co-release both glutamate and GLP1 from axon varicosities and terminals in the hypothalamus and other brain regions.

Vesicular neurotransmitter transporters: an approach for studying transporters with purified proteins

Vesicular storage and subsequent release of neurotransmitters are the key processes of chemical signal transmission. In this process, vesicular neurotransmitter transporters are responsible for loading the signaling molecules. The use of a "clean biochemical" approach with purified, recombinant transporters has helped in the identification of novel vesicular neurotransmitter transporters and in the analysis of the control of signal transmission.

VGLUTs in Peripheral Neurons and the Spinal Cord: Time for a Review

Vesicular glutamate transporters (VGLUTs) are key molecules for the incorporation of glutamate in synaptic vesicles across the nervous system, and since their discovery in the early 1990s, research on these transporters has been intense and productive. This review will focus on several aspects of VGLUTs research on neurons in the periphery and the spinal cord. Firstly, it will begin with a historical account on the evolution of the morphological analysis of glutamatergic systems and the pivotal role played by the discovery of VGLUTs. Secondly, and in order to provide an appropriate framework, there will be a synthetic description of the neuroanatomy and neurochemistry of peripheral neurons and the spinal cord. This will be followed by a succinct description of the current knowledge on the expression of VGLUTs in peripheral sensory and autonomic neurons and neurons in the spinal cord. Finally, this review will address the modulation of VGLUTs expression after nerve and tissue insult, their physiological relevance in relation to sensation, pain, and neuroprotection, and their potential pharmacological usefulness.

Glutamate transporter 1: target for the treatment of alcohol dependence

Emerging evidence indicates that many aspects of alcohol and drug dependence involve changes in glutamate transmission. A number of studies have reported that drugs of abuse, including alcohol and cocaine, alter glutamate transport. Extracellular glutamate is regulated by a number of glutamate transporters in various brain regions. Of these transporters, glutamate transporter (GLT1) is a key player in the removal of most of the extracellular glutamate. Similar to neurodegenerative disease models, in which there is dysfunction of the glutamatergic excitatory system, the role of GLT1 has been tested in drug dependence models that show dysfunction of glutamate transmission. We and others have recently found that ceftriaxone, an FDA-approved drug known to elevate GLT1 expression, attenuates cue-induced cocaine relapse. Moreover, we recently found that alcohol-preferring rats treated with ceftriaxone showed a significant dosedependent reduction in alcohol consumption. We also demonstrated that ceftriaxone-induced upregulation of GLT1 expression was associated with increases in glutamate uptake in Huntington's disease mouse model. Importantly, ceftriaxone is currently in clinical trials for the treatment of amyotrophic lateral sclerosis. This review provides information about the potential therapeutic role of GLT1 for the treatment of alcohol abuse and dependence.

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.

Haploinsufficiency of VGluT1 but not VGluT2 impairs extinction of spatial preference and response suppression

The excitatory neurotransmitter l-glutamate is transported into synaptic vesicles by vesicular glutamate transporters (VGluTs) to transmit glutamatergic signals. Changes in their expression have been linked to various brain disorders including schizophrenia, Parkinson's, and Alzheimer's disease. Deleting either the VGluT1 or VGluT2 gene leads to profound developmental and neurological complications and early death, but mice heterozygous for VGluT1 or VGluT2 are viable and thrive. Acquisition, retention and extinction of conditioned visuospatial and emotional responses were compared between VGluT1(+/-) and VGluT2(+/-) mice, and their wildtype littermates, using different water maze procedures, appetitive scheduled conditioning, and conditioned fear protocols. The distinct brain expression profiles of the VGluT1 and -2 isoforms particularly in telencephalic structures, such as neocortex, hippocampus and striatum, are reflected in very specific behavioral changes. VGluT2(+/-) mice were unimpaired in spatial learning tasks and fear extinction. Conversely, VGluT1(+/-) mice displayed spatial extinction learning deficits and markedly impaired fear extinction. These data indicate that VGluT1, but not VGluT2, plays a role in the neural processes underlying inhibitory learning.

Glutamatergic projection from the nucleus incertus to the septohippocampal system

Recent findings support a relevant role of the nucleus incertus in the control of the hippocampal activity through the modulation of theta rhythm. Previous studies from our group have shown that this nucleus is a critical relay between reticularis pontis oralis and the medial septum/diagonal band, regarded as the main activator and the pacemaker of the hippocampal oscillations, respectively. Besides, the nucleus incertus is highly linked to activated states related to the arousal response. The neurotransmission of the nucleus incertus, however, remains uncertain. Only GABA and the neuromodulator relaxin 3 are usually considered to be involved in its contribution to the septohippocampal system. In this work, we have analyzed the existence of an excitatory projection from the nucleus incertus to the medial septum. We have found a group of glutamatergic neurons in the nucleus incertus projecting to the medial septum. Moreover, we were able to describe a segregated distribution of calbindin and calretinin neurons. While calretinin expression was restricted to the nucleus incertus pars compacta, calbindin positive neurons where observed both in the pars dissipata and the pars compacta of the nucleus. The present work provides innovative data supporting an excitatory component in the pontoseptal pathway.

Expression of vesicular glutamate transporters type 1 and 2 in sensory and autonomic neurons innervating the mouse colorectum

Vesicular glutamate transporters (VGLUTs) have been extensively studied in various neuronal systems, but their expression in visceral sensory and autonomic neurons remains to be analyzed in detail. Here we studied VGLUTs type 1 and 2 (VGLUT(1) and VGLUT(2) , respectively) in neurons innervating the mouse colorectum. Lumbosacral and thoracolumbar dorsal root ganglion (DRG), lumbar sympathetic chain (LSC), and major pelvic ganglion (MPG) neurons innervating the colorectum of BALB/C mice were retrogradely traced with Fast Blue, dissected, and processed for immunohistochemistry. Tissue from additional naïve mice was included. Previously characterized antibodies against VGLUT(1) , VGLUT(2) , and calcitonin gene-related peptide (CGRP) were used. Riboprobe in situ hybridization, using probes against VGLUT(1) and VGLUT(2) , was also performed. Most colorectal DRG neurons expressed VGLUT(2) and often colocalized with CGRP. A smaller percentage of neurons expressed VGLUT(1) . VGLUT(2) -immunoreactive (IR) neurons in the MPG were rare. Abundant VGLUT(2) -IR nerves were detected in all layers of the colorectum; VGLUT(1) -IR nerves were sparse. A subpopulation of myenteric plexus neurons expressed VGLUT2 protein and mRNA, but VGLUT1 mRNA was undetectable. In conclusion, we show 1) that most colorectal DRG neurons express VGLUT(2) , and to a lesser extent, VGLUT(1) ; 2) abundance of VGLUT2-IR fibers innervating colorectum; and 3) a subpopulation of myenteric plexus neurons expressing VGLUT(2). Altogether, our data suggests a role for VGLUT(2) in colorectal glutamatergic neurotransmission, potentially influencing colorectal sensitivity and motility.

Bovine neuronal vesicular glutamate transporter activity is inhibited by ergovaline and other ergopeptines

l-Glutamate (Glu) is a major excitatory neurotransmitter responsible for neurotransmission in the vertebrate central nervous system. Vesicular Glu transporters VGLUT1 and VGLUT2 concentrate (50mM) Glu [Michaelis constant (measuring affinity), or K(m),=1 to 4mM] into synaptic vesicles (SV) for subsequent release into the synaptic cleft of glutamatergic neurons. Vesicular Glu transporter activity is dependent on vacuolar H(+)-ATPase function. Previous research has shown that ergopeptines contained in endophyte-infected tall fescue interact with dopaminergic and serotoninergic receptors, thereby affecting physiology regulated by these neuron types. To test the hypothesis that ergopeptine alkaloids inhibit VGLUT activity of bovine cerebral SV, SV were isolated from cerebral tissue of Angus-cross steers that were naive to ergot alkaloids. Immunoblot analysis validated the enrichment of VGLUT1, VGLUT2, synaptophysin 1, and vacuolar H(+)-ATPase in purified SV. Glutamate uptake assays demonstrated the dependence of SV VGLUT-like activity on the presence of ATP, H(+)-gradients, and H(+)-ATPase function. The effect of ergopeptines on VGLUT activity was evaluated by ANOVA. Inhibitory competition (IC(50)) experiments revealed that VGLUT-mediated Glu uptake (n=9) was inhibited by ergopeptine alkaloids: bromocriptine (2.83±0.59μM)<ergotamine (20.5±2.77μM)<ergocornine (114±23.1μM)<ergovaline (137±6.55μM). Subsequent ergovaline kinetic inhibition analysis (n=9; Glu=0.05, 0.10, 0.50, 1, 2, 4, 5mM) demonstrated no change in apparent K(m). However, the maximum reaction rate (V(max)) of Glu uptake was decreased when evaluated in the presence of 50, 100, and 200μM ergovaline, suggesting that ergovaline inhibited SV VGLUT activity through a noncompetitive mechanism. The findings of this study suggest cattle with fescue toxicosis may have a decreased glutamatergic neurotransmission capacity due to consumption of ergopeptine alkaloids.

A vesicular transporter that mediates aspartate and glutamate neurotransmission

Aspartate, an excitatory amino acid, is known to be stored in synaptic vesicles and exocytosed from some neurons to perform aspartergic neurotransmission. Through in vitro reconstitution, we found that sialin, a lysosomal sialic acid exporter, is responsible for the vesicular storage of aspartate in hippocampal neurons and pinealocytes. Mutations found in Salla disease cause decreased aspartate transport activity without affecting sialic acid transport. Thus, sialin is a multifunctional transporter. It is possible that people with Salla disease lose the ability of aspartergic neurotransmission, and this could explain why Salla disease involves severe neurological defects.

From glutamate co-release to vesicular synergy: vesicular glutamate transporters

Recent data indicate that 'classical' neurotransmitters can also act as co-transmitters. This notion has been strengthened by the demonstration that three vesicular glutamate transporters (vesicular glutamate transporter 1 (VGLUT1), VGLUT2 and VGLUT3) are present in central monoamine, acetylcholine and GABA neurons, as well as in primarily glutamatergic neurons. Thus, intriguing questions are raised about the morphological and functional organization of neuronal systems endowed with such a dual signalling capacity. In addition to glutamate co-release, vesicular synergy - a process leading to enhanced packaging of the 'primary' transmitter - is increasingly recognized as a major property of the glutamatergic co-phenotype. The behavioural relevance of this co-phenotype is presently the focus of considerable interest.

Exciting times beyond the brain: metabotropic glutamate receptors in peripheral and non-neural tissues

Metabotropic glutamate (mGlu) receptors are G-protein-coupled receptors expressed primarily on neurons and glial cells, where they are located in the proximity of the synaptic cleft. In the central nervous system (CNS), mGlu receptors modulate the effects of l-glutamate neurotransmission in addition to that of a variety of other neurotransmitters. However, mGlu receptors also have a widespread distribution outside the CNS that has been somewhat neglected to date. Based on this expression, diverse roles of mGlu receptors have been suggested in a variety of processes in health and disease including controlling hormone production in the adrenal gland and pancreas, regulating mineralization in the developing cartilage, modulating lymphocyte cytokine production, directing the state of differentiation in embryonic stem cells, and modulating gastrointestinal secretory function. Understanding the role of mGlu receptors in the periphery will also provide a better insight into potential side effects of drugs currently being developed for neurological and psychiatric conditions. This review summarizes the new potential roles of mGlu receptors and raises the possibility of novel pharmacological targets for various disorders.

Disturbed neurotransmitter transporter expression in Alzheimer's disease brain

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by memory loss and behavioral and psychological symptoms of dementia. An imbalance of different neurotransmitters--glutamate, acetylcholine, dopamine, and serotonin--has been proposed as the neurobiological basis of behavioral symptoms in AD. The molecular changes associated with neurotransmission imbalance in AD are not clear. We hypothesized that altered reuptake of neurotransmitters by vesicular glutamate transporters (VGLUTs), excitatory amino acid transporters (EAATs), the vesicular acetylcholine transporter (VAChT), the serotonin reuptake transporter (SERT), or the dopamine reuptake transporter (DAT) are involved in the neurotransmission imbalance in AD. We tested this hypothesis by examining protein and mRNA levels of these transporters in postmortem prefrontal cortex from 10 AD patients and 10 matched non-AD controls. Compared with controls, protein and mRNA levels of VGLUTs, EAAT1-3, VAChT, and SERT were reduced significantly in AD. Expression of DAT and catechol O-methyltransferase was unchanged. Reduced VGLUTs and EAATs may contribute to an alteration in glutamatergic recycling, and reduced SERT could exacerbate depressive symptoms in AD. The reduced VAChT expression could contribute to the recognized cholinergic deficit in AD. Altered neurotransmitter transporters could contribute to the pathophysiology of AD and are potential targets for therapy.

Glutamatergic elements in an excitability and circumnutation mechanism

In plants, an electrical potential and circumnutation disturbances are a part of a response to environmental and internal stimuli. Precise relations between electrical potential changes and circumnutation mechanisms are unclear. We have found recently that glutamate (Glu) injection into Helianthus annuus stem induced a series of action potentials (APs) and a transient decrease in circumnutation activity. A theoretical explanation for this finding is discussed here taking into considerations data about the ion mechanism of AP and circumnutation as well as about the metabolic and signaling pathways of glutamate and their possible interactions.

A transgenic mouse line for molecular genetic analysis of excitatory glutamatergic neurons

Excitatory glutamatergic neurons are part of most of the neuronal circuits in the mammalian nervous system. We have used BAC-technology to generate a BAC-Vglut2::Cre mouse line where Cre expression is driven by the vesicular glutamate transporter 2 (Vglut2) promotor. This BAC-Vglut2::Cre mouse line showed specific expression of Cre in Vglut2 positive cells in the spinal cord with no ectopic expression in GABAergic or glycinergic neurons. This mouse line also showed specific Cre expression in Vglut2 positive structures in the brain such as thalamus, hypothalamus, superior colliculi, inferior colliculi and deep cerebellar nuclei together with nuclei in the midbrain and hindbrain. Cre-mediated recombination was restricted to Cre expressing cells in the spinal cord and brain and occurred as early as E 12.5. Known Vglut2 positive neurons showed normal electrophysiological properties in the BAC-Vglut2::Cre transgenic mice. Altogether, this BAC-Vglut2::Cre mouse line provides a valuable tool for molecular genetic analysis of excitatory neuronal populations throughout the mouse nervous system.

Role of mechanical stress-induced glutamate signaling-associated molecules in cytodifferentiation of periodontal ligament cells

In this study, we analyzed the effects of tensile mechanical stress on the gene expression profile of in vitro-maintained human periodontal ligament (PDL) cells. A DNA chip analysis identified 17 up-regulated genes in human PDL cells under the mechanical stress, including HOMER1 (homer homolog 1) and GRIN3A (glutamate receptor ionotropic N-methyl-d-aspartate 3A), which are related to glutamate signaling. RT-PCR and real-time PCR analyses revealed that human PDL cells constitutively expressed glutamate signaling-associated genes and that mechanical stress increased the expression of these mRNAs, leading to release of glutamate from human PDL cells and intracellular glutamate signal transduction. Interestingly, exogenous glutamate increased the mRNAs of cytodifferentiation and mineralization-related genes as well as the ALP (alkaline phosphatase) activities during the cytodifferentiation of the PDL cells. On the other hand, the glutamate signaling inhibitors riluzole and (+)-MK801 maleate suppressed the alkaline phosphatase activities and mineralized nodule formation during the cytodifferentiation and mineralization. Riluzole inhibited the mechanical stress-induced glutamate signaling-associated gene expressions in human PDL cells. Moreover, in situ hybridization analyses showed up-regulation of glutamate signaling-associated gene expressions at tension sites in the PDL under orthodontic tooth movement in a mouse model. The present data demonstrate that the glutamate signaling induced by mechanical stress positively regulates the cytodifferentiation and mineralization of PDL cells.

Tricks of the trade used to accelerate high-resolution structure determination of membrane proteins

The rate at which X-ray structures of membrane proteins are solved is on a par with that of soluble proteins in the late 1970s. There are still many obstacles facing the membrane protein structural community. Recently, there have been several technical achievements in the field that have started to dramatically accelerate structural studies. Here, we summarize these so-called 'tricks-of-the-trade' and include case studies of several mammalian transporters.

Genetic inactivation of the vesicular glutamate transporter 2 (VGLUT2) in the mouse: what have we learnt about functional glutamatergic neurotransmission?

During the past decade, three proteins that possess the capability of packaging glutamate into presynaptic vesicles have been identified and characterized. These three vesicular glutamate transporters, VGLUT1-3, are encoded by solute carrier genes Slc17a6-8. VGLUT1 (Slc17a7) and VGLUT2 (Slc17a6) are expressed in glutamatergic neurons, while VGLUT3 (Slc17a8) is expressed in neurons classically defined by their use of another transmitter, such as acetylcholine and serotonin. As glutamate is both a ubiquitous amino acid and the most abundant neurotransmitter in the adult central nervous system, the discovery of the VGLUTs made it possible for the first time to identify and specifically target glutamatergic neurons. By molecular cloning techniques, different VGLUT isoforms have been genetically targeted in mice, creating models with alterations in their glutamatergic signalling. Glutamate signalling is essential for life, and its excitatory function is involved in almost every neuronal circuit. The importance of glutamatergic signalling was very obvious when studying full knockout models of both VGLUT1 and VGLUT2, none of which were compatible with normal life. While VGLUT1 full knockout mice die after weaning, VGLUT2 full knockout mice die immediately after birth. Many neurological diseases have been associated with altered glutamatergic signalling in different brain regions, which is why conditional knockout mice with abolished VGLUT-mediated signalling only in specific circuits may prove helpful in understanding molecular mechanisms behind such pathologies. We review the recent studies in which mouse genetics have been used to characterize the functional role of VGLUT2 in the central nervous system.

Demonstration of vesicular glutamate transporter-1 in corticotroph cells in the anterior pituitary of the rat

Recent immunohistochemical studies of the rat adenohypophysis identified type-2 vesicular glutamate transporter (VGLUT2), a marker for glutamatergic neuronal phenotype, in high percentages of adenohypophysial gonadotrophs and thyrotrophs. The presence and molecular identity of amino acid neurotransmitters in the remaining hormone producing cell types are unknown. In the present study we addressed the putative synthesis of another glutamatergic marker, VGLUT1 by adenohypophysial cells. Immunohistochemical studies revealed VGLUT1 immunoreactivity in a small subset of polygonal medium-sized cells in the anterior lobe. Western blot analysis revealed a single major 60 kDa protein band in the adenohypophysis. Furthermore, the expression of VGLUT1 mRNA was confirmed by reverse transcription-polymerase chain reaction followed by sequence analysis of the amplicon. In contrast with rats which only showed VGLUT1 signal in the anterior lobe of the pituitary, mice contained high levels of VGLUT1 immunoreactivity in the intermediate, in addition to the anterior lobe. No signal was present in VGLUT1-knockout mice, providing evidence for specificity. In rats, results of colocalization studies with dual-immunofluorescent labeling provided evidence for VGLUT1 immunoreactivity in 45.9% of corticotrophs and 7.7% of luteinizing hormone beta-immunopositive gonadotrophs. Cells of the other peptide hormone phenotypes were devoid of VGLUT1 signal. A few cells in the adenohypophysis expressed both VGLUT1 and VGLUT2 immunoreactivities. The presence of the glutamate markers VGLUT1 and VGLUT2 in distinct populations of peptide hormone-secreting hypophysial cells highly indicates the involvement of endogenous glutamate release in autocrine/paracrine regulatory mechanisms. The biological function of adenohypophysial glutamate will require clarification.

Nitrate and glutamate as environmental cues for behavioural responses in plant roots

As roots explore the soil, they encounter a complex and fluctuating environment in which the different edaphic resources (water and nutrients) are heterogeneously distributed in space and time. Many plant species are able to respond to this heterogeneity by modifying their root system development, such that they colonize the most resource-rich patches of soil. The complexities of these responses, and their dependence on the implied ability to perceive and integrate multiple external signals, would seem to amply justify the term 'behaviour'. This review will consider the types of behaviour that are elicited in roots of Arabidopsis thaliana by exposure to variations in the external concentrations and distribution of two different N compounds, nitrate and glutamate. Molecular genetic studies have revealed an intricate N regulatory network at the root tip that is responsible for orchestrating changes in root growth rate and root architecture to accommodate variations in the extrinsic and intrinsic supply of N. The review will discuss what is known of the genetic basis for these responses and speculate on their physiological and ecological significance.

Docetaxel inhibits bone resorption through suppression of osteoclast formation and function in different manners

Osteoclasts are formed from the monocyte-macrophage lineage in response to receptor activator of nuclear factor kappaB ligand (RANKL) expressed by osteoblasts. Bone is the most common site of breast cancer metastasis, and osteoclasts play roles in the metastasis. The taxane-derived compounds paclitaxel and docetaxel are used for the treatment of malignant diseases, including breast cancer. Here we explored the effects of docetaxel on osteoclastic bone resorption in mouse culture systems. Osteoclasts were formed within 6 days in cocultures of osteoblasts and bone marrow cells treated with 1,25-dihydroxyvitamin D(3) plus prostaglandin E(2). Docetaxel at 10(-8) M inhibited osteoclast formation in the coculture when added for the entire culture period or for the first 3 days. Docetaxel, even at 10(-6) M added for the final 3 days, failed to inhibit osteoclast formation. Osteoprotegerin, a decoy receptor of RANKL, completely inhibited osteoclast formation when added for the final 3 days. Docetaxel at 10(-8) M inhibited the proliferation of osteoblasts and bone marrow cells. RANKL mRNA expression induced by 1,25-dihydroxyvitamin D(3) plus prostaglandin E(2) in osteoblasts was not affected by docetaxel even at 10(-6) M. Docetaxel at 10(-6) M, but not at 10(-8) M, inhibited pit-forming activity of osteoclasts cultured on dentine. Actin ring formation and L: -glutamate secretion by osteoclasts were also inhibited by docetaxel at 10(-6) M. Thus, docetaxel inhibits bone resorption in two different manners: inhibition of osteoclast formation at 10(-8) M and of osteoclast function at 10(-6) M. These results suggest that taxanes have beneficial effects in the treatment of bone metastatic cancers.

VGluT2 expression in painful Achilles and patellar tendinosis: evidence of local glutamate release by tenocytes

The pathogenesis of chronic tendinopathy is unclear. We have previously measured high intratendinous levels of glutamate in patients with tendinosis, suggesting potential roles of glutamate in the modulation of pain, vascular function, and degenerative changes including apoptosis of tenocytes. However, the origin of free glutamate found in tendon tissue is completely unknown. Surgical biopsies of pain-free normal tendons and tendinosis tendons (Achilles and patellar) were examined immunohistochemically using antibodies against vesicular glutamate transporters (VGluT1 and VGluT2), as indirect markers of glutamate release. In situ hybridization for VGluT2 mRNA was also conducted. Specific immunoreactions for VGluT2, but not VGluT1, could be consistently detected in tenocytes. However, there were interindividual variations in the levels of immunoreactivity. The level of immunoreaction for VGluT2 was higher in tendinosis tendons compared to normal tendons (p < 0.05). In situ hybridization of VGluT2 demonstrated that mRNA was localized in a similar pattern as the protein, with marked expression by certain tenocytes, particularly those showing abnormal appearances. Reactivity for VGluT1 and -2 was absent from nerves and vessel structures in both normal and painful tendons. The current data demonstrate that tenocytes may be involved in the regulation of extracellular glutamate levels in tendons. Specifically, the observations suggest that free glutamate may be locally produced and released by tenocytes, rather than by peripheral neurons. Excessive free glutamate is expected to impact a variety of autocrine and paracrine functions important in the development of tendinosis, including tenocyte proliferation and apoptosis, extracellular matrix metabolism, nociception, and blood flow.

Identification of a vesicular nucleotide transporter

ATP is a major chemical transmitter in purinergic signal transmission. Before secretion, ATP is stored in secretory vesicles found in purinergic cells. Although the presence of active transport mechanisms for ATP has been postulated for a long time, the proteins responsible for its vesicular accumulation remains unknown. The transporter encoded by the human and mouse SLC17A9 gene, a novel member of an anion transporter family, was predominantly expressed in the brain and adrenal gland. The mouse and bovine counterparts were associated with adrenal chromaffin granules. Proteoliposomes containing purified transporter actively took up ATP, ADP, and GTP by using membrane potential as the driving force. The uptake properties of the reconstituted transporter were similar to that of the ATP uptake by synaptic vesicles and chromaffin granules. Suppression of endogenous SLC17A9 expression in PC12 cells decreased exocytosis of ATP. These findings strongly suggest that SLC17A9 protein is a vesicular nucleotide transporter and should lead to the elucidation of the molecular mechanism of ATP secretion in purinergic signal transmission.

The ontogenic expressions of multiple vesicular glutamate transporters during postnatal development of rat pineal gland

The pineal gland expresses vesicular glutamate transporters 1 and 2 (VGLUT1 and VGLUT2), which are thought to transport glutamate into synaptic-like microvesicles in the pinealocytes. Recently, we reported that the rat pineal gland also expresses VGLUT1v which is a novel variant of VGLUT1 during the perinatal period. To explore the biological significance of these VGLUT expressions in pineal development, we studied the ontogeny of VGLUT in this gland by in situ hybridization, immunohistochemistry and quantitative reverse transcription-polymerase chain reaction (RT-PCR) using rats. Histological analysis revealed that intensities of VGLUT1 hybridization signal and immunostaining drastically increase by postnatal day (P) 7, whereas VGLUT2 expression exhibits high levels of mRNA and protein at birth and decreases gradually from P7 onward. Quantitative RT-PCR analysis supported these histological observations, showing that expressions of VGLUT1 and VGLUT2 exhibit opposite patterns to each other. Coinciding with VGLUT1-upregulation, RT-PCR data showed that expressions of dynamin 1 and endophilin 1, which are factors predictably involved in the endocytotic recovery of VGLUT1-associated vesicle, are also increased by P7. Quantitative RT-PCR analysis of VGLUT1v demonstrated that its mRNA expression is upregulated by P7, kept at the same level until P14, and apparently decreased at P21, suggesting its functional property required for a certain developmental event. Moreover, a comparison of mRNA expressions at daytime and nighttime revealed that neither VGLUT1 nor VGLUT1v shows any difference in both P7 and P21 glands, whereas VGLUT2 is significantly lower at daytime than at nighttime at P21 but not P7, the time point at which the melatonin rhythm is not yet generated. The present study shows that expressions of these VGLUT types are differentially regulated during postnatal pineal development, each presumably participating in physiologically distinct glutamatergic functions.

Expression of the vesicular glutamate transporters-1 and -2 in adult mouse dorsal root ganglia and spinal cord and their regulation by nerve injury

The expression of two vesicular glutamate transporters (VGLUTs), VGLUT1 and VGLUT2, was studied with immunohistochemistry in lumbar dorsal root ganglia (DRGs), the lumbar spinal cord and the skin of the adult mouse. About 12% and 65% of the total number of DRG neuron profiles (NPs) expressed VGLUT1 and VGLUT2, respectively. VGLUT1-immunoreactive (IR) NPs were usually medium- to large-sized, in contrast to a majority of small- or medium-sized VGLUT2-IR NPs. Most VGLUT1-IR NPs did not coexpress calcitonin gene-related peptide (CGRP) or bound isolectin B4 (IB4). In contrast, approximately 31% and approximately 42% of the VGLUT2-IR DRG NPs were also CGRP-IR or bound IB4, respectively. Conversely, virtually all CGRP-IR and IB4-binding NPs coexpressed VGLUT2. Moderate colocalization between VGLUT1 and VGLUT2 was also observed. Sciatic nerve transection induced a decrease in the overall number of VGLUT1- and VGLUT2-IR NPs (both ipsi- and contralaterally) and, in addition, a parallel, unilateral increase of VGLUT2-like immunoreactivity (LI) in a subpopulation of mostly small NPs. In the dorsal horn of the spinal cord, strong VGLUT1-LI was detected, particularly in deep dorsal horn layers and in the ventral horns. VGLUT2-LI was abundant throughout the gray spinal matter, 'radiating' into/from the white matter. A unilateral dorsal rhizotomy reduced VGLUT1-LI, while apparently leaving unaffected the VGLUT2-LI. Transport through axons for both VGLUTs was confirmed by their accumulation after compression of the sciatic nerve or dorsal roots. In the hind paw skin, abundant VGLUT2-IR nerve fibers were observed, sometimes associated with Merkel cells. Lower numbers of VGLUT1-IR fibers were also detected in the skin. Some VGLUT1-IR and VGLUT2-IR fibers were associated with hair follicles. Based on these data and those by Morris et al. [Morris JL, Konig P, Shimizu T, Jobling P, Gibbins IL (2005) Most peptide-containing sensory neurons lack proteins for exocytotic release and vesicular transport of glutamate. J Comp Neurol 483:1-16], we speculate that virtually all DRG neurons in adult mouse express VGLUTs and use glutamate as transmitter.

Synapse-specific localization of vesicular glutamate transporters in the rat olfactory bulb

Vesicular glutamate transporters (VGLUTs) mediate the packaging of the excitatory neurotransmitter glutamate into synaptic vesicles. Three VGLUT subtypes have so far been identified, with distinct expression patterns in the adult brain. Here, we investigated the spatial distribution of the three VGLUTs in the rat olfactory bulb, a brain region containing a variety of glutamate synapses, both axodendritic and dendrodendritic. Using multilabelling confocal microscopy and electron microscopic immunocytochemistry, we showed that each VGLUT isoform has a highly selective localization in olfactory bulb synapses. VGLUT1 is present at dendrodendritic synapses established by the output neurones (mitral and tufted cells) with bulbar interneurones in the glomerular layer and external plexiform layer, as well as in axonal synapses of the granule cell layer. By contrast, VGLUT2 is strongly expressed in axon terminals of olfactory sensory neurones, which establish synapses with second-order neurones in the glomerular neuropil. VGLUT2 is also found in the outer part of the external plexiform layer and in the granule cell layer but colocalizes only partially with VGLUT1. Finally, we showed that VGLUT3 is exclusively located in the glomerular neuropil, where it colocalizes extensively with the vesicular inhibitory amino acid transporter vesicular GABA transporter, suggesting that it is associated with a subset of inhibitory synapses. Together, these observations extend previous findings on VGLUT distribution in the forebrain, and suggest that each VGLUT subtype has a specific function in the distinct features of axodendritic and dendrodendritic synapses that characterize the olfactory bulb circuit.

Inhibitors of glutamate transport modulate distinct patterns in brain metabolism

High affinity uptake of glutamate plays a major role in the termination of excitatory neurotransmission. Identification of the ramifications of transporter function is essential to understand the diseases in which defective excitatory amino acid transporters (EAAT) have been implicated. In this work we incubated Guinea pig cortical tissue slices with [3-(13)C]pyruvate and major currently available glutamate uptake inhibitors and studied the resultant metabolic sequelae by (13)C and (1)H NMR spectroscopy using a multivariate statistical approach. Perturbation of glutamate uptake produced significant effects on metabolic flux through the Krebs cycle, and on glutamate/glutamine cycling rates, with this effect accounting for 76% of the variation in the total data set. The effects of all inhibitors were separable from each other along three major principal components. The competitive inhibitor L-CCG III ((2S,1'S,2'R)-2-carboxycyclopropyl)glycine) differed most from the other inhibitors, showing negative weightings on both the first and second principal components, whereas the EAAT2-specific inhibitor dihydrokainate (DHK) showed metabolic patterns similar to that of anti-endo-3,4-methanopyrolidine dicarboxylate but separate from those of DL-threo-beta-benzyloxyaspartate (TBOA) and L-trans-pyrrolidine-2,4-dicarboxylate (L-tPDC). This indicates that different inhibition mechanisms or different colocalisation of the separate transporter subtypes with glutamate receptors can produce significantly different metabolic and functional outcomes for the brain.

Distribution of neuropeptide S receptor mRNA and neurochemical characteristics of neuropeptide S-expressing neurons in the rat brain

Neuropeptide S (NPS) and its receptor (NPSR) constitute a novel neuropeptide system that is involved in regulating arousal and anxiety. The NPS precursor mRNA is highly expressed in a previously undescribed group of neurons located between the locus coeruleus (LC) and Barrington's nucleus. We report here that the majority of NPS-expressing neurons in the LC area and the principal sensory trigeminal nucleus are glutamatergic neurons, whereas many NPS-positive neurons in the lateral parabrachial nucleus coexpress corticotropin-releasing factor (CRF). In addition, we describe a comprehensive map of NPSR mRNA expression in the rat brain. High levels of expression are found in areas involved in olfactory processing, including the anterior olfactory nucleus, the endopiriform nucleus, and the piriform cortex. NPSR mRNA is expressed in several regions mediating anxiety responses, including the amygdaloid complex and the paraventricular hypothalamic nucleus. NPSR mRNA is also found in multiple key regions of sleep neurocircuitries, such as the thalamus, the hypothalamus, and the preoptic region. In addition, NPSR mRNA is strongly expressed in major output and input regions of hippocampus, including the parahippocampal regions, the lateral entorhinal cortex, and the retrosplenial agranular cortex. Multiple hypothalamic nuclei, including the dorsomedial and the ventromedial hypothalamic nucleus and the posterior arcuate nucleus, express high levels of NPSR mRNA, indicating that NPS may regulate energy homeostasis. These data suggest that the NPS system may play a key role in modulating a variety of physiological functions, especially arousal, anxiety, learning and memory, and energy balance.