Uptake of glutamate into synaptic vesicles by an inorganic phosphate transporter

Genetics of sensory mechanotransduction

The molecular mechanisms for the transduction of light and chemical signals in animals are fairly well understood. In contrast, the processes by which the senses of touch, balance, hearing, and proprioception are transduced are still largely unknown. Biochemical approaches to identify transduction components are difficult to use with mechanosensory systems, but genetic approaches are proving more successful. Genetic research in several organisms has demonstrated the importance of cytoskeletal, extracellular, and membrane components for sensory mechanotransduction. In particular, researchers have identified channel proteins in the DEG/ENaC and TRP families that are necessary for signaling in a variety of mechanosensory cells. Proof that these proteins are components of the transduction channel, however, is incomplete.

Identification of a new member of type I Na/phosphate co-transporter in the rat kidney

AIM

To clone new phosphate transporters from the kidney.

METHODS

EST bank was screened for new members of Na-dependent inorganic phosphate (P(i)) co-transporter. To obtain a full-length clone, a rat kidney cDNA library was screened with the EST clone as a probe.

RESULTS

We isolated a cDNA that encodes a type I Na/P(i) co-transporter. Based on the sequence homology and tissue distribution, it will be the rat ortholog of human NPT4 (Solute Carrier family 17, member A3, SLC17A3) and is designated rNPT4. When expressed in Xenopus oocytes, Na/P(i) co-transport activity was demonstrated. In Northern blotting analysis, rNPT4 was selectively expressed in the liver and kidney. In immunohistochemical analysis, rNPT4 was localized at the apical membrane of the proximal tubules (S2, S3) in the rat kidney. The electron microscopic studies confirmed this finding.

CONCLUSION

The present study revealed the expression of a new type I Na/P(i) co-transporter at the brush border membrane of the proximal tubules in the kidney.

VMAT2 binding is elevated in dopa-responsive dystonia: visualizing empty vesicles by PET

Dopa-responsive dystonia (DRD) is a lifelong disorder in which dopamine deficiency is not associated with neuronal loss and therefore it is an ideal human model for investigating the compensatory changes that occur in response to this biochemical abnormality. Using positron emission tomography (PET), we examined the (+/-)-alpha-[(11)C]dihydrotetrabenazine ([(11)C]DTBZ) binding potential of untreated DRD patients and normal controls. Two other PET markers of presynaptic nigrostriatal function, d-threo-[(11)C]methylphenidate ([(11)C]MP) and 6-[(18)F]fluoro-L-dopa ([(18)F]-dopa), and [(11)C]raclopride were also used in the study. We found increased [(11)C]DTBZ binding potential in the striatum of DRD patients. By contrast, no significant changes were detected in either [(11)C]MP binding potential or [(18)F]-dopa uptake rate constant. In addition, we found evidence for increased dopamine turnover in one DRD patient by examining changes in [(11)C]raclopride binding potential in relation to levodopa treatment. We propose that the increase in [(11)C]DTBZ binding likely reflects the dramatic decrease in the intravesicular concentration of dopamine that occurs in DRD; upregulation of vesicular monoamine transporter type 2 (VMAT2) expression may also contribute. Our findings suggest that the striatal expression of VMAT2 (as estimated by [(11)C]DTBZ binding) is not coregulated with dopamine synthesis. This is in keeping with a role for VMAT2 in other cellular processes (i.e., sequestration and release from the cell of potential toxic products), in addition to its importance for the quantal release of monoamines.

Amino acid transporters: roles in amino acid sensing and signalling in animal cells

Amino acid availability regulates cellular physiology by modulating gene expression and signal transduction pathways. However, although the signalling intermediates between nutrient availability and altered gene expression have become increasingly well documented, how eukaryotic cells sense the presence of either a nutritionally rich or deprived medium is still uncertain. From recent studies it appears that the intracellular amino acid pool size is particularly important in regulating translational effectors, thus, regulated transport of amino acids across the plasma membrane represents a means by which the cellular response to amino acids could be controlled. Furthermore, evidence from studies with transportable amino acid analogues has demonstrated that flux through amino acid transporters may act as an initiator of nutritional signalling. This evidence, coupled with the substrate selectivity and sensitivity to nutrient availability classically associated with amino acid transporters, plus the recent discovery of transporter-associated signalling proteins, demonstrates a potential role for nutrient transporters as initiators of cellular nutrient signalling. Here, we review the evidence supporting the idea that distinct amino acid "receptors" function to detect and transmit certain nutrient stimuli in higher eukaryotes. In particular, we focus on the role that amino acid transporters may play in the sensing of amino acid levels, both directly as initiators of nutrient signalling and indirectly as regulators of external amino acid access to intracellular receptor/signalling mechanisms.

Functional G-protein heterotrimers are associated with vesicles of putative glutamatergic terminals: implications for regulation of transmitter uptake

Changes in the vesicular transmitter content modulate synaptic strength and may contribute to synaptic plasticity. Several transporters mediating transmitter uptake into small synaptic vesicles (SSVs) have been identified but their regulation is largely unknown. Here we show by quantitative immunoelectron microscopy that the heterotrimeric G-protein subunits Galphao(2), Galpha(q/11), Gbeta(2), and Ggamma(7) are associated with vesicle-containing areas in terminals of cerebellar parallel fibers. These terminals also contain the vesicular glutamate transporter 1 (VGLUT1). In contrast, SSVs of climbing fiber terminals that contain VGLUT2 express one of the Gbeta-subunits Gbeta(1), Gbeta(3), or Gbeta(4), Ggamma(7), and one Galpha-subunit, probably Galphao(2). Glutamate uptake into cerebellar SSVs was inhibited by more than 50% by GMppNp, an activator of G proteins. Thus, vesicle populations with different subtypes of vesicular glutamate transporters contain functional G proteins with distinct subunit profiles. Heterotrimeric G proteins may play an important role in the control of vesicular filling.

Functional properties and cellular distribution of the system A glutamine transporter SNAT1 support specialized roles in central neurons

Glutamine, the preferred precursor for neurotransmitter glutamate and GABA, is likely to be the principal substrate for the neuronal System A transporter SNAT1 in vivo. We explored the functional properties of SNAT1 (the product of the rat Slc38a1 gene) by measuring radiotracer uptake and currents associated with SNAT1 expression in Xenopus oocytes and determined the neuronal-phenotypic and cellular distribution of SNAT1 by confocal laser-scanning microscopy alongside other markers. We found that SNAT1 mediates transport of small, neutral, aliphatic amino acids including glutamine (K0.5 approximately 0.3 mm), alanine, and the System A-specific analogue 2-(methylamino)isobutyrate. Amino acid transport is driven by the Na+ electrochemical gradient. The voltage-dependent binding of Na+ precedes that of the amino acid in a simultaneous transport mechanism. Li+ (but not H+) can substitute for Na+ but results in reduced Vmax. In the absence of amino acid, SNAT1 mediates Na+-dependent presteady-state currents (Qmax approximately 9 nC) and a nonsaturable cation leak with selectivity Na+, Li+ >> H+, K+. Simultaneous flux and current measurements indicate coupling stoichiometry of 1 Na+ per 1 amino acid. SNAT1 protein was detected in somata and proximal dendrites but not nerve terminals of glutamatergic and GABAergic neurons throughout the adult CNS. We did not detect SNAT1 expression in astrocytes but detected its expression on the luminal membranes of the ependyma. The functional properties and cellular distribution of SNAT1 support a primary role for SNAT1 in glutamine transport serving the glutamate/GABA-glutamine cycle in central neurons. Localization of SNAT1 to certain dopaminergic neurons of the substantia nigra and cholinergic motoneurons suggests that SNAT1 may play additional specialized roles, providing metabolic fuel (via alpha-ketoglutarate) or precursors (cysteine, glycine) for glutathione synthesis.

Alpha-neurexins couple Ca2+ channels to synaptic vesicle exocytosis

Synapses are specialized intercellular junctions in which cell adhesion molecules connect the presynaptic machinery for neurotransmitter release to the postsynaptic machinery for receptor signalling. Neurotransmitter release requires the presynaptic co-assembly of Ca2+ channels with the secretory apparatus, but little is known about how synaptic components are organized. Alpha-neurexins, a family of >1,000 presynaptic cell-surface proteins encoded by three genes, link the pre- and postsynaptic compartments of synapses by binding extracellularly to postsynaptic cell adhesion molecules and intracellularly to presynaptic PDZ domain proteins. Using triple-knockout mice, we show that alpha-neurexins are not required for synapse formation, but are essential for Ca2+-triggered neurotransmitter release. Neurotransmitter release is impaired because synaptic Ca2+ channel function is markedly reduced, although the number of cell-surface Ca2+ channels appears normal. These data suggest that alpha-neurexins organize presynaptic terminals by functionally coupling Ca2+ channels to the presynaptic machinery.

Brain synaptic junctional proteins at the acrosome of rat testicular germ cells

Proteins of the presynaptic exocytic machinery have been found associated with the acrosome of male germ cells, suggesting that the sperm acrosome reaction and neurotransmission at chemical synapses may share some common mechanisms. To substantiate this hypothesis, we studied the expression and ultrastructural localization of prominent pre- and postsynaptic protein components in rat testis. The presynaptic membrane trafficking proteins SV2 and complexin, the vesicular amino acid transporters VGLUT and VIAAT, the postsynaptic scaffolding protein ProSAP/Shank, and the postsynaptic calcium-sensor protein caldendrin, could be identified in germ line cells. Immunogold electron microscopy revealed an association of these proteins with the acrosome. In addition, evidence was obtained for the expression of the plasmalemmal glutamate transporters GLT1 and GLAST in rat sperm. The novel finding that not only presynaptic proteins, which are believed to be involved in membrane fusion processes, but also postsynaptic elements are present at the acrosome sheds new light on its structural organization. Moreover, our data point to a possible role for neuroactive amino acids in reproductive physiology.

Presynaptic control of quantal size: kinetic mechanisms and implications for synaptic transmission and plasticity

Although the strength of quantal synaptic transmission is jointly controlled by pre- and post-synaptic mechanisms, the presynaptic mechanisms remain substantially less well characterized. Recent studies reveal that a single package of neurotransmitter is generally insufficient to activate all available postsynaptic receptors, whereas the sum of transmitter from multiple vesicles can result in receptor saturation. Thus, depending upon the number of vesicles released, a given synaptic pathway might be either 'reliable' or 'unreliable'. A lack of receptor saturation in turn makes it possible to modify quantal size by altering the flux of transmitter through the synaptic cleft. Studies are now illuminating several new mechanisms behind the regulation of this transmitter flux--characteristics that control how transmitter is loaded into vesicles, how it is released and the manner by which it interacts with postsynaptic receptors.

Subtype switching of vesicular glutamate transporters at parallel fibre-Purkinje cell synapses in developing mouse cerebellum

Two subtypes of the vesicular glutamate transporter are expressed differentially in two excitatory afferents synapsing on to Purkinje cells: VGluT1 (BNPI) in axon terminals of cerebellar granule cells (i.e. parallel fibres; PFs) and VGluT2 (DNPI) in those of the inferior olivary neurons (climbing fibres; CFs). In the present study, we examined their expression in the developing mouse cerebellum. By in situ hybridization, the inferior olivary nucleus selectively expressed VGluT2 mRNA through postnatal life. In the cerebellum, both subtypes were transcribed in the external and internal granular layers during the first postnatal week. Thereafter, VGluT1 mRNA showed marked upregulation in the internal granular layer, whereas VGluT2 mRNA disappeared from the external and internal granular layers by the end of the third postnatal week. By immunohistochemistry, CF terminals consistently exhibited VGluT2 immunoreactivity in the postnatal cerebellum. By contrast, in the first 10 days of postnatal life, VGluT2 predominated in PF terminals, despite the transcription of both transporters in developing granule cells. During the second 10 days, VGluT2 in PF terminals was replaced with VGluT1 from deep regions of the molecular layer upwards, correlating with dendritic translocation of CFs. This replacement was accomplished by postnatal day 30. Taking that late-borne PFs are laid down successively on earlier ones in the molecular layer, the deep-to-superficial replacement represents maturation-linked switching from VGluT2 to VGluT1 in individual PFs, and is likely to be regulated at both the transcription and translation levels.

Association of gephyrin with synaptic and extrasynaptic GABAA receptors varies during development in cultured hippocampal neurons

Several studies have reported extrasynaptic clusters of GABAA receptors in hippocampal neurons. Yet their functional relevance as well as their evolution in relation with gephyrin during synaptogenesis remain unknown. We have analyzed the expression pattern of the main proteins of the GABAergic synapses during synaptogenesis in cultured hippocampal neurons. We found that GABAergic terminals, characterized by VIAAT and GAD-65 expression, differentiated 3 to 7 days after the glutamatergic endings. At the postsynaptic side, the GABAAR- beta3 subunit was first diffuse and then clustered when GABAergic terminals differentiated and gephyrin formed large clusters. Colocalization of these proteins was high and increased with development. At later stages, GABAAR beta3 clusters colocalized with gephyrin at synaptic but also at extrasynaptic sites. GABAAR gamma2 subunits were directly expressed as clusters which were first extrasynaptic and not associated with gephyrin. Subsequently, the GABAAR gamma2 subunits associated with gephyrin at synaptic and/or extrasynaptic sites. Our data indicate that formation of GABAAR gamma2 subunit clusters is gephyrin independent.

Differential distribution of vesicular glutamate transporters in the rat cerebellar cortex

The chemical organization of excitatory axon terminals in the rat cerebellar cortex was examined by immunocytochemistry and in situ hybridization histochemistry of vesicular glutamate transporters 1 and 2 (VGluT1 and VGluT2). Chemical depletion of the inferior olivary complex neurons by 3-acetylpyridine treatment almost completely removed VGluT2 immunoreactivity from the molecular layer, leaving VGluT1 immunoreactivity apparently intact. On the other hand, neuronal deprivation of the cerebellar cortex by kainic acid injection induced a large loss of VGluT1 immunoreactivity in the molecular layer. In the cerebellar granular layer, both VGluT1 and VGluT2 immunoreactivities were found in mossy fiber terminals, and the two immunoreactivities were mostly colocalized in single-axon terminals. Signals for mRNA encoding VGluT2 were found in the inferior olivary complex, and those for VGluT1 and VGluT2 mRNAs were observed in most brainstem precerebellar nuclei sending mossy fibers, such as the pontine, pontine tegmental reticular, lateral reticular and external cuneate nuclei. These results indicate that climbing and parallel fibers selectively use VGluT2 and VGluT1, respectively, whereas mossy fibers apply both VGluT1 and VGluT2 together to accumulate glutamate into synaptic vesicles. Since climbing-fiber and parallel-fiber terminals are known to make depressing and facilitating synapses, respectively, VGluT1 and VGluT2 might have distinct properties associated with those synaptic characteristics. Thus, it would be the next interesting issue to determine whether mossy-fiber terminals co-expressing VGluT1 and VGluT2 show synaptic facilitation or depression.

Vesicular glutamate transporter 2 immunoreactivity in putative vagal mechanosensor terminals of mouse and rat esophagus: indication of a local effector function?

Intraganglionic laminar endings (IGLEs) represent the major vagal afferent structures throughout the gastrointestinal tract. Previous ultrastructural investigations have revealed synaptic contacts of IGLEs on myenteric neurons. Thus, in addtion to functioning probably as mechanosensors, IGLEs may also synaptically influence myenteric neurons. In search of clues for potential transmitters in IGLEs, we investigated, by combined neuronal tracing and immunocytochemistry in the esophagus, the correlation between IGLEs and vesicular glutamate transporter 2 (VGLUT2), which is considered a reliable marker for glutamatergic neurons. In rat esophagus, IGLEs were immunostained with calretinin. In the mouse, anterograde wheat germ agglutinin/horseradish peroxidase (WGA-HRP) tracing from nodose ganglion was used in order to label esophageal IGLEs. Confocal laser scanning microscopy demonstrated that VGLUT2 immunoreactivity was highly colocalized with synaptophysin and that both calretinin and tyramide amplified WGA-HRP in rat and mouse esophagus, respectively. No colocalization was found with calcitonin gene-related peptide, a marker for spinal primary afferents. Thus, VGLUT2 is found in vagal afferent endings in the esophagus, suggesting that glutamate is contained in, and probably released from, synaptic vesicles previously described in IGLEs. Functional evidence pending, this finding is in favor of a local effector function of IGLEs onto myenteric neurons.

Characterization of vesicular glutamate transporter in pancreatic alpha - and beta -cells and its regulation by glucose

Glutamate has been suggested to play an important role in the release of insulin and glucagon from pancreatic cells via exocytosis. Vesicular glutamate transporter is a rate-limiting step for glutamate release and is involved in the glutamate-evoked exocytosis. Two vesicular glutamate transporters (VGLUT1 and -2) have recently been cloned from the brain. In this report, we first functionally characterized vesicular glutamate transporter in cultured pancreatic alpha- and beta-cells, and then detected mRNA expression of VGLUT1 and -2 in these cells. We also investigated the effect of high or low level of glucose on vesicular glutamate transport in cultured pancreas cells. Our results suggest that both alpha- and beta-cells contain functional vesicular glutamate transporter. The transport characteristics are similar to the cloned neuronal VGLUT1 and -2 in regard to ATP dependence, substrate specificity, kinetics, and chloride dependence. VGLUT2 mRNA is expressed in both alpha- and beta-cells, whereas VGLUT1 is only expressed in beta-cells. High (12.8 mM) and low (2.8 mM) concentrations of glucose increased vesicular glutamate transport in beta- and alpha-cells, respectively. VGLUT2 mRNA was significantly increased in beta- and alpha-cells by high and low glucose concentration, respectively. This increase in VGLUT2 mRNA was suppressed by actinomycin D. We conclude that both alpha- and beta-cells possess functional vesicular glutamate transporters regulated by alteration in glucose concentration, partly via the transcriptional mechanism.

From genes to integrative physiology: ion channel and transporter biology in Caenorhabditis elegans

The stunning progress in molecular biology that has occurred over the last 50 years drove a powerful reductionist approach to the study of physiology. That same progress now forms the foundation for the next revolution in physiological research. This revolution will be focused on integrative physiology, which seeks to understand multicomponent processes and the underlying pathways of information flow from an organism's "parts" to increasingly complex levels of organization. Genetically tractable and genomically defined nonmammalian model organisms such as the nematode Caenorhabditis elegans provide powerful experimental advantages for elucidating gene function and the molecular workings of complex systems. This review has two main goals. The first goal is to describe the experimental utility of C. elegans for investigating basic physiological problems. A detailed overview of C. elegans biology and the experimental tools, resources, and strategies available for its study is provided. The second goal of this review is to describe how forward and reverse genetic approaches and direct behavioral and physiological measurements in C. elegans have generated novel insights into the integrative physiology of ion channels and transporters. Where appropriate, I describe how insights from C. elegans have provided new understanding of the physiology of membrane transport processes in mammals.

Expression of vesicular glutamate transporters, VGluT1 and VGluT2, in axon terminals of nociceptive primary afferent fibers in the superficial layers of the medullary and spinal dorsal horns of the rat

We examined immunohistochemically whether the vesicular glutamate transporters (VGluTs), VGluT1 and VGluT2, might be expressed in synaptic terminals of nociceptive primary afferent fibers within laminae I and II of the medullary and spinal dorsal horns of the rat. VGluT1 immunoreactivity (IR) was intense in the inner part of lamina II but weak in lamina I and the outer part of lamina II. VGluT2-IR was most intense in lamina I and the outer part of lamina II. Expression of VGluTs in synaptic terminals was confirmed by dual immunofluorescence histochemistry for VGluTs and synaptophysin. Expression of VGluTs in axon terminals of primary afferent fibers terminating in laminae I and II was also confirmed immunohistochemically after unilateral dorsal rhizotomy. The dual immunofluorescence histochemistry indicated expression of VGluTs in substance P (SP)-containing axon terminals in lamina I and the outer part of lamina II. Electron microscopy confirmed the coexpression of VGluTs and SP in axon terminals within laminae I and II; VGluTs was associated with round synaptic vesicles at the asymmetric synapses. It was further observed that isolectin IB4, a marker for unmyelinated axons, often bound with VGluT2-immunopositive structures but rarely with VGluT1-immunopositive structures in lamina II. Thus, the results indicated in laminae I and II of the medullary and spinal dorsal horns that both VGluT1 and VGluT2 were expressed in axon terminals of primary afferent fibers, including SP-containing nociceptive fibers and that VGluT in unmyelinated primary afferent fibers terminating in lamina II was primarily VGluT2.

Glycolysis and glutamate accumulation into synaptic vesicles. Role of glyceraldehyde phosphate dehydrogenase and 3-phosphoglycerate kinase

Glucose is the major source of brain energy and is essential for maintaining normal brain and neuronal function. Hypoglycemia causes impaired synaptic transmission. This occurs even before significant reduction in global cellular ATP concentration, and relationships among glycolysis, ATP supply, and synaptic transmission are not well understood. We demonstrate that the glycolytic enzymes glyceraldehyde phosphate dehydrogenase (GAPDH) and 3-phosphoglycerate kinase (3-PGK) are enriched in synaptic vesicles, forming a functional complex, and that synaptic vesicles are capable of accumulating the excitatory neurotransmitter glutamate by harnessing ATP produced by vesicle-bound GAPDH/3-PGK at the expense of their substrates. The GAPDH inhibitor iodoacetate suppressed GAPDH/3-PGK-dependent, but not exogenous ATP-dependent, [(3)H]glutamate uptake into isolated synaptic vesicles. It also decreased vesicular [(3)H]glutamate content in the nerve ending preparation synaptosome; this decrease was reflected in reduction of depolarization-induced [(3)H]glutamate release. In contrast, oligomycin, a mitochondrial ATP synthase inhibitor, had minimal effect on any of these parameters. ADP at concentrations above 0.1 mm inhibited vesicular glutamate and dissipated membrane potential. This suggests that the coupled GAPDH/3-PGK system, which converts ADP to ATP, ensures maximal glutamate accumulation into presynaptic vesicles. Together, these observations provide insight into the essential nature of glycolysis in sustaining normal synaptic transmission.

Postnatal development of the glutamate vesicular transporter VGLUT1 in rat cerebral cortex

The expression of the vesicular glutamate transporter VGLUT1 in the rat neocortex was studied during postnatal development using immunocytochemistry and Western blotting. At all ages, VGLUT immunoreactivity is localized to puncta that coexpress the presynaptic marker synaptophysin. VGLUT1 immunoreactivity is faint at birth, increases in the subplate during the first postnatal week, invades the supragranular layers in the second week and reaches the adult pattern at P20-P30. Its spatial and temporal maturation patterns suggest that VGLUT1 may be the vesicular transporter in developing corticocortical connections.

Phosphorylation of proteins in neuron terminals: specificity depends on coincidental signaling

We investigate the role of neuronal coincidental signaling mediated by the second messengers, on phosphorylation of three major proteins of neurosecretory vesicles. Our data show that different combinations of coincidental signaling generate specific pattern of phosphoproteins and not strictly additional effects. This suggests that an added phosphate on a site might 'mask' or 'unmask' the next sites for specific kinases and phosphatases action by inducing conformation change or protein association. We show that a function of vesicles such as the uptake of glutamate is highly regulated by coincidental signaling.

Distribution of vesicular glutamate transporter-2 messenger ribonucleic Acid and protein in the septum-hypothalamus of the rat

The excitatory neurotransmitter glutamate is involved in the control of most, perhaps all, neuroendocrine systems, yet the sites of glutamatergic neurons and their processes are unknown. Here, we used in situ hybridization and immunohistochemistry for the neuron-specific vesicular glutamate transporter-2 (VGLUT2) to identify the neurons in female rats that synthesize the neurotransmitter glutamate as well as their projections throughout the septum-hypothalamus. The results show that glutamatergic neurons are present in the septum-diagonal band complex and throughout the hypothalamus. The preoptic area and ventromedial and dorsomedial nuclei are particularly rich in glutamatergic neurons, followed by the supraoptic, paraventricular, and arcuate nuclei, whereas the suprachiasmatic nucleus does not express detectable amounts of VGLUT2 mRNA. Immunoreactive neurites are seen in very high densities in all regions analyzed, particularly in the preoptic region, followed by the ventromedial, dorsomedial, and arcuate nuclei as well as the external layer of the median eminence, whereas the mammillary complex does not exhibit VGLUT2 immunoreactivity. Many VGLUT2 immunoreactive fibers also contained synaptophysin, suggesting that the transporter is indeed localized to presynaptic terminals. Together, the results identify glutamatergic cell bodies throughout the septum-hypothalamus in region-specific patterns and show that glutamatergic nerve terminals are present in very large numbers such that most neurons in these brain regions can receive glutamatergic input. We examined the GnRH system as an example of a typical neuroendocrine system and could show that the GnRH perikarya are closely apposed by many VGLUT2-immunoreactive boutons, some of which also contained synaptophysin. The presence of VGLUT2 mRNA-containing cells in specific nuclei of the hypothalamus indicates that many neuroendocrine neurons coexpress glutamate as neurotransmitter, in addition to neuropeptides. These systems include the oxytocin, vasopressin, or CRH neurons as well as many others in the periventricular and mediobasal hypothalamus. The presence of VGLUT2 mRNA in steroid-sensitive regions of the hypothalamus, such as the anteroventral periventricular, paraventricular, or ventromedial nuclei indicates that gonadal and adrenal steroid can directly alter the functions of these glutamatergic neurons.

Vesicular neurotransmitter transporter expression in developing postnatal rodent retina: GABA and glycine precede glutamate

Vesicular transporters regulate the amount and type of neurotransmitter sequestered into synaptic vesicles and, hence, the kind of signal transmitted to postsynaptic neurons. Glutamate is the prominent excitatory neurotransmitter in retina; GABA and glycine are the main inhibitory neurotransmitters. Little is known about the ontogeny of vesicular neurotransmission in retina. We investigated expression of glutamatergic [vesicular glutamate transporter 1 (VGLUT1)] and GABA/glycinergic [vesicular GABA/glycine transporter (VGAT)] vesicular transporters in postnatal retina. VGLUT1 labels glutamatergic synapses. VGLUT1 and synaptic vesicle 2 colocalized to photoreceptor terminals. VGLUT1 colocalized with PKC to rod bipolar terminals and to ON bipolar terminals in metabotropic glutamate receptor 6+/- mice. Developmentally, VGAT expression precedes VGLUT1. In rat and mouse retina, VGAT occurred in the inner retina by postnatal day 1 (P1). In rat retina, VGLUT1 was in the outer retina by P5-P7 and the inner retina by P7. In the mouse retina, VGLUT1 expression was in the outer retina by P3 and the inner retina by P5. Both rat and mouse retina had an adult pattern of VGLUT1 expression by P14. VGLUT1 expression precedes ribbon synapses, which are first observed in the inner retina at P11 (Fisher, 1979) in mouse and P13 (Horsburgh and Sefton, 1987) in rat. The ribbon synapse marker RIBEYE was not detected in inner retina of P5 or P7 rat. Spontaneous EPSCs in mouse ganglion cells were recorded as early as P7. Together, these findings indicate that vesicular GABA and glycine transmission precedes vesicular glutamate transmission in developing rodent retina. Furthermore, vesicular glutamate transmission likely occurs before ribbon synapse formation in the inner retina.

Secretory granule-mediated co-secretion of L-glutamate and glucagon triggers glutamatergic signal transmission in islets of Langerhans

L-Glutamate is believed to function as an intercellular transmitter in the islets of Langerhans. However, critical issues, i.e. where, when and how L-glutamate appears, and what happens upon stimulation of glutamate receptors in the islets, remain unresolved. Vesicular glutamate transporter 2 (VGLUT2), an isoform of the vesicular glutamate transporter essential for neuronal storage of L-glutamate, is expressed in alpha cells (Hayashi, M., Otsuka, M., Morimoto, R., Hirota, S., Yatsushiro, S., Takeda, J., Yamamoto, A., and Moriyama, Y. (2001) J. Biol. Chem. 276, 43400-43406). Here we show that VGLUT2 is specifically localized in glucagon-containing secretory granules but not in synaptic-like microvesicles in alpha TC6 cells, clonal alpha cells, and islet alpha cells. VGLUT1, another VGLUT isoform, is also expressed and localized in secretory granules in alpha cells. Low glucose conditions triggered co-secretion of stoichiometric amounts of L-glutamate and glucagon from alpha TC6 cells and isolated islets, which is dependent on temperature and Ca(2+) and inhibited by phentolamine. Similar co-secretion of L-glutamate and glucagon from islets was observed upon stimulation of beta-adrenergic receptors with isoproterenol. Under low glucose conditions, stimulation of glutamate receptors facilitates secretion of gamma-aminobutyric acid from MIN6 m9, clonal beta cells, and isolated islets. These results indicate that co-secretion of L-glutamate and glucagon from alpha cells under low glucose conditions triggers GABA secretion from beta cells and defines the mode of action of L-glutamate as a regulatory molecule for the endocrine function. To our knowledge, this is the first example of secretory granule-mediated glutamatergic signal transmission.

Behavioral plasticity in C. elegans: paradigms, circuits, genes

Life in the soil is an intellectual and practical challenge that the nematode Caenorhabditis elegans masters by utilizing 302 neurons. The nervous system assembled by these 302 neurons is capable of executing a variety of behaviors, some of respectable complexity. The simplicity of the nervous system, its thoroughly characterized structure, several sets of well-defined behaviors, and its genetic amenability combined with its isogenic background make C. elegans an attractive model organism to study the genetics of behavior. This review describes several behavioral plasticity paradigms in C. elegans and their underlying neuronal circuits and then goes on to review the forward genetic analysis that has been undertaken to identify genes involved in the execution of these behaviors. Lastly, the review outlines how reverse genetics and genomic approaches can guide the analysis of the role of genes in behavior and why and how they will complement the forward genetic analysis of behavior.

Co-expression of vesicular glutamate transporters (VGLUT1 and VGLUT2) and their association with synaptic-like microvesicles in rat pinealocytes

A vesicular glutamate transporter (VGLUT) is responsible for the accumulation of l-glutamate in synaptic vesicles in glutamatergic neurons. Two isoforms, VGLUT1 and VGLUT2, have been identified, which are complementarily expressed in these neurons. Mammalian pinealocytes, endocrine cells for melatonin, are also glutamatergic in nature, accumulate l-glutamate in synaptic-like microvesicles (SLMVs), and secrete it through exocytosis. Although the storage of l-glutamate in SLMVs is mediated through a VGLUT, the molecular nature of the transporter is less understood. We recently observed that VGLUT2 is expressed in pinealocytes. In the present study, we show that pinealocytes also express VGLUT1. RT-PCR and northern blot analyses indicated expression of the VGLUT1 gene in pineal gland. Western blotting with specific antibodies against VGLUT1 indicated the presence of VGLUT1 in pineal gland. Indirect immunofluorescence microscopy with a section of pineal gland and cultured cells indicated that VGLUT1 and VGLUT2 are co-localized with process terminal regions of pinealocytes. Furthermore, immunoelectronmicroscopy as well as subcellular fractionation studies revealed that both VGLUT1 and VGLUT2 are specifically associated with SLMVs. These results indicate that both VGLUTs are responsible for storage of l-glutamate in SLMVs in pinealocytes. Pinealocytes are the first exception as to complementary expression of VGLUT1 and VGLUT2.

Expression of vesicular glutamate transporters in rat lumbar spinal cord, with a note on dorsal root ganglia

Three vesicular glutamate transporters (VGLUTs) have been recently identified and their distribution has been mapped in various brain areas. In the present study, we used morphological approaches to investigate their expression in the rat lumbar spinal cord and dorsal root ganglia. Our results show a complementary distribution of VGLUT-expressing fibers in the spinal cord, with no overlapping in nerve endings. In the dorsal horn, VGLUT1 is most abundant in mechanosensory/proprioceptive deep afferent fibers. VGLUT2 and VGLUT3 are expressed only at moderate levels in primary sensory afferent fibers and are not used by central projections of nociceptive neurons. VGLUT1 and VGLUT2 mRNAs are mainly segregated in superficial laminae but colocalized in deeper laminae. Weak expression of VGLUT3 mRNA is only detected in deep laminae. The colocalization of VGLUT1 and VGLUT2 transcripts in most sensory neurons of the dorsal root ganglia is not in agreement with the clear segregation between the proteins in their spinal projections. Such a discrepancy suggests targeting mechanisms specific for each transporter and/or a distinct regulation of their translation. In the ventral horn, the expression of VGLUT1 and VGLUT2 mRNAs in motoneuron perikarya suggests the possible unexpected role of glutamate in the vertebrate neuromuscular junction. These results demonstrate the existence of different subpopulations of glutamate nerve terminals in the rat lumbar spinal cord and suggest that functionally distinct subsets of excitatory glutamatergic neuronal networks are involved in sensory processing and motor control.

Relationship between two types of vesicular glutamate transporters and neurokinin-1 receptor-immunoreactive neurons in the pre-Bötzinger complex of rats: light and electron microscopic studies

Our previous study demonstrated GABAergic and glycinergic synapses onto neurokinin-1 receptor (NK1R)-immunoreactive (ir) neurons in the pre-Bötzinger complex (pre-BötC), the hypothesized kernel of normal respiratory rhythmogenesis. In the present study, we aimed to identify glutamatergic synapses onto NK1R-ir pre-BötC neurons, as excitatory synaptic transmission is a prerequisite to normal respiratory rhythmogenesis. Two types of vesicular glutamate transporters (VGLUT), VGLUT1 and VGLUT2, have been recently implicated in glutamate-mediated transmission. The present study used immunofluorescence and immunogold-silver staining to determine the relationship between the transporters and NK1R-ir neurons in the pre-BötC of adult rats. Under the confocal laser-scanning microscope, VGLUT2-ir boutons were found to be widely distributed in the pre-BötC, some of which were in close apposition to NK1R-ir somas and dendrites. VGLUT1-ir boutons were relatively rare and only a few were found to be in close apposition to NK1R-ir somas and dendrites. Electron microscopic observation revealed that approximately 41% of VGLUT2-ir terminals were in close apposition to, or made asymmetric synapses with NK1R-ir somas and dendrites in the pre-BötC. On the other hand, 50.5% of NK1R-ir dendrites were closely apposed to, or synapsed with VGLUT2-ir terminals. Occasionally, VGLUT1-ir terminals were found in close apposition to NK1R-ir somas or dendrites, but we were unable to identify synapses between them. The present findings provide the morphological basis for excitatory synaptic inputs onto NK1R-ir neurons in the pre-BötC. VGLUT2 may be involved in a dominant excitatory synaptic pathway for normal respiratory rhythmogenesis.

Inspiratory augmenting bulbospinal neurons express both glutamatergic and enkephalinergic phenotypes

Many of the inspiratory augmenting (I-AUG) neurons of the rostral ventral respiratory group (rVRG) are premotor neurons that excite phrenic motor neurons during inspiration, probably by releasing glutamate. In the present study, we demonstrate that these neurons are indeed glutamatergic, in that their cell bodies contain vesicular glutamate transporter-2 (VGLUT2) mRNA and spinal terminals from neurons in the region of the rVRG contain VGLUT2 protein. We also demonstrate by using parallel in situ hybridization and immunocytochemical evidence that most rVRG inspiratory premotor neurons are enkephalinergic. After iontophoretic deposits of biotinylated dextran amine (BDA) in the area of the rVRG, many BDA-labeled terminals in the ventral horn of cervical spinal cord (C4-C5) were immunoreactive for enkephalin and VGLUT2. Injections of Fluoro-Gold amidst phrenic motor neurons in C4-C5 labeled neurons in the area of the rVRG that contained both VGLUT2 mRNA and preproenkephalin (PPE) mRNA as revealed by double in situ hybridization. Thirty-eight bulbospinal I-AUG neurons were recorded in the rVRG and filled with biotinamide by using the juxtacellular labeling technique. Every biotinamide-filled cell tested was positively labeled for VGLUT2 mRNA (n = 14), and most of the cells tested in a separate population exhibited PPE mRNA (16/18). We conclude that most of the phrenic inspiratory premotor neurons of the rVRG are glutamatergic neurons that may also release enkephalins.

The expression of vesicular glutamate transporters VGLUT1 and VGLUT2 in neurochemically defined axonal populations in the rat spinal cord with emphasis on the dorsal horn

Two vesicular glutamate transporters, VGLUT1 and VGLUT2, have recently been identified, and it has been reported that they are expressed by largely nonoverlapping populations of glutamatergic neurons in the brain. We have used immunocytochemistry with antibodies against both transporters, together with markers for various populations of spinal neurons, in an attempt to identify glutamatergic interneurons in the dorsal horn of the mid-lumbar spinal cord of the rat. The great majority (94-100%) of nonprimary axonal boutons that contained somatostatin, substance P or neurotensin, as well as 85% of those that contained enkephalin, were VGLUT2-immunoreactive, which suggests that most dorsal horn neurons that synthesize these peptides are glutamatergic. In support of this, we found that most somatostatin- and enkephalin-containing boutons (including somatostatin-immunoreactive boutons that lacked calcitonin gene-related peptide and were therefore probably derived from local interneurons) formed synapses at which AMPA receptors were present. We also investigated VGLUT expression in central terminals of primary afferents. Myelinated afferents were identified with cholera toxin B subunit; most of those in lamina I were VGLUT2-immunoreactive, whereas all those in deeper laminae were VGLUT1-immunoreactive, and some (in laminae III-VI) appeared to contain both transporters. However, peptidergic primary afferents that contained substance P or somatostatin (most of which are unmyelinated), as well as nonpeptidergic C fibres (identified with Bandeiraea simplicifolia isolectin B4) showed low levels of VGLUT2-immunoreactivity, or were not immunoreactive with either VGLUT antibody. As all primary afferents are thought to be glutamatergic, this raises the possibility that unmyelinated afferents, most of which are nociceptors, express a different vesicular glutamate transporter.

Molecular cloning and functional identification of mouse vesicular glutamate transporter 3 and its expression in subsets of novel excitatory neurons

We have cloned and functionally characterized a third isoform of a vesicular glutamate transporter (VGLUT3) expressed on synaptic vesicles that identifies a distinct glutamatergic system in the brain that is partly and selectively promiscuous with cholinergic and serotoninergic transmission. Transport activity was specific for glutamate, was H(+)-dependent, was stimulated by Cl(-) ion, and was inhibited by Rose Bengal and trypan blue. Northern analysis revealed higher mRNA levels in early postnatal development than in adult brain. Restricted patterns of mRNA expression were observed in presumed interneurons in cortex and hippocampus, and projection systems were observed in the lateral and ventrolateral hypothalamic nuclei, limbic system, and brainstem. Double in situ hybridization histochemistry for vesicular acetylcholine transporter identified VGLUT3 neurons in the striatum as cholinergic interneurons, whereas VGLUT3 mRNA and protein were absent from all other cholinergic cell groups. In the brainstem VGLUT3 mRNA was concentrated in mesopontine raphé nuclei. VGLUT3 immunoreactivity was present throughout the brain in a diffuse system of thick and thin beaded varicose fibers much less abundant than, and strictly separated from, VGLUT1 or VGLUT2 synapses. Co-existence of VGLUT3 in VMAT2-positive and tyrosine hydroxylase -negative varicosities only in the cerebral cortex and hippocampus and in subsets of tryptophan hydroxylase-positive cell bodies and processes in differentiating primary raphé neurons in vitro indicates selective and target-specific expression of the glutamatergic/serotoninergic synaptic phenotype.

Subtype-specific expression of group III metabotropic glutamate receptors and Ca2+ channels in single nerve terminals

The release properties of glutamatergic nerve terminals are influenced by a number of factors, including the subtype of voltage-dependent calcium channel and the presence of presynaptic autoreceptors. Group III metabotropic glutamate receptors (mGluRs) mediate feedback inhibition of glutamate release by inhibiting Ca(2+) channel activity. By imaging Ca(2+) in preparations of cerebrocortical nerve terminals, we show that voltage-dependent Ca(2+) channels are distributed in a heterogeneous manner in individual nerve terminals. Presynaptic terminals contained only N-type (47.5%; conotoxin GVIA-sensitive), P/Q-type (3.9%; agatoxin IVA-sensitive), or both N- and P/Q-type (42.6%) Ca(2+) channels, although the remainder of the terminals (6.1%) were insensitive to these two toxins. In this preparation, two mGluRs with high and low affinity for l(+)-2-amino-4-phosphonobutyrate were identified by immunocytochemistry as mGluR4 and mGluR7, respectively. These receptors were responsible for 22.2 and 24.1% reduction of glutamate release, and they reduced the Ca(2+) response in 24.4 and 30.3% of the nerve terminals, respectively. Interestingly, mGluR4 was largely (73.7%) located in nerve terminals expressing both N- and P/Q-type Ca(2+) channels, whereas mGluR7 was predominantly (69.9%) located in N-type Ca(2+) channel-expressing terminals. This specific coexpression of different group III mGluRs and Ca(2+) channels may endow synaptic terminals with distinct release properties and reveals the existence of a high degree of presynaptic heterogeneity.

Decoding of polymodal sensory stimuli by postsynaptic glutamate receptors in C. elegans

The C. elegans polymodal ASH sensory neurons detect mechanical, osmotic, and chemical stimuli and release glutamate to signal avoidance responses. To investigate the mechanisms of this polymodal signaling, we have characterized the role of postsynaptic glutamate receptors in mediating the response to these distinct stimuli. By studying the behavioral and electrophysiological properties of worms defective for non-NMDA (GLR-1 and GLR-2) and NMDA (NMR-1) receptor subunits, we show that while the osmotic avoidance response requires both NMDA and non-NMDA receptors, the response to mechanical stimuli only requires non-NMDA receptors. Furthermore, analysis of the EGL-3 proprotein convertase provides additional evidence that polymodal signaling in C. elegans occurs via the differential activation of postsynaptic glutamate receptor subtypes.

GABAergic and glutamatergic terminals differentially influence the organization of GABAergic synapses in rat cerebellar granule cells in vitro

Synapse formation in CNS neurons requires appropriate sorting and clustering of neurotransmitter receptors and associated proteins at postsynaptic sites. In GABAergic synapses, clustering of GABA(A) receptors requires gephyrin, but it is not known whether presynaptic signals are also involved in this process. To investigate this issue, we analyzed the subcellular distribution of GABA(A) receptors and gephyrin in primary cultures of cerebellar granule cells, by comparing cells receiving GABAergic input with cells devoid of such afferents. Using immunofluorescence staining, we show that the GABA(A) receptor alpha1 and gamma2 subunit, but not alpha6 or delta subunit, form clusters co-localized with gephyrin in granule cell neurites, irrespective of the presence of GABAergic axons. GABAergic terminals typically were surrounded by groups of gephyrin clusters, pointing to the presence of multiple synaptic sites. In contrast, in neurites devoid of GABAergic input, gephyrin clusters were distributed at random and apposed to glutamatergic terminals, suggesting the formation of mismatched synapses. Both populations of gephyrin clusters were co-localized with GABA(A) receptor subunits, indicating that these proteins are associated also in non-GABAergic synapses. To determine whether signaling mediated by GABA(A) receptors is required for the formation of appropriately matched gephyrin clusters, cultures were treated chronically with bicuculline, or with either muscimol or 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol. All these treatments failed to influence the distribution of gephyrin clusters. We conclude that although GABAergic presynaptic terminals have a preponderant influence on the distribution of gephyrin clusters in dendrites of cerebellar granule cells, GABA transmission is dispensable for postsynaptic clustering of gephyrin and GABA(A) receptors and for the formation of appropriately matched GABAergic synapses.

Increase in cellular glutamate levels stimulates exocytosis in pancreatic beta-cells

Glutamate has been implicated as an intracellular messenger in the regulation of insulin secretion in response to glucose. Here we demonstrate by measurements of cell capacitance in rat pancreatic beta-cells that glutamate (1 mM) enhanced Ca2+-dependent exocytosis. Glutamate (1 mM) also stimulated insulin secretion from permeabilized rat beta-cells. The effect was dose-dependent (half-maximum at 5.1 mM) and maximal at 10 mM glutamate. Glutamate-induced exocytosis was stronger in rat beta-cells and clonal INS-1E cells compared to beta-cells isolated from mice and in parental INS-1 cells, which correlated with the expressed levels of glutamate dehydrogenase. Glutamate-induced exocytosis was inhibited by the protonophores FCCP and SF6847, by the vacuolar-type H+-ATPase inhibitor bafilomycin A(1) and by the glutamate transport inhibitor Evans Blue. Our data provide evidence that exocytosis in beta-cells can be modulated by physiological increases in cellular glutamate levels. The results suggest that stimulation of exocytosis is associated with accumulation of glutamate in the secretory granules, a process that is dependent on the transgranular proton gradient.

Glial uptake of neurotransmitter glutamate from the extracellular fluid studied in vivo by microdialysis and (13)C NMR

Glial uptake of neurotransmitter glutamate (GLU) from the extracellular fluid was studied in vivo in rat brain by (13)C NMR and microdialysis combined with gas-chromatography/mass-spectrometry. Brain GLU C5 was (13)C enriched by intravenous [2,5-(13)C]glucose infusion, followed by [(12)C]glucose infusion to chase (13)C from the small glial GLU pool. This leaves [5-(13)C]GLU mainly in the large neuronal metabolic pool and the vesicular neurotransmitter pool. During the chase, the (13)C enrichment of whole-brain GLU C5 was significantly lower than that of extracellular GLU (GLU(ECF)) derived from exocytosis of vesicular GLU. Glial uptake of neurotransmitter [5-(13)C]GLU(ECF) was monitored in vivo through the formation of [5-(13)C,(15)N]GLN during (15)NH(4)Ac infusion. From the rate of [5-(13)C,(15)N]GLN synthesis (1.7 +/- 0.03 micromol/g/h), the mean (13)C enrichment of extracellular GLU (0.304 +/- 0.011) and the (15)N enrichment of precursor NH(3) (0.87 +/- 0.014), the rate of synthesis of GLN (V'(GLN)), derived from neurotransmitter GLU(ECF), was determined to be 6.4 +/- 0.44 micromol/g/h. Comparison with V(GLN) measured previously by an independent method showed that the neurotransmitter provides 80-90% of the substrate GLU pool for GLN synthesis. Hence, under our experimental conditions, the rate of 6.4 +/- 0.44 micromol/g/h also represents a reasonable estimate for the rate of glial uptake of GLU(ECF), a process that is crucial for protecting the brain from GLU excitotoxicity.

Differences in ratios of GABA, glycine and glutamate immunoreactivities in nerve terminals on rat hindlimb motoneurons: a possible source of post-synaptic variability

Previous pharmacological and physiological data on GABA and glycine receptor-dependent components of miniature inhibitory post-synaptic currents show that the electrophysiological characteristics of synaptic transmission from inhibitory synapses on spinal motoneurons are highly variable. Although post-synaptic factors are thought to be the major underlying cause of this variability, quantitative immunohistochemical data suggest that the transmitter content of afferents also vary from terminal to terminal. To examine whether ratios of amino acid staining densities vary similar to those of components of post-synaptic currents mediated by the corresponding receptors, we quantified immunogold labeling for GABA, glycine and the major excitatory transmitter, glutamate, in nerve terminals contacting the dendrites of motoneurons retrogradely labeled from the rat hindlimb muscle, biceps femoris. Nearly all terminals (94%) were immunoreactive for at least one amino acid and 64% of these contained two or three amino acids. All possible combinations of GABA, glycine and glutamate labeling were found. Over 70% of the terminals contained glycine, of which 60% also labeled for GABA. Of these GABA/glycine boutons, 40% also had glutamate. Half of all terminals contained GABA, but terminals immunoreactive for GABA alone were extremely rare. Immunoreactivity for glutamate occurred in 48% of all terminals and nearly 60% of these also contained glycine. Labeling densities for GABA, glycine and glutamate varied over a wide range from terminal to terminal. We hypothesize that this diversity in amino acid content may be a major underlying cause of variability in GABA- and glycine receptor-mediated components of miniature inhibitory post-synaptic currents in motoneurons.

Expression of inorganic phosphate/vesicular glutamate transporters (BNPI/VGLUT1 and DNPI/VGLUT2) in the cerebellum and precerebellar nuclei of the rat

Expression of inorganic phosphate/vesicular glutamate transporters (BNPI/VGLUT1 and DNPI/VGLUT2) was studied in the cerebellum and precerebellar nuclei of rats using immunohistochemistry and in situ hybridization. DNPI/VGLUT2-stained mossy fibers were principally seen in the vermis (lobules I and VIII-X) and flocculus, whereas BNPI/VGLUT1-stained mossy fibers were localized throughout the cortex. Some vermal and floccular mossy fibers were stained for both transporters. High levels of DNPI/VGLUT2 mRNA hybridization signals were demonstrated in many neurons throughout the vestibular nuclear complex as well as the lateral reticular, external cuneate, inferior olivary and deep cerebellar nuclei. Significant BNPI/VGLUT1 mRNA signals were demonstrated in the lateral reticular nucleus and vestibular nuclear complex but not in the inferior olivary nucleus, indicating that climbing fibers have DNPI/VGLUT2 only. These results show that DNPI/VGLUT2 is expressed preferentially to vestibulo-, reticulo- and cuneocerebellar neurons, some of which also possess BNPI/VGLUT1, suggesting some differential and co-operative functions between DNPI/VGLUT2 and BNPI/VGLUT1 in the cerebellum.

The identification of vesicular glutamate transporter 3 suggests novel modes of signaling by glutamate

Quantal release of the principal excitatory neurotransmitter glutamate requires a mechanism for its transport into secretory vesicles. Within the brain, the complementary expression of vesicular glutamate transporters (VGLUTs) 1 and 2 accounts for the release of glutamate by all known excitatory neurons. We now report the identification of VGLUT3 and its expression by many cells generally considered to release a classical transmitter with properties very different from glutamate. Remarkably, subpopulations of inhibitory neurons as well as cholinergic interneurons, monoamine neurons, and glia express VGLUT3. The dendritic expression of VGLUT3 by particular neurons also indicates the potential for retrograde synaptic signaling. The distribution and subcellular location of VGLUT3 thus suggest novel modes of signaling by glutamate.

Differential expression of two distinct vesicular glutamate transporters in the rat retina

Expression and cellular localization of vesicular glutamate transporters (BNPI and DNPI) were studied in the rat retina. RT-PCR showed expression of both transporter mRNAs. hybridization demonstrated BNPI mRNA signals in the inner segments of photoreceptors and the inner nuclear layer, whereas DNPI mRNA signals were confined to the ganglion cell layer. Punctate BNPI immunoreactivity was localized in the inner and outer plexiform layers, and weak DNPI immunoreactivity was detectable only in some cells and fibers of the ganglion cell layer. The present study suggests that BNPI exists in photoreceptors and bipolar cells, while DNPI is present in ganglion cells, as specific systems in distinct glutamatergic neurons of the retina.

Update on the glutamatergic neurotransmitter system and the role of excitotoxicity in amyotrophic lateral sclerosis

Excitotoxicity may play a role in certain disorders of the motor system thought to be caused by environmentally acquired toxins, including lathyrism and domoic acid poisoning. Motor neurons appear to be particularly susceptible to toxicity mediated via alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-kainate receptors. There is a body of evidence implicating glutamatergic toxicity as a contributory factor in the selective neuronal injury occurring in amyotrophic lateral sclerosis (ALS). Interference with glutamate-mediated toxicity is so far the only neuroprotective therapeutic strategy that has shown benefit in terms of slowing disease progression in ALS patients. Biochemical studies have shown decreased glutamate levels in central nervous system (CNS) tissue and increased levels in the cerebrospinal fluid (CSF) of ALS patients. CSF from ALS patients is toxic to neurons in culture, apparently via a mechanism involving AMPA receptor activation. There is evidence for altered expression and function of glial glutamate transporters in ALS, particularly excitatory amino acid transporter 2 (EAAT2). Abnormal splice variants of EAAT2 have been detected in human CNS. Mitochondrial dysfunction may contribute to excitotoxicity in ALS. Induction of neuronal nitric oxide synthase and cyclooxygenase 2 in ALS may also lead to significant interactions with regulation of the glutamate transmitter system. Certain features of motor neurons may predispose them to the neurodegenerative process in ALS, such as the cell size, mitochondrial activity, neurofilament content, and relative lack of certain calcium-binding proteins and molecular chaperones. Motor neurons appear vulnerable to toxicity mediated by calcium-permeable AMPA receptors. The relatively low expression of the glutamate receptor 2 (GluR2) AMPA receptor subunit and the high current density caused by the large number and density of cell surface AMPA receptors are potentially important factors that may predispose to such toxicity.

Expression profiling with oligonucleotide arrays: technologies and applications for neurobiology

DNA microarrays have been used in applications ranging from the assignment of gene function to analytical uses in prognostics. However, the detection sensitivity, cross hybridization, and reproducibility of these arrays can affect experimental design and data interpretation. Moreover, several technologies are available for fabrication of oligonucleotide microarrays. We review these technologies and performance attributes and, with data sets generated from human brain RNA, present statistical tools and methods to analyze data quality and to mine and visualize the data. Our data show high reproducibility and should allow an investigator to discern biological and regional variability from differential expression. Although we have used brain RNA as a model system to illustrate some of these points, the oligonucleotide arrays and methods employed in this study can be used with cell lines, tissue sections, blood, and other fluids. To further demonstrate this point, we provide data generated from total RNA sample sizes of 200 ng.

Aberrant reduction of an inhibitory protein factor in a rat epileptic model

Certain forms of seizure involve excessive glutamate transmission. We have recently identified a protein, referred to as the inhibitory protein factor (IPF), which potently inhibits glutamate uptake into isolated synaptic vesicles. In an effort to understand the mechanism underlying excessive glutamate transmission associated with seizure, we have analyzed IPF content in various brain regions of the spontaneously epileptic rat, SER (tm/tm, zi/zi), the absence-seizure tremor rat, TM (tm/tm), and the seizure-free control rats zitter ZI (zi/zi) and Wistar tremor control, each at 13 weeks of age. IPF content was found to be markedly reduced in the hippocampus, but not in the other brain regions, of SER, compared to the control and TM rats. TM rats also exhibited reduced IPF content compared to seizure-free controls. These changes appear developmentally regulated; no such alteration was observed in 8-week-old rats, which rarely show seizure. These observations indicate that an aberrant decrease in IPF is associated with certain forms of seizure; this decrease could lead to an abnormal increase in the amount of exocytotically released glutamate through its excessive accumulation in synaptic vesicles.

Facilitation of glutamate release by ionotropic glutamate receptors in osteoblasts

Constitutive expression of mRNA was seen for the vesicular glutamate transporter brain-specific Na(+)-dependent inorganic phosphate cotransporter (BNPI), but not differentiation-associated Na(+)-dependent inorganic phosphate cotransporter, in rat calvarial osteoblasts cultured for 7 and 21 days in vitro (DIV). Three different agonists for ionotropic glutamate receptors (iGluR) at 1mM, as well as 50mM KCl, significantly increased the release of endogenous L-glutamate from osteoblasts cultured for 7DIV when determined 5 min after the addition by using a high performance liquid chromatograph. The inhibitor of desensitization of DL-alpha-amino-3-hydroxy-5-methylisoxasole-4-propionate (AMPA) receptors cyclothiazide significantly potentiated and prolonged the release of endogenous L-glutamate evoked by AMPA in a dose-dependent manner. The release evoked by AMPA was significantly prevented by the addition of an AMPA receptor antagonist as well as by the removal of Ca(2+) ions. These results suggest that endogenous L-glutamate could be released from intracellular vesicular constituents associated with BNPI through activation of particular iGluR subtypes expressed in cultured rat calvarial osteoblasts.

Molecular cloning and functional characterization of human vesicular glutamate transporter 3

Glutamate is the major excitatory neurotransmitter in the mammalian CNS. It is loaded into synaptic vesicles by a proton gradient-dependent uptake system and is released by exocytosis upon stimulation. Recently, two mammalian isoforms of a vesicular glutamate transporter, VGLUT1 and VGLUT2, have been identified, the expression of which enables quantal release of glutamate from glutamatergic neurons. Here, we report a novel isoform of a human vesicular glutamate transporter (hVGLUT3). The predicted amino acid sequence of hVGLUT3 shows 72% identity to both hVGLUT1 and hVGLUT2. hVGLUT3 functions as a vesicular glutamate transporter with similar properties to the other isoforms when it is heterologously expressed in a neuroendocrine cell line. Although mammalian VGLUT1 and VGLUT2 exhibit a complementary expression pattern covering all glutamatergic pathways in the CNS, expression of hVGLUT3 overlaps with them in some brain areas, suggesting molecular diversity that may account for physiological heterogeneity in glutamatergic synapses.

A third vesicular glutamate transporter expressed by cholinergic and serotoninergic neurons

Two proteins previously known as Na(+)-dependent phosphate transporters have been identified recently as vesicular glutamate transporters (VGLUT1 and VGLUT2). Together, VGLUT1 and VGLUT2 are operating at most central glutamatergic synapses. In this study, we characterized a third vesicular glutamate transporter (VGLUT3), highly homologous to VGLUT1 and VGLUT2. Vesicles isolated from endocrine cells expressing recombinant VGLUT3 accumulated l-glutamate with bioenergetic and pharmacological characteristics similar, but not identical, to those displayed by the type-1 and type-2 isoforms. Interestingly, the distribution of VGLUT3 mRNA was restricted to a small number of neurons scattered in the striatum, hippocampus, cerebral cortex, and raphe nuclei, in contrast to VGLUT1 and VGLUT2 transcripts, which are massively expressed in cortical and deep structures of the brain, respectively. At the ultrastructural level, VGLUT3 immunoreactivity was concentrated over synaptic vesicle clusters present in nerve endings forming asymmetrical as well as symmetrical synapses. Finally, VGLUT3-positive neurons of the striatum and raphe nuclei were shown to coexpress acetylcholine and serotonin transporters, respectively. Our study reveals a novel class of glutamatergic nerve terminals and suggests that cholinergic striatal interneurons and serotoninergic neurons from the brainstem may store and release glutamate.

Combinatorial expression of TRPV channel proteins defines their sensory functions and subcellular localization in C. elegans neurons

C. elegans OSM-9 is a TRPV channel protein involved in sensory transduction and adaptation. Here, we show that distinct sensory functions arise from different combinations of OSM-9 and related OCR TRPV proteins. Both OSM-9 and OCR-2 are essential for several forms of sensory transduction, including olfaction, osmosensation, mechanosensation, and chemosensation. In neurons that express both OSM-9 and OCR-2, tagged OCR-2 and OSM-9 proteins reside in sensory cilia and promote each other's localization to cilia. In neurons that express only OSM-9, tagged OSM-9 protein resides in the cell body and acts in sensory adaptation rather than sensory transduction. Thus, alternative combinations of TRPV proteins may direct different functions in distinct subcellular locations. Animals expressing the mammalian TRPV1 (VR1) channel in ASH nociceptor neurons avoid the TRPV1 ligand capsaicin, allowing selective, drug-inducible activation of a specific behavior.

GABAergic terminals are required for postsynaptic clustering of dystrophin but not of GABA(A) receptors and gephyrin

In rat hippocampal cultures, we show by multilabeling immunocytochemistry that pyramidal cells, which receive little or no GABAergic input, mistarget alpha2-GABA(A) receptors and gephyrin to glutamatergic terminals. This mismatch does not occur in neurons innervated by numerous GABAergic terminals. A similar phenomenon has been reported for isolated autaptic hippocampal neurons (Rao et al., 2000). GABAergic synapses typically form multiple release sites apposed to GABA(A) receptor and gephyrin clusters. Remarkably, dystrophin, a protein highly abundant in skeletal muscle membranes, is extensively colocalized with alpha2-GABA(A) receptors exclusively opposite GABAergic terminals. In addition, selective apposition of syntrophin and beta-dystroglycan to GABAergic presynaptic terminals suggests that the entire dystrophin-associated protein complex (DPC) clusters at GABAergic synapses. In contrast to gephyrin and GABA(A) receptors, DPC proteins are not mistargeted to glutamatergic synapses, indicating independent clustering mechanisms. This was confirmed in hippocampal neurons cultured from GABA(A) receptor gamma2 subunit-deficient mice. Clustering of GABA(A) receptor and gephyrin in these neurons was strongly impaired, whereas clustering of dystrophin and associated proteins was unaffected by the absence of the gamma2 subunit. Our results indicate that accumulation of dystrophin and DPC proteins at GABAergic synapses occurs independently of postsynaptic GABA(A) receptors and gephyrin. We suggest that selective signaling from GABAergic terminals contributes to postsynaptic clustering of dystrophin.

Ubiquitin and AP180 regulate the abundance of GLR-1 glutamate receptors at postsynaptic elements in C. elegans

Regulated delivery and removal of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptors (GluRs) from postsynaptic elements has been proposed as a mechanism for regulating synaptic strength. Here we test the role of ubiquitin in regulating synapses that contain a C. elegans GluR, GLR-1. GLR-1 receptors were ubiquitinated in vivo. Mutations that decreased ubiquitination of GLR-1 increased the abundance of GLR-1 at synapses and altered locomotion behavior in a manner that is consistent with increased synaptic strength. By contrast, overexpression of ubiquitin decreased the abundance of GLR-1 at synapses and decreased the density of GLR-1-containing synapses, and these effects were prevented by mutations in the unc-11 gene, which encodes a clathrin adaptin protein (AP180). These results suggest that ubiquitination of GLR-1 receptors regulates synaptic strength and the formation or stability of GLR-1-containing synapses.