Inhibition of gamma-aminobutyrate and glycine uptake into synaptic vesicles

Presynaptic control of inhibitory neurotransmitter content in VIAAT containing synaptic vesicles

In mammals, fast inhibitory neurotransmission is carried out by two amino acid transmitters, γ-aminobutyric acid (GABA) and glycine. The higher brain uses only GABA, but in the spinal cord and brain stem both GABA and glycine act as inhibitory signals. In some cases GABA and glycine are co-released from the same neuron where they are co-packaged into synaptic vesicles by a shared vesicular inhibitory amino acid transporter, VIAAT (also called vGAT). The vesicular content of all other classical neurotransmitters (eg. glutamate, monoamines, acetylcholine) is determined by the presence of a specialized vesicular transporter. Because VIAAT is non-specific, the phenotype of inhibitory synaptic vesicles is instead predicted to be dependent on the relative concentration of GABA and glycine in the cytosol of the presynaptic terminal. This predicts that changes in GABA or glycine supply should be reflected in vesicle transmitter content but as yet, the mechanisms that control GABA versus glycine uptake into synaptic vesicles and their potential for modulation are not clearly understood. This review summarizes the most relevant experimental data that examines the link between GABA and glycine accumulation in the presynaptic cytosol and the inhibitory vesicle phenotype. The accumulated evidence challenges the hypothesis that vesicular phenotype is determined simply by the competition of inhibitory transmitter for VIAAT and instead suggest that the GABA/glycine balance in vesicles is dynamically regulated.

The alanine-serine-cysteine-1 (Asc-1) transporter controls glycine levels in the brain and is required for glycinergic inhibitory transmission

Asc-1 (SLC7A10) is an amino acid transporter whose deletion causes neurological abnormalities and early postnatal death in mice. Using metabolomics and behavioral and electrophysiological methods, we demonstrate that Asc-1 knockout mice display a marked decrease in glycine levels in the brain and spinal cord along with impairment of glycinergic inhibitory transmission, and a hyperekplexia-like phenotype that is rescued by replenishing brain glycine. Asc-1 works as a glycine and L-serine transporter, and its transport activity is required for the subsequent conversion of L-serine into glycine in vivo. Asc-1 is a novel regulator of glycine metabolism and a candidate for hyperekplexia disorders.

Vesicular GABA transporter (VGAT) transports β-alanine

Vesicular GABA transporter (VGAT) is expressed in GABAergic and glycinergic neurons, and is responsible for vesicular storage and subsequent exocytosis of these inhibitory amino acids. In this study, we show that VGAT recognizes β-alanine as a substrate. Proteoliposomes containing purified VGAT transport β-alanine using Δψ but not ΔpH as a driving force. The Δψ-driven β-alanine uptake requires Cl(-). VGAT also facilitates Cl(-) uptake in the presence of β-alanine. A previously described VGAT mutant (Glu213Ala) that disrupts GABA and glycine transport similarly abrogates β-alanine uptake. These findings indicated that VGAT transports β-alanine through a mechanism similar to those for GABA and glycine, and functions as a vesicular β-alanine transporter. Vesicular GABA transporter (VGAT) is expressed in GABAergic and glycinergic neurons, and is responsible for vesicular storage and subsequent exocytosis of these inhibitory amino acids. In the present study, we showed that proteoliposomes containing purified VGAT transport β-alanine using Δψ as a driving force. VGAT also facilitates Cl(-) uptake. Our findings indicated that VGAT functions as a vesicular β-alanine transporter.

Storage and uptake of D-serine into astrocytic synaptic-like vesicles specify gliotransmission

Glial cells are increasingly recognized as active players that profoundly influence neuronal synaptic transmission by specialized signaling pathways. In particular, astrocytes have been shown recently to release small molecules, such as the amino acids l-glutamate and d-serine as "gliotransmitters," which directly control the efficacy of adjacent synapses. However, it is still controversial whether gliotransmitters are released from a cytosolic pool or by Ca(2+)-dependent exocytosis from secretory vesicles, i.e., by a mechanism similar to the release of synaptic vesicles in synapses. Here we report that rat cortical astrocytes contain storage vesicles that display morphological and biochemical features similar to neuronal synaptic vesicles. These vesicles share some, but not all, membrane proteins with synaptic vesicles, including the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) synaptobrevin 2, and contain both l-glutamate and d-serine. Furthermore, they show uptake of l-glutamate and d-serine that is driven by a proton electrochemical gradient. d-Serine uptake is associated with vesicle acidification and is dependent on chloride. Whereas l-serine is not transported, serine racemase, the synthesizing enzyme for d-serine, is anchored to the membrane of the vesicles, allowing local generation of d-serine. Finally, we reveal a previously unexpected mutual vesicular synergy between d-serine and l-glutamate filling in glia vesicles. We conclude that astrocytes contain vesicles capable of storing and releasing d-serine, l-glutamate, and most likely other neuromodulators in an activity-dependent manner.

Vesicular uptake and exocytosis of L-aspartate is independent of sialin

The mechanism of release and the role of l-aspartate as a central neurotransmitter are controversial. A vesicular release mechanism for l-aspartate has been difficult to prove, as no vesicular l-aspartate transporter was identified until it was found that sialin could transport l-aspartate and l-glutamate when reconstituted into liposomes. We sought to clarify the release mechanism of l-aspartate and the role of sialin in this process by combining l-aspartate uptake studies in isolated synaptic vesicles with immunocyotchemical investigations of hippocampal slices. We found that radiolabeled l-aspartate was taken up into synaptic vesicles. The vesicular l-aspartate uptake, relative to the l-glutamate uptake, was twice as high in the hippocampus as in the whole brain, the striatum, and the entorhinal and frontal cortices and was not inhibited by l-glutamate. We further show that sialin is not essential for exocytosis of l-aspartate, as there was no difference in ATP-dependent l-aspartate uptake in synaptic vesicles from sialin-knockout and wild-type mice. In addition, expression of sialin in PC12 cells did not result in significant vesicle uptake of l-aspartate, and depolarization-induced depletion of l-aspartate from hippocampal nerve terminals was similar in hippocampal slices from sialin-knockout and wild-type mice. Further, there was no evidence for nonvesicular release of l-aspartate via volume-regulated anion channels or plasma membrane excitatory amino acid transporters. This suggests that l-aspartate is exocytotically released from nerve terminals after vesicular accumulation by a transporter other than sialin.

Development of synaptic inhibition in glycine transporter 2 deficient mice

Mice deficient for the neuronal glycine transporter subtype 2 (GlyT2) die during the second postnatal week after developing neuromotor deficiencies, which resembles severe forms of human hyperekplexia. This phenotype has been attributed to a dramatic reduction in glycinergic neurotransmission. In the present study we analyzed the development of GABAergic and glycinergic synaptic transmission in GlyT2-knockout mice during early postnatal life. Anti-glycine immunohistochemistry in spinal cord and brainstem slices and whole-cell voltage-clamp recordings of glycinergic inhibitory postsynaptic currents (IPSCs) from hypoglossal motoneurons revealed strikingly reduced levels of synaptic glycine already at birth. Since GABA and glycine use the same vesicular inhibitory amino acid transporter (VIAAT or VGAT) we also analysed GABAergic neurotransmission. No increase of GABA immunoreactivity was observed in the spinal cord and brainstem of GlyT2(-/-) mice at any stage of postnatal development. Correspondingly no up-regulation of GABAergic IPSCs was detected in GlyT2(-/-) hypoglossal motoneurons. These data suggest that in the first postnatal week, loss of the glycine transporter 2 is neither compensated by glycine de-novo synthesis nor by up-regulation of the GABAergic transmission in GlyT2(-/-) mice.

Co-localisation of markers for glycinergic and GABAergic neurones in rat nucleus of the solitary tract: implications for co-transmission

Immunoreactive structures visualised with antibodies to glycine were prominent in areas of the nucleus of the solitary tract (NTS) surrounding the tractus solitarius, but scarcer in medial and ventral areas of the nucleus. This contrasted with a higher density, more homogenous distribution of structures labelled for gamma-aminobutyric acid (GABA). Immunolabelling of adjacent semi-thin sections nonetheless indicated a close correspondence between cells and puncta labelled by glycine and GABA antisera in certain NTS areas. With post-embedding electron microscopic immunolabelling, synaptic terminals with high, presumed transmitter levels of glycine were discriminated from terminals containing low, metabolic levels by quantitative analysis of gold particle labelling densities. In a random sample of terminals, 28.5% qualified on this basis as glycinergic (compared to 44.4% GABAergic); these glycinergic terminals targeted mainly dendritic structures and contained pleomorphic vesicles and symmetrical synapses. Serial section analysis revealed few terminals (5.2%) immunoreactive for glycine alone, with 82% of glycinergic terminals also containing high levels of GABA immunoreactivity. No evidence for co-localisation of glycine and glutamate was found. Light, confocal and electron microscopic labelling with antibodies to proteins specific for glycine and GABA synthesis, release and uptake confirmed that glycinergic terminals also containing GABA are found predominantly in more lateral areas of NTS, despite glycine receptors and the 'glial' glycine transporter (GLYT1) being expressed throughout all areas of the nucleus. The data suggest that synaptic terminals in certain functionally distinct areas of NTS co-release both inhibitory amino acids, which may account for the previously reported differential inhibitory effects of glycine and GABA on NTS neurones.

Plasmalemmal and vesicular gamma-aminobutyric acid transporter expression in the developing mouse retina

Plasmalemmal and vesicular gamma-aminobutyric acid (GABA) transporters influence neurotransmission by regulating high-affinity GABA uptake and GABA release into the synaptic cleft and extracellular space. Postnatal expression of the plasmalemmal GABA transporter-1 (GAT-1), GAT-3, and the vesicular GABA/glycine transporter (VGAT) were evaluated in the developing mouse retina by using immunohistochemistry with affinity-purified antibodies. Weak transporter immunoreactivity was observed in the inner retina at postnatal day 0 (P0). GAT-1 immunostaining at P0 and at older ages was in amacrine and displaced amacrine cells in the inner nuclear layer (INL) and ganglion cell layer (GCL), respectively, and in their processes in the inner plexiform layer (IPL). At P10, weak GAT-1 immunostaining was in Müller cell processes. GAT-3 immunostaining at P0 and older ages was in amacrine cells and their processes, as well as in Müller cells and their processes that extended radially across the retina. At P10, Müller cell somata were observed in the middle of the INL. VGAT immunostaining was present at P0 and older ages in amacrine cells in the INL as well as processes in the IPL. At P5, weak VGAT immunostaining was also observed in horizontal cell somata and processes. By P15, the GAT and VGAT immunostaining patterns appear similar to the adult immunostaining patterns; they reached adult levels by about P20. These findings demonstrate that GABA uptake and release are initially established in the inner retina during the first postnatal week and that these systems subsequently mature in the outer retina during the second postnatal week.

Vesicular neurotransmitter transporters as targets for endogenous and exogenous toxic substances

Exocytotic release of neurotransmitters requires their accumulation inside preformed secretory vesicles. Distinct vesicular transport activities translocate classical transmitters into synaptic vesicles energized by a H+ electrochemical gradient (Delta(mu(H+))), with subtle but important differences in dependence on the electrical and chemical components. The vesicular transporters also interact with toxic compounds and drugs. They mediate neuroprotection by sequestering toxic compounds as well as neurotransmitters into vesicles, reducing their concentration in the cytosol where they may have detrimental effects. Both therapeutic agents and psychostimulants interfering with vesicular transport have yielded insight into the pathogenesis of psychiatric as well as neurodegenerative diseases. Thus, specific inhibitors have helped to characterize both the physiological role and mechanism of vesicular neurotransmitter transport.

Corelease of GABA/glycine in lamina-X of the spinal cord of neonatal rats

Spinal-cord slices from neonatal rats were used to record lamina-X neurons using the patch-clamp technique under whole cell recording configuration. Lamina-X surrounds the central canal of the spinal cord and contains sympathetic preganglionic neurons of the central autonomic nucleus. Miniature inhibitory postsynaptic currents were recorded in the presence of tetrodotoxin and kynurenic acid to block action potential-dependent transmitter release and glutamatergic transmissions, respectively. We recorded mixed gamma-amino-n-butyric acid/glycine miniature synaptic currents suggesting that gamma-amino-n-butyric acid and glycine can be coreleased from the same single synaptic vesicles, and that this corelease can be detected by the postsynaptic cell. In addition, acetylcholine can induce the release of gamma-amino-n-butyric acid/glycine by acting presynaptically at nicotinic receptors located on the gamma-amino-n-butyric acid ergic/glycinergic terminals.

The transporters GlyT2 and VIAAT cooperate to determine the vesicular glycinergic phenotype

The mechanisms that specify the vesicular phenotype of inhibitory interneurons in vertebrates are poorly understood because the two main inhibitory transmitters, glycine and GABA, share the same vesicular inhibitory amino acid transporter (VIAAT) and are both present in neurons during postnatal development. We have expressed VIAAT and the plasmalemmal transporters for glycine and GABA in a neuroendocrine cell line and measured the quantal release of glycine and GABA using a novel double-sniffer patch-clamp technique. We found that glycine is released from vesicles when VIAAT is coexpressed with either the neuronal transporter GlyT2 or the glial transporter GlyT1. However, GlyT2 was more effective than GlyT1, probably because GlyT2 is unable to operate in the reverse mode, which gives it an advantage in maintaining the high cytosolic glycine concentration required for efficient vesicular loading by VIAAT. The vesicular inhibitory phenotype was gradually altered from glycinergic to GABAergic through mixed events when GABA is introduced into the secretory cell and competes for uptake by VIAAT. Interestingly, the VIAAT ortholog from Caenorhabditis elegans (UNC-47), a species lacking glycine transmission, also supports glycine exocytosis in the presence of GlyT2, and a point mutation of UNC-47 that abolishes GABA transmission in the worm confers glycine specificity. Together, these results suggest that an increased cytosolic availability of glycine in VIAAT-containing terminals was crucial for the emergence of glycinergic transmission in vertebrates.

A shared vesicular carrier allows synaptic corelease of GABA and glycine

The type of vesicular transporter expressed by a neuron is thought to determine its neurotransmitter phenotype. We show that inactivation of the vesicular inhibitory amino acid transporter (Viaat, VGAT) leads to embryonic lethality, an abdominal defect known as omphalocele, and a cleft palate. Loss of Viaat causes a drastic reduction of neurotransmitter release in both GABAergic and glycinergic neurons, indicating that glycinergic neurons do not express a separate vesicular glycine transporter. This loss of GABAergic and glycinergic synaptic transmission does not impair the development of inhibitory synapses or the expression of KCC2, the K+ -Cl- cotransporter known to be essential for the establishment of inhibitory neurotransmission. In the absence of Viaat, GABA-synthesizing enzymes are partially lost from presynaptic terminals. Since GABA and glycine compete for vesicular uptake, these data point to a close association of Viaat with GABA-synthesizing enzymes as a key factor in specifying GABAergic neuronal phenotypes.

Neurochemical Characterization of a Neuroprotective Compound fromParawixia bistriataSpider Venom That Inhibits Synaptosomal Uptake of GABA and Glycine

The major contribution of this work is the isolation of a neuroprotective compound referred to as 2-amino-5-ureidopentanamide (FrPbAII) (M(r) = 174) from Parawixia bistriata spider venom and an investigation of its mode of action. FrPbAII inhibits synaptosomal GABA uptake in a dose-dependent manner and probably does not act on Na(+), K(+), and Ca(2+) channels, GABA(B) receptors, or gamma-aminobutyrate:alpha-ketoglutarate aminotransferase enzyme; therefore, it is not directly dependent on these structures for its action. Direct increase of GABA release and reverse transport are also ruled out as mechanisms of FrPbAII activities as well as unspecific actions on pore membrane formation. Moreover, FrPbAII is selective for GABA and glycine transporters, having slight or no effect on monoamines or glutamate transporters. According to our experimental glaucoma data in rat retina, FrPbAII is able to cross the blood-retina barrier and promote effective protection of retinal layers submitted to ischemic conditions. These studies are of relevance by providing a better understanding of neurochemical mechanisms involved in brain function and for possible development of new neuropharmacological and therapeutic tools.

Changes in vesicular glutamate transporter 2, vesicular GABA transporter and vesicular acetylcholine transporter labeling of sacrocaudal motoneurons in the spastic rat

Spasticity of the midline musculature can significantly hinder performing transfers and lead to development of pressure sores. Currently, significant gaps exist in our knowledge of the pathophysiology involved in spasticity development following SCI, especially regarding the axial musculature. The goals of this study were: (1) to determine the effects of S(2) transection on the number and distribution of glutamatergic, GABAergic and cholinergic inputs on more caudal motoneurons, (2) to correlate these changes with the development of spasticity within the tail musculature, which are the caudal counterparts to the axial musculature. Animals with S(2) spinal transection were tested behaviorally for the progression of spasticity within the tail musculature. At 1, 2, 4, or 12 weeks post-injury, the animals were sacrificed and temporal changes in glutamatergic, GABAergic, and cholinergic inputs to sacrocaudal motoneurons were assessed using antibodies for the specific vesicular transporter of each neurotransmitter and confocal microscopy. At 1 week post-injury, when the tail musculature demonstrated decreased responsiveness, an overall increase in the ratio of excitatory to inhibitory input to sacrocaudal motoneurons was observed. From 2 to 12 weeks post-injury, when the tail musculature demonstrated increased reflex behavior, an overall decrease in the ratio of excitatory to inhibitory inputs was observed. Additionally, from 2 to 12 weeks following spinal transection, a progressive loss of cholinergic labeling of sacrocaudal motoneurons was observed. The increase in the overall level of excitation with a concomitant loss of cholinergic influence following spinal transection could, in part, explain the development of spasticity within the tail musculature.

Glycine--an important neurotransmitter and cytoprotective agent

BACKGROUND

Glycine, the simplest of the amino acids, is an essential component of important biological molecules, a key substance in many metabolic reactions, the major inhibitory neurotransmitter in the spinal cord and brain stem, and an anti-inflammatory, cytoprotective, and immune modulating substance.

MATERIAL AND METHODS

Based on available literature, we discuss some of the important biological properties of glycine. In addition, we describe some clinical disorders where glycine plays a central role, either as an essential structural element, or through its metabolism or receptors.

RESULTS

The past few years have witnessed a broadening of glycine research. The traditional prime interest in aspects related to its role as an inhibitory neurotransmitter in the central nervous system has been expanded to equally emphasize other organs and tissues. With the demonstration of glycine-gated chloride channels on neurons in the central nervous system, on most leukocytes, and subsequently on other cells as well, a unifying mechanism of action accounting for many of the widespread effects of glycine has been found.

CONCLUSIONS

Glycine is a simple, easily available, and inexpensive substance with few and innocuous side-effects. The diversity of biological activities is well documented in the literature. Despite this, glycine has only gained a modest place in clinical medicine.

Pharmacological and biochemical aspects of GABAergic neurotransmission: pathological and neuropsychobiological relationships

1. The GABAergic neurotransmission has been implicated in the modulation of many neural networks in forebrain, midbrain and hindbrain, as well as, in several neurological disorders. 2. The complete comprehension of GABA system neurochemical properties and the search for approaches in identifying new targets for the treatment of neural diseases related to GABAergic pathway are of the extreme relevance. 3. The present review will be focused on the pharmacology and biochemistry of the GABA metabolism, GABA receptors and transporters. In addition, the pathological and psychobiological implications related to GABAergic neurotransmission will be considered.

Distinct profiles of refilling of inhibitory neurotransmitters into presynaptic terminals projecting to spinal neurones in immature rats

Corelease of glycine and GABA from the single synaptic terminal (synaptic bouton) is well accepted in immature rat spinal cord and brainstem. However, it raises the question of how glycine and GABA are accumulated in the same synaptic vesicles and coreleased. To address this issue, spontaneous miniature inhibitory postsynaptic currents (mIPSCs) and focally evoked IPSCs (eIPSCs) mediated via a single synapse were recorded from synaptic bouton preparations of the rat immature sacral dorsal commissural nucleus (SDCN) neurones by whole-cell patch recording. Focal stimulation of a single synaptic bouton revealed that three different quantal releases occur from a single synaptic bouton: i.e. pure glycine, pure GABA, and mixed. Prolonged treatment with bafilomycin A1, a vacuolar-type H+/ATPase inhibitor, to the SDCN neurone greatly suppressed frequency and amplitude of the mIPSCs. During washing out of bafilomycin A1, complete recovery in the amplitude of glycinergic mIPSCs was observed, while that of GABAergic and mixed mIPSCs was incomplete. These observations indicate that three types of vesicles coexist in single synaptic terminals, and that refilling of glycine into the synaptic vesicle predominantes over GABA after pretreatment with bafilomycin A1 in immature rats. This could be explained by the decrease in the cytosolic concentration of GABA, or by the presence of subtypes of vesicular inhibitory amino acid transporter in the synaptic vesicle membrane.

Gene structure and alternative splicing of the mouse glycine transporter type-2

The type-2 glycine transporter GLYT2 is expressed in glycinergic neurons and is involved in the termination of inhibitory neurotransmission at strychnine-sensitive glycinergic synapses. We isolated cDNA of a GLYT2 isoform, GLYT2a, from mouse brain, and found that it contains a coding sequence of 798 amino acids. We also isolated and characterized the mouse GLYT2 (mGLYT2) gene, which was found to be divided into 18 exons and spread over 55 kb. 5'-rapid amplification of cDNA ends analyses demonstrated the existence of another two isoforms, mGLYT2b and mGLYT2c, in addition to mGLYT2a. Both mGLYT2b and mGLYT2c would produce a protein eight amino acids shorter than mGLYT2a. Analysis of the genomic clones encompassing the 5'-exons revealed that the three transcripts arose from a single gene by alternative splicing. RT-PCR analysis indicated that all three mGLYT2 isoforms were expressed at high levels in brain stem and spinal cord. These data will be useful for investigating the function of GLYT2 proteins and glycinergic neurons by gene targeting experiments.

The SLC32 transporter, a key protein for the synaptic release of inhibitory amino acids

The SLC32 family comprises a single member: the vesicular inhibitory amino acid transporter (VIAAT) or vesicular GABA transporter (VGAT). It belongs to a eukaryotic-specific superfamily of H(+)-coupled amino acid transporters, which also comprises the mammalian SLC36 and SLC38 transporters. VIAAT exchanges GABA or glycine for protons. It is present on synaptic vesicles of GABAergic and glycinergic neurons, and in some endocrine cells, where it ensures the H(+)-ATPase-driven uptake, and subsequent exocytotic release, of inhibitory amino acids. Despite a similar function in vesicular neurotransmitter loading, VIAAT is not related to the vesicular glutamate transporter (VGLUT, SLC17) or the vesicular monoamine transporter/vesicular acetylcholine transporter (VMAT/VACHT, SLC18) proteins.

Membrane glycine transport proteins

Structurally, the simplest amino acid is glycine, and it has a number of important yet distinct functions in the body. This review focuses on the different transport systems and the associated carrier proteins for glycine that are responsible for its movement across biological membranes. Transport proteins in the class GLYT appear to be the most specific for glycine. However, the B0+ system also carries significant amounts of glycine. Other amino acid transport systems capable of carrying small amounts of glycine are ASC, asc and system L. In addition, an ATP-dependent transport process exists that takes up glycine into synaptic vesicles at nerve endings. This is known as the vesicular inhibitory amino acid transporter since, in addition to glycine, it can transport possibly two other inhibitory neurotransmitters.

Redistribution of neuroactive amino acids in hippocampus and striatum during hypoglycemia: a quantitative immunogold study

Postembedding immunocytochemistry was used to localize aspartate, glutamate, gamma-aminobutyric acid (GABA), and glutamine in hippocampus and striatum during normo- and hypoglycemia in rat. In both brain regions, hypoglycemia caused aspartatelike immunoreactivity to increase. In hippocampus, this increase was evident particularly in the terminals of known excitatory afferents-in GABA-ergic neurons and myelinated axons. Aspartate was enriched along with glutamate in nerve terminals forming asymmetric synapses on spines and with GABA in terminals forming symmetric synapses on granule and pyramidal cell bodies. In both types of terminal, aspartate was associated with clusters of synaptic vesicles. Glutamate and glutamine immunolabeling were markedly reduced in all tissue elements in both brain regions, but less in the terminals than in the dendrosomatic compartments of excitatory neurons. In glial cells, glutamine labeling showed only slight attenuation. The level of GABA immunolabeling did not change significantly during hypoglycemia. The results support the view that glutamate and glutamine are used as energy substrates in hypoglycemia. Under these conditions both excitatory and inhibitory terminals are enriched with aspartate, which may be released from these nerve endings and thus contribute to the pattern of neuronal death characteristic of hypoglycemia.

Constitutive phosphorylation of the vesicular inhibitory amino acid transporter in rat central nervous system

gamma-Aminobutyric acid (GABA) and glycine are stored into synaptic vesicles by a recently identified vesicular inhibitory amino acid transporter [VIAAT, also called vesicular GABA transporter (VGAT)]. Immunoblotting analysis revealed that rat brain VIAAT migrated as a doublet during sodium dodecyl sulfate-polyacrylamide gel electrophoresis, with a predominant slower band in all areas examined except olfactory bulb and retina. The slower band corresponded to a phosphorylated form of VIAAT as it was converted to the faster one by treating brain homogenates with alkaline phosphatase or with an endogenous phosphatase identified as type 2A protein-serine/threonine phosphatase using okadaic acid. In contrast, the recombinant protein expressed in COS-7 or PC12 cells co-migrated with the faster band of the brain doublet and was insensitive to alkaline phosphatase. To investigate the influence of VIAAT phosphorylation on vesicular neurotransmitter loading, purified synaptic vesicles were treated with alkaline phosphatase and assayed for amino acid uptake. However, neither GABA nor glycine uptake was affected by VIAAT phosphorylation. These results indicate that VIAAT is constitutively phosphorylated on cytosolic serine or threonine residues in most, but not all, regions of the rat brain. This phosphorylation does not regulate the vesicular loading of GABA or glycine, suggesting that it is involved at other stages of the synaptic vesicle life cycle.

Immunoisolation of GABA-specific synaptic vesicles defines a functionally distinct subset of synaptic vesicles

Synaptic vesicles from mammalian brain are among the best characterized trafficking organelles. However, so far it has not been possible to characterize vesicle subpopulations that are specific for a given neurotransmitter. Taking advantage of the recent molecular characterization of vesicular neurotransmitter transporters, we have used an antibody specific for the vesicular GABA transporter (VGAT) to isolate GABA-specific synaptic vesicles. The isolated vesicles are of exceptional purity as judged by electron microscopy. Immunoblotting revealed that isolated vesicles contain most of the major synaptic vesicle proteins in addition to VGAT and are devoid of vesicular monoamine and acetylcholine transporters. The vesicles are 10-fold enriched in GABA uptake activity when compared with the starting vesicle fraction. Furthermore, glutamate uptake activity and glutamate-induced but not chloride-induced acidification are selectively lost during immunoisolation. We conclude that the population of GABA-containing synaptic vesicles is separable and distinct from vesicle populations transporting other neurotransmitters.

gamma-aminobutyric acid mimetic drugs differentially inhibit the dopaminergic response to cocaine

Dopaminergic activity in the mesocorticolimbic system is associated with reinforcing properties of psychostimulant drugs. We previously demonstrated that increased gamma-aminobutyric acid (GABA)-ergic activity produced by gamma-vinyl GABA [D,L-4-amino-hex-5-enoic acid (Vigabatrin(R))], an irreversible inhibitor of GABA-transaminase, attenuated cocaine, nicotine, heroin, alcohol, and methamphetamine-induced increases in extracellular nucleus accumbens dopamine as well as behaviors associated with these biochemical changes. In the present study, using in vivo microdialysis techniques, we compared three different strategies to increase GABAergic activity in order to modulate cocaine-induced increase in extracellular dopamine. Our data demonstrate that the anticonvulsant 1-(2-(((diphenylmethylene)amino)oxy)ethyl)-1,2,5, 6-tetrahydro-3-pyridinecarboxylic acid hydrochloride (NNC-711), a GABA uptake inhibitor, dose and time dependently diminished increases in extracellular dopamine following acute cocaine challenge. Furthermore, we demonstrated that cyclized analogue of vigabatrin, a competitive reversible GABA-transaminase inhibitor, is a more potent inhibitor of cocaine-induced dopamine increase than vigabatrin. Our data suggest that in addition to irreversible inhibition of GABA transaminase, inhibition of GABA uptake represent another potentially effective, indirect strategy for the treatment of cocaine abuse.

Presence of the vesicular inhibitory amino acid transporter in GABAergic and glycinergic synaptic terminal boutons

The characterization of the Caenorhabditis elegans unc-47 gene recently allowed the identification of a mammalian (gamma)-amino butyric acid (GABA) transporter, presumed to be located in the synaptic vesicle membrane. In situ hybridization data in rat brain suggested that it might also take up glycine and thus represent a general Vesicular Inhibitory Amino Acid Transporter (VIAAT). In the present study, we have investigated the localization of VIAAT in neurons by using a polyclonal antibody raised against the hydrophilic N-terminal domain of the protein. Light microscopy and immunocytochemistry in primary cultures or tissue sections of the rat spinal cord revealed that VIAAT was localized in a subset (63-65%) of synaptophysin-immunoreactive terminal boutons; among the VIAAT-positive terminals around motoneuronal somata, 32.9% of them were also immunoreactive for GAD65, a marker of GABAergic presynaptic endings. Labelling was also found apposed to clusters positive for the glycine receptor or for its associated protein gephyrin. At the ultrastructural level, VIAAT immunoreactivity was restricted to presynaptic boutons exhibiting classical inhibitory features and, within the boutons, concentrated over synaptic vesicle clusters. Pre-embedding detection of VIAAT followed by post-embedding detection of GABA or glycine on serial sections of the spinal cord or cerebellar cortex indicated that VIAAT was present in glycine-, GABA- or GABA- and glycine-containing boutons. Taken together, these data further support the view of a common vesicular transporter for these two inhibitory transmitters, which would be responsible for their costorage in the same synaptic vesicle and subsequent corelease at mixed GABA-and-glycine synapses.

The vesicular GABA transporter, VGAT, localizes to synaptic vesicles in sets of glycinergic as well as GABAergic neurons

A transporter thought to mediate accumulation of GABA into synaptic vesicles has recently been cloned (McIntire et al., 1997). This vesicular GABA transporter (VGAT), the first vesicular amino acid transporter to be molecularly identified, differs in structure from previously cloned vesicular neurotransmitter transporters and defines a novel gene family. Here we use antibodies specific for N- and C-terminal epitopes of VGAT to localize the protein in the rat CNS. VGAT is highly concentrated in the nerve endings of GABAergic neurons in the brain and spinal cord but also in glycinergic nerve endings. In contrast, hippocampal mossy fiber boutons, which although glutamatergic are known to contain GABA, lack VGAT immunoreactivity. Post-embedding immunogold quantification shows that the protein specifically associates with synaptic vesicles. Triple labeling for VGAT, GABA, and glycine in the lateral oliva superior revealed a higher expression of VGAT in nerve endings rich in GABA, with or without glycine, than in others rich in glycine only. Although the great majority of nerve terminals containing GABA or glycine are immunopositive for VGAT, subpopulations of nerve endings rich in GABA or glycine appear to lack the protein. Additional vesicular transporters or alternative modes of release may therefore contribute to the inhibitory neurotransmission mediated by these two amino acids.

Cloning of a functional vesicular GABA and glycine transporter by screening of genome databases

The unc-47 locus of Caenorhabditis elegans has been suggested to encode a synaptic vesicle GABA transporter. Here we used hydropathy plot analysis to identify a candidate vesicular GABA transporter in genomic sequences derived from a region of the physical map comprising unc-47. A mouse homologue was identified and cloned from EST database information. In situ hybridization in rat brain revealed codistribution with both GABAergic and glycinergic neuronal markers. Moreover, expression in COS-7 and PC12 cells induced an intracellular, glycine-sensitive GABA uptake activity. These observations, consistent with previous data on GABA and glycine uptake by synaptic vesicles, demonstrate that the mouse clone encodes a vesicular inhibitory amino acid transporter.

Identification and characterization of the vesicular GABA transporter

Synaptic transmission involves the regulated exocytosis of vesicles filled with neurotransmitter. Classical transmitters are synthesized in the cytoplasm, and so must be transported into synaptic vesicles. Although the vesicular transporters for monoamines and acetylcholine have been identified, the proteins responsible for packaging the primary inhibitory and excitatory transmitters, gamma-aminobutyric acid (GABA) and glutamate remain unknown. Studies in the nematode Caenorhabditis elegans have implicated the gene unc-47 in the release of GABA. Here we show that the sequence of unc-47 predicts a protein with ten transmembrane domains, that the gene is expressed by GABA neurons, and that the protein colocalizes with synaptic vesicles. Further, a rat homologue of unc-47 is expressed by central GABA neurons and confers vesicular GABA transport in transfected cells with kinetics and substrate specificity similar to those previously reported for synaptic vesicles from the brain. Comparison of this vesicular GABA transporter (VGAT) with a vesicular transporter for monoamines shows that there are differences in the bioenergetic dependence of transport, and these presumably account for the differences in structure. Thus VGAT is the first of a new family of neurotransmitter transporters.

Pharmacologic characteristics of excitatory gamma-amino-butyric acid (GABA) receptors in a snail neuron

1. The pharmacologic characteristics of excitatory gamma-aminobutyric acid (GABA) receptors, termed muscimol II type GABA receptors, found in a giant neuron type, v-LCDN (ventral-left cerebral distinct neuron), of an African giant snail (Achatina fulica Férussac), were studied using the mammalian GABA receptor agonists, antagonists and synergists and GABA uptake inhibitor using the voltage clamp technique. 2. GABA and its agonists, ejected by brief pressure, produced an inward current (Iin) of the following order of potency: trans-t-aminocrotonic acid (TACA) > GABA > muscimol > isoguvacine > 5-aminopentanoic acid and cis-4-aminocrotonic acid (CACA). (+/-)-Baclofen and 3-aminopropylphosphonic acid (APPA) were ineffective. The Iin values produced by GABA, TACA, isoguvacine and CACA were stable for at least 60 min, whereas the Iin induced by muscimol was not. 3. According to the dose-response curves of GABA, TACA, isoguvacine and CACA, measured by the varied pressure duration method, the ED50 value of CACA was larger than those of the other compounds, and Emax of TACA was larger than that of GABA, whereas Emax values of isoguvacine and CACA were smaller. 4. The perfusion of beta-alanine, pentobarbital and 5-aminopentanoic acid inhibited the Iin induced by GABA, whereas (-)-bicuculline, pitrazepin, diazepam and 2-hydroxysaclofen had no effect. 5. From the effects of beta-alanine on the dose-response curves of GABA, measured by the varied pressure duration method, beta-alanine competitively inhibited the Iin caused by GABA. According to the effects of pentobarbital on the dose-response curves of GABA, this drug noncompetitively inhibited the Iin using the varied pressure duration method, and partly competitively and partly noncompetitively using the Y-tube method. The effects of 5-aminopentanoic acid on the dose-response curves of GABA indicated that this drug noncompetitively inhibited the Iin using the varied pressure duration method, and partly noncompetitively and partly uncompetitively using the Y-tube method. 6. The pharmacologic features of the Achatina muscimol II type GABA receptors were similar to those of mammalian GABAC (GABAp1) receptors, except for the effects of pentobarbital.

Amino acid neurotransmission: dynamics of vesicular uptake

Glutamate, GABA and glycine, the major neurotransmitters in CNS, are taken up and stored in synaptic vesicles by a Mg(2+)-ATP dependent process. The main driving force for vesicular glutamate uptake is the membrane potential, whereas both the membrane potential and the proton gradient contribute to the uptake of GABA and glycine. Glutamate is taken up by a specific transporter with no affinity for aspartate. Evans blue and related dyes are competitive inhibitors of the uptake of glutamate. GABA, beta-alanine, and glycine are taken up by the same family of transporter molecules. Aspartate, taurine, and proline are not taken up by any synaptic vesicle preparations. It is suggested that vesicular uptake and release are characteristics that identify these amino acids as neurotransmitters. We also discuss that "quanta" in the brain are not necessarily related the content of neurotransmitter in the synaptic vesicles, but rather to postsynaptic events.

A study of the uptake of glutamate, gamma-aminobutyric acid (GABA), glycine and beta-alanine in synaptic brain vesicles from fish and avians

The ATP-dependent uptake of amino acids into synaptic vesicles isolated from mammalian brain is well characterized. To determine whether these characteristics are fundamental to the vesicular uptake system, synaptic vesicles were isolated from brains of the vertebrate species, rainbow trout and chicken and assayed for glutamate, gamma-aminobutyric acid (GABA) and glycine uptake activity. Uptake was dependent upon temperature, Mg2+ and ATP and was also strongly inhibited by the alkylating agent N-ethylmaleimide which is known to inhibit the ATPase, confirming that this was an energy requiring process. Interestingly GABA and beta-alanine were inhibitors of vesicular uptake of glycine in both species. Likewise the uptake of GABA was inhibited by glycine and beta-alanine. Glutamate, GABA, glycine and beta-alanine were all taken up into vesicles from both trout and chicken, and the uptake ratios were similar to the corresponding uptake ratios in synaptic vesicles from rat. These results indicate that the synaptic vesicle uptake system for glutamate, GABA and glycine uptake system is conserved throughout the vertebrate class both in respect to ATP-dependency and substrate specificity.

Effects of taurine, homotaurine and GABA on hypothalamic and striatal dopamine metabolism

To elucidate the effects of taurine on hypothalamic and striatal dopaminergic neurotransmission we compared its effects to those of gamma-aminobutyric acid (GABA) and homotaurine (a GABAA-receptor agonist) on hypothalamic and striatal concentrations of dopamine (DA) and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and, in the case of striatum, 3-methoxytyramine (3-MT) in rats. In addition, hypothalamic and striatal 5-hydroxytryptamine (5-HT) und 5-hydroxyindoleacetic acid, hypothalamic noradrenaline (NA) and 3-methoxy-4-hydroxyphenylglycol sulfate, and pituitary DA concentrations were also measured. The amino acids were injected into the lateral brain ventricles of conscious male rats in doses of 10 and 36 mumol/rat, and rat were sacrificed 15 and 60 min later, respectively. Homotaurine (by 11%) but not the other two amino acids elevated striatal DA, whereas hypothalamic DA was increased by both taurine (36%) and homotaurine (31%). All three amino acids at 36 mumol elevated striatal DOPAC, homotaurine (51%) more than taurine (31%) or GABA (30%), and hypothalamic DOPAC, both taurine (102%) and homotaurine (82%) clearly more than GABA (34%). Neither striatal nor hypothalamic HVA was altered by any of the amino acids. At 10 mumol the amino acids decreased striatal 3-MT by about 40%. At 36 mumol taurine and homotaurine reduced 3-MT by about 70%, whereas increasing the dose of GABA did not further reduce 3-MT. Both taurine and homotaurine at 36 mumol decreased hypothalamic NA content. Neither hypothalamic nor striatal 5-HT metabolism was altered. In the neurointermediate lobe of the pituitary gland taurine at 10 mumol but not at 36 mumol slightly (20%) increased DA.(ABSTRACT TRUNCATED AT 250 WORDS)

The ontogeny of the uptake systems for glutamate, GABA, and glycine in synaptic vesicles isolated from rat brain

The ontogeny of the uptake of glutamate, GABA and glycine into synaptic vesicles isolated from rat brain has been investigated. The vesicular uptake of the three amino acids increased with developmental age in parallel with synaptogenesis, indicating a functional role of uptake of the amino acids by synaptic vesicles in the nerve terminals. Uptake of the amino acids by plasma membrane particles (synaptosomes) in brain homogenate showed a somewhat different developmental profile. The uptake of glutamate increased markedly with developmental time, while the uptake of GABA showed only a slight increase. Uptake of glycine by plasma membrane particles was very low and therefore not registered. The observed developmental increase in uptake of glycine by synaptic vesicles isolated from brain, supports previous reports indicating that glycine can be taken up by vesicles from non-glycine terminals.

Inhibition of L-glutamate uptake into synaptic vesicles

The effects of different agents similar in structure to glutamate were tested for inhibition of the vesicular uptake of L-glutamate. Kainate and L-homocysteate turned out to be non-competitive inhibitors of the L-glutamate uptake. Kainate was not taken up by the vesicle fraction. The vesicular uptake of gamma-aminobutyric acid (GABA) was also inhibited by kainate and L-homocysteate. Kynurenate, on the other hand, strongly inhibited the uptake of L-glutamate, whereas the uptake of GABA was hardly affected. L-alpha-Aminoadipate and D-glutamate inhibited the uptake of L-glutamate, whereas L- and D-aspartate and L-cysteate only weakly inhibited the uptake of L-glutamate. GABA, glycine, L-serine and taurine did not inhibit the uptake of L-glutamate.

The ontogeny of the uptake systems for glycine, GABA and glutamate in synaptic vesicles isolated from rat spinal cord-medulla

Synaptic vesicles have been isolated from rat spinal cord-medulla at different postnatal ages, and the ontogeny of the uptake of glycine, gamma-aminobutyric acid (GABA) and glutamate has been investigated. The accumulation of the 3 amino acids increased with increasing time after birth reaching adult level at about postnatal day 30. This developmental increase probably parallels the synaptogenesis and suggests a functional role of the uptake of the amino acids into synaptic vesicles in the nerve terminals. The developmental time course for these vesicular uptake systems was totally different from those of the corresponding plasma membrane uptakes.