Glutamate uptake into synaptic vesicles of bovine cerebral cortex and electrochemical potential difference of proton across the membrane

Hofmeister Series: From Aqueous Solution of Biomolecules to Single Cells and Nanovesicles

Hofmeister effects play a critical role in numerous physicochemical and biological phenomena, including the solubility and/or accumulation of proteins, the activities of enzymes, ion transport in biochannels, the structure of lipid bilayers, and the dynamics of vesicle opening and exocytosis. This minireview focuses on how ionic specificity affects the physicochemical properties of biomolecules to regulate cellular exocytosis, vesicular content, and nanovesicle opening. We summarize recent progress in further understanding Hofmeister effects on biomacromolecules and their applications in biological systems. These important steps have increased our understanding of the Hofmeister effects on cellular exocytosis, vesicular content, and nanovesicle opening. Increasing evidence is firmly establishing that the ions along the Hofmeister series play an important role in living organisms that has often been ignored.

Anionic Species Regulate Chemical Storage in Nanometer Vesicles and Amperometrically Detected Exocytotic Dynamics

Hofmeister effects have often been ignored in living organisms, although they affect the activity and functions of biological molecules. Herein, amperometry has been applied to show that the vesicular content, dynamics of exocytosis and vesicles opening, depend on the anionic species treatment. Compared to 100 μM Cl^- treated chromaffin cells, a similar number of catecholamine molecules is released after chaotropic anions (ClO₄^- and SCN^-) treatment, even though the vesicular catecholamine content significantly increases, suggesting a lower release fraction. In addition, there are opposite effects on the dynamics of vesicles release (shorter duration) and vesicle opening (longer duration) for chaotropic anions treated cells. Our results show anion-dependent vesicle release, vesicle opening, and vesicular content, providing understanding of the pharmacological and pathological processes induced by inorganic ions.

Protons Regulate Vesicular Glutamate Transporters through an Allosteric Mechanism

The quantal nature of synaptic transmission requires a mechanism to transport neurotransmitter into synaptic vesicles without promoting non-vesicular efflux across the plasma membrane. Indeed, the vesicular transport of most classical transmitters involves a mechanism of H(+) exchange, which restricts flux to acidic membranes such as synaptic vesicles. However, vesicular transport of the principal excitatory transmitter glutamate depends primarily on membrane potential, which would drive non-vesicular efflux, and the role of protons is unclear. Adapting electrophysiology to record currents associated with the vesicular glutamate transporters (VGLUTs), we characterize a chloride conductance that is gated by lumenal protons and chloride and supports glutamate uptake. Rather than coupling stoichiometrically to glutamate flux, lumenal protons and chloride allosterically activate vesicular glutamate transport. Gating by protons serves to inhibit what would otherwise be substantial non-vesicular glutamate efflux at the plasma membrane, thereby restricting VGLUT activity to synaptic vesicles.

The glutamate uptake system in presynaptic vesicles: further characterization of structural requirements for inhibitors and substrates

Noncyclic fluorine-substituted and cyclic analogs of glutamic acid were tested for their ability to inhibit glutamate uptake in isolated bovine presynaptic vesicles, in order to assess the specific structural requirements of the glutamate translocation system in the vesicle membrane. Cyclic analogs that permitted close interaction between the positive and negative charges of the glutamate molecule were effective inhibitors; maximum inhibitory potency was observed with L-trans-1-aminocyclopentane-1,3-dicarboxylic acid (L-t-ACPD), while D-t-ACPD was less active. Analogs with a larger or smaller ring (as in trans-1-aminocyclohexane-1,3-dicarboxylic acid or trans-1-aminocyclobutane-1,3-dicarboxylic acid) were also inhibitory, but somewhat less so. trans-ACPD was also taken up by the vesicles with a time course and ATP dependence similar to uptake of glutamate, and this uptake was inhibited by glutamate. The K(m) value for t-ACPD uptake was similar to its K(i) for inhibition of glutamate uptake, while its rate of uptake was lower than that of glutamate. Fluorine-substituted noncyclic analogs with substitutions at the 4-carbon were less effective than glutamic acid itself, although 4,4-difluoroglutamic acid was equal in activity to the unsubstituted compound. Inhibition by these derivatives appeared to be competitive in nature, and they probably were also transported by the vesicle uptake system.

Ethanolamine and related amino alcohols increase basal and evoked release of [3H]-D-aspartic acid from synaptosomes by enhancing the filling of synaptic vesicles

This research examines the effects of ethanolamine and other amino alcohols on the dynamics of acridine orange (AO), oxonol V, and [3H]-D-aspartic acid in synaptic preparations isolated from mammalian brain. Ethanolamine concentration-dependently enhanced AO release from synaptosomes. Similar effects were observed with methylethanolamine and dimethylethanolamine, but not choline. The enhancement of AO efflux by ethanolamine was independent of extrasynaptosomal calcium (in contrast to KCl-induced AO efflux), was unaffected by tetrodotoxin and did not involve depolarization of the synaptosomal plasma membrane. KCl was unable to release AO from synaptosomes following exposure to ethanolamine, however ethanolamine and other amino alcohols were found to enhance both basal and KCl-evoked release of [3H]-D-aspartic acid from synaptosomes. Using isolated synaptic vesicles we demonstrate that amino alcohols are able to 1) abolish the ATP-dependent intravesicular proton concentration (i.e. stimulate efflux of AO) in a similar way to carbonyl cyanide m-chlorophenylhydrazone (CCCP), 2) increase the ATP-supported transvesicular membrane potential (i.e. quench oxonol V fluorescence) in contrast to CCCP and 3) enhance intravesicular uptake of [3H]-D-aspartic acid. These results suggest that positively charged, membrane impermeant amino alcohol species are generated within synaptic vesicles as they sequester protons. Cationic forms of these amino alcohols boost the transvesicular electrical potential which increases transmitter uptake into synaptic vesicles and facilitates enhancement of basal and evoked release of transmitter. Our data suggest a potential role for ethanolamine and related amino alcohols in the regulation of synaptic vesicle filling. These findings may also have relevance to neuropathophysiological states involving altered production of ethanolamine.

Guanine derivatives modulate L-glutamate uptake into rat brain synaptic vesicles

Glutamate uptake into synaptic vesicles is driven by a proton electrochemical gradient generated by a vacuolar H(+)-ATPase and stimulated by physiological concentrations of chloride. This uptake plays an important role in glutamatergic transmission. We show here that vesicular glutamate uptake is selectively inhibited by guanine derivatives, in a time- and concentration-dependent manner. Guanosine, GMP, GDP, guanosine-5'-O-2-thiodiphosphate, GTP, or 5'-guanylylimidodiphosphate (GppNHp) inhibited glutamate uptake in 1.5 and 3 min incubations, however, when incubating for 10 min, only GTP or GppNHp displayed such inhibition. By increasing ATP concentrations, the inhibitory effect of GTP was no longer observed, but GppNHp still inhibited glutamate uptake. In the absence of ATP, vesicular ATPase can hydrolyze GTP in order to drive glutamate uptake. However, 5mM GppNHp inhibited ATP hydrolysis by synaptic vesicle preparations. GTP or GppNHp decreased the proton electrochemical gradient, whereas the other guanine derivatives did not. Glutamate saturation curves were assayed in order to evaluate the specificity of inhibition of the vesicular glutamate carrier by the guanine derivatives. The maximum velocity of the initial rate of glutamate uptake was decreased by all guanine derivatives. These results indicate that, although GppNHp can inhibit ATPase activity, guanine derivatives are more likely to be acting through interaction with vesicular glutamate carrier.

Chloride-dependent inhibition of vesicular glutamate uptake by alpha-keto acids accumulated in maple syrup urine disease

Maple syrup urine disease is a metabolic disorder caused by mutations of the branched chain keto acid dehydrogenase complex, leading to accumulation of alpha-keto acids and their amino acid precursors in the brain. We now report that alpha-ketoisovaleric, alpha-keto-beta-methyl-n-valeric and alpha-ketoisocaproic acids accumulated in the disease inhibit glutamate uptake into rat brain synaptic vesicles. The alpha-keto acids did not affect the electrochemical proton gradient across the membrane, suggesting that they interact directly with the vesicular glutamate carrier. Chloride anions have a biphasic effect on glutamate uptake. Low concentrations activate the uptake (0.2 to 8 mM), while higher concentrations are inhibitory. The alpha-keto acids inhibited glutamate uptake by a new mechanism, involving a change in the chloride dependence for the activation of glutamate uptake. The activation of glutamate uptake by low chloride concentrations was shifted toward higher concentrations of the anion in the presence of alpha-keto acids. Inhibition by alpha-keto acids was abolished at high chloride concentrations (20 to 80 mM), indicating that alpha-keto acids specifically change the stimulatory effect of low chloride concentrations. High glutamate concentrations also reduced the inhibition by alpha-keto acids, an effect observed both in the absence and in the presence of low chloride concentrations. The results suggest that in addition to their possible pathophysiological role in maple syrup urine disease, alpha-keto acids are valuable tools to study the mechanism of vesicular transport of glutamate.

Uptake of L-glutamate into synaptic vesicles: competitive inhibition by dyes with biphenyl and amino- and sulphonic acid-substituted naphthyl groups

The specificity of the vesicular L-glutamate carrier was characterized using dyes with biphenyl and amino- and sulphonic acid substituted naphthyl groups, structurally similar to the specific vesicular L-glutamate inhibitor Evans Blue. The dye Trypan Blue was the most potent inhibitor; the IC50 value was determined to be 49 nM. Naphthol Blue Black, Reactive Blue 2, Benzopurpurin 4B, Ponceau SS, Direct Blue 71 and Acid red 114 were also highly potent inhibitors with IC50 values from 330 to 1670 nM (series 1). The dyes were competitive inhibitors of vesicular glutamate uptake, and acted therefore on the glutamate transporter. Their IC50 values for the vesicular uptake of gamma-aminobutyric acid (GABA) were all higher than 20 microM. They had no effect on synaptosomal uptake of glutamate. Furthermore, we have also found several other dyes with IC50 values for the vesicular uptake of glutamate ranging between 1 and 30 microM and for gamma-aminobutyric acid higher than 50 microM (series 2). The most potent inhibitor Trypan Blue contains a biphenyl group, linked by azo groups to side chains containing sulphonic, amino and/or hydroxyl groups coupled to a naphthalene ring system. Trypan Blue and Evans Blue are by molecular mechanics, shown to have planar structures with conjugated double bonds throughout the structure. The other dyes, which were less effective, had phenyl and/or naphthalene groups linked by an azo group. We have also tested a series of amino and/or hydroxyl naphthalene di-/sulphonic acids that correspond to the side chains of the most potent dyes, but they had no effect on glutamate nor on gamma-aminobutyric acid uptake. We conclude that the inhibitory action of these compounds is strictly dependent of the complete molecule.

Bilirubin inhibits transport of neurotransmitters in synaptic vesicles

Uptake of neurotransmitters into synaptic vesicles occurs through specific transport proteins which are driven by an ATPase-generated electrochemical force consisting of a proton gradient and a membrane potential. In this study we examined the effects of bilirubin, a well known neurotoxic agent, on the vesicle uptake both of [3H]dopamine (which is driven mostly by the proton gradient) and [3H]glutamate (which is driven mostly by the membrane potential), and compared these to the vesicular proton gradient, which was estimated by analyzing the uptake of [14C]methylamine. Bilirubin inhibited the uptake of both dopamine and glutamate (p < 0.01), with an identical dose-response curve for both transmitters. Inhibition was detected readily at 75 microM. The effects of bilirubin were dependent on the concentration of vesicles in the assay, suggesting that the concentration of bilirubin in the membranes and not the water phase was important. Bilirubin also decreased uptake-dependent efflux of dopamine from the vesicles. In contrast, bilirubin had no effect on the vesicular proton gradient, as measured by methylamine uptake. Our results show that bilirubin has essentially identical inhibitory effects on the uptake of both a monoamine transmitter and an amino acid transmitter into synaptic vesicles, but does not influence the vesicular H+-ATPase or proton translocation. Our data suggest an inhibitory interaction between bilirubin and several transport proteins in synaptic vesicle membranes.

Glutamate transport and storage in synaptic vesicles

Glutamate plays an important metabolic role in virtually every vertebrate cell. In particular, glutamate is the most common excitatory neurotransmitter in the vertebrate central nervous system. As such, the mechanism by which glutamate is diverted from its normal metabolic activities toward its role as a neurotransmitter has, in recent years, been systematically investigated. In glutamatergic nerve endings, synaptic vesicles accumulate and store a proportion of the cellular glutamate pool and, in response to appropriate signals, release glutamate into the synaptic cleft by exocytosis. Glutamate accumulation is accomplished by virtue of a glutamate uptake system present in the synaptic vesicle membrane. The uptake system consists of a transport protein, remarkably specific for glutamate, and a vacuolar-type H+-ATPase, which provides the coupling between ATP hydrolysis and glutamate transport. The precise manner in which the glutamate transporter and H+-ATPase operate is currently the subject of debate. Recent data relevant to this debate are reviewed in this article. Additionally, pharmacological agents thought to specifically interact with the vesicular glutamate transporter are discussed. Finally, a newly discovered, endogenous inhibitor of vesicular uptake, inhibitory protein factor (IPF), is discussed with some speculations as to its potential role as a presynaptic modulator of neurotransmission.

The effect of arachidonic acid and free fatty acids on vesicular uptake of glutamate and gamma-aminobutyric acid

The manner in which arachidonic acid and other free fatty acids influence the vesicular uptake of glutamate and gamma-aminobutyric acid (GABA) has been investigated. The cis-polyunsaturated fatty acid arachidonic acid (20:4), eicosapentanoic acid (20:5) and linolenic acid (18:3) at 150 nmol/mg protein (50 microM) inhibited the vesicular uptake of glutamate and GABA more than 70%. Reduced inhibition of vesicular uptake was seen with the cis-monounsaturated fatty acid oleic acid (18:1) and the trans-mono-unsaturated fatty acid elaidic acid (18:1). The saturated fatty acids stearic acid (16:0) and arachidic acid (20:0) had no significant effect on the uptake. The inhibition of vesicular uptake by arachidonic acid was prevented by the addition of fatty acid free bovine serum albumin. Arachidonic acid inhibited in a dose-dependent manner the generation of the transmembrane pH gradient of the synaptic vesicles. This inhibition was proportional to the inhibition of the vesicular uptake of glutamate and GABA. The saturated fatty acid arachidic acid showed no inhibition of delta pH generation. Arachidonic acid at 200 nmol/mg of protein did not increase the uptake-independent leakage of glutamate and GABA from the vesicles, showing that the effect of arachidonic acid is not caused by an unspecific detergent effect. These results suggest that arachidonic acid and other polyunsaturated fatty acids are acting like proton-ionophores on the vesicular uptake of these neurotransmitters. This finding may have implications for the increased fatty acid concentration during pathological conditions like ischemia and in long term potentiation.

A protein factor that inhibits ATP-dependent glutamate and gamma-aminobutyric acid accumulation into synaptic vesicles: purification and initial characterization

Glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, is transported into and stored in synaptic vesicles. We have purified to apparent homogeneity a protein from brain cytosol that inhibits glutamate and gamma-aminobutyric acid uptake into synaptic vesicles and have termed this protein "inhibitory protein factor" (IPF). IPF refers to three distinct proteins with relative molecular weights of 138,000 (IPF alpha), 135,000 (IPF beta), and 132,000 (IPF gamma), respectively. Gel filtration and sedimentation data suggest that all three proteins share an elongated structure, identical Stokes radius (60 A), and identical sedimentation coefficient (4.3 S). Using these values and a partial specific volume of 0.716 ml/g, we determined the native molecular weight for IPF alpha to be 103,000. Partial sequence analysis shows that IPF alpha is derived from alpha fodrin, a protein implicated in several diverse cellular activities. IPF alpha inhibits ATP-dependent glutamate uptake into purified synaptic vesicles with an IC50 of approximately 26 nM, while showing no ability to inhibit ATP-independent uptake at concentrations up to 100 nM. Moreover, IPF alpha inhibited neither norepinephrine uptake into chromaffin vesicles nor Na+-dependent glutamate uptake into synaptosomes. However, IPF alpha inhibited uptake of gamma-aminobutyric acid into synaptic vesicles derived from spinal cord, suggesting that inhibition may not be limited to glutamatergic systems. We propose that IPF could be a novel component of a presynaptic regulatory system. Such a system might modulate neurotransmitter accumulation into synaptic vesicles and thus regulate the overall efficacy of neurotransmission.

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.

Characterization of ATP transport into chromaffin granule ghosts. Synergy of ATP and serotonin accumulation in chromaffin granule ghosts

ATP is an excitatory neurotransmitter that is stored and cosecreted with catecholamines from cells of the adrenal medulla. While the transport of catecholamines into chromaffin granule ghosts has been extensively characterized, there is little information on the mechanism of ATP transport into these structures. Here we show that ATP transport is driven by the electrical component of the electrochemical proton gradient created by the chromaffin granule membrane H+-ATPase, and that the accumulated nucleotide is released from the vesicles by inhibition of the H+-ATPase. GTP and UTP are also substrates for this transporter, distinguishing it from the mitochondrial ADP/ATP exchanger. Accumulation of ADP and ATP (rather than exchange with intravesicular ATP) is demonstrated by high pressure liquid chromatography measurements. The anion transport inhibitor 4,4-diisothiocyanatostilbene-2,2-disulfonic acid (Ki = 27 microM) inhibits ATP transport, while atractyloside, the inhibitor of the mitochondrial ATP/ADP exchanger, is a very poor inhibitor. Finally, we have demonstrated a synergy between the accumulation of ATP and that of serotonin (i.e. more of each solute accumulates when the two are accumulated together), supporting the view that there is an interaction between serotonin and ATP that reduces their effective concentration within the ghosts.

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.

Co-localization of glutamate and homocysteic acid immunoreactivities in human photoreceptor terminals

Consecutive semithin sections of human retinae were treated with antisera recognizing fixed homocysteic acid, glutamate or glutamine. Photoreceptor terminals displayed a co-localization of glutamate-like and homocysteic acid-like immunoreactivities. This was confirmed in the electron microscope by immunogold cytochemistry. A quantitative analysis of the immunogold labelling indicated that glutamate and homocysteic acid occurred at higher concentrations in the terminals than in outer parts of the receptor cells. No such gradient was found for glutamine immunoreactivity, which was concentrated in Müller cell processes. These processes were also labelled by the homocysteic acid antiserum, although less intensely than were the photoreceptor terminals. Control experiments suggested that the homocysteic acid antiserum visualized a pool of authentic homocysteic acid, although it could not be excluded that part of this pool had been generated by non-enzymatic oxidation of precursor molecules. Homocysteic acid immunoreactivity was also demonstrated in photoreceptor terminals of baboon. The present data indicate that primate photoreceptor terminals contain homocysteic acid in addition to glutamate and open up the possibility that homocysteic acid is released as a glutamate co-agonist at photoreceptor synapses.

Ammonia added in vitro, but not moderate hyperammonemia in vivo, stimulates glutamate uptake and H(+)-ATPase activity in synaptic vesicles of the rat brain

The uptake of radiolabelled neurotransmitters: glutamate (GLU), GABA, and dopamine (DA) and the activity of the vacuolar type H(+)-pumping ATPase (H(+)-ATPase), were measured in crude synaptic vesicles treated in vitro with a neurotoxic (3 mM) dose of NH4+ (acetate or chloride), or isolated from rats with a moderate increase of brain ammonia (to approximately 0.6 mM) induced by i.p. administration of ammonium acetate (HA rats) or a hepatotoxin-thioacetamide (HE rats). In vitro treatment with ammonium salts increased the sodium-independent, chloride-dependent uptake of GLU but did not stimulate the uptake of GABA or DA. The in vitro treatment also stimulated the H(+)-ATPase activity. Since H(+)-ATPase generates the electrochemical gradient driving synaptic vesicular neurotransmitter transport, its stimulation by ammonia may have facilitated GLU uptake. However the GLU specificity of the effect must be related to other factors differentially affecting GLU uptake and the uptake of other neurotransmitters. Enhanced GLU accumulation in the synaptic vesicles may contribute to the increase of synaptic GLU exocytosis previously reported to accompany acute increases of brain ammonia to toxic levels. However, GLU uptake and H(+)-ATPase activity, but also the uptake of GABA and DA, were unchanged in synaptic vesicles prepared from rats with HA or HE. This indicates that changes in GLU and/or GABA release reported for moderate hyperammonemic conditions must be elicited by factors unrelated to the synaptic vesicular transport of the amino acids.

Effect of intermittent mild hypoxia and drug treatment on synaptosomal nonmitochondrial ATPase activities

Synaptosomal nonmitochondrial ATPases linked to the energy-utilizing systems were evaluated in cerebral cortex from normoxic rats and rats submitted to mild intermittent normobaric hypoxia [12 hr daily exposure to N2:O2 (90:10) mixture for 4 weeks]. The activities of Na+,K(+)-ATPase; high- and low-affinity Ca(2+)-ATPase; basal Mg(2+)-ATPase; and Ca2+, Mg(2+)-ATPase were assayed in synaptosomes and synaptosomal subfractions, namely, synaptosomal plasma membranes and synaptic vesicles. The evaluations were performed either in normoxic rats or in hypoxic rats submitted to 4-week treatment with saline (controls) or a vasodilator agent (papaverine), an energy-metabolism interfering agent (theniloxazine), a calcium blocker (nicardipine), and a lipid-metabolism interfering agent (phosphatidylcholine) in order to define the plasticity and the selective changes in individual ATPases. In synaptosomes from rat cerebral cortex, the enzyme adaptation to the daily mild intermittent hypoxia for 4 weeks was characterized by an increase in the activity of Mg(2+)-ATPase, concomitant with a decrease in the activities of Na+,K(+)-ATPase, high-affinity Ca(2+)-ATPase, and Ca2+, Mg(2+)-ATPase. In hypoxic rats the enzyme adaptation to the 4-week treatment with phosphatidylcholine was characterized by an increase in Ca2+, Mg(2+)-ATPase activity and a decrease in Mg(2+)-ATPase activity. The action involves the enzymatic form located in the synaptic plasma membranes. In hypoxic rats the adaptation to the 4 week treatment with nicardipine was characterized by an increase in high-affinity Ca(2+)-ATPase activity, while the 4-week-treatment with theniloxazine induced an increase in Na+,K(+)-ATPase activity. The actions of both nicardipine and theniloxazine were related to the enzymatic forms located in the synaptic plasma membranes. The effects on the biophase induced by the sequential cycles of hypoxia/normoxia and the treatment with the various agents tested should also be related to the changes induced in the activity of some synaptosomal ATPases.

Glutamate uptake system in the presynaptic vesicle: glutamic acid analogs as inhibitors and alternate substrates

A variety of naturally occurring amino acids, their isomers, and synthetic analogs were tested for their ability to inhibit uptake of [3H]glutamate into presynaptic vesicles from bovine cerebral cortex. Strongest inhibition (Ki < 1mM) was observed for trans-1-aminocyclopentane-1,3-dicarboxylic acid (t-ACPD) and erythro-4-methyl-L-glutamic acid (MGlu), while 4-methylene-L-glutamic acid (MeGlu) was only moderately inhibitory (Ki = approximately 3mM), indicating that the synaptic vesicle glutamate translocator has higher affinity for trans-ACPD and MGlu than for glutamate. A few other amino acids, e.g., 4-hydroxyglutamic acid, S-carboxyethyl cysteine, and 5-fluorotryptophan, were slightly inhibitory; all L- and DL-isomers of protein amino acids and longer chain acidic amino acids were without measurable inhibition. Potassium tetrathionate and S-sulfocysteine exhibited strong to moderate noncompetitive or irreversible inhibition. Inhibition by t-ACPD, MGlu, or MeGlu was competitive with glutamic acid. Each of these competitive inhibitors was also taken up by the vesicle preparation in an ATP-dependent manner, as indicated by their being recovered unchanged from filtered vesicles. Similar results were obtained with reconstituted vesicles, while glutamate uptake by partially purified rat synaptosomes was inhibited only by MGlu. These results indicate that the glutamate translocator of presynaptic vesicles has stringent structural requirements distinct from those of the plasma membrane translocator and the metabotropic type of postsynaptic glutamate receptor. They further suggest possible structural requirements of pharmacologically significant compounds that can substitute for glutamic acid in the presynaptic side of glutamatergic synapses, thus serving to moderate or control glutamate excitation and associated excitotoxic effects in these neurons.

Added ATP influences some responses of rat synaptosomes to glutamate

ATP added externally to rat synaptosomes activated uptake of both Ca2+ and glutamate which was partially accounted for by the uptake phenomena of synaptic vesicles and mitochondria, as shown by using specific inhibitors of the latter. Increasing concentrations of glutamate stimulated Ca2+ entry linearly, as shown by using 45Ca or a Ca-specific electrode. The processes of glutamate and Ca2+ uptake shared some common features and their ATP-dependence may be correlated with an ouabain-insensitive synaptosomal ectonucleotidase activity measured by a 31P-NMR or a luminometric technique. The ATP hydrolysis catalysed by the synaptosomes was activated by both Ca2+ and glutamate. The present synaptosomal activities may represent a model for studying the modulatory effects of ATP on the glutamatergic neurotransmission.

ATP-stimulated glutamate-dependent calcium uptake by rat synaptosomes

The entry of Ca2+ in rat synaptosomes was followed with a Ca(2+)-selective electrode. Extracellular ATP is necessary for the entry which is a function of synaptosomal protein, free Ca2+ and glutamate concentrations. Ketamine, glycine and kainate have negligible effect while quisqualate slightly inhibits the uptake of Ca2+ in the presence of glutamate. The added ATP is hydrolyzed by the synaptosomes through an ouabain-insensitive ecto-ATPase affected by the presence of Ca2+, glutamate and, to a slight extent, NMDA.

Transport of gamma-aminobutyrate and L-glutamate into synaptic vesicles. Effect of different inhibitors on the vesicular uptake of neurotransmitters and on the Mg2(+)-ATPase

The uptakes of gamma-aminobutyrate (GABA) and L-glutamate into synaptic vesicles isolated from rat brain were compared with respect to the effects of 4-acetamido-4'-isothiocyanostilbene-2,2'- disulphonic acid (SITS), 4,4'-di-isothiocyanostilbene-2,2'-disulphonic acid (DIDS) and 5-nitro-2-(3-phenylpropylamino)benzoic acid (N144), agents known to block anion channels. The uptake of glutamate was inhibited by low micromolar concentrations of SITS, DIDS and N144. GABA uptake was much less sensitive to these agents than was glutamate uptake. SITS and N144 inhibited the vacuolar H(+)-ATPase of synaptic vesicles to a smaller extent than the glutamate uptake. The uptake of GABA was not affected by the permeant anions Cl- and Br-, whereas the uptake of glutamate was highly stimulated by low concentrations of these ions. The uptakes of both glutamate and GABA were inhibited by similar, but not identical, concentrations of the lipophilic anion SCN-.

2,6-diisopropylphenol, a general anesthetic, inhibits glutamate action on rat synaptosomes

2,6-diisopropylphenol (propofol), a general intravenous anesthetic, inhibits the glutamate-dependent Ca2+ entry in rat synaptosomes with an approximate IC50 of 3.0 x 10(-5) M. Propofol, at concentrations above 10(-6) M, also inhibits the ATP-dependent uptake of glutamate in the presence of Ca2+, with an approximate IC50 of 3.5 x 10(-5) M, while it only has a slight inhibitory effect on the release of glutamate. The ouabain-insensitive synaptosomal ATPase is strongly inhibited by propofol, with an IC50 of about 2.5 x 10(-6) M, at concentrations which do not affect the luciferase system.

Artificially imposed electrical potentials drive L-glutamate uptake into synaptic vesicles of bovine cerebral cortex

L-Glutamate is a major excitatory neurotransmitter in the central nervous system. MgATP-dependent glutamate uptake and H(+)-pumping ATPase activity were reported in highly purified synaptic vesicles [Naito & Ueda (1983) J. Biol. Chem. 258, 696-699; Shioi, Naito & Ueda (1989) Biochem. J. 258, 499-504], and it is hypothesized that an electrochemical H+ gradient across the vesicle membrane, the so-called protonmotive force, elicits the neurotransmitter uptake. An inside-positive diffusion potential across the vesicle membrane was established with valinomycin plus Rb+. This artificial electrical potential promoted the uptake of glutamate, but not aspartate, in the synaptic vesicles prepared from bovine cerebral cortex. The uptake was inhibited by the protonmotive-force dissipators carbonyl cyanide p-trifluoro-methoxyphenylhydrazone or nigericin, and was enhanced by concomitant imposition of a pH jump (alkalinization) in the external medium. Subcellular and subvesicular distributions showed the uptake system to be predominantly associated with small synaptic vesicles. The results support the hypothesis that glutamate uptake into synaptic vesicles is coupled with a H+ efflux down the electrochemical potential gradient, which is generated by H(+)-pumping ATPase.

Accumulated glutamate levels in the synaptic vesicle are not maintained in the absence of active transport

We have investigated factors which may affect accumulated glutamate levels in synaptic vesicles and glutamate efflux. Agents which dissipate the electrochemical proton gradient resulted in a rapid reduction of steady-state vesicular glutamate levels, which was prevented by N-ethylmaleimide. Glutamate efflux was found to occur even in the presence of an electrochemical proton gradient, but was effectively inhibited by N-ethylmaleimide. These results suggest that accumulated glutamate levels in synaptic vesicles are not maintained unless glutamate is taken up continuously by an active transport mechanism, and they could provide an explanation for the lack of convincing evidence for the enrichment of endogenous glutamate in isolated synaptic vesicles.

Glutamate uptake into synaptic vesicles: competitive inhibition by bromocriptine

The ATP-dependent uptake of L-glutamate into synaptic vesicles has been well characterized, implicating a key role for synaptic vesicles in glutamatergic neurotransmission. In the present study, we provide evidence that vesicular glutamate uptake is selectively inhibited by the peptide-containing halogenated ergot bromocriptine. It is the most potent inhibitor of the agents tested: the IC50 was determined to be 22 microM. The uptake was also inhibited by other ergopeptines such as ergotamine and ergocristine, but with less potency. Ergots devoid of the peptide moiety, however, such as ergonovine, lergotrile, and methysergide, had little or no effect. Although bromocriptine is known to elicit dopaminergic and serotonergic effects, its inhibitory effect on vesicular glutamate uptake was not mimicked by agents known to interact with dopamine and serotonin receptors. Kinetic data suggest that bromocriptine competes with glutamate for the glutamate binding site on the glutamate translocator. It is proposed that this inhibitor could be useful as a prototype probe in identifying and characterizing the vesicular glutamate translocator, as well as in developing a more specific inhibitor of the transport system.