Uptake of L-[3H] glutamic acid by crude and purified synaptic vesicles from rat brain

Rat brain synaptic vesicles suspended in a medium comprised of potassium tartrate displayed saturable accumulation of L-[³H] glutamic acid at 37° (Km 2.0 × 10^?4M; 311±13 pmol/mg protein), which was stable for periods up to 60 min. The accumulation was temperature sensitive and partially ATP-dependent, uptake levels being reduced to 18.7±0.8 pmol/mg protein at 4°, and to 141±4 pmol/mg protein in the absence of ATP. Fractionation of a crude vesicle preparation on a discontinuous sucrose gradient demonstrated the accumulation to be specifically associated with the synaptic vesicle fraction.     

ATP-regulated neuronal catecholamine uptake: a new mechanism

Uptake of the catecholamines (CA), dopamine (DA) and norepinephrine (NE) into synaptosomes prepared from rat and bovine brains was potentiated by ATP (from 0.1 to 5.0 mM) in a dose-dependent manner. Other nucleotides, particularly the nonhydrolyzable ATP analogs beta,gamma-imidoadenosine-5'-triphosphate (AMP-PNP) and beta,gamma-methyladenosine-5'-triphosphate (AMP-PCP) also potentiated [3H]DA and [3H]NE uptake. Several endogenous 5'-nucleotide triphosphates (e.g. GTP, UTP and CTP) potentiated [3H]CA uptake, but were less effective than ATP. Among the ATP metabolites, only ADP potentiated uptake whereas AMP and adenosine did not. [3H]Dopamine uptake measured in Krebs bicarbonate buffer had a Km of 2.1 microM and a Vmax of 163.9 pmol/mg prot./min. In presence of ATP, [3H]DA uptake had much higher affinity (Km = 0.56 microM) and larger capacity (Vmax = 333 pmol/mg prot./min) than uptake in absence of added ATP. Furthermore, [3H]DA uptake in presence of ATP had faster rate of uptake, and was independent of temperature while in absence of added ATP it was temperature-dependent. This ATP-dependent [3H]DA uptake was retained by synaptosomal ghosts that were obtained after lysing the striatal synaptosomes and removing their contents of synaptic vesicles and mitochondria. It is proposed that, in addition to the carrier-mediated (neuronal) uptake of CA, there is neuronal uptake that is regulated by ATP and inhibited by cocaine, which may be more relevant for terminating the synaptic action of CA because of its faster rate of uptake and larger capacity.     

SYNAPTIC VESICLES ISOLATED FROM RAT HEART: l-[3H]NOREPINEPHRINE UPTAKE PROPERTIES1

A fraction containing synaptic vesicles was isolated from rat heart by differential centrifugation, and the uptake of l-[³H]norepinephrine was studied in vitro., Uptake was highly dependent upon time and temperature, and was linear for 6 min at 30° or 4 min at 37°C. About 80% of the measured uptake required both ATP and Mg²⁺ and was inhibited by nanomolar concentrations of reserpine; no inhibition was obtained with cocaine. These properties are characteristic of storage vesicle uptake as opposed to synaptic membrane uptake. Uptake of norepinephrine was saturable and displayed a single K_m value of 2 μM. The uptake was completely stereospecific, as unlabeled dl-norepinephrine was less than half as effective as unlabeled l-norepinephrine in reducing uptake of l-[³H]norepinephrine. Norepinephrine uptake could be inhibited by various phenethylamines and indoleamines following the rank order: reserpine > harmaline > 5-hydroxytryptamine > dopamine > norepinephrine. The vesicle preparation also incorporated [³H]5-hydroxytryptamine and [³H]dopamine. 5-Hydroxytryptamine uptake displayed a K_m of 0.5 μM and a maximal uptake equivalent to that seen with norepineph-rine; dopamine uptake followed complex kinetics. Administration of reserpine in vivo or destruction of sympathetic neurons by long-term guanethidine treatment both eliminated the ability of the preparation to take up norepinephrine. Synaptic vesicles of cardiac sympathetic neurons thus resemble vesicles prepared from other central and peripheral catecholaminergic tissues; this method may be used readily to examine drug effects on rat heart synaptic vesicle function.     

Delta pH-dependent and -independent uptake of [3H]dopamine by synaptic vesicles isolated from rat brain

The dopamine (DA)-translocating mechanism of synaptic vesicles isolated from rat brain has been studied in the presence of an artificially imposed delta pH on the vesicle membranes (acidic inside with respect to the external medium) without the aid of ATP-Mg2+. Under the experimental conditions, [3H]DA uptake by the synaptic vesicles was driven by two different, i.e. delta pH-dependent and -independent processes. Both processes appeared to be carrier-mediated based on the inhibition by NEM (N-ethylmaleimide), an -SH reagent, and by nomifensine, a DA uptake blocker at nerve terminals. The DA carrier of the vesicles was similar to that of the nerve terminal plasma membrane with respect to their susceptibility to nomifensine. The delta pH-dependent uptake was transient and most of the incorporated DA was easily lost from the vesicles. On the other hand, the delta pH-independent uptake increased with time and the amine was retained in the vesicles. The initial rate of the delta pH-independent uptake was lower than that of the delta pH-dependent one but their extents were comparable with each other. These results indicate that rat brain synaptic vesicles have a DA uptake system requiring no ATP hydrolysis. A preparation of synaptic vesicles used here exhibited an inwardly directed proton translocation in the presence of ATP-Mg2+ when monitored by following changes in the fluorescence of ANS (8-anilino-1-naphthalene sulfonate). However, the time course of the delta pH-generation was not influenced by the addition of 1 mM DA.(ABSTRACT TRUNCATED AT 250 WORDS)     

Partial Purification and Characterization of the Vacuolar H+-ATPase of Mammalian Synaptic Vesicles

Several major proteins of synaptic vesicles from rat or cow brain sediment as a large complex on sucrose density gradients when solubilized in nonionic detergents. A vacuolar H(+)-ATPase identified by sensitivity to bafilomycin A1 appears to be associated with this oligomeric protein complex. Two subunits of this complex, synaptic vesicle proteins S and U, correspond to the 57-kDa (B) and 39-kDa accessory (Ac39) subunits, respectively, of bovine chromaffin granule vacuolar H(+)-ATPase as shown by Western immunoblot analysis. The five subunits of the oligomeric complex constitute approximately 20% of the total protein of rat brain synaptic vesicles. Taken together, these results strongly suggest that the abundant, multisubunit complex partially purified from brain synaptic vesicles by density gradient centrifugation is a vacuolar H(+)-ATPase. Bafilomycin A1 completely blocks proton pumping in rat brain synaptic vesicles as measured by [14C]methylamine uptake and also blocks catecholamine accumulation measured by [3H]dopamine uptake. Moreover, ATPase activity, [14C]methylamine uptake, and [3H]dopamine uptake are inhibited by bafilomycin A1 at similar I50 values of approximately 1.7 nmol/mg of protein. These findings indicate that the vacuolar H(+)-ATPase is essential for proton pumping as well as catecholamine uptake by mammalian synaptic vesicles.     

Reserpine sensitivity of catecholamine metabolism in murine neuroblastoma clone N1E-115.

—Neuroblastoma cells of clone NIE-115, originally obtained from the murine tumor C1300, resemble normal noradrenergic neurons in that they have high levels of tyrosine 3-monooxygenase (EC 1.14.16.2; l -tyrosine, tetrahydropteridine: oxygen oxidoreductase (3-hydroxylating)) and dopamine β-monooxygenase (EC 1.14.17.1; 3,4-dihydroxyphenylethylamine, ascorbate: oxygen oxidoreductase (β-hydroxylating)) activities, dense core versicles (100–300 nm in dia), long neurites and excitable membranes. These studies show that reserpine, a blocker of vesicular uptake in noradrenergic neurons, inhibits the accumulation and storage of catecholamines, as well as the conversion of dopamine to NE in neuroblastoma cells. Differentiated monolayer cultures took up [³H]dopamine [10⁻⁴] at a rate of 37 pmol/min per mg protein. Reserpine [5 × 10⁻⁵m ] did not affect the initial rate of uptake, but reduced the extent of uptake at saturation by 60%. Chromatographic examination of cell extracts showed that dopamine was converted to NE in control cultures, but not in reserpine treated cultures. Cells labelled with [³H]dopamine for 60 min and then exposed to release buffer without dopamine for an additional 60 min, retained approximately 40% of the label, 10% as dopamine and 30% as NE. Thirty-five per cent of the radioactivity retained was found, after homogenization and high speed centrifugation, to be associated with a particulate, subcellular fraction. Reserpine, present during release incubations, also reduced the ability of cells to store catecholamines. These results show that N1E-115 cells synthesize and store NE by reactions similar to those in normal noradrenergic neurons.     

p-Chloroamphetamine induces serotonin release through serotonin transporters.

p-Chloroamphetamine (PCA) interacts with serotonin transporters in two membrane vesicle model systems by competing with serotonin for transport and stimulating efflux of accumulated serotonin. In plasma membrane vesicles isolated from human platelets, PCA competes with [3H]imipramine for binding to the serotonin transporter with a KD of 310 nM and competitively inhibits serotonin transport with a KI of 4.8 nM. [3H]Serotonin efflux from plasma membrane vesicles is stimulated by PCA in a Na(+)-dependent and imipramine-sensitive manner characteristic of transporter-mediated exchange. In membrane vesicles isolated from bovine adrenal chromaffin granules, PCA competitively inhibits ATP-dependent [3H]serotonin accumulation with a KI of 1.7 microM and, at higher concentrations, stimulates efflux of accumulated [3H]serotonin. Stimulation of vesicular [3H]serotonin efflux is due in part to dissipation of the transmembrane pH difference (delta pH) generated by ATP hydrolysis. Part of PCA's ability to stimulate efflux may be due to its transport by the vesicular amine transporter. Flow dialysis experiments demonstrated uptake of [3H]PCA into chromaffin granule membrane vesicles in response to the delta pH generated in the presence of Mg2+ and ATP. In plasma membrane vesicles, no accumulation was observed using an NaCl gradient as the driving force. We conclude that rapid nonmediated efflux of transported PCA prevents accumulation unless PCA is trapped inside by a low internal pH.     

Hyperforin inhibits vesicular uptake of monoamines by dissipating pH gradient across synaptic vesicle membrane

Extracts of Hypericum perforatum (St. John's wort) have antidepressant properties in depressed patients and exert antidepressant-like action in laboratory animals. The phloroglucinol derivative hyperforin has become a topic of interest, as this Hypericum component is a potent inhibitor of monoamines reuptake. The molecular mechanism by which hyperforin inhibits monoamines uptake is yet unclear. In the present study we try to clarify the mechanism by which hyperforin inhibits the synaptic vesicle transport of monoamines. The pH gradient across the synaptic vesicle membrane, induced by vacuolar type H(+)-ATPase, is the major driving force for vesicular monoamines uptake and storage. We suggest that hyperforin, like the protonophore FCCP, dissipates an existing Delta pH generated by an efflux of inwardly pumped protons. Proton transport was measured by acridine orange fluorescence quenching. Adding Mg-ATP to a medium containing 130 mM KCl and synaptic vesicles caused an immediate decrease in fluorescence of acridine orange and the addition of 1 microM FCCP abolished this effect. H(+)-ATPase dependent proton pumping was inhibited by hyperforin in a dose dependent manner (IC(50) = 1.9 x 10(-7) M). Hyperforin acted similarly to the protonophore FCCP, abolishing the ATP induced fluorescence quenching (IC(50) = 4.3 x 10(-7) M). Hyperforin and FCCP had similar potencies for inhibiting rat brain synaptosomal uptake of [3H]monoamines as well as vesicular monoamine uptake. The efflux of [3H]5HT from synaptic vesicles was sensitive to both drugs, thus 50% of preloaded [3H]5HT was released in the presence of 2.1 x 10(-7) M FCCP and 4 x 10(-7) M hyperforin. The effect of hyperforin on the pH gradient in synaptic vesicle membrane may explain its inhibitory effect on monoamines uptake, but could only partially explain its antidepressant properties.     

Glucocorticoid-recognizing and -effector sites in rat liver plasma membrane. Kinetics of corticosterone uptake by isolated membrane vesicles--I. Binding and transport.

To gain insight into the mechanisms governing cellular uptake of glucocorticoids, we studied the binding and membrane transport of corticosterone (B) on a highly purified plasma membrane fraction from rat liver that was homogenized using a gentle, isotonic procedure. The fraction was mostly in the form of right-side out and osmotically active vesicles that were free of intracellular glucocorticoid receptors (GCR), transcortin (CBG) and ATP. Our uptake and binding studies carried out at 22 degrees C with [3H]B in physiological concentrations resulted in the following findings: (1) unlabeled B competed with [3H]B for uptake by the membrane vesicles; half-maximal competition of specific uptake was achieved with a 10- to 11-fold molar excess of unlabeled B. (2) [3H]B uptake was a saturable process of unusual kinetics (multiple sigmoidity); modified Scatchard plots revealed three significantly different apparent Kd-values of 1.3, 4.7 and 17.3 nM, corresponding to free B in the blood of non-stressed rats (4-16 nM). (3) Osmotic shrinkage of the vesicles led to a linear decrease in specific uptake, while non-specific uptake was independent of vesicle volume. Passive diffusion of [3H]B took place in leaky, but not in intact, vesicles. Reversible binding to, and mediated transport through, the membrane were interdependent parts of a strongly linked process. B was accumulated inside the vesicle up a concentration gradient by an active transport that followed first-order kinetics (Kt:3.9 nM); for its statistically reliable mathematical formulation and kinetic analysis, a replot was developed that revealed that relative accumulation increased with decreasing external hormone concentration. (4) Comparative binding studies disclosed that the apparent Kd-values (86.5 +/- 7.3 and 77.0 +/- 14.3 nM, respectively) of the [3H]B interactions with CBG and GCR did not differ (P greater than 0.3). These findings permit the conclusion that a plasma membrane-inserted carrier for B, effectively operating at physiological concentrations in the blood, is involved in a functional and regulatory manner in the biological action of glucocorticoids.     

ACTIVE UPTAKE OF [3H]5-HT BY SYNAPTIC VESICLES FROM RAT BRAIN

The question of whether synaptic vesicles accumulate [³H]5-HT by an active process was investigated in a mixed population of vesiclcs from whole rat brain. The temperature dependence and the effect of metabolic inhibitors were studied in synaptosomal suspensions and vesicular fractions. Arrhenius plots for synaptosomes differed from those for vesicles as did the temperature coefficients for these two fractions. For synaptosomes the Q₁₀ was 7 and for vesicles 1.6. However, if ATP was added to the incubation, the temperature dependence of vesicular amine accumulation became manifest; the Arrhenius plot resembled that of synaptosomes and the Q₁₀ was greater than 20 indicating strong temperature dependence. In the presence of ATP, vesicular uptake was stimulated approx 8-fold. Ouabain, dinitrophenol and NEM inhibited synaptosomal uptake but failed to affect [³H]5-HT accumulation by vesicles in the absence of ATP. When ATP was added, vesicular uptake was also blocked by NEM but was unaffected by either ouabain or DNP. Total observed uptake consisted of two components, one ATP-dependent and one nonsaturable and ATP-independent. The active process had a K_m= 1.25 × 10^?7 M and could be completely blocked by either 10^?3 M or 10^?7 M-reserpine. Active vesicular [³H]5-HT uptake was magnesium dependent and was inhibited by sodium and potassium. Cation effects on uptake were specific and could not be accounted for by either changes in osmotic pressure or ionic strength. It was concluded that synaptic vesicles from whole rat brain accumulate [³H]5-HT by an active process.     

Lobeline Displaces [3H]Dihydrotetrabenazine Binding and Releases [3H]Dopamine from Rat Striatal Synaptic Vesicles: Comparison with d-Amphetamine

Abstract: Lobeline, an alkaloid from Indian tobacco (Lobelia inflata), is classified as a nicotinic agonist and is currently used as a smoking cessation agent. However, our previous in vitro studies demonstrate that lobeline does not act as a nicotinic agonist but alters presynaptic dopamine (DA) storage by potently inhibiting DA uptake into synaptic vesicles. Recently, d-amphetamine has been reported to act at the level of the synaptic vesicle to alter presynaptic function. The present in vitro studies further elucidate the mechanism of lobeline's action and compare its effects with those of d-amphetamine. [³H]Dihydrotetrabenazine ([³H]DTBZ), used routinely to probe a high-affinity binding site on the vesicular monoamine transporter (VMAT2), bound to vesicle membranes from rat striatum with a K_D of 1.67 nM and B_max of 8.68 pmol/mg of protein. Lobeline inhibited [³H]DTBZ binding with an IC₅₀ of 0.90 µM, consistent with its previously reported IC₅₀ of 0.88 µM for inhibition of [³H]DA uptake into vesicles. These results suggest that lobeline specifically interacts with DTBZ sites on VMAT2 to inhibit DA uptake into synaptic vesicles. Interestingly, d-amphetamine inhibited [³H]DTBZ binding to vesicle membranes with an IC₅₀ of 39.4 µM, a concentration 20 times greater than reported for inhibition of VMAT2 function, suggesting that d-amphetamine interacts with a different site than lobeline on VMAT2 to inhibit monoamine uptake. Kinetic analysis of [³H]DA release from [³H]DA-preloaded synaptic vesicles in the absence of drug revealed a t_1/2 of 2.12 min. Lobeline and d-amphetamine evoked [³H]DA release with EC₅₀ values of 25.3 and 2.22 µM, respectively. At a concentration 10 times the EC₅₀, lobeline and d-amphetamine significantly decreased the t_1/2 of [³H]DA release to 1.58 and 1.48 min, respectively. Thus, in contrast to d-amphetamine, which is equipotent in inhibiting DA uptake and promoting release from the synaptic vesicles, lobeline more potently (28-fold) inhibits DA uptake (via an interaction with the DTBZ site on VMAT2) than it evokes DA release to redistribute presynaptic DA storage.     

Glutamate uptake by brain synaptic vesicles. Energy dependence of transport and functional reconstitution in proteoliposomes

The dependence of glutamate uptake on ATP-generated proton electrochemical potential was studied in a highly purified preparation of synaptic vesicles from rat brain. At low chloride concentration (4 mM), the proton pump present in synaptic vesicles generated a large membrane potential (inside-positive), associated with only minor acidification. Under these conditions, the rate of L-[3H]glutamate uptake was maximal. In addition, L-glutamate induced acidification of the vesicle interior. D-Glutamate produced only 40% of the effect, and L-aspartate or gamma-aminobutyric acid produced less than 5%. The initial rate of glutamate-induced acidification increased with increasing glutamate concentration. It was saturable and showed first-order kinetics (KM = 0.32 mM). Correspondingly, L-glutamate induced a small reduction in the membrane potential. The rate of ATP hydrolysis was unaffected. In comparison, glutamate had no effect on acidification or membrane potential in resealed membranes of chromaffin granules. At high chloride concentration (150 mM), the vesicular proton pump generated a large pH difference, associated with a small change in membrane potential. Under these conditions, uptake of L-[3H]glutamate by synaptic vesicles was low. For reconstitution, vesicle proteins were solubilized with the detergent sodium cholate, supplemented with brain phospholipids, and incorporated into liposomes. Proton pump and glutamate uptake activities of the proteoliposomes showed properties similar to those of intact vesicles indicating that the carrier was reconstituted in a functionally active form. It is concluded that glutamate uptake by synaptic vesicles is dependent on the membrane potential and that all components required for uptake are integral parts of the vesicle membrane.     

Sulfation and constitutive secretion of dopamine beta-hydroxylase from rat pheochromocytoma (PC12) cells

The biosynthesis and secretion of dopamine beta-hydroxylase were investigated by radiolabeling rat pheochromocytoma (PC12) cells in culture. Intracellular dopamine beta-hydroxylase from a crude chromaffin vesicle fraction and secreted dopamine beta-hydroxylase from culture medium were immunoprecipitated using antiserum made against purified bovine soluble dopamine beta-hydroxylase. Analysis of the immunoprecipitated enzyme on sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that: 1) the membrane-bound form of the hydroxylase from crude secretory vesicle membrane extracts contained two nonidentical subunits in approximately stoichiometric amounts (Mr = 77,000 and 73,000); 2) the soluble hydroxylase from the lysate of these secretory vesicles was composed predominantly of a single subunit (Mr = 73,000); and 3) the hydroxylase secreted into the medium under resting conditions was also composed of a single subunit (approximate Mr = 73,000). All subunits of the multiple forms of hydroxylase were glycoproteins. Under resting conditions, the rate of secretion of hydroxylase was approximately 6% of total cellular enzyme/15 min. The secreted form of the hydroxylase incorporated [35S]sulfate, whereas no significant [35S]sulfate was incorporated into the cellular forms of enzyme. We propose that in addition to the dopamine beta-hydroxylase which is found in catecholamine storage vesicles and released during stimulus-coupled exocytosis, PC12 cells also have a constitutive secretory pathway for dopamine beta-hydroxylase and that the enzyme released by this second pathway is sulfated.     

Dynamic Storage of Dopamine in Rat Brain Synaptic Vesicles In Vitro

Abstract: The dynamics of catecholamine storage were studied in highly purified, small synaptic vesicles from rat brain both during active uptake or after inhibiting uptake with reserpine, tetrabenazine, or removal of external dopamine. To assess turnover during active uptake, synaptic vesicles were allowed to accumulate [³H]dopamine ([³H]DA) for ～10 min and then diluted 20-fold into a solution containing unlabeled DA under conditions such that active uptake could continue. After dilution, [³H]DA was lost with single exponential kinetics at a half-time of ～4 min at 30°C in 8 mM Cl^? medium, in which both voltage and H⁺ gradients are present in the vesicles. In 90 mM Cl^? medium, in which high H⁺ and Cl^? gradients but no voltage gradient are present, [³H]DA escaped at a half-time of ～7 min. In both high and low Cl^? media, ～40% of [³H]DA efflux was blocked by reserpine or tetrabenazine. The residual efflux also followed first-order kinetics. These results indicate that two efflux pathways were present, one dependent on DA uptake (and thus on the presence of external DA) and the other independent of uptake, and that both pathways function regardless of the type of electrochemical H⁺ gradient in the vesicles. The presence of both uptake-dependent and -independent efflux was observed in experiments using DA-free medium, instead of uptake inhibitors, to prevent uptake. Uptake-independent efflux showed molecular selectivity for catecholamines; [¹⁴C]DA was lost about three times faster than [³H]norepinephrine after adding tetrabenazine directly (without dilution) to vesicles that had taken up comparable amounts of each amine. In addition, the first-order rate constant for uptake-independent efflux showed little change over a 60-fold range of internal DA concentrations, which suggests that this pathway had a high transport capacity. All efflux was blocked at 0°C, suggesting that efflux did not occur through a large pore. There was little or no change in the proton gradient in synaptic vesicles, monitored by [¹⁴C]methylamine equilibration, during the experimental manipulations used here. Thus, the driving force for catecholamine uptake remained approximately constant. The physiological role of uptake-independent efflux could be to allow the monoamine content of synaptic vesicles to be regulated over a time range of minutes and, thereby, control the amount released by exocytosis. These results imply that catecholamines turn over with a half-time of minutes during active uptake by brain synaptic vesicles in vitro.     

Synaptic Vesicle-bound Pyruvate Kinase can Support Vesicular Glutamate Uptake

Glucose metabolism is essential for normal brain function and plays a vital role in synaptic transmission. Recent evidence suggests that ATP synthesized locally by glycolysis, particularly via glyceraldehyde 3-phosphate dehydrogenase/3-phosphoglycerate kinase, is critical for synaptic transmission. We present evidence that ATP generated by synaptic vesicle-associated pyruvate kinase is harnessed to transport glutamate into synaptic vesicles. Isolated synaptic vesicles incorporated [³H]glutamate in the presence of phosphoenolpyruvate (PEP) and ADP. Pyruvate kinase activators and inhibitors stimulated and reduced PEP/ADP-dependent glutamate uptake, respectively. Membrane potential was also formed in the presence of pyruvate kinase activators. “ATP-trapping” experiments using hexokinase and glucose suggest that ATP produced by vesicle-associated pyruvate kinase is more readily used than exogenously added ATP. Other neurotransmitters such as GABA, dopamine, and serotonin were also taken up into crude synaptic vesicles in a PEP/ADP-dependent manner. The possibility that ATP locally generated by glycolysis supports vesicular accumulation of neurotransmitters is discussed. Atsuhiko Ishida—On leave from the Department of Biochemistry, Asahikawa Medical College, Asahikawa, Japan.     

Differences in L-[3H]Glutamate Accumulation and Endogenous L-Glutamate Content in Synaptic Vesicles from Mice Selectively Bred for Differences in Ethanol Sensitivity

The ATP-stimulated accumulation of L-[3H]glutamate by whole brain synaptic vesicle preparations from long-sleep and short-sleep mice, lines selectively bred for difference in sleep time response to acute ethanol administration, was examined. L-[3H]Glutamate accumulation in vesicles from short-sleep mice was approximately twice that observed in vesicles from long-sleep mice at three glutamate loading concentrations. The vesicular content of endogenous L-glutamate in crude and enriched vesicle preparations from short-sleep mice was approximately 1.5-fold higher than in vesicles from long-sleep mice. The data suggest that L-glutamate associated with synaptic vesicles may serve a role in glutamate neurosecretion.     

Analysis of VMAT2 binding after methamphetamine or MPTP treatment: disparity between homogenates and vesicle preparations

[3H]Dihydrotetrabenazine ([3H]DTBZ), a specific ligand for the vesicular monoamine transporter (VMAT2), has been used to characterize the integrity of monoaminergic nerve terminals in experimental animals and humans. The purpose of the present studies was to compare the loss of VMAT2 binding with the loss of other neurochemical markers of the dopamine (DA) nerve terminals in mice treated with neurotoxic doses of methamphetamine (METH) or MPTP. Profound decreases (> or =70%) in DA content, tyrosine hydroxylase activity, and PH]carbomethoxy-3-(4-fluorophenyl)tropane binding to the DA transporter were observed in striatal homogenates at both 1 and 6 days after exposure to the neurotoxins. It is surprising that no significant loss of [3H]DTBZ binding in the homogenates was observed at 1 day after exposure, although a significant loss (-50%) was apparent 6 days later. However, in isolated vesicle preparations, [3H]DTBZ binding and active [3H]DA uptake were markedly reduced (>70%) at 1 day. These observations indicate that vesicle function is compromised at an early time point after exposure to neurotoxic insult. Furthermore, the changes in [H]DTBZ binding in homogenates may not be a sensitive indicator of early damage to synaptic vesicles, although homogenate binding reliably identifies a loss of VMAT2 at later times.     

Isoosmotic isolation of rat brain synaptic vesicles, some of which contain tyrosine hydroxylase

Rat brain synaptic vesicles were isoosmotically isolated and examined for Mg(2+)-ATPase [EC 3.6.1.3.] and tyrosine hydroxylase [EC 1.14.16.2.] associated with the synaptic vesicles. Synaptosomes in 0.32 M sucrose were disrupted by freezing and thawing treatment, and the cytosol fraction was fractionated on a Sephacryl S-500 column with a mean exclusion size of 200 nm. Peak I at the void volume was a mixture of large vesicular membranes, small amounts of synaptic vesicles and coated vesicles, etc. Peak II consisted of non- and granulated synaptic vesicles of 35-40 nm diameter, and peak III of soluble proteins. The synaptic vesicles in peak II reacted with antibodies against the H(+)-ATPase A-subunit, vesicular acetylcholine transporter, and vesicular monoamine transporter. However, they showed little Mg(2+)-ATPase activity. Tyrosine hydroxylase was observed in either peak II or III on blotting with an anti-tyrosine hydroxylase antibody. These results imply that tyrosine hydroxylase exists in soluble and bound forms to synaptic vesicles in nerve terminals.     

Styrene and styrene oxide affect the transport of dopamine in purified rat striatal synaptic vesicles

Animal and human studies suggest a dopamine-mediated effect of styrene neurotoxicity. To date, mechanisms of cerebral membrane transport of neurotransmitter amines in the presence of styrene in relation to its neurotoxicity have not been addressed properly. So, the present study has examined to test the hypothesis that dopaminergic malfunction in vesicular transport is a critical component in styrene-induced neurotoxicity in rats. Both styrene and its intermediate reactive metabolite, styrene oxide antagonized the in vitro striatal binding of [3H] tyramine, a putative marker of the vesicular transporter for dopamine. Both styrene and styrene oxide potently inhibited the uptake of [3H] dopamine in purified synaptic vesicles prepared from rat brain striata, in a dose-related manner, with inhibitory constants (Ki) 2.5 and 2.2 microM respectively. However, neither styrene nor styrene oxide significantly increased the basal efflux of [3H] dopamine that has been preloaded into striatal vesicles in vitro. On the other hand, both styrene and styrene oxide have failed to significantly inhibit the uptake of either [3H] norepinephrine, or [3H] serotonin into striatal synaptic vesicles. It is concluded that both styrene and styrene oxide are capable of producing impairments in dopaminergic transport in purified striatal synaptic vesicles, an effect which may be a critical component in styrene-induced neurotoxicity.     

Ouabain binding and coupled sodium, potassium, and chloride transport in isolated transverse tubules of skeletal muscle

The affinity and number of binding sites of [3H]ouabain to isolated transverse (T) tubules were determined in the absence and presence of deoxycholate. In both conditions the KD was approximately 53 nM while deoxycholate increased the number of binding sites from 3.5 to 37 pmol/mg protein. We concluded that the ouabain binding sites were located primarily on the inside of the isolated vesicle and that the vesicles were impermeable to ouabain. ATP induced a highly active Na+ accumulation by the T tubules which increased Na+ in the T tubular lumen by almost 200 nmol/mg protein. The accumulation had an initial fast phase lasting 2-3 min and a subsequent slow phase which continued for at least 40 min. The rate of the initial fast phase indicated a turnover number of 20 Na+/s. The Na+ accumulation was prevented by monensin but was unaffected by valinomycin. Ouabain did not influence Na+ uptake, but digitoxin inhibited it. At low K+ the accumulation of Na+ was reduced 3.7-fold below the value at 50 mM K+. 86Rb, employed as a tracer to detect K+, showed a first phase of K+ release while Na+ was accumulated. After 2-3 min, K+ was reaccumulated while Na+ continued to increase in the lumen. T tubules accumulated Cl- on addition of ATP. This suggested that ATP initiated an exchange of Na+ for K+ followed by uptake of Na+ and K+ accompanied by Cl-.