Expression of a putative vesicular acetylcholine transporter facilitates quantal transmitter packaging

Effects of reduced vesicular filling on synaptic transmission in rat hippocampal neurones

The consequence of reduced uptake of neurotransmitters into synaptic vesicles on synaptic transmission was examined in rat hippocampal slices and culture using bafilomycin A1 (Baf), a potent and specific blocker of the vacuolar-type (V-type) ATPase, which eliminates the driving force for the uptake of both glutamate and GABA into synaptic vesicles. After incubation with Baf, both the amplitude and frequency of GABAergic miniature inhibitory postsynaptic currents (mIPSCs) were reduced in the slice preparation. Similar effects were seen with glutamatergic miniature excitatory postsynaptic currents (mEPSCs) and GABAergic mIPSCs from cultured neurons. This result indicates that vesicular content is reduced by Baf. The dramatic reduction in the frequency of mPSCs could result either from the exocytosis of empty vesicles or from a mechanism which prevents the exocytosis of depleted vesicles. Vesicle cycling was directly examined using confocal imaging with FM 1-43. In the presence of Baf, vesicles could still be endocytosed and they were released at the same probability as from control untreated synapses. Prolonged high-frequency electrical stimulation of synapses in culture failed to alter the amplitude of mEPSCs, suggesting that the filling of vesicles is rapid compared to the rate of vesicle recycling during repetitive synaptic stimulation. Profound release of glutamate with alpha-latrotoxin did cause a small, but reproducible, reduction in quantal size. These results indicate that decreasing the amount of glutamate and GABA in synaptic vesicles reduces quantal size. Furthermore, the probability of vesicle exocytosis appears to be entirely independent of the state of filling of the vesicle. However, even during high-frequency action potential-evoked release of glutamate, quantal size remained unchanged.

Functional implications of the noradrenergic-cholinergic switch induced by retinoic acid in NB69 neuroblastoma cells

Some neuroblastoma cell lines change their neurotransmitter phenotype from noradrenergic to cholinergic under retinoic acid treatment. Such "neurotransmitter switch" seems to be a consequence of changes in the expression and activity of the biosynthetic machinery for both neurotransmitters. In this study, we have characterized this "neurotransmitter switch" induced by retinoic acid in a human neuroblastoma cell line (NB69) showing catecholaminergic characteristics. Retinoic acid treatment reduced tyrosine hydroxylase activity and noradrenaline levels in NB69 cells but did not modify the expression of this enzyme. Moreover, the calcium-dependent release of [(3)H]noradrenaline in control cells was highly reduced by retinoic acid treatment. On the other hand, NB69 cells treated with retinoic acid enhanced the expression of choline acetyltransferase and acquired the capability to release [(3)H]acetylcholine in a calcium-dependent way. In addition, we found that the expression of the vesicular monoamine transporter 2 (VMAT2) and the vesicular acetylcholine transporter (VAChT) was increased in those cells treated with retinoic acid. Immunostaining revealed that retinoic acid treatment changed the cellular distribution of both vesicular monoamine transporter 2 and vesicular acetylcholine transporter. In conclusion, retinoic acid induces a noradrenergic to cholinergic switch in NB69 cells by acting at several levels of the neurotransmitter phenotypic expression.

A phosphorylation site regulates sorting of the vesicular acetylcholine transporter to dense core vesicles

Vesicular transport proteins package classical neurotransmitters for regulated exocytotic release, and localize to at least two distinct types of secretory vesicles. In PC12 cells, the vesicular acetylcholine transporter (VAChT) localizes preferentially to synaptic-like microvesicles (SLMVs), whereas the closely related vesicular monoamine transporters (VMATs) localize preferentially to large dense core vesicles (LDCVs). VAChT and the VMATs contain COOH-terminal, cytoplasmic dileucine motifs required for internalization from the plasma membrane. We now show that VAChT undergoes regulated phosphorylation by protein kinase C on a serine (Ser-480) five residues upstream of the dileucine motif. Replacement of Ser-480 by glutamate, to mimic the phosphorylation event, increases the localization of VAChT to LDCVs. Conversely, the VMATs contain two glutamates upstream of their dileucine-like motif, and replacement of these residues by alanine conversely reduces sorting to LDCVs. The results provide some of the first information about sequences involved in sorting to LDCVs. Since the location of the transporters determines which vesicles store classical neurotransmitters, a change in VAChT trafficking due to phosphorylation may also influence the mode of transmitter release.

Regulation of quantal size by presynaptic mechanisms

Quantal size is often modeled as invariant, although it is now well established that the number of transmitter molecules released per synaptic vesicle during exocytosis can be modulated in central and peripheral synapses. In this review, we suggest why presynaptically altered quantal size would be important at social synapses that provide extrasynaptic neurotransmitter. Current techniques used to measure quantal size are reviewed with particular attention to amperometry, the first approach to provide direct measurement of the number of molecules and kinetics of presynaptic quantal release, and to CNS dopamine neuronal terminals. The known interventions that alter quantal size at the presynaptic locus are reviewed and categorized as (1) alteration of transvesicular free energy gradients, (2) modulation of vesicle transmitter transporter activity, (3) modulation of fusion pore kinetics, (4) altered transmitter degranulation, and (5) changes in synaptic vesicle volume. Modulation of the number of molecules released per quantum underlies mechanisms of drug action of L-DOPA and the amphetamines, and seems likely to be involved in both normal synaptic modification and disease states. Statistical analysis for examining quantal size and data presentation is discussed. We include detailed information on performing nonparametric resampling statistical analysis, the Kolmogorov-Smirnov test for two populations, and random walk simulations using spreadsheet programs.

The neuronal monoamine transporter VMAT2 is regulated by the trimeric GTPase Go(2)

Monoamines such as noradrenaline and serotonin are stored in secretory vesicles and released by exocytosis. Two related monoamine transporters, VMAT1 and VMAT2, mediate vesicular transmitter uptake. Previously we have reported that in the rat pheochromocytoma cell line PC 12 VMAT1, localized to peptide-containing secretory granules, is controlled by the heterotrimeric G-protein Go(2). We now show that in BON cells, a human serotonergic neuroendocrine cell line derived from a pancreatic tumor expressing both transporters on large, dense-core vesicles, VMAT2 is even more sensitive to G-protein regulation than VMAT1. The activity of both transporters is only downregulated by Galphao(2), whereas comparable concentrations of Galphao(1) are without effect. In serotonergic raphe neurons in primary culture VMAT2 is also downregulated by pertussis toxin-sensitive Go(2). By electron microscopic analysis from prefrontal cortex we show that VMAT2 and Galphao(2) associate preferentially to locally recycling small synaptic vesicles in serotonergic terminals. In addition, Go(2)-dependent modulation of VMAT2 also works when using a crude synaptic vesicle preparation from this brain area. We conclude that regulation of monoamine uptake by the heterotrimeric G proteins is a general feature of monoaminergic neurons that controls the content of both large, dense-core and small synaptic vesicles.

Modification of cysteines reveals linkage to acetylcholine and vesamicol binding sites in the vesicular acetylcholine transporter of Torpedo californica

Properties of cysteinyl residues in the vesicular acetylcholine transporter (VAChT) of synaptic vesicles isolated from Torpedo californica were probed. Cysteine-specific reagents of different size and polarity were used and the effects on [3H]vesamicol binding determined. The vesamicol dissociation constant increased 1,000-fold after reaction with p-chloromercuriphenylsulfonate or phenylmercury acetate, but only severalfold after reaction with relatively small methylmercury chloride or methylmethanethiosulfonate (MMTS). Methylmercury chloride, but not MMTS, protected binding from phenylmercury acetate. Thus, two classes of cysteines react to affect vesamicol binding. Class 1 reacts with only organomercurials, and class 2 reacts with both organomercurials and MMTS. Quantitative analysis of the competition between p-chloromercuriphenylsulfonate and VAChT ligands was possible after defining second-order reaction conditions. The results indicate that each cysteinyl class probably contains a single residue. Acetylcholine protects cysteine 1, but apparently does not protect cysteine 2. Vesamicol, which binds to a different site than acetylcholine does, apparently protects both cysteines, suggesting that it induces a conformational change. The relatively large reagent glutathione removes a substituent from cysteine 1, but not cysteine 2, suggesting that cysteine 2 is deeper in the transporter than cysteine 1 is. The complete sequence of T. californica VAChT is given, and possible identities of cysteines 1 and 2 are discussed.

Targeted neutralization of calcineurin, by expression of an inhibitor peptide under the control of a cholinergic specific promoter in PC12 cells, promotes neurite outgrowth in the presence of NGF

We have characterized a region of the mouse vesicular acetylcholine transporter(VAChT)/choline acetyltransferase (ChAT) gene locus that serves as a cholinergic-specific promoter for the expression of both VAChT and ChAT genes, as well as a reporter gene (LacZ) in vivo. We have used this promoter to direct the expression of an inhibitor peptide, derived from the calcineurin (CalN) autoregulatory domain, to directly neutralize the function of CalN to define the role of this Ca2+/Calmodulin regulated phosphatase in neurite outgrowth. Targeted inhibition of CalN promotes neurite outgrowth in PC12 cells in the presence of NGF, as early as 24 h after transfection. Inhibition of CalN-mediated enhancement of neurite outgrowth in PC12 cells reaches a maximum effect within the first 4 to 6 days after transfection, and does not cause adverse effects when highly expressed for up to 12 days. Cyclosporin A, a nontargeted CalN inhibitor, increases the number of neurites in mock transfected cells by 1.5 fold, while in transfected PC12 cells, the expression of the CalN inhibitor peptide increases the neurite number by 1.8 fold. These data demonstrate that CalN is an important regulator of the neurotrophic response in cholinergic cells and may prove valuable in developing treatment strategies to promote recovery from neurological injury.

Reconstitution of mediatophore-supported quantal acetylcholine release

Synaptic transmission of a nerve impulse is an extremely rapid event relying on transfer of brief chemical impulses from one cell to another. This transmission is dependent upon Ca2+ and known to be quantal, which led to the widely accepted vesicular hypothesis of neurotransmitter release. However, at least in the case of rapid synaptic transmission the hypothesis has been found difficult to reconcile with a number of observations. In this article, we shall review data from experiments dealing with reconstitution of quantal and Ca2+-dependent acetylcholine release in: i) proteoliposomes, ii) Xenopus oocytes, and iii) release-deficient cell lines. In these three experimental models, release is dependent on the expression of the mediatophore, a protein isolated from the plasma membrane of cholinergic nerve terminals of the Torpedo electric organ. We shall discuss the role of mediatophore in quantal acetylcholine release, its possible involvement in morphological changes affecting presynaptic membrane during the release, and its interactions with others proteins of the cholinergic nerve terminal.

Recycling and refilling of transmitter quanta at the frog neuromuscular junction

1. Fluorescent dyes have been used at the frog neuromuscular junction to label synaptic vesicular membrane. Retrieved membrane is reformed into vesicles, which are released along with pre-existing vesicles. Consequently, if vesicular refilling with acetylcholine (ACh) is depressed by inhibitors, two sizes of quanta should be released: normal and smaller. As recycling continues the fraction of smaller size quanta should increase exponentially. 2. We enhanced the rate of quantal release by elevating the K+ concentration. The principal inhibitors were (-)-vesamicol (VES), hemicholinium-3 (HC3), and NH4+. Quantal size measurements were fitted to one and to two cumulative lognormal probability distribution functions. When two fitted better, the statistical significance assessment took into account the three additional parameters used in calculating the fit. 3. After recycling in the presence of inhibitor, many sets were fitted better by two lognormal functions. As recycling continued, the fraction of the miniature endplate potential voltage-time integrals ( MEPPs) in the larger sub-population decreased exponentially. 4. The size of the releasable pool was estimated by counting the quanta released by carbonyl cyanide m-chlorophenylhydrazone (CCCP). This was compared to pool sizes calculated from the inhibitor experiments. The two estimates of pool size were indistinguishable, with mean values ranging from about 170,000 to 270,000. 5. With all of the treatments tested, the means of the sizes in the smaller sub-population of MEPPs were about 1/3 those of the larger sub-populations. 6. Recycling synaptic vesicles appear to be incorporated into the releasable pool from which they have roughly the same probability of release as the pre-existing vesicles.

Differential quantal release of histamine and 5-hydroxytryptamine from mast cells of vesicular monoamine transporter 2 knockout mice

The recent availability of mice lacking the neuronal form of the vesicular monoamine transporter 2 (VMAT2) affords the opportunity to study its roles in storage and release. Carbon fiber microelectrodes were used to measure individual secretory events of histamine and 5-hydroxytryptamine (5-HT) from VMAT2-expressing mast cells as a model system for quantal release. VMAT2 is indispensable for monoamine storage because mast cells from homozygous (VMAT2(-/-)) mice, while undergoing granule-cell fusion, do not release monoamines. Cells from heterozygous animals (VMAT2(+/-)) secrete lower amounts of monoamine per granule than cells from wild-type controls. Investigation of corelease of histamine and 5-HT from granules in VMAT2(+/-) cells revealed 5-HT quantal size was reduced more than that of histamine. Thus, although vesicular transport is the limiting factor determining quantal size of 5-HT and histamine release, intragranular association with the heparin matrix also plays a significant role.

Synaptic vesicle biogenesis

Synaptic vesicles, which have been a paradigm for the fusion of a vesicle with its target membrane, also serve as a model for understanding the formation of a vesicle from its donor membrane. Synaptic vesicles, which are formed and recycled at the periphery of the neuron, contain a highly restricted set of neuronal proteins. Insight into the trafficking of synaptic vesicle proteins has come from studying not only neurons but also neuroendocrine cells, which form synaptic-like microvesicles (SLMVs). Formation and recycling of synaptic vesicles/SLMVs takes place from the early endosome and the plasma membrane. The cytoplasmic machinery of synaptic vesicle/SLMV formation and recycling has been studied by a variety of experimental approaches, in particular using cell-free systems. This has revealed distinct machineries for membrane budding and fission. Budding is mediated by clathrin and clathrin adaptors, whereas fission is mediated by dynamin and its interacting protein SH3p4, a lysophosphatidic acid acyl transferase.

The role of the synthetic enzyme GAD65 in the control of neuronal gamma-aminobutyric acid release

We have studied GABAergic synaptic transmission in retinal ganglion cells and hippocampal pyramidal cells to determine, at a cellular level, what is the effect of the targeted disruption of the gene encoding the synthetic enzyme GAD65 on the synaptic release of gamma-aminobutyric acid (GABA). Neither the size nor the frequency of GABA-mediated spontaneous inhibitory postsynaptic currents (IPSCs) were reduced in retina or hippocampus in GAD65-/- mice. However, the release of GABA during sustained synaptic activation was substantially reduced. In the retina both electrical- and K(+)-induced increases in IPSC frequency were depressed without a change in IPSC amplitude. In the hippocampus the transient increase in the probability of inhibitory transmitter release associated with posttetanic potentiation was absent in the GAD65-/- mice. These results indicate that during and immediately after sustained stimulation the increase in the probability of transmitter release is not maintained in GAD65-/- mice. Such a finding suggests a decrease in the size or refilling kinetics of the releasable pool of vesicles, and various mechanisms are discussed that could account for such a defect.

Regulation of transmitter release from retinal bipolar cells

Mb1 bipolar cells (ON-type cells) of the goldfish retina have exceptionally large (approximately 10 microns in diameter) presynaptic terminals, and thus, are suitable for investigating presynaptic mechanisms for transmitter release. Using enzymatically dissociated Mb1 bipolar cells under whole-cell voltage clamp, we measured the Ca2+ current (ICa), the intracellular free Ca2+ concentration ([Ca2+]i), and membrane capacitance changes associated with exocytosis and endocytosis. Release of transmitter (glutamate) was monitored electrophysiologically by a glutamate receptor-rich neuron as a probe. L-type Ca2+ channels were localized at the presynaptic terminals. The presynaptic [Ca2+]i was strongly regulated by cytoplasmic Ca2+ buffers, the Na(+)-Ca2+ exchanger and the Ca2+ pump in the plasma membrane. Once ICa was activated, a steep Ca2+ gradient was created around Ca2+ channels; [Ca2+]i increased to approximately 100 microM at the fusion sites of synaptic vesicles whereas up to approximately 1 microM at the cytoplasm. The short delay (approximately 1 ms) of exocytosis and the lack of prominent asynchronous release after the termination of ICa suggested a low-affinity Ca2+ fusion sensor for exocytosis. Depending on the rate of Ca2+ influx, glutamate was released in a rapid phasic mode as well as a tonic mode. Multiple pools of synaptic vesicles as well as vesicle cycling seemed to support continuous glutamate release. Activation of protein kinase C increased the size of synaptic vesicle pool, resulting in the potentiation of glutamate release. Goldfish Mb1 bipolar cells may still be an important model system for understanding the molecular mechanisms of transmitter release.

Membrane trafficking of neurotransmitter transporters in the regulation of synaptic transmission

Many psychoactive drugs influence the transport of neurotransmitters across biological membranes, suggesting that the physiological regulation of neurotransmitter transport might contribute to normal and perhaps abnormal behaviour. Over the past few years, molecular characterization of the neurotransmitter transporters has enabled investigation of their subcellular location and regulation. The analysis of location suggests that membrane trafficking has an important role in the normal function of these proteins. One of the major regulatory mechanisms also involves changes in localization that might contribute to synaptic plasticity. This article discusses recent work on the membrane trafficking of neurotransmitter transporters and its role in regulating their activity.

Arginine analogs modify signal detection by neurons in the visual cortex

Nitric oxide (NO) modulates neurotransmitter release, induction of long-term synaptic potentiation and depression, and activity levels of neurons. However, it is not known whether NO contributes to the ability of the CNS to distinguish sensory signals from background noise and/or extract sensory information with greater reliability. We addressed these questions in the visual cortex, in vivo, using electrophysiological recording and analysis of signal detection from individual neurons. This was combined with microiontophoretic application of arginine analogs that either upregulate or downregulate the brain's endogenous NO-generating pathways or compounds that produce exogenous NO. Protocols that enhance NO levels generally increased the number of action potentials per trial evoked by visual stimuli, improved signal detection, and decreased the coefficient of variation of visually evoked responses, whereas NO-reducing protocols predominantly had complementary effects. Control experiments demonstrate that these effects are likely attributable to the specific ability of these arginine compounds to modify NO levels versus other nonspecific effects. Differential effects between neighboring cells and between single-cell receptive subfields suggest that these actions have a significant direct neural component versus exclusively operating indirectly on neurons through the central vascular actions of NO.

Antibodies against cysteine string proteins inhibit evoked neurotransmitter release at Xenopus neuromuscular junctions

Cysteine string proteins (CSPs) are evolutionarily conserved proteins that are associated with synaptic vesicles and other regulated secretory organelles. To investigate the role of CSPs in vertebrate neuromuscular transmission, we introduced anti-CSP antibodies into the cell bodies of Xenopus spinal motor neurons that form synapses with embryonic muscle cells in culture. These antibodies produced a rapid (within 3-6 min), and in most cases complete, inhibition of stimulus-dependent neurotransmitter secretion. However, spontaneous neurotransmitter release was stable (both in frequency and amplitude) throughout the period of antibody exposure. Several control experiments validated the specificity of the anti-CSP antibody effects. First, the anti-CSP antibody actions were not mimicked either by antibodies against another synaptic vesicle protein SV2, or by nonspecific immunoglobins. Second, heat treatment of the anti-CSP antibodies eliminated their effect on evoked secretion. Third, immunoblot experiments showed that the anti-CSP and anti-SV2 antibodies were highly selective for their respective antigens in these Xenopus cultures. We conclude from these results that CSPs are vital constituents of the pathway for regulated neurotransmitter release in vertebrates. Moreover, the selective inhibition of evoked, but not spontaneous transmitter release by anti-CSP antibodies indicates that there is a fundamental difference in the machinery that mediates these secretory processes.

Phospholipase C-gamma and phosphoinositide 3-kinase mediate cytoplasmic signaling in nerve growth cone guidance

Expression of rat TrkA in Xenopus spinal neurons confers responsiveness of these neurons to nerve growth factor (NGF) in assays of neuronal survival and growth cone chemotropism. Mutational analysis indicates that coactivation of phospholipase C-gamma (PLC-gamma) and phosphoinositide 3-kinase (PI3-kinase) by specific cytoplasmic domains of TrkA is essential for triggering chemoattraction of the growth cone in an NGF gradient. Uniform exposure of TrkA-expressing neurons to NGF resulted in a cross-desensitization of turning responses induced by a gradient of netrin-1, brain-derived neurotrophic factor (BDNF), or myelin-associated glycoprotein (MAG) but not by a gradient of collapsin-1/semaphorin III/D or neurotrophin-3 (NT-3). These results, together with the effects of pharmacological inhibitors, support the notion that there are common cytosolic signaling pathways for two separate groups of guidance cues, one of which requires coactivation of PLC-gamma and PI3-kinase pathways.

Acetylcholine synthesis and quantal release reconstituted by transfection of mediatophore and choline acetyltranferase cDNAs

Neuroblastoma N18TG-2 cells cannot synthesize or release acetylcholine (ACh), and do not express proteins involved in transmitter storage and vesicle fusion. We restored some of these functions by transfecting N18TG-2 cells with cDNAs of either rat choline acetyltransferase (ChAT), or Torpedo mediatophore 16-kDa subunit, or both. Cells transfected only with ChAT synthesized but did not release ACh. Cells transfected only with mediatophore expressed Ca2+-dependent ACh release provided they were previously filled with the transmitter. Cell lines produced after cotransfection of ChAT and mediatophore cDNAs released the ACh that was endogenously synthesized. Synaptic-like vesicles were found neither in native N18TG-2 cells nor in ChAT-mediatophore cotransfected clones, where all the ACh content was apparently cytosolic. Furthermore, restoration of release did not result from enhanced ACh accumulation in intracellular organelles consecutive to enhanced acidification by V-ATPase, as Torpedo 16 kDa transfection did not increase, but decreased the V-ATPase-driven proton transport. Using ACh-sensitive Xenopus myocytes for real-time recording of evoked release, we found that cotransfected cells released ACh in a quantal manner. We compared the quanta produced by ChAT-mediatophore cotransfected clones to those produced by clones transfected with mediatophore alone (artificially filled with ACh). The time characteristics and quantal size of currents generated in the myocyte were the same in both conditions. However, cotransfected cells released a larger proportion of their initial ACh store. Hence, expression of mediatophore at the plasma membrane seems to be necessary for quantal ACh release; the process works more efficiently when ChAT is operating as well, suggesting a functional coupling between ACh synthesis and release.

Genetics of synaptic vesicle function: toward the complete functional anatomy of an organelle

Synaptic transmission starts with the release of neurotransmitters by exocytosis of synaptic vesicles. As a relatively simple organelle with a limited number of components, synaptic vesicles are in principle accessible to complete structural and functional genetic analysis. At present, the majority of synaptic vesicle proteins has been characterized, and many have been genetically analyzed in mice, Drosophila, and Caenorhabditis elegans. These studies have shown that synaptic vesicles contain proteins with diverse structures and functions. Although the genetic studies are as yet unfinished, they promise to lead to a full description of synaptic vesicles as macromolecular machines involved in all aspects of presynaptic neurotransmitter release.

Presynaptic depolarization facilitates neurotrophin-induced synaptic potentiation

Neurotrophins have been proposed to participate in activity-dependent modifications of neuronal connectivity and synaptic efficacy. Preferential strengthening of active inputs requires restriction of putative neurotrophin-mediated synaptic potentiation to active synapses. Here we report that potentiation of synaptic efficacy by brain-derived neurotrophic factor (BDNF) is greatly facilitated by presynaptic depolarization at developing neuromuscular synapses. Brief depolarization in the presence of low-level BDNF results in a marked potentiation of both evoked and spontaneous synaptic transmission, whereas exposure to either BDNF or depolarization alone is without effect. This potentiation depends on the relative timing of depolarization and reflects an enhancement of transmitter secretion from the presynaptic neuron. Thus synapses made by active inputs may be selectively strengthened by secreted neurotrophins as part of activity-dependent refinement of developing connections or of mature synapses.

Elimination of zinc from synaptic vesicles in the intact mouse brain by disruption of the ZnT3 gene

The mammalian protein ZnT3 resides on synaptic vesicle membranes of zinc-containing neurons, suggesting its possible role in vesicular zinc transport. We show here that histochemically reactive zinc, corresponding to the zinc found within synaptic vesicles, was undetectable in the brains of mice with targeted disruption of the ZnT3 gene. Total zinc levels in the hippocampus and cortex of these mice were reduced by about 20%. The ultrastructure of mossy fiber boutons, which normally store the highest levels of vesicular zinc, was unaffected. Mice with one normal ZnT3 allele had reduced levels of ZnT3 protein on synaptic vesicle membranes and had intermediate amounts of vesicular zinc. These results demonstrate that ZnT3 is required for transport of zinc into synaptic vesicles and suggest that vesicular zinc concentration is determined by the abundance of ZnT3.

Neurotransmitter secretion along growing nerve processes: comparison with synaptic vesicle exocytosis

In mature neurons, synaptic vesicles continuously recycle within the presynaptic nerve terminal. In developing axons which are free of contact with a postsynaptic target, constitutive membrane recycling is not localized to the nerve terminal; instead, plasma membrane components undergo cycles of exoendocytosis throughout the whole axonal surface (Matteoli et al., 1992; Kraszewski et al., 1995). Moreover, in growing Xenopus spinal cord neurons in culture, acetylcholine (ACh) is spontaneously secreted in the quantal fashion along the axonal shaft (Evers et al., 1989; Antonov et al., 1998). Here we demonstrate that in Xenopus neurons ACh secretion is mediated by vesicles which recycle locally within the axon. Similar to neurotransmitter release at the presynaptic nerve terminal, ACh secretion along the axon could be elicited by the action potential or by hypertonic solutions. We found that the parameters of neurotransmitter secretion at the nerve terminal and at the middle axon were strikingly similar. These results lead us to conclude that, as in the case of the presynaptic nerve terminal, synaptic vesicles involved in neurotransmitter release along the axon contain a complement of proteins for vesicle docking and Ca2+-dependent fusion. Taken together, our results support the idea that, in developing axons, the rudimentary machinery for quantal neurotransmitter secretion is distributed throughout the whole axonal surface. Maturation of this machinery in the process of synaptic development would improve the fidelity of synaptic transmission during high-frequency stimulation of the presynaptic cell.

Variability of neurotransmitter concentration and nonsaturation of postsynaptic AMPA receptors at synapses in hippocampal cultures and slices

To understand the elementary unit of synaptic communication between CNS neurons, one must know what causes the variability of quantal postsynaptic currents and whether unitary packets of transmitter saturate postsynaptic receptors. We studied single excitatory synapses between hippocampal neurons in culture. Focal glutamate application at individual postsynaptic sites evoked currents (I(glu)) with little variability compared with quantal excitatory postsynaptic currents (EPSCs). The maximal I(glu) was >2-fold larger than the median EPSC. Thus, variations in [glu]cleft are the main source of variability in EPSC size, and glutamate receptors are generally far from saturation during quantal transmission. This conclusion was verified by molecular antagonism experiments in hippocampal cultures and slices. The general lack of glutamate receptor saturation leaves room for increases in [glu]cleft as a mechanism for synaptic plasticity.

Mutational analysis of aspartate residues in the transmembrane regions and cytoplasmic loops of rat vesicular acetylcholine transporter

The vesicular acetylcholine transporter (VAChT) is responsible for the transport of the neurotransmitter acetylcholine (ACh) into synaptic vesicles using an electrochemical gradient to drive transport. Rat VAChT has a number of aspartate residues within its predicted transmembrane domains (TM) and cytoplasmic loops, which may play important structural or functional roles in acetylcholine transport. In order to identify functional charged residues, site-directed mutagenesis of rVAChT was undertaken. No effect on ACh transport was observed when any of the five aspartate residues in the cytoplasmic loop were converted to asparagine. Similarly, changing Asp-46 (D46N) in TM1 or Asp-255 (D255N) in TM6 had no effect on ACh transport or vesamicol binding. However, replacement of Asp-398 in TM10 with Asn completely eliminated both ACh transport and vesamicol binding. The conservative mutant D398E retained transport activity, but not vesamicol binding, suggesting this residue is critical for transport. Mutation of Asp-193 in TM4 did not affect ACh transport activity; however, vesamicol binding was dramatically reduced. With mutant D425N of TM11 transport activity for ACh was completely blocked, without an effect on vesamicol binding. Activity was not restored in the conservative mutant D425E, suggesting the side chain as well as the negative charge of Asp-425 is important for substrate binding. These mutants, as well as mutant D193N, clearly dissociated ACh binding and transport from vesamicol binding. These data suggest that Asp-398 in TM10 and Asp-425 in TM11 are important for ACh binding and transport, while Asp-193 and Asp-398 in TM4 and TM10, respectively, are involved in vesamicol binding.

Identification and transgenic analysis of a murine promoter that targets cholinergic neuron expression

Choline acetyltransferase (ChAT) is a specific phenotypic marker of cholinergic neurons. Previous reports showed that different upstream regions of the ChAT gene are necessary for cell type-specific expression of reporter genes in cholinergic cell lines. The identity of the mouse ChAT promoter region controlling the establishment, maintenance, and plasticity of the cholinergic phenotype in vivo is not known. We characterized a promoter region of the mouse ChAT gene in transgenic mice, using beta-galactosidase (LacZ) as a reporter gene. A 3,402-bp segment from the 5'-untranslated region of the mouse ChAT gene (from -3,356 to +46, +1 being the translation initiation site) was sufficient to direct the expression of LacZ to selected neurons of the nervous system; however, it did not provide complete cholinergic specificity. A larger fragment (6,417 bp, from -6,371 to +46) of this region contains the requisite regulatory elements that restrict expression of the LacZ reporter gene only in cholinergic neurons of transgenic mice. This 6.4-kb DNA fragment encompasses 633 bp of the 5'-flanking region of the mouse vesicular acetylcholine transporter (VAChT), the entire open reading frame of the VAChT gene, contained within the first intron of the ChAT gene, and sequences upstream of the start coding sequences of the ChAT gene. This promoter will allow targeting of specific gene products to cholinergic neurons to evaluate the mechanisms of diseases characterized by dysfunction of cholinergic neurons and will be valuable in design strategies to correct those disorders.

Synaptic vesicle size and number are regulated by a clathrin adaptor protein required for endocytosis

Clathrin-mediated endocytosis is thought to involve the activity of the clathrin adaptor protein AP180. However, the role of this protein in endocytosis in vivo remains unknown. Here, we show that a mutation that eliminates an AP180 homolog (LAP) in Drosophila severely impairs the efficiency of synaptic vesicle endocytosis and alters the normal localization of clathrin in nerve terminals. Most importantly, the size of both synaptic vesicles and quanta is significantly increased in lap mutants. These results provide novel insights into the molecular mechanism of endocytosis and reveal a role for AP180 in regulating vesicle size through a clathrin-dependent reassembly process.

Submillisecond kinetics of glutamate release from a sensory synapse

Exocytosis-mediated glutamate release from ribbon-type synaptic terminals of retinal bipolar cells was studied using AMPA receptors and simultaneous membrane capacitance measurements. Release onset (delay <0.8 ms) and offset were closely tied to Ca2+ channel opening and closing. Asynchronous release was not copious and we estimate that there are approximately 5 Ca2+ channels per docked synaptic vesicle. Depending on Ca2+ current amplitude, release occurred in a single fast bout or in two successive bouts with fast and slow onset kinetics. The second, slower bout may reflect a mobilization rate of reserve vesicles toward fusion sites that is accelerated by increasing Ca2+ influx. Bipolar cell synaptic ribbons thus are remarkably versatile signal transducers, capable of transmitting rapidly changing sensory input, as well as sustained stimuli, due to their large pool of releasable vesicles.

From the cholinergic gene locus to the cholinergic neuron

The cholinergic gene locus (CGL) was first identified in 1994 as the site (human chromosome 10q11.2) at which choline acetyltransferase and a functional vesicular acetylcholine transporter are co-localized. Here, we present recent neuroanatomical, developmental, and evolutionary insights into the chemical coding of cholinergic neurotransmission that have been gleaned from the study of the CGL, and its protein products VAChT and ChAT, which comprise a synthesis-sequestration pathway that functionally defines the cholinergic phenotype.

Constitutive secretion of exogenous neurotransmitter by nonneuronal cells: implications for neuronal secretion

Fibroblasts in cell culture were loaded with exogenous neurotransmitter acetylcholine (ACh). ACh secretion from loaded cells was detected by whole-cell patch clamp recordings from Xenopus myocytes manipulated into contact with ACh-loaded cells. Two different approaches were used for ACh loading. In the first approach, fibroblasts were incubated in the culture medium containing ACh. Recordings from myocytes revealed fast inward currents that resemble miniature endplate currents found at neuromuscular synapses. The currents observed in recordings from myocytes were due to exocytosis of ACh-containing vesicles. Although exogenous ACh penetrated through the plasma membrane of fibroblasts during incubation and was present in the cytoplasm at detectable levels, cytoplasmic ACh did not contribute to the quantal ACh secretion. In the second approach, exogenous ACh was loaded into the cytoplasm of fibroblasts by microinjection. Under these experimental conditions, fibroblasts also exhibited spontaneous quantal ACh secretion. Analysis of the exocytotic events in fibroblasts following two different protocols of ACh loading revealed that the vesicular compartments responsible for uptake of exogenous ACh are associated with the endocytic recycling pathway. Extrapolation of our results to neuronal cells suggest that in cholinergic neurons, in addition to genuine synaptic vesicles, ACh can be secreted by the vesicles participating in endosomal membrane recycling.

D2-Like dopamine autoreceptor activation reduces quantal size in PC12 cells

D2-like dopamine autoreceptors regulate dopamine release and are implicated in important actions of antipsychotic drugs and rewarding behaviors. To directly observe the effects of D2 autoreceptors on exocytic neurotransmitter release, we measured quantal release of dopamine from pheochromocytoma PC12 cells that express D2 and D4 autoreceptors. High potassium-evoked secretion in PC12 cells produced a unimodal population of quantal sizes. We found that exposures to the D2-like agonist quinpirole that inhibited tyrosine hydroxylase activity by approximately 50% also reduced quantal size by approximately 50%. The reduced quantal size was blocked by the D2 antagonist sulpiride and reversed by L-DOPA. Quinpirole also decreased the frequency of stimulation-evoked quantal release. Together, these findings indicate effects on quantal neurotransmission by D2-like dopamine autoreceptors previously distinguished as synthesis-modulating autoreceptors that regulate tyrosine hydroxylase activity versus impulse-regulating autoreceptors that modulate membrane potential. The results also provide an initial demonstration of a receptor-mediated mechanism that alters quantal size.

Presynaptic recording of quanta from midbrain dopamine neurons and modulation of the quantal size

The observation of quantal release from central catecholamine neurons has proven elusive because of the absence of evoked rapid postsynaptic currents. We adapted amperometric methods to observe quantal release directly from axonal varicosities of midbrain dopamine neurons that predominantly contain small synaptic vesicles. Quantal events were elicited by high K+ or alpha-latrotoxin, required extracellular Ca2+, and were abolished by reserpine. The events indicated the release of 3000 molecules over 200 microsec, much smaller and faster events than quanta associated with large dense-core vesicles previously recorded in vertebrate preparations. The number of dopamine molecules per quantum increased as a population to 380% of controls after glial-derived neurotrophic factor (GDNF) exposure and to 350% of controls after exposure to the dopamine precursor L-dihydroxyphenylalanine (L-DOPA). These results introduce a means to measure directly the number of transmitter molecules released from small synaptic vesicles of CNS neurons. Moreover, quantal size was not an invariant parameter in CNS neurons but could be modulated by neurotrophic factors and altered neurotransmitter synthesis.

Vesicular neurotransmitter transport and the presynaptic regulation of quantal size

Specific transport activities package classical neurotransmitters into secretory vesicles for release by regulated exocytosis, but the proteins responsible for the vesicular transport of neurotransmitters are still being identified. One family of proteins includes vesicular transporters for monoamines and acetylcholine. Genetic manipulation in cells and in mice now shows that changes in the expression of these proteins can alter the amount of neurotransmitter stored per synaptic vesicle, the amount released and behavior. Although the mechanisms responsible for regulating these transporters in vivo remains unknown, recent work has demonstrated the potential for regulation by changes in intrinsic activity and in location. In addition, a recently identified vesicular transporter for GABA defines a novel family of proteins that mediates the packaging of amino acid neurotransmitters.

Dissociation of the vesicular acetylcholine transporter domains important for high-affinity transport recognition, binding of vesamicol and targeting to synaptic vesicles

Chimeras between the human vesicular acetylcholine transporter (hVAChT) and the neuronal isoform of the human vesicular monoamine transporter (hVMAT2) have been constructed and stably expressed in a rat pheochromocytoma cell line (PC12) in an effort to identify cholinergic-specific domains of VAChT. Examination of the transport properties of a chimera in which the N-terminal portion (up to putative transmembrane domain II and including the lumenal glycosylated loop) of hVAChT was replaced with hVMAT2 sequences (2/V@NheI) revealed that its apparent affinity for acetylcholine (ACh) was reduced approximately seven-fold compared to wild-type. However, the affinity of this chimera for vesamicol did not significantly differ from hVAChT. Similarly, the 2/V@NheI chimera retained its preferential targeting to the small synaptic-like vesicles found in PC12 cells in agreement with our recently reported observations that the synaptic vesicle targeting domain resides in the cytoplasmic tail of VAChT.

Structure and organization of the Drosophila cholinergic locus

The Drosophila cholinergic locus is composed of two distinct genetic functions: choline acetyltransferase (ChAT; EC 2.3.1.6), the enzyme catalyzing biosynthesis of neurotransmitter acetylcholine (ACh), and the vesicular ACh transporter (VAChT), the synaptic vesicle membrane protein which pumps transmitter into vesicles. Both genes share a common first exon and the remainder of the VAChT gene contains a single coding exon residing entirely within the first intron of ChAT. RNase protection analysis indicates that all Drosophila VAChT specific transcripts contain the shared first exon and suggests common transcriptional control for ChAT and VAChT. Similar types of genomic organization have been evolutionarily conserved for cholinergic loci in nematodes and vertebrates, and may operate to ensure coordinate expression of these functionally related genes in the same cells. The relative levels of Drosophila ChAT and VAChT mRNA differ, however, in different tissues or in Cha mutants, indicating that independent regulation of ChAT and VAChT transcripts may occur post-transcriptionally. The predicted Drosophila VAChT protein is composed of 578 amino acids and contains 12 conserved putative transmembrane domains. Full-length VAChT cDNA is 7.2 kilobase long and has unusually long 5'- and 3'-untranslated regions (UTR). The 5'-UTR contains a GTG ChAT translational initiation codon along with three other potential ATG initiation codons. These features of the VAChT 5'-UTR region suggest that a ribosome scanning model may not be used for VAChT translation initiation.

Vesicular transport regulates monoamine storage and release but is not essential for amphetamine action

To assess the role of exocytotic release in signaling by monoamines, we have disrupted the neuronal vesicular monoamine transporter 2 (VMAT2) gene. VMAT2-/- mice move little, feed poorly, and die within a few days after birth. Monoamine cell groups and their projections are indistinguishable from those of wild-type littermates, but the brains of mutant mice show a drastic reduction in monoamines. Using midbrain cultures from the mutant animals, amphetamine but not depolarization induces dopamine release. In vivo, amphetamine increases movement, promotes feeding, and prolongs the survival of VMAT2-/- animals, indicating that precise, temporally regulated exocytotic release of monoamine is not required for certain complex behaviors. In addition, the brains of VMAT2 heterozygotes contain substantially lower monoamine levels than those of wild-type littermates, and depolarization induces less dopamine release from heterozygous than from wild-type cultures, suggesting that VMAT2 expression regulates monoamine storage and release.

Vesicular neurotransmitter transporters. Potential sites for the regulation of synaptic function

Neurotransmission depends on the regulated release of chemical transmitter molecules. This requires the packaging of these substances into the specialized secretory vesicles of neurons and neuroendocrine cells, a process mediated by specific vesicular transporters. The family of genes encoding the vesicular transporters for biogenic amines and acetylcholine have recently been cloned. Direct comparison of their transport characteristics and pharmacology provides information about vesicular transport bioenergetics, substrate feature recognition by each transporter, and the role of vesicular amine storage in the mechanism of action of psychopharmacologic and neurotoxic agents. Regulation of vesicular transport activity may affect levels of neurotransmitter available for neurosecretion and be an important site for the regulation of synaptic function. Gene knockout studies have determined vesicular transport function is critical for survival and have enabled further evaluation of the role of vesicular neurotransmitter transporters in behavior and neurotoxicity. Molecular analysis is beginning to reveal the sites involved in vesicular transporter function and the sites that determine substrate specificity. In addition, the molecular basis for the selective targeting of these transporters to specific vesicle populations and the biogenesis of monoaminergic and cholinergic synaptic vesicles are areas of research that are currently being explored. This information provides new insights into the pharmacology and physiology of biogenic amine and acetylcholine vesicular storage in cardiovascular, endocrine, and central nervous system function and has important implications for neurodegenerative disease.

Potentiation of developing synapses by postsynaptic release of neurotrophin-4

The hypothesis that synaptic functions can be regulated by neurotrophins secreted from the postsynaptic cell was examined in Xenopus nerve-muscle cultures. Neuromuscular synapses formed on myocytes overexpressing neurotrophin-4 (M+ synapses) exhibited a higher level of spontaneous synaptic activity and enhanced evoked synaptic transmission as compared to those formed on normal control myocytes (M- synapses). The NT-4 effects involve a potentiation of presynaptic transmitter secretion as well as a lengthening of the mean burst duration of postsynaptic low conductance acetylcholine channels. Repetitive stimulation of either the presynaptic neuron or the postsynaptic myocyte led to a potentiation of synaptic transmission at M+ synapses. All potentiation effects of NT-4 overexpression were abolished by the extracellular presence of TrkB-IgG but not by the presence of TrkA-IgG, indicating that postsynaptic secretion of NT-4 was responsible for the synaptic modification.