Overexpression of synaptotagmin modulates short-term synaptic plasticity at developing neuromuscular junctions

Rab3 and synaptotagmin proteins in the regulation of vesicle fusion and neurotransmitter release

Ca²⁺-triggered neurotransmitter release involves complex regulatory mechanisms, including a series of protein-protein interactions. Three proteins, synaptobrevin (VAMP), synaptosomal-associated protein of 25kDa (SNAP-25) and syntaxin, constitute the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) core complex that plays key roles in controlling vesicle fusion and exocytosis. Many other proteins participate in the regulation of the processes via direct and/or indirect interaction with the SNARE complex. Although much effort has been made, the regulatory mechanism for exocytosis is still not completely clear. Accumulated evidence indicates that the small GTPase Rab3 and synaptotagmin proteins play important regulatory roles during vesicle fusion and neurotransmitter release. This review outlines our present understanding of the two regulatory proteins, with the focus on the interaction of Rab3 with synaptotagmin in the regulatory process.

Drugs used to treat bipolar disorder act via microRNAs to regulate expression of genes involved in neurite outgrowth

BACKGROUND

The drugs commonly used to treat bipolar disorder have limited efficacy and drug discovery is hampered by the paucity of knowledge of the pathophysiology of this disease. This study aims to explore the role of microRNAs in bipolar disorder and understand the molecular mechanisms of action of commonly used bipolar disorder drugs.

METHODS

The transcriptional effects of bipolar disorder drug combination (lithium, valproate, lamotrigine and quetiapine) in cultured human neuronal cells were studied using next generation sequencing. Differential expression of genes (n=20) and microRNAs (n=6) was assessed and the differentially expressed microRNAs were confirmed with TaqMan MicroRNA Assays. The expression of the differentially expressed microRNAs were inhibited to determine bipolar disorder drug effects on their target genes (n=8). Independent samples t-test was used for normally distributed data and Kruskal-Wallis/Mann-Whitney U test was used for data not distributed normally. Significance levels were set at p<0.05.

RESULTS

We found that bipolar disorder drugs tended to increase the expression of miR-128 and miR-378 (p<0.05). Putative target genes of these microRNAs targeted pathways including those identified as "neuron projection development" and "axonogenesis". Many of the target genes are inhibitors of neurite outgrowth and neurogenesis and were downregulated following bipolar disorder drug combination treatment (all p<0.05). The bipolar disorder drug combination tended to decrease the expression of the target genes (NOVA1, GRIN3A, and VIM), however this effect could be reversed by the application of microRNA inhibitors.

CONCLUSIONS

We conclude that at a transcriptional level, bipolar disorder drugs affect several genes in concert that would increase neurite outgrowth and neurogenesis and hence neural plasticity, and that this effect is mediated (at least in part) by modulation of the expression of these two key microRNAs.

Synaptotagmin-1 enables frequency coding by suppressing asynchronous release in a temperature dependent manner

To support frequency-coded information transfer, mammalian synapses tightly synchronize neurotransmitter release to action potentials (APs). However, release desynchronizes during AP trains, especially at room temperature. Here we show that suppression of asynchronous release by Synaptotagmin-1 (Syt1), but not release triggering, is highly temperature sensitive, and enhances synchronous release during high-frequency stimulation. In Syt1-deficient synapses, asynchronous release increased with temperature, opposite to wildtype synapses. Mutations in Syt1 C2B-domain polybasic stretch (Syt1 K326Q,K327Q,K331Q) did not affect synchronization during sustained activity, while the previously observed reduced synchronous response to a single AP was confirmed. However, an inflexible linker between the C2-domains (Syt1 9Pro) reduced suppression, without affecting synchronous release upon a single AP. Syt1 9Pro expressing synapses showed impaired synchronization during AP trains, which was rescued by buffering global Ca²⁺ to prevent asynchronous release. Hence, frequency coding relies on Syt1's temperature sensitive suppression of asynchronous release, an aspect distinct from its known vesicle recruitment and triggering functions.

Revisiting medial preoptic area plasticity induced in male mice by sexual experience

Sexual experience in male rodents, induced by a first exposure to a receptive female, improves efficiency of following copulations. In mice, the mechanisms supporting this improvement are poorly understood. We characterized molecular modifications of the mouse hypothalamic medial preoptic area (mPOA), the main integrative structure for male sexual behaviour, after a single mating event. This paradigm induced long-lasting behavioural improvements and mPOA morphological changes, evidenced by dendritic spine maturation and an increase in the acetylated and tri-methylated forms of histone H3. Ejaculation affected testosterone, progesterone and corticosterone levels in both naive and experienced mice, but sexual experience did not modify basal plasma or hypothalamic levels of steroids. In contrast to studies carried out in rats, no changes were observed, either in the nitrergic system, or in sex steroid receptor levels. However, levels of glutamate- and calcium-associated proteins, including PSD-95, calbindin and the GluN1 subunit of the NMDA receptor, were increased in sexually experienced male mice. The Iba-1 microglial marker was up-regulated in these animals suggesting multicellular interactions induced within the mPOA by sexual experience. In conclusion, plasticity mechanisms induced by sexual experience differ between rat and mouse, even if in both cases they converge to potentiation of the mPOA network.

Proteome Analysis of Potential Synaptic Vesicle Cycle Biomarkers in the Cerebrospinal Fluid of Patients with Sporadic Creutzfeldt-Jakob Disease

Sporadic Creutzfeldt-Jakob disease (sCJD) is the most frequent fatal human prion disease with a rapid progression and unknown mechanism. The synaptic vesicle (SV) cycle pathway has been a hot research field associated with many neurodegenerative diseases that affect synaptic function and thus may affect pathogenesis of the disorder. Here, we used the iTRAQ-based proteomic method and a KEGG pathway enrichment analysis to meticulously analyze all pathways involved in sCJD disease. In total, 1670 proteins were validated in pooled cerebrospinal fluid (CSF) from 20 patients with sCJD compared with that from 13 patients without CJD. The demographic analysis demonstrated that 557 proteins were upregulated and 595 proteins were downregulated with a 1.5-fold change, and 690 proteins involved in 39 pathways changed significantly (p ≤ 0.05) according to the enrichment analysis. The SV cycle pathway and proteins involved were further evaluated, and 14 proteins were confirmed to participate in the SV cycle pathway due to increased expression. Six key proteins, such as AP2A1, SYT1, SNAP25, STXBP1, CLTB, and Rab3a, showed the same trend by western blot as detected by iTRAQ. This is the first study to use high-throughput proteomics to accurately identify and quantify proteins in the SV cycle pathway of a neurodegenerative disease. These results will help define the mechanism and provide new insight into the pathogenetic factors involved in the SV cycle pathway in patients with sCJD. We hope that promising biomarkers can be identified in the CSF of patients with sCJD and other neurodegenerative disorders to help predict disease progression.

SUMO1 Affects Synaptic Function, Spine Density and Memory

Small ubiquitin-like modifier-1 (SUMO1) plays a number of roles in cellular events and recent evidence has given momentum for its contributions to neuronal development and function. Here, we have generated a SUMO1 transgenic mouse model with exclusive overexpression in neurons in an effort to identify in vivo conjugation targets and the functional consequences of their SUMOylation. A high-expressing line was examined which displayed elevated levels of mono-SUMO1 and increased high molecular weight conjugates in all brain regions. Immunoprecipitation of SUMOylated proteins from total brain extract and proteomic analysis revealed ~95 candidate proteins from a variety of functional classes, including a number of synaptic and cytoskeletal proteins. SUMO1 modification of synaptotagmin-1 was found to be elevated as compared to non-transgenic mice. This observation was associated with an age-dependent reduction in basal synaptic transmission and impaired presynaptic function as shown by altered paired pulse facilitation, as well as a decrease in spine density. The changes in neuronal function and morphology were also associated with a specific impairment in learning and memory while other behavioral features remained unchanged. These findings point to a significant contribution of SUMO1 modification on neuronal function which may have implications for mechanisms involved in mental retardation and neurodegeneration.

The cytoplasmic domain of rat synaptotagmin I enhances synaptic transmission

Synaptotagmin, an integral membrane protein of synaptic vesicles, functions as a calcium sensor in the temporal control of neurotransmitter release. Although synaptotagmin facilitates lipid membrane fusion in biochemical experiments, overexpression of synaptotagmin inhibits neurotransmission. A facilitatory effect of synaptotagmin on synaptic transmission was never observed. To determine whether synaptotagmin may accelerate synaptic transmission in vivo, we injected the cytoplasmic domain of rat synaptotagmin I (CD-syt) into crayfish motor axons and tested the effect of CD-syt on synaptic response. We confirmed that CD-syt accelerates neuromuscular transmission. The injected preparation had larger synaptic potentials with shorter rise time. Experiments with varying calcium concentrations showed that CD-syt increased the maximum synaptic response of the neuromuscular synapses. Further tests on short-term plasticity of neuromuscular synapses revealed that CD-syt increases the release probability of the release-ready vesicles.

Brief alcohol exposure alters transcription in astrocytes via the heat shock pathway

Astrocytes are critical for maintaining homeostasis in the central nervous system (CNS), and also participate in the genomic response of the brain to drugs of abuse, including alcohol. In this study, we investigated ethanol regulation of gene expression in astrocytes. A microarray screen revealed that a brief exposure of cortical astrocytes to ethanol increased the expression of a large number of genes. Among the alcohol-responsive genes (ARGs) are glial-specific immune response genes, as well as genes involved in the regulation of transcription, cell proliferation, and differentiation, and genes of the cytoskeleton and extracellular matrix. Genes involved in metabolism were also upregulated by alcohol exposure, including genes associated with oxidoreductase activity, insulin-like growth factor signaling, acetyl-CoA, and lipid metabolism. Previous microarray studies performed on ethanol-treated hepatocyte cultures and mouse liver tissue revealed the induction of almost identical classes of genes to those identified in our microarray experiments, suggesting that alcohol induces similar signaling mechanisms in the brain and liver. We found that acute ethanol exposure activated heat shock factor 1 (HSF1) in astrocytes, as demonstrated by the translocation of this transcription factor to the nucleus and the induction of a family of known HSF1-dependent genes, the heat shock proteins (Hsps). Transfection of a constitutively transcriptionally active Hsf1 construct into astrocytes induced many of the ARGs identified in our microarray study supporting the hypothesis that HSF1 transcriptional activity, as part of the heat shock cascade, may mediate the ethanol induction of these genes. These data indicate that acute ethanol exposure alters gene expression in astrocytes, in part via the activation of HSF1 and the heat shock cascade.

Early-life lead exposure affects the activity of TNF-alpha and expression of SNARE complex in hippocampus of mouse pups

This study aims to investigate the effects of maternal lead exposure on learning and memory ability and the protein expression of TNF-alpha and SNARE complex (SNAP-25, VAMP-2, and Syntaxin 1A) in hippocampus of mice offspring. Pb exposure was initiated from beginning of gestation to weaning. Pb acetate administered in drinking solutions was dissolved in distilled deionized water at 0.1%, 0.5%, and 1% groups, respectively. On the PND21, the learning and memory ability of mouse pups was tested by water maze test, and the Pb levels in their blood and hippocampus were also determined. The protein expression of TNF-alpha and SNARE complex in hippocampus was measured by immunohistochemistry and Western blotting. The Pb levels in blood and hippocampus of all exposure groups were significantly higher than control group (P < 0.05). In the water maze test, the performances of 0.5% and 1% groups were worse than that of control group (P < 0.05). The expression of TNF-alpha, Syntaxin 1A, and VAMP-2 was increased in Pb-exposed groups comparing control group (P < 0.05), but the expression of SNAP-25 was decreased (P < 0.05). Up-regulation of TNF-alpha and disturbance of SNARE expression in the hippocampus of pups may contribute to impairment of learning and memory ability associated with maternal Pb exposure.

The regulation of neuronal gene expression by alcohol

In recent years there has been an explosion of interest in how genes regulate alcohol drinking and contribute to alcoholism. This work has been stimulated by the completion of the human and mouse genome projects and the resulting availability of gene microarrays. Most of this work has been performed in drinking animals, and has utilized the extensive genetic variation among different mouse strains. At the same time, a much smaller amount of effort has gone into the in vitro study of the mechanisms underlying the regulation of individual genes by alcohol. These studies at the cellular and sub-cellular level are beginning to reveal the ways in which alcohol can interact with the transcriptional, translational and post-translational events inside the cell. Detailed studies of the promoter regions within several individual alcohol-responsive genes (ARGs) have been performed and this work has uncovered intricate signaling pathways that may be generalized to larger groups of ARGs. In the last few years several distinct ARGs have been identified from 35,000 mouse genes, by both the "top-down" approach (ex vivo gene arrays) and the "bottom-up" methods (in vitro promoter analysis). These divergent methodologies have converged on a surprisingly small number of genes encoding ion channels, receptors, transcription factors and proteins involved in synaptic function and remodeling. In this review we will describe some of the most interesting cellular and microarray work in the field, and will outline specific examples of genes for which the mechanisms of regulation by alcohol are now somewhat understood.

Short-term plasticity of small synaptic vesicle (SSV) and large dense-core vesicle (LDCV) exocytosis

Synaptic plasticity results from changes in the strength of synaptic transmission upon repetitive stimulation. The amount of neurotransmitter released from presynaptic terminals can regulate short-term plasticity that lasts for a few minutes. This review focuses on short-term plasticity of small synaptic vesicle (SSV) and large dense-core vesicle (LDCV) exocytosis. Whereas SSVs contain classical neurotransmitters and activate ion channels, LDCVs contain neuropeptides and hormones which primarily activate G protein-coupled receptors (GPCRs). Thus, LDCV exocytosis is mainly associated with modulation of synaptic activity and cannot induce synaptic activity by itself. As in SSV exocytosis, repetitive stimulation leads to short-term enhancement of LDCV exocytosis: i.e., activity-dependent potentiation (ADP) which represents potentiation of neurotransmitter release. Short-term plasticity of SSV exocytosis results from Ca2+ accumulation, but ADP of LDCV exocytosis does not. Here, we review the signaling mechanisms and differences of short-term plasticity in exocytotic processes of SSV and LDCV.

Impaired synaptogenesis and long-term modulation of behavior following postnatal elevation of GABA levels in mice

Antiepileptic drugs acting through the potentiation of GABAergic pathways have adverse effects on brain development. Increased risk of impaired intellectual development has been reported in children born to women treated for epilepsy during pregnancy. We have previously shown, in mice, that treatment with the antiepileptic drug vigabatrin (GVG) on postnatal days 4-14 delays reflex development in the newborn and impairs learning and memory in the adult. Here, we report the time course in which postnatal GVG treatment induced behavioral changes in an open field test and had a detrimental developmental effect on recognition memory in mice. Furthermore, GVG treatment significantly modulated the expression of synaptobrevin/vesicle-associated membrane protein (VAMP) II and synaptotagmin (Synt) I. A short-term decrease in the expression of these proteins was followed by a long-term elevation in their expression in both the hippocampus and the cerebral cortex. In contrast, no changes were detected in the levels of Synt II or in the vesicular GABA transporter. The over-expression of VAMP II and Synt I in the GVG-treated mice was associated with a significant decrease in the basal field excitatory postsynaptic potentials (fEPSP) and modulated the response to repeated stimulation. The changes observed in synaptogenesis may explain the behavioral impairment induced by postnatal GVG treatment and may suggest a possible mechanism for the detrimental effect of antiepileptic drugs acting through elevation of GABA levels.

Fusion of synaptic vesicles and plasma membrane in the presence of synaptosomal soluble proteins

Fusion between synaptic vesicles and plasma membranes isolated from rat brain synaptosomes is regarded as a model of neurosecretion. The main aim of current study is to investigate whether the synaptosomal soluble proteins are essential members of Ca(2+)-triggered fusion examined in this system. Fusion experiments were performed using fluorescent dye octadecylrhodamine B, which was incorporated into synaptic vesicle membranes at self-quenching concentration. The fusion of synaptic vesicles, containing marker octadecylrhodamine B, with plasma membranes was detected by dequenching of the probe fluorescence. Membrane fusion was not found in Ca(2+)-supplemented buffer solution, but was initiated by the addition of the synaptosomal soluble proteins. When soluble proteins were treated with trypsin, they lost completely the fusion activity. These experiments confirmed that soluble proteins of synaptosomes are sensitive to Ca(2+) signal and essential for membrane fusion. The experiments, in which members of fusion process were treated with monoclonal antibodies raised against synaptotagmin and synaptobrevin, have shown that antibodies only partially inhibited fusion of synaptic vesicles and plasma membranes in vitro. These results indicate that other additional component(s), which may or may not be related to synaptobrevin or synaptotagmin, mediate this process. It can be assumed that fusion of synaptic vesicles with plasma membranes in vitro depends upon the complex interaction of a large number of protein factors.

Genetic analysis of synaptotagmin 2 in spontaneous and Ca2+-triggered neurotransmitter release

Synaptotagmin 2 resembles synaptotagmin 1, the Ca2+ sensor for fast neurotransmitter release in forebrain synapses, but little is known about synaptotagmin 2 function. Here, we describe a severely ataxic mouse strain that harbors a single, destabilizing amino-acid substitution (I377N) in synaptotagmin 2. In Calyx of Held synapses, this mutation causes a delay and a decrease in Ca2+-induced but not in hypertonic sucrose-induced release, suggesting that synaptotagmin 2 mediates Ca2+ triggering of evoked release in brainstem synapses. Unexpectedly, we additionally observed in synaptotagmin 2 mutant synapses a dramatic increase in spontaneous release. Synaptotagmin 1-deficient excitatory and inhibitory cortical synapses also displayed a large increase in spontaneous release, demonstrating that this effect was shared among synaptotagmins 1 and 2. Our data suggest that synaptotagmin 1 and 2 perform equivalent functions in the Ca2+ triggering of action potential-induced release and in the restriction of spontaneous release, consistent with a general role of synaptotagmins in controlling 'release slots' for synaptic vesicles at the active zone.

Role of synaptotagmin in Ca2+-triggered exocytosis

The Ca(2+)-binding synaptic-vesicle protein synaptotagmin I has attracted considerable interest as a potential Ca(2+) sensor that regulates exocytosis from neurons and neuroendocrine cells. Recent studies have shed new light on the structure, biochemical/biophysical properties and function of synaptotagmin, and the emerging view is that it plays an important role in both exocytosis and endocytosis. At least a dozen additional isoforms exist, some of which are expressed outside of the nervous system, suggesting that synaptotagmins might regulate membrane traffic in a variety of cell types. Here we provide an overview of the members of this gene family, with particular emphasis on the question of whether and how synaptotagmin I functions during the final stages of membrane fusion: does it regulate the Ca(2+)-triggered opening and dilation of fusion pores?

Direct, Ca2+-dependent interaction between tubulin and synaptotagmin I: a possible mechanism for attaching synaptic vesicles to microtubules

The synaptic vesicle protein synaptotagmin I probably plays important roles in the synaptic vesicle cycle. However, the mechanisms of its action remain unclear. In this study, we have searched for cytoplasmic proteins that interact with synaptotagmin I. We found that the cytoskeletal protein tubulin directly and stoichiometrically bound to recombinant synaptotagmin I. The binding depended on mm Ca(2+), and 1 mol of tubulin dimer bound 2 mol of synaptotagmin I with half-maximal binding at 6.6 microm tubulin. The Ca(2+) dependence mainly resulted from Ca(2+) binding to the Ca(2+) ligands of synaptotagmin I. The C-terminal region of beta-tubulin and both C2 domains of synaptotagmin I were involved in the binding. The YVK motif in the C2 domains of synaptotagmin I was essential for tubulin binding. Tubulin and synaptotagmin I were co-precipitated from the synaptosome extract with monoclonal antibodies to tubulin and SNAP-25 (synaptosome-associated protein of 25 kDa), indicating the presence of tubulin/synaptotagmin I complex and tubulin binding to synaptotagmin I in SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complexes. Synaptotagmin I promoted tubulin polymerization and bundled microtubules in the presence of Ca(2+). These results suggest that direct interaction between synaptotagmin I and tubulin provides a mechanism for attaching synaptic vesicles to microtubules in high Ca(2+) concentrations.

Short-term synaptic plasticity

Synaptic transmission is a dynamic process. Postsynaptic responses wax and wane as presynaptic activity evolves. This prominent characteristic of chemical synaptic transmission is a crucial determinant of the response properties of synapses and, in turn, of the stimulus properties selected by neural networks and of the patterns of activity generated by those networks. This review focuses on synaptic changes that result from prior activity in the synapse under study, and is restricted to short-term effects that last for at most a few minutes. Forms of synaptic enhancement, such as facilitation, augmentation, and post-tetanic potentiation, are usually attributed to effects of a residual elevation in presynaptic [Ca(2+)]i, acting on one or more molecular targets that appear to be distinct from the secretory trigger responsible for fast exocytosis and phasic release of transmitter to single action potentials. We discuss the evidence for this hypothesis, and the origins of the different kinetic phases of synaptic enhancement, as well as the interpretation of statistical changes in transmitter release and roles played by other factors such as alterations in presynaptic Ca(2+) influx or postsynaptic levels of [Ca(2+)]i. Synaptic depression dominates enhancement at many synapses. Depression is usually attributed to depletion of some pool of readily releasable vesicles, and various forms of the depletion model are discussed. Depression can also arise from feedback activation of presynaptic receptors and from postsynaptic processes such as receptor desensitization. In addition, glial-neuronal interactions can contribute to short-term synaptic plasticity. Finally, we summarize the recent literature on putative molecular players in synaptic plasticity and the effects of genetic manipulations and other modulatory influences.

Ultrastructural basis of synaptic transmission between endbulbs of Held and bushy cells in the rat cochlear nucleus

Auditory nerve fibres make large excitatory synaptic contacts, the endbulbs of Held, with bushy cells in the anteroventral cochlear nucleus (AVCN). We have used serial-section electron microscopy to reconstruct seven endbulbs of Held in contact with three different AVCN bushy cells from a 25-day-old rat, as a basis for interpreting our previous physiological results at this connection. Four endbulbs of Held contacting the same bushy cell were completely reconstructed. The number of separate synaptic specializations within these endbulbs varied from 85 to 217, with a mean of 155. Detailed measurements were obtained from high magnification segments of four endbulbs contacting three different bushy cells. Large variability was found in the size of synaptic specializations within an individual endbulb. The size of postsynaptic densities (PSDs) varied between endbulbs (mean PSD area 0.03, 0.07, 0.07 and 0.18 microm(2); n = 4 endbulbs). The number of morphologically docked vesicles at individual specializations within the same endbulb varied considerably (between 1 and 102). The mean number of morphologically docked vesicles per specialization differed between endbulbs (mean numbers of docked vesicles per specialization = 2.1, 3.7, 5.3, 14.8; n = 4 endbulbs). Despite these large differences, the density of docked vesicles per square micron of PSD was similar between endbulbs (54, 80, 81, 83 docked vesicles per microm(2); n = 4 endbulbs). Within an endbulb, a linear relationship was found between the number of docked vesicles and PSD area, and between PSD area and the number of undocked vesicles within 150 nm of the active zone. The ratio of undocked vesicles (< 150 nm) to docked vesicles ranged from 2 to 5 in different endbulbs (n = 4 endbulbs). These structural observations are discussed in relation to the functional properties of synaptic transmission between endbulbs of Held and bushy cells in the AVCN.

Molecular mechanisms of neurotransmitter release

The release of neurotransmitter from neurons represents one of the pivotal events in synaptic transmission. Neurotransmitters are released from synaptic vesicles in presynaptic neurons in response to neural activity, diffuse across the synaptic cleft, and bind specific receptors in order to bring about changes in postsynaptic neurons. Some of the molecular processes that govern neurotransmitter release are now becoming better understood. The steps involved can be broken down into two partially overlapping presynaptic cycles, the neurotransmitter cycle and the synaptic vesicle cycle. The neurotransmitter cycle involves transmitter biosynthesis, storage, reuptake, and degradation. The synaptic vesicle cycle involves targeting to the nerve terminal, docking, fusion, endocytosis, and recycling. Biochemical and structural studies have yielded important insight into our understanding of each of these two cycles. Further, both pharmacological and genetic interference with either of these cycles results in profound alterations in synaptic transmission and behavior, demonstrating the crucial role of neurotransmitter release.

Postsynaptic target specificity of neurotrophin-induced presynaptic potentiation

The role of the target cell in neurotrophin-induced modifications of glutamatergic synaptic transmission was examined in cultured hippocampal neurons. Brain-derived neurotrophic factor (BDNF) induced rapid and persistent potentiation of evoked glutamate release when the postsynaptic neuron was glutamatergic, or excitatory (E-->E), but not when it was GABAergic, or inhibitory (E-->1). This target-specific action of BDNF was also found at divergent outputs of a single presynaptic neuron innervating both glutamatergic and GABAergic neurons, suggesting that individual terminals can be independently modified. Surprisingly, BDNF increased the frequency of miniature postsynaptic currents at both E-->E and E-->I, although it had no effect on evoked currents at E-->I. Finally, potentiation by neurotrophin-3 (NT-3) was also target specific. The selective effect at E-->E suggests that retrograde signaling by the postsynaptic target cell endows a localized presynaptic action of neurotrophins.

Calcium- and activity-dependent synaptic plasticity

Calcium ions play crucial signaling roles in many forms of activity-dependent synaptic plasticity. Recent presynaptic [Ca2+]i measurements and manipulation of presynaptic exogenous buffers reveal roles for residual [Ca2+]i following conditioning stimulation in all phases of short-term synaptic enhancement. Pharmacological manipulations implicate mitochondria in post-tetanic potentiation. New evidence supports an influence of Ca2+ in replacing depleted vesicles after synaptic depression. In addition, high-resolution measurements of [Ca2+]i in dendritic spines show how Ca2+ can encode the precise relative timing of presynaptic input and postsynaptic activity and generate long-term synaptic modifications of opposite polarity.

Synaptic vesicle docking and fusion

Neurotransmitter secretion shares many features with constitutive membrane trafficking. In both cases, vesicles are targeted to a specific acceptor membrane and fuse via a series of protein-protein interactions. Recent work has added to the list of protein complexes involved and is beginning to define the order in which they act. The rapid fusion, precise regulation and plasticity characteristic of synaptic exocytosis probably results from the addition of specialized regulators.

Reconstitution of regulated exocytosis in cell-free systems: a critical appraisal

Regulated exocytosis involves the tightly controlled fusion of a transport vesicle with the plasma membrane. It includes processes as diverse as the release of neurotransmitters from presynaptic nerve endings and the sperm-triggered deposition of a barrier preventing polyspermy in oocytes. Cell-free model systems have been developed for studying the biochemical events underlying exocytosis. They range from semi-intact permeabilized cells to the reconstitution of membrane fusion from isolated secretory vesicles and their target plasma membranes. Interest in such cell-free systems has recently been reinvigorated by new evidence suggesting that membrane fusion is mediated by a basic mechanism common to all intracellular fusion events. In this chapter, we review some of the literature in the light of these new developments and attempt to provide a critical discussion of the strengths and limitations of the various cell-free systems.

Novel domain-specific actions of amyloid precursor protein on developing synapses

The effect of the secretory form of amyloid precursor protein (sAPP) on synaptic transmission was examined by using developing neuromuscular synapses in Xenopus cell cultures. The frequency of spontaneous postsynaptic currents (SSCs) was reduced by the addition of sAPP, whereas the amplitude of impulse-evoked postsynaptic currents (ESCs) was increased by sAPP. These opposing effects on spontaneous versus evoked release were separated by using the specific domain of APP. The C-terminal fragment of sAPP (CAPP) only reduced SSC frequency and did not affect ESCs. By contrast, the N-terminal fragment of sAPP (NAPP) did not affect SSC frequency but did increase ESC amplitude. The reduction of SSC frequency by sAPP appears to be mediated by activation of potassium channels through a cGMP-dependent pathway, whereas the increase of ESC amplitude is mediated by a different pathway involving activation of protein kinase(s). These results suggest the potential role of sAPP as a modulator of synaptic activity by two specific domains.

Effect of chronic haloperidol treatment on synaptic protein mRNAs in the rat brain

Chronic haloperidol treatment caused significant decreases in the levels of synaptotagmin I and IV, synaptobrevin II, syntaxin 1A and Rab 3A mRNAs in the nucleus accumbens but not in the prefrontal cortex medial field, striatum, substantia nigra and ventral tegmental area. No significant changes in SNAP 25 and synaptophysin mRNA levels were observed in any brain region examined. The reduced expression of synaptic proteins may be related to haloperidol-induced depolarization block of mesolimbic dopamine neurons.