The Experts below are selected from a list of 207 Experts worldwide ranked by ideXlab platform
Milton P Charlton - One of the best experts on this subject based on the ideXlab platform.
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cholesterol dependent kinase activity regulates Transmitter Release from cerebellar synapses
The Journal of Neuroscience, 2010Co-Authors: Alex J Smith, Shuzo Sugita, Milton P CharltonAbstract:Changes in membrane cholesterol content can alter protein kinase activity, however, it is not known whether kinases regulating Transmitter Release are sensitive to membrane cholesterol content. Here we have used the cholesterol extracting agent methyl-β-cyclodextrin to measure the effects of acute cholesterol reduction on Transmitter Release from cultured cerebellar neurons. Cholesterol depletion increased the frequency of spontaneous Transmitter Release without altering the amplitude and time course of mEPSCs. Evoked Transmitter Release was decreased by cholesterol extraction and the paired pulse ratio was also decreased. Alterations in synaptic transmission were not associated with failure of action potential generation or changes in presynaptic Ca 2+ signaling. Both the increase in mEPSC frequency and the change in paired pulse ratio were blocked by the broad spectrum protein kinase inhibitor staurosporine. The increase in mEPSC frequency was also sensitive to selective inhibitors of PKC and PKA. Our results therefore demonstrate that the activity of presynaptic protein kinases that regulate spontaneous and evoked neuroTransmitter Release is sensitive to changes of membrane cholesterol content.
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cholesterol and synaptic Transmitter Release at crayfish neuromuscular junctions
The Journal of Physiology, 2006Co-Authors: Orit Zamir, Milton P CharltonAbstract:During exocytosis of synaptic Transmitters, the fusion of highly curved synaptic vesicle membranes with the relatively planar cell membrane requires the coordinated action of several proteins. The role of membrane lipids in the regulation of Transmitter Release is less well understood. Since it helps to control membrane fluidity, alteration of cholesterol content may alter the fusibility of membranes as well as the function of membrane proteins. We assayed the importance of cholesterol in Transmitter Release at crayfish neuromuscular junctions where action potentials can be measured in the preterminal axon. Methyl-β-cyclodextrin (MβCD) depleted axons of cholesterol, as shown by reduced filipin labelling, and cholesterol was replenished by cholesterol–MβCD complex (Ch-MβCD). MβCD blocked evoked synaptic transmission. The lack of postsynaptic effects of MβCD on the time course and amplitude of spontaneous postsynaptic potentials or on muscle resting potential allowed us to focus on presynaptic mechanisms. Intracellular presynaptic axon recordings and focal extracellular recordings at individual boutons showed that failure of Transmitter Release was correlated with presynaptic hyperpolarization and failure of action potential propagation. All of these effects were reversed when cholesterol was replenished with Ch-MβCD. However, focal depolarization of presynaptic boutons and administration of a Ca2+ ionophore both triggered Transmitter Release after cholesterol depletion. Therefore, both presynaptic Ca2+ channels and Ca2+-dependent exocytosis functioned after cholesterol depletion. The frequency of spontaneous quantal Transmitter Release was increased by MβCD but recovered when cholesterol was reintroduced. The increase in spontaneous Release was not through a calcium-dependent mechanism because it persisted with intense intracellular calcium chelation. In conclusion, cholesterol levels in the presynaptic membrane modulate several key properties of synaptic Transmitter Release.
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Cholesterol and synaptic Transmitter Release at crayfish neuromuscular junctions.
The Journal of physiology, 2005Co-Authors: Orit Zamir, Milton P CharltonAbstract:During exocytosis of synaptic Transmitters, the fusion of highly curved synaptic vesicle membranes with the relatively planar cell membrane requires the coordinated action of several proteins. The role of membrane lipids in the regulation of Transmitter Release is less well understood. Since it helps to control membrane fluidity, alteration of cholesterol content may alter the fusibility of membranes as well as the function of membrane proteins. We assayed the importance of cholesterol in Transmitter Release at crayfish neuromuscular junctions where action potentials can be measured in the preterminal axon. Methyl-beta-cyclodextrin (MbetaCD) depleted axons of cholesterol, as shown by reduced filipin labelling, and cholesterol was replenished by cholesterol-MbetaCD complex (Ch-MbetaCD). MbetaCD blocked evoked synaptic transmission. The lack of postsynaptic effects of MbetaCD on the time course and amplitude of spontaneous postsynaptic potentials or on muscle resting potential allowed us to focus on presynaptic mechanisms. Intracellular presynaptic axon recordings and focal extracellular recordings at individual boutons showed that failure of Transmitter Release was correlated with presynaptic hyperpolarization and failure of action potential propagation. All of these effects were reversed when cholesterol was replenished with Ch-MbetaCD. However, focal depolarization of presynaptic boutons and administration of a Ca2+ ionophore both triggered Transmitter Release after cholesterol depletion. Therefore, both presynaptic Ca2+ channels and Ca2+-dependent exocytosis functioned after cholesterol depletion. The frequency of spontaneous quantal Transmitter Release was increased by MbetaCD but recovered when cholesterol was reintroduced. The increase in spontaneous Release was not through a calcium-dependent mechanism because it persisted with intense intracellular calcium chelation. In conclusion, cholesterol levels in the presynaptic membrane modulate several key properties of synaptic Transmitter Release.
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Different VAMP/Synaptobrevin Complexes for Spontaneous and Evoked Transmitter Release at the Crayfish Neuromuscular Junction
Journal of Neurophysiology, 1998Co-Authors: Dorota A. Raciborska, William S. Trimble, Milton P CharltonAbstract:Hua, Shao-Ying, Dorota A. Raciborska, William S. Trimble, and Milton P. Charlton. Different VAMP/synaptobrevin complexes for spontaneous and evoked Transmitter Release at the crayfish neuromuscular junction. J. Neurophysiol. 80: 3233–3246, 1998. Although vesicle-associated membrane protein (VAMP/synaptobrevin) is essential for evoked neuroTransmitter Release, its role in spontaneous Transmitter Release remains uncertain. For instance, many studies show that tetanus toxin (TeNT), which cleaves VAMP, blocks evoked Transmitter Release but leaves some spontaneous Transmitter Release. We used recombinant tetanus and botulinum neurotoxin catalytic light chains (TeNT-LC, BoNT/B-LC, and BoNT/D-LC) to examine the role of VAMP in spontaneous Transmitter Release at neuromuscular junctions (nmj) of crayfish. Injection of TeNT-LC into presynaptic axons removed most of the VAMP immunoreactivity and blocked evoked Transmitter Release without affecting nerve action potentials or Ca2+ influx. The frequency of spontaneous Transmitter Release was little affected by the TeNT-LC when the evoked Transmitter Release had been blocked by >95%. The spontaneous Transmitter Release left after TeNT-LC treatment was insensitive to increases in intracellular Ca2+. BoNT/B-LC, which cleaves VAMP at the same site as TeNT-LC but uses a different binding site, also blocked evoked Release but had minimal effect on spontaneous Release. However, BoNT/D-LC, which cleaves VAMP at a different site from the other two toxins but binds to the same position on VAMP as TeNT, blocked both evoked and spontaneous Transmitter Release at similar rates. The data indicate that different VAMP complexes are employed for evoked and spontaneous Transmitter Release; the VAMP used in spontaneous Release is not readily cleaved by TeNT or BoNT/B. Because the exocytosis that occurs after the action of TeNT cannot be increased by increased intracellular Ca2+, the final steps in neuroTransmitter Release are Ca2+ independent.
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presynaptic calcium signals and Transmitter Release are modulated by calcium activated potassium channels
The Journal of Neuroscience, 1992Co-Authors: Richard Robitaille, Milton P CharltonAbstract:The regulation of synaptic transmission by Ca(2+)-activated potassium (gKca) channels was investigated at the frog neuromuscular junction (nmj). Charybdotoxin (CTX), a blocker of certain types of gKca channels, induced a twofold increase of Transmitter Release. Similar results were obtained with purified natural toxin, synthetic toxin, and recombinant toxin. Apamin, a blocker of a different type of gKca channel, did not alter Transmitter Release. CTX was ineffective after intraterminal Ca2+ buffering was increased by application of the membrane-permeant Ca2+ buffer dimethyl-BAPTA-AM. By itself, the permeant buffer first caused a slight increase in Transmitter Release before Release was eventually decreased. This increase of Release did not occur when the buffer was applied in the presence of CTX or Ba2+, another gKca channel blocker. Stimulus-evoked entry of Ca2+ in nerve terminals, detected with the fluorescent Ca2+ indicator FLUO-3, was increased after blockade of gKca channels by CTX. CTX had no effect on the amount or the time course of synaptic depression. The results are consistent with the hypothesis that CTX-sensitive gKca channels normally narrow the presynaptic action potential and thus, by indirectly regulating Ca2+ entry, can serve as powerful modulators of evoked Transmitter Release. In order to affect presynaptic action potentials, the gKca channels must be located close to Ca2+ channels.
Alan Fine - One of the best experts on this subject based on the ideXlab platform.
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calcium stores in hippocampal synaptic boutons mediate short term plasticity store operated ca2 entry and spontaneous Transmitter Release
Neuron, 2001Co-Authors: Nigel J Emptage, Christopher M Reid, Alan FineAbstract:Evoked Transmitter Release depends upon calcium influx into synaptic boutons, but mechanisms regulating bouton calcium levels and spontaneous Transmitter Release are obscure. To understand these processes better, we monitored calcium transients in axons and presynaptic terminals of pyramidal neurons in hippocampal slice cultures. Action potentials reliably evoke calcium transients in axons and boutons. Calcium-induced calcium Release (CICR) from internal stores contributes to the transients in boutons and to paired-pulse facilitation of EPSPs. Store depletion activates store-operated calcium channels, influencing the frequency of spontaneous Transmitter Release. Boutons display spontaneous Ca2+ transients; blocking CICR reduces the frequency of these transients and of spontaneous miniature synaptic events. Thus, spontaneous Transmitter Release is largely calcium mediated, driven by Ca2+ Release from internal stores. Bouton store Release is important for short-term synaptic plasticity and may also contribute to long-term plasticity.
Ralf Schneggenburger - One of the best experts on this subject based on the ideXlab platform.
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Ca2+ channels and Transmitter Release at the active zone
Cell Calcium, 2012Co-Authors: Ralf Schneggenburger, Olexiy KochubeyAbstract:Ca2+-dependent Transmitter Release is the most important signaling mechanism for fast information transfer between neurons. Transmitter Release takes places at highly specialized active zones with sub-micrometer dimension, which contain the molecular machinery for vesicle docking and -fusion, as well as a high density of voltage-gated Ca2+ channels. In the absence of direct evidence for the ultrastructural localization of Ca2+ channels at CNS synapses, important insights into Ca2+ channel-vesicle coupling has come from functional experiments relating presynaptic Ca2+ current and Transmitter Release, at large and accessible synapses like the calyx of Held. First, high slope values in log-log plots of Transmitter Release versus presynaptic Ca2+ current indicate that multiple Ca2+ channels are involved in Release control of a single vesicle. Second, Release kinetics in response to step-like depolarizations revealed fast- and slowly releasable sub-pools of vesicles. FRP and SRP, which, according to the "positional" model, are distinguished by a differential proximity to Ca2+ channels. Considering recent evidence for a rapid conversion of SRP- to FRP vesicles, however, we highlight that multivesicular Release events and clearance of vesicle membrane from the active zone must be taken into account when interpreting kinetic Release data. We conclude that the careful kinetic analysis of Transmitter Release at presynaptically accessible and molecularly targeted synapses has the potential to yield important insights into the molecular physiology of Transmitter Release. (C) 2012 Elsevier Ltd. All rights reserved.
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the timing of phasic Transmitter Release is ca2 dependent and lacks a direct influence of presynaptic membrane potential
Proceedings of the National Academy of Sciences of the United States of America, 2003Co-Authors: Felix Felmy, Erwin Neher, Ralf SchneggenburgerAbstract:Ca2+ influx through voltage-gated Ca2+ channels and the resulting elevation of intracellular Ca2+ concentration, [Ca2+]i, triggers Transmitter Release in nerve terminals. However, it is controversial whether in addition to the opening of Ca2+ channels, membrane potential directly affects Transmitter Release. Here, we assayed the influence of membrane potential on Transmitter Release at the calyx of Held nerve terminals. Transmitter Release was evoked by presynaptic Ca2+ uncaging, or by presynaptic Ca2+ uncaging paired with presynaptic voltage-clamp depolarizations to +80 mV, under pharmacological block of voltage-gated Ca2+ channels. Such a change in membrane potential did not alter the Ca2+ dependence of Transmitter Release rates or synaptic delays. We also found, by varying the amount of Ca2+ influx during Ca2+ tail-currents, that the time course of phasic Transmitter Release is not invariant to changes in Release probability. Rather, the time difference between peak Ca2+ current and peak Transmitter Release became progressively shorter with increasing Ca2+ current amplitude. When this time difference was plotted as a function of the estimated local [Ca2+]i at the sites of vesicle fusion, a slope of ≈100 μs per 10 μM [Ca2+]i was found, in reasonable agreement with a model of cooperative Ca2+ binding and vesicle fusion. Thus, the amplitude and time course of the [Ca2+]i signal at the sites of vesicle fusion controls the timing and the amount of Transmitter Release, both under conditions of brief periods of Ca2+ influx, as well as during step-like elevations of [Ca2+]i produced by Ca2+ uncaging.
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probing the intracellular calcium sensitivity of Transmitter Release during synaptic facilitation
Neuron, 2003Co-Authors: Felix Felmy, Erwin Neher, Ralf SchneggenburgerAbstract:Abstract In nerve terminals, residual Ca 2+ remaining from previous activity can cause facilitation of Transmitter Release by a mechanism that is still under debate. Here we show that the intracellular Ca 2+ sensitivity of Transmitter Release at the calyx of Held is largely unchanged during facilitation, which leaves an increased microdomain Ca 2+ signal as a possible mechanism for facilitation. We measured the Ca 2+ dependencies of facilitation, as well as of Transmitter Release, to estimate the required increment in microdomain Ca 2+ . These measurements show that linear summation of residual and microdomain Ca 2+ accounts for only 30% of the observed facilitation. However, a small degree of supralinearity in the summation of intracellular Ca 2+ signals, which might be caused by saturation of cytosolic Ca 2+ buffer(s), is sufficient to explain facilitation at this CNS synapse.
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intracellular calcium dependence of Transmitter Release rates at a fast central synapse
Nature, 2000Co-Authors: Ralf Schneggenburger, Erwin NeherAbstract:Calcium-triggered fusion of synaptic vesicles and neuroTransmitter Release are fundamental signalling steps in the central nervous system. It is generally assumed that fast Transmitter Release is triggered by elevations in intracellular calcium concentration ([Ca2+]i) to at least 100 µM near the sites of vesicle fusion1,2,3,4,5. For synapses in the central nervous system, however, there are no experimental estimates of this local [Ca2+]i signal. Here we show, by using calcium ion uncaging in the large synaptic terminals of the calyx of Held, that step-like elevations to only 10 µM [Ca2+] i induce fast Transmitter Release, which depletes around 80% of a pool of available vesicles in less than 3 ms. Kinetic analysis of Transmitter Release rates after [Ca2+]i steps revealed the rate constants for calcium binding and vesicle fusion. These show that transient (around 0.5 ms) local elevations of [Ca2+]i to peak values as low as 25 µM can account for Transmitter Release during single presynaptic action potentials. The calcium sensors for vesicle fusion are far from saturation at normal Release probability. This non-saturation, and the high intracellular calcium cooperativity in triggering vesicle fusion, make fast synaptic transmission very sensitive to modulation by changes in local [Ca2+]i.
Nigel J Emptage - One of the best experts on this subject based on the ideXlab platform.
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presynaptic nmdars in the hippocampus facilitate Transmitter Release at theta frequency
Neuron, 2010Co-Authors: Lindsay Mcguinness, Chanel Taylor, Ruth D T Taylor, Tobias Langenhan, M Hart, Helen C Christian, Patricia W Tynan, Peter Donnelly, Nigel J EmptageAbstract:Summary A rise in [Ca 2+ ] i provides the trigger for neuroTransmitter Release at neuronal boutons. We have used confocal microscopy and Ca 2+ sensitive dyes to directly measure the action potential-evoked [Ca 2+ ] i in the boutons of Schaffer collaterals. This reveals that the trial-by-trial amplitude of the evoked Ca 2+ transient is bimodally distributed. We demonstrate that "large" Ca 2+ transients occur when presynaptic NMDA receptors are activated following Transmitter Release. Presynaptic NMDA receptor activation proves critical in producing facilitation of transmission at theta frequencies. Because large Ca 2+ transients "report" Transmitter Release, their frequency on a trial-by-trial basis can be used to estimate the probability of Release, p r . We use this novel estimator to show that p r increases following the induction of long-term potentiation.
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calcium stores in hippocampal synaptic boutons mediate short term plasticity store operated ca2 entry and spontaneous Transmitter Release
Neuron, 2001Co-Authors: Nigel J Emptage, Christopher M Reid, Alan FineAbstract:Evoked Transmitter Release depends upon calcium influx into synaptic boutons, but mechanisms regulating bouton calcium levels and spontaneous Transmitter Release are obscure. To understand these processes better, we monitored calcium transients in axons and presynaptic terminals of pyramidal neurons in hippocampal slice cultures. Action potentials reliably evoke calcium transients in axons and boutons. Calcium-induced calcium Release (CICR) from internal stores contributes to the transients in boutons and to paired-pulse facilitation of EPSPs. Store depletion activates store-operated calcium channels, influencing the frequency of spontaneous Transmitter Release. Boutons display spontaneous Ca2+ transients; blocking CICR reduces the frequency of these transients and of spontaneous miniature synaptic events. Thus, spontaneous Transmitter Release is largely calcium mediated, driven by Ca2+ Release from internal stores. Bouton store Release is important for short-term synaptic plasticity and may also contribute to long-term plasticity.
Richard Bertram - One of the best experts on this subject based on the ideXlab platform.
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A simple model of Transmitter Release and facilitation
Neural Computation, 1997Co-Authors: Richard BertramAbstract:We describe a model of synaptic Transmitter Release and presynaptic facilitation that is based on activation of Release sites by single Ca2+ C microdomains. Facilitation is due to Ca2+ that remains...
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A Simple Model of Transmitter Release and Facilitation
Neural Computation, 1997Co-Authors: Richard BertramAbstract:We describe a model of synaptic Transmitter Release and presynaptic facilitation that is based on activation of Release sites by single Ca2+ C microdomains. Facilitation is due to Ca2+ that remains bound to Release sites between impulses. This model is inherently stochastic, but deterministic equations can be derived for the mean Release. The number of equations required to describe the mean Release is small, so it is practical to use the model with models of neuronal electrical activity to investigate the effects of different input spike patterns on presynaptic facilitation. We use it in conjunction with a model of dopamine-secreting neurons of the basal ganglia to demonstrate that Transmitter Release is greater when the neuron bursts than when it spikes continuously, due to the greater facilitation generated by the bursting impulse pattern. Finally, a minimal form of the model is described that is coupled to simple models of postsynaptic receptors and passive membrane to compute the postsynaptic voltage response to a train of presynaptic stimuli. This form of the model is appropriate for neural network simulations.
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single domain bound calcium hypothesis of Transmitter Release and facilitation
Journal of Neurophysiology, 1996Co-Authors: Richard Bertram, Arthur Sherman, Elis F StanleyAbstract:1. We describe a model of Transmitter Release that is based on the finding that Release can be gated during the opening of individual Ca2+ channels, suggesting that the Release site can be activate...
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Single-domain/bound calcium hypothesis of Transmitter Release and facilitation
Journal of Neurophysiology, 1996Co-Authors: Richard Bertram, Arthur Sherman, Elis F StanleyAbstract:1. We describe a model of Transmitter Release that is based on the finding that Release can be gated during the opening of individual Ca2+ channels, suggesting that the Release site can be activate...
