The Experts below are selected from a list of 291 Experts worldwide ranked by ideXlab platform
Joshua R Sanes - One of the best experts on this subject based on the ideXlab platform.
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Agrin and Synaptic Laminin Are Required to Maintain Adult Neuromuscular Junctions
PLOS ONE, 2012Co-Authors: Melanie A. Samuel, Gregorio Valdez, Juan Carlos Tapia, Jeff W. Lichtman, Joshua R SanesAbstract:As synapses form and mature the Synaptic partners produce organizing molecules that regulate each other’s differentiation and ensure precise apposition of pre- and post-Synaptic specializations. At the skeletal neuromuscular junction (NMJ), these molecules include agrin, a nerve-derived organizer of postSynaptic differentiation, and Synaptic laminins, muscle-derived organizers of preSynaptic differentiation. Both become concentrated in the Synaptic Cleft as the NMJ develops and are retained in adulthood. Here, we used mutant mice to ask whether these organizers are also required for Synaptic maintenance. Deletion of agrin from a subset of adult motor neurons resulted in the loss of acetylcholine receptors and other components of the postSynaptic apparatus and Synaptic Cleft. Nerve terminals also atrophied and eventually withdrew from muscle fibers. On the other hand, mice lacking the preSynaptic organizer laminin-α4 retained most of the Synaptic Cleft components but exhibited Synaptic alterations reminiscent of those observed in aged animals. Although we detected no marked decrease in laminin or agrin levels at aged NMJs, we observed alterations in the distribution and organization of these Synaptic Cleft components suggesting that such changes could contribute to age-related Synaptic disassembly. Together, these results demonstrate that pre- and post-Synaptic organizers actively function to maintain the structure and function of adult NMJs.
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preSynaptic calcium channels and α3 integrins are complexed with Synaptic Cleft laminins cytoskeletal elements and active zone components
Journal of Neurochemistry, 2010Co-Authors: Steven S Carlson, Gregorio Valdez, Joshua R SanesAbstract:Pre- and postSynaptic elements of developing chemical synapses provide signals that organize differentiation of post- and preSynaptic structures, respectively. At the skeletal neuromuscular junction (NMJ), one prominent organizer of nerve terminals is laminin 421, a heterotrimer of α4, β2 and γ1 subunits. Laminin 421 is synthesized by muscle fibers and inserted into the basal lamina that occupies the Synaptic Cleft between nerve and muscle membranes. In mature muscle, the α4, and β2 subunits are highly concentrated in the Synaptic Cleft, along with the α5 subunit, while the γ1 subunit is also present in laminin 211 (α2/β1/γ1), a major component of the basal lamina that coats the remainder of the muscle fiber surface (Patton et al., 1997). In mice with a targeted mutation of the laminin β2 (lamb2) gene, preSynaptic maturation is dramatically disrupted and homozygous mutants die by the time of weaning (Nishimune et al., 2004; Noakes et al., 1995). Mice with a targeted mutation of the lama4 gene are outwardly healthy, but exhibit disruption of the precise apposition between preSynaptic neurotransmitter release sites (active zones) and postSynaptic invaginations (junctional folds) (Patton et al., 2001). The more severe phenotype of lamb2 compared to lama4 mutants presumably reflects the fact that laminin α4 is present only in laminin 421, whereas laminin β2 is present in three distinct laminin heterotrimers, laminins 221, 421 and 521 (Patton et al., 1997); thus, considerable laminin β2 is present at synapses in lama4 mutants. Together, these results suggest that the laminin α4 and β2 chains play distinct roles in the development on the NMJ, and raise the question of what receptors that they interact with on the nerve terminal. We recently showed that one receptor is the voltage-gated calcium channel (VGCC), which was already known to play a central role in neurotransmitter secretion: the VGCC opens in response to membrane depolarization, allowing the influx of calcium ions that trigger neurotransmitter release (Kandel, et al., 2000). In addition, interaction of the pore-forming VGCC α subunit (Cavα) with Synaptic laminin is required for formation and stabilization of active zones in early postnatal life (Nishimune, et al., 2004). It remains to be determined, however, how Cavα links to active zones. Here, we address this issue by isolating and identifying intracellular proteins associated with the VGCC in nerve terminals. To this end, we used the Torpedo electric organ. This tissue contains nicotinic cholinergic synapses that are similar to NMJs in many respects, but are far larger and more abundant. Synapses in the electric organ occupy most of one surface of slender electroplaque cells, whereas NMJs occupy only ~0.1% of the surface of large, cylindrical muscle fibers. Consequently, the concentration of Synaptic molecules such as postSynaptic acetylcholine receptors (AChRs) is >100X higher in electric than in muscle. Indeed, many key components of NMJs were originally purified from electric organ; these include AChRs, acetylcholinesterase, agrin, rapsyn, and several Synaptic vesicle proteins (Hall, 1992). Likewise, use of electric organ allowed us to isolate a VGCC-associated complex in amounts sufficient to identify several of its major components by mass spectroscopy. Our results suggest that the preSynaptic Cavα anchors a complex of cytoskeletal and active zone-associated proteins. Our previous studies demonstrated that the preSynaptic Cavα subunit binds directly to laminin β2 in the laminin 421 heterotrimer (Nishimune et al., 2004). As noted above, however, the distinct phenotypes of targeted lama4 and lamb2 mutants suggest distinct roles for laminins β2 and α4, implying the existence of a laminin α4 binding protein on nerve terminals. In the second part of this study, we used electric organ to seek such a protein. We focused on integrins, which are the best-studied laminin receptors in both neural and non-neural tissues, and are known to interact with laminin α subunits (Suzuki, et al., 2005). Among several integrin subunits present at the NMJ (Martin et al., 1995), integrin α3 was of particular interest because it is concentrated near active zones in frog nerve terminals (Cohen, et al., 2000), and might explain the mislocalization of active zones in mouse lama4 mutants (Patton, et al., 2001). Indeed, we show that laminin 421 interacts with integrin α3 and that this integrin, in turn, interacts intracellularly with at least one component of the VGCC-associated complex. Thus, our results define two paths by which laminin 421 in the Synaptic Cleft can provide developmentally important signals to motor axons.
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Synaptic laminin prevents glial entry into the Synaptic Cleft
Nature, 1998Co-Authors: Bruce L Patton, Arlene Y Chiu, Joshua R SanesAbstract:PreSynaptic and postSynaptic membranes directly oppose each other at chemical synapses, minimizing the delay in transmitting information across the Synaptic Cleft. ExtraSynaptic neuronal surfaces, in contrast, are almost entirely covered by processes from glial cells1. The exclusion of glial cells from the Synaptic Cleft, and the long-term stability of synapses, presumably result in large part from the tight adhesion between preSynaptic and postSynaptic elements2,3. Here we show that there is another requirement for Synaptic maintenance: glial cells of the skeletal neuromuscular synapse, Schwann cells, are actively inhibited from entering the Synaptic Cleft between the motor nerve terminal and the muscle fibre. One inhibitory component is laminin 11, a heterotrimeric glycoprotein that is concentrated in the Synaptic Cleft4. Regulation of an inhibitory interaction between glial cells and Synaptic Cleft components may contribute to Synaptic rearrangements, and loss of this inhibition may underlie the loss of synapses that results from injury to the postSynaptic cell5,6,7,8,9,10,11,12.
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the Synaptic Cleft of the neuromuscular junction
Seminars in Developmental Biology, 1995Co-Authors: Joshua R SanesAbstract:The basal lamina (BL) that ensheaths each skeletal musclefiber passes between the pre- and postSynaptic membranes at the neuromuscular junction. A consequence of this arrangement is that BL comprises much of the Synaptic Cleft material of this synapse. Some of the cues that guide differentiation and regeneration of both pre- and postSynaptic components are stably associated with Synaptic BL. This paper reviews studies aimed at identifying these components and learning how they work.
Leonid P Savtchenko - One of the best experts on this subject based on the ideXlab platform.
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nanoscale diffusion in the Synaptic Cleft and beyond measured with time resolved fluorescence anisotropy imaging
Scientific Reports, 2017Co-Authors: Kaiyu Zheng, Leonid P Savtchenko, Thomas P Jensen, James A Levitt, Klaus Suhling, Dmitri A RusakovAbstract:Neural activity relies on molecular diffusion within nanoscopic spaces outside and inside nerve cells, such as Synaptic Clefts or dendritic spines. Measuring diffusion on this small scale in situ has not hitherto been possible, yet this knowledge is critical for understanding the dynamics of molecular events and electric currents that shape physiological signals throughout the brain. Here we advance time-resolved fluorescence anisotropy imaging combined with two-photon excitation microscopy to map nanoscale diffusivity in ex vivo brain slices. We find that in the brain interstitial gaps small molecules move on average ~30% slower than in a free medium whereas inside neuronal dendrites this retardation is ~70%. In the Synaptic Cleft free nanodiffusion is decelerated by ~46%. These quantities provide previously unattainable basic constrains for the receptor actions of released neurotransmitters, the electrical conductance of the brain interstitial space and the limiting rate of molecular interactions or conformational changes in the Synaptic microenvironment.
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the optimal height of the Synaptic Cleft
Proceedings of the National Academy of Sciences of the United States of America, 2007Co-Authors: Leonid P Savtchenko, Dmitri A RusakovAbstract:Signal integration in the brain is determined by the size and kinetics of rapid Synaptic responses. The latter, in turn, depends on the concentration profile of neurotransmitter in the Synaptic Cleft. According to a traditional view, narrower Clefts should correspond to higher intraCleft concentrations of neurotransmitter, and therefore to the enhanced activation of Synaptic receptors. Here, we argue that narrowing the Cleft also increases electrical resistance of the intraCleft medium and therefore reduces local receptor currents. We employ detailed theoretical analyses and Monte Carlo simulations to propose that these two contrasting phenomena result in a relatively narrow range of Cleft heights at which the Synaptic receptor current reaches its maximum. Over a physiological range of Synaptic parameters, the “optimum” height falls between ≈12 and 20 nm. This range is consistent with the structure of central synapses reported by electron microscopy. Therefore, our results suggest that a simple fundamental principle may underlie the Synaptic Cleft architecture: to maximize Synaptic strength.
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glutamate escape from a tortuous Synaptic Cleft of the hippocampal mossy fibre synapse
Neurochemistry International, 2004Co-Authors: Leonid P Savtchenko, Dmitri A RusakovAbstract:Abstract The time course of neurotransmitter in the Synaptic Cleft contributes substantially to the fast kinetics of Synaptic signalling. Hippocampal mossy fibres (MFs), a well-characterised excitatory pathway from dentate granule cells to the hippocampus proper, form large glutamatergic synapses at branched spiny structures in CA3 pyramidal cell dendrites. To what extent transmission at these synapses is affected by retarded glutamate clearance from the large tortuous Synaptic Cleft is not known. Here, we propose a simple geometrical approximation representing the ‘typical’ geometry of thorny excrescences that form the tortuous Cleft interface at a MF synapse. We then employ Monte Carlo simulations to monitor movements of 3000 individual glutamate molecules released within the Cleft. The results predict that, in the absence of neuronal glutamate transporters, it should take approximately 10 ms for 50% and 60–70 ms for 90% of glutamate molecules to escape the MF synapse.
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effect of voltage drop within the Synaptic Cleft on the current and voltage generated at a single synapse
Biophysical Journal, 2000Co-Authors: Leonid P Savtchenko, Sergey N Antropov, S M KorogodAbstract:In a model of a single synapse with a circular contact zone and a single concentric zone containing receptor-gated channels, we studied the dependence of the Synaptic current on the Synaptic Cleft width and on the relative size of the receptor zone. During Synaptic excitation, the extracellular current entered the Cleft and flowed into the postSynaptic cell through receptor channels distributed homogeneously over the receptor zone. The membrane potential and channel currents were smaller toward the Cleft center if compared to the Cleft edges. This radial gradient was due to the voltage drop produced by the Synaptic current on the Cleft resistance. The total Synaptic current conducted by the same number of open channels was sensitive to changes in the receptor zone radius and the Cleft width. We conclude that Synaptic geometry may affect Synaptic currents by defining the volume resistor of the Cleft. The in-series connection of the resistances of the intraCleft medium and the receptor channels plays the role of the Synaptic voltage divider. This voltage dividing effect should be taken into account when the conductance of single channels or Synaptic contacts is estimated from experimental measurements of voltage-current relationships.
Donatella Contini - One of the best experts on this subject based on the ideXlab platform.
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Synaptic Cleft microenvironment influences potassium permeation and Synaptic transmission in hair cells surrounded by calyx afferents in the turtle
The Journal of Physiology, 2020Co-Authors: Donatella ContiniAbstract:Key points In central regions of vestibular semicircular canal epithelia, the [K+ ] in the Synaptic Cleft ([K+ ]c ) contributes to setting the hair cell and afferent membrane potentials; the potassium efflux from type I hair cells results from the interdependent gating of three conductances. Elevation of [K+ ]c occurs through a calcium-activated potassium conductance, GBK , and a low-voltage-activating delayed rectifier, GK(LV) , that activates upon elevation of [K+ ]c . Calcium influx that enables quantal transmission also activates IBK , an effect that can be blocked internally by BAPTA, and externally by a CaV 1.3 antagonist or iberiotoxin. Elevation of [K+ ]c or chelation of [Ca2+ ]c linearizes the GK(LV) steady-state I-V curve, suggesting that the outward rectification observed for GK(LV) may result largely from a potassium-sensitive relief of Ca2+ inactivation of the channel pore selectivity filter. Potassium sensitivity of hair cell and afferent conductances allows three modes of transmission: quantal, ion accumulation and resistive coupling to be multiplexed across the synapse. Abstract In the vertebrate nervous system, ions accumulate in diffusion-limited Synaptic Clefts during ongoing activity. Such accumulation can be demonstrated at large appositions such as the hair cell-calyx afferent synapses present in central regions of the turtle vestibular semicircular canal epithelia. Type I hair cells influence discharge rates in their calyx afferents by modulating the potassium concentration in the Synaptic Cleft, [K+ ]c , which regulates potassium-sensitive conductances in both hair cell and afferent. Dual recordings from Synaptic pairs have demonstrated that, despite a decreased driving force due to potassium accumulation, hair cell depolarization elicits sustained outward currents in the hair cell, and a maintained inward current in the afferent. We used kinetic and pharmacological dissection of the hair cell conductances to understand the interdependence of channel gating and permeation in the context of such restricted extracellular spaces. Hair cell depolarization leads to calcium influx and activation of a large calcium-activated potassium conductance, GBK , that can be blocked by agents that disrupt calcium influx or buffer the elevation of [Ca2+ ]i , as well as by the specific KCa 1.1 blocker iberiotoxin. Efflux of K+ through GBK can rapidly elevate [K+ ]c , which speeds the activation and slows the inactivation and deactivation of a second potassium conductance, GK(LV) . Elevation of [K+ ]c or chelation of [Ca2+ ]c linearizes the GK(LV) steady-state I-V curve, consistent with a K+ -dependent relief of Ca2+ inactivation of GK(LV) . As a result, this potassium-sensitive hair cell conductance pairs with the potassium-sensitive hyperpolarization-activated cyclic nucleotide-gated channel (HCN) conductance in the afferent and creates resistive coupling at the Synaptic Cleft.
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Synaptic Cleft microenvironment influences potassium permeation and Synaptic transmission in hair cells surrounded by calyx afferents in the turtle
The Journal of Physiology, 2019Co-Authors: Donatella ContiniAbstract:KEY POINTS: In central regions of vestibular semicircular canal epithelia, the [K+ ] in the Synaptic Cleft ([K+ ]c ) contributes to setting the hair cell and afferent membrane potentials; the potassium efflux from type I hair cells results from the interdependent gating of three conductances. Elevation of [K+ ]c occurs through a calcium-activated potassium conductance, GBK , and a low-voltage-activating delayed rectifier, GK(LV) , that activates upon elevation of [K+ ]c . Calcium influx that enables quantal transmission also activates IBK , an effect that can be blocked internally by BAPTA, and externally by a CaV 1.3 antagonist or iberiotoxin. Elevation of [K+ ]c or chelation of [Ca2+ ]c linearizes the GK(LV) steady-state I-V curve, suggesting that the outward rectification observed for GK(LV) may result largely from a potassium-sensitive relief of Ca2+ inactivation of the channel pore selectivity filter. Potassium sensitivity of hair cell and afferent conductances allows three modes of transmission: quantal, ion accumulation and resistive coupling to be multiplexed across the synapse. ABSTRACT: In the vertebrate nervous system, ions accumulate in diffusion-limited Synaptic Clefts during ongoing activity. Such accumulation can be demonstrated at large appositions such as the hair cell-calyx afferent synapses present in central regions of the turtle vestibular semicircular canal epithelia. Type I hair cells influence discharge rates in their calyx afferents by modulating the potassium concentration in the Synaptic Cleft, [K+ ]c , which regulates potassium-sensitive conductances in both hair cell and afferent. Dual recordings from Synaptic pairs have demonstrated that, despite a decreased driving force due to potassium accumulation, hair cell depolarization elicits sustained outward currents in the hair cell, and a maintained inward current in the afferent. We used kinetic and pharmacological dissection of the hair cell conductances to understand the interdependence of channel gating and permeation in the context of such restricted extracellular spaces. Hair cell depolarization leads to calcium influx and activation of a large calcium-activated potassium conductance, GBK , that can be blocked by agents that disrupt calcium influx or buffer the elevation of [Ca2+ ]i , as well as by the specific KCa 1.1 blocker iberiotoxin. Efflux of K+ through GBK can rapidly elevate [K+ ]c , which speeds the activation and slows the inactivation and deactivation of a second potassium conductance, GK(LV) . Elevation of [K+ ]c or chelation of [Ca2+ ]c linearizes the GK(LV) steady-state I-V curve, consistent with a K+ -dependent relief of Ca2+ inactivation of GK(LV) . As a result, this potassium-sensitive hair cell conductance pairs with the potassium-sensitive hyperpolarization-activated cyclic nucleotide-gated channel (HCN) conductance in the afferent and creates resistive coupling at the Synaptic Cleft.
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accumulation of k in the Synaptic Cleft modulates activity by influencing both vestibular hair cell and calyx afferent in the turtle
The Journal of Physiology, 2017Co-Authors: Donatella Contini, Steven D PriceAbstract:Key points In the Synaptic Cleft between type I hair cells and calyceal afferents, K+ ions accumulate as a function of activity, dynamically altering the driving force and permeation through ion channels facing the Synaptic Cleft. High-fidelity Synaptic transmission is possible due to large conductances that minimize hair cell and afferent time constants in the presence of significant membrane capacitance. Elevated potassium maintains hair cells near a potential where transduction currents are sufficient to depolarize them to voltages necessary for calcium influx and Synaptic vesicle fusion. Elevated potassium depolarizes the postSynaptic afferent by altering ion permeation through hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, and contributes to depolarizing the afferent to potentials where a single EPSP (quantum) can generate an action potential. With increased stimulation, hair cell depolarization increases the frequency of quanta released, elevates [K+]Cleft and depolarizes the afferent to potentials at which smaller and smaller EPSPs would be sufficient to trigger APs. Abstract Fast neurotransmitters act in conjunction with slower modulatory effectors that accumulate in restricted Synaptic spaces found at giant synapses such as the calyceal endings in the auditory and vestibular systems. Here, we used dual patch-clamp recordings from turtle vestibular hair cells and their afferent neurons to show that potassium ions accumulating in the Synaptic Cleft modulated membrane potentials and extended the range of information transfer. High-fidelity Synaptic transmission was possible due to large conductances that minimized hair cell and afferent time constants in the presence of significant membrane capacitance. Increased potassium concentration in the Cleft maintained the hair cell near potentials that promoted the influx of calcium necessary for Synaptic vesicle fusion. The elevated potassium concentration also depolarized the postSynaptic neuron by altering ion permeation through hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. This depolarization enabled the afferent to reliably generate action potentials evoked by single AMPA-dependent EPSPs. Depolarization of the postSynaptic afferent could also elevate potassium in the Synaptic Cleft, and would depolarize other hair cells enveloped by the same neuritic process increasing the fidelity of neurotransmission at those synapses as well. Collectively, these data demonstrate that neuronal activity gives rise to potassium accumulation, and suggest that potassium ion action on HCN channels can modulate neurotransmission, preserving the fidelity of high-speed Synaptic transmission by dynamically shifting the resting potentials of both preSynaptic and postSynaptic cells.
M I Glavinovic - One of the best experts on this subject based on the ideXlab platform.
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molecular dynamics simulations of glutamate diffusion in membrane bound Synaptic Cleft
Biophysical Journal, 2009Co-Authors: Sean Cory, M I GlavinovicAbstract:Diffusion of the transmitter in the Synaptic Cleft critically influences the amplitude and the time course of quantal events and thus strongly affects the Synaptic efficacy. However, the value of the diffusion constant remains speculative. In a confined space diffusion of ions and molecules should in general be slower as they do not move exclusively in their solvated space, but also interact with walls of the Synaptic Cleft, which are fixed. Indeed molecular dynamics simulations demonstrated that the diffusion of glutamate- (and water) in the Cleft formed by two single wall carbon sheets is slower, but only for Clefts narrower than those of synapses in the central nervous system. To provide a more realistic assessment we simulate the diffusion of glutamate- (and water) in the Cleft formed by lipid bilayers. The glutamate- molecules are layered near the bilayer, and partly interdigitate with it, and the interfacial glutamate--bilayer interactions modulate the speed of glutamate diffusion. Water molecules also interdigitate with the bilayer, which masks the layering near the bilayer. The diffusion of glutamate and water in the Cleft bound by lipid bilayers is influenced by factors similar to those observed in the Cleft bound by carbon sheets - Cleft separation and charge on the atoms of the wall. Finally, the movement of atoms of the lipid bilayer (evaluated by ‘freezing’ the positions of all atoms of the bilayer) also affects the diffusion of glutamate- and water. In conclusion this study provides a more realistic evaluation of spatial distribution and diffusion of glutamate- and water in the Synaptic Cleft and how they are influenced by the interactions with the membrane.
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molecular dynamics simulations of glutamate diffusion in Synaptic Cleft
Critical Reviews in Neurobiology, 2006Co-Authors: Sean Cory, M I GlavinovicAbstract:Diffusion of transmitters in the Synaptic Cleft critically infl uences Synaptic effi - cacy by affecting both the amplitude and the time course of quantal events, but the value of the diffusion constant is speculative. In this study, we use molecular dynamics simulations to de- termine how the spatial confi nement and membrane charges affect the diffusion constants of glutamate- and water as well as general properties of their diffusion. The Synaptic Cleft is rep- resented as the space enclosed by two single-wall carbon sheets. Both water and especially glu- tamate- are concentrated near the pore wall, where the concentration of glutamate can reach 30-50 times the mean value and the concentration of water can reach 2-8 times the mean value. Such spatial profi les of glutamate- contradict the classical notions of diffusion on which both continuous and Monte Carlo simulations are built. The layering of glutamate- and water mol- ecules suggests that the interfacial glutamate-Cleft wall (or water-Cleft wall) interactions may critically regulate their diffusion in the Cleft. Indeed, the effective longitudinal diffusion con- stant of glutamate- is steeply dependent on the Cleft width, but only when the Cleft is very nar- row (< 5 nm). Therefore, even for a Cleft as narrow as at the glutamatergic synapse in the cen- tral nervous system, the effective diffusion constant of glutamate- will not be much lower than free diffusion in the bulk solution due to confi nement. The effective diffusion constant of wa- ter is considerably less sensitive to Cleft width over the same range of Cleft widths than is glu- tamate, but is also higher than that of glutamate. Finally, the layering of glutamate- and water and their effective diffusion constants are largely independent of how the Cleft wall is charged. In conclusion, in the confi ned space of the Synaptic Cleft, glutamate- is layered near the wall. Consequently, its diffusion constant becomes dependent on the Cleft width. However, the dif- fusion of glutamate- is slower than its free diffusion in water only if the Cleft is very narrow. If the width of the Cleft is consistent with that determined by morphometric studies in the central nervous system, glutamate- diffusion should not be slowed by confi nement and is thus likely to be similar to that in free solution.
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monte carlo simulation of vesicular release spatiotemporal distribution of glutamate in Synaptic Cleft and generation of postSynaptic currents
Pflügers Archiv: European Journal of Physiology, 1999Co-Authors: M I GlavinovicAbstract:The release of vesicular glutamate, spatiotemporal changes in glutamate concentration in the Synaptic Cleft and the subsequent generation of fast excitatory postSynaptic currents at a hippocampal synapse were modeled using the Monte Carlo method. It is assumed that glutamate is released from a spherical vesicle through a cylindrical fusion pore into the Synaptic Cleft and that S-α-amino-3-hydroxy -5-methyl-4-isoxazolepropionic acid (AMPA) receptors are uniformly distributed postSynaptically. The time course of change in vesicular concentration can be described by a single exponential, but a slow tail is also observed though only following the release of most of the glutamate. The time constant of decay increases with vesicular size and a lower diffusion constant, and is independent of the initial concentration, becoming markedly shorter for wider fusion pores. The Cleft concentration at the fusion pore mouth is not negligible compared to vesicular concentration, especially for wider fusion pores. Lateral equilibration of glutamate is rapid, and within ≈50 µs all AMPA receptors on average see the same concentration of glutamate. Nevertheless the single-channel current and the number of channels estimated from mean-variance plots are unreliable and different when estimated from rise- and decay-current segments. Greater saturation of AMPA receptor channels provides higher but not more accurate estimates. Two factors contribute to the variability of postSynaptic currents and render the mean-variance nonstationary analysis unreliable, even when all receptors see on average the same glutamate concentration. Firstly, the variability of the instantaneous Cleft concentration of glutamate, unlike the mean concentration, first rapidly decreases before slowly increasing; the variability is greater for fewer molecules in the Cleft and is spatially nonuniform. Secondly, the efficacy with which glutamate produces a response changes with time. Understanding the factors that determine the time course of vesicular content release as well as the spatiotemporal changes of glutamate concentration in the Cleft is crucial for understanding the mechanism that generates postSynaptic currents.
Angus R Silver - One of the best experts on this subject based on the ideXlab platform.
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modulation of glutamate mobility reveals the mechanism underlying slow rising ampar epscs and the diffusion coefficient in the Synaptic Cleft
Neuron, 2004Co-Authors: Thomas A Nielsen, David A Digregorio, Angus R SilverAbstract:Abstract Fast- and slow-rising AMPA receptor-mediated EPSCs occur at central synapses. Fast-rising EPSCs are thought to be mediated by rapid local release of glutamate. However, two controversial mechanisms have been proposed to underlie slow-rising EPSCs: prolonged local release of transmitter via a fusion pore, and spillover of transmitter released rapidly from distant sites. We have investigated the mechanism underlying slow-rising EPSCs and the diffusion coefficient of glutamate in the Synaptic Cleft (D glut ) at cerebellar mossy fiber-granule cell synapses using a combination of diffusion modeling and patch-clamp recording. Simulations show that modulating D glut has different effects on the peak amplitudes and time courses of EPSCs mediated by these two mechanisms. Slowing diffusion with the macromolecule dextran slowed slow-rising EPSCs and had little effect on their amplitude, indicating that glutamate spillover underlies these currents. Our results also suggest that under control conditions D glut is approximately 3-fold lower than in free solution.
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glutamate uptake from the Synaptic Cleft does not shape the decay of the non nmda component of the Synaptic current
Neuron, 1993Co-Authors: Monique Sarantis, Laura Ballerini, Barbara Miller, Angus R Silver, Malcolm Edwards, David AttwellAbstract:To study the role of glutamate uptake at central glutamatergic synapses, we used the uptake blocker L-transpyrrolidine-2,4-dicarboxylate (PDC). The effects of PDC on the glutamate uptake current in salamander retinal glia indicated that PDC competes with glutamate for transport on the uptake carrier and that 300 muM PDC should significantly reduce the uptake of glutamate during the Synaptic current. In isolated rat hippocampal neurons, 300 muM PDC did not affect non-N-methyl-D-aspartate (NMDA) receptor currents, but reduced NMDA receptor currents by 30%. In hippocampal and cerebellar slices, whereas 300 muM PDC reduced the NMDA component of excitatory Synaptic currents by 50%, it reduced the non-NMDA component only slightly with no change in its decay time constant. Thus, the decay rate of the non-NMDA component is not set by the rate of glutamate uptake from the Synaptic Cleft into the preSynaptic terminal.