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.     

L-trans-PDC enhances hippocampal neuronal activity by stimulating glial glutamate release independently of blocking transporters

The glutamate transporter inhibitor, L-trans-pyrrolidine-2,4-dicarboxylic acid (PDC) reversibly enhanced hippocampal neuronal activity in the rat and mouse dentate gyrus. The PDC action was still found in mice lacking the glial glutamate transporter GLT-1. PDC did not influence the rate of spontaneous miniature excitatory postsynaptic currents and spontaneous inhibitory postsynaptic currents, ionotropic glutamate receptor currents, or GABA-evoked currents in cultured rat hippocampal neurons. PDC increased glutamate released from cultured hippocampal astrocytes from normal rats, normal mice, and GLT-1 knock-out mice, that is not inhibited by deleting extracellular Na(+), while the drug had no effect on the release from cultured rat hippocampal neurons. The results of the present study thus suggest that PDC stimulates glial glutamate release by a mechanism independent of inhibiting glutamate transporters, which perhaps causes an increase in synaptic glutamate concentrations, in part responsible for the enhancement in hippocampal neuronal activity.     

Presynaptic inhibition of miniature excitatory synaptic currents by baclofen and adenosine in the hippocampus.

Presynaptic inhibition of neurotransmitter release is thought to be mediated by a reduction of axon terminal Ca2+ current. We have compared the actions of several known inhibitors of evoked glutamate release with the actions of the Ca2+ channel antagonist Cd2+ on action potential-independent synaptic currents recorded from CA3 neurons in hippocampal slice cultures. Baclofen and adenosine decreased the frequency of miniature excitatory postsynaptic currents (mEPSCs) without affecting the distribution of their amplitudes. Cd2+ blocked evoked synaptic transmission, but had no effect on the frequency or amplitude of either mEPSCs or inhibitory postsynaptic currents (IPSCs). Inhibition of presynaptic Ca2+ current therefore appears not to be required for the inhibition of glutamate release by adenosine and baclofen. Baclofen had no effect on the frequency of miniature IPSCs, indicating that gamma-aminobutyric acid B-type receptors exert distinct presynaptic actions at excitatory and inhibitory synapses.     

Simultaneous Release of Multiple Vesicles from Rods Involves Synaptic Ribbons and Syntaxin 3B

First proposed as a specialized mode of release at sensory neurons possessing ribbon synapses, multivesicular release has since been described throughout the central nervous system. Many aspects of multivesicular release remain poorly understood. We explored mechanisms underlying simultaneous multivesicular release at ribbon synapses in salamander retinal rod photoreceptors. We assessed spontaneous release presynaptically by recording glutamate transporter anion currents (I_A(glu)) in rods. Spontaneous I_A(glu) events were correlated in amplitude and kinetics with simultaneously measured miniature excitatory postsynaptic currents in horizontal cells. Both measures indicated that a significant fraction of events is multiquantal, with an analysis of I_A(glu) revealing that multivesicular release constitutes ∼30% of spontaneous release events. I_A(glu) charge transfer increased linearly with event amplitude showing that larger events involve greater glutamate release. The kinetics of large and small I_A(glu) events were identical as were rise times of large and small miniature excitatory postsynaptic currents, indicating that the release of multiple vesicles during large events is highly synchronized. Effects of exogenous Ca²⁺ buffers suggested that multiquantal, but not uniquantal, release occurs preferentially near Ca²⁺ channels clustered beneath synaptic ribbons. Photoinactivation of ribbons reduced the frequency of spontaneous multiquantal events without affecting uniquantal release frequency, showing that spontaneous multiquantal release requires functional ribbons. Although both occur at ribbon-style active zones, the absence of cross-depletion indicates that evoked and spontaneous multiquantal release from ribbons involve different vesicle pools. Introducing an inhibitory peptide into rods to interfere with the SNARE protein, syntaxin 3B, selectively reduced multiquantal event frequency. These results support the hypothesis that simultaneous multiquantal release from rods arises from homotypic fusion among neighboring vesicles on ribbons and involves syntaxin 3B.     

Fast removal of synaptic glutamate by postsynaptic transporters

Glutamate transporters are believed to remove glutamate from the synaptic cleft only slowly because they cycle slowly. However, we show that when glutamate binds to postsynaptic transporters at the cerebellar climbing fiber synapse, it evokes a conformation change and inward current that reflect glutamate removal from the synaptic cleft within a few milliseconds, a time scale much faster than the overall cycle time. Contrary to present models, glutamate removal does not require binding of an extracellular proton, and the time course of transporter anion conductance activation differs from that of glutamate removal. The charge movement associated with glutamate removal is consistent with the majority of synaptically released glutamate being removed from the synaptic cleft by postsynaptic transporters.     

Neutralizing aspartate 83 modifies substrate translocation of excitatory amino acid transporter 3 (EAAT3) glutamate transporters

Excitatory amino acid transporters (EAATs) terminate glutamatergic synaptic transmission by removing glutamate from the synaptic cleft into neuronal and glial cells. EAATs are not only secondary active glutamate transporters but also function as anion channels. Gating of EAAT anion channels is tightly coupled to transitions within the glutamate uptake cycle, resulting in Na(+)- and glutamate-dependent anion currents. A point mutation neutralizing a conserved aspartic acid within the intracellular loop close to the end of transmembrane domain 2 was recently shown to modify the substrate dependence of EAAT anion currents. To distinguish whether this mutation affects transitions within the uptake cycle or directly modifies the opening/closing of the anion channel, we used voltage clamp fluorometry. Using three different sites for fluorophore attachment, V120C, M205C, and A430C, we observed time-, voltage-, and substrate-dependent alterations of EAAT3 fluorescence intensities. The voltage and substrate dependence of fluorescence intensities can be described by a 15-state model of the transport cycle in which several states are connected to branching anion channel states. D83A-mediated changes of fluorescence intensities, anion currents, and secondary active transport can be explained by exclusive modifications of substrate translocation rates. In contrast, sole modification of anion channel opening and closing is insufficient to account for all experimental data. We conclude that D83A has direct effects on the glutamate transport cycle and that these effects result in changed anion channel function.     

Removal of extracellular chloride suppresses transmitter release from photoreceptor terminals in the mudpuppy retina

Removal of extracellular Cl- has been shown to suppress light-evoked voltage responses of ON bipolar and horizontal cells, but not photoreceptors or OFF bipolar cells, in the amphibian retina. A substantial amount of experimental evidence has demonstrated that the photoreceptor transmitter, L-glutamate, activates cation, not Cl-, channels in these cells. The mechanism for Cl-free effects was therefore reexamined in a superfused retinal slice preparation from the mudpuppy (Necturus maculosus) using whole-cell voltage and current clamp techniques. In a Cl-free medium, light-evoked currents were maintained in rod and cone photoreceptors but suppressed in horizontal, ON bipolar, and OFF bipolar cells. Changes in input resistance and dark current in bipolar and horizontal cells were consistent with the hypothesis that removal of Cl- suppresses tonic glutamate release from photoreceptors. The persistence of light-evoked voltage responses in OFF bipolar cells, despite the suppression of light-evoked currents, is due to a compensatory increase in input resistance. Focal application of hyperosmotic sucrose to photoreceptor terminals produced currents in bipolar and horizontal cells arising from two sources: (a) evoked glutamate release and (b) direct actions of the hyperosmotic solution on postsynaptic neurons. The inward currents resulting from osmotically evoked release of glutamate in OFF bipolar and horizontal cells were suppressed in a Cl-free medium. For ON bipolar cells, both the direct and evoked components of the hyperosmotic response resulted in outward currents and were thus difficult to separate. However, in some cells, removal of extracellular Cl- suppressed the outward current consistent with a suppression of presynaptic glutamate release. The results of this study suggest that removal of extracellular Cl- suppresses glutamate release from photoreceptor terminals. Thus, it is possible that control of [Cl-] in and around photoreceptors may regulate glutamate release from these cells.     

Kinetics of Synaptic Transmission at Ribbon Synapses of Rods and Cones

The ribbon synapse is a specialized structure that allows photoreceptors to sustain the continuous release of vesicles for hours upon hours and years upon years but also respond rapidly to momentary changes in illumination. Light responses of cones are faster than those of rods and, mirroring this difference, synaptic transmission from cones is also faster than transmission from rods. This review evaluates the various factors that regulate synaptic kinetics and contribute to kinetic differences between rod and cone synapses. Presynaptically, the release of glutamate-laden synaptic vesicles is regulated by properties of the synaptic proteins involved in exocytosis, influx of calcium through calcium channels, calcium release from intracellular stores, diffusion of calcium to the release site, calcium buffering, and extrusion of calcium from the cytoplasm. The rate of vesicle replenishment also limits the ability of the synapse to follow changes in release. Post-synaptic factors include properties of glutamate receptors, dynamics of glutamate diffusion through the cleft, and glutamate uptake by glutamate transporters. Thus, multiple synaptic mechanisms help to shape the responses of second-order horizontal and bipolar cells.     

Spillover of glutamate onto synaptic AMPA receptors enhances fast transmission at a cerebellar synapse

Diffusion of glutamate from the synaptic cleft can activate high-affinity receptors, but is not thought to contribute to fast AMPA receptor-mediated transmission. Here, we show that single AMPA receptor EPSCs at the cerebellar mossy fiber-granule cell connection are mediated by both direct release of glutamate and rapid diffusion of glutamate from neighboring synapses. Immunogold localization revealed that AMPA receptors are located exclusively in postsynaptic densities, indicating that spillover of glutamate occurs between synaptic contacts. Spillover currents contributed half the synaptic charge and exhibited little trial-to-trial variability. We propose that spillover of glutamate improves transmission efficacy by both increasing the amplitude and duration of the EPSP and reducing fluctuations arising from the probabilistic nature of transmitter release.     

Clearance of glutamate inside the synapse and beyond.

The heated debate over the level of postsynaptic receptor occupancy by transmitter has not been extinguished - indeed, new evidence is fanning the flames. Recent experiments using two-photon microscopy suggest that the concentration of glutamate in the synaptic cleft does not attain levels previously suggested. In contrast, recordings from glial cells and studies of extrasynaptic receptor activation indicate that significant quantities of glutamate escape from the cleft following exocytosis. Determining the amount of glutamate efflux from the synaptic cleft and the distance it diffuses is critical to issues of synaptic specificity and the induction of synaptic plasticity.     

Analytical description of the activation of multi-state receptors by continuous neurotransmitter signals at brain synapses.

Chemical synaptic transmission is a fundamental component of interneuronal communications in the central nervous system (CNS). Discharge of a presynaptic vesicle containing a few thousand molecules (a quantum) of neurotransmitter into the synaptic cleft generates a transmitter concentration signal that drives postsynaptic ion-channel receptors. These receptors exhibit multiple states, with state transition kinetics dependent on neurotransmitter concentration. Here, a novel and simple analytical approach for describing gating of multi-state receptors by signals with complex continuous time courses is used to describe the generation of glutamate-mediated quantal postsynaptic responses at brain synapses. The neurotransmitter signal, experienced by multi-state N-methyl-D-aspartate (NMDA)- and L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-type glutamate receptors at specific points in a synaptic cleft, is approximated by a series of step functions of different intensity and duration and used to drive a Markovian, multi-state kinetic scheme that describes receptor gating. Occupancy vectors at any point in time can be computed interatively from the occupancy vectors at the times of steps in transmitter concentration. Multi-state kinetic schemes for both the low-affinity AMPA subtype of glutamate receptor and for the high-affinity NMDA subtype are considered, and expected NMDA and AMPA components of synaptic currents are calculated. The amplitude of quantal responses mediated by postsynaptic receptor clusters having specific spatial distributions relative to foci of quantal neurotransmitter release is then calculated and related to the displacement between the center of the postsynaptic receptor cluster and the focus of synaptic vesicle discharge. Using this approach we show that the spatial relation between the focus of release and the center of the postsynaptic receptor cluster affects synaptic efficacy. We also show how variation in this relation contributes to variation in synaptic current amplitudes.     

Ectopic release of synaptic vesicles

Exocytosis of synaptic vesicles is generally assumed to occur only at ultrastructurally defined presynaptic active zones. If release is restricted to these sites, receptors not located within the synaptic cleft must be activated by transmitter that diffuses out of the cleft or not be activated at all. Here we report that AMPA receptor-mediated quantal events resulting from climbing fiber release are observed in Bergmann glial cells in the cerebellar cortex. These quantal events are not coincident with quanta recorded in neighboring Purkinje cells which receive input from the same climbing fiber. As Bergmann glial membranes are excluded from the synaptic cleft, we propose that exocytosis can occur from climbing fiber release sites located directly across from Bergmann glial membranes. Such ectopic release may account for the majority of the Bergmann glial AMPA response evoked by climbing fiber stimulation.     

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.     

Control of Cleft Glutamate Concentration and Glutamate Spill-Out by Perisynaptic Glia: Uptake and Diffusion Barriers

Most glutamatergic synapses in the mammalian central nervous system are covered by thin astroglial processes that exert a dual action on synaptically released glutamate: they form physical barriers that oppose diffusion and they carry specific transporters that remove glutamate from the extracellular space. The present study was undertaken to investigate the dual action of glia by means of computer simulation. A realistic synapse model based on electron microscope data and Monte Carlo algorithms were used for this purpose. Results show (1) that physical obstacles formed by glial processes delay glutamate exit from the cleft and (2) that this effect is efficiently counteracted by glutamate uptake. Thus, depending on transporter densities, the presence of perisynaptic glia may result in increased or decreased glutamate transient in the synaptic cleft. Changes in temporal profiles of cleft glutamate concentration induced by glia differentially impact the response of the various synaptic and perisynaptic receptor subtypes. In particular, GluN2B- and GluN2C-NMDA receptor responses are strongly modified while GluN2A-NMDA receptor responses are almost unaffected. Thus, variations in glial transporter expression may allow differential tuning of NMDA receptors according to their subunit composition. In addition, simulation data suggest that the sink effect generated by transporters accumulation in the vicinity of the release site is the main mechanism limiting glutamate spill-out. Physical obstacles formed by glial processes play a comparatively minor role.     

Modelling zinc changes at the hippocampal mossy fiber synaptic cleft

Zinc, a transition metal existing in very high concentrations in the hippocampal mossy fibers from CA3 area, is assumed to be co-released with glutamate and to have a neuromodulatory role at the corresponding synapses. The synaptic action of zinc is determined both by the spatiotemporal characteristics of the zinc release process and by the kinetics of zinc binding to sites located in the cleft area, as well as by their concentrations. This work addresses total, free and complexed zinc concentration changes, in an individual synaptic cleft, following single, short and long periods of evoked zinc release. The results estimate the magnitude and time course of the concentrations of zinc complexes, assuming that the dynamics of the release processes are similar to those of glutamate. It is also considered that, for the cleft zinc concentrations used in the model (≤ 1 μM), there is no postsynaptic zinc entry. For this reason, all released zinc ends up being reuptaken in a process that is several orders of magnitude slower than that of release and has thus a much smaller amplitude. The time derivative of the total zinc concentration in the cleft is represented by the difference between two alpha functions, corresponding to the released and uptaken components. These include specific parameters that were chosen assuming zinc and glutamate co-release, with similar time courses. The peak amplitudes of free zinc in the cleft were selected based on previously reported experimental cleft zinc concentration changes evoked by single and multiple stimulation protocols. The results suggest that following a low amount of zinc release, similar to that associated with one or a few stimuli, zinc clearance is mainly mediated by zinc binding to the high-affinity sites on the NMDA receptors and to the low-affinity sites on the highly abundant GLAST glutamate transporters. In the case of higher zinc release brought about by a larger group of stimuli, most zinc binding occurs essentially to the GLAST transporters, having the corresponding zinc complex a maximum concentration that is more than one order of magnitude larger than that for the high and low affinity NMDA sites. The other zinc complexes considered in the model, namely those formed with sites on the AMPA receptors, calcium and K_ATP channels and with ATP molecules, have much smaller contributions to the synaptic zinc clearance.     

Postfusional control of quantal current shape

Whether glutamate is released rapidly, in an all-or-none manner, or more slowly, in a regulated manner, is a matter of debate. We analyzed the time course of excitatory postsynaptic currents (EPSCs) at glutamatergic neuromuscular junctions of Drosophila and found that the decay phase of EPSCs was protracted to a variable extent. The protraction was more pronounced in evoked and spontaneous quantal EPSCs than in action potential-evoked multiquantal EPSCs; reduced in quantal EPSCs from endophilin null mutants, which maintain release via kiss-and-run; and dependent on synaptotagmin isoform, calcium, and protein phosphorylation. Our data indicate that glutamate is released from individual synaptic vesicles for milliseconds through a fusion pore. Quantal glutamate discharge time course depends on presynaptic calcium inflow and the molecular composition of the release machinery.     

Asymmetry of glia near central synapses favors presynaptically directed glutamate escape

Recent findings demonstrate that synaptically released excitatory neurotransmitter glutamate activates receptors outside the immediate synaptic cleft and that the extent of such extrasynaptic actions is regulated by the high affinity glutamate uptake. The bulk of glutamate transporter systems are evenly distributed in the synaptic neuropil, and it is generally assumed that glutamate escaping the cleft affects pre- and postsynaptic receptors to a similar degree. To test whether this is indeed the case, we use quantitative electron microscopy and establish the stochastic pattern of glial occurrence in the three-dimensional (3D) vicinity of two common types of excitatory central synapses, stratum radiatum synapses in hippocampus and parallel fiber synapses in cerebellum. We find that the occurrence of glia postsynaptically is strikingly higher (3-4-fold) than presynaptically, in both types of synapses. To address the functional consequences of this asymmetry, we simulate diffusion and transport of synaptically released glutamate in these two brain areas using a detailed 3D compartmental model of the extracellular space with glutamate transporters arranged unevenly, in accordance with the obtained experimental data. The results predict that glutamate escaping the synaptic cleft is 2-4 times more likely to activate presynaptic compared to postsynaptic receptors. Simulations also show that postsynaptic neuronal transporters (EAAT4 type) at dendritic spines of cerebellar Purkinje cells exaggerate this asymmetry further. Our data suggest that the perisynaptic environment of these common central synapses favors fast presynaptic feedback in the information flow while preserving the specificity of the postsynaptic input.     

Fusion pore modulation as a presynaptic mechanism contributing to expression of long-term potentiation

Working on the idea that postsynaptic and presynaptic mechanisms of long-term potentiation (LTP) expression are not inherently mutually exclusive, we have looked for the existence and functionality of presynaptic mechanisms for augmenting transmitter release in hippocampal slices. Specifically, we asked if changes in glutamate release might contribute to the conversion of 'silent synapses' that show N-methyl-D-aspartate (NMDA) responses but no detectable alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) responses, to ones that exhibit both. Here, we review experiments where NMDA receptor responses provided a bioassay of cleft glutamate concentration, using opposition between peak [glu](cleft )and a rapidly reversible antagonist, L-AP5. We discuss findings of a dramatic increase in peak [glu](cleft) upon expression of pairing-induced LTP (Choi). We present simulations with a quantitative model of glutamatergic synaptic transmission that includes modulation of the presynaptic fusion pore, realistic cleft geometry and a distributed array of postsynaptic receptors and glutamate transporters. The modelling supports the idea that changes in the dynamics of glutamate release can contribute to synaptic unsilencing. We review direct evidence from Renger et al., in accord with the modelling, that trading off the strength and duration of the glutamate transient can markedly alter AMPA receptor responses with little effect on NMDA receptor responses. An array of additional findings relevant to fusion pore modulation and its proposed contribution to LTP expression are considered.     

Regulation of neurotransmitter release by metabotropic glutamate receptors

The G protein-coupled metabotropic glutamate (mGlu) receptors are differentially localized at various synapses throughout the brain. Depending on the receptor subtype, they appear to be localized at presynaptic and/or postsynaptic sites, including glial as well as neuronal elements. The heterogeneous distribution of these receptors on glutamate and nonglutamate neurons/cells thus allows modulation of synaptic transmission by a number of different mechanisms. Electrophysiological studies have demonstrated that the activation of mGlu receptors can modulate the activity of Ca(2+) or K(+) channels, or interfere with release processes downstream of Ca(2+) entry, and consequently regulate neuronal synaptic activity. Such changes evoked by mGlu receptors can ultimately regulate transmitter release at both glutamatergic and nonglutamatergic synapses. Increasing neurochemical evidence has emerged, obtained from in vitro and in vivo studies, showing modulation of the release of a variety of transmitters by mGlu receptors. This review addresses the neurochemical evidence for mGlu receptor-mediated regulation of neurotransmitters, such as excitatory and inhibitory amino acids, monoamines, and neuropeptides.     

Cholesterol as a key player in the balance of evoked and spontaneous glutamate release in rat brain cortical synaptosomes

Membrane rafts are domains enriched in sphingolipids, glycolipids and cholesterol that are able to compartmentalize cellular processes. Noteworthy, many proteins have been assigned to membrane rafts including those related to the control of the synaptic vesicle release machinery, which is a important step for neurotransmission between synapses. In this work, we have investigated the role of cholesterol in key steps of glutamate release in isolated nerve terminals (synaptosomes) from rat brain cortices. Incubation of synaptosomes with methyl-β-cyclodextrin (MβCD) induced glutamate release in a dose-dependent fashion. HγCD, a cyclodextrin with low affinity for cholesterol, had no significant effect on spontaneous glutamate release. When we evaluated the effects of MβCD on glutamate release induced by depolarizing stimuli, we observed that MβCD treatment inhibited the KCl-evoked glutamate release. The glutamate release induced by MβCD was not altered by treatment with EGTA nor with EGTA-AM. The KCl-evoked glutamate release was no further inhibited when synaptosomes were incubated with MβCD in the absence of calcium. We therefore investigated whether the cholesterol removal by MβCD changes intrasynaptosomal sodium and calcium levels. Our results suggested that the cholesterol removal effect on spontaneous and evoked glutamate release might be upstream to sodium and calcium entry through voltage-activated channels. We therefore tested if MβCD would have a direct effect on synaptic vesicle exocytosis and we showed that cholesterol removal by MβCD induced spontaneous exocytosis and inhibited synaptic vesicle exocytosis evoked by depolarizing stimuli. Lastly, we investigated the effect of protein kinase inhibitors on the spontaneous exocytosis evoked by MβCD and we observed a statistically significant reduction of synaptic vesicles exocytosis. In conclusion, our work shows that cholesterol removal facilitates protein kinase activation that favors spontaneous synaptic vesicles and consequently glutamate release in isolated nerve terminals.