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R Angus Silver

Publications and source records attributed to R Angus Silver.

12 recordsLinked to original sources
PubMed2006

Fast vesicle reloading and a large pool sustain high bandwidth transmission at a central synapse.

What limits the rate at which sensory information can be transmitted across synaptic connections in the brain? High-frequency signalling is restricted to brief bursts at many central excitatory synapses, whereas graded ribbon-type synapses can sustain release and transmit information at high rates. Here we investigate transmission at the cerebellar mossy fibre terminal, which can fire at over 200 Hz for sustained periods in vivo, yet makes few synaptic contacts onto individual granule cells. We show that connections between mossy fibres and granule cells can sustain high-frequency signalling at physiological temperature. We use fluctuation analysis and pharmacological block of desensitization to identify the quantal determinants of short-term plasticity and combine these with a short-term plasticity model and cumulative excitatory postsynaptic current analysis to quantify the determinants of sustained high-frequency transmission. We show that release is maintained at each release site by rapid reloading of release-ready vesicles from an unusually large releasable pool of vesicles (approximately 300 per site). Our results establish that sustained vesicular release at high rates is not restricted to graded ribbon-type synapses and that mossy fibres are well suited for transmitting broad-bandwidth rate-coded information to the input layer of the cerebellar cortex.

Animals↗
PubMed2005

Errors in the estimation of the variance: implications for multiple-probability fluctuation analysis.

Synapses play a crucial role in information processing in the brain. Amplitude fluctuations of synaptic responses can be used to extract information about the mechanisms underlying synaptic transmission and its modulation. In particular, multiple-probability fluctuation analysis can be used to estimate the number of functional release sites, the mean probability of release and the amplitude of the mean quantal response from fits of the relationship between the variance and mean amplitude of postsynaptic responses, recorded at different probabilities. To determine these quantal parameters, calculate their uncertainties and the goodness-of-fit of the model, it is important to weight the contribution of each data point in the fitting procedure. We therefore investigated the errors associated with measuring the variance by determining the best estimators of the variance of the variance and have used simulations of synaptic transmission to test their accuracy and reliability under different experimental conditions. For central synapses, which generally have a low number of release sites, the amplitude distribution of synaptic responses is not normal, thus the use of a theoretical variance of the variance based on the normal assumption is not a good approximation. However, appropriate estimators can be derived for the population and for limited sample sizes using a more general expression that involves higher moments and introducing unbiased estimators based on the h-statistics. Our results are likely to be relevant for various applications of fluctuation analysis when few channels or release sites are present.

Animals↗
PubMed2005

Rapid vesicular release, quantal variability, and spillover contribute to the precision and reliability of transmission at a glomerular synapse.

The amplitude and shape of EPSC waveforms are thought to be important determinants of information processing and storage in the brain, yet relatively little is known about the origins of EPSC variability or how it affects synaptic signaling. We investigated the stochastic determinants of AMPA receptor-mediated EPSC variability at cerebellar mossy fiber-granule cell (MF-GC) connections by combining multiple-probability fluctuation analysis (MPFA) and deconvolution methods. The properties of MF connections with a single release site and the effects of the rapidly equilibrating competitive antagonist kynurenic acid on EPSCs suggest that receptors are not saturated by glutamate during a quantal event and that quanta sum linearly over a wide range of release probabilities. MPFA revealed an average of five vesicular release sites per MF-GC connection. Our results show that the time course of vesicular release is rapid (decay, tau = 75 micros) and independent of release probability, introducing little jitter in the shape or timing of the quantal component of the EPSC at physiological temperature. Moreover, the peak vesicular release rate per release site after an action potential (AP) (approximately 3 ms(-1)) is substantially higher than previously reported for central synapses. Interaction of amplitude fluctuations arising from quantal release and quantal size with the slower, low variability spillover-mediated current produce substantial variability in EPSC shape. Our simulations of MF-GC transmission suggest that quantal variability and transmitter spillover extend the voltage from which AP threshold can be crossed, improving reliability, and that fast vesicular release allows precise signaling across MF connections with heterogeneous weights.

Animals↗
PubMed2004

Modulation of glutamate mobility reveals the mechanism underlying slow-rising AMPAR EPSCs and the diffusion coefficient in the synaptic cleft.

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 (Dglut) at cerebellar mossy fiber-granule cell synapses using a combination of diffusion modeling and patch-clamp recording. Simulations show that modulating Dglut 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 Dglut is approximately 3-fold lower than in free solution.

Amino Acids↗
PubMed2004

Tonically active GABA A receptors: modulating gain and maintaining the tone.

GABAA receptors not only respond to the local release of GABA from presynaptic terminals, but can also mediate a persistent 'tonic current'. This reflects the activation of high-affinity GABAA receptors by ambient GABA concentrations. Tonic GABAA-receptor-mediated signalling occurs in different brain regions, shows cell-type-specific differences in magnitude and pharmacology, and changes during brain development. Some clues to the adaptive significance of this phenomenon are beginning to emerge: in cerebellar granule cells, it alters the gain of transmission of rate-coded sensory information; in the hippocampus, it acts in a cell-type-specific manner to regulate the excitability of the network. Because tonic conductances can be modulated by changes in GABA release and uptake, and by modulators of high-affinity GABAA receptors including neurosteroids, this phenomenon provides a potentially important new window onto neuronal information processing and pathological states such as epilepsy.

Action Potentials↗
PubMed2003

Estimation of nonuniform quantal parameters with multiple-probability fluctuation analysis: theory, application and limitations.

Synapses are a key determinant of information processing in the central nervous system. Investigation of the mechanisms underlying synaptic transmission at central synapses is complicated by the inaccessibility of synaptic contacts and the fact that their temporal dynamics are governed by multiple parameters. Multiple-probability fluctuation analysis (MPFA) is a recently developed method for estimating quantal parameters from the variance and mean amplitude of evoked steady-state synaptic responses recorded under a range of release probability conditions. This article describes the theoretical basis and the underlying assumptions of MPFA, illustrating how a simplified multinomial model can be used to estimate mean quantal parameters at synapses where quantal size and release probability are nonuniform. Interpretations of the quantal parameter estimates are discussed in relation to uniquantal and multiquantal models of transmission. Practical aspects of this method are illustrated including a new method for estimating quantal size and variability, approaches for optimising data collection, error analysis and a method for identifying multivesicular release. The advantages and limitations of investigating synaptic function with MPFA are explored and contrasted with those for traditional quantal analysis and more recent optical quantal analysis methods.

Algorithms↗
PubMed2003

High-probability uniquantal transmission at excitatory synapses in barrel cortex.

The number of vesicles released at excitatory synapses and the number of release sites per synaptic connection are key determinants of information processing in the cortex, yet they remain uncertain. Here we show that the number of functional release sites and the number of anatomically identified synaptic contacts are equal at connections between spiny stellate and pyramidal cells in rat barrel cortex. Moreover, our results indicate that the amount of transmitter released per synaptic contact is independent of release probability and the intrinsic release probability is high. These properties suggest that connections between layer 4 and layer 2/3 are tuned for reliable transmission of spatially distributed, timing-based signals.

Action Potentials↗
PubMed2003

Shunting inhibition modulates neuronal gain during synaptic excitation.

Neuronal gain control is important for processing information in the brain. Shunting inhibition is not thought to control gain since it shifts input-output relationships during tonic excitation rather than changing their slope. Here we show that tonic inhibition reduces the gain and shifts the offset of cerebellar granule cell input-output relationships during frequency-dependent excitation with synaptic conductance waveforms. Shunting inhibition scales subthreshold voltage, increasing the excitation frequency required to attain a particular firing rate. This reduces gain because frequency-dependent increases in input variability, which couple mean subthreshold voltage to firing rate, boost voltage fluctuations during inhibition. Moreover, synaptic time course and the number of inputs also influence gain changes by setting excitation variability. Our results suggest that shunting inhibition can multiplicatively scale rate-coded information in neurons with high-variability synaptic inputs.

Action Potentials↗
PubMed2003

The density of AMPA receptors activated by a transmitter quantum at the climbing fibre-Purkinje cell synapse in immature rats.

We aimed to estimate the number of AMPA receptors (AMPARs) bound by the quantal transmitter packet, their single-channel conductance and their density in the postsynaptic membrane at cerebellar Purkinje cell synapses. The synaptic and extrasynaptic AMPARs were examined in Purkinje cells in 2- to 4-day-old rats, when they receive synaptic inputs solely from climbing fibres (CFs). Evoked CF EPSCs and whole-cell AMPA currents displayed roughly linear current-voltage relationships, consistent with the presence of GluR2 subunits in synaptic and extrasynaptic AMPARs. The mean quantal size, estimated from the miniature EPSCs (MEPSCs), was approximately 300 pS. Peak-scaled non-stationary fluctuation analysis of spontaneous EPSCs and MEPSCs gave a weighted-mean synaptic channel conductance of approximately 5 pS (approximately 7 pS when corrected for filtering). By applying non-stationary fluctuation analysis to extrasynaptic currents activated by brief glutamate pulses (5 mM), we also obtained a small single-channel conductance estimate for extrasynaptic AMPARs (approximately 11 pS). This approach allowed us to obtain a maximum open probability (Po,max) value for the extrasynaptic receptors (Po,max = 0.72). Directly resolved extrasynaptic channel openings in the continued presence of glutamate exhibited clear multiple-conductance levels. The mean area of the postsynaptic density (PSD) of these synapses was 0.074 microm2, measured by reconstructing electron-microscopic (EM) serial sections. Postembedding immunogold labelling by anti-GluR2/3 antibody revealed that AMPARs are localised in PSDs. From these data and by simulating error factors, we estimate that at least 66 AMPARs are bound by a quantal transmitter packet at CF-Purkinje cell synapses, and the receptors are packed at a minimum density of approximately 900 microm-2 in the postsynaptic membrane.

Age Factors↗
PubMed2002

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.

Aging↗
PubMed2002

Synaptic connections between layer 4 spiny neurone-layer 2/3 pyramidal cell pairs in juvenile rat barrel cortex: physiology and anatomy of interlaminar signalling within a cortical column.

Whole-cell voltage recordings were obtained from 64 synaptically coupled excitatory layer 4 (L4) spiny neurones and L2/3 pyramidal cells in acute slices of the somatosensory cortex ('barrel' cortex) of 17- to 23-days-old rats. Single action potentials (APs) in the L4 spiny neurone evoked single unitary EPSPs in the L2/3 pyramidal cell with a peak amplitude of 0.7 +/- 0.6 mV. The average latency was 2.1 +/- 0.6 ms, the rise time was 0.8 +/- 0.3 ms and the decay time constant was 12.7 +/- 3.5 ms. The percentage of failures of an AP in a L4 spiny neurone to evoke a unitary EPSP in the L2/3 pyramidal cell was 4.9 +/- 8.8 % and the coefficient of variation (c.v.) of the unitary EPSP amplitude was 0.27 +/- 0.13. Both c.v. and percentage of failures decreased with increased average EPSP amplitude. Postsynaptic glutamate receptors (GluRs) in L2/3 pyramidal cells were of the N-methyl-D-aspartate (NMDA) receptor (NMDAR) and the non-NMDAR type. At -60 mV in the presence of extracellular Mg2+ (1 mM), 29 +/- 15 % of the EPSP voltage-time integral was blocked by NMDAR antagonists. In 0 Mg2+, the NMDAR/AMPAR ratio of the EPSC was 0.50 +/- 0.29, about half the value obtained for L4 spiny neurone connections. Burst stimulation of L4 spiny neurones showed that EPSPs in L2/3 pyramidal cells depressed over a wide range of frequencies (1-100 s(-1) ). However, at higher frequencies (30 s(-1)) EPSP summation overcame synaptic depression so that the summed EPSP was larger than the first EPSP amplitude in the train. The number of putative synaptic contacts established by the axonal collaterals of the L4 projection neurone with the target neurone in layer 2/3 varied between 4 and 5, with an average of 4.5 +/- 0.5 (n = 13 pairs). Synapses were established on basal dendrites of the pyramidal cell. Their mean geometric distance from the pyramidal cell soma was 67 +/- 34 microm (range, 16-196 microm). The results suggest that each connected L4 spiny neurone produces a weak but reliable EPSP in the pyramidal cell. Therefore transmission of signals to layer 2/3 is likely to have a high threshold requiring simultaneous activation of many L4 neurons, implying that L4 spiny neurone to L2/3 pyramidal cell synapses act as a gate for the lateral spread of excitation in layer 2/3.

Action Potentials↗