R. Angus Silver

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.

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 μm (range, 16–196 μm). 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.

NeuroML: a language for describing data driven models of neurons and networks with a high degree of biological detail

Biologically detailed single neuron and network models are important for understanding how ion channels, synapses and anatomical connectivity underlie the complex electrical behavior of the brain. While neuronal simulators such as NEURON, GENESIS, MOOSE, NEST, and PSICS facilitate the development of these data-driven neuronal models, the specialized languages they employ are generally not interoperable, limiting model accessibility and preventing reuse of model components and cross-simulator validation. To overcome these problems we have used an Open Source software approach to develop NeuroML, a neuronal model description language based on XML (Extensible Markup Language). This enables these detailed models and their components to be defined in a standalone form, allowing them to be used across multiple simulators and archived in a standardized format. Here we describe the structure of NeuroML and demonstrate its scope by converting into NeuroML models of a number of different voltage- and ligand-gated conductances, models of electrical coupling, synaptic transmission and short-term plasticity, together with morphologically detailed models of individual neurons. We have also used these NeuroML-based components to develop an highly detailed cortical network model. NeuroML-based model descriptions were validated by demonstrating similar model behavior across five independently developed simulators. Although our results confirm that simulations run on different simulators converge, they reveal limits to model interoperability, by showing that for some models convergence only occurs at high levels of spatial and temporal discretisation, when the computational overhead is high. Our development of NeuroML as a common description language for biophysically detailed neuronal and network models enables interoperability across multiple simulation environments, thereby improving model transparency, accessibility and reuse in computational neuroscience.

Estimated conductance of glutamate receptor channels activated during EPSCs at the cerebellar mossy fiber-granule cell synapse

We have analyzed the variance associated with the decay of the non-NMDA receptor component of synaptic currents, recorded from mossy fiber-granule cell synapses in cerebellar slices, to obtain a conductance estimate for the synaptic channel. Current fluctuations arising from the random channel gating properties were separated from those arising from the fluctuations in the population of channels by subtracting the mean excitatory postsynaptic current (EPSC) waveform scaled to the EPSC peak amplitude. A weighted mean single-channel conductance of approximately 20 pS was determined from the relationship between the mean current and the variance around the mean during the decay of evoked and spontaneous synaptic currents. This result suggests that high conductance non-NMDA channels, such as the 10-30 pS glutamate receptor channel previously characterized in granule cells, carry the majority of the fast component of the EPSC at this synapse. In addition, our data are consistent with the activation of surprisingly few (approximately 10) non-NMDA channels by a single packet of transmitter.

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.

Unveiling synaptic plasticity: a new graphical and analytical approach

Short-term synaptic plasticity has a key role in information processing in the CNS, whereas memories can be formed through long-lasting changes in synaptic strength. Despite the importance of these phenomena, it remains difficult to determine whether a synaptic modulation is expressed at a presynaptic or postsynaptic site. This article describes a new approach that, in its simplest form, can identify the site of expression by direct graphical means. A more-sophisticated form of the technique can quantify functional synaptic properties and determine which of these properties is altered following a modulation of synaptic strength.

Synaptic depression enables neuronal gain control

To act as computational devices, neurons must perform mathematical operations as they transform synaptic and modulatory input into output firing rate. Experiments and theory indicate that neuronal firing typically represents the sum of synaptic inputs, an additive operation, but multiplication of inputs is essential for many computations. Multiplication by a constant produces a change in the slope, or gain, of the input–output relationship, amplifying or scaling down the sensitivity of the neuron to changes in its input. Such gain modulation occurs in vivo, during contrast invariance of orientation tuning, attentional scaling, translation-invariant object recognition, auditory processing and coordinate transformations. Moreover, theoretical studies highlight the necessity of gain modulation in several of these tasks. Although potential cellular mechanisms for gain modulation have been identified, they often rely on membrane noise and require restrictive conditions to work. Because nonlinear components are used to scale signals in electronics, we examined whether synaptic nonlinearities are involved in neuronal gain modulation. We used synaptic stimulation and the dynamic-clamp technique to investigate gain modulation in granule cells in acute slices of rat cerebellum. Here we show that when excitation is mediated by synapses with short-term depression (STD), neuronal gain is controlled by an inhibitory conductance in a noise-independent manner, allowing driving and modulatory inputs to be multiplied together. The nonlinearity introduced by STD transforms inhibition-mediated additive shifts in the input–output relationship into multiplicative gain changes. When granule cells were driven with bursts of high-frequency mossy fibre input, as observed in vivo, larger inhibition-mediated gain changes were observed, as expected with greater STD. Simulations of synaptic integration in more complex neocortical neurons suggest that STD-based gain modulation can also operate in neurons with large dendritic trees. Our results establish that neurons receiving depressing excitatory inputs can act as powerful multiplicative devices even when integration of postsynaptic conductances is linear.

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 (∼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.

Glutamate uptake from the synaptic cleft does not shape the decay of the non-NMDA component of the synaptic current

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 microM PDC should significantly reduce the uptake of glutamate during the synaptic current. In isolated rat hippocampal neurons, 300 microM 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 microM 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.

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.