K+ efflux through postsynaptic NMDA receptors suppresses local astrocytic glutamate uptake.

Glutamatergic transmission prompts K+ efflux through postsynaptic NMDA receptors. The ensuing hotspot of extracellular K+ elevation depolarizes presynaptic terminal, boosting glutamate release, but whether this also affects glutamate uptake in local astroglia has remained an intriguing question. Here, we find that the pharmacological blockade, or conditional knockout, of postsynaptic NMDA receptors suppresses use-dependent increase in the amplitude and duration of the astrocytic glutamate transporter current (IGluT ), whereas blocking astrocytic K+ channels prevents the duration increase only. Glutamate spot-uncaging reveals that astrocyte depolarization, rather than extracellular K+ rises per se, is required to reduce the amplitude and duration of IGluT . Biophysical simulations confirm that local transient elevations of extracellular K+ can inhibit local glutamate uptake in fine astrocytic processes. Optical glutamate sensor imaging and a two-pathway test relate postsynaptic K+ efflux to enhanced extrasynaptic glutamate signaling. Thus, repetitive glutamatergic transmission triggers a feedback loop in which postsynaptic K+ efflux can transiently facilitate presynaptic release while reducing local glutamate uptake.

Electrodiffusion phenomena in neuroscience: a neglected companion.

The emerging technological revolution in genetically encoded molecular sensors and super-resolution imaging provides neuroscientists with a pass to the real-time nano-world. On this small scale, however, classical principles of electrophysiology do not always apply. This is in large part because the nanoscopic heterogeneities in ionic concentrations and the local electric fields associated with individual ions and their movement can no longer be ignored. Here, we review basic principles of molecular electrodiffusion in the cellular environment of organized brain tissue. We argue that accurate interpretation of physiological observations on the nanoscale requires a better understanding of the underlying electrodiffusion phenomena.

Conductance of porous media depends on external electric fields.

In obstacle-filled media, such as extracellular or intracellular lumen of brain tissue, effective ion-diffusion permeability is a key determinant of electrogenic reactions. Although this diffusion permeability is thought to depend entirely on structural features of the medium, such as porosity and tortuosity, brain tissue shows prominent nonohmic properties, the origins of which remain poorly understood. Here, we explore Monte Carlo simulations of ion diffusion in a space filled with overlapping spheres to predict that diffusion permeability of such media decreases with stronger external electric fields. This dependence increases with lower medium porosity while decreasing with radial (two-dimensional or three-dimensional) compared with homogenous (one-dimensional) fields. We test our predictions empirically in an electrolyte chamber filled with microscopic glass spheres and find good correspondence with our predictions. A theoretical insight relates this phenomenon to a disproportionately increased dwell time of diffusing ions at potential barriers (or traps) representing geometric obstacles when the field strength increases. The dependence of medium ion-diffusion permeability on electric field could be important for understanding conductivity properties of porous materials, in particular for the accurate interpretation of electric activity recordings in brain tissue.

Extracellular GABA waves regulate coincidence detection in excitatory circuits.

KEY POINTS: Rapid changes in neuronal network activity trigger widespread waves of extracellular GABA in hippocampal neuropil. Elevations of extracellular GABA narrow the coincidence detection window for excitatory inputs to CA1 pyramidal cells. GABA transporters control the effect of extracellular GABA on coincidence detection. Small changes in the kinetics of dendritic excitatory currents amplify when reaching the soma. ABSTRACT: Coincidence detection of excitatory inputs by principal neurons underpins the rules of signal integration and Hebbian plasticity in the brain. In the hippocampal circuitry, detection fidelity is thought to depend on the GABAergic synaptic input through a feedforward inhibitory circuit also involving the hyperpolarisation-activated Ih current. However, afferent connections often bypass feedforward circuitry, suggesting that a different GABAergic mechanism might control coincidence detection in such cases. To test whether fluctuations in the extracellular GABA concentration [GABA] could play a regulatory role here, we use a GABA 'sniffer' patch in acute hippocampal slices of the rat and document strong dependence of [GABA] on network activity. We find that blocking GABAergic signalling strongly widens the coincidence detection window of direct excitatory inputs to pyramidal cells whereas increasing [GABA] through GABA uptake blockade shortens it. The underlying mechanism involves membrane-shunting tonic GABAA receptor current; it does not have to rely on Ih but depends strongly on the neuronal GABA transporter GAT-1. We use dendrite-soma dual patch-clamp recordings to show that the strong effect of membrane shunting on coincidence detection relies on nonlinear amplification of changes in the decay of dendritic synaptic currents when they reach the soma. Our results suggest that, by dynamically regulating extracellular GABA, brain network activity can optimise signal integration rules in local excitatory circuits.

ARACHNE: A neural-neuroglial network builder with remotely controlled parallel computing.

Creating and running realistic models of neural networks has hitherto been a task for computing professionals rather than experimental neuroscientists. This is mainly because such networks usually engage substantial computational resources, the handling of which requires specific programing skills. Here we put forward a newly developed simulation environment ARACHNE: it enables an investigator to build and explore cellular networks of arbitrary biophysical and architectural complexity using the logic of NEURON and a simple interface on a local computer or a mobile device. The interface can control, through the internet, an optimized computational kernel installed on a remote computer cluster. ARACHNE can combine neuronal (wired) and astroglial (extracellular volume-transmission driven) network types and adopt realistic cell models from the NEURON library. The program and documentation (current version) are available at GitHub repository https://github.com/LeonidSavtchenko/Arachne under the MIT License (MIT).

Tonic GABAA conductance bidirectionally controls interneuron firing pattern and synchronization in the CA3 hippocampal network.

The spiking output of interneurons is key for rhythm generation in the brain. However, what controls interneuronal firing remains incompletely understood. Here we combine dynamic clamp experiments with neural network simulations to understand how tonic GABAA conductance regulates the firing pattern of CA3 interneurons. In baseline conditions, tonic GABAA depolarizes these cells, thus exerting an excitatory action while also reducing the excitatory postsynaptic potential (EPSP) amplitude through shunting. As a result, the emergence of weak tonic GABAA conductance transforms the interneuron firing pattern driven by individual EPSPs into a more regular spiking mode determined by the cell intrinsic properties. The increased regularity of spiking parallels stronger synchronization of the local network. With further increases in tonic GABAA conductance the shunting inhibition starts to dominate over excitatory actions and thus moderates interneuronal firing. The remaining spikes tend to follow the timing of suprathreshold EPSPs and thus become less regular again. The latter parallels a weakening in network synchronization. Thus, our observations suggest that tonic GABAA conductance can bidirectionally control brain rhythms through changes in the excitability of interneurons and in the temporal structure of their firing patterns.

Equal levels of pre- and postsynaptic potentiation produce unequal outcomes.

Which proportion of the long-term potentiation (LTP) expressed in the bulk of excitatory synapses is postsynaptic and which presynaptic remains debatable. To understand better the possible impact of either LTP form, we explored a realistic model of a CA1 pyramidal cell equipped with known membrane mechanisms and multiple, stochastic excitatory axo-spinous synapses. Our simulations were designed to establish an input-output transfer function, the dependence between the frequency of presynaptic action potentials triggering probabilistic synaptic discharges and the average frequency of postsynaptic spiking. We found that, within the typical physiological range, potentiation of the postsynaptic current results in a greater overall output than an equivalent increase in presynaptic release probability. This difference grows stronger at lower input frequencies and lower release probabilities. Simulations with a non-hierarchical circular network of principal neurons indicated that equal increases in either synaptic fidelity or synaptic strength of individual connections also produce distinct changes in network activity, although the network phenomenology is likely to be complex. These observations should help to interpret the machinery of LTP phenomena documented in situ. This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

Glutamate-Transporter Unbinding in Probabilistic Synaptic Environment Facilitates Activation of Distant NMDA Receptors.

Once outside the synaptic cleft, the excitatory neurotransmitter glutamate is rapidly bound by its high-affinity transporters, which are expressed in abundance on the surface of perisynaptic astroglia. While this binding and the subsequent uptake of glutamate constrain excitatory transmission mainly within individual synapses, there is growing evidence for the physiologically important extrasynaptic actions of glutamate. However, the mechanistic explanation and the scope of such actions remain obscure. Furthermore, a significant proportion of glutamate molecules initially bound by transporters could be released back into the extracellular space before being translocated into astrocytes. To understand the implications of such effects, we simulated the release, diffusion, and transporter and receptor interactions of glutamate molecules in the synaptic environment. The latter was represented via trial-by-trial stochastic generation of astroglial and neuronal elements in the brain neuropil (overlapping spheroids of varied sizes), rather than using the 'average' morphology, thus reflecting the probabilistic nature of neuropil architectonics. Our simulations predict significant activation of high-affinity receptors, such as receptors of the NMDA type, at distances beyond half-micron from the glutamate release site, with glutamate-transporter unbinding playing an important role. These theoretical predictions are consistent with recent glutamate imaging data, thus lending support to the concept of significant volume-transmitted actions of glutamate in the brain.

Volume-transmitted GABA waves pace epileptiform rhythms in the hippocampal network.

Mechanisms that entrain and pace rhythmic epileptiform discharges remain debated. Traditionally, the quest to understand them has focused on interneuronal networks driven by synaptic GABAergic connections. However, synchronized interneuronal discharges could also trigger the transient elevations of extracellular GABA across the tissue volume, thus raising tonic conductance (Gtonic) of synaptic and extrasynaptic GABA receptors in multiple cells. Here, we monitor extracellular GABA in hippocampal slices using patch-clamp GABA "sniffer" and a novel optical GABA sensor, showing that periodic epileptiform discharges are preceded by transient, region-wide waves of extracellular GABA. Neural network simulations that incorporate volume-transmitted GABA signals point to a cycle of GABA-driven network inhibition and disinhibition underpinning this relationship. We test and validate this hypothesis using simultaneous patch-clamp recordings from multiple neurons and selective optogenetic stimulation of fast-spiking interneurons. Critically, reducing GABA uptake in order to decelerate extracellular GABA fluctuations-without affecting synaptic GABAergic transmission or resting GABA levels-slows down rhythmic activity. Our findings thus unveil a key role of extrasynaptic, volume-transmitted GABA in pacing regenerative rhythmic activity in brain networks.

Increased Extrasynaptic Glutamate Escape in Stochastically Shaped Probabilistic Synaptic Environment.

Excitatory synapses in the brain are often surrounded by nanoscopic astroglial processes that express high-affinity glutamate transporters at a high surface density. This ensures that the bulk of glutamate leaving the synaptic cleft is taken up for its subsequent metabolic conversion and replenishment in neurons. Furthermore, variations in the astroglial coverage of synapses can thus determine to what extent glutamate released into the synaptic cleft could activate its receptors outside the cleft. The biophysical determinants of extrasynaptic glutamate actions are complex because they involve a competition between transporters and target receptors of glutamate in the tortuous space of synaptic environment. To understand key spatiotemporal relationships between the extrasynaptic landscapes of bound and free glutamate, we explored a detailed Monte Carlo model for its release, diffusion, and uptake. We implemented a novel representation of brain neuropil in silico as a space filled with randomly scattered, overlapping spheres (spheroids) of distributed size. The parameters of perisynaptic space, astroglial presence, and glutamate transport were constrained by the empirical data obtained for the 'average' environment of common cortical synapses. Our simulations provide a glimpse of the perisynaptic concentration landscapes of free and transporter-bound glutamate relationship, suggesting a significant tail of space-average free glutamate within 3 ms post-release.

Buffering by Transporters Can Spare Geometric Hindrance in Controlling Glutamate Escape.

The surface of astrocyte processes that often surround excitatory synapses is packed with high-affinity glutamate transporters, largely preventing extrasynaptic glutamate escape. The shape and prevalence of perisynaptic astroglia vary among brain regions, in some cases providing a complete isolation of synaptic connections from the surrounding tissue. The perception has been that the geometry of perisynaptic environment is therefore essential to preventing extrasynaptic glutamate escape. To understand to what degree this notion holds, we modelled brain neuropil as a space filled with a scatter of randomly sized, overlapping spheres representing randomly shaped cellular elements and intercellular lumen. Simulating release and diffusion of glutamate molecules inside the interstitial gaps in this medium showed that high-affinity transporters would efficiently constrain extrasynaptic spread of glutamate even when diffusion passages are relatively open. We thus estimate that, in the hippocampal or cerebellar neuropil, the bulk of glutamate released by a synaptic vesicle is rapidly bound by transporters (or high-affinity target receptors) mainly in close proximity of the synaptic cleft, whether or not certain physiological or pathological events change local tissue geometry.

Release probability increases towards distal dendrites boosting high-frequency signal transfer in the rodent hippocampus.

Dendritic integration of synaptic inputs involves their increased electrotonic attenuation at distal dendrites, which can be counterbalanced by the increased synaptic receptor density. However, during network activity, the influence of individual synapses depends on their release fidelity, the dendritic distribution of which remains poorly understood. Here, we employed classical optical quantal analyses and a genetically encoded optical glutamate sensor in acute hippocampal slices of rats and mice to monitor glutamate release at CA3-CA1 synapses. We find that their release probability increases with greater distances from the soma. Similar-fidelity synapses tend to group together, whereas release probability shows no trends regarding the branch ends. Simulations with a realistic CA1 pyramidal cell hosting stochastic synapses suggest that the observed trends boost signal transfer fidelity, particularly at higher input frequencies. Because high-frequency bursting has been associated with learning, the release probability pattern we have found may play a key role in memory trace formation.

Outwardly rectifying tonically active GABAA receptors in pyramidal cells modulate neuronal offset, not gain.

Hippocampal pyramidal cell excitability is regulated both by fast synaptic inhibition and by tonically active high-affinity extrasynaptic GABA(A) receptors. The impact of tonic inhibition on neuronal gain and offset, and thus on information processing, is unclear. Offset is altered by shunting inhibition, and the gain of a neuronal response to an excitatory input can be modified by changing the level of "background" synaptic noise. Therefore, tonic activation of GABA(A) receptors would be expected to modulate offset and, in addition, to alter gain through a shunting effect on synaptic noise. Here we show that tonically active GABA(A) receptors in CA1 pyramidal cells show marked outward rectification, while the peaks of IPSCs exhibit a linear current-voltage relationship. As a result, tonic GABA(A) receptor-mediated currents have a minimal effect upon subthreshold membrane potential variation due to synaptic noise, but predominantly affect neurons at spiking threshold. Consistent with this, tonic GABA(A) receptor-mediated currents in pyramidal cells exclusively affect offset and not gain. Modulation of tonically active GABA(A) receptors by fluctuations in extracellular GABA concentrations or neuromodulators acting on high-affinity receptors potentially provides a powerful mechanism to alter neuronal offset independently of neuronal gain.

Noisy Synaptic Conductance: Bug or a Feature?

More often than not, action potentials fail to trigger neurotransmitter release. And even when neurotransmitter is released, the resulting change in synaptic conductance is highly variable. Given the energetic cost of generating and propagating action potentials, and the importance of information transmission across synapses, this seems both wasteful and inefficient. However, synaptic noise arising from variable transmission can improve, in certain restricted conditions, information transmission. Under broader conditions, it can improve information transmission per release, a quantity that is relevant given the energetic constraints on computing in the brain. Here we discuss the role, both positive and negative, synaptic noise plays in information transmission and computation in the brain.

Author Correction: Disentangling astroglial physiology with a realistic cell model in silico.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Disentangling astroglial physiology with a realistic cell model in silico.

Electrically non-excitable astroglia take up neurotransmitters, buffer extracellular K+ and generate Ca2+ signals that release molecular regulators of neural circuitry. The underlying machinery remains enigmatic, mainly because the sponge-like astrocyte morphology has been difficult to access experimentally or explore theoretically. Here, we systematically incorporate multi-scale, tri-dimensional astroglial architecture into a realistic multi-compartmental cell model, which we constrain by empirical tests and integrate into the NEURON computational biophysical environment. This approach is implemented as a flexible astrocyte-model builder ASTRO. As a proof-of-concept, we explore an in silico astrocyte to evaluate basic cell physiology features inaccessible experimentally. Our simulations suggest that currents generated by glutamate transporters or K+ channels have negligible distant effects on membrane voltage and that individual astrocytes can successfully handle extracellular K+ hotspots. We show how intracellular Ca2+ buffers affect Ca2+ waves and why the classical Ca2+ sparks-and-puffs mechanism is theoretically compatible with common readouts of astroglial Ca2+ imaging.

Bilateral processing in chemical synapses with electrical 'ephaptic' feedback: a theoretical model.

I have developed a detailed biophysical model of the chemical synapse which hosts voltage-dependent presynaptic ion channels and takes into account the capacitance of synaptic membranes. I find that at synapses with a relatively large cleft resistance (e.g., mossy fiber or giant calyx synapse) the rising postsynaptic current could activate, within the synaptic cleft, electrochemical phenomena that induce rapid widening of the presynaptic action potential (AP). This mechanism could boost fast Ca(2+) entry into the terminal thus increasing the probability of subsequent synaptic releases. The predicted difference in the AP waveforms generated inside and outside the synapse can explain the previously unexplained fast capacitance transient recorded in the postsynaptic cell at the giant calyx synapse. I propose therefore the mechanism of positive ephaptic feedback that acts between the postsynaptic and presynaptic cell contributing to the basal synaptic transmission at large central synapses. This mechanism could also explain the supralinear voltage dependence of EPSCs recorded at hyperpolarizing membrane potentials in low extracellular calcium concentration.

Nanoscale diffusion in the synaptic cleft and beyond measured with time-resolved fluorescence anisotropy imaging.

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.

Glutamate escape from a tortuous synaptic cleft of the hippocampal mossy fibre synapse.

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.

Regulation of rhythm genesis by volume-limited, astroglia-like signals in neural networks.

Rhythmic activity of the brain often depends on synchronized spiking of interneuronal networks interacting with principal neurons. The quest for physiological mechanisms regulating network synchronization has therefore been firmly focused on synaptic circuits. However, it has recently emerged that synaptic efficacy could be influenced by astrocytes that release signalling molecules into their macroscopic vicinity. To understand how this volume-limited synaptic regulation can affect oscillations in neural populations, here we explore an established artificial neural network mimicking hippocampal basket cells receiving inputs from pyramidal cells. We find that network oscillation frequencies and average cell firing rates are resilient to changes in excitatory input even when such changes occur in a significant proportion of participating interneurons, be they randomly distributed or clustered in space. The astroglia-like, volume-limited regulation of excitatory synaptic input appears to better preserve network synchronization (compared with a similar action evenly spread across the network) while leading to a structural segmentation of the network into cell subgroups with distinct firing patterns. These observations provide us with some previously unknown insights into the basic principles of neural network control by astroglia.