Tight Coupling of Astrocyte pH Dynamics to Epileptiform Activity Revealed by Genetically Encoded pH Sensors

Astrocytes can both sense and shape the evolution of neuronal network activity and are known to possess unique ion regulatory mechanisms. Here we explore the relationship between astrocytic intracellular pH dynamics and the synchronous network activity that occurs during seizure-like activity. By combining confocal and two-photon imaging of genetically encoded pH reporters with simultaneous electrophysiological recordings, we perform pH measurements in defined cell populations and relate these to ongoing network activity. This approach reveals marked differences in the intracellular pH dynamics between hippocampal astrocytes and neighboring pyramidal neurons in rodent in vitro models of epilepsy. With three different genetically encoded pH reporters, astrocytes are observed to alkalinize during epileptiform activity, whereas neurons are observed to acidify. In addition to the direction of pH change, the kinetics of epileptiform-associated intracellular pH transients are found to differ between the two cell types, with astrocytes displaying significantly more rapid changes in pH. The astrocytic alkalinization is shown to be highly correlated with astrocytic membrane potential changes during seizure-like events and mediated by an electrogenic Na+/HCO3- cotransporter. Finally, comparisons across different cell-pair combinations reveal that astrocytic pH dynamics are more closely related to network activity than are neuronal pH dynamics. This work demonstrates that astrocytes exhibit distinct pH dynamics during periods of epileptiform activity, which has relevance to multiple processes including neurometabolic coupling and the control of network excitability.

Disruption of layer-specific visual processing in a model of focal neocortical epilepsy

Silvestro Micera, Matteo Vissani

The epileptic brain is the result of a sequence of events transforming normal neuronal populations into hyperexcitable networks supporting recurrent seizure generation. These modifications are known to induce fundamental alterations of circuit function and, ultimately, of behavior. However, how hyperexcitability affects information processing in cortical sensory circuits is not yet fully understood. Here, we investigated interlaminar alterations in sensory processing of the visual cortex in a mouse model of focal epilepsy. We found three main circuit dynamics alterations in epileptic mice: (i) a spreading of visual contrast-driven gamma modulation across layers, (ii) an increase in firing rate that is layer-unspecific for excitatory units and localized in infragranular layers for inhibitory neurons, and (iii) a strong and contrast-dependent locking of firing units to network activity. Altogether, our data show that epileptic circuits display a functional disruption of layer-specific organization of visual sensory processing, which could account for visual dysfunction observed in epileptic subjects. Understanding these mechanisms paves the way to circuital therapeutic interventions for epilepsy.

OXFORD UNIV PRESS INC2022

The In-Silico Neocortical Microcircuit

Michael Reimann

The neocortex is one of the most important brain regions, occupying more than 80% of the brain volume in mammals and contributing massively to its capability of perceiving and interpreting then environment. While it has been extensively studied, it is still largely unclear how it accomplishes the enormous computational tasks it is faced with. A number of structural features are conserved across the cortex, such as its vertical organization in six individual layers with a large number of morphologically unique neuron types. The exceptional horizontal uniformity, lead to the hypothesis of horizontally repeated unitary microcircuits, the idea that the cortex consists of a number of relatively uniform horizontally repeated modules, each with their own function in the concert of neural activity. While a rough mapping of function to cortical region is virtually identical between individuals and very similar between species, the extraordinary plasticity of the cortex allows functions of damaged parts to be taken over by neighboring regions. This indicates that flexible, "reprogrammable" modules implement the functions of the cortex. The Blue Brain Project (BBP) aims to build a facility for the collection and integration of available data on the fundamental structure and operating principles of the cortex to build and continuously refine a detailed, bottom-up model of the unitary microcircuit. It can then be used as a building block for the construction and simulation of larger cortical regions. A bottom-upmodel such as the one of the BBP aims to match the object being represented primarily on an underlying physical level. As such it is guaranteed to be anchored in real world physical constraints, this however comes with the disadvantage that on a higher level the behavior of the model is undetermined. The goal of this thesis is to study and characterize the model to learnmore about its function and what determines it. In order to ensure the biological relevance, this endeavour was to be tightly linked with comparisons to and validations against relevant experimental data. I concluded that the simulation of extracellular electrical signals are the right tool to link characterization and validation, due to their great importance in in-vitro work. I authored several improvements to the model to ensure the biological accuracy of the computed extracellular signals, the most important being the derivation of the local connectome inside the modeled microcircuit. Next, I implemented software for the calculation of extracellular signals during the course of a simulation on a BlueGene/P supercomputer. The method for the calculation is flexible and can discern between the contributions of individual cells. In addition to validating the signal against in-vitro data, this allowed us to analyze the composition of the signals in a way that is impossible outside of a simulation. The method revealed the crucial role of active currents in the local field potential, i.e. the relatively slow deflections in extracellular signals. We also extensively analyzed and quantified the varying contributions of individual cell types and membrane mechanisms to extracellular signals at different frequencies.

EPFL2014

Orchestration of oscillatory activity to improve visual motion discrimination

Roberto Felipe Salamanca Giron

This thesis describes advances in the use of novel configurations of non-invasive brain stimulation over the visual system allowing to modulate of modifying electro-physiological activity, interregional interactions and by it, visual behavior such as motion discrimination capacity in healthy subjects. We have implemented three experimental protocols that include the application of a motion discrimination and integration task in combination with bifocal transcranial Alternating Current Stimulation (i.e. tACS) over the primary visual cortex (i.e. V1) and medio-temporal areas (i.e. V5), while varying some of the âorchestraâ parameters in each study. The common objective pursued in the three studies presented in the upcoming chapters was to evaluate physiological changes induced by tACS combined with the visual task, leading to enhanced visual performance expressed by the accurate distinction of the generalized movement orientation of a kinetogram. After introducing the scientific rationale of this thesis in Chapter 1, Chapter 2 describes whether applying two phase-shifted (Alpha) É-tACS conditions (Anti-Phase and In-Phase tACS) within the V1-V5 network were able to positively modulate behavior compared to Sham tACS. Our results suggest that the active Anti-Phase condition significantly increased visual motion discrimination compared to In-phase tACS which rather tended to decrease performances. These two case scenarios were associated with opposite changes in Alpha-Gamma oscillatory modulation (i.e. V1 phase â V5 amplitude coupling) determined by multichannel EEG. Based on these findings, in Chapter 3, we describe testing the effects of modulating Alpha-Gamma interregional interaction. Hence, two conditions V1ÉV5Æ tACS and vice versa, V1ÆV5É tACS, were behaviorally and electrophysiologically evaluated. The results suggested that there was a common electrophysiological feature between the two Verum tACS, which contrasted with Sham tACS, expressed through WPLIÆ (i.e. Weighted Phase Locking Value) connectivity. Furthermore, WPLIÉ and ZPAC V1amplitude â V5phase (i.e. Z-scored Phase Amplitude Coupling) were the inter-areal mechanisms in which both Verum conditions differed to explain the variance of their corresponding group behavior. However, the electrophysiological changes did not lead to significant difference in behavioral measures.In Chapter 4, we combined the knowledge gained in the first two studies and thus, we time-locked, short bursts of phase-shifted É-tACS to the visual stimulus onset. This permitted to find out that, despite the phase difference between the tACS conditions (i.e. In-Phase vs. Anti-Phase), there was a generalized augmentation of the performance after verum stimulation compared to the results with Sham. This amelioration was generally associated with changes in causal PSI (i.e. Phase Slope Index) flows in Æ, whereas specifically the Î-Æ modulation permitted to explain the differences in behavior between Verum and Sham. Moreover, dynamic PSI-causal Î² bottom-up and top-down flows revealed the mechanisms behind each type of Verum stimulation. These studies provided first interesting evidence that physiology-inspired bifocal tACS applied to the visual network might be used to modulate visual behavior and respective underlying mechanisms. The induced electrophysiological and behavioural effects achieved are complex and need to be studied in more details in upcoming studies.