京都大学 / 0048 / 新制・課程博士 / 博士(生命科学) / 甲第20531号 / 生博第373号 / 新制||生||49(附属図書館) / 京都大学大学院生命科学研究科高次生命科学専攻 / (主査)教授 松田 道行, 教授 上村 匡, 教授 見学 美根子 / 学位規則第4条第1項該当 / Doctor of Philosophy in Life Sciences / Kyoto University / DFAM

This thesis proposes a novel set of generic and compact biologically plausible VLSI (Very Large Scale Integration) neural circuits, suitable for implementing a parallel VLSI network that closely resembles the function of a small-scale neocortical network. The proposed circuits include a cortical neuron, two different long-term plastic synapses and four different short-term plastic synapses. These circuits operate in accelerated-time, where the time scale of neural responses is approximately three to four orders of magnitude faster than the biological-time scale of the neuronal activities, providing higher computational throughput in computing neural dynamics. Further, a novel biological-time cortical neuron circuit with similar dynamics as of the accelerated-time neuron is proposed to demonstrate the feasibility of migrating accelerated-time circuits into biological-time circuits. The fabricated accelerated-time VLSI neuron circuit is capable of replicating distinct firing patterns such as regular spiking, fast spiking, chattering and intrinsic bursting, by tuning two external voltages. It reproduces biologically plausible action potentials. This neuron circuit is compact and enables implementation of many neurons in a single silicon chip. The circuit consumes extremely low energy per spike (8pJ). Incorporating this neuron circuit in a neural network facilitates diverse non-linear neuron responses, which is an important aspect in neural processing. Two of the proposed long term plastic synapse circuits include spike-time dependent plasticity (STDP) synapse, and dopamine modulated STDP synapse. The short-term plastic synapses include excitatory depressing, inhibitory facilitating, inhibitory depressing, and excitatory facilitating synapses. Many neural parameters of short- and long- term synapses can be modified independently using externally controlled tuning voltages to obtain distinct synaptic properties. Having diverse synaptic dynamics in a network facilitates richer network behaviours such as learning, memory, stability and dynamic gain control, inherent in a biological neural network. To prove the concept in VLSI, different combinations of these accelerated-time neural circuits are fabricated in three integrated circuits (ICs) using a standard 0.35 µm CMOS technology. Using first two ICs, functions of cortical neuron and STDP synapses have been experimentally verified. The third IC, the Cortical Neural Layer (CNL) Chip is designed and fabricated to facilitate cortical network emulations. This IC implements neural circuits with a similar composition to the cortical layer of the neocortex. The CNL chip comprises 120 cortical neurons and 7 560 synapses. Many of these CNL chips can be combined together to form a six-layered VLSI neocortical network to validate the network dynamics and to perform neural processing of small-scale cortical networks. The proposed neuromorphic systems can be used as a simulation acceleration platform to explore the processing principles of biological brains and also move towards realising low power, real-time intelligent computing devices and control systems.

The majority of fast excitatory neurotransmission in the brain occurs at glutamatergic synapses. The extensive dendritic arborisations of pyramidal neurons in the neocortex and hippocampus harbor thousands of synaptic connections, each formed on tiny protrusions called dendritic spines. Spine synapses are rapidly established during early postnatal development – a key period in neural circuit assembly – and are subject to dynamic activity-dependent plasticity mechanisms that are believed to underlie neural information storage and processing for learning and memory. Recent decades have seen remarkable progress in identifying diverse plasticity mechanisms responsible for regulating synapse structure and function, and in understanding the processes underlying computation of synaptic inputs in the dendrites of individual neurons. These advances have strengthened our understanding of the biological mechanisms underlying brain function but, not surprisingly, they have also raised many new questions. Using a combination of whole-cell electrophysiology, 2-photon imaging and glutamate uncaging in rodent brain slice preparations, I have helped to document the subtype-specific regulation of glutamate receptors during a homeostatic form of synaptic plasticity at CA1 pyramidal neurons of the hippocampus, and have discovered novel synaptic calcium dynamics during a critical period of neural circuit formation. First, we found that during a homeostatic response to prolonged inactivity, both AMPA and NMDA subtypes of glutamate receptors undergo a switch in subunit composition at synapses, but exhibit a divergence in their subcellular localization at extrasynaptic regions of the plasma membrane (this work was published in the Journal of Neuroscience in 2013). In separate series of experiments using 2-photon calcium imaging, I discovered a functional coupling between NMDA receptor activation and intracellular calcium release at dendritic spines and dendrites that is selectively expressed during a critical period of synapse formation. This synaptic calcium signaling mechanism enabled the transformation of distinct spatiotemporal patterns of synaptic input into salient biochemical signals, and is thus apt to locally regulate synapse development along individual dendritic branches. Consistent with this hypothesis, I found evidence for non-random clustering of synapse development between neighboring dendritic spines. Together, these experimental results expand the current understanding of the dynamics of synapse function during homeostatic plasticity and early postnatal development. --- Les synapses glutamatergiques soutiennent la majorité de la neurotransmission excitatrice rapide du cerveau. Des milliers de ces synapses, localisées sur de minuscules saillies appelées épines dendritiques, décorent les vastes arborisations dendritiques des neurones pyramidaux du néocortex et de l'hippocampe. Ces synapses sont formées tôt lors du développement postnatal et sont soumises à des mécanismes dynamiques de plasticité qui sous-tendent, croit-on, les capacités d'apprentissage et de mémoire du cerveau. Les dernières décennies ont vu des progrès remarquables dans l'identification de divers mécanismes de régulation de la structure et de la fonction des synapses sur différentes échelles de temps, et dans la compréhension des processus qui régissent l’intégration des inputs synaptiques au niveau des dendrites individuelles. Ces progrès ont renforcé notre compréhension des éléments fondamentaux régissant la fonction cérébrale et ont ouvert de nouvelles voies d’investigations neurophysiologiques. En utilisant une combinaison d’électrophysiologie cellulaire, d'imagerie à deux-photons et de photolibération de glutamate sur des neurones pyramidaux de la région CA1 de l'hippocampe de rats, j’ai contribué à la découverte et à la caractérisation de nouvelles régulations des récepteurs du glutamate durant la plasticité synaptique homéostatique. J’ai également découvert un nouveau type de dynamique de calcium synaptique relié à une organisation spatiale du développement des synapses pendant une période critique de l’ontogénie des circuits neuronaux. Dans la première étude, nous avons constaté que lors d'une plasticité de type homéostatique induite par une inactivité prolongée, les récepteurs de glutamate de types AMPA et NMDA sont soumis à un changement important dans la composition de leurs sous-unités. De plus, nous avons observé un ciblage différentiel de ces récepteurs vers des compartiments subcellulaires spécifiques des neurones. Dans une série d'expériences séparée utilisant l’imagerie calcique à deux-photons, j’ai découvert un couplage fonctionnel durant le développent entre l'activation des récepteurs NMDA et une libération de calcium intracellulaire qui envahit tant les épines dendritiques que les dendrites. J’ai également trouvé que ce mécanisme de signalisation de calcium synaptique transforme des motifs spatiotemporels d’activités synaptiques spécifiques en signaux biochimiques post-synaptiques de manière à potentiellement réguler l’organisation spatiale des synapses durant le développement. Conformément à cette hypothèse, j’ai observé des manifestations fonctionnelles claires de regroupement dans l’espace de synapses de forces similaires le long de branches dendritiques individuelles. Ensemble, ces résultats expérimentaux élargissent notre compréhension actuelle de de la fonction des synapses durant la plasticité homéostatique ainsi que durant le développement postnatal du cerveau. En étudiant les mécanismes neurophysiologiques de base, il sera possible d'avoir un aperçu plus profond du fonctionnement du cerveau et de ses pathologies.

Studies over the past two decades have demonstrated that astrocytes are tightly associated with neurons and play pivotal roles in neural circuit development, operation, and adaptation in health and disease. Nevertheless, precisely how astrocytes integrate diverse neuronal signals, modulate neural circuit structure and function at multiple temporal and spatial scales, and influence animal behavior or disease through aberrant excitation and molecular output remains unclear. This Perspective discusses how new and state-of-the-art approaches, including fluorescence indicators, optoand chemogenetic actuators, genetic targeting tools, quantitative behavioral assays, and computational methods, might help resolve these longstanding questions. It also addresses complicating factors in interpreting astrocytes' role in neural circuit regulation and animal behavior, such as their heterogeneity, metabolism, and inter-glial communication. Research on these questions should provide a deeper mechanistic understanding of astrocyte-neuron assemblies' role in neural circuit function, complex behaviors, and disease.

Spontaneous travelling waves of neuronal activity are a prominent feature throughout the developing brain and have been shown to be essential for achieving normal function, but the mechanism of their action on post-synaptic connections remains unknown. A well-known and widespread mechanism for altering synaptic strengths is spike-timing dependent plasticity (STDP), whereby the temporal relationship between the pre- and post-synaptic spikes determines whether a synapse is strengthened or weakened. Here, I answer the theoretical question of how these two phenomenon interact: what types of connectivity patterns can emerge when travelling waves drive a downstream area that implements STDP, and what are the critical features of the waves and the plasticity rules that shape these patterns? I then demonstrate how the theory can be applied to the development of the visual system, where retinal waves are hypothesised to play a role in the refinement of downstream connections. My major findings are as follows. (1) Mathematically, STDP translates the correlated activity of travelling waves into coherent patterns of synaptic connectivity; it maps the spatiotemporal structure in waves into a spatial pattern of synaptic strengths, building periodic structures into feedforward circuits. This is analogous to pattern formation in reaction diffusion systems. The theory reveals a role for the wave speed and time scale of the STDP rule in determining the spatial frequency of the connectivity pattern. (2) Simulations verify the theory and extend it from one-dimensional to two-dimensional cases, and from simplified linear wavefronts to more complex realistic and noisy wave patterns. (3) With appropriate constraints, these pattern formation abilities can be harnessed to explain a wide range of developmental phenomena, including how receptive fields (RFs) in the visual system are refined in size and topography and how simple-cell and direction selective RFs can develop. The theory is applied to the visual system here but generalises across different brain areas and STDP rules. The theory makes several predictions that are testable using existing experimental paradigms.

L’anxiété et l’exposition aux contaminants environnementaux sont actuellement deux enjeux internationaux majeurs en santé publique. Plusieurs études ont examiné empiriquement le développement des troubles intériorisés, soit ses corrélats neuronaux et ses facteurs de risque. Plus récemment, l’exposition prénatale et postnatale aux contaminants environnementaux a été soulevé comme un facteur de risque au développement de l’anxiété. Bien que les Inuits du Nunavik sont parmis les plus exposés aux contaminants environnementaux dans le monde et semble particulièrement à risque de développer des troubles intériorisés, la prévalence de l’anxiété et ses facteurs de risque demeurent à être clarifié dans cette population. Les corrélats neuronaux pouvant sous-tendre les liens entre une exposition aux contaminants environnementaux et l’anxiété chez l’humain demeure aussi à être étudié. Le premier article de ma thèse présente une revue de littérature afin de mettre en lumière les associations entre les troubles intériorisés et une exposition prénatale ainsi que postnatale au plomb, au mercure et aux biphényles polychlorés (BPC) dans diverses populations. Le deuxième article étudie empiriquement la présence d’anxiété auprès des adolescents Inuit du Nunavik ainsi que ses principaux facteurs de risque lors du développement, dont l’exposition prénatale et postnatale aux contaminants environnementaux. Cette étude a permis de confirmer des niveaux d’anxiété très élevé et de souligner les facteurs de risque pouvant y contribuer tels qu’être une femme, avoir un moindre quotient intellectuel, être plus exposé au mercure durant les périodes prénatale et posnatale, vivre davantage d’insécurité alimentaire, avoir un plus faible apport vitaminique et avoir été victime davantage d’intimidation. Finalement, le troisième article examine le fonctionnement du circuit neuronal de la peur avec l’imagerie par raisonance magnétique fonctionnelle (IRMf), grâce à une tâche de conditionnement et d’extinction de la peur, selon l’exposition prénatale et postnatale aux contaminants environnementaux. Des différences d’activation dans le cortex préfrontal ont ainsi été retrouvées, soit pour l’exposition prénatale aux BPC dans le cortex orbitofrontal lors du conditionnement ainsi que lors de l’extinction pour l’exposition prénatale au mercure dans le cortex cingulaire antérieur et l’exposition présente au plomb dans le cortex préfrontal dorsolatéral. Tous ces résultats convergent vers des pistes intéressantes pour la compréhension, la prévention et l’intervention. / Anxiety and exposure to environmental contaminants are currently two major international issues in public health. Inuit of Nunavik appear at risk of developing psychological difficulties and more prone to be exposed to environmental contaminants. Several studies have empirically examined the development of anxiety, namely its neural correlates, as well as its risk factors. Environmental contaminants have recently emerged as contributing to the development of internalized disorders. The generalization of such risk factors remains to be validated within the Inuit population of Nunavik. Although closely related to anxiety, the relationship between exposure to environmental contaminants during development and the functioning of the neural circuit of fear remains to be examined. First, we reviewed the literature about association of internalized symptoms-related with prenatal and postnatal exposure to lead, mercury and polychlorinated biphenyls (PCB) in various populations. Next, we empirically examined anxiety levels in adolescent Inuit population of Nunavik and his risk factors, included environmental contaminants. This allowed to highlight very high levels of anxiety, as well as the implication of several risk factors, such as being a woman, higher exposure to mercury during prenatal and postnatal periods, more food insecurity, lower vitamin intake and more bullying experiences. Finally, we explored the functioning of the neural circuitry of fear with a task of conditioning and extinguishing fear, using functional magnetic resonance imaging, according to prenatal and postnatal exposure to environmental contaminants. Activation differences in prefrontal cortex were found, which are in the orbitofrontal cortex for prenatal exposure to PCB during fear conditioning, as well as during fear extinction in the anterior cingular cortex for prenatal exposure to mercury and in the dorsolateral prefrontal cortex for current lead exposure. All these results converge on interesting avenues for understanding, prevention and intervention.

<p>Neuromorphic computing endeavors to develop computational systems capable of emulating the brain’s capacity to execute intricate tasks concurrently and with remarkable energy efficiency. By utilizing new bioinspired computing architectures, these systems have the potential to revolutionize high-performance computing and enable local, low-energy computing for sensors and robots. Organic and soft materials are particularly attractive for neuromorphic computing as they offer biocompatibility, low-energy switching, and excellent tunability at a relatively low cost. Additionally, organic materials provide physical flexibility, large-area fabrication, and printability.</p><p>This doctoral dissertation showcases the research conducted in fabricating a comprehensive spiking organic neuron, which serves as the fundamental constituent of a circuit system for neuromorphic computing. The major contribution of this dissertation is the development of the organic, flexible neuron composed of spiking synapses and somas utilizing ultra-low voltage organic field-effect transistors (OFETs) for information processing. The synaptic and somatic circuits are implemented using physically flexible and biocompatible organic electronics necessary to realize the Polymer Neuromorphic Circuitry. An Axon-Hillock (AH) somatic circuit was fabricated and analyzed, followed by the adaptation of a log-domain integrator (LDI) synaptic circuit and the fabrication and analysis of a differential-pair integrator (DPI). Finally, a spiking organic neuron was formed by combining two LDI synaptic circuits and one AH synaptic circuit, and its characteristics were thoroughly examined. This is the first demonstration of the fabrication of an entire neuron using solid-state organic materials over a flexible substrate with integrated complementary OFETs and capacitors.</p>