Circuit mechanisms underlying cortical communications

为了了解大脑皮层通信中远程连接的原理，我们在2013财年集中开展了以下三个项目。 不同的远程输入到感觉皮质的功能连接旨在实现对反馈/自上而下的投射到初级感觉皮质的功能连接的机械性理解。我们系统地研究了初级体感皮质(S1)不同脑区到不同神经元类型的突触强度，并确定了初级感觉皮质如何利用输入区域依赖的、优先招募的特定类型的GABA能中间神经元来解析来自不同反馈投影的信息。基于这一框架，我们研究了在不同类型的GABA能中间神经元(INS)中，神经发育(NDD)高危基因Syngap1(突触Ras GTPase激活蛋白1)发生突变的转基因小鼠中，皮层对S1的反馈投射是如何改变的。Syngap1编码突触Ras GTPase激活蛋白1(Syngap1)，该蛋白参与突触后密度的形成和兴奋性突触强度的活性调节。人类患者中的Syngap1突变通常在被诊断为智力残疾(ID)、精神分裂症和自闭症谱系障碍的个体中发现。虽然Syngap1在兴奋性神经元中的重要性已被广泛研究，但Syngap1在皮质GABA能神经元中的作用尚不清楚。我们询问了NDD危险基因之一Syngap1在不同的GABA能IN亚型中的中断是否以及如何导致输入区域依赖的皮质通讯障碍。我们在皮质-皮质相互作用中发现了多层异常，包括谷氨酸能突触传递、局部网络振荡、相关皮质区域之间的同步性以及感觉知觉。 第二个项目《皮层去抑制回路的发育机制》的目标是探讨早期发育过程中皮层去抑制连接突触特异性的发育机制。由血管活性肠肽(VIP)阳性的GABA能中间神经元(INS)介导的去抑制是在所有皮质区域都发现的一个强大的回路基序。VIP INS抑制其他类型的皮质GABA能INS，但其对树突靶向生长抑素(SST)阳性INS的抑制尤其强烈，导致锥体神经元的去抑制。这一皮层去抑制回路基序已被证明在感觉运动整合、选择性注意、增益控制和回路可塑性中发挥重要作用。然而，在早期发育过程中，这个强大的回路模体出现在整个皮质中的机制在很大程度上是未知的。正在进行的工作是研究哪些因素对VIP INS到SST INS的早期发育过程中强抑制性连接的稳定性和可塑性至关重要。我们发现VIP INS优先与SST INS形成突触联系，并且这种抑制-抑制联系的出现是由突触前VIP INS的活性控制的。VIP INS在发育早期的自发活动永久地影响了成年期S1的自上而下的调节。 第三个项目，大脑皮层网络的结构和功能组织，旨在了解支配感觉皮层主神经元功能异质性的原理。脑区内和脑区之间的神经元连接为神经元功能提供了支架。虽然大脑皮层神经元的连通性已经在大尺度水平上被绘制出来，但在单个神经元水平上将连通性规则与活动模式联系起来仍然是具有挑战性的。我们使用体内双光子钙成像、神经药理学、基于单细胞的单突触输入示踪和光遗传学工具，研究了皮质神经元功能异质性的解剖连接规则。我们表征了在单个神经元和跨神经元群体中自发运动期间S1中行为状态的神经表征。表征独立于感觉反馈，随着时间的推移而稳定，对神经调制的药物抑制很强，但对谷氨酸能传递没有影响。对自发运动中具有不同活动轮廓的单个神经元的全脑突触前网络的分析揭示了解剖学输入的特征模式。尽管在单细胞水平上全脑输入的汇聚程度很高，但对行为状态更敏感的神经元从运动皮质区域获得的输入比例较小，而来自丘脑核团的输入比例较大。丘脑输入的光遗传抑制抑制了行为状态相关的活动。我们的研究表明，编码行为状态的皮质神经元接收到典型的全脑输入，而预先配置的网络限制了神经元的功能。

To understand the principles of long-range connectivity in cortical communication, our efforts have focused on the following three projects during FY23. Functional connectivity of diverse long-range inputs to sensory cortex is aimed at achieving a mechanistic understanding of the functional connectivity of feedback / top-down projections to the primary sensory cortex. We have systematically examined the synaptic strength from different brain areas to diverse neuronal types in the primary somatosensory cortex (S1) and determined how the primary sensory cortex uses input-area-dependent, preferential recruitment of specific types of GABAergic interneurons to parse information from diverse feedback projections. Based on this framework, we investigate how cortical feedback projections to S1 are altered in a transgenic mouse line in which SynGAP1 (synaptic Ras GTPase-activating protein 1), one of the neurodevelopmental (NDD) high risk genes, is mutated in different types of GABAergic interneurons (INs). SynGAP1 encodes the synaptic Ras GTPase-activating protein 1 (SynGAP1), a crucial protein involved in the formation of postsynaptic densities and in the activity-dependent regulation of excitatory synaptic strength. SynGAP1 mutations in human patients are commonly found in individuals diagnosed with intellectual disability (ID), schizophrenia and autism spectrum disorder. While the importance of SynGAP1 in the excitatory neurons has been extensively studied, the role of SynGAP1 in cortical GABAergic neurons is largely unknown. We asked whether and how the disruptions of SynGAP1, one of NDD risk genes, in different GABAergic IN subtypes lead to the impairment of input-area-dependent corticocortical communication. We found multiple layers of abnormality in cortico-cortical interactions including glutamatergic synaptic transmission, oscillation in a local network, synchrony between relevant cortical areas, and sensory perception. The goal of the second project, development mechanism of cortical disinhibitory circuits is to address the developmental mechanism of synaptic specificity of cortical disinhibitory connections during early development. Disinhibition mediated by vasoactive intestinal polypeptide (VIP)-positive GABAergic interneurons (INs) is a robust circuit motif found in all cortical areas. VIP INs inhibit other types of cortical GABAergic INs, but its inhibition of dendrite-targeting somatostatin (SST)-positive INs is particularly strong, leading to the disinhibition of pyramidal neurons. This cortical disinhibitory circuit motif has been shown to play an important role in sensorimotor integration, selective attention, gain control, and circuit plasticity. However, the mechanisms by which this robust circuit motif emerges throughout the cortex during early development is largely unknown. Ongoing work investigates which factors are critical for the stability and plasticity of strong inhibitory connections from VIP INs to SST INs during early development. We found that VIP INs preferentially form synaptic connections to SST INs earlier than to other cell types, and that the emergence of this inhibitory-to-inhibitory connection is governed by activity of the presynaptic VIP INs. The spontaneous activity of VIP INs during early development permanently affects the top-down modulation of S1 during adulthood. The third project, the structural and functional organization of cortical subnetworks, is aimed at understanding the principles that govern the functional heterogeneity of principal neurons in sensory cortex. Neuronal connections within and across brain areas provide the scaffolding for neuronal function. While the connectivity of cortical neurons has been mapped at a macroscale level, linking connectivity rules with activity patterns at the level of single neurons remains challenging. We investigated anatomical wiring rules for the functional heterogeneity of cortical neurons using in vivo two-photon calcium imaging, neuropharmacology, single-cell based monosynaptic input tracing, and optogenetic tools. We characterized the neural representations of behavioral state in S1 during spontaneous movements in both single neurons and across neuronal populations. Representations were independent of sensory feedback, stable over time, and robust to pharmacologic inhibition of neuromodulatory, but not glutamatergic transmission. Analysis of brain-wide presynaptic networks of single neurons with distinct activity profiles during spontaneous movements revealed characteristic patterns of anatomical input. Despite the high degree of convergence from brain-wide inputs at the single-cell level, neurons more sensitive to behavioral state received a smaller proportion of inputs from motor cortical areas and a larger proportion of inputs from thalamic nuclei. Optogenetic inhibition of thalamic inputs suppressed behavioral state-related activity. Our study suggest that behavior state-encoding cortical neurons receive a characteristic brain-wide inputs, and that preconfigured networks constrain neuronal function.