The blueprint of GABAergic synapses onto GABAergic interneurons: which disinhibitory cortical circuits are laid out in a cortical column?

兴奋-抑制平衡是新皮层突触回路中正确信息处理所必需的关键机制（Isaacson和Scanziani，2011）。已知不同类型的GABA能中间神经元在兴奋性主神经元的体树突膜的特定亚细胞部分上建立对称突触，即轴突起始段上的轴-轴（或枝状）细胞、体周的篮状细胞和树突树的各个部分上的树突靶向中间神经元（即Martinotti细胞）。然而，GABA能中间神经元之间的连接是否遵循相同的布线规则仍然是一个悬而未决的问题，但我们最近获得的间接证据表明可能是这种情况。此外，由于抑制性神经元的抑制（以及下游神经元的去抑制）是一种新出现的皮层回路基序，因此充分阐明哪些类型的GABA能神经元可以与哪些其他类型的GABA能神经元发生突触前作用是很重要的。因此，我们希望通过使用光遗传学转导的动物（PVcre、SOMcre和VIPcre小鼠系中的cre依赖性ChR 2表达）以中间神经元亚型特异性方式测试连接性。最重要的是，随后我们将对三个主要的中间神经元亚群进行配对的全细胞记录（即篮状细胞，BC; Martinotti细胞，MC;和双极/双簇细胞，BPC），并确定以下前-后对的基本功能特性：MC/BC，BPC/BC，BC最终靶向锥体细胞的体周区域; BC/BPC、MC/BPC，BPC最终靶向锥体细胞的中间树突树; BC/MC、BPC/MC，MC最终靶向锥体细胞的远端树突树。通过用生物胞素（或荧光染料）填充这些神经元对，并对其进行定量和三维重建，我们将能够追踪突触前细胞的轴突到突触后神经元的体树突结构域。因此，将使用Neurolucida软件绘制推定突触的数量和位置。这些接触的一个子集将被验证为相关的光和电子显微镜制备的突触。这些研究将揭示神经抑制连接的功能和亚细胞结构逻辑。此外，在这些基础实验中获得的见解将指导体内方法，这些方法针对哪些输入选择哪些去抑制电路基序，以及如何将其用于目标导向行为的感觉信息处理。

Excitation-inhibition balance is a crucial mechanism that is necessary for proper information processing in synaptic circuits of the neocortex (Isaacson and Scanziani, 2011). It is known that different types of GABAergic interneurons establish symmetric synapses on specific subcellular portions of the somatodendritic membrane of excitatory principal neurons, i.e. axo-axonic (or Chandelier) cells on the axon initial segment, basket cells perisomatically and dendrite-targeting interneurons (i.e. Martinotti cells) on various portions of the dendritic tree. However, whether connectivity amongst GABAergic interneurons themselves follows the same wiring rules, is still an open question but we have recently obtained indirect evidence that this might be the case. Furthermore, since inhibition of inhibitory neurons (and thus disinhibition of downstream neurons) is a newly emerging cortical circuit motif, it is important to fully elucidate which types of GABAergic neurons can be presynaptic to which other types. Thus, we want to test for connectivity in an interneuron subtype-specific manner by using optogenetically transduced animals (cre-dependent ChR2-expression in PVcre, SOMcre and VIPcre mouse lines). Most importantly, subsequently we will perform paired whole cell recordings of three main interneuron subpopulations (i.e. basket cells, BC; Martinotti cells, MC; and bipolar/bitufted cells, BPC) contained in these transgenic mouse lines and determine the elementary functional properties of the following pre-post pairs: MC/BC, BPC/BC, with BC finally targeting the perisomatic region of pyramidal cells; BC/BPC, MC/BPC, with BPC finally targeting the intermediate dendritic trees of pyramidal cells; BC/MC, BPC/MC, with MC finally targeting the distal dendritic trees of pyramidal cells. By filling these pairs of neurons with biocytin (or fluorescent dyes) and reconstructing them quantitatively and three-dimensionally, we will be able to trace the axon of the presynaptic cell onto the somatodendritic domain of the postsynaptic neuron. Thus, the number and location of putative synapses will be mapped with Neurolucida software. A subset of these contacts will be verified as synapses in correlated light and electron microscopic preparations. These studies will shed light onto the functional and subcellular structural logic of inhibitory-inhibitory connections. Moreover, insights gained in these ground-laying experiments will guide in vivo approaches directed toward which inputs select which disinhibitory circuit motif and how this is used in sensory information processing for goal-directed behavior.