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正确处理信息所必需的关键机制（Isaacson and Scanziani, 2011）。已知不同类型的gaba能中间神经元在兴奋性主神经元体树突膜的特定亚细胞部分建立对称突触，即轴突初始段的轴突-轴突（或Chandelier）细胞，树突树周围的篮状细胞和树突树不同部分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，但我们最近获得了可能是这种情况的间接证据。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能神经元可以与其他类型的突触前突触是很重要的。因此，我们希望通过光遗传转导动物（在PVcre、SOMcre和VIPcre小鼠系中cree依赖的chr2表达）来测试中间神经元亚型特异性方式的连通性。最重要的是，随后我们将对这些转基因小鼠系中包含的三个主要的中间神经元亚群（即篮细胞，BC； Martinotti细胞，MC；和双极/沥青细胞，BPC）进行成对的全细胞记录，并确定以下前后对的基本功能特性：MC/BC， BPC/BC，其中BC最终靶向锥体细胞的周围区域；BC/BPC, MC/BPC，其中BPC最终靶向锥体细胞的中间树突状树；BC/MC, BPC/MC，其中MC最终靶向锥体细胞的远端树突状树。通过用生物细胞素（或荧光染料）填充这些成对的神经元，并定量地和三维地重建它们，我们将能够追踪突触前细胞的轴突到突触后神经元的体突结构域。因此，假定的突触的数量和位置将用Neurolucida软件绘制出来。这些接触的一个子集将在相关的光学和电子显微镜制备中被验证为突触。这些研究将揭示抑制-抑制连接的功能和亚细胞结构逻辑。此外，在这些奠基实验中获得的见解将指导体内方法，指导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.