Hippocampal inhibitory neuronal circuit organization

描述（由申请人提供）：海马体含有不同类型的GABA能抑制性神经元。以前的工作表明，这些抑制性神经元的不同亚型具有不同的功能，但它们对海马回路的调节规则仍有待确定。我们假设，抑制性神经元之间的功能差异是由不同类型的抑制性神经元的不同回路连接造成的。拟议的研究的目标是映射本地和远程直接突触连接到主要的抑制性神经元类型在小鼠海马CA 1区。我们假设特定类型的抑制性神经元选择性地接收来自不同脑区的局部和远端兴奋性突触输入，并且这些输入中的每一个对特定抑制性神经元的差异有助于它们对海马靶神经元的抑制性调节。具体而言，（1）我们的目标是通过激光扫描光刺激（LSPS）来识别特定类型的抑制性海马神经元的局部兴奋性连接。我们结合LSPS与活脑切片中抑制性神经元的全细胞记录，将海马内兴奋性输入源映射到最多的抑制性细胞类型，包括表达小清蛋白（PV+）的篮状细胞、表达胆囊收缩素（CCK+）的篮状细胞、轴-轴细胞和表达生长抑素（SOM+）的定向-腔隙分子（O-LM）细胞。我们将测试的假设，轴-轴细胞，PV+和CCK+篮细胞接受不同强度的兴奋性输入从CA 3与CA 1支持他们的前馈和反馈抑制CA 1网络活动。我们还将测试的假设，O-LM细胞只接受兴奋性锥体细胞在CA 1和严格执行局部反馈抑制。(2)我们的目标是确定长距离突触连接到选定组的抑制性海马神经元与一种新的狂犬病为基础的跟踪系统和光遗传学刺激。我们将使用在选定的抑制性神经元组中表达Cre（Cre重组酶）的敲入小鼠系（例如，PV-Cre、SOM-Cre或CCK-Cre）以限制狂犬病感染和对完整大脑中每个选定细胞群的单突触逆行追踪。狂犬病追踪之后将进行通道视紫红质辅助电路映射，以在功能上表征与每个靶向Cre表达细胞群内已鉴定细胞类型的远端连接的特异性。我们将测试的假设，PV+抑制细胞，但不是SOM+抑制细胞，收到强大的远距离连接从内嗅皮层和内侧隔，CCK+细胞群有很强的直接突触连接与杏仁核。这些研究应该建立海马回路内主要抑制性神经元的操作规则。

DESCRIPTION (provided by applicant): The hippocampus contains diverse types of GABAergic inhibitory neurons. Previous work suggests that different subtypes of these inhibitory neurons have distinct functions, but the rules for their regulation of the hippocampal circuit remain to be determined. We hypothesize that functional differences between inhibitory neurons result from distinct circuit connections of different types of inhibitory neurons. The goal of the proposed studies is to map local and long-range direct synaptic connections to major inhibitory neuronal types in CA1 of the mouse hippocampus. We hypothesize that specific types of inhibitory neurons selectively receive local and distant excitatory synaptic inputs from different brain regions, and that each of these inputs to specific inhibitory neurons differentially contributes to their inhibitory regulation of hippocampal target neurons. Specifically, (1) we aim to identify local excitatory connections to specific types of inhibitory hippocampal neurons by laser scanning photostimulation (LSPS). We have combined LSPS with whole-cell recordings from inhibitory neurons in living brain slices to map intrahippocampal sources of excitatory input to the most numerous inhibitory cell types including parvalbumin-expressing (PV+) basket cells, cholecystokinin- expressing (CCK+) basket cells, axo-axonic cells, and somatostatin-expressing (SOM+) oriens-lacunosum moleculare (O-LM) cells. We will test the hypothesis that axo-axonic cells, PV+ and CCK+ basket cells receive differential strength of excitatory input from CA3 vs. CA1 to support their feedforward and feedback inhibition of CA1 network activity. We will also test the hypothesis that O-LM cells only receive excitation from excitatory pyramidal cells in CA1 and strictly perform local feedback inhibition. (2) We aim to identify long-range synaptic connections to selected groups of inhibitory hippocampal neurons with a novel rabies-based tracing system and optogenetic stimulation. We will use knock-in mouse lines that express Cre (Cre recombinase) in selected groups of inhibitory neurons (e.g., PV-Cre, SOM-Cre or CCK-Cre) to limit rabies infection and monosynaptic retrograde tracing to each selected cell group in the intact brain. The rabies tracing will be followed by channelrhodopsin-assisted circuit mapping to functionally characterize the specificity of distant connections to identified cell type within each targeted Cre-expressing cell group. We will test the hypotheses that PV+ inhibitory cells, but not SOM+ inhibitory cells, receive strong distant connections from entorhinal cortex and the medial septum, and that the CCK+ cell group has strong direct synaptic connections with the amygdala. These studies should establish the operational rules of the major classes of inhibitory neurons within the hippocampal circuit.