Hippocampal inhibitory neuronal circuit organization

描述(申请人提供)：海马区含有不同类型的GABA能抑制神经元。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于不同类型抑制性神经元的不同电路连接造成的。这项研究的目的是定位局部和远程直接突触连接到小鼠海马区CA1中的主要抑制性神经元类型。我们假设特定类型的抑制性神经元选择性地接受来自不同脑区的局部和远程兴奋性突触输入，并且这些输入到特定抑制性神经元的每一种都不同地参与了它们对海马靶神经元的抑制调节。具体地说，(1)我们的目标是通过激光扫描光刺激(LSPs)识别与特定类型抑制性海马神经元的局部兴奋性连接。我们将LSP与活体脑片中抑制性神经元的全细胞记录相结合，将兴奋性输入的海马区来源映射到最大量的抑制性细胞类型，包括副蛋白表达(PV+)篮细胞、CCK+篮细胞、轴突细胞和生长抑素表达(SOM+)定向腔隙分子(O-LM)细胞。我们将验证轴突细胞、PV+和CCK+篮子细胞从CA3和CA1接受不同强度的兴奋性输入的假设，以支持它们对CA1网络活动的前馈和反馈抑制。我们还将验证O-LM细胞只接受CA1区兴奋性锥体细胞的兴奋并严格执行局部反馈抑制的假设。(2)我们的目标是利用一种新的基于狂犬病的追踪系统和光遗传刺激来识别与选定的抑制性海马神经元组的远程突触连接。我们将使用在选定的抑制性神经元组(如PV-Cre、SOM-Cre或CCK-Cre)中表达Cre(Cre重组酶)的敲入鼠系来限制狂犬病感染，并对完整大脑中的每个选定细胞组进行单突触逆行追踪。狂犬病追踪之后，将进行通道视紫红质辅助的电路映射，以从功能上表征远距离连接到每个目标Cre表达细胞组中已识别的细胞类型的特异性。我们将检验这样的假设，即PV+抑制细胞，而不是SOM+抑制细胞，从内嗅皮层和内侧隔接受强大的远距离联系，CCK+细胞群与杏仁核有强大的直接突触联系。这些研究应该建立起海马环路内主要类别抑制性神经元的运作规则。

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.