Hippocampal inhibitory neuronal circuit organization

描述（由申请人提供）：海马中含有多种gaba能抑制神经元。先前的研究表明，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。我们假设抑制神经元之间的功能差异是由不同类型的抑制神经元的不同回路连接引起的。本研究的目标是绘制小鼠海马CA1中主要抑制性神经元类型的局部和远程直接突触连接。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大脑区域的局部和远端兴奋性突触输入，并且每种输入到特定抑制性神经元的差异有助于它们对海马目标神经元的抑制调节。具体来说，(1)我们的目标是通过激光扫描光刺激（LSPS）识别局部兴奋性连接到特定类型的抑制性海马神经元。我们将LSPS与活脑片中抑制性神经元的全细胞记录相结合，将海马内兴奋性输入的来源映射到大多数抑制性细胞类型，包括表达小蛋白（PV+）篮细胞、表达胆囊收缩素（CCK+）篮细胞、轴突-轴突细胞和表达生长抑素（SOM+）取向-空白分子（O-LM）细胞。我们将验证轴-轴突细胞、PV+和CCK+篮细胞接受来自CA3和CA1的不同强度的兴奋性输入的假设，以支持它们对CA1网络活性的前馈和反馈抑制。我们还将验证O-LM细胞只接受CA1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)我们的目标是通过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群的远距离突触连接。我们将使用在选定的抑制性神经元组（例如PV-Cre， SOM-Cre或CCK-Cre）中表达Cre （Cre重组酶）的敲入小鼠系来限制狂犬病感染和对完整大脑中每个选定细胞组的单突触逆行追踪。狂犬病追踪将随后进行通道视紫红质辅助电路制图，以功能表征每个靶向cre表达细胞组中与鉴定细胞类型的远端连接的特异性。我们将测试PV+抑制细胞，而不是SOM+抑制细胞，接受来自内嗅皮层和内侧隔的强远端连接，CCK+细胞组与杏仁核有强直接突触连接的假设。这些研究应该建立海马回路中主要种类的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.