Systematic analysis of the neural circuits connecting the lower-order sensory neuropils and higher-order integrative regions of the Drosophila brain

在昆虫中，视觉信息在视叶中进行处理，并传递到中央大脑。虽然人们对视叶内的神经回路进行了广泛的研究，但对视叶和中央脑之间的联系知之甚少。为了了解中央脑神经元如何读取视觉信息，以及什么样的离心神经元将控制信号从中央脑发送到视叶，我们对连接黑腹果蝇视叶和中央脑的视觉投射神经元进行了系统的分析。通过筛选大约4,000个GAL4增强子陷阱菌株，我们确定了44条途径。通过表达胞浆和突触前靶向的荧光报告，研究每条通路的整体形态和信息的方向。一个规范的命名系统被引入来描述中央大脑的投射区域。我们特别集中于14个特别来自小叶的视觉投射神经元。小叶内有8条小径形成柱状树枝，其余6条小径形成切向或树状树枝。11条是向心通路，其中9条终止于前脑腹外侧部。每条柱状通路的末端在前脑腹外侧部的不同区域形成肾小球样结构。大脑后外侧原脑和视结节各由一条向心路径贡献。另一条通路连接大脑两侧的小叶。两条离心式通路将信号从后外侧前脑传递到小叶。

In insects, visual information is processed in the optic lobe and conveyed to the central brain. Although neural circuits within the optic lobe have been studied extensively, relatively little is known about the connection between the optic lobe and the central brain. To understand how visual information is read by the neurons of the central brain, and what kind of centrifugal neurons send the control signal from the central brain to the optic lobe, we performed a systematic analysis of the visual projection neurons that connect the optic lobe and the central brain of Drosophila melanogaster. By screening about 4,000 GAL4 enhancer-trap strains, we identified 44 pathways. The overall morphology and the direction of information of each pathway were investigated by expressing cytoplasmic and presynapsis-targeted fluorescent reporters. A canonical nomenclature system was introduced to describe the area of projection in the central brain. We especially concentrated on the 14 visual projection neurons arising specifically from the lobula. Eight pathways form columnar arborization in the lobula, whereas the remaining six form tangential or tree-like arborization. Eleven are centripetal pathways, among which nine terminate in the ventrolateral protocerebrum. Terminals of each columnar pathway form glomerulus-like structure in different areas of the ventrolateral protocerebrum. The posterior lateral protocerebrum and the optic tubercle were each contributed by a single centripetal pathway. Another pathway connects the lobula on each side of the brain. Two centrifugal pathways convey signals from the posterior lateral protocerebrum to the lobula.