The torus longitudinalis (TL) is a midbrain structure unique to ray finned fish. Although previously implicated in orienting behaviors elicited by changes in ambient lighting, the role of TL in visual processing is not well understood. TL is known to be reciprocally connected to tectum and is the only known source of synaptic input to the stratum marginalis (SM) layer of the tectal neuropil. Conversely, tectal pyramidal neurons (PyrNs) are the only identified tectal neuron population that forms a dendrite in SM. In this study we identify a zebrafish gal4 transgenic that specifically labels TL neurons that project to SM. We demonstrate that the axonal TL projection to SM in zebrafish is glutamatergic. Consistent with these axons synapsing directly onto PyrNs, SM-targeted dendrites of PyrNs contain glutamatergic postsynaptic specializations. Sparse genetic labeling of individual TL axons and PyrN dendrites enabled quantitative morphometric analysis that revealed 1) large, sparsely branched TL axons in SM and 2) small, densely innervated PyrN dendrites in SM. Together this unique combination of morphologies support a wiring diagram in which TL inputs to PyrNs exhibit a high degree of convergence. We propose that this convergence functions to generate large, compound visual receptive fields in PyrNs. This quantitative anatomical data will instruct future functional studies aimed at identifying the precise contribution of TL-PyrN circuitry to visual behavior.

中文翻译：

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斑马鱼直肠的锥体神经元从环圆环中接收高度收敛的输入。

纵向花托（TL）是中鳍鱼鳍特有的中脑结构。尽管以前牵涉到由环境照明的变化引起的定向行为，但还没有很好地理解TL在视觉处理中的作用。已知TL相互连接到顶盖，并且是向顶盖神经桩的边缘层（SM）层的突触输入的唯一已知来源。相反，圆锥形锥体神经元（PyrNs）是唯一确定的在SM中形成树突的圆锥形神经元种群。在这项研究中，我们确定了斑马鱼gal4转基因，该斑马鱼gal4特别标记了投射到SM的TL神经元。我们证明了到斑马鱼SM的轴突TL投影是谷氨酸能。与这些突触直接突触到PyrNs上一致，PyrNs的SM靶向树突含有谷氨酸能突触后特化。单个TL轴突和PyrN树突的稀疏遗传标记启用了定量形态分析，揭示了1）SM中的大而稀疏分支的TL轴突和2）SM中小的，密集的神经支配的PyrN树突。这种独特的形态组合在一起支持了一个接线图，其中PyrN的TL输入表现出高度的收敛性。我们建议这种收敛功能，以在PyrNs中生成大型的复合视觉感受野。这种定量的解剖数据将指导未来的功能研究，旨在确定TL-PyrN电路对视觉行为的精确贡献。这种独特的形态组合在一起支持了一个接线图，其中PyrN的TL输入表现出高度的收敛性。我们建议这种收敛功能，以在PyrNs中生成大型的复合视觉感受野。这种定量的解剖数据将指导未来的功能研究，旨在确定TL-PyrN电路对视觉行为的精确贡献。这种独特的形态组合在一起支持了一个接线图，其中PyrN的TL输入表现出高度的收敛性。我们建议这种收敛功能，以在PyrNs中生成大型的复合视觉感受野。这种定量的解剖数据将指导未来的功能研究，旨在确定TL-PyrN电路对视觉行为的精确贡献。