Trophic interactions during visual system development

描述(申请人提供)：拟议的研究将检查视网膜投射神经元与其中心目标建立精确和功能联系的机制。神经营养因子调节视神经轴突树枝形成以及突触形成和稳定的假说将在活体发育的大脑中得到验证。具体地说，神经营养因子脑源性神经营养因子(BDNF)控制轴突和树突树枝形成以及突触形成/稳定的细胞和分子机制将在活体麻醉的蝌蚪中进行检测。非洲爪哇的视觉系统提供了一个独特的可接触的脊椎动物模型，在该模型中，神经元连接的发展可以在完整的胚胎中随着时间的推移进行跟踪。胚胎操作和活体成像技术(延时共聚焦显微镜和钙成像技术)将结合在一起，实时跟踪突触前和突触后伙伴的形态分化，以及视网膜和其目标视顶盖之间突触连接的形成。通过对单个视轴突终枝和/或顶盖神经元树突枝进行成像，同时对完整动物的突触位置进行可视化，将检验以下假设： BDNF通过促进视轴突和靶顶盖神经元之间的突触稳定来调节发育中的视网膜输入之间的活性依赖性竞争。 靶源性脑源性神经营养因子直接影响视轴突终末调节轴突树枝形成和突触形成。脑源性神经营养因子引起视轴突杆复杂性的改变导致顶盖神经元突触复杂性的结构改变。 脑源性神经营养因子诱导树枝状视轴突内细胞内钙离子水平的局部变化，这些变化负责突触的稳定和新分支的形成。 这些研究结果将对活脑中突触发生的基本机制提供有价值的见解，并将进一步加深我们对控制视觉通路发展的机制的理解，这些机制对维持正常的视觉功能至关重要。

DESCRIPTION (provided by applicant): The proposed studies will examine the mechanisms by which retinal projection neurons make precise and functional connections with their central targets. The hypothesis that neurotrophic factors modulate optic axon arborization, and synapse formation and stabilization will be tested in the live developing brain. Specifically, the cellular and molecular mechanisms by which the neurotrophin brain-derived neurotrophic factor (BDNF) controls axon and dendritic arborization, and synapse formation/stabilization will be examined in live, anesthetized tadpoles. The Xenopus laevis visual system offers a uniquely accessible vertebrate model in which the development of neuronal connections can be followed over time in the intact embryo. Embryologic manipulations and in vivo imaging techniques (time-lapse confocal microscopy and calcium imaging techniques) will be combined to follow, in real time, the morphological differentiation of pre- and post-synaptic partners and the formation of synaptic connections between the retina and its target optic tectum. By imaging individual optic axon terminal arbors and/or tectal neuron dendritic arbors while simultaneously visualizing synaptic sites in the intact animal, the following hypotheses will be tested: BDNF modulates activity-dependent competition between developing retinal inputs by promoting synapse stabilization between optic axons and target tectal neurons. Target-derived BDNF directly influences optic axon terminals to modulate axon arborization and synapse formation. The BDNF-elicited changes in optic axon arbor complexity lead to structural changes in tectal neuron synaptic complexity. BDNF induces localized changes in intracellular calcium levels in arborizing optic axons that are responsible for synapse stabilization and the formation of new branches. The results of these studies will provide valuable insights into fundamental mechanisms of synaptogenesis in the living brain, and will further our understanding of the mechanisms that control the development of visual pathways, that are critically important in the maintenance of normal visual function.