Activity-dependent circuit formation in the neocortex

在发育中的视皮层中，来自上层细胞的轴突沿着相同的层行进并形成分支和突触连接。迄今为止的形态学和生理学研究表明，水平连接的形成依赖于神经元的活动。然而，电活动如何影响单个轴突的分支尚不清楚。此外，这一过程的分子机制几乎是未知的。我们通过使用皮质切片的器官型培养制剂来解决这个问题。一种新的轴突示踪方法，其中一小部分的神经元标记与绿色荧光蛋白也被开发。这些方法使我们能够长时间观察单个的轴突。首先，来自上层皮层细胞的轴突在体外以类似于体内的方式形成分支。其次，分枝伴随着生长和收缩，表明分支的形成是一个动态过程。第三，当整体神经元活性被药物治疗抑制时，分支被下调。最后，结果表明，RhoA，一个小的GTdR，参与细胞内的机制，将电信号转换为轴突形态。

In the developing visual cortex axons from upper layer cells travel along the same layers and form branches and synaptic connections. To date morphological and physiological studies have indicated that the formation of the horizontal connections is dependent on neuronal activity. However, how electrical activity influences branching of individual axons is not known. In addition, the molecular mechanisms underlying this process are almost unknown. We address this issue by using organotypic culture preparations of cortical slices. A new axon tracing method in which a small fraction of neurons are labeled with GFP was also developed. These methods enable us to observe individual axon arbors for a long period.In this study we have obatined the following results. First, axons from cortical cells in the upper layers formed branches in vitro in a fashion which is similar to that in vivo. Second, branching was accompanied by growth and retraction, suggesting that branch formation is a dynamic process. Third, branching was down-regulated when overall neuronal activity was suppressed by pharmacological treatments. Finally, the result suggests that RhoA, a small GTPase, is involved in intracellular mechanisms which convert electrical signals to the axon morphology.