Activity-dependent circuit formation in the neocortex

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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.