Development of visual cortical axons: Layer-specific effects of extrinsic influences and activity blockade

During normal cortical development, individual pyramidal neurons form intra cortical axonal arbors that are specific for particular cortical layers. Py ramidal neurons within layer 6 are able to develop layer-specific projectio ns in cultured slices of ferret visual cortex, indicating that extrinsic in fluences, including patterned visual activity, are not required (Dantzker a nd Callaway [1998] J Neurosci 18:4145-4154). However, when spontaneous acti vity is blocked in cultures with tetrodotoxin, layer 6 pyramidal neurons fa il to preferentially target their axons to layer 4. To determine whether me chanisms that regulate the development of layer 6 pyramidal neuron arbors c an be generalized to pyramidal neurons in other layers, we examined the dev elopment of layer 5 and layer 2/3 pyramidal neurons in cultured slices of f erret visual cortex prepared on postnatal day 14 or 15. Layer 5 pyramidal n eurons developed layer-specific axonal arbors during 5-7 days in vitro. How ever, unlike layer 6 pyramidal neurons, layer 5 pyramidal neurons formed la yer-specific axonal arbors in the presence of tetrodotoxin. In contrast to layer 5 and layer 6 pyramidal neurons, layer 2/3 pyramidal neurons did not form appropriate layer-specific projections during 5-7 days in vitro. Taken together, these data suggest that the development of layer-specific axons is regulated by different mechanisms for neurons in different layers and ca nnot be categorically classified as either activity-dependent or independen t. Instead, the type of pyramidal neuron, the layers targeted, and the type of activity must be considered. J. Comp. Neurol. 430: 321-331, 2001. (C) 2 001 Wiley-Liss, Inc.