Guidance of circumferentially growing axons by netrin-dependent and -independent floor plate chemotropism in the vertebrate brain.

Netrin-1, a diffusible signal secreted by floor plate cells at the ventral midline of the vertebrate CNS, can attract ventrally migrating axons and repel a subset of dorsally migrating axons in the spinal cord and rostral hindbrain in vitro. Whether netrin-1 can act as a global cue to guide all circumferentially migrating axons is, however, unknown. Here, we show that netrin-1 can attract alar plate axons that cross the floor plate along its entire rostrocaudal axis. Dorsally directed axons forming the posterior commissure are, however, repelled by the floor plate by a netrin-independent mechanism. These results suggest that netrin-1 functions as a global guidance cue for attraction to the midline. Moreover, floor plate-mediated chemorepulsion may also operate generally to direct dorsal migrations, but its molecular basis may involve both netrin-dependent and -independent mechanisms.