Local directional cues control growth polarity of dopaminergic axons alongthe rostrocaudal axis

The vertebrate CNS is composed of a variety of longitudinal axonal tracts e xtending rostrally and caudally. Although recent studies have demonstrated that chemoattraction and chemorepulsion play key roles in axon guidance alo ng the circumferential axis in the neural tube of the vertebrate, mechanism s of axonal elongation along the longitudinal axis, and most importantly, w hat determines rostrocaudal polarity of axonal growth, remains unknown. Her e, we examined the mechanism that guides midbrain dopaminergic axons rostra lly, using flat wholemount preparations of embryonic rat brain both in vivo and in vitro. At embryonic day 11 (E11) and early stage E12, dopaminergic neurons in the ventral midbrain extended short axons dorsally. By middle stage E12, these axons had increased in number, some deflecting rostrally and others caudall y. At E13, almost all axons showed rostrally oriented growth heading toward the forebrain targets. In in vitro whole-mount preparations prepared from an E12 embryo and cultured for 24 hr, these axons showed rostrally oriented growth, but when they were forced to grow on substratum of reversed rostro caudal polarity, they turned abruptly and grew following the polarity of th e reversed midbrain substratum. These results suggest that local directiona l cues in the midbrain guide these axons rostrally and support the idea tha t substratum-associated polarized cues play an important role in axon guida nce along the longitudinal axis.