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Local Directional Cues Control Growth Polarity of Dopaminergic Axons Along the Rostrocaudal Axis

Shin-ichiro Nakamura¹, Yasuko Ito¹, Ryuichi Shirasaki^{1, 2}, and Fujio Murakami^{1, 2, 3}

¹ Laboratory of Neuroscience, Division of Biophysical Engineering, Graduate School of Engineering Science, and ² Core Research for Evolutional Science and Technology/Murakami Laboratory, Center for Advanced Research Projects, Osaka University, Machikaneyama 1-3, Toyonaka 560-8531, Japan, and ³ Division of Behavior and Neurobiology, National Institute for Basic Biology, Myodaiji-cho, Okazaki 444-8585, Japan

The vertebrate CNS is composed of a variety of longitudinal axonal tracts extending rostrally and caudally. Although recent studies have demonstrated that chemoattraction and chemorepulsion play key roles in axon guidance along the circumferential axis in the neural tube of the vertebrate, mechanisms of axonal elongation along the longitudinal axis, and most importantly, what determines rostrocaudal polarity of axonal growth, remains unknown. Here, we examined the mechanism that guides midbrain dopaminergic axons rostrally, using flat whole-mount 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 caudally. 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 rostrocaudal polarity, they turned abruptly and grew following the polarity of the reversed midbrain substratum. These results suggest that local directional cues in the midbrain guide these axons rostrally and support the idea that substratum-associated polarized cues play an important role in axon guidance along the longitudinal axis.

Key words: rat embryo; whole-mount culture; local directional cue; dopaminergic neuron; tyrosine hydroxylase (TH); polarity; longitudinal axis

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Copyright © 2000 Society for Neuroscience  0270-6474/00/20114112-08$05.00/0

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