The Development of Abnormal Axon Trajectories after Rotation of
One Eye in Xenopus
Yujin
Guo and
Susan B.
Udin
Department of Physiology and Biophysics, State University of New
York, Buffalo, New York 14214
The targeting of isthmotectal axons in the Xenopus
binocular pathway is guided by both activity-dependent cues and
activity-independent cues. Abnormal visual activity induced by
unilateral eye rotation overrides activity-independent cues and causes
isthmotectal axons to arborize at new locations during a critical
period of development that ends ~3 months postmetamorphosis (PM).
Horseradish peroxidase staining of isthmotectal axons reveals that they
normally run rostrocaudally in the tectum; in contrast, those axons in
animals with early eye rotation have circuitous trajectories. In this paper, by studying the trajectories and branching patterns of isthmotectal axons at different times after eye rotation, we aimed to
investigate when and how activity cues determine the projection pattern
of isthmotectal axons. As suggested by electrophysiological recording,
isthmotectal axons initially grow normally and make arbors according to
activity-independent cues despite the presence of abnormal visual
input. Our findings demonstrate that the development of abnormal
trajectories starts by 2 weeks PM in response to eye rotation and is a
protracted process. It begins in the tectal regions in which the
initial connections of isthmotectal axons are first formed according to
activity-independent cues. At transitional stages (5 and 10 weeks),
axons with arbors at two different locations are observed, with
locations corresponding to the old and new termination sites,
respectively. Later, at 10 weeks of age, the fainter horseradish
peroxidase staining in arbors at old termination sites suggests that
the older arbors are undergoing withdrawal.
Key words:
optic tectum; Xenopus; activity-dependent
synaptic modification; unilateral eye rotation; axon trajectories; arbors; development; nucleus isthmi
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