Regeneration of an abnormal ipsilateral visuotectal projection in Xenopus is delayed by the presence of optic fibres from the other eye

Abstract

Optic fiber regeneration was studied by [3H]proline autoradiography and by mapping electrophysiologically the direct visuotectal projections to the contralateral and ipsilateral tecta 12-16 wk after sectioning the right optic nerve in Xenopus 2 to 4 wk after metamorphosis. The experiments were carried out in 3 groups: (A) optic nerve section in newly metamorphosed animals with embryonic left-eye enucleation; (B) optic nerve section with simultaneous left-eye enucleation; and (C) optic nerve section with delayed left-eye enucleation 5-31 days prior to sacrifice. In all but 3 animals regenerated optic fibers were demonstrated morphologically both in the contralateral and ipsilateral tecta. The contralateral visuotectal projection was fully restored within the 12-16 wk in all animals. In animals with embryonic enucleation or in those where the enucleation was carried out simultaneously with optic nerve section, a direct ipsilateral visuotectal projection was established at the same time as the restoration of the contralateral projection. In contrast, no direct ipsilateral visuotectal projection was detectable in the presence of optic fibers from the other eye in the doubly innervated tecta. However, 14-31 days after the removal of the incumbent optic fibers by left-eye enucleation a direct visuotectal projection from the right eye to the ipsilateral tectum was established, and its polarity corresponded to the normal contralateral retinotectal projection. The apparent disparity between morphological and electrophysiological findings in the doubly innervated tectum suggests that superimposed optic fibers are unable to form normal synaptic relations with tectal neurons during early regeneration, delaying the establishment of the induced direct ipsilateral visuotectal projection.