Acceleration by NMDA treatment of visually induced map reorganization in juvenile Xenopus after larval eye rotation

Abstract

Each tectal lobe of Xenopus frogs receives two topographic maps, one via the ipsilateral eye and one via the contralateral eye. The alignment of the ipsilateral map with the contralateral map depends upon binocular visual input during a critical period that extends from late tadpole to early juvenile stages. Rotation of one eye during the critical period leads to reorganization of the ipsilateral map, which eventually comes back into alignment with the contralateral map despite the abnormal eye position. The ipsilateral eye's map initially develops as if there had been no alteration in eye position; there is a delay of 4-6 weeks before reorganization can be detected by electrophysiological mapping. In this paper, the possible role of the NMDA receptor in the delay in reorganization is addressed. The degree of NMDA receptor activation may need to be above some threshold level to trigger reorganization. If NMDA receptor activation normally is below that level until after the first month postmetamorphosis, then exogenous NMDA might boost the process sufficiently to start the reorganization process sooner than usual. In order to test this possibility, the left eye of tadpoles was rotated and NMDA was applied to the right tectal lobe for 3-5 weeks, starting at 1 week postmetamorphosis. Electrophysiological mapping demonstrated that reorganization takes place more rapidly than in untreated frogs or frogs treated with vehicle only. This result is consistent with the interpretation that the activation of the NMDA receptor is a rate-limiting step in the activity-dependent matching of binocular maps in Xenopus tectum.