ACCELERATION BY NMDA TREATMENT OF VISUALLY INDUCED MAP REORGANIZATIONIN JUVENILE XENOPUS AFTER LARVAL EYE ROTATION

Each tectal lobe of Xenopus frogs receives two topographic maps, one v ia the ipsilateral eye and one via the contralateral eye. The alignmen t of the ipsilateral map with the contralateral map depends upon binoc ular visual input during a critical period that extends from late tadp ole to early juvenile stages. Rotation of one eye during the critical period leads to reorganization of the ipsilateral map, which eventuall y comes back into alignment with the contralateral map despite the abn ormal 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 m apping. In this paper, the possible role of the NMDA receptor in the d elay in reorganization is addressed. The degree of NMDA receptor activ ation may need to be above some threshold level to trigger reorganizat ion. If NMDA receptor activation normally is below that level until af ter the first month postmetamorphosis, then exogenous NMDA might boost the process sufficiently to start the reorganization process sooner t han 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 wee ks, starting at 1 week postmetamorphosis. Electrophysiological mapping demonstrated that reorganization takes place more rapidly than in unt reated frogs or frogs treated with vehicle only. This result is consis tent with the interpretation that the activation of the NMDA receptor is a rate-limiting step in the activity-dependent matching of binocula r maps in Xenopus tectum. (C) 1994 John Wiley & Sons, Inc.