Binocular maps in Xenopus tectum: Visual experience and the development of isthmotectal topography.

Xenopus frogs have a prominent binocular field that develops as a consequence of the migration of the eyes during the remodeling of the head during and after metamorphosis. In the optic tectum, a topographic representation of the ipsilateral eye develops during this same period. It is relayed indirectly, via the nucleus isthmi. In the early stages of binocular development, the topographic matching of the ipsilateral input to the retinotectal input from the contralateral eye is largely governed by chemical cues, but the ultimate determinant of the ipsilateral map is binocular visual input. Visual input is such a dominant factor that abnormal visual input resulting from unilateral eye rotation can induce isthmotectal axons to alter their trajectories dramatically, even shifting their terminal zones from one pole of the tectum to the other. This plasticity normally is high only during a 3-4-month critical period of late tadpole-early juvenile life, but the critical period can be extended indefinitely by dark-rearing. N-methyl-D-aspartate (NMDA) receptors are involved in this process; plasticity can be blocked or promoted by chronic treatment with NMDA antagonists or agonists, respectively. Cholinergic nicotinic receptors on retinotectal axons are likely to play an essential role as well. Modifications in the polysialylation of neural cell adhesion molecule are correlated with the state of plasticity. The circuitry underlying binocular plasticity is not yet fully understood but has proved not to be a simple convergence of ipsilateral and contralateral inputs onto the same targets.