Polysialylated neural cell adhesion molecule and plasticity of ipsilateral connections in Xenopus tectum

The optic tectum of Xenopus offers a readily manipulated system for testing the hypothesis that polysialylation of the neural cell adhesion molecule is associated with axonal plasticity. Axons relaying input to the tectum from the ipsilateral eye employ visual input to establish a topographic map in register with the contralateral map, despite naturally-occurring or surgically-induced repositioning of the eyes. This capacity for activity-dependent refinement or re-organization of the ipsilateral map is normally confined to a period between about one and four months postmetamorphosis but can be restored in adults by local application of N-methyl-D aspartate to the tectum. In addition, dark-rearing prolongs plasticity indefinitely. We have used immunohistochemical staining with antibodies to polysialic acid to determine whether conditions of high plasticity are correlated with high levels of polysialylated neural cell adhesion molecule in the tectum. We find that the staining level is high in tecta from one to three month postmetamorphic frogs but is low both before and after this period. Thus, in normal Xenopus frogs, anti-polysialic acid staining is heavier in the period of high plasticity than in the preceding or following postmetamorphic periods. As a further test of this relationship, we examined brains of adults with experimentally-induced plasticity. Tecta of N-methyl-D-aspartate-treated adults and of dark-reared adults showed higher levels of staining than did the tecta of normally-reared adults. These results also support the hypothesis that the presence of high levels of polysialic acid on neural cell adhesion molecules is causally related to activity-related changes in axonal growth patterns.