Alterations in the Xenopus retinotectal projection by antibodies to Xenopus N-CAM.

The patterned neural projection from the eye to the optic tectum of lower vertebrates (the retinotectal projection) has been proposed to be ordered by interactions between the optic nerve fibers and their surrounding tissues. To investigate the role of one such defined cell interaction, agarose implants containing antibodies to the neural cell adhesion molecule, N-CAM, were inserted into the tectum of the African clawed frog, Xenopus laevis. Both monoclonal and polyclonal antibodies against N-CAM reversibly and specifically distorted the pattern of the retinotectal projection, decreasing the precision of the projection as determined by electrophysiological techniques as well as decreasing the density of retinal innervation of the tectum and the branching of single axons as determined by horseradish peroxidase tracing. The anatomical effects became maximal at 4 to 6 days after implantation and returned to undetectable levels by 2 weeks, whereas the physiological effects became maximal by 8 to 10 days and a normal physiological map was reestablished within 4 weeks. The results are consistent with the hypothesis that anti-N-CAM antibodies perturb the ongoing growth and retraction of the terminal arbors of the optic nerve fibers, such that a region of the tectum becomes largely denuded of fibers. The physiological defects may then be a consequence both of the initial retraction of optic nerve terminals and of the rapid ingrowth of the perturbed and neighboring optic nerve fibers into the denuded region after the antibodies were cleared from the tectum. These results support the concept of a major role for N-CAM-mediated adhesion during map regeneration and maintenance.