The role of nitric oxide in development of topographic precision in the retinotectal projection of chick

The axonal projection from the retina to the tectum exhibits a precise topo graphic order in the mature chick such that neighboring ganglion cells send axons to neighboring termination zones in the contralateral tectum. The in itial pattern formed during development is much less organized and is refin ed to the adult pattern during a discrete period of development. Refinement includes elimination of radically aberrant projections, such as those from the temporal side of the retina to posterior regions of the tectum, as wel l as a more subtle improvement in the topographic precision of the projecti on. The enzyme that synthesizes nitric oxide is expressed at high levels in the tectum during the developmental period in which the topography improve s. Pharmacological blockade of nitric oxide synthesis during this period pr evented elimination of topographically inappropriate retinotectal projectio ns in a dose-dependent manner. This effect could not be duplicated by treat ment of embryos with a vasoconstrictor, indicating that vascular changes we re not a factor. These results show that nitric oxide is involved in refine ment of the topography of the retinotectal projection as well as in other a spects of refinement of this projection in developing chick.