Distinct Developmental Programs Displayed by the Xenopus Tadpole Accessory Optic System and Retinotectal Projection.

The retinotectal projection, the direct synapse between retinal ganglion cells (RGCs) of the eye and tectal neurons of the optic tectum, is a major component of the amphibian visual system. A model of circuit formation, this projection has been studied in detail. There are, however, other retinorecipient targets that also comprise the amphibian visual system such as the pretectum and ventral midbrain tegmentum. Understanding how these other components of the visual system form and function will lead to a more comprehensive understanding of how the visual system, as a whole, assembles and functions. Toward this aim, here we describe the functional development of the Xenopus tadpole accessory optic system (AOS), a direct synaptic connection between RGC axons and the basal optic nucleus of the midbrain tegmentum. The AOS is highly conserved across vertebrates. It functions as the sensory side of the optokinetic and optomotor reflexes, compensatory eye and body movements, respectively, that stabilize the visual scene as the organism moves through it. Using an isolated brain preparation and whole-cell electrophysiological approaches, we compared the development of the AOS and retinotectal projection. We found that these two retinofugal projections display distinct developmental programs, which appear to mirror their different functions. Retinotectal synapses moved through a dynamic phase of previously described NMDA receptor-dependent refinement, a process that is known to sharpen the retinotopic map and thereby visual acuity. In contrast, the AOS synapse appeared more stable and activity independent across development, indicative of a hardwired circuit, built to support reflexive optic behaviors.