Development and function of the Xenopus tadpole retinotegmental projection

University Of Wyoming, Laramie WY

Investigators

Abstract

New-born neurons (brain cells) have the amazing ability to self-assemble into circuits – groups of functionally connected neurons. These circuits give rise to our perceptions, thoughts, emotions, and behaviors. Understanding how neurons self-assemble into circuits is an important aim in the field of neuroscience, and it is the overall aim of this project. This would be difficult to study in mammals with their billions of neurons, complex circuits, and variable behaviors. This project studies a more experimentally tractable circuit: the developing visual system of the African Clawed Frog (Xenopus) tadpole. Previous studies on the tadpole visual system have focused on the retinotectal projection, a circuit consisting of the synapse between the retinal ganglion cells (RGCs) in the eye and the optic tectum, a major visual processing center on the dorsal (top) surface of the amphibian midbrain. In preparatory work for the current project, we characterized a second projection between the retinal ganglion cells from the Xenopus eye and the ventral (bottom) midbrain called the retinotegmental projection. Preliminary data indicate that these two projections are built differently and carry out different functions within the visual system. The goal of this project is to provide a detailed description of the development and function of the retinotegmental projection. Overall, this work will contribute important new insights about how neural circuits form and carry out specific functions. Educational and broader impact activities for this project include teaching a “Vision and Art” course for non-science undergraduates, and an upper-level developmental science lab course in which students design and carry out experiments to determine how environmental factors impact the developing Xenopus embryo. How neurons self-assemble into circuits that give rise to behaviors is a fundamental question in neuroscience. For decades, the Xenopus tadpole retinotectal projection – the synapse between the retinal ganglion cells (RGCs) in the eye and the midbrain optic tectum – has served as a popular model to study this question. But the retinotectal projection is only one of several components of the vertebrate visual system. Our recent work shows that color-dependent phototaxis in tadpoles does not require the optic tectum, but does require the tegmentum, a region of the midbrain that lies ventral to the optic tectum. We also found several color-tuned neuronal populations in the tegmentum. Through additional anatomical and electrophysiological studies, we identified a retinotegmental projection, a direct projection from the RGCs to the midbrain tegmental neurons. This projection appears to lack the developmental plasticity displayed by the retinotectal projection, suggesting a more hard-wired circuit. A highly conserved retinotegmental projection, termed the accessory optic system (AOS), has been described across a wide range of adult vertebrates, from frogs to humans. It is associated with optomotor and optokinetic responses – reflexive body and eye movements, respectively, that stabilize vision as the organism moves through space. It is likely that the retinotegmental projection we are studying in the tadpole is, at least in part, the AOS. Through a combination of electrophysiology-based approaches and circuit tracing, this work will provide a detailed description of the development and differentiation of the Xenopus tadpole retinotegmental projection and its role in processing visual stimuli. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.