Assessing direction selectivity map development in the retina

Project Summary Visual information is processed through a set of neural circuits that organize into functional maps distributed throughout the thalamus, midbrain, and cerebral cortex. However, little is known about how circuits develop and organize in the retina, where this information processing begins. The goal of this proposal is to determine the mechanisms underlying the development of the circuits that mediate direction selectivity (DS) in the retina. Classic studies show that the distribution of preferred directions, referred to as the DS map, align with the cardinal axes–– superior-inferior and anterior- posterior. However, recent characterization has shown that the DS map follows the axes defined by optic flow. As a result, the DS map across the adult retina changes as a function of location, where the clusters are orthogonal to one another closer to the optic nerve and become skewed as distance from the optic nerve increases. This map is present at eye opening. How this complex organization arises prior to eye opening is not known. This prompts an investigation of the developmental factors that contribute to the formation of DS maps. Direction-selective ganglion cells (DSGCs) respond robustly to motion in a preferred direction and weakly to motion in the opposite, or null, direction. In order to achieve this computation, DSGCs receive greater synaptic inhibition during null direction motion from starburst amacrine cells (SACs) via precise wiring patterns. Interestingly, during the developmental period where DSGCs are wiring up with SACs, the retina is spontaneously active. This activity presents itself as waves propagating across the surface of the retina––termed retinal waves. In this proposal, I will explore the role of retinal waves, specifically waves driven by cholinergic signaling, in the development of DS maps. Additionally, I propose to investigate the synaptic basis underlying the formation of this distinct organization across the retinal surface. As a first step towards understanding whether retinal waves influence DS map formation, I will use two-photon population calcium imaging, genetic tools, and pharmacology to assess how the DS map develops in the presence and absence of patterned spontaneous activity across development. To achieve this, I will use a mouse model where cholinergic waves are severely disrupted by knocking out the β2 subunit of the nicotinic acetylcholine receptor (Aim 1). Moreover, given the extent to which asymmetric inhibition is necessary for directional tuning, I propose that the tuning of inhibitory inputs onto DSGCs will change at varied locations in the retina, to account for the skewing of preferred directions. To test this, I will use two-photon-targeted voltage clamp recordings to unmask the synaptic basis of this organization (Aim 2). These findings will provide key insights into the mechanisms that underlie this precise organization during development.