- Project Summary - The goal of this proposal is to determine the role of activity in the development of the circuits that mediate direction selectivity in the retina. Direction selective ganglion cells (DSGCs) fire many action potentials in response to light stimuli moving in a preferred direction and few action potentials to light moving in the opposite, or null, direction. Our lab has used population calcium imaging of DSGCs, whose preferred directions tightly cluster around the four cardinal axes of visual space, to show that depriving animals of visual experience reduces the clustering of preferred directions. Dark-reared adult DSGCs were instead broadly distributed in their preferred directions, similar to DSGCs tuning observed at eye opening. However, the mechanism by which dark-rearing prevents clustering remains unknown. This prompts an investigation of the role of visual experience in the maturation of mechanisms for the direction-selective computation. There are two important circuit elements for direction selectivity. First, asymmetric release of gamma- aminobutyric acid (GABA) from starburst amacrine cells dendrites confers direction selective tuning to DSGCs through asymmetric synaptic wiring. Second, DGSC dendrites integrate inputs in a directional manner. This second mechanism is revealed in a subtype of DSGC, the ventral-preferring DSGCs, which exhibit inhibitory- independent directional tuning, speculated to arise from their asymmetric dendrites. In this proposal, I explore the contribution of synaptic and dendritic mechanisms to directional tuning across development. I focus on these ventral-preferring DSGCs to dissect the relative contributions of asymmetric inhibition and asymmetric dendrites to directional tuning during development. Asymmetric inhibitory input from starburst amacrine cells has been shown to establish directional tuning in DSGCs around the time of eye-opening, by forming more synapses on the null side. As a first step towards understanding the contribution of synaptic mechanisms for establishing directional tuning, in Aim 1, I will use electrophysiology, pharmacology and cellular reconstructions to examine the contribution of inhibitory input on directional tuning during development (Aim 1.1, 1.2). Next, I will test whether activity, mediated by asymmetric inhibitory input, is necessary for establishing directional tuning and asymmetric dendrites in a mouse model where that lacks functional GABA release in SACs (Aim 1.3). To understand the contribution of asymmetric dendrites to inhibitory-independent tuning of DSGCs, in Aim 2, I propose to use simultaneous 2-photon calcium imaging and visual stimulation of dendrites. I ask whether active conductances in the dendrites of DSGCs exist, and if so, I propose...