Project Summary/Abstract Vision is the most valuable of human senses for interpreting the world, and visual inputs are profoundly shaped and transformed within the retina to extract a scene?s most salient characteristics. The conversion of visual input to neuronal output begins with rod and cone photoreceptors, the first, light-sensitive neurons of the visual system. The properties of neurotransmission at the first synapse in the retina begin to constrain and shape the information that is delivered from photoreceptors to downstream retinal neurons, and eventually, the visual cortex for perception. The overall goal of this research proposal is to characterize key properties of neurotransmission in rods and cones in mouse retina. Neurotransmission from photoreceptors, like all other neurons, is a Ca2+-dependent process. In lower vertebrates, the characteristics of Ca2+-triggered vesicular release from photoreceptors exhibit unique properties. The broad goals of this research proposal are to test the hypothesis that mammalian photoreceptors share these unique characteristics and to identify the molecular Ca2+ sensors that mediate synaptic release from mouse photoreceptors. Specific Aim 1A will test the hypothesis that the Ca2+ dependence of vesicular release from mouse photoreceptors is triggered by a high- affinity, low-cooperativity mechanism like that found in amphibian photoreceptors. The goal of Specific Aim 1B is to describe the kinetics of vesicular release from mouse rods and cones, with the hypothesis that both rods and cones exhibit fast release but only rods exhibit prominent slow release. Specific Aim 2 will test the hypothesis that one or more candidate synaptotagmin isoforms is the Ca2+ sensor that operates in mammalian photoreceptors. To pursue these aims, ex vivo (slice electrophysiology) and in vivo (electroretinography) experiments will be performed using wild-type mice and transgenic strains with rod- and cone-specific inactivation of the most likely synaptotagmin isoforms suggested by preliminary data. The research proposed is an essential first step in using transgenic mouse models to study normal and pathological function at photoreceptor synapses. Characterizing mouse photoreceptor neurotransmission will help us to understand fundamental retinal processes in mammals, which is required to develop realistic human vision restoration therapies. The proposed fellowship training plan will provide extensive research and clinical training to help the applicant reach his ultimate career goal of becoming a successful physician-scientist.