Summary The long-term objective of this project is to understand how the unique structure of the mammalian cone photoreceptor synapse determines its function in vision. The outer segments of cones, which are specialized for daylight vision, convert absorbed photons into a voltage signal. In an essential step for visual perception, the voltage signal spreads to the photoreceptor synaptic terminal where it gates Ca2+ channels that control the release of the transmitter glutamate onto postsynaptic bipolar and horizontal cells. The cone synapse is a site at which parallel processing begins in the visual system. A single cone contacts ~14 bipolar cell types, which in turn carry signals to over 40 different types of retinal output neurons, the ganglion cells, in the inner retina. The mammalian cone terminal has a complex and stereotyped spatial organization that incorporates two unique structures, the invaginating and basal junctions. Transmitter is released from sites atop ~twenty 200-400 nm long membrane invaginations that open onto a ~3 µm diameter basal surface. “On” bipolar cells, cells that respond at light onset, and horizontal cells contact cones within invaginations, whereas “Off” bipolar cells, which respond at light offset, contact at the base, although there are minor exceptions to these rules. The different processing functions of basal and invaginating contacts are poorly understood. By recording from cones and postsynaptic bipolar cells in slices from the cone-dominant ground squirrel retina, the Aims seek to understand how the cone visual signal is decomposed by the ~14 different types of bipolar cells at basal and invaginating cone contacts. The first aim seeks to identify novel types of Off bipolar cells and to characterize their visual functions. An understanding of parallel processing at the cone synapse relies on a complete identification of the bipolar cell types and a characterization of their signaling properties. The second Aim addresses the function of Off bipolar cell kainate receptor desensitization during visual stimuli. Receptor desensitization during strong stimuli may mediate an adaptation that aids visual processing by matching the full dynamic range of the cone synapse to the level of light intensity variation in the visual stream. A third Aim addresses the visual function of basal synapses made by On bipolar cells; a type of connection that goes against the general pattern described above. Experiments will determine whether basally contacting On bipolar cells mediate a high threshold signaling pathway like that mediated by basally contacting Off bipolar cells. Finally, a 4th aim proposes to use a new super-resolution microscopy technique, MINFLUX, to visualize the proteins that link the cone to bipolar cells across the synaptic cleft at invaginating contacts. The completion of this work will help us understand the mechanisms underlying the first step in parallel synaptic processing in the visual system. In addition, t...