Calcium-regulated release of neurotransmitter is a fundamental feature of synaptic communication. How this process is regulated presynaptically is not yet fully understood, particularly at the ribbon-style synapses of retinal photoreceptors and bipolar cells. These synapses differ from conventional synapses in several important ways, including the expression of unusual isoforms of key synaptic proteins and the absence of other synaptic proteins. Thus, it is not clear to what extent knowledge of presynaptic mechanisms obtained in other model systems can be extrapolated to these neurons. Furthermore, mutations in some of these unusual proteins are associated with human retinal disease. The long-term goal of this research project is to understand how visual information is transferred across the retina. In this proposal, we examine mechanisms of exocytosis and synaptic vesicle dynamics in bipolar cells and photoreceptors of the vertebrate retina. This information will reveal aspects of presynaptic design that allows these neurons to release glutamate rapidly and for extended periods of time. A combination of biophysical, ultrastructural, and molecular approaches are used. Specific goals are to examine the mechanisms underlying the requirement for ATP hydrolysis in maintaining neurotransmitter release and endocytosis in ON-bipolar cells. Because endocytosis may contribute to the supply of active zone synaptic vesicles in addition to preserving membrane surface area, mechanisms of membrane retrieval in bipolar cells and photoreceptors will also be examined. To allow us to determine in what ways photoreceptors are adapted for tonic release and to define those presynaptic mechanisms that are hallmarks of retinal ribbon synapses, synaptic vesicle dynamics in photoreceptors will be examined and compared with those of bipolar cells and conventional synapses. Finally, we will develop the mouse bipolar cell as a preparation for biophysical study of presynaptic mechanisms. This will position us to study presynaptic mechanisms at the molecular level and examine the role of specific synaptic proteins implicated in disorders of vision. ? ?
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