Cone photoreceptors tonically release glutamate in the dark. Light hyperpolarizes cones and causes a graded reduction in glutamate release. Understanding how cones regulate transmitter release is crucial for understanding how visual information is transmitted to bipolar and horizontal cells. Previous studies have indirectly inferred presynaptic release from cones by measuring postsynaptic responses from these cells. However, many fundamental questions remain because, until now, there has been no direct measure of cone transmitter release. Among the key questions are: 1) What is the spatio-temporal pathway taken by synaptic vesicles as they journey between endocytosis and exocytosis in the cone terminal? 2) What is the quantitative relationship between presynaptic voltage, Ca2+ concentration, and exocytosis? 3) What is the rate of release from cones in the light and dark? 4) How is release modulated by feedback synapses and modulatory transmitters in the outer retina? 5) What is the size and extent of the lateral-inhibition contrast signal on arrays of cone terminals in the retina? Our goal is to answer these questions by directly measuring presynaptic release from cones. We will use the activity-dependent uptake and release of fluorescent lipophilic dyes, including FM1-43, as indicators of endocytosis and exocytosis. Preliminary results show that uptake and release of the dyes into cone terminals are Ca2+ and depolarization-dependent. Electron microscopic studies show localization of """"""""photoconverted"""""""" dye to synaptic vesicles. FM1-43 release occurs rapidly in the dark, is suppressed by light, and is affected by synaptic feedback from HCs. We have monitored FM 1-43 release from individually dissociated cones, groups of cones in retinal slices, and 2-dimensional arrays of cone terminals in intact retinal flat mounts. These preparations allow us to answer subcellular-to systems-level questions about the regulation of cone neurotransmitter release. These results will provide fundamental information about the mechanisms of normal vision and will provide insights about synaptic dysfunction that occur in retinal diseases.
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