Bipolar cell and amacrine cell synapses are key components of the vertebrate retinal circuitry with multiple specialized functions. The underlying cellular mechanisms that control transmitter output from the ribbon-type synapses of bipolar cells and the conventional inhibitory synapses of amacrine cells, under vastly different ambient light conditions, are still poorly understood in the mammalian retina. Here we propose to do a series of patch-clamp electrophysiology and calcium imaging experiments on single amacrine cells of the mouse retina. This allows us to measure both presynaptic Ca2+ currents and evoked changes in membrane capacitance that assay synaptic vesicle exocytosis in real time from a living cell. We will apply calcium imaging and membrane capacitance techniques to the AII amacrine cell, a major interneuron in the mammalian retina that releases glycine at conventional active zones that contain a large cluster of synaptic vesicles. We will determine the overall capacity for exocytosis of a single AII amacrine cell. We will test the hypothesis that AII amacrine cells contain three distinct readily releasable pools of vesicles that have different sizes and kinetics of exocytosis. The mechanisms that maintain and modulate glycine release and short-term plasticity from AII amacrine cells are not known. We will test the hypothesis that cAMP levels modulate the kinetics of glycine release, the size of the readily releasable vesicle pool, and the short-term plasticity at AII amacrine cell synapses. We will also perform these measurements on AII amacrine cells during early postnatal development. We will test the hypothesis that the coupling between vesicles and the Ca2+ sensor for exocytosis becomes tighter during early development as Ca2+ currents increase in size and as Ca2+ channels become more colocalized with docked vesicles at the active zones of AII amacrine cells. Finally, using a combination of voltage-clamp and current- clamp recordings we will measure the degree of exocytosis from single AII amacrine cells that is evoked by physiological stimuli, namely, light stimuli of different intensities. In summary, the results of this grant proposal will provide a better understanding of how AII amacrine cells modulate bipolar cell terminal release via dynamic and plastic inhibitory synapses.

Public Health Relevance

Millions of Americans suffer from significant visual disorders and blindness. Many of these impairments are due to retinal diseases that degenerate photoreceptors, like macular degeneration and retinitis pigmentosa, but spare ganglion cells. A prosthetic device that stimulates the remaining neurons might restore sight to some blind people. By studying the basic mechanisms that modulate retinal synapses, and their capacity for exocytosis, we will obtain new insights on how to inhibit and excite ganglion cells. This knowledge will lead to better strategies for stimulating ganglion cells artificially with a retinal prosthesis. Our studies will thus eventually aid future designs of more efficient retinal prosthesi devices.

National Institute of Health (NIH)
National Eye Institute (NEI)
Research Project (R01)
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Neurotransporters, Receptors, and Calcium Signaling Study Section (NTRC)
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Greenwell, Thomas
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Oregon Health and Science University
Schools of Medicine
United States
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