Using optical sensors to measure synaptic glutamate release from retinal rod photoreceptor cells

University Of Nebraska Medical Center, Omaha NE

Investigators

Abstract

Project Summary Light responses generated by photon capture in rod photoreceptor cells must be transmitted at their synapses to downstream neurons. Transmission at the first synapse in the retina thus determines the information about light available to the visual system. A remarkable ability of the human visual system is the ability to detect dim light flashes consisting of only a few photons. This sensitivity originates with the ability of rods to respond to single photons but requires nearly noiseless transmission of small single photon responses through the retina and brain. A critical first step in this process is transmission from rods to rod bipolar cells. Fluorescent glutamate sensors have provided important insights into circuit mechanisms in other parts of the retina and CNS but have not been successfully used to study release from rods. To overcome this, we developed a genetically-modified mouse line allowing constitutive expression of an improved glutamate sensor, iGluSnFr3, specifically in rods. In this proposal, we plan to characterize the strengths and limitations of using iGluSnFr3 to measure glutamate at rod synapses. Electrophysiological recordings from tiny mammalian rods are exceptionally challenging so providing an optical approach for studying glutamate release from rods will pave the way for future work by many laboratories on basic and disease-related questions of photoreceptor function. Release rates at rod synapses are slow and this makes it difficult for a post-synaptic rod bipolar cell to discriminate a lengthy random pause in release from a genuinely lengthy pause caused by a single photon response. One way to overcome this would be for release to occur at regular, not random, intervals. Results from our laboratory using an indirect measure of glutamate release suggested that synaptic release from rods is surprisingly regular, not random as generally thought. Optical measurements using iGluSnFr3 offer the opportunity to directly measure release from dark-adapted rods in intact retina and test the physiological significance of this regularity. Establishing that release from rods occurs at regular intervals would be a fundamental change in our understanding of how visual information is encoded at the first synapse in the retina and suggest many future experiments.