Function of Basal Synapses at Mammalian Photoreceptors

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 opsin proteins in the outer segments of cones convert absorbed light into a voltage signal. In a necessary step for visual perception, the voltage signal spreads to the photoreceptor synaptic terminal where it gates a Ca2+ channel that controls the release of the transmitter glutamate onto postsynaptic bipolar and horizontal cells. Parallel processing in the visual system begins at the cone synapse. Each cone terminal communicates with ~14 anatomically distinct bipolar cell types at two structurally unique contacts termed invaginating and basal. Transmitter is not released at basal contacts, but instead at sites near the top of each of a cone’s 20-40 invaginations. Released glutamate must then diffuse over an extracellular path of 200 – 500 nm to reach the dendrites of basally contacting bipolar cells. Recent results suggest that a long diffusion path can introduce a threshold that eliminates the low-amplitude noise associated with random fluctuations in cone transmitter release in the dark. At the same time, the threshold permits the cone to transmit signals resulting from larger release events coordinated by a change in illumination. Using electro- and opto-physiological techniques, this proposal addresses two mechanisms that increase the threshold nonlinearity at basal contacts: First, at least one type of Off bipolar cell expresses receptors with an unusually high EC50 for glutamate (~1.5 mM); and second, basally located glutamate transporters provide saturable binding sites that can deplete cleft glutamate under dark release conditions. Specific Aim 1 addresses the mechanisms and functions of the threshold nonlinearity at the cone to cb1a bipolar cell basal synapse. Experiments will determine how transporter glutamate binding and kainate receptor properties contribute to nonlinear sign