Molecular mechanisms of photoreceptor synaptic transmission

Voltage changes resulting from the absorption of photons and triggering of a second-messenger cascade in outer segments of rod and cone photoreceptors are encoded at the synaptic terminal and transmitted to second-order bipolar and horizontal cells. In this chapter, we survey the unique structural, molecular, and functional features of photoreceptor synapses that enable them to encode and transmit light responses. We begin by describing the anatomy of the photoreceptor synapse, focusing on a unique structure called the synaptic ribbon, which is present in only a handful of other primary sensory synapses and plays an important role in priming glutamate-laden vesicles for release. We then discuss the locations and functional roles of many of the key proteins present at the synapse. Although many of these are shared in common with conventional nonribbon synapses, some important differences appear to contribute to the unique signaling capabilities of photoreceptors. We then explore the properties of synaptic transmission by rods and cones, noting the ways in which calcium-dependent regulation of the rate of vesicle fusion enables the synapse to encode different features of the visual scene. Although this chapter is focused on the molecular mechanisms of synaptic transmission in healthy retinas, we make note of how synaptic defects can lead to vision loss. For example, forms of X-linked congenital stationary night blindness can arise from mutations in the gene encoding the voltage-gated calcium channels present at rod and cone synapses.