Transmission at the Mammalian Cone Photoreceptor Basal Synapse

Abstract

Cone photoreceptor synaptic terminals have both an unusual structure and an unusual function. The unusual structure is called the basal contact. At these contacts, the membranes of a cone and a postsynaptic cell, usually an Off bipolar cell, come into close apposition, but the stigmata of synaptic transmission, presynaptic clusters of vesicles and active zones, are absent. Rather, the cone transmitter, glutamate, is thought to be released solely at synaptic ribbons located atop invaginations, and must diffuse 200–500 nm to the nearest basal contacts. The unusual function is that the cone synapse transmits voltage signals that are hyperpolarizing, graded, and frequently very small. How does the unusual structure of the basal contact relate to its function in transmission?