The Cone Pedicle, the First Synapse in the Retina

Abstract

Cone pedicles are the output synapses of cone photoreceptors and transfer the light signal onto the dendrites of bipolar and horizontal cells. In the macaque monkey retina, cone pedicles contain between 20 and 45 ribbon synapses (triads), which are the release sites for glutamate, the cone transmitter [11, 13, 19, 22, 41]. Triads comprise a presynaptic ribbon, two horizontal dendrites as lateral elements, and one or two ON-cone bipolar cell dendrites as central elements (Fig. 1). Flat contacts are formed in most instances by the dendrites of OFF-cone bipolar cells at the cone pedicle base [19]. Glutamate released continuously at the synaptic ribbons acts not only through direct contacts but to an even greater extent through diffusion to the appropriate glutamate receptors (GluRs) [72].