The synaptic architecture of AMPA receptors at the cone pedicle of the primate retina

Cone pedicles, the output synapses of cone photoreceptors, transfer the light signal onto the dendrites of bipolar and horizontal cells. Cone pedicles contain between 20 and 45 ribbon synapses (triads) which are the release sites for glutamate, the cone transmitter. Several hundred postsynaptic dendrites contact individual cone pedicles, and we studied the glutamate receptors expressed and clustered at these contacts, particularly the AMPA receptor subunits. Using immunocytochemistry and confocal imaging we were able to resolve individual triads within the cone pedicles by light microscopy. We studied their differences in L/M- and S-cones, and we counted the number of triads per pedicle across the retina. The presynaptic matrix protein bassoon, the synapse-associated membrane protein P84, and peanut agglutinin were used to specifically label synaptic ribbons, invaginating dendrites of horizontal cells and invaginating dendrites of ON-cone bipolar cells, respectively. Pre- and post-embedding immunocytochemistry and electron microscopy were used to localize the AMPA receptor subunits at the cone pedicle base. They were aggregated at three different postsynaptic sites: at horizontal cell invaginating contacts, at bipolar cell flat contacts, and at desmosome-like junctions underneath the cone pedicles. We also performed double-labeling experiments with the triad-specific markers and the antibodies against the AMPA receptor subunits. AMPA receptors were preferentially expressed by horizontal cells, and to a lesser extent by OFF-cone bipolar cells. We did not observe any cone-selective expression of AMPA receptor subunits postsynaptic to L/M- or S-cones, suggesting AMPA receptors are not the key to understanding trichromatic signaling in the primate retina.