Cone pedicles, the synaptic terminals of cone photoreceptors, are connected
in the macaque monkey retina to several hundred postsynaptic dendrites. Us
ing light and electron microscopy, we found underneath each cone pedicle a
laminated distribution of dendritic processes of bipolar and horizontal cel
ls. Superimposed were three strata of glutamate receptor (GluR) aggregates,
including a novel layer of glutamate receptors clustered at desmosome-like
junctions. They are, most likely, postsynaptic densities on horizontal cel
l dendrites. GABAA and GABA, receptors are aggregated on bipolar cell dendr
ites in a narrow band underneath the cone pedicle. Glutamate released from
cone pedicles and GABA released from horizontal cell dendrites act not only
through direct synaptic contacts but also (more so) through diffusion to t
he appropriate receptors.