Localization of kainate receptors at the cone pedicles of the primate retina

In the macaque monkey retina cone pedicles, the output synapses of cone pho toreceptors, contain between 20 and 45 ribbon synapses (triads), which are the release sites for glutamate, the cone transmitter. Several hundred post synaptic dendrites contact individual cone pedicles, and we studied the glu tamate receptors expressed and clustered at these contacts, particularly th e kainate receptor subunits GluR5, GluR6/7, and KA2. Pre- and postembedding immunocytochemistry and electron microscopy were used to localize GluR5 an d GluR6/7 to specific synaptic contacts at the cone pedicle base. The GluR5 subunit was aggregated at bipolar cell flat contacts. The GluR6/7 subunit was aggregated at bipolar cell flat contacts and at the desmosome-like junc tions formed by horizontal cell processes underneath the cone pedicles. KA2 immunoreactivity was observed at the invaginating dendritic tips of ON-con e and rod bipolar cells, which we interpret as a cross-reactivity of the KA 2 antiserum with some other, unknown protein of the monkey retina. Kainate receptors are preferentially expressed by OFF-cone bipolar cells and to a l esser extent by horizontal cells. We also performed double-labeling experim ents with the ribbon-specific marker bassoon and with antibodies against Gl uR5 and GluR6/7 in order to define the position of the flat bipolar cell co ntacts with respect to the triads. There was a tendency of GluR6/7 clusters to represent triad-associated contacts, whereas GluR5 clusters represented non-triad-associated contacts. The GluR5 and GluR6/7 subunits were cluster ed at different bipolar cell contacts. We studied a possible cone-selective expression of the kainate receptor subunits by double labeling cone pedicl es for the S-cone opsin and for the different receptor subunits. We observe d a reduced expression of both GluR5 and GluR6/7 at the S-cone pedicles. Th e reduced expression of GluR6/7 was analyzed in more detail and it appears to be a consequence of a horizontal cell-specific expression: H1 horizontal cells express GluR6/7, whereas H2 horizontal cells, which preferentially i nnervate S-cones, show no expression of GluR6/7. J. Comp. Neurol. 436:471-4 86, 2001. (C) 2001 Wiley-Liss, Inc.