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B. Fyk-Kolodziej, A. Dzhagaryan, P. Qin, R.G. Pourcho; Differential Localization of AMPA- and Kainate-type Glutamate Receptors and their Anchoring Proteins in the Outer Cat Retina . Invest. Ophthalmol. Vis. Sci. 2003;44(13):5160.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Although horizontal cells have been shown to express both AMPA- and kainate-type receptors, their response properties are dominated by AMPA receptors. The goal of the present study was to determine the spatial relationship of AMPA- and kainate- type receptor subunits at photoreceptor to horizontal cell contacts . In a further effort to visualize sites of AMPA/kainate activity, we also localized two of the postsynaptic density proteins which are putative receptor anchoring or targeting molecules. Methods: Cat retinas were processed for post-embedding immunocytochemical visualization of the AMPA subunits GluRs 2/3 and 4 and the kainate subunit GluR6. Pre-embedding methods were used to localize GRIP and PSD-95 . Results: Post-embedding analysis of cone pedicles confirmed the presence of GluR2/3 and GluR4 in invaginating horizontal cell processes and showed the two subunits to be colocalized at the site of closest proximity of the horizontal cells. Since staining with the GluR2/3 antiserum represents GluR3 in horizontal cells, the present results suggest that A-type horizontal cells express a heteromeric combination of GluR3 and GluR4 whereas B-type horizontal cells which lack GluR4 may have homomeric GluR3 receptors. In both types of horizontal cells, the AMPA receptors are positioned just beneath the synaptic ribbon of the cones. In contrast, GluR6 was localized to the upper margin of the horizontal cell processes, slightly lateral to the synaptic ribbon. A similar relationship was observed at rod spherules where B-type horizontal cells exhibit GluR2/3 staining just subjacent to the ribbon and GluR6 staining somewhat more laterally. Among the anchoring proteins PSD-95 immunoreactivity was seen to be presynaptic in both rod and cone photoreceptor terminals so is not a factor in the targeting of receptors within horizontal cells. GRIP was localized to somas of both A- and B-horizontal cells and was seen postsynaptically at both cone pedicles and rod spherules. Staining for GRIP was particularly intense in the axon terminal processes of B-horizontal cells at their sites of invagination of rod spherules. Conclusions: The present results are consistent with the dominant AMPA responses of horizontal cells with a secondary role for kainate receptors. PSD-95 appears not to anchor either AMPA or kainate subunits in the cat retina; however, GRIP is well positioned to anchor or target glutamate receptors in the outer retina.
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