Localization of mGluR6 to dendrites of ON bipolar cells in primate retina

We prepared antibodies selective for the C-terminus of the human mGluR6 rec eptor and used confocal and electron microscopy to study the patterns of im munostaining in retina of monkey, cat, and rabbit. In all three species pun ctate stain was restricted to the outer plexiform layer. Ln monkey, stain w as always observed in the central element of the postsynaptic "triad" of ro d and cone terminals. In monkey peripheral retina, stain was seen only in c entral elements, but; in the fovea, stain was also observed in some dendrit es contacting the base of the cone terminal. S-cone terminals, identified b y staining for S opsin, showed staining of postsynaptic dendrites. These we re identified as dendrites of the ON S-cone bipolar cell by immunostaining for the marker cholecystokinin precursor. The staining pattern suggests tha t all types of ON bipolar cells, despite their marked differences in functi on, express a single isoform of mGluR6. Ultrastructurally, mGluRG was locat ed not on the tip of the central element, near the site of vesicle release, but on its base at the mouth of the invagination, 400-800 nm from the rele ase site. Thus, the mGluRG receptors of ON bipolar cells lie at about the s ame distance fi-om sites of vesicle release as the iGluR receptors of OFF b ipolar cells at the basal contacts. (C) 2000 Wiley-Liss, Inc.