Localization of the AMPA subunit GluR2 in the outer plexiform layer of goldfish retina

L-glutamate, the photoreceptor neurotransmitter, depolarizes horizontal cel ls and OFF bipolar cells by ionotropic AMPA-glutamate receptors. The AMPA-r eceptor subunit (GluR4) is localized to dendrites of OFF bipolar cells in g oldfish retina. Here, we used immunohistochemical techniques to identify AM PA-receptor subunits on horizontal cell dendrites. A monoclonal antibody ag ainst rat GluR2, with high sequence homology to the recently cloned goldfis h GluR2a receptor, was used for light- and electron-microscopical immunocyt ochemistry. Light- and dark-adapted retinas were analyzed, with no major di fference in results. GluR2-immunoreactivity (IR) was restricted to a narrow band in the outer plexiform layer, in which it appeared as bright dome-sha ped structures amidst numerous puncta. At the ultrastructural level, GluR2- IR was found in horizontal cell dendrites that invaginated cones and rods. Dendrites of OFF bipolar cells were not labeled. GluR2-IR was present mostl y in horizontal cell dendrites that were the lateral elements of the triad, rather than in dendrites that were the central elements. In light-adapted retinas, GluR2-IR was found in many horizontal cell spinules. GluR2-IR was observed, on occasion, in a mixed rod/cone (Mb) ON bipolar cell process tha t innervated rod spherules. Verification of the Mb ON bipolar cell was made by protein kinase C and metabotropic mGluR1 alpha immunolabeling. The pres ence of GluR2-IR in lateral elements suggests that lateral horizontal cell dendrites are postsynaptic to cones rather than only sites of feedback inhi bition. All horizontal cell types express the GluR2 subunit, uniquely diffe rentiating themselves from OFF bipolar cells that express the GluR4 subunit . This differentiation most likely has a major influence on the glutamate p harmacology and response kinetics of these cell types to glutamate. J. Comp . Neurol. 441:155-167, 2001. (C) 2001 Wiley-Liss, Inc.