COMPARTMENTAL LOCALIZATION OF A METABOTROPIC GLUTAMATE-RECEPTOR (MGLUR7) - 2 DIFFERENT ACTIVE-SITES AT A RETINAL SYNAPSE

The distribution of the metabotropic glutamate receptor 7 (mGluR7) was studied in the rat retina using a specific antiserum. Punctate immuno fluorescence that corresponded to synaptic localization was present ex clusively in the inner plexiform layer. Double-labeling experiments su ggested that mGluR7 is expressed at the synaptic terminals of certain cone bipolar cells. Electron microscopy showed that mGluR7 was present both presynaptically, as an autoreceptor in cone bipolar cell ribbon synapses, and postsynaptically in amacrine cells. There are usually tw o postsynaptic processes at a bipolar cell ribbon synapse; however, th e presynaptic aggregation of mGluR7 was restricted to one half of the active zone and therefore was opposed to only one of the postsynaptic processes. This selective localization of mGluR7 could differentially regulate the glutamate release from the ribbon synapse, thus leading t o a differential activation of the postsynaptic neurons.