Immunolocalization of the mGluR1b splice variant of the metabotropic glutamate receptor 1 at parallel fiber-Purkinje cell synapses in the rat cerebellar cortex

Several metabotropic glutamate receptor (mGluR) subtypes have been identifi ed in the cerebellar cortex that are targeted to different compartments in cerebellar cells. In this study, preembedding immunocytochemical methods fo r electron microscopy were used to investigate the subcellular distribution of the mGluR1b splice variant in the rat cerebellar cortex. Dendritic spin es of Purkinje cells receiving parallel fiber synaptic terminals were immun oreactive for mGluR1b, With a preembedding immunogold method, similar to 25 % of the mGluR1b immunolabeling was observed perisynaptically within 60 nm from the edge of the postsynaptic densities, Values of extrasynaptic gold p articles beyond the first 60 nm were maintained at between 10 and 18% along the whole intracellular surface of the dendritic spine membranes of Purkin je cells. For comparison, the distribution of mGluR1a was studied. A predom inant (similar to 37%) perisynaptic localization of mGluR1a was seen in den dritic spines of Purkinje cells, dropping the extrasynaptic labeling to 15% in the 60-120-nm bin from the edge of the postsynaptic specialization, Our results reveal that mGluR1b and mGluR1a are localized to the same subcellu lar compartments in Purkinje cells but that the densities of the perisynapt ic and extrasynaptic pools were different for both isoforms. The compartmen talization of mGluR1b and mGluR1a might serve distinct requirements in cere bellar neurotransmission.