Several metabotropic glutamate receptor (mGluR) subtypes have been identified in the cerebellar cortex that are targeted to different compartments in cerebellar cells. In this study, preembedding immunocytochemical methods for electron microscopy were used to investigate the subcellular distribution of the mGluR1b splice variant in the rat cerebellar cortex. Dendritic spines of Purkinje cells receiving parallel fiber synaptic terminals were immunoreactive for mGluR1b. With a preembedding immunogold method, ϳ25% of the mGluR1b immunolabeling was observed perisynaptically within 60 nm from the edge of the postsynaptic densities. Values of extrasynaptic gold particles beyond the first 60 nm were maintained at between 10 and 18% along the whole intracellular surface of the dendritic spine membranes of Purkinje cells. For comparison, the distribution of mGluR1a was studied. A predominant (ϳ37%) perisynaptic localization of mGluR1a was seen in dendritic 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 subcellular compartments in Purkinje cells but that the densities of the perisynaptic and extrasynaptic pools were different for both isoforms. The compartmentalization of mGluR1b and mGluR1a might serve distinct requirements in cerebellar neurotransmission.

Studies indicate that metabotropic glutamate receptors (mGluRs) may play a role in spinal sensory transmission. We examined the cellular and subcellular distribution of the mGluR subtype 4a in spinal tissue by means of a specific antiserum and immunocytochemical techniques for light and electron microscopy. A dense plexus of mGluR4a-immunoreactive elements was seen in the dorsal horn, with an apparent accumulation in lamina II. The immunostaining was composed of sparse immunoreactive fibres and punctate elements. No perikaryal staining was seen. Immunostaining for mGluR4a was detected in small to medium-sized cells but not in large cells in dorsal root ganglia. At the electron microscopic level, superficial dorsal horn laminae demonstrated numerous immunoreactive vesicle-containing profiles. Labelling was present in the cytoplasmic matrix, but accretion of immunoreaction product to presynaptic specialisations was commonly observed. Axolemmal labelling was confirmed by using a preembedding immunogold technique, which revealed distinctive deposits of gold immunoparticles along presynaptic thickenings with an average centre-to-centre distance of 41 nm (41.145 +/- 13.59). Immunoreactive terminals often formed synaptic contacts with dendritic profiles immunonegative for mGluR4a. Immunonegative dendritic profiles were observed in apposition to both mGluR4a-immunoreactive and immunonegative terminals. Diffuse immunoperoxidase reaction product was also detected in dendritic profiles, some of which were contacted by mGluR4a-immunoreactive endings, but only occasionally were they observed to accumulate immunoreaction product along the postsynaptic density. Terminals immunoreactive for mGluR4a also formed axosomatic contacts. The present results reveal that mGluR4a subserves a complex spinal circuitry to which the primary afferent system seems to be a major contributor.

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