Metabotropic glutamate receptors are associated with non-synaptic appendages of unipolar brush cells in rat cerebellar cortex and cochlear nuclear complex

Unipolar brush cells (UBCs) are a class of small neurons that are densely c oncentrated in the granular layers of the vestibulocerebellar cortex and do rsal cochlear nucleus. The UBCs form giant synapses with individual mossy f ibre rosettes on the dendrioles which make up their brush formations and ar e provided with numerous, unusual non-synaptic appendages. In accord with t he glutamatergic nature of mossy fibres, our previous post-embedding immuno cytochemical studies indicated that various ionotropic glutamate receptor s ubunits are localized at the post-synaptic densities of the giant synapses, whereas the non-synaptic appendages are immunonegative. On the contrary, t he metabotropic glutamate receptors mGluR1 alpha and mGluR2/3 are situated at the non-synaptic appendages and are lacking at the post-synaptic densiti es. Other authors, however, have shown that antibodies to these metabotropi c receptors stain both appendages and post-synaptic densities. In the prese nt study, we have re-evaluated the distribution of metabotropic glutamate r eceptors in the UBCs of the cerebellum and the cochlear nuclear complex by light and electron microscopic pre-embedding immunocytochemistry with subty pe-specific antibodies. We confirm that UBCs dendritic brushes are densely immunostained by antibody to mGluR1 alpha particularly in the cerebellum an d that antibody to mGluR2/3 labels at least a percentage of the UBC brushes in both the cerebellum and cochlear nuclei. At the ultrastructural level, it appears that mGluR1 alpha and mGluR2/3 immunoreactivities are not associ ated with the post-synaptic densities of the giant mossy fibre-UBC synapses , but instead are concentrated on the non-synaptic appendages of the cerebe llar UBCs. The non-synaptic appendages, therefore, may be an important aven ue for regulating the excitability of UBCs and mediating glutamate effects on their still unknown intracellular signal transduction cascades. We also show that the pre-synaptic densities of UBC dendrodendritic junctions are m GluR2/3 positive. As previously demonstrated, antibodies to mGluR1 alpha an d mGluR2/3 label subsets of Golgi cells. Antibody to mGluR5 does not stain UBCs in the cerebellum and cochlear nucleus and reveals the somatodendritic compartment of Golgi cells situated in the core of the cerebellar granular layer, whilst cochlear nucleus Golgi cells are mGluR5 negative.