Glutamate receptor subunits at mossy fiber-unipolar brush cell synapses: light and electron microscopic immunocytochemical study in cerebellar cortex of rat and cat.

The present study provides a survey of the immunolocalization of ionotropic glutamate receptor subunits throughout the rat and cat cerebellar cortex, with emphasis on the unipolar brush cell (UBC), a hitherto neglected cerebellar cell that is densely concentrated in the granular layer of the vestibulocerebellum and that forms giant synapses with mossy fibers. An array of nine previously characterized antibodies has been used, each of which stained a characteristic profile of cerebellar cells. The UBCs of both rat and cat were strongly immunostained by an antibody against the alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate (AMPA) receptor subunits, GluR2 and GluR3; were moderately immunostained by a monoclonal antibody to kainate receptor subunits, GluR5/6/7; were weakly immunostained by antibodies to NR1 subunits; and were not stained by antibodies to GluR1, GluR4, GluR6/7, KA-2, and NR2A/B. Postsynaptic densities of the giant mossy fiber-UBC synapses were GluR2/3, GluR5/6/7, and NR1 immunoreactive. The other cerebellar neurons were all immunolabeled to some extent with the GluR2/3 and NR1 antibodies. In addition, Purkinje cells were immunopositive for GluR1 and GluR5/6/7; granule cells were immunopositive for GluR5/6/7, GluR6/7, KA-2, and NR2A/B. The Golgi-Bergmann glia was densely stained by GluR1 and GluR4 antibodies, whereas astrocytes of the granular layer were stained by the GluR4 antiserum. Data provided herein may guide further electrophysiological and pharmacological studies of cerebellar cells in general and the UBCs in particular.