Differential expression of calretinin and metabotropic glutamate receptor mGluR1α defines subsets of unipolar brush cells in mouse cerebellum

The unipolar brush cell (UBC) is a type of glutamatergic interneuron in the granular layer of the cerebellum. The UBC brush and a single mossy fiber (MF) terminal contact each other within a cerebellar glomerulus, forming a giant synapse. Many UBCs receive input from extrinsic MFs, whereas others are innervated by intrinsic mossy terminals formed by the axons of other UBCs. In all mammalian species so far examined, the vestibulocerebellum is enriched of UBCs that are strongly immunoreactive for the calcium binding protein calretinin (CR) in both the somatodendritic and axonal compartment. UBCs have postsynaptic ionotropic glutamate receptors and extrasynaptic metabotropic glutamate receptors that immunocytochemically highlight their somatodendritic compartment and brush, respectively. In this study on the mouse cerebellum, we present evidence that immunoreactivities to CR and mGluRlalpha define two distinct UBC subsets with partly overlapping distributions in lobule X (the nodulus). In sections double-labeled for CR and mGluRlalpha, the patterns of distributions of CR+/mGluRlalpha(-) UBCs and CR-/mGluRlalpha(+) UBCs differed along the medio-lateral and dorsoventral axes of the folium. Moreover, mGluRlalpha(+) UBCs outnumbered CR+ UBCs. Both UBC subsets were mGluR2/3, GluR2/3, and NMDAR1 immunoreactive. The different distribution patterns of the two UBC subsets within lobule X suggest that expression of CR or mGluRlalpha by UBCs may be afferent-specific and related to the terminal fields of different vestibular MF afferents.