Unipolar brush cell axons form a large system of intrinsic mossy fibers inthe postnatal vestibulocerebellum

The unipolar brush cells (UBCs), a class of neurons recently identified in the granular layer of the vestibulocerebellum, receive excitatory synaptic input from mossy fibers (MFs) in the form of a giant glutamatergic synapse. UBCs are provided with axons that bear synaptic endings situated at the ce nter of glomeruli, similar to cerebellar MF afferents. A single MF stimulus evokes a prolonged train of action potentials in the UBC (Rossi et al., 19 95), which is presumably distributed to postsynaptic targets. Knowledge of the synaptic connections of UBC axons is essential to define the role of th ese cells in the integration of vestibular signals in the cerebellar circui try. To evaluate these connections, the nodulus (folium X) was isolated from ver mal slices of postnatal day 8 mice, cultured for 2-4 or 15-30 days in vitro , and studied by electron and fluorescence microscopy. The peak of degenera tion of extrinsic MF terminals, which have been severed from the parent cel l bodies, was observed at 2 days in vitro (DIV). Quantification of degenera ting and nondegenerating (e.g., intrinsic) MF terminals indicated that abou t half of the MF terminals were provided by local UBC axons synapsing on de ndrites of granule cells and other UBCs. The proportion of nondegenerating vs, degenerating MF terminals terminating on UBCs also indicated that appro ximately two-thirds of the intrinsic MFs are involved in UBC-UBC connection s. In long-term cultures, the granular layer appeared well preserved and th e UBC axons formed an extensive system of MF collaterals. It is suggested t hat UBCs may act by spatially amplifying vestibular inputs carried by extri nsic MFs. (C) 2000 Wiley-Liss, Inc.