Unipolar brush cells are a class of interneurons in the granular layer of the 
mammalian cerebellum that receives excitatory mossy fiber synaptic input in the 
form of a giant glutamatergic synapse. Previously, it was shown that the 
unipolar brush cell axon branches within the granular layer, giving rise to 
large terminals. Single mossy fiber stimuli evoke a prolonged burst of firing in 
unipolar brush cells, which would be distributed to postsynaptic targets within 
the granular layer. Knowledge of the ultrastructure of the unipolar brush cell 
terminals and of the cellular identity of its postsynaptic targets is required 
to understand how unipolar brush cells contribute to information processing in 
the cerebellar circuit. To investigate the unipolar brush cell axon and its 
targets, unipolar brush cells were patch-clamped in fresh parasagittal slices 
from rat cerebellar vermis with electrodes filled with Lucifer Yellow and 
Biocytin, and examined by confocal fluorescence and electron microscopy. 
Biocytin was localized with diaminobenzidine chromogen or gold-conjugated, 
silver-intensified avidin. Light microscopic examination revealed a single thin 
axon emanating from the unipolar brush cell soma that gave rise to 2-3 axon 
collaterals terminating in mossy fiber-like rosettes in the granular layer, 
typically within a few hundred microm of the soma. In some cases, axon 
collaterals crossed the white matter within the same folium before terminating 
in the adjacent granular layer. Electron microscopic examination of serial 
ultrathin sections revealed that proximal unipolar brush cell axons and axon 
collaterals were unmyelinated and devoid of synaptic contacts. However, the 
rosette-shaped enlargements of each collateral formed the central component of 
glomeruli where they were surrounded by dendrites of granule cells and/or other 
unipolar brush cells, with which they formed asymmetric synaptic contacts. A 
long-latency repetitive burst of polysynaptic activity was observed in granule 
cells in this cerebellar region following white matter stimulation. The unipolar 
brush cell axons, therefore, form a system of cortex-intrinsic mossy fibers. The 
results indicate that synaptic excitation of unipolar brush cells by mossy 
fibers will drive a large population of granule cells, and thus will contribute 
a powerful form of distributed excitation within the basic circuit of the 
cerebellar cortex.