The unipolar brush cell (UBC), a small interneuron occurring at high density in 
the granular layer of the mammalian vestibulocerebellum, receives a giant 
glutamatergic synapse from a single mossy fiber (MF) rosette, usually on a brush 
of dendritic branchlets. MF stimulation produces a current in the UBC several 
orders of magnitude greater in duration than at other glutamatergic synapses. We 
assumed that the cytoskeleton would have a special role in plasticity of the 
MF-UBC synapse. Neurofilaments and microtubules are enriched in the UBC 
somatodendritic compartment but are conspicuously absent in close proximity to 
the giant synapse, where standard electron microscopy reveals a 
granulo-flocculent material. Because osmium tetroxide fixation during sample 
preparation for standard electron microscopy destabilizes actin filaments, we 
hypothesized that this subsynaptic granulo-flocculent material is actin-based. 
After actin stabilization, we observed prominent, but loosely organized, bundles 
of microfilaments at the subsynaptic region of the MF-UBC synapse that linked 
the postsynaptic density with the cytoskeletal core of the dendritic branchlets. 
Confocal fluorescence microscopy and pre- and postembedding immunogold labeling 
with phalloidin and actin antibodies showed that these microfilaments consist of 
f-actin and contain little beta-actin. This extraordinary postsynaptic actin 
apparatus is ideally situated to form a dynamic framework for glutamate 
receptors and other postsynaptic molecules, and to mediate activity-dependent 
plastic rearrangements of the giant synapse.