Epidermal growth factor receptor pathway substrate 8 (Eps8) is a widely 
expressed multidomain signaling protein that coordinates two disparate 
GTPase-dependent mechanisms: actin reorganization via Ras/Rac pathways and 
receptor trafficking via Rab5. Expression of Eps8, the gene encoding the 
founding member of the Eps8 family of proteins, was found in cerebellum by 
virtual Northern analysis and in situ hybridization. Because the cerebellum has 
a well-known cellular architecture and is a favored model to study synaptic 
plasticity and actin dynamics, we sought to analyze Eps8 localization in rat 
cerebellar neurons and synapses by light and electron microscopy. Specificity of 
Eps8-antibody was demonstrated by immunoblots and in brain sections. In 
cerebellum, unipolar brush cells (UBCs) were densely Eps8 immunopositive and 
granule cells were moderately immunostained. In both types of neuron 
immunoreaction product was localized to the somatodendritic and axonal 
compartments. Postsynaptic immunostained foci were demonstrated in the glomeruli 
in correspondence of the synapses formed by mossy fiber terminals with granule 
cell and UBC dendrites. These foci appeared especially evident in the UBC brush, 
which contains an extraordinary postsynaptic apparatus of actin microfilaments 
facing synaptic junctions of the long and segmented varieties. Eps8 
immunoreactivity was conspicuously absent in Purkinje cells and their actin-rich 
dendritic spines, in all types of inhibitory interneurons of the cerebellum, 
cerebellar nuclei neurons, and astrocytes. In conclusion, Eps8 protein in 
cerebellum is expressed exclusively by excitatory cortical interneurons and is 
intracellularly compartmentalized in a cell-class specific manner. This is the 
first demonstration of the presence of a member of the Eps8 protein family in 
UBCs and its enrichment at postsynaptic sites.