Cerebellar unipolar brush cells are targets of primary vestibular afferents: an experimental study in the gerbil

The unipolar brush cell (UBC) is an excitatory glutamatergic interneuron, s ituated in the cerebellar granular layer, that itself receives excitatory s ynaptic input on its dendritic brush from a single mossy fiber terminal in the form of a giant glutamatergic synapse. The UBC axon branches within the granular layer, giving rise to large terminals that synapse with both gran ule cell and UBC dendrites within glomeruli and resemble in morphological a nd functional terms those formed by extrinsic mossy fibers. So far, the onl y demonstrated extrinsic afferents to the UBC are the choline acetyltransfe rase (ChAT)-positive mossy fibers, some of which originate from the medial and descending vestibular nuclei. To ascertain whether UBCs are innervated by primary vestibular fibers. we performed a tract-tracing light and electr on microscopic study of the vestibulocerebellum in gerbils. Macular and can al vestibular end-organs were individually labeled by injection of biotinyl ated dextran amine. After an appropriate survival time, gerbils were then p rocessed for light and electron microscopic analysis of central vestibular projections. In the nodulus and uvula, labeled primary vestibular fibers fo rmed mossy terminals synapsing with both granule cells and UBCs in all of t he injected gerbils. Thus, innervation of UBCs by extrinsic mossy fibers ca rrying static and dynamic vestibular signals represents the first synapse o f networks that contribute a powerful form of distributed excitation in the granular layer.