Unipolar brush cells in the mammalian cerebellar cortex

The unipolar brush cell (UBC) is a novel neuronal cell type of the cerebellar cortex. Intermediate in size between the granule and Golgi cells, it is endowed with a striking morphology and unique synaptic properties. A dendrite emerges from its cell body and ends in a tuft of dendrioles which engulf a single mossy fiber afferent. The synapse it forms with the mossy fiber is among the most powerful in the CNS: A single mossy fiber stimulus evokes long-lasting excitatory postsynaptic currents, prolonged depolarizations and a train of action potentials. These properties are a function of the extensive synaptic apposition at the mossy fiber-UBC synapse. These synaptic specializations occur in a variety of forms, ranging from nearly uninterrupted and unusually large synapses, to a series of discontinuous small synapses. ^ In this series of studies we attempted to understand the role of UBCs in cerebellar functions by comparing its distribution in a variety of animals and by identifying its postsynaptic targets and some of its afferents. We demonstrate that UBCs are preferentially distributed in the vestibulocerebellum and in the ventromedial portion of the paraflocculus continuous to the flocculus—regions involved with postural, visual and collic motor functions. In larger mammals, this distribution expands to all vermal folia as well as regions of the intermediate and lateral hemispheral cortex, suggesting that the acquisition of complex movements requires more microcircuits involving UBCs. Within these microcircuits, UBCs give rise to a network of cortex-intrinsic mossy fibers which target granule cells and other UBCs, providing a powerful form of distributed excitation within the cerebellar cortex. Thus, identifying inputs to UBCs becomes crucial. Here, we have identified primary vestibular afferents as one of those inputs. The configuration of the mossy fiber-UBC synapse is reminiscent of perforated synapses, suggesting that they may undergo plasticity changes. We show that postsynaptic actin is concentrated in the underlying subsynaptic region and links the postsynaptic density to the cytoskeletal core of the UBC dendriole. At this location, actin is specially poised to mediate plastic rearrangements of the mossy fiber-UBC synapse.