THE UNIPOLAR BRUSH CELLS OF THE RAT CEREBELLAR CORTEX AND COCHLEAR NUCLEUS ARE CALRETININ-POSITIVE - A STUDY BY LIGHT AND ELECTRON-MICROSCOPIC IMMUNOCYTOCHEMISTRY

Cell class-specific markers are powerful tools for the study of indivi dual neuronal populations. The peculiar unipolar brush cells of the ma mmalian cerebellar cortex have only recently been definitively identif ied by means of the Golgi method, and we have explored markers of cere bellar neurons with the purpose of facilitating the analysis of this n ew cell population and, especially, its distribution and ultrastructur al features. By light microscopic immunocytochemistry, we demonstrate that, in the rat, the unipolar brush cells are the cortical neurons th at are most densely immunostained with antiserum to calretinin, a rece ntly discovered calcium-binding protein. The unipolar brush cells are highly concentrated in the flocculo-nodular lobe, the ventral uvula an d the ventral paraflocculus, occur at relatively high density in the l ingula, at moderate-to-low density in other folia of the vermis and in the narrow intermediate cortex, and at low to very low density, with the exception of a few hot spots, in the lateral regions of the cerebe llar hemispheres and in the dorsal paraflocculus. Unipolar brush cells are also found in the cochlear nucleus. In addition to the unipolar b rush cells, calretinin antibody distinctly stains certain messy fibers , and weakly to moderately stains other cerebellar elements, such as g ranule neurons and climbing fibers. In the lobules containing high den sities of unipolar brush cells, the granule cell bodies and the parall el fibers are much less immunoreactive, and there are many more densel y immunostained messy fibers than in the lobules, where these cells ar e rare, which suggests some relationships between these elements. In t he cerebellar nuclei, small neurons are densely immunostained, while l arge neurons are immunonegative. The unipolar brush cells reside nearl y exclusively in the granular layer. They are small neurons, intermedi ate in size between granule cells and Golgi cells, and their features are remarkably similar across all lobules. They usually have a single, relatively thick dendrite of varying length that terminates in a brus h-like tip consisting of several short branchlets. Utilizing a pre-emb edding protocol, we have identified unipolar brush cells with the elec tron microscope. The cytoplasm of these cells is partially obscured by the electron dense product of calretinin immunoreaction in all region s of the soma and processes. The cells are often covered with non-syna ptic appendages and contain a peculiar cytoplasmic inclusion consistin g of ringlet subunits. Other characteristic components are numerous ne urofilaments, mitochondria and large, dense-core vesicles. Individual brushes enter one or two glomeruli, where the dendritic branchlets est ablish an unusually extensive synapse with messy fiber rosettes. In ad dition to their contact with the messy rosettes, the branchlets are po stsynaptic to boutons presumably belonging to the axonal plexus of Gol gi cells and are also presynaptic to small dendrites, displaying small , clear synaptic vesicles at the site of contact. The distinct calreti nin-like immunoreactivity of the unipolar brush cells may be related t o strong calcium influx at their extensive synapses with the messy fib er rosettes.