Auditory input to cerebellar unipolar brush cells

This proposal investigates the hypothesis that a heretofore unknown auditory pathway projects to unipolar brush cells (UBCs) of the cerebellar flocculus and paraflocculus (FL/PFL). This hypothesis is based upon three different and converging sets of preliminary data. The first line of evidence is provided by in vivo extracellular electrophysiological recordings from the gerbil flocculus showing a short latency, frequency-dependent response to acoustic stimulation. Interestingly, the firing pattern of the responder neuron closely resembles the peculiar firing pattern of UBCs. The second line of evidence shows that auditory stimulation leads to c-Fos expression in floccular neurons that have spatial distribution compatible with that of UBCs. The third evidence was obtained from injection of biotinylated dextran amine (BDA), an anterograde tracer dye, into the cochlear spiral ganglion. We found that BDA labeled fibers terminated in FL/PFL and contacted UBCs. Thus multiple sets of converging data strongly suggest that cerebellar UBCs receive auditory inputs. This finding is further supported by the fact that the only CNS region outside the cerebellar cortex where UBCs are expressed are the cochlear nuclei. The goal of this proposal is two-fold. We will perform further in-vivo electrophysiological recordings and examine auditory stimulation-induced c-Fos staining to confirm the existence and define the properties of the cerebellar responses to auditory stimulation (Specific Aim 1), and we will perform anatomical tracing experiments to determine the origin of the afferent cerebellar fibers carrying the auditory information (Specific Aim 2). Besides confirming this novel auditory pathway, our experiments will also determine the connectivity of cerebellar unipolar brush cells, which is still largely unknown. Cerebellar UBCs are classified into the less numerous type I and the more abundant type II neurons. Type I UBCs of the cerebellum are innervated by primary and secondar