Timothy S. Balmer - US grants

? DESCRIPTION (provided by applicant): The dorsal cochlear nucleus (DCN) integrates diverse multisensory inputs with auditory nerve signals to compute the location of a sound relative to body position. Loss of auditory input to DCN contributes to enhancement of multisensory input and could lead to hyperactivity in DCN, an event highly associated with tinnitus. Elucidating the multisensory circuitry of DCN is critical for both understanding auditory processing and pathophysiology that occurs in tinnitus. Multisensory input from disparate brain regions is carried to DCN by mossy fibers (MFs). MFs innervate granule cells and unipolar brush cells (UBCs). UBCs are glutamatergic interneurons that receive a single MF input at their brush-like dendrite and project to ensembles of granule cells whose parallel fiber axons modulate the activity of principal (fusiform) cells of the DCN. Recently, the Trussell lab has shown that UBCs respond to MF inputs by either markedly increasing or decreasing firing for seconds, depending on their ON or OFF subtype. ON UBCs could amplify the signal from a single MF input, synchronizing and enhancing the firing of numerous postsynaptic granule cells. Thus, in addition to amplifying specific multisensory channels of unknown origin, UBCs could contribute to pathologically enhanced DCN activity that is associated with tinnitus. To understand how multisensory integration occurs it is essential to identify the nature of the inputs to UBCs and how UBCs transmit information to other cells in the DCN circuit. The purpose of this proposal is to test: (1) where the neurons that project MFs to UBCs originate and what sensory modalities they represent, and (2) how the information they provide is integrated in the DCN circuit. In Aim 1 I will use cutting-edge anatomical tracing methods to identify MF projections that innervate UBCs. This will elucidate the source and sensory modality of the signals processed by UBCs. Several candidate regions that project MFs to DCN carry proprioceptive, motor and higher-level auditory feedback information, but it is unclear whether they innervate UBCs. I will Identify which sources innervate UBCs and whether both ON and OFF UBC subtypes are targeted. In Aim 2 I will characterize the function of identified MF input to UBCs electrophysiologically by expressing channelrhodopsin in UBC projecting sources. Using 2-photon microscopy I will define the spatial projection pattern of UBC axons within DCN and make paired electrophysiological recordings to test how MF input is transformed between UBCs and postsynaptic granule cells. This research will identify a major missing piece of the DCN circuit: what information UBCs process and their effect on the granule cell system. Transformation of this information by UBCs is likely to play a major role in multisensory integration and sound source localization in DCN. Because of UBCs' potential role in the amplification of excitatory signals, this work may provide insights into tinnitus, a common disorder associated with DCN hyperactivity.

? DESCRIPTION (provided by applicant): The dorsal cochlear nucleus (DCN) integrates diverse multisensory inputs with auditory nerve signals to compute the location of a sound relative to body position. Loss of auditory input to DCN contributes to enhancement of multisensory input and could lead to hyperactivity in DCN, an event highly associated with tinnitus. Elucidating the multisensory circuitry of DCN is critical for both understanding auditory processing and pathophysiology that occurs in tinnitus. Multisensory input from disparate brain regions is carried to DCN by mossy fibers (MFs). MFs innervate granule cells and unipolar brush cells (UBCs). UBCs are glutamatergic interneurons that receive a single MF input at their brush-like dendrite and project to ensembles of granule cells whose parallel fiber axons modulate the activity of principal (fusiform) cells of the DCN. Recently, the Trussell lab has shown that UBCs respond to MF inputs by either markedly increasing or decreasing firing for seconds, depending on their ON or OFF subtype. ON UBCs could amplify the signal from a single MF input, synchronizing and enhancing the firing of numerous postsynaptic granule cells. Thus, in addition to amplifying specific multisensory channels of unknown origin, UBCs could contribute to pathologically enhanced DCN activity that is associated with tinnitus. To understand how multisensory integration occurs it is essential to identify the nature of the inputs to UBCs and how UBCs transmit information to other cells in the DCN circuit. The purpose of this proposal is to test: (1) where the neurons that project MFs to UBCs originate and what sensory modalities they represent, and (2) how the information they provide is integrated in the DCN circuit. In Aim 1 I will use cutting-edge anatomical tracing methods to identify MF projections that innervate UBCs. This will elucidate the source and sensory modality of the signals processed by UBCs. Several candidate regions that project MFs to DCN carry proprioceptive, motor and higher-level auditory feedback information, but it is unclear whether they innervate UBCs. I will Identify which sources innervate UBCs and whether both ON and OFF UBC subtypes are targeted. In Aim 2 I will characterize the function of identified MF input to UBCs electrophysiologically by expressing channelrhodopsin in UBC projecting sources. Using 2-photon microscopy I will define the spatial projection pattern of UBC axons within DCN and make paired electrophysiological recordings to test how MF input is transformed between UBCs and postsynaptic granule cells. This research will identify a major missing piece of the DCN circuit: what information UBCs process and their effect on the granule cell system. Transformation of this information by UBCs is likely to play a major role in multisensory integration and sound source localization in DCN. Because of UBCs' potential role in the amplification of excitatory signals, this work may provide insights into tinnitus, a common disorder associated with DCN hyperactivity.

PROJECT SUMMARY / ABSTRACT The unipolar brush cell (UBC) is an excitatory interneuron cell type found in the dorsal cochlear nucleus (DCN) and vestibular cerebellum. UBCs are positioned within the circuit to amplify and prolong signals and likely play a major role in multisensory integration, sound source localization and cancellation of self-generated sounds. The UBC has a characteristic large dendritic brush that slows the diffusion of glutamate from the syn- apse and prolongs excitatory signals. Multisensory input is carried to UBCs by mossy fibers. The Trussell Lab has found that there are two types of UBCs- one responds to glutamate released from mossy fibers with an increase in firing (ON UBCs) and another that responds with a decrease in firing (OFF UBCs). I have recently discovered that in the vestibular cerebellum, ON UBCs receive direct primary afferent input from vestibular ganglion cells, whereas OFF UBCs receive indirect secondary input from vestibular nuclei. UBCs are also pre- sent in the dorsal cochlear nucleus (DCN), but their mossy fiber inputs are unknown. Aim 1 will use innovative tools and approaches to identify sources of input to UBC subtypes in DCN. Aim 2 will test the UBCs? synaptic responses to those inputs. Aim 3, the R00 phase, will test the function and circuitry of UBCs in vivo. This proposal utilizes cutting-edge approaches under mentors and consultants with expertise in their use. In Aim 1, motivated by my findings in the vestibular system, I will test the hypothesis that in DCN primary sensory mossy fibers project to ON UBCs and secondary mossy fibers project to OFF UBCs. This Aim utilizes mouse genetics to target ON or OFF UBCs specifically, and monosynaptic transmission of pseudotyped rabies virus to identify presynaptic mossy fiber sources. In Aim 2 the pathways identified will be validated by expressing channelrhodopsin in the projecting sources and recording postsynaptic currents in UBCs using in vitro electro- physiology. Aim 3 during the R00 phase will combine the skills I have learned as a postdoc with my back- ground using in vivo electrophysiology. The function of UBCs in the DCN circuit will be tested by recording from the principal output (fusiform) neurons during optogenetic activation of ON or OFF UBCs. The role of UBCs in gating multisensory inputs to DCN will be tested by inhibiting ON or OFF UBCs optogenetically. This research will clarify the role of these fascinating excitatory interneurons in multisensory integration. Be- cause of UBCs? potential role in the amplification of excitatory signals, this work may provide insights into an etiology of tinnitus.