Susan Shore, Ph.D. - US grants

DESCRIPTION: (Adapted from applicant's abstract): We have demonstrated that the trigeminal ganglion sends a projection to the auditory brain stem and to the cochlear vasculature. The terminations in the brainstem end in granular and magnocellular regions of the ventral cochlear nucleus (VCN) and at the locations of olivocochlear neurons in the superior olivary complex. The cochlear portion of the innervation modulates blood flow within the cochlea. However, the function of the brainstem projection is unknown. Preliminary findings suggest that the projection to the VCN is excitatory. The axons of granule' cells (parallel fibers) project to the dorsal cochlear nucleus (DCN). Therefore, excitation by the trigeminal innervation could affect most of the output neurons of the cochlear nucleus (CN). A series of studies is designed to elucidate the function of the CN portion of this new projection and define its neurotransmitters: The trigeminal ganglion will be electrically stimulated while observing the responses of single units in the CN. The candidate neurotransmitters, Substance P and Nitric Oxide, will be evaluated using immunocytochemistry and neuropharmacology. A role for the trigeminal ganglion in the generation and modulation of tinnitus will be explored: As many as two thirds of tinnitus patients are able modulate their tinnitus by clenching the jaw or touching the skin on the face, areas innervated by the trigeminal ganglion. Others can attribute the onset of tinnitus to a somatic insult in the head and neck region ("somatic tinnitus"). On the assumption that increased spontaneous rate is a manifestation of tinnitus, its modulation by somatosensory input to the CN could play a role in the generation and modulation of tinnitus. The hypothesis wifi be tested by electrically stimulating the trigeminal ganglion while recording spontaneous activity in single units of the CN. Identifying the neurotransmitter used in this pathway could then set the stage for later drug treatments.

The long term goal of this research program is to elucidate both the normal role of non-auditory inputs to the cochlear nucleus (CN), and how alterations of these inputs after deafness may lead to a re-organization of bimodal interactions that result in the perception of phantom sounds or tinnitus. A significant finding from this funding period is that stimulating trigeminal neurons can dramatically suppress acoustically driven responses in dorsal CN (DCN) units. This suggests that somatosensory- auditory integration may be involved in the suppression of internally-generated sounds such as self vocalization or respiration. Investigating the mechanisms underlying this bimodal integration will allow us to appreciate the role of the DCN in improving the detection of external, novel stimuli. Aim 1 will describe the projections from the spinal trigeminal nucleus (Sp5) and lateral reticular formation (RF) to the CN. Electrically stimulating these regions will enable us to learn more about their contributions to auditory processing within the DCN. Aim two will determine the mechanisms underlying bimodal integration in the DCN. The hypotheses invoked to explain the long lasting integration include long term depression/potentiation, transient potassium channel activation, or GABAB receptor activation. These hypotheseswill be tested using multichannel recording probes, enabling us to record simultaneously from a large number of units. The focus will be on the changes in temporal firing patterns of DCN units in response to trigeminal stimulation. Aim three will explore our recent observation that DCN units become more sensitive to trigeminal stimulation after cochlear damage: Changes in the temporal firing patterns of DCN units may reflect altered intrinsic membrane properties or an increase in the number of trigeminal inputs following cochlear damage. The hypothesis that trigeminal innervation may increase following noise damage will be investigated using vesicular glutamate and GAP-43/synaptophysin/synapsin1 immuno cytochemistry combined with tract tracing. Increased synchrony between neurons may be a correlate of tinnitus and changes in regularity and synchrony with trigeminal stimulation may be a correlate of somatic tinnitus. Thus, examining the disruption of bimodal integration following cochlear damage will allow us to elucidate the mechanisms underlying tinnitus, and thus provide insights to guide clinical intervention.

DESCRIPTION (provided by applicant): Glutamatergic somatosensory projections to the cochlear nucleus (CN) originate in trigeminal and dorsal column systems and terminate primarily in the CN granule cell domain (GCD). Stimulating these inputs alters spontaneous and sound-driven responses in principal neurons of the dorsal and ventral CN for extended periods of time. This long-term bimodal alteration is enhanced after unilateral deafness and could explain why patients are able to modulate their tinnitus by somatic maneuvers such as jaw clenching. The aims of this proposal are to determine the physiological and molecular mechanisms underlying long-term suppression and enhancement of CN responses by somatosensory projection neurons and their implications for tinnitus generation and modulation. Aim 1a will examine long-term synaptic plasticity as a mechanism underlying bimodal enhancement and suppression in fusiform and bushy cells in normal and noise-damaged guinea pigs. We hypothesize that bimodal enhancement will predominate in noise-damaged animals with physiological correlates (increased spontaneous rates and synchrony) and behavioral evidence of tinnitus using the gap-detection tinnitus screening method (Aim 1b). Aim 2 will examine the hypothesis that the predominance of bimodal enhancement in animals with tinnitus (preliminary data) is a result of up-regulation of specific Vglut2- positive somatosensory endings in the CN after deafness. In Aim 2a, tract-tracing and immunocytochemical studies will determine the precise origins and endings of the upregulated inputs in mouse. Aim 2b will utilize Vglut2-deficient mice to test the hypothesis that Vglut2+/- mice will be resistant to tinnitus induction. Mice will be tested for tinnitus using gap-detection before and after narrow-band noise overexposure. Preliminary data indicate that compared to matched wild-types, the Vglut2+/- mice show significantly less evidence of tinnitus, supporting this hypothesis. Aim 2c will then explore the involvement of the fibroblast growth factor, FGF22, as a postsynaptic signal for presynaptic upregulation of somatosensory mossy fibers to the CN after deafness. Our studies strongly implicate the somatosensory system, not only in the modulation, but also in the generation of tinnitus. Not surprisingly, more than half of tinnitus patients (~20 million) can modulate their tinnitus with somatic maneuvers, or attribute its onset to a somatosensory injury. Investigating underlying mechanisms in somatosensory-auditory integration after cochlear damage will allow us to elucidate the changes that contribute to tinnitus, and thus provide insights leading to successful interventions.

Project Summary This training program for predoctoral students and postdoctoral fellows provides an interdisciplinary education and research experience in hearing, balance and chemical senses (HBCS). Predoctoral training is based in one of the 15 academic departments and degree-granting graduate programs with which the 21 training faculty are associated. Doctoral thesis research and postdoctoral research training reside primarily within the laboratories of the mentors. Our affiliated faculty provide outstanding training in each of their corresponding disciplines: bioengineering, cellular and molecular biology, genetics, neurophysiology and optogenetics, developmental biology, and biopsychology of sensory processing. The HBCS program integrates investigators in auditory, vestibular, and chemosensory research with trainees from across these disciplines and thereby fosters innovative training opportunities into mechanisms and disorders of hearing, balance, and taste and olfaction. Cross-disciplinary integration is made possible by the training program through: a) introductory and advanced courses in sensory systems; b) seminars relevant to hearing, balance and chemical senses from experts within and outside the University of Michigan; c) student and faculty seminars, journal clubs and research forums; d) exposure to clinical research issues in Otolaryngology and clinical/translational resources available at the University of Michigan Medical School, and e) training in research standards and ethics. In addition, mentee projects usually include a co-mentor from a different laboratory within or across departments to provide complementary expertise. There is a strong emphasis on equity and diversity, which is fostered by bimonthly meetings for women and minority students. Support for 4 predoctoral and 3 postdoctoral trainees is requested. Predoctoral trainees will be selected from the most highly qualified graduate students in the training faculty's affiliated departments and programs. Interest and motivation for research in sensory systems will be an important selection criterion. Postdoctoral trainees (Ph.D., M.D., or D.D.S.) will be selected from applicants with a strong commitment to sensory biology and promising research backgrounds.

Abstract The dorsal cochlear nucleus (DCN) integrates auditory and somatosensory information through circuitry that modulates activity of the principal output neurons of the circuit, the fusiform cells. Fusiform cells receive somatosensory information via synapses on their apical dendrites and acoustic information via their basal dendrites. When somatosensory activation is combined with sound, the circuit can be strengthened or weakened depending on the order of the bimodal stimuli. This process is called stimulus timing dependent plasticity. In the condition of phantom sound perception, or tinnitus, the DCN circuitry is strengthened to increase the firing rates and synchrony of fusiform cells. This proposal seeks to investigate the effects of manipulations on the circuitry of this first auditory brainstem nucleus by implementing a bimodal sound + electrical stimulus paradigm to alter the circuit in normal and pathological conditions. Animal and human studies will be conducted to optimize the stimulation parameters/dosages necessary to weaken the circuit and thus ameliorate the pathological condition. This will ultimately lead to treatments for neural disorders involving hyperactive or hyper synchronous circuits such as those identified for tinnitus or Parkinson's disease.

DESCRIPTION (provided by applicant): This training program for predoctoral students and postdoctoral fellows provides an interdisciplinary education and research experience in hearing, balance and chemical senses (HBCS). Predoctoral training is based in one of the 16 academic departments and degree-granting graduate programs with which the 23 training faculty are associated. Doctoral thesis research and postdoctoral research training reside primarily within the laboratories of the mentors. Our affiliated faculty provides outstanding training in each of their corresponding disciplines: bioengineering, cellular and molecular biology, genetics, molecular and neurophysiology, developmental biology, and biopsychology of sensory processing. The HBCS program integrates investigators in auditory, vestibular, and chemosensory research with trainees from across these disciplines and thereby fosters innovative training opportunities into mechanisms and disorders of hearing, balance, taste and smell. Cross- disciplinary integration is made possible by the training program through: a) introductory and advanced courses in sensory systems; b) seminars relevant to hearing, balance and chemical senses from experts within and outside the University of Michigan; c) student and faculty seminars, journal clubs and research forums; d) exposure to clinical research issues in Otolaryngology and clinical/translational resources available at the University of Michigan Medical School, and e) training in research standards and ethics. Support for 4 predoctoral and 3 postdoctoral trainees is requested. Predoctoral trainees will be selected from the most highly qualified graduate students in the training faculty's affiliated departments and programs. Interest and motivation for research in sensory systems will be an important selection criterion. Postdoctoral trainees (Ph.D., M.D., or D.D.S.) will be selected from applicants with a strong commitment to sensory biology and promising research backgrounds.

Abstract The concept of ?hidden? hearing loss challenges the idea that temporary threshold shifts (TTS) reflect a return to normal hearing. Recent studies indicate that after noise-exposure that produces TTS, and thus clinically 'normal' audiograms, there is nonetheless permanent damage to auditory nerve fiber (ANF) synapses with cochlear inner hair cells. Hidden hearing loss is a potential major health issue, as human temporal bone and ABR studies suggest it is common in humans. The remaining perceptual deficits in humans with clinically normal audiograms reflect temporal coding problems likely due to loss of the high threshold, low spontaneous rate ANFs, which are preferentially affected after TTS. The primary central targets of high-threshold ANFs reside in the small cell cap (SCC) of the cochlear nucleus (CN). High-threshold ANFs and their SCC targets display large dynamic ranges and superior suprathreshold tuning and temporal coding, which are essential for speech perception in noisy environments. The SCC occupies a large proportion of the CN in humans and is therefore poised to play a major role in central mechanisms of hidden hearing loss. The SCC is unique also as a putative recipient and projection area of medial olivocochlear (MOC) neurons. The overall hypothesis of this proposal is that the SCC plays a major role in suprathreshold sound coding and that this coding is highly susceptible to degradation by hidden hearing loss. The goal of this series of studies is to elucidate the cochlea- SCC-MOC circuit in normal and noise-damaged animals with hidden hearing loss, using state-of-the-art optogenetics, multichannel single unit physiology, tract tracing and sophisticated immunohistochemical methods.