Recent animal studies have revealed substantial irreversible neural degeneration following the temporary noise-induced elevation of the auditory threshold, i.e., temporary threshold shift (TTS). Despite no hair cell loss and recovery of threshold sensitivity, exposed animals demonstrated extensive loss of synaptic connections between cochlear hair cells and auditory nerve terminals and delayed degeneration of spiral ganglion cells and their central projections (Kujawa and Liberman, 2009; Lin, et al., 2011; for review, Kujawa and Liberman, 2015). Single-unit recordings indicate that high-threshold auditory nerve fibers with lower spontaneous rates are preferentially lost under these conditions (Furman, et al., 2013), suggesting that this type of noise-induced damage could negatively impact the ability to process supra-threshold sounds, especially in the presence of masking noise, though a loss would not likely be detected using threshold audiometry (for review, Plack, et al., 2014).
The focus of this NIDCD workshop was to identify barriers to, and opportunities in, the area of noise-induced synaptopathy and to articulate unmet research needs as the scientific community looks to translate these animal studies into the realm of the human auditory system and the clinic.
Participants
The workshop was organized by Janet Cyr, Ph.D., and Amy Donahue, Ph.D., in the NIDCD Division of Scientific Programs and co-chaired by Sharon Kujawa, Ph.D., and M. Charles Liberman, Ph.D., from Massachusetts Eye and Ear Infirmary/Harvard Medical School. Drs. Kujawa and Liberman moderated the meeting and discussions.
Sixteen auditory scientists from the U.S. and abroad were invited to bring their expertise to the discussion (see participant list). Two invited participants, Alice Suter, Ph.D., and Rick Davis, Ph.D., were unable to attend the face-to-face meeting but were involved in pre-workshop discussions. In addition, Dan Tagle, Ph.D., from the NIH’s National Center for Advancing Translational Sciences (NCATS) was a guest speaker. The workshop was open to all interested members of the scientific community and the public.
Pre-Workshop Discussions
In preparation for the workshop, invited participants held subgroup conference calls focused on the following four areas related to noise-induced synaptopathy: animal studies, human psychophysics, human electrophysiology, and mechanisms/therapies. During these calls, the subgroups delineated the current state of knowledge in these specific areas and began to identify knowledge gaps and barriers to filling those gaps. The subgroups also compiled a list of pertinent scientific articles to be read by all participants before the workshop. Subgroup conference calls were open to all invited workshop participants.
Workshop Format
The workshop (see agenda) began at 8 a.m. on May 4, 2015, with welcoming remarks from NIDCD Director James F. Battey, Jr., M.D., Ph.D., followed by introductory comments from Dr. Cyr. Dr. Liberman facilitated introductions of the invited workshop participants and Dr. Kujawa presented the workshop charge, which included a request to delineate a list of long-term and short-term research recommendations in the area of noise-induced cochlear synaptopathy. These workshop recommendations will help to guide NIDCD staff as well as the larger research community.
Dr. Tagle, NCATS, gave the first presentation, providing an overview of some of the challenges of translating animal studies to humans. Dr. Liberman then presented the current state of the auditory field regarding noise-induced synaptopathy in animal models and recent preliminary work demonstrating similar findings in human temporal bones. Each of the four subgroups then presented a synopsis of their pre-meeting discussions with an emphasis on the current state of scientific knowledge and the identified challenges, obstacles, and unanswered questions. Extensive group discussion followed each presentation as the panel members identified additional knowledge gaps and research needs.
Day two of the workshop began with additional whole-group discussions, followed by smaller breakout group discussions to generate specific research recommendations. The two breakout groups were:
- Group 1 (Mechanisms and Potential Therapies): Drs. Glowatzki, Green, Kujawa, Liberman, and Lustig
- Group 2 (Diagnostics): Drs. Brungart, Gallun, Heinz, Konrad-Martin, Lauer, McAlpine, Oxenham, Plack, and Shinn-Cunningham
Members of the audience were welcome to observe the breakout group discussions. When the breakout groups had developed a written list of recommendations, the entire workshop panel reconvened to discuss and refine the overall workshop recommendations. A draft of the workshop summary and recommendations was disseminated to workshop participants, who provided final input and edits.
Workshop Recommendations
The primary questions and research needs identified by the workshop panel are listed below. Recommendations were broken into subtopics and represent the discussions of the panel during the two-day workshop. The recommendations are not intended to be all-inclusive and are presented in no particular order.
Mechanisms and Potential Therapies for Noise-Induced Synaptopathy:
Primary Questions: Mechanisms
- What are the mechanisms of normal pre- and post-synaptic transmission?
- What are the pre-synaptic release mechanisms?
- Which post-synaptic ion channels and pumps modulate afferent activity?
- What are the differences underlying the properties of high- vs low-spontaneous rate fibers, and the molecular signals driving their developmental differentiation?
- What is the basis of efferent modulation of dendritic activity—mechanisms of excitation and inhibition?
- What are the special features of inner hair cell (IHC) synaptic transmission, and how do ribbon synapses differ across hair cell organs and vertebrate species?
- What are the mechanisms of noise-induced cochlear synapse damage?
- What is the phenomenology of primary neural degeneration?
- What other cochlear insults lead to synaptopathy?
- Do all TTS-inducing noise exposures produce synaptopathy; is synaptopathy worse when hair cells are also damaged; does impulse noise also cause synaptopathy? If impulse noise causes synaptopathy, is the mechanism the same as that for long exposures?
- What are the dynamics of synapse damage—what is the sequence of structural and ultrastructural changes?
- What are the receptor dynamics and trafficking in the post-synaptic bouton regarding postsynaptic receptors, especially NMDA-type and Ca2+-permeable AMPA-type receptors?
- How is calcium homeostasis regulated in the post-synaptic bouton?
- What are the roles of inflammatory processes, immune processes, and vascular changes in synaptopathy?
- What are the contributions of glutamate excitotoxicity, recycling, and uptake in synaptopathy?
- What is the role of reactive oxygen species and/or osmotic stress in synaptopathy?
- Are there protective effects of prior noise exposure?
- Do olivocochlear efferent fibers protect from synaptopathy?
- What is the phenomenology of primary neural degeneration?
- What are the mechanisms of IHC synapse assembly and synapse regeneration?
- Which signals drive the normal developmental process of IHC synapse assembly?
- Is there IHC synapse reassembly after loss and what are the barriers to synapse reassembly in the adult cochlea?
- Which neurotrophic factors support synaptogenesis and how are their synthesis and release affected by acoustic trauma?
Primary Questions: Potential Therapies
- What are the in vitro requirements for neurite extension and synapse regeneration?
- Can drug discovery identify small molecules that can rescue cochlear synaptopathy?
- What is the therapeutic window (trauma-treatment interval) for in vivo neurotrophin-based synapse regeneration?
- What is the functional status of neurotrophin-rescued synapses?
- What is the permeability of the round window to drugs and viruses?
- What is the efficacy of various slow-release polymers and hydrogels for drug/molecule delivery into the ear?
- What is the efficacy of various viral vectors and promoters in targeting inner ear structures/cell types?
Technical and Infrastructure Needs
- Utilize high-resolution calcium imaging of hair cell/auditory-nerve terminals synapses, e.g., with GCaMP, coupled with pharmacologic and genetic manipulation.
- Implement/develop i) fast voltage and calcium sensors to monitor firing in auditory nerve fibers and ii) glutamate and pH detection methods to monitor IHC synaptic release in vitro.
- Identify gene expression differences among the different spontaneous-rate spiral ganglion neuron groups to facilitate the development of cell-specific drivers for selective genetic silencing/activation of specific fiber types in vivo.
- Develop minimally invasive in vivo imaging to visualize spiral ganglion cells, peripheral axons, synapses, etc., in animal models.
- Standardize rigorous synaptic histological analyses across laboratories (e.g., core facilities/collaborative networks).
Diagnostics for Noise-Induced Synaptopathy:
Primary Questions: Diagnosis
- What is the prevalence of noise-induced synaptopathy in humans?
- Are there perceptual consequences of noise-induced synaptopathy? If so, what are they and how can they best be measured?
- What is the most appropriate diagnostic test(s) for synaptopathy? What “gold standard” measure of synaptopathy should be used to determine the sensitivity and specificity of diagnostic tests?
- Does noise-induced synaptopathy underlie patient-reported difficulties in understanding speech in noise in the absence of hearing loss?
- Does noise-induced synaptopathy contribute to patient-reported difficulties in understanding speech in noise when hearing loss is also present?
- What is the relationship between noise-induced synaptopathy and age-related hearing loss?
- Is there a relationship between noise-induced synaptopathy and tinnitus?
- What are the implications of noise-induced synaptopathy for noise exposure regulations?
To begin to answer these primary questions in the area of diagnostics, the panel considered the research needs, challenges, and approaches/considerations to address them in the areas of behavior, electrophysiology, modeling, and imaging:
Behavioral Studies
Needs and Identified Challenges:
- Utilize animal behavior studies (informed by human psychoacoustics and computational modeling) for before and after comparisons following noise-induced synaptopathy. Consider:
- Interactions between noise exposure and aging (including the use of aging models).
- Potential interactions between “pure” synaptopathy and threshold elevating pathologies.
- Interactions of synaptopathy with other models of neural loss (e.g., ototoxic insults).
- Utilize human psychoacoustics/speech studies to investigate noise-induced synaptopathy. Challenges with these studies include:
- Minimizing inter-subject variability, including motivation, cognition, genetic heterogeneity, etc.
- Interactions between age-related and noise-induced damage.
Approaches and Considerations:
- Behavioral tasks should consider:
- Specificity of fiber loss (e.g. low- vs high-spontaneous fibers).
- Potential effects of neural adaptation to sound statistics.
- Characteristic frequency specificity.
- Role of efferent fibers.
- Relevance to speech understanding in challenging conditions.
- Supra-threshold stimulation/testing.
- Speech understanding in noise/babble.
- Subjective measures include perceived hearing handicaps and listening effort.
- Validation, sensitivity, specificity, and test/retest reliability.
- Approaches include:
- Longitudinal studies (military personnel, musicians, etc.).
- Collections of large datasets—add to existing data banks or create new collections, such as using smartphone apps/mobile technology for testing pre-/post-concert attendance.
- Increased use of human temporal bones:
- Improve pre-mortem characterizations of the general health and hearing health of donors, including noise exposure and physical/ auditory test results, to enhance the study of pre-mortem auditory performance vs. post-mortem synaptopathy/histopathology.
- Promote temporal bone donation programs to specific targeted populations—such as members of the military or musicians—who may have a history of auditory testing and increased likelihood of noise exposure.
Electrophysiology Studies
Needs and Identified Challenges:
- Compare various electrophysiological measures for diagnosing synaptopathy.
- Challenges include:
- Individual differences
- Mixed (sensory, neural) involvements
- Consequences of peripheral damage on the central auditory system.
Approaches and Considerations:
- Optimize electrophysiological measures for sensitivity, specificity, time efficiency, test/re-test reliability, and validation. Potential measures for humans and animals include:
- Frequency following response (monaural and binaural, modulation frequencies)
- Auditory brainstem response (Wave I/ Wave V, amplitude, latency)
- Middle ear muscle reflex
- Medial olivocochlear bundle reflex
- Use differential measures to reduce within- and between-subject variability.
- Consider information contained within the responses (e.g., variability, not just mean responses).
Modeling Studies
Needs and Identified Challenges:
- Employ computational modeling to make testable predictions for behavioral and physiological responses in animals and humans with synaptopathy, including speech and non-speech stimuli.
Approaches and Considerations:
- Use simple signal-detection (information) theory expectations as a baseline.
- Extend to physiologically realistic models, including efferent effects, and potential differences between low- and high-spontaneous-rate fibers.
- Extend the model to consider individuals who do not have a “pure” synaptopathy, but rather have mixed sensory and neural injuries.
Imaging Studies
Needs and Identified Challenges:
- New/refined techniques for high-resolution, minimally invasive imaging of human auditory structures in vivo to detect damage at the cellular and subcellular level.
Approaches and Considerations:
- Diffusion tensor imaging of auditory nerve
- Fiber-optic tubes
- Other magnetic resonance imaging/positron emission tomography techniques
