Glen K. Martin - US grants

Distortion-product emissions (DPEs) have been shown to be sensitive indicators of cochlear function. Several features of these particular otacoustic emissions make them important candidates for the measurement of progressive hearing loss. Thus, DPEs appear to uniquely test outer hair-cell function, they are simply measured by inserting a miniature microphone system into the external ear canal, and they can precisely measure the ear's response to subtle gradations in stimulus frequency and intensity. All these positive features suggest that DPEs may someday become a powerful test of hearing impairment. The outcome of a small pilot study we recently completed on both normally hearing and hearing-impaired ears supports the potential utility of DPEs in a clinical setting. The present study proposes first to extend our initial data base to include a larger sampling of normal hearing subjects so that reliable descriptive statistical analyses can be performed. Sources of individual variability in "normal" ears will be investigated by relating DPEs to other emission types and fine- resolution measures of hearing. In addition, statistical and technical procedures will be investigated to assess their effectiveness in reducing variability in DPE measures and related noise floors in order to increase the dynamic range of the response measure. Secondly, patient populations representing several common progressive-hearing disorders will be tested for DPEs in order to relate these measures to the frequency pattern and degree of hearing loss. An attempt will also be made to determine if the etiology of the hearing disorder significantly influences the DPE results. Finally, selected patients will be monitored over the requested 5-yr project period to establish the ability of DPEs to describe the course of their progressive-hearing losses.

Recent research in mice with age-related hearing loss has established that an augmented acoustic environment (AAE) consisting of moderate-level pulsed noise bursts presented for 12 hrs per day can, as measured by auditory brainstem responses (ABRs) and spiral ganglion-cell counts, dramatically retard the functional and histological changes typically associated with such genetically-based accelerated aging processes. Whether these ameliorating influences also disrupt the associated degeneration of sensory receptor cells has not yet been determined. The primary goal of the proposed study is to assess to what extent, if any, an AAE affects the progressive sensory-cell loss in mice that exhibit rapid-onset presbycusis. This aim will be accomplished using functional and histological methods in two strains of mice, consisting of the prematurely aging C57s and the normal CBAs, by, respectively, monitoring distortion-product otoacoustic emissions (DPOAEs) that reflect the normal micromechanical activity of outer hair cells (OHCs), and evaluating hair-cell presence in plastic-embedded whole mounts of the cochlea. The C57 strain shows progressive sensory-cell degeneration that is advanced by age 6 mo, whereas the CBA strain exhibits normal cochlear function and sensory-cell presence up to age 18 mo. The planned experiments will determine if DPOAEs and cochlear histopathology in the C57s differ in an experimental group exposed to an AAE as compared to the identical measures obtained for control C57s housed under normal vivarium conditions. The outcome of the AAE treatment will also be compared to similar groups of CBA mice, which are not expected to show any changes as a result of exposure to the same acoustically enriched environment or vivarium conditions. ABRs will also be obtained in order to facilitate comparisons of these findings to the earlier results as well as to determine if they improve in the absence of significant AAE-induced cochlear effects. The finding that an AAE can preserve OHC function and prevent or retard hair-cell degeneration would have far-reaching implications for plasticity of auditory sensory receptors, and indicate that the adverse effects of certain genetic forms of hearing loss can be deliberately reversed.

The planned studies are designed to develop a rapid means of phenotyping mice according to the functional status of their peripheral ears. The initial experiments in Years 1-1.5 focus on developing a protocol using distortion-product otoacoustic emissions (DPOAEs) that is suitable for screening large numbers of mice for cochlear abnormalities. Toward this goal, 2f1-f2 DPOAEs will be obtained at three levels in the form of DPgrams that will test at 10 points per octave over a frequency extent that ranges from a geometric mean of 5.6 to 48.5 kHz (f2=60 kHz). These measures will be recorded from both ears of mice representing 40 unique strains, with n=12 mice from each strain. In addition, such DPOAE-screening data will be obtained at two ages in the same mice, ie, at 2 and 5 mo of age. The database created by these data will be analyzed in order to determine the 5th percentile of DPOAE levels in normal CBA-type mice and the 95th percentile in mice, such as the C57, with impaired cochlear function. The percentile data will be used to devise a standardized chart upon which data for individual mice undergoing mutagenesis can be plotted in order to determine if cochlear function is normal or impaired. In association with these experiments, a commercial instrument will be developed with a leading manufacturer of auditory-research equipment, which can be made available in the marketplace so that the scientific community will have access to the ability to determine the status of cochlear function in unknown mice. The second stage of the project, to be conducted in another group of mice from Years 1.5 to 3 of the study, is aimed at establishing a similar database for these same 40 mouse strains based on their susceptibility to noise-induced hearing loss. To accomplish this goal, following the acquisition of baseline DP-grams, at 3 mo mice will be exposed to a standard octave band noise known to cause permanent decrements in DPOAE levels, and then re-tested 2 wks later in order to document the effects of noise overexposure on cochlear function. Percentiles from these exposed mouse groups will form the basis of another standardized chart for determining the susceptibility of individual mice to acoustic overstimulation. Together, both projects will establish the usefulness of DPOAE testing for the purpose of developing simple and rapid screening methods of determining the status of cochlear function in individual mice. The resulting data will be made widely available on the internet in the form of databases and the relevant training manuals.

The planned studies are designed to develop a rapid means of phenotyping mice according to the functional status of their peripheral ears. The initial experiments in Years 1-1.5 focus on developing a protocol using distortion-product otoacoustic emissions (DPOAEs) that is suitable for screening large numbers of mice for cochlear abnormalities. Toward this goal, 2f1-f2 DPOAEs will be obtained at three levels in the form of DPgrams that will test at 10 points per octave over a frequency extent that ranges from a geometric mean of 5.6 to 48.5 kHz (f2=60 kHz). These measures will be recorded from both ears of mice representing 40 unique strains, with n=12 mice from each strain. In addition, such DPOAE-screening data will be obtained at two ages in the same mice, ie, at 2 and 5 mo of age. The database created by these data will be analyzed in order to determine the 5th percentile of DPOAE levels in normal CBA-type mice and the 95th percentile in mice, such as the C57, with impaired cochlear function. The percentile data will be used to devise a standardized chart upon which data for individual mice undergoing mutagenesis can be plotted in order to determine if cochlear function is normal or impaired. In association with these experiments, a commercial instrument will be developed with a leading manufacturer of auditory-research equipment, which can be made available in the marketplace so that the scientific community will have access to the ability to determine the status of cochlear function in unknown mice. The second stage of the project, to be conducted in another group of mice from Years 1.5 to 3 of the study, is aimed at establishing a similar database for these same 40 mouse strains based on their susceptibility to noise-induced hearing loss. To accomplish this goal, following the acquisition of baseline DP-grams, at 3 mo mice will be exposed to a standard octave band noise known to cause permanent decrements in DPOAE levels, and then re-tested 2 wks later in order to document the effects of noise overexposure on cochlear function. Percentiles from these exposed mouse groups will form the basis of another standardized chart for determining the susceptibility of individual mice to acoustic overstimulation. Together, both projects will establish the usefulness of DPOAE testing for the purpose of developing simple and rapid screening methods of determining the status of cochlear function in individual mice. The resulting data will be made widely available on the internet in the form of databases and the relevant training manuals.

Description: The proposed studies focus on the contribution of the cochlear-efferent system in the susceptibility to noise and the phenomenon of "sound conditioning." Several hypotheses are proposed to determine whether the susceptibility of the ear can be experimentally manipulated through sound conditioning, if the cochlear-efferent system is involved in sound conditioning and whether auditory or systemic factors are involved in the process. In addition, the molecular bases of protection from sound-overexposure will be explored. The functional measure employed in the experiment is the 2f1-f2 DPOAE and its alteration by presentation of a pure tone presented to the opposite ear. The change in DPOAE with binaural stimulation is proposed as a measure of "efferent strength," which appears to vary among individual animals and humans. Three hypotheses are proposed. In the first, awake rabbits will be used as subjects to determine if the efferent system plays a role in 'sound conditioning', whether the effect is ear-specific and whether other systemic factors are involved. The second hypothesis tests the extent to which "sound conditioning" is apparent in mice that have varying susceptibility to noise-induced hearing loss. The third hypothesis tests whether susceptibility to hearing loss or the prevention from it is associated with specific molecules in the cochlea.