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P.I.: Glen K Martin PhD Lab: Otoacoustic Emissions Laboratory Location: University of Miami Ear Institute (M805) Description: Our laboratory has extensive experience is using distortion-products otoacoustic emissions (DPOAEs) for measuring cochlear function. DPOAEs are sounds generated by the outer hair cells (OHCs), one group of inner ear receptor cells. These sounds are created as a by-product of nonlinear amplification of basilar membrane motion by the electromotile micromovements of OHCs. DPOAEs can be recorded noninvasively by placing a miniature microphone in the outer ear canal, and averaging the ear-canal sound pressure in response to two stimulating tones typically called f1 (lower frequency) and f2 (higher frequency). In the process of the OHC nonlinear amplification, new frequencies are generated, thus, the origin of the term 'distortion' products. The largest and most commonly measured DPOAE is at the 2f1-f2 frequency. The OHCs are the most vulnerable receptor cell in the cochlea and, when damaged by noise, ototoxic drugs, or aging, they degenerate. In the case of many mutant mouse strains, DPOAEs can no longer be measured, thus, inferring these mice have varying degrees of deafness. Consequently, these emitted responses are ideal for the rapid screening of cochlear function. In fact, otoacoustic emissions, in general, of which there are several types, beside DPOAEs, have been found to be so reliable, that they have been adopted universally as a means of screening hearing in newborn infants. Screening Protocol: Our screening procedure for evaluating ear function is based upon the non-invasive measurement of distortion-product otoacoustic emissions (DPOAEs), which specifically document the sound-induced activity of the outer hair cells of the cochlear, representing the most abundant and vulnerable of the hearing receptors. These measures provide a rapid means of phenotyping mice according to the functional status of the cochlea. DPOAEs will be obtained at three levels in the form of DP-grams that test, at 10 points per octave, a frequency extent that ranges from approximately 6 to 54 kHz, ie, over 80% of the frequency range of the mouse cochlea. The DPOAEs will be recorded from both ears of mice representing approximately 40 strains, with n=12 mice per strain. In addition, such DPOAE-screening data will be obtained at two ages in the same strain, ie, at 2 and 5 mo of age. The database created by these data will be analyzed 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. These percentile data will be used to devise a standardized chart upon which DP-gram data for individual mice undergoing mutagenesis can be plotted to determine specifically, if their cochlear function is normal or impaired. Most of these strains have been tested previously (Zheng et al, 1995. Assessment of hearing in 80 inbred strains of mice by ABR threshold analysis. Hear Res. 130, 94-107) with auditory brainstem response (ABR) techniques, so that we have a means of validating our DPOAE findings. The second stage of the project is aimed at establishing a similar database, for many of these same 40 strains, based on their susceptibility to noise-induced hearing loss. To accomplish this goal, following the acquisition of baseline DP-grams at 3 mo of age, will be exposed to a standard octave band noise known to cause permanent decrements in DPOAE levels, and then re-tested 2 wks later to document the effects of noise over-exposure on cochlear function. Percentiles from the exposed mouse groups will form the basis of another standardized chart for determining the susceptibility of individual mice to acoustic overstimulation. Contact: Selected References: 1. Probst R, Lonsbury-Martin BL, Martin GK. A review of otoacoustic emissions. J Acoust Soc Am 89:2027-2067, 1991. 2. Jimenez AM, Stagner BB, Martin GK, Lonsbury-Martin BL. Age-related loss of distortion-product otoacoustic emissions in four mouse strains. Hear Res 138:91-105, 1999. |