An anatomically based frequency–place map for the mouse cochlea
Introduction
Permanent threshold shifts have many histological correlates, most commonly hair cell loss, a variety of intracellular changes, stereociliary changes and nerve fiber loss. To be able to identify subtle structural alterations in cochleas having decreased auditory function but little hair cell loss, it is necessary to develop a frequency–place map of the organ of Corti (OC). Such maps have been created for the cochleas of humans (von Békésy, 1949), cats (Schuknecht, 1953), chinchillas (Eldredge et al., 1981), and guinea pigs (Koenig, 1949). The goal of the present study was to develop an anatomically based frequency–place map for the mouse OC.
Ehret (1975) generated a frequency–place map for the mouse using behavioral conditioning to measure critical bands at various frequencies. This map is theoretical because it is based on the assumptions that the mouse cochlea is 7 mm long and that all critical bands cover the same linear distance on the basilar membrane (i.e., 1 mm) (e.g., Fletcher, 1940, Greenwood, 1961, Ehret, 1975). Our companion study (Ou et al., 2000) has shown that the OC length in the C57BL/CBA mouse is variable, although the mice are genetically homogeneous. Furthermore, if each critical band covers the same linear distance, the basilar membrane must have a uniform gradation in stiffness. This assumption has not been confirmed experimentally.
Previous anatomically based mapping studies can be divided into three main types: (1) direct observation of the point of maximal vibration of the basilar membrane when using different stimulus frequencies (e.g., von Békésy, 1949); (2) injection with a retrograde tracer into eighth nerve fibers having different characteristic frequencies (e.g., Liberman, 1982, Horikawa and Armstrong, 1988); (3) correlation of histological lesions in the OC with permanent audiometric threshold shifts (e.g., Eldredge et al., 1981).
In order to determine temporary and permanent changes in auditory function after noise exposure, it was important to use a non-invasive method so that auditory function could be measured repeatedly in individual mice. Evoked auditory brainstem responses (ABRs) were used because of their simplicity, repeatability, and the existence of baseline ABR data in different strains of mice (Zheng et al., 1999). Plastic-embedded flat preparations were used to obtain quantitative histopathological data throughout the cochlea to provide detailed information with which to make function–position matches.
Section snippets
Noise exposure, ABR threshold shifts, cochlear processing, and microscopic examination
Details of the noise exposures, ABR testing and determination of threshold shift, histological processing, and microscopic examination are described in the companion paper and Nordmann et al. (2000). In brief, the mice used to develop the frequency–place map were 26 C57BL/CBA F1 mice aged 2–3 months when first tested (3–4 months at time of death) that were purchased from The Jackson Laboratory. The protocol for the care and utilization of the animals in this study was reviewed and approved by
Results
Twenty-six of the original 39 noise-exposed mice (Ou et al., 2000) developed ‘notched’ auditory threshold shift patterns – i.e., patterns with focal regions of PTS and adjacent regions of total recovery of thresholds. Frequency–place matches could be made for 22 of these mice. From each mouse with a notched audiogram, one cochlea was analyzed quantitatively by phase contrast microscopy.
Discussion
The experimentally based map of the mouse OC detailed here was developed using a well-established method of mapping (i.e., correlating permanent threshold shifts with cochlear pathology). This map is in good agreement with the theoretical map of Ehret (1975) and suggests that Ehret’s mapping method using critical bands is valid. As a result, comments can be made about the assumptions essential to Ehret’s study. Ehret assumed that all critical bands cover equivalent distances on the basilar
Acknowledgements
This work was supported by the National Organization for Hearing Research, the American Heart Association, and the Department of Otolaryngology at Washington University School of Medicine. We gratefully acknowledge the excellent technical assistance provided by Mr. Thomas J. Watkins.
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Present address: Department of Otolaryngology, VMB Hearing Research Center, Box 357923, University of Washington Medical School, Seattle, WA 98195-7923, USA.