CD1 HEARING-IMPAIRED MICE - I - DISTORTION-PRODUCT OTOACOUSTIC EMISSION LEVELS, COCHLEAR FUNCTION AND MORPHOLOGY

The levels of distortion product otoacoustic emissions (DPOAEs) were m easured in a strain of hearing-impaired mutant mice (CD1) at various s tages of outer hair cell impairment and compared to those of a control inbred strain (CBA/J). Parallel measurements of cochlear potentials a nd auditory brainstem evoked responses (ABRs) were performed and surfa ce preparations of organs of Corti were observed using phalloidin stai ning of filamentous actin. Comparison of DPOAEs (elicited by stimulus levels of 60 and 70 dB SPL) with standard functional tests allowed the categorization df CD 1 ears into two groups on the basis of the prese nce or absence of DPOAE, which corresponded to mean ABR thresholds gre ater or less than 40 dB nHL respectively. When adopting ABR threshold as the gold standard, this procedure yielded rates of false-positives and -negatives ranging from 5 to 16%. However, individual predictions of electrophysiological function from DPOAE levels were not accurate, owing to their large variance, and attempts to optimize stimulus level s did not reduce this variance. In contrast, the profiles of DPOAE lev el vs. f(2 )exhibited large correlations with ABR threshold profiles a s a function of f(2). It was also noteworthy that the mean levels of D POAEs in CD1 mice recorded in frequency intervals with normal ABR thre sholds were significantly smaller than those of CBA/J mice. Although h earing loss was revealed early both by DPOAEs and by other functional tests, surface preparations often remained normal until about 3-4 mont hs of age. (C) 1998 Elsevier Science B.V. All rights reserved.