The Aging of the Middle Ear in 129S6/SvEvTac and CBA/CaJ Mice: Measurements of Umbo Velocity, Hearing Function, and the Incidence of Pathology

0 Virginia Merrill Bloedel Hearing Research Center, University of Washington , Seattle, WA 98195, USA 1 Department of Otology and Laryngology, Harvard Medical School , Boston, MA 02114, USA 2 Massachusetts Eye and Ear Infirmary 3 Eaton-Peabody Laboratory , Massachusetts Eye and Ear Infirmary, Boston, MA 02114, USA 4 Department of Audiology , Massachusetts Eye and Ear Infirmary, Boston, MA 02114, USA 5 Department of Otolaryngology/Head and Neck Surgery, University of Washington School of Medicine , Seattle, WA 98195, USA Measurements of umbo velocity and auditory brainstem response (ABR) were made on two different strains of mice, 129S6/SvEvTac (129S6) and CBA/ CaJ (CB), within three different age ranges. The velocity measurements were made with a laser Doppler vibrometer using a semiclosed sound delivery system; the frequency range of accurate velocity measurements is from 1 to 21 kHz. The visual detection threshold of the ABR was determined at selected frequencies between 2 and 32 kHz. The velocity results suggest a small but significant change in umbo velocity with age in both strains, between the youngest (1.5-3 months) and mid-aged (12-14 months) groups. There is also a clear difference in the umbo velocity in the youngest animals of the two strains, with the 129S6 having more sensitive middle-ear function than the CB. These results support the existence of a small age-related loss in middle-ear sensitivity in the mouse that was first described in the BALB/6J strain. The age-related changes in middleear function observed in both CB and 129S6 are much smaller than the age-related decreases in ABR. Our results also describe a statistically significant increase in the incidence of middle-ear pathology with age in the 129S6. - The use of molecular and genetic techniques in investigations of auditory function has led to a huge increase in the number of experimental studies that make use of various strains of mice. These studies include investigations of the development of the ear (Huangfu and Saunders 1983; Doan et al. 1994), the sensitivity of the ear to acoustic trauma (Li and Borg 1993; Yoshida et al. 2000; Willott et al. 2000), and the aging of the ear (Li and Borg 1991; Doan et al. 1996a). All of these studies depend on the definition of normal function, and it has been repeatedly demonstrated that the ears of different strains of mice show different patterns of development and aging (Jimenez et al. 1999; Zheng et al. 1999). The CBA/CaJ (CB) strain has been used extensively as the standard for normal hearing in studies of hearing function and how it changes with age (Li and Borg 1991; Willott et al. 1992; Jimenez et al. 1999). While 129-derived strains, e.g., 129S6/SvEvTac (129S6), were little used in earlier studies of mouse audition, they are becoming quite common in auditory studies (e.g., Zheng et al. 1999; Yoshida et al. 2000) and are prominent in the generation of knockout mice for auditory and nonauditory applications. Since it has been demonstrated previously that there are strain-dependent differences in the function of the developing and aging auditory system of different mice (Li and Borg 1991; Jimenez et al. 1999), it is important to understand the bases for these differences. The results in this report are part of a larger study of how differences in the genetic programming within the inner ear affect susceptibility to hearing loss with age and noise exposure. The larger study includes measurements of cochlear and peripheral auditory function, including distortion-product otoacoustic emissions (DPOAE) and auditory brainstem responses (ABR), in aging mice of different genetic backgrounds (Kujawa et al. 2001, 2002). In performing preliminary studies, three questions arose: (1) Are there any age-related alterations in middle-ear function in the different mouse strains? (2) Is there any correlation between age-related changes in the middle ear and other measures of auditory function? (3) Are there age- and strain-related differences in the susceptibility to middle-ear disease? The first question has been addressed in one mouse strain by Doan et al. (1996a,b), who used measurements of umbo velocity to determine age-related changes in middle-ear sound transmission and found a small (610 dB) reduction in velocity in old BALB/c mice in response to tones of 412 kHz. This result suggests that a 610 dB reduction in auditory sensitivity in aging mice is related to a loss in middleear sound transfer. We investigate this question in two other mouse strains also using measurements of the sound-induced velocity near the center of the tympanic membrane (close to the umbo). This velocity, which is at the input to the ossicular system, is a measure of the transmission of sound between the external ear and the inner ear. The second question is addressed by investigating the degree of correlation between the age-related changes in umbo velocity we observed with changes in the sound-evoked auditory brainstem response (ABR), a measure of the sensitivity of the entire auditory periphery to sound. Correlations between the age-dependent ABR and umbo velocity have been shown in some developing animals (Relkin and Saunders 1981; Doan et al. 1994, 1996b). With regard to the third question, McGinn et al. (1992) noted an increased incidence of otitis media in CBA/J with little incidence in CBA/CaJ (the CB of our study), and our own observations suggest that the 129S6 is more susceptible to middle-ear disorders than CB, especially in older mice. The purpose of this Number of normal ears for ABR testing study is to investigate these questions regarding agerelated changes in middle-ear function and the susceptibility to middle-ear disease in the CB and the 129S6 strains. The two strains of mice (CB and 129S6) were investigated in three different age groups: (1) 1.53 months, (2) 1214 months, and (3) 1824 months. Mice reach sexual maturity within 3.54 weeks after birth and can live for 2.53.5 years in captivity; therefore, our three groups represent young adults, middle-aged, and old mice. All mice were born and reared in an AAALAC-approved Specific Pathogen Free (SPF) facility at the University of Washington from stock originally obtained from Jackson Labs (CB) and Taconic (129S6). All breeding and husbandry procedures were approved by the Animal Care and Use Committee of the University of Washington. The mice from the two strains were housed, fed, and otherwise treated identically and were airfreighted to the Massachusetts Eye and Ear Infirmary several days to two weeks prior to electrophysiologic and mechanoacoustic measurements. Upon arrival at the Massachusetts Eye and Ear Infirmary Animal Facility, the mice were kept in a dedicated isolation room until the time of the measurements. Data from two subsets of the bred and transported mice were used in our measurements. A larger subset with normal middle ears that included from 14 to 83 mice in each of the three age groups (...truncated)