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J mouse model of presbycusis
Hearing Research 134 (1999) 29^38
Distortion product otoacoustic emissions in the CBA/J mouse model of presbycusis Kourosh Parham a
a;
*, Xiao-Ming Sun
a;b
, D.O. Kim
a;b
Division of Otolaryngology, Department of Surgery, University of Connecticut Health Center, Farmington, CT 06030-1110, USA b Department of Communication Science, University of Connecticut, Storrs, CT, USA Received 24 September 1998; received in revised form 20 March 1999; accepted 26 March 1999
Abstract CBA mice do not exhibit age-related loss of auditory sensitivity or cochlear pathology until relatively late in life. Therefore, this strain is believed to be an excellent animal model for the examination of the effects of age on the cochlea. To evaluate the effects of age on outer hair cell function, 2f1 3f2 distortion product otoacoustic emissions (DPOAEs) were measured for f2 between 8 and 16 kHz in CBA/J mice between 1 and 25 months of age. CBA mice exhibited mild age-related changes in DPOAE level and detection threshold at 17 months of age, and changes of 20^40 dB by 25 months of age. The DPOAE level decreased and detection threshold increased with age in a frequency-dependent manner, starting at high frequencies and eventually extending to low frequencies. The range of frequencies in which notches were observed in the DPOAE input/output (I/O) functions extended toward lower frequencies by 17 months of age. Notches were absent in the I/O functions of 25-month-old mice. The present results for a frequency range of 8^16 kHz suggest that age has modest effects on outer hair cell function in CBA mice. ß 1999 Elsevier Science B.V. All rights reserved. Key words: Distortion product otoacoustic emission; Aging; Mouse; Hearing loss; Outer hair cell
1. Introduction Human presbycusis, hearing loss due to increasing chronological age, is characterized by elevation of hearing thresholds, usually beginning with high frequencies and gradually progressing to lower frequencies. Because of the complex interactions of genetic and environmental variables that can contribute to age-related hearing loss, human presbycusis is a challenging research topic, particularly the late-onset, progressive hearing impairment, which is the most common type of presbycusis in humans (for review see Willott, 1991). The di¤culties faced by investigations of presbycusis in human subjects, including limitations on evaluating the correlation between functional and histological data of the same ear, have prompted the utilization of animal models to gain a better understanding of presbycusis.
* Corresponding author. Tel.: +1 (860)679-2554; Fax: +1 (860)679-2451; E-mail: [email protected]
A popular animal model for the study of aging in the auditory system is the inbred CBA mouse strain. Agerelated changes in the auditory system of CBA mice have been investigated behaviorally (Parham and Willott, 1988 ; Willott et al., 1994), anatomically (e.g. Henry and Chole, 1980; Li and Hultcrantz, 1994; Shone et al., 1991; Spongr et al., 1997 ; Willott et al., 1987, 1988, 1991) and electrophysiologically, including single neuron (e.g. Walton et al., 1998 ; Willott, 1986; Willott et al., 1988, 1991) and auditory evoked potential recordings (e.g. Henry, 1982; Henry and Chole, 1980; Hunter and Willott, 1987 ; Li and Borg, 1991 ; Shone et al., 1991; Sjostrom and Anniko, 1990 ; Wenngren and Anniko, 1988). These studies have demonstrated that the auditory system of the CBA mouse does not display signi¢cant behavioral, anatomical or physiological changes until relatively late in life. Histopathological studies are in general agreement that age-related hearing loss is associated with a progressive degeneration of cochlear hair cells and spiral ganglion cells, usually beginning with and most severely
0378-5955 / 99 / $ ^ see front matter ß 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 5 9 5 5 ( 9 9 ) 0 0 0 5 9 - 3
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a¡ecting the outer hair cells (OHCs) in both humans (e.g. Bredberg, 1968; Johnsson and Hawkins, 1972, 1979 ; Schuknecht, 1964, 1974; Schuknecht and Gacek, 1993) and CBA mice (e.g. Henry and Chole, 1980 ; Li and Hultcrantz, 1994; Spongr et al., 1997). The OHCs are thought to underlie the generation of otoacoustic emissions (OAEs). OAEs are low-intensity sounds detectable in the ear canal (Kemp, 1978). The association of OAEs with OHC function is supported by numerous experimental studies (e.g. Anderson and Kemp, 1979 ; Brown et al., 1989; Guinan, 1986; Kemp and Brown, 1984 ; Kim et al., 1980; Liberman et al., 1996; Lonsbury-Martin et al., 1987; Martin et al., 1987; Mountain, 1980 ; Schmiedt, 1986; Siegel and Kim, 1982; Zurek et al., 1982), including ¢ndings in mutant mice (Horner et al., 1985; Schrott et al., 1991). The impact of age on OHCs and the relationship between OAEs and OHCs suggests that OAEs can serve as a useful tool for studying age-related hearing loss. By permitting assessment of peripheral function, OAEs also have the potential to help separate out the peripheral and central contributions to presbycusis. Investigations on the e¡ects of age on various types of OAEs in humans have been initiated (e.g. Bon¢ls et al., 1988 ; Collet et al., 1990; Lonsbury-Martin et al., 1991 ; Stover and Norton, 1993). As pointed out above, these studies need to be complemented by evaluation of OAEs in animal models to make up for the disadvantages inherent in the human study of presbycusis. In mouse models, the general features of distortion product otoacoustic emissions (DPOAEs) have been documented in C57BL/6J mice (C57) (Jimenez et al., 1997 ; Parham, 1997 ; Parham et al., 1996; Sun et al., 1997), which show an early-onset, progressive pattern of severe hearing loss. The current study documents DPOAEs in CBA mice which show hearing loss only at old ages thus allowing an opportunity to assess the e¡ects of aging on OHC function more completely. Preliminary results of this study were previously reported (Parham et al., 1996; Sun et al., 1996). 2. Materials and methods 2.1. Subjects CBA/JNia mice were purchased from the National Institute of Aging (NIA)/Charles River Laboratories (Kingston, NY) at three age groups : 1 month (male), 17 months (female) and 21 months (female). Each group included ¢ve mice. Because no signi¢cant e¡ect of gender on the auditory sensitivity of CBA mice has been identi¢ed (Hunter and Willott, 1987), and to overcome the limitations on availability of aged mice, gender among age groups of CBA mice used in this study
was not maintained. Upon arrival, the mice were housed at the University of Connecticut Health Center's vivarium. The mice received at 1 month of age were tested at 1, 4, 8 and 13 months of age. Mice received at 17 months of age were tested at 17, 21 and 23 months of age and the mice received at 21 months of age were tested at 21, 23 and 25 months of age. Both ears of each mouse were tested. Because of either death or signs of middle ear problems during the course of this study, the ¢nal numbers of ears from which useful data were collected for the di¡erent age groups were : 1 month, n = 10; 8 months, n = 8; 17 months, n = 6; 21 months, n = 9; 23 months, n = 4; and 25 months, n = 2. Since the results for the mice up to 13 months of age were generally similar, to avoid unnecessary cluttering of the presentation, the data from the 4- and 13-month-old groups of mice were not included in the subsequent ¢gures and analyses. The care and use of animal subjects reported in this study were approved by the Animal Care Committee of the University of Connecticut Health Center (ACC 920032-932). 2.2. Recording system and protocol 2.2.1. Distortion product otoacoustic emissions The DPOAE recording system, including stimulus generation and calibration, has been described in detail elsewhere (e.g. Parham, 1997 ; Smurzynski and Kim, 1992). Brie£y, this system consists of an IBM PC-compatible computer, an Ariel DSP-16 board containing two-channel 16-bit D/A and A/D converters installed in a 486 PC, two ER-2 earphones and an ER-10B low noise microphone system (including an ampli¢er which provided 40 dB of gain). The 2f1 3f2 DPOAEs were measured. For each ear, DPOAEs were recorded as a function of stimulus level, i.e. the input/output (I/O) function. DPOAE level was recorded in response to f2 from 8 to 16 kHz spaced at 1/4 octave intervals, i.e. 8, 9.5, 11.3, 13.4 and 16 kHz, and two additional f2 , 12 and 14.4 kHz. The primary frequency ratio, f2 /f1 , was kept constant at 1.2. At each frequency pair, primary tone levels were incremented from L2 = 5 dB to 65 dB SPL (re: 20 WPa) in 5 dB steps with L1 3L2 = 10 dB. 2.2.2. Auditory brainstem responses (ABRs) As a measure of auditory sensitivity, ABRs were recorded for each ear at all age groups except the 23month-old mice. Details of the ABR recording instrumentation are described elsewhere (Parham, 1997). At each of the frequencies, 4, 8, 9.5, 10, 11.3, 12, 13.4, 14, 14.4, 16, 24 and 32 kHz, ABR threshold, de¢ned as the minimum sound pressure level (SPL) for which
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any ABR wave was reliably observed, was determined. 2.3. Test procedure For both recordings of ABRs and DPOAEs, the animal was anesthetized by an intraperitoneal injection of ketamine/xylazine (0.12 and 0.01 mg/g body weight, respectively). When needed, supplementary doses (20% of the initial dose) were administered every 50^60 min. Prior to both of the recordings, the ear canals and ear drums were otoscopically inspected for signs of obstruction or infection, and only those with clear outer and middle ears were used. Animals were placed in a 2.5U3.5U3 ft sound-treated chamber (Industrial Acoustics Corporation 1204). A heating pad, which was controlled by an Animal Blanket Control Unit (Harvard Apparatus), maintained the mouse body temperature within 1³ of 36³C. DPOAEs and ABRs were tested in two sessions separated by 1 week. Each recording session lasted approximately 1 h. 2.4. Statistical analyses Di¡erences in DPOAE levels among the age groups of 1, 8, 17 and 21 months were evaluated for statistical signi¢cance (P 6 0.05). The groups of 23 and 25 months of age were not included in the statistical analyses because of their small sample size. Two-way repeated analyses of variance (ANOVAs) were used. The ANOVAs included age and a repeated independent measure, such as f2 . Signi¢cant interactions were followed up by performing one-way ANOVAs. Signi¢cant main e¡ects were evaluated with Tukey post hoc pairwise comparisons. 3. Results 3.1. Age-related changes in ABR threshold The mean ABR thresholds of CBA mice, in the 4^32 kHz frequency region, are presented in Fig. 1. There was no obvious change in ABR threshold with increasing age until 21 months of age in the frequency region below 16 kHz. For 16 kHz and above, ABR threshold was elevated at 8 months of age, and continued to increase gradually with age. 3.2. Age-related changes in DPOAE I/O function The mean 2f1 3f2 DPOAE levels as a function of the primary tone level, i.e. the I/O function, in the 8^16 kHz frequency region for six age groups of CBA mice are shown in Fig. 2. In the 1-month-old mice, the gen-
Fig. 1. Mean ABR thresholds as a function of frequency and age in the CBA/J mice. Error bars in this and subsequent ¢gures represent the S.E.M.
eral shape of the I/O functions was characterized by a monotonically increasing DPOAE level (slope was about 1 dB/dB, see below) with increasing primary tone level at all frequencies except at f2 = 13.4 and 14.4 kHz, where `notches' (i.e. nonmonotonicities) were present at L2 near 50 dB SPL. High levels of DPOAEs were recorded, particularly for 11.3^14.4 kHz, where DPOAEs reached 40 dB SPL. The averaged DPOAE levels of the 1-month group ranged from 10 to 55 dB less than the stimulus level (L2 ) depending on f2 . At 8 months of age, CBA mice did not show any changes in the DPOAE I/O functions in comparison to the 1-month-old mice. Starting at 17 months of age, this strain showed some frequency-dependent changes. At the low frequencies (8 and 9.5 kHz) the I/O function of 17-month-old mice shifted to the left relative to that of the 1-month-old. At frequencies s 12 kHz the 17-month-old functions shifted to the right. The I/O functions were shifted to the right across all frequencies by 25 months of age. Another notable age-related change is the emergence of notches in the I/O functions of 17-month-old mice at L2 = 55 dB SPL for f2 = 11.3 and 12 kHz. Such notches were absent in I/O functions of the younger mice at those frequencies. By 25 months of age, I/O functions sharply shifted to the right, notches disappeared, and the slope became steeper (see below for frequencies at and above 11.3 kHz. Fig. 3 shows the slope of the 2f1 3f2 DPOAE I/O functions as a function of age. Slopes were calculated in the region of the I/O functions where at least three data points were above the noise £oor, but below the L2
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Fig. 2. The mean 2f1 3f2 DPOAE I/O functions of the aging CBA mice at f2 = 8^16 kHz. The solid lines without error bars represent the mean noise level as a function of stimulus level.
at which DPOAE levels saturated. In the youngest mice, the slopes ranged from 0.94 to 1.38. Slopes increased with increasing f2 and age. Age-related changes in DPOAE I/O function slopes were frequency-dependent. By 17 months of age, the slope increased at f2 = 13.4 and 14.4 kHz, but did not at f2 = 8 and 16 kHz. At 25 months of age the slope increased at most of the frequencies. Increased slope implies that the DPOAE signals produced by the low-level primary tones decreased more than those produced by the highlevel primaries. 3.3. Age-related changes in DPOAE detection threshold The 2f1 3f2 DPOAE detection thresholds for individ-
ual ears (de¢ned as the minimum primary level, L2 in this case, required to produce a response that was 3 dB above the noise £oor followed by further growth of DPOAE level as primary levels increased) were derived from the I/O functions. The mean data of the six groups of CBA mice at f2 are shown in Fig. 4 (top panel). DPOAE detection thresholds decreased with increasing frequency. For the 1-month-old mice, the lowest detection threshold was about L2 = 5 dB SPL at f2 = 14 and 16 kHz, and the highest threshold was 37 dB SPL at 8 kHz. The change in DPOAE detection thresholds of the aged mice relative to the 1-month-old mice is displayed in the bottom panel of Fig. 4. For f2 s 11.3 kHz, detection thresholds increased slightly (about 5 dB) at
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Fig. 3. The slope of 2f1 3f2 DPOAE I/O functions as a function of age and frequency in the CBA mice.
17 months of age, and continued to increase with increasing age. For f2 6 11.3 kHz, the detection thresholds for 8-month-old mice were lower than those of the 1-month-old mice, and did not increase until 21 months of age. At 21 months of age detection thresholds were elevated ( 6 15 dB) for the entire frequency range tested. At 25 months of age, CBA mice displayed elevations of detection thresholds (about 20^40 dB), which were more severe in response to higher primary frequencies. Statistically signi¢cant age-related change in DPOAE detection thresholds occurred at 17 months of age at 13.4 and 14.4 kHz relative to the younger groups and extended to the remaining frequencies by 21 months of age. Fig. 5 shows the relationship between the change in DPOAE detection thresholds (relative to those of 1month-old mice, data from Fig. 4) and the slope of the I/O functions (data from Fig. 3). The data points for the mice at 21 months of age and older were distributed in the upper-right quadrant relative to intersecting dotted lines representing no change in the respective measures. This distribution indicates that there was a correlation between the elevation of the DPOAE detection thresholds and the increase of the I/O function slopes in aged mice. This trend is more evident in 25-month-old mice. This result implies that the change in the I/O functions for the aged mice relative to those of young mice was not simply a parallel shift to the right.
Fig. 4. Top panel: The mean 2f1 3f2 DPOAE detection thresholds derived from the I/O functions of the aging CBA mice as a function of frequency. Bottom panel: The change in mean DPOAE detection thresholds of the aged mice relative to those of the 1-month-old mice.
3.4. Age-related changes in DPOAE level Fig. 6 (panels a, c, e and g) displays the mean 2f1 3f2 DPOAE level as a function of f2 at four primary levels (L2 = 20, 30, 40 and 50 dB SPL) in six age groups of CBA mice. The changes in DPOAE level for the aged CBA mice relative to the 1-month-old mice are shown in panels b, d, f and h. Age-related changes of DPOAE level were dependent upon primary frequencies and levels. At 8 months of age, the DPOAE levels were similar to those of 1-month-old mice. However, 17-month-old mice showed decreased DPOAE levels at f2 s 11.3 kHz for all primary levels, and increased DPOAE levels at f2 6 11.3 kHz for L2 v30 dB SPL. At 21 months of age, the decreased DPOAE levels were present at most of
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Fig. 5. The relationship between the change in 2f1 3f2 DPOAE detection thresholds and the change in the I/O function slopes of the aging CBA mice.
the tested conditions. At 25 months of age, the DPOAE-vs.-f2 functions became nearly £at, and the DPOAE levels decreased by 20^40 dB. At this age, for the lower L2 values, DPOAE levels were at or near the noise £oor levels. Statistically signi¢cant agerelated changes of DPOAE levels in CBA mice, including decreased and increased levels at frequencies above and below 11.3 kHz, respectively, could be detected at 17 months of age with L2 = 20^40 dB SPL and L1 3L2 = 10 dB. The reduction of the DPOAE level at 21 months of age was found to be statistically signi¢cant at almost all frequencies tested for L2 = 30 and 40 dB SPL. 4. Discussion The main ¢ndings of the present study for a frequency range of 8^16 kHz were: (1) CBA mice exhibited mild age-related changes in DPOAE level and detection threshold at 17 months of age, and changes of 20^40 dB by 25 months of age; (2) DPOAE level of CBA mice decreased and detection threshold increased with age in a frequency-dependent manner, starting at high frequencies and eventually extending to low frequencies. Prior to the present study, DPOAE data were available only for young CBA mice which were used as controls in studies of cochlear function in hearing impaired strains (Horner et al., 1985; Schrott et al., 1991 ; Le Calvez et al., 1998a,b). Keeping in mind that the
median life span of CBA mice is 24 months (Willott et al., 1987, 1988), the present results suggest that CBA mice do not show signi¢cant alteration of the DPOAE measures until late in life. This strain exhibits mild DPOAE changes after about 70% of its median life span and only moderate changes at a very old age. It should be noted that although the CBA mouse does not exhibit signi¢cant hearing loss over much of its life span, it does exhibit retinal degeneration early in life (see Green, 1989). A recent study by Spongr et al. (1997) reported quantitative measures of hair cell loss in CBA mice throughout their life span. In comparing their histological results with multi-unit neural thresholds recorded from the inferior colliculus of CBA mice (Willott, 1986), Spongr et al. noted a discrepancy. They noted signi¢cant elevations of neural thresholds in a frequency range corresponding to a region in the cochlea where little hair cell loss was found. Spongr et al. speculated that the surviving hair cells in this region of the cochlea may be functioning abnormally. The present study provides direct experimental evidence in support of this view. We observed signi¢cant changes in CBA DPOAE characteristics through 25 months of age in the 8^16 kHz region. In this region of the CBA mouse cochlea (V46^60% distance from apex), little hair cell loss (e.g. OHC loss 6 15.2%) was noted through 26 months of age (Spongr et al., 1997). Since OAEs are a functional measure of OHCs (see Section 1), it can be concluded that the function of the remaining OHCs in the aged CBA ear is impaired. OHC dysfunction in the aged CBA cochlea could arise from direct and indirect e¡ects of age on OHC function. The function of OHCs might be considered to consist of several processes, such as hair bundle coupling, apical conductance change, electrochemical gradient (receptor current), electrical impedance and force generating motor elements (e.g. Patuzzi et al., 1989; Patuzzi and Rajan, 1992). Disruptions of CBA OHC function could arise from direct e¡ects of age on some or all of the above processes. Aging could also indirectly alter OHC function through changes in other cochlear structures. For example, changes in the stria vascularis (SV) are a common ¢nding in the cochleas of the aged humans (e.g. Engstrom et al., 1987; Schuknecht, 1974) and various animals (e.g. Bohne et al., 1990 ; Keithley et al., 1992; Schulte and Schmiedt, 1992 ; Shimada et al., 1998), including the mouse (e.g. Mikaelian, 1979; Willott, 1991). The SV helps to maintain the ionic homeostasis in the endolymph which is believed to be necessary for normal OHC function. Thus disruption of the SV function could indirectly alter OHC function. Recent work in quiet-aged gerbils indicates that alterations in microvasculature precede the degeneration of SV which in turn precedes age-re-
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Fig. 6. The mean 2f1 3f2 DPOAE levels as a function of frequency at (a) L2 = 20, (c) L2 = 30, (e) L2 = 40 and (g) L2 = 50 dB SPL of the aging CBA mice. The changes in DPOAE levels of the aged mice relative to those of the 1-month-old mice are shown for (b) L2 = 20, (d) L2 = 30, (f) L2 = 40 and (h) L2 = 50 dB SPL.
lated alteration of the endocochlear potential and auditory thresholds (e.g. Gratton and Schulte, 1995; Gratton et al., 1997). Another interesting age-related ¢nding in CBA mice is associated with the nonmonotonicity (i.e. notches) of the DPOAE I/O growth functions. The range of frequencies in which notches were observed in the CBA I/O functions extended from a narrow range of f2 = 13.4^14.4 kHz at 1 month to f2 = 11.3^14.4 kHz by 17 months of age. The new notches of the older mice appeared in a frequency region (i.e. f2 = 11.3 and 12 kHz) where DPOAE level and detection threshold were essentially unaltered. With further aging, the notches disappeared as DPOAE detection threshold in-
creased and DPOAE level decreased. This latter ¢nding is consistent with the disappearance of notches in the DPOAE I/O functions of 12-month-old C57 mice which also showed deteriorations in DPOAE level and detection threshold (Parham, 1997). Notches in the DPOAE I/O functions have been documented in other species (e.g. Brown, 1987 ; Kim, 1980 ; Kossl, 1992; Lonsbury-Martin et al., 1987; Whitehead et al., 1992a ; Wiederhold et al., 1986; Zurek et al., 1982) and have been attributed to an interaction between multiple sources giving rise to phase cancellation (e.g. Brown, 1987; Kim, 1980 ; Zwicker, 1986) including an interaction between a low-level source and a high-level source (Mills and Rubel, 1994 ; Norton and
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Rubel, 1990 ; Whitehead et al., 1992a,b). Both emergence and disappearance of the notches in the I/O functions of the aged mice imply altered interaction between the various DPOAE sources. An altered interaction between the various DPOAE sources with age is further supported by another ¢nding in the DPOAEs of aged mice. The slope of the I/O functions tended to increase with age in both CBA (the present report) and C57 (Parham, 1997) mice. The more rapid growth of DPOAE I/O function at an older age may be attributed to an unmasked high-level source of DPOAEs, analogous to conditions where the more vulnerable low-level source has been disrupted in animals with experimentally induced cochlear insults (e.g. Lonsbury-Martin et al., 1987; Mills et al., 1993). A recent preliminary report by Doan et al. (1996) documented functional changes in the middle ear of aged mice. Doan et al. (1996), using laser interferometry in 3- and 24-month-old BALB/c mice, reported up to 8 dB decreases in umbo velocity in the 5^13 kHz range with age. This change is likely, they argued, to be associated with increased sti¡ness of the middle ear. Since OAEs are dependent on both forward and backward transmission of stimuli through the middle ear, it is possible that increased sti¡ness of the middle ear ossicles contributed to the changes in the DPOAEs of aged CBA (the present report) and C57 mice (Parham, 1997). However, the magnitude of age-related DPOAE changes appears to be much greater than that expected from the age-related increase in sti¡ness of the middle ear apparatus alone. Although the contributions of agerelated changes of the inner and middle ears are additive in the DPOAEs of CBA and C57 mice, the middle ear is believed to play a relatively minor role. Interestingly, for f2 6 11.3 kHz, a small decrease in DPOAE detection thresholds and a small increase in DPOAE suprathreshold levels were noted by 8 and 17 months of age. These changes were not re£ected in the ABR thresholds of CBA mice. Similarly, previous studies did not ¢nd lower ABR thresholds or multi-unit neural thresholds in older CBA mice (e.g. Li and Borg, 1991; Willott, 1986). A similar trend in the DPOAE measures was reported in C57 mice (Parham, 1997). The reason for this improvement in the DPOAE response is not clear at this point. Parham (1997) speculated that the improvement in the DPOAE response in mice may re£ect continued maturation of OHCs in the cochlea, which a¡ects the generation of OAEs, and/or the maturation of the middle ear structure, which affects the transmission of the sound signals back and forth in recording OAEs. The former hypothesis is currently being investigated by examining developmental changes in DPOAEs of CBA mouse pups starting at the onset of hearing (Parham et al., 1997). The latter hypothesis could be evaluated by examining middle ear
function in intermediate age groups (e.g. 8 months of age) of mice, in a study similar to that of Doan et al. (1996). Comparing the present CBA results with our previous C57 ¢ndings (Parham, 1997) yields several interesting points. As emphasized above, 17-month-old CBA mice showed relatively mild DPOAE changes. In contrast, 18-month-old C57 mice had striking reduction of DPOAE levels (up to 50 dB) and elevations of DPOAE detection thresholds (up to 40 dB), both of which were more severe at high frequencies (Parham, 1997). This, of course, is a re£ection of the previously documented changes in auditory sensitivity and cochlear pathology which start at an earlier age in C57 mice (about 3^4 months of age; e.g. Henry, 1983 ; Henry and Chole, 1980 ; Hunter and Willott, 1987; Li and Borg, 1991; Li and Hultcrantz, 1994 ; Spongr et al., 1997 ; Willott, 1986). It is instructive to note that after the initial signs of age-related changes in DPOAEs at a given frequency, the rates of their progression are similar between the two strains. This is best illustrated by comparing results obtained under identical stimulus conditions (e.g. compare Figs. 2, 4 and 6 of the present paper with those of Figs. 8, 9 and 10 of Parham, 1997). For example, at f2 = 14.4 kHz at 17^25 months in CBA and 12^20 months in C57 mice (i.e. over an 8-month span) the mean DPOAE detection thresholds increased from 7 to 42 dB SPL and from 2 to 45 dB SPL, respectively. To state the above observation in di¡erent words, although the disruption of OHC function clearly starts much later in CBA mice, it does not seem to progress more rapidly (or slowly) after onset than in C57 mice. This conclusion implies that the aged OHCs appear to be no more vulnerable to the age-related pathological processes than relatively younger ears are. This is consistent with the conclusions of studies on e¡ects of noise exposure on aging CBA and C57 mice (Shone et al., 1991 ; Li, 1992) and aged chinchillas (Sun et al., 1994). Setting aside the di¡erence in the age of onset of DPOAE changes, the similarity in the characteristics of age-related changes in DPOAEs of CBA and C57 mice is surprising because of the genotypic and phenotypic di¡erences between the two strains. First, the agerelated hearing loss of C57 mice is attributed to a peripherally acting gene, Ahl, mapped to chromosome 10 (Erway et al., 1993; Johnson et al., 1997; Willott and Erway, 1998). CBA mice lack this allele. Second, the pattern and quantity of hair cell loss in the cochleas of C57 and CBA mice are strikingly di¡erent (Spongr et al., 1997). For example, age-related loss of OHCs in C57 mice progresses primarily in a basoapical direction, in contrast, that of CBA mice progresses primarily in an apicobasal direction. Reconciliation of these divergent ¢ndings has important implications for under-
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standing the e¡ects of age on cochlear function. Given the similarity in the rate of progression of DPOAE changes between the two strains, future e¡ort needs to be directed at uncovering age-related changes in cochlear structure which are common between the two strains. Acknowledgements This research was supported by grants from the Sandoz Foundation for Gerontological Research, National Institute on Aging (NIH Grant P60-AG13631-01), Department of Surgery and Research Advisory Committee, University of Connecticut Health Center. References Anderson, S.D., Kemp, D.T., 1979. The evoked cochlear mechanical response in laboratory primates. Arch. Otorhinolaryngol. 224, 47^ 54. Bohne, B.A., Grunner, M.M., Harding, G.W., 1990. Morphological correlates of aging in the chinchilla cochlea. Hear. Res. 11, 41^53. Bon¢ls, P., Bertrand, Y., Uzel, A., 1988. Evoked otoacoustic emissions: Normative data and presbycusis. Audiology 27, 27^35. Bredberg, G., 1968. Cellular pattern and nerve supply of the human organ of Corti. Acta Otolaryngol. 236 (Suppl.), 1^135. Brown, A.M., 1987. Acoustic distortion from rodent ears: A comparison of response from rats, guinea pigs and gerbils. Hear. Res. 31, 25^38. Brown, A.M., McDowell, B., Forge, A., 1989. Acoustic distortion products can be used to monitor the e¡ects of chronic gentamicin treatment. Hear. Res. 19, 191^198. Collet, L., Moulin, A., Gartner, M., Morgon, A., 1990. Age-related changes in evoked otoacoustic emissions. Ann. Otol. Rhinol. Laryngol. 99, 993^997. Doan, D.E., Erulkar, J.S., Saunders, J.C., 1996. Functional change in the aging mouse middle ear. Hear. Res. 97, 174^177. Engstrom, B., Hillerdal, M., Laurell, G., 1987. Selected pathological ¢ndings in the human cochlea. Acta Otolaryngol. 436, 110^116. Erway, L.C., Willott, J.F., Archer, J.R., Harrison, D.E., 1993. Genetics of age-related hearing loss in mice: I. Inbred and F1 hybrid strains. Hear. Res. 65, 125^132. Gratton, M.A., Schulte, B.A., 1995. Alterations in microvasculature are associated with atrophy of the stria vascularis in quiet-aged gerbils. Hear. Res. 82, 44^52. Gratton, M.A., Schulte, B.A., Smythe, N.M., 1997. Quanti¢cation of the stria vascularis and strial caplillary areas in quiet-reared young and aged gerbils. Hear. Res. 114, 1^9. Green, M.C., 1989. Catolog of mutant genes and polymorphic loci. In: Lyon, M.F., Searle, A.G. (Eds.), Genetic Variants and Strains of the Laboratory Mouse, Oxford University Press, Oxford, pp. 12^403. Guinan, J.J., Jr., 1986. E¡ect of e¡erent neural activity on cochlear mechanics. Scand. Audiol. Suppl. 25, 53^62. Henry, K.R., 1982. Age-related changes in sensitivity of the postpubertal ear to acoustic trauma. Hear. Res. 8, 285^294. Henry, K.R., 1983. Ageing and audition. In: Willott, J.F. (Ed.), The Auditory Psychobiology of the Mouse. Charles C. Thomas, Spring¢eld, IL, pp. 470^493. Henry, K.R., Chole, R.A., 1980. Genotypic di¡erences in behavioral,
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physiological and anatomical expressions of age-related hearing loss in the laboratory mouse. Audiology 19, 369^383. Horner, K.C., Lenoir, M., Bock, G.R., 1985. Distortion product otoacoustic emissions in hearing-impaired mutant mice. J. Acoust. Soc. Am. 78, 1603^1611. Hunter, K.P., Willott, J.F., 1987. Aging and the auditory brainstem response in mice with severe or minimal presbycusis. Hear. Res. 30, 207^218. Jimenez, A.M., Stagner, B.B., Martin, G.K., Lonsbury-Martin, B.L., 1997. Patterns of distortion-product otoacoustic emissions in aging C57BL/6J mice. Assoc. Res. Otolaryngol. Abstr. 20, 101. Johnson, K.R., Erway, L.C., Cook, S.A., Willott, J.F., Zheng, Q.Y., 1997. A major gene a¡ecting age-related hearing loss in C57BL/6J mice. Hear. Res. 114, 83^92. Johnsson, L.-G., Hawkins, J.E., 1972. Sensory and neural degeneration with aging, as seen in microdissection of the human inner ear. Ann. Otol. Rhinol. Otolaryngol. 81, 179^193. Johnsson, L.-G., Hawkins, J.E., 1979. Age-related degeneration of the inner ear. In: Han, S.S., Coons, D.H. (Eds.), Special Senses in Aging. Institute of Gerontology, Ann Arbor, MI, pp. 119^135. Keithley, E.M., Ryan, A.F., Feldman, M.L., 1992. Cochlear degeneration in aged rats of four strains. Hear. Res. 59, 171^178. Kemp, D.T., 1978. Stimulated acoustic emissions from within the human auditory system. J. Acoust. Soc. Am. 64, 1386^1391. Kemp, D.T., Brown, A.M., 1984. Ear canal acoustic and round window electrical correlates of 2f1 3f2 distortion generated in the cochlea. Hear. Res. 13, 39^46. Kim, D.O., 1980. Cochlear mechanics: Implications of electrophysiological and acoustical observations. Hear. Res. 2, 297^317. Kim, D.O., Molnar, C.E., Matthews, J.W., 1980. Cochlear mechanics: Nonlinear behavior behavior in two-tone responses as re£ected in cochlear-nerve-¢ber responses and in ear-canal sound pressure. J. Acoust. Soc. Am. 67, 1704^1721. Kossl, M., 1992. High frequency distortion products from the ears of two bat species Megaderma lyra and Carollia prespicillata. Hear. Res. 59, 156^164. Le Calvez, S., Avan, P., Gilain, L., Romand, R., 1998a. CD1 hearingimpaired mice. I: Distortion product otoacoustic emission levels, cochlear function and morphology. Hear. Res. 120, 37^50. Le Calvez, S., Guilhaume, A., Romand, R., Aran, J.M., Avan, P., 1998b. CD1 hearing-impaired mice. II. Group latencies and optimal f2/f1 ratios of distortion product otoacoustic emissions, and scanning electron microscopy. Hear. Res. 120, 51^61. Li, H.-S., 1992. In£uence of genotype and age on acute acoustic trauma and recovery in CBA/Ca and C57BL/6J mice. Acta Otolaryngol. 112, 965^967. Li, H.-S., Borg, E., 1991. Age-related loss of auditory sensitivity in two mouse genotypes. Acta Otolaryngol. 111, 827^834. Li, H.-S., Hultcrantz, M., 1994. Age-related degeneration of the organ of Corti in two genotypes of mice. Otorhinolaryngology 56, 61^ 67. Liberman, M.C., Puria, S., Guinan, J.J.Jr., 1996. The ipsilaterally evoked olivocochlear re£ex causes rapid adaptation of the 2f1 3f2 distortion product otoacoustic emission. J. Acoust. Soc. Am. 99, 3572^3584. Lonsbury-Martin, B.L., Martin, G.K., Probst, R., Coats, A.C., 1987. Acoustic distortion products in rabbit ear canal. I. Basic features and physiological vulnerability. Hear. Res. 28, 173^189. Lonsbury-Martin, B.L., Cutler, W.M., Martin, G.K., 1991. Evidence for the in£uence of aging on distortion-product otoacoustic emissions in humans. J. Acoust. Soc. Am. 89, 1749^1759. Martin, G.K., Lonsbury-Martin, B.L., Probst, R., Scheinin, S.A., Coats, A.C., 1987. Acoustic distortion products in rabbit ear canal. II. Sites of origin revealed by suppression contours and puretone exposures. Hear. Res. 28, 191^208.
HEARES 3239 19-7-99
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Mikaelian, D.O., 1979. Development and degeneration of hearing in the C57BL/6J mouse: Relation of electrophysiologic responses from the round window and cochlear nucleus to cochlear anatomy and behavioral responses. Laryngoscope 89, 1^15. Mills, D.M., Rubel, E.W., 1994. Variation of distortion product otoacoustic emissions with furosemide injection. Hear. Res. 77, 183^ 199. Mills, D.M., Norton, S.J., Rubel, E.W., 1993. Vulnerability and adaptation of distortion product otoacoustic emissions to endocochlear potential variation. J. Acoust. Soc. Am. 94, 2108^2122. Mountain, D.C., 1980. Changes in endolymphatic potential and crossed olivocochlear bundle stimulation alters cochlear mechanics. Science 210, 71^72. Norton, S.J., Rubel, E.W., 1990. Active and passive ADP components in mammalian and avian ears. In: Dallos, P., Geisler, C.D., Matthews, J.W., Ruggero, M.A., Steele, C.R. (Eds.), Mechanics and Biophysics of Hearing. Springer-Verlag, New York, pp. 219^226. Parham, K., 1997. Distortion product otoacoustic emissions in the C57BL/6J mouse model of age-related hearing loss. Hear. Res. 112, 216^234. Parham, K., Willott, D.O., 1988. Acoustic startle response in young and aging C57BL/6J and CBA/J mice. Behav. Neurosci. 102, 881^ 886. Parham, K., Sun, X., Kim, D.O., 1996. Examination of optimal stimulus parameters for distortion product otoacoustic emissions in mouse models of presbycusis. Assoc. Res. Otolaryngol. Abstr. 19, 26. Parham, K., Gerber, A., McKowen, A., Sun, X.-M., Kim, D.O., 1997. Distortion product otoacoustic emissions of the DBA/2J mouse with early-onset progressive hearing loss. Assoc. Res. Otolaryngol. Abstr. 20, 194. Patuzzi, R., Rajan, R., 1992. Additivity of threshold elevations produced by disruption of outer hair cell function. Hear. Res. 60, 165^177. Patuzzi, R., Yates, G.K., Johnstone, B.M., 1989. The origin of the low-frequency microphonic in the ¢rst cochlear turn of the guinea pig. Hear. Res. 39, 177^188. Schmiedt, R.A., 1986. Acoustic distortion in the ear canal, I. Cubic di¡erence tones: E¡ects of acute noise injury. J. Acoust. Soc. Am. 79, 1481^1490. Schrott, A., Puel, J.-L., Rebillard, G., 1991. Cochlear origin of 2f1 3f2 distortion products assessed by using 2 types of mutant mice. Hear. Res. 52, 245^254. Schuknecht, H.F., 1964. Further observations on the pathology of presbycusis. Arch. Otolaryngol. 80, 369^382. Schuknecht, H.F., 1974. Pathology of the Ear. Harvard University Press, Cambridge, MA. Schuknecht, H.F., Gacek, M.R., 1993. Cochlear pathology in presbycusis. Ann. Otol. Rhinol. Laryngol. 102, 1^16. Schulte, B.A., Schmiedt, R., 1992. Lateral wall Na, K-ATPase and endocochlear potentials decline with age in quiet-reared gerbils. Hear. Res. 61, 35^46. Shimada, A., Ebisu, M., Morita, T., Takeuchi, T., Umemura, T., 1998. Age-related changes in the cochlea and cochlear nuclei of dogs. J. Vet. Med. Sci. 60, 41^48. Shone, G., Altschuler, R.A., Miller, J.M., Nuttall, A.L., 1991. The e¡ect of noise exposure on the aging ear. Hear. Res. 56, 173^178. Siegel, J.H., Kim, D.O., 1982. E¡erent neural control of cochlear mechanics? Olivocochlear bundle stimulation a¡ects cochlear biomechanical nonlinearity. Hear. Res. 6, 171^182. Sjostrom, B., Anniko, M., 1990. Variability in genetically induced age-related impairment of auditory brainstem response thresholds. Acta Otolaryngol. 109, 353^360.
Smurzynski, J., Kim, D.O., 1992. Distortion-product and click-evoked otoacoustic emissions of normally hearing adults. Hear. Res. 58, 227^240. Spongr, V.P., Flood, D.G., Frisina, R.D., Salvi, R.J., 1997. Quantitative measures of hair cells in CBA and C57BL/6 mice throughout their life spans. J. Acoust. Soc. Am. 101, 3546^3553. Stover, L., Norton, S.J., 1993. The e¡ects of aging on otoacoustic emissions. J. Acoust. Soc. Am. 94, 2670^2681. Sun, J.C., Bohne, B.A., Harding, G.W., 1994. Is the older ear more susceptible to noise damage? Laryngoscope 104, 1251^1258. Sun, X., Parham, K., Kim, D.O., 1996. Distortion product otoacoustic emissions of an animal model of presbycusis, the CBA/J mouse. Assoc. Res. Otolaryngol. Abstr. 19, 25. Sun, X.-M., Parham, K., Kim, D.O., 1997. E¡ects of stimulus parameters on distortion product otoacoustic emissions in the C57BL/6J mouse with age-related hearing loss. Assoc. Res. Otolaryngol. Abstr. 20, 101. Walton, J.P., Frisina, R.D., O'Neill, W.E., 1998. Age-related alteration in processing of temporal sound features in the auditory midbrain of the CBA mouse. J. Neurosci. 18, 2764^2776. Wenngren, B.-I., Anniko, M., 1988. A frequency-speci¢c auditory brainstem response technique exempli¢ed in the determination of age-related auditory thresholds. Acta Otolaryngol. 106, 238^243. Whitehead, M.L., Lonsbury-Martin, B.L., Martin, G.K., 1992a. Evidence for two discrete source of 2f1 3f2 distortion-product otoacoustic emission in rabbit: I. Di¡erential dependence on stimulus parameters. J. Acoust. Soc. Am. 91, 1587^1607. Whitehead, M.L., Lonsbury-Martin, B.L., Martin, G.K., 1992b. Evidence for two discrete source of 2f1 3f2 distortion-product otoacoustic emission in rabbit: II. Di¡erential physiological vulnerability. J. Acoust. Soc. Am. 92, 2662^2682. Wiederhold, M.L., Mahoney, J.W., Kellogg, D.L., 1986. Acoustic overstimulation reduces 2f1 3f2 cochlear emissions at all levels in the cat. In: Allen, J.B., Hall, J.L., Hubbard, A., Neely, S.T., Tubis, A. (Eds.), Peripheral Auditory Mechanisms. Springer, Berlin, pp. 322^329. Willott, J.F., 1986. E¡ects of aging, hearing loss, and anatomical location on threshold of inferior colliculus neurons in C57BL/6 and CBA mice. J. Neurophysiol. 56, 391^480. Willott, J.F., 1991. Aging and the Auditory System: Anatomy, Physiology, and Psychophysics. Singlar Publishing, San Diego, CA. Willott, J.F., Erway, L.C., 1998. Genetics of age-related hearing loss in mice. IV. Cochlear pathology and hearing loss in 25 BXD recommbinant inbred mouse strains. Hear. Res. 119, 27^36. Willott, J.F., Jackson, L.M., Hunter, K.P., 1987. Morphometric study of the anteroventral cochlear nucleus of two mouse models of presbycusis. J. Comp. Neurol. 260, 472^480. Willott, J.F., Parham, K., Hunter, K.P., 1988. Response properties of inferior colliculus neurons in young and very old CBA/J mice. Hear. Res. 37, 1^14. Willott, J.F., Parham, K., Hunter, K.P., 1991. Comparison of the auditory sensitivity of neurons in the cochlear nucleus and inferior colliculus of young and aging C57BL/6J and CBA/J mice. Hear. Res. 53, 78^94. Willott, J.F., Carlson, S., Chen, H., 1994. Prepulse inhibition of the startle response in mice: relationship to hearing loss and auditory system plasticity. Behav. Neurosci. 108, 703^713. Zurek, P.M., Clark, W.W., Kim, D.O., 1982. The behavior of acoustic distortion products in the ear canals of chinchillas with normal or damaged ears. J. Acoust. Soc. Am. 72, 774^780. Zwicker, E., 1986. Suppression and (2f13f2) di¡erence tones in a nonlinear cochlear preprocessing model with active feedback. J. Acoust. Soc. Am. 80, 163^176.
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Distortion product otoacoustic emissions in the CBA/J mouse model of presbycusis Kourosh Parham a
a;
*, Xiao-Ming Sun
a;b
, D.O. Kim
a;b
Division of Otolaryngology, Department of Surgery, University of Connecticut Health Center, Farmington, CT 06030-1110, USA b Department of Communication Science, University of Connecticut, Storrs, CT, USA Received 24 September 1998; received in revised form 20 March 1999; accepted 26 March 1999
Abstract CBA mice do not exhibit age-related loss of auditory sensitivity or cochlear pathology until relatively late in life. Therefore, this strain is believed to be an excellent animal model for the examination of the effects of age on the cochlea. To evaluate the effects of age on outer hair cell function, 2f1 3f2 distortion product otoacoustic emissions (DPOAEs) were measured for f2 between 8 and 16 kHz in CBA/J mice between 1 and 25 months of age. CBA mice exhibited mild age-related changes in DPOAE level and detection threshold at 17 months of age, and changes of 20^40 dB by 25 months of age. The DPOAE level decreased and detection threshold increased with age in a frequency-dependent manner, starting at high frequencies and eventually extending to low frequencies. The range of frequencies in which notches were observed in the DPOAE input/output (I/O) functions extended toward lower frequencies by 17 months of age. Notches were absent in the I/O functions of 25-month-old mice. The present results for a frequency range of 8^16 kHz suggest that age has modest effects on outer hair cell function in CBA mice. ß 1999 Elsevier Science B.V. All rights reserved. Key words: Distortion product otoacoustic emission; Aging; Mouse; Hearing loss; Outer hair cell
1. Introduction Human presbycusis, hearing loss due to increasing chronological age, is characterized by elevation of hearing thresholds, usually beginning with high frequencies and gradually progressing to lower frequencies. Because of the complex interactions of genetic and environmental variables that can contribute to age-related hearing loss, human presbycusis is a challenging research topic, particularly the late-onset, progressive hearing impairment, which is the most common type of presbycusis in humans (for review see Willott, 1991). The di¤culties faced by investigations of presbycusis in human subjects, including limitations on evaluating the correlation between functional and histological data of the same ear, have prompted the utilization of animal models to gain a better understanding of presbycusis.
* Corresponding author. Tel.: +1 (860)679-2554; Fax: +1 (860)679-2451; E-mail: [email protected]
A popular animal model for the study of aging in the auditory system is the inbred CBA mouse strain. Agerelated changes in the auditory system of CBA mice have been investigated behaviorally (Parham and Willott, 1988 ; Willott et al., 1994), anatomically (e.g. Henry and Chole, 1980; Li and Hultcrantz, 1994; Shone et al., 1991; Spongr et al., 1997 ; Willott et al., 1987, 1988, 1991) and electrophysiologically, including single neuron (e.g. Walton et al., 1998 ; Willott, 1986; Willott et al., 1988, 1991) and auditory evoked potential recordings (e.g. Henry, 1982; Henry and Chole, 1980; Hunter and Willott, 1987 ; Li and Borg, 1991 ; Shone et al., 1991; Sjostrom and Anniko, 1990 ; Wenngren and Anniko, 1988). These studies have demonstrated that the auditory system of the CBA mouse does not display signi¢cant behavioral, anatomical or physiological changes until relatively late in life. Histopathological studies are in general agreement that age-related hearing loss is associated with a progressive degeneration of cochlear hair cells and spiral ganglion cells, usually beginning with and most severely
0378-5955 / 99 / $ ^ see front matter ß 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 5 9 5 5 ( 9 9 ) 0 0 0 5 9 - 3
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a¡ecting the outer hair cells (OHCs) in both humans (e.g. Bredberg, 1968; Johnsson and Hawkins, 1972, 1979 ; Schuknecht, 1964, 1974; Schuknecht and Gacek, 1993) and CBA mice (e.g. Henry and Chole, 1980 ; Li and Hultcrantz, 1994; Spongr et al., 1997). The OHCs are thought to underlie the generation of otoacoustic emissions (OAEs). OAEs are low-intensity sounds detectable in the ear canal (Kemp, 1978). The association of OAEs with OHC function is supported by numerous experimental studies (e.g. Anderson and Kemp, 1979 ; Brown et al., 1989; Guinan, 1986; Kemp and Brown, 1984 ; Kim et al., 1980; Liberman et al., 1996; Lonsbury-Martin et al., 1987; Martin et al., 1987; Mountain, 1980 ; Schmiedt, 1986; Siegel and Kim, 1982; Zurek et al., 1982), including ¢ndings in mutant mice (Horner et al., 1985; Schrott et al., 1991). The impact of age on OHCs and the relationship between OAEs and OHCs suggests that OAEs can serve as a useful tool for studying age-related hearing loss. By permitting assessment of peripheral function, OAEs also have the potential to help separate out the peripheral and central contributions to presbycusis. Investigations on the e¡ects of age on various types of OAEs in humans have been initiated (e.g. Bon¢ls et al., 1988 ; Collet et al., 1990; Lonsbury-Martin et al., 1991 ; Stover and Norton, 1993). As pointed out above, these studies need to be complemented by evaluation of OAEs in animal models to make up for the disadvantages inherent in the human study of presbycusis. In mouse models, the general features of distortion product otoacoustic emissions (DPOAEs) have been documented in C57BL/6J mice (C57) (Jimenez et al., 1997 ; Parham, 1997 ; Parham et al., 1996; Sun et al., 1997), which show an early-onset, progressive pattern of severe hearing loss. The current study documents DPOAEs in CBA mice which show hearing loss only at old ages thus allowing an opportunity to assess the e¡ects of aging on OHC function more completely. Preliminary results of this study were previously reported (Parham et al., 1996; Sun et al., 1996). 2. Materials and methods 2.1. Subjects CBA/JNia mice were purchased from the National Institute of Aging (NIA)/Charles River Laboratories (Kingston, NY) at three age groups : 1 month (male), 17 months (female) and 21 months (female). Each group included ¢ve mice. Because no signi¢cant e¡ect of gender on the auditory sensitivity of CBA mice has been identi¢ed (Hunter and Willott, 1987), and to overcome the limitations on availability of aged mice, gender among age groups of CBA mice used in this study
was not maintained. Upon arrival, the mice were housed at the University of Connecticut Health Center's vivarium. The mice received at 1 month of age were tested at 1, 4, 8 and 13 months of age. Mice received at 17 months of age were tested at 17, 21 and 23 months of age and the mice received at 21 months of age were tested at 21, 23 and 25 months of age. Both ears of each mouse were tested. Because of either death or signs of middle ear problems during the course of this study, the ¢nal numbers of ears from which useful data were collected for the di¡erent age groups were : 1 month, n = 10; 8 months, n = 8; 17 months, n = 6; 21 months, n = 9; 23 months, n = 4; and 25 months, n = 2. Since the results for the mice up to 13 months of age were generally similar, to avoid unnecessary cluttering of the presentation, the data from the 4- and 13-month-old groups of mice were not included in the subsequent ¢gures and analyses. The care and use of animal subjects reported in this study were approved by the Animal Care Committee of the University of Connecticut Health Center (ACC 920032-932). 2.2. Recording system and protocol 2.2.1. Distortion product otoacoustic emissions The DPOAE recording system, including stimulus generation and calibration, has been described in detail elsewhere (e.g. Parham, 1997 ; Smurzynski and Kim, 1992). Brie£y, this system consists of an IBM PC-compatible computer, an Ariel DSP-16 board containing two-channel 16-bit D/A and A/D converters installed in a 486 PC, two ER-2 earphones and an ER-10B low noise microphone system (including an ampli¢er which provided 40 dB of gain). The 2f1 3f2 DPOAEs were measured. For each ear, DPOAEs were recorded as a function of stimulus level, i.e. the input/output (I/O) function. DPOAE level was recorded in response to f2 from 8 to 16 kHz spaced at 1/4 octave intervals, i.e. 8, 9.5, 11.3, 13.4 and 16 kHz, and two additional f2 , 12 and 14.4 kHz. The primary frequency ratio, f2 /f1 , was kept constant at 1.2. At each frequency pair, primary tone levels were incremented from L2 = 5 dB to 65 dB SPL (re: 20 WPa) in 5 dB steps with L1 3L2 = 10 dB. 2.2.2. Auditory brainstem responses (ABRs) As a measure of auditory sensitivity, ABRs were recorded for each ear at all age groups except the 23month-old mice. Details of the ABR recording instrumentation are described elsewhere (Parham, 1997). At each of the frequencies, 4, 8, 9.5, 10, 11.3, 12, 13.4, 14, 14.4, 16, 24 and 32 kHz, ABR threshold, de¢ned as the minimum sound pressure level (SPL) for which
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any ABR wave was reliably observed, was determined. 2.3. Test procedure For both recordings of ABRs and DPOAEs, the animal was anesthetized by an intraperitoneal injection of ketamine/xylazine (0.12 and 0.01 mg/g body weight, respectively). When needed, supplementary doses (20% of the initial dose) were administered every 50^60 min. Prior to both of the recordings, the ear canals and ear drums were otoscopically inspected for signs of obstruction or infection, and only those with clear outer and middle ears were used. Animals were placed in a 2.5U3.5U3 ft sound-treated chamber (Industrial Acoustics Corporation 1204). A heating pad, which was controlled by an Animal Blanket Control Unit (Harvard Apparatus), maintained the mouse body temperature within 1³ of 36³C. DPOAEs and ABRs were tested in two sessions separated by 1 week. Each recording session lasted approximately 1 h. 2.4. Statistical analyses Di¡erences in DPOAE levels among the age groups of 1, 8, 17 and 21 months were evaluated for statistical signi¢cance (P 6 0.05). The groups of 23 and 25 months of age were not included in the statistical analyses because of their small sample size. Two-way repeated analyses of variance (ANOVAs) were used. The ANOVAs included age and a repeated independent measure, such as f2 . Signi¢cant interactions were followed up by performing one-way ANOVAs. Signi¢cant main e¡ects were evaluated with Tukey post hoc pairwise comparisons. 3. Results 3.1. Age-related changes in ABR threshold The mean ABR thresholds of CBA mice, in the 4^32 kHz frequency region, are presented in Fig. 1. There was no obvious change in ABR threshold with increasing age until 21 months of age in the frequency region below 16 kHz. For 16 kHz and above, ABR threshold was elevated at 8 months of age, and continued to increase gradually with age. 3.2. Age-related changes in DPOAE I/O function The mean 2f1 3f2 DPOAE levels as a function of the primary tone level, i.e. the I/O function, in the 8^16 kHz frequency region for six age groups of CBA mice are shown in Fig. 2. In the 1-month-old mice, the gen-
Fig. 1. Mean ABR thresholds as a function of frequency and age in the CBA/J mice. Error bars in this and subsequent ¢gures represent the S.E.M.
eral shape of the I/O functions was characterized by a monotonically increasing DPOAE level (slope was about 1 dB/dB, see below) with increasing primary tone level at all frequencies except at f2 = 13.4 and 14.4 kHz, where `notches' (i.e. nonmonotonicities) were present at L2 near 50 dB SPL. High levels of DPOAEs were recorded, particularly for 11.3^14.4 kHz, where DPOAEs reached 40 dB SPL. The averaged DPOAE levels of the 1-month group ranged from 10 to 55 dB less than the stimulus level (L2 ) depending on f2 . At 8 months of age, CBA mice did not show any changes in the DPOAE I/O functions in comparison to the 1-month-old mice. Starting at 17 months of age, this strain showed some frequency-dependent changes. At the low frequencies (8 and 9.5 kHz) the I/O function of 17-month-old mice shifted to the left relative to that of the 1-month-old. At frequencies s 12 kHz the 17-month-old functions shifted to the right. The I/O functions were shifted to the right across all frequencies by 25 months of age. Another notable age-related change is the emergence of notches in the I/O functions of 17-month-old mice at L2 = 55 dB SPL for f2 = 11.3 and 12 kHz. Such notches were absent in I/O functions of the younger mice at those frequencies. By 25 months of age, I/O functions sharply shifted to the right, notches disappeared, and the slope became steeper (see below for frequencies at and above 11.3 kHz. Fig. 3 shows the slope of the 2f1 3f2 DPOAE I/O functions as a function of age. Slopes were calculated in the region of the I/O functions where at least three data points were above the noise £oor, but below the L2
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Fig. 2. The mean 2f1 3f2 DPOAE I/O functions of the aging CBA mice at f2 = 8^16 kHz. The solid lines without error bars represent the mean noise level as a function of stimulus level.
at which DPOAE levels saturated. In the youngest mice, the slopes ranged from 0.94 to 1.38. Slopes increased with increasing f2 and age. Age-related changes in DPOAE I/O function slopes were frequency-dependent. By 17 months of age, the slope increased at f2 = 13.4 and 14.4 kHz, but did not at f2 = 8 and 16 kHz. At 25 months of age the slope increased at most of the frequencies. Increased slope implies that the DPOAE signals produced by the low-level primary tones decreased more than those produced by the highlevel primaries. 3.3. Age-related changes in DPOAE detection threshold The 2f1 3f2 DPOAE detection thresholds for individ-
ual ears (de¢ned as the minimum primary level, L2 in this case, required to produce a response that was 3 dB above the noise £oor followed by further growth of DPOAE level as primary levels increased) were derived from the I/O functions. The mean data of the six groups of CBA mice at f2 are shown in Fig. 4 (top panel). DPOAE detection thresholds decreased with increasing frequency. For the 1-month-old mice, the lowest detection threshold was about L2 = 5 dB SPL at f2 = 14 and 16 kHz, and the highest threshold was 37 dB SPL at 8 kHz. The change in DPOAE detection thresholds of the aged mice relative to the 1-month-old mice is displayed in the bottom panel of Fig. 4. For f2 s 11.3 kHz, detection thresholds increased slightly (about 5 dB) at
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Fig. 3. The slope of 2f1 3f2 DPOAE I/O functions as a function of age and frequency in the CBA mice.
17 months of age, and continued to increase with increasing age. For f2 6 11.3 kHz, the detection thresholds for 8-month-old mice were lower than those of the 1-month-old mice, and did not increase until 21 months of age. At 21 months of age detection thresholds were elevated ( 6 15 dB) for the entire frequency range tested. At 25 months of age, CBA mice displayed elevations of detection thresholds (about 20^40 dB), which were more severe in response to higher primary frequencies. Statistically signi¢cant age-related change in DPOAE detection thresholds occurred at 17 months of age at 13.4 and 14.4 kHz relative to the younger groups and extended to the remaining frequencies by 21 months of age. Fig. 5 shows the relationship between the change in DPOAE detection thresholds (relative to those of 1month-old mice, data from Fig. 4) and the slope of the I/O functions (data from Fig. 3). The data points for the mice at 21 months of age and older were distributed in the upper-right quadrant relative to intersecting dotted lines representing no change in the respective measures. This distribution indicates that there was a correlation between the elevation of the DPOAE detection thresholds and the increase of the I/O function slopes in aged mice. This trend is more evident in 25-month-old mice. This result implies that the change in the I/O functions for the aged mice relative to those of young mice was not simply a parallel shift to the right.
Fig. 4. Top panel: The mean 2f1 3f2 DPOAE detection thresholds derived from the I/O functions of the aging CBA mice as a function of frequency. Bottom panel: The change in mean DPOAE detection thresholds of the aged mice relative to those of the 1-month-old mice.
3.4. Age-related changes in DPOAE level Fig. 6 (panels a, c, e and g) displays the mean 2f1 3f2 DPOAE level as a function of f2 at four primary levels (L2 = 20, 30, 40 and 50 dB SPL) in six age groups of CBA mice. The changes in DPOAE level for the aged CBA mice relative to the 1-month-old mice are shown in panels b, d, f and h. Age-related changes of DPOAE level were dependent upon primary frequencies and levels. At 8 months of age, the DPOAE levels were similar to those of 1-month-old mice. However, 17-month-old mice showed decreased DPOAE levels at f2 s 11.3 kHz for all primary levels, and increased DPOAE levels at f2 6 11.3 kHz for L2 v30 dB SPL. At 21 months of age, the decreased DPOAE levels were present at most of
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Fig. 5. The relationship between the change in 2f1 3f2 DPOAE detection thresholds and the change in the I/O function slopes of the aging CBA mice.
the tested conditions. At 25 months of age, the DPOAE-vs.-f2 functions became nearly £at, and the DPOAE levels decreased by 20^40 dB. At this age, for the lower L2 values, DPOAE levels were at or near the noise £oor levels. Statistically signi¢cant agerelated changes of DPOAE levels in CBA mice, including decreased and increased levels at frequencies above and below 11.3 kHz, respectively, could be detected at 17 months of age with L2 = 20^40 dB SPL and L1 3L2 = 10 dB. The reduction of the DPOAE level at 21 months of age was found to be statistically signi¢cant at almost all frequencies tested for L2 = 30 and 40 dB SPL. 4. Discussion The main ¢ndings of the present study for a frequency range of 8^16 kHz were: (1) CBA mice exhibited mild age-related changes in DPOAE level and detection threshold at 17 months of age, and changes of 20^40 dB by 25 months of age; (2) DPOAE level of CBA mice decreased and detection threshold increased with age in a frequency-dependent manner, starting at high frequencies and eventually extending to low frequencies. Prior to the present study, DPOAE data were available only for young CBA mice which were used as controls in studies of cochlear function in hearing impaired strains (Horner et al., 1985; Schrott et al., 1991 ; Le Calvez et al., 1998a,b). Keeping in mind that the
median life span of CBA mice is 24 months (Willott et al., 1987, 1988), the present results suggest that CBA mice do not show signi¢cant alteration of the DPOAE measures until late in life. This strain exhibits mild DPOAE changes after about 70% of its median life span and only moderate changes at a very old age. It should be noted that although the CBA mouse does not exhibit signi¢cant hearing loss over much of its life span, it does exhibit retinal degeneration early in life (see Green, 1989). A recent study by Spongr et al. (1997) reported quantitative measures of hair cell loss in CBA mice throughout their life span. In comparing their histological results with multi-unit neural thresholds recorded from the inferior colliculus of CBA mice (Willott, 1986), Spongr et al. noted a discrepancy. They noted signi¢cant elevations of neural thresholds in a frequency range corresponding to a region in the cochlea where little hair cell loss was found. Spongr et al. speculated that the surviving hair cells in this region of the cochlea may be functioning abnormally. The present study provides direct experimental evidence in support of this view. We observed signi¢cant changes in CBA DPOAE characteristics through 25 months of age in the 8^16 kHz region. In this region of the CBA mouse cochlea (V46^60% distance from apex), little hair cell loss (e.g. OHC loss 6 15.2%) was noted through 26 months of age (Spongr et al., 1997). Since OAEs are a functional measure of OHCs (see Section 1), it can be concluded that the function of the remaining OHCs in the aged CBA ear is impaired. OHC dysfunction in the aged CBA cochlea could arise from direct and indirect e¡ects of age on OHC function. The function of OHCs might be considered to consist of several processes, such as hair bundle coupling, apical conductance change, electrochemical gradient (receptor current), electrical impedance and force generating motor elements (e.g. Patuzzi et al., 1989; Patuzzi and Rajan, 1992). Disruptions of CBA OHC function could arise from direct e¡ects of age on some or all of the above processes. Aging could also indirectly alter OHC function through changes in other cochlear structures. For example, changes in the stria vascularis (SV) are a common ¢nding in the cochleas of the aged humans (e.g. Engstrom et al., 1987; Schuknecht, 1974) and various animals (e.g. Bohne et al., 1990 ; Keithley et al., 1992; Schulte and Schmiedt, 1992 ; Shimada et al., 1998), including the mouse (e.g. Mikaelian, 1979; Willott, 1991). The SV helps to maintain the ionic homeostasis in the endolymph which is believed to be necessary for normal OHC function. Thus disruption of the SV function could indirectly alter OHC function. Recent work in quiet-aged gerbils indicates that alterations in microvasculature precede the degeneration of SV which in turn precedes age-re-
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Fig. 6. The mean 2f1 3f2 DPOAE levels as a function of frequency at (a) L2 = 20, (c) L2 = 30, (e) L2 = 40 and (g) L2 = 50 dB SPL of the aging CBA mice. The changes in DPOAE levels of the aged mice relative to those of the 1-month-old mice are shown for (b) L2 = 20, (d) L2 = 30, (f) L2 = 40 and (h) L2 = 50 dB SPL.
lated alteration of the endocochlear potential and auditory thresholds (e.g. Gratton and Schulte, 1995; Gratton et al., 1997). Another interesting age-related ¢nding in CBA mice is associated with the nonmonotonicity (i.e. notches) of the DPOAE I/O growth functions. The range of frequencies in which notches were observed in the CBA I/O functions extended from a narrow range of f2 = 13.4^14.4 kHz at 1 month to f2 = 11.3^14.4 kHz by 17 months of age. The new notches of the older mice appeared in a frequency region (i.e. f2 = 11.3 and 12 kHz) where DPOAE level and detection threshold were essentially unaltered. With further aging, the notches disappeared as DPOAE detection threshold in-
creased and DPOAE level decreased. This latter ¢nding is consistent with the disappearance of notches in the DPOAE I/O functions of 12-month-old C57 mice which also showed deteriorations in DPOAE level and detection threshold (Parham, 1997). Notches in the DPOAE I/O functions have been documented in other species (e.g. Brown, 1987 ; Kim, 1980 ; Kossl, 1992; Lonsbury-Martin et al., 1987; Whitehead et al., 1992a ; Wiederhold et al., 1986; Zurek et al., 1982) and have been attributed to an interaction between multiple sources giving rise to phase cancellation (e.g. Brown, 1987; Kim, 1980 ; Zwicker, 1986) including an interaction between a low-level source and a high-level source (Mills and Rubel, 1994 ; Norton and
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Rubel, 1990 ; Whitehead et al., 1992a,b). Both emergence and disappearance of the notches in the I/O functions of the aged mice imply altered interaction between the various DPOAE sources. An altered interaction between the various DPOAE sources with age is further supported by another ¢nding in the DPOAEs of aged mice. The slope of the I/O functions tended to increase with age in both CBA (the present report) and C57 (Parham, 1997) mice. The more rapid growth of DPOAE I/O function at an older age may be attributed to an unmasked high-level source of DPOAEs, analogous to conditions where the more vulnerable low-level source has been disrupted in animals with experimentally induced cochlear insults (e.g. Lonsbury-Martin et al., 1987; Mills et al., 1993). A recent preliminary report by Doan et al. (1996) documented functional changes in the middle ear of aged mice. Doan et al. (1996), using laser interferometry in 3- and 24-month-old BALB/c mice, reported up to 8 dB decreases in umbo velocity in the 5^13 kHz range with age. This change is likely, they argued, to be associated with increased sti¡ness of the middle ear. Since OAEs are dependent on both forward and backward transmission of stimuli through the middle ear, it is possible that increased sti¡ness of the middle ear ossicles contributed to the changes in the DPOAEs of aged CBA (the present report) and C57 mice (Parham, 1997). However, the magnitude of age-related DPOAE changes appears to be much greater than that expected from the age-related increase in sti¡ness of the middle ear apparatus alone. Although the contributions of agerelated changes of the inner and middle ears are additive in the DPOAEs of CBA and C57 mice, the middle ear is believed to play a relatively minor role. Interestingly, for f2 6 11.3 kHz, a small decrease in DPOAE detection thresholds and a small increase in DPOAE suprathreshold levels were noted by 8 and 17 months of age. These changes were not re£ected in the ABR thresholds of CBA mice. Similarly, previous studies did not ¢nd lower ABR thresholds or multi-unit neural thresholds in older CBA mice (e.g. Li and Borg, 1991; Willott, 1986). A similar trend in the DPOAE measures was reported in C57 mice (Parham, 1997). The reason for this improvement in the DPOAE response is not clear at this point. Parham (1997) speculated that the improvement in the DPOAE response in mice may re£ect continued maturation of OHCs in the cochlea, which a¡ects the generation of OAEs, and/or the maturation of the middle ear structure, which affects the transmission of the sound signals back and forth in recording OAEs. The former hypothesis is currently being investigated by examining developmental changes in DPOAEs of CBA mouse pups starting at the onset of hearing (Parham et al., 1997). The latter hypothesis could be evaluated by examining middle ear
function in intermediate age groups (e.g. 8 months of age) of mice, in a study similar to that of Doan et al. (1996). Comparing the present CBA results with our previous C57 ¢ndings (Parham, 1997) yields several interesting points. As emphasized above, 17-month-old CBA mice showed relatively mild DPOAE changes. In contrast, 18-month-old C57 mice had striking reduction of DPOAE levels (up to 50 dB) and elevations of DPOAE detection thresholds (up to 40 dB), both of which were more severe at high frequencies (Parham, 1997). This, of course, is a re£ection of the previously documented changes in auditory sensitivity and cochlear pathology which start at an earlier age in C57 mice (about 3^4 months of age; e.g. Henry, 1983 ; Henry and Chole, 1980 ; Hunter and Willott, 1987; Li and Borg, 1991; Li and Hultcrantz, 1994 ; Spongr et al., 1997 ; Willott, 1986). It is instructive to note that after the initial signs of age-related changes in DPOAEs at a given frequency, the rates of their progression are similar between the two strains. This is best illustrated by comparing results obtained under identical stimulus conditions (e.g. compare Figs. 2, 4 and 6 of the present paper with those of Figs. 8, 9 and 10 of Parham, 1997). For example, at f2 = 14.4 kHz at 17^25 months in CBA and 12^20 months in C57 mice (i.e. over an 8-month span) the mean DPOAE detection thresholds increased from 7 to 42 dB SPL and from 2 to 45 dB SPL, respectively. To state the above observation in di¡erent words, although the disruption of OHC function clearly starts much later in CBA mice, it does not seem to progress more rapidly (or slowly) after onset than in C57 mice. This conclusion implies that the aged OHCs appear to be no more vulnerable to the age-related pathological processes than relatively younger ears are. This is consistent with the conclusions of studies on e¡ects of noise exposure on aging CBA and C57 mice (Shone et al., 1991 ; Li, 1992) and aged chinchillas (Sun et al., 1994). Setting aside the di¡erence in the age of onset of DPOAE changes, the similarity in the characteristics of age-related changes in DPOAEs of CBA and C57 mice is surprising because of the genotypic and phenotypic di¡erences between the two strains. First, the agerelated hearing loss of C57 mice is attributed to a peripherally acting gene, Ahl, mapped to chromosome 10 (Erway et al., 1993; Johnson et al., 1997; Willott and Erway, 1998). CBA mice lack this allele. Second, the pattern and quantity of hair cell loss in the cochleas of C57 and CBA mice are strikingly di¡erent (Spongr et al., 1997). For example, age-related loss of OHCs in C57 mice progresses primarily in a basoapical direction, in contrast, that of CBA mice progresses primarily in an apicobasal direction. Reconciliation of these divergent ¢ndings has important implications for under-
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standing the e¡ects of age on cochlear function. Given the similarity in the rate of progression of DPOAE changes between the two strains, future e¡ort needs to be directed at uncovering age-related changes in cochlear structure which are common between the two strains. Acknowledgements This research was supported by grants from the Sandoz Foundation for Gerontological Research, National Institute on Aging (NIH Grant P60-AG13631-01), Department of Surgery and Research Advisory Committee, University of Connecticut Health Center. References Anderson, S.D., Kemp, D.T., 1979. The evoked cochlear mechanical response in laboratory primates. Arch. Otorhinolaryngol. 224, 47^ 54. Bohne, B.A., Grunner, M.M., Harding, G.W., 1990. Morphological correlates of aging in the chinchilla cochlea. Hear. Res. 11, 41^53. Bon¢ls, P., Bertrand, Y., Uzel, A., 1988. Evoked otoacoustic emissions: Normative data and presbycusis. Audiology 27, 27^35. Bredberg, G., 1968. Cellular pattern and nerve supply of the human organ of Corti. Acta Otolaryngol. 236 (Suppl.), 1^135. Brown, A.M., 1987. Acoustic distortion from rodent ears: A comparison of response from rats, guinea pigs and gerbils. Hear. Res. 31, 25^38. Brown, A.M., McDowell, B., Forge, A., 1989. Acoustic distortion products can be used to monitor the e¡ects of chronic gentamicin treatment. Hear. Res. 19, 191^198. Collet, L., Moulin, A., Gartner, M., Morgon, A., 1990. Age-related changes in evoked otoacoustic emissions. Ann. Otol. Rhinol. Laryngol. 99, 993^997. Doan, D.E., Erulkar, J.S., Saunders, J.C., 1996. Functional change in the aging mouse middle ear. Hear. Res. 97, 174^177. Engstrom, B., Hillerdal, M., Laurell, G., 1987. Selected pathological ¢ndings in the human cochlea. Acta Otolaryngol. 436, 110^116. Erway, L.C., Willott, J.F., Archer, J.R., Harrison, D.E., 1993. Genetics of age-related hearing loss in mice: I. Inbred and F1 hybrid strains. Hear. Res. 65, 125^132. Gratton, M.A., Schulte, B.A., 1995. Alterations in microvasculature are associated with atrophy of the stria vascularis in quiet-aged gerbils. Hear. Res. 82, 44^52. Gratton, M.A., Schulte, B.A., Smythe, N.M., 1997. Quanti¢cation of the stria vascularis and strial caplillary areas in quiet-reared young and aged gerbils. Hear. Res. 114, 1^9. Green, M.C., 1989. Catolog of mutant genes and polymorphic loci. In: Lyon, M.F., Searle, A.G. (Eds.), Genetic Variants and Strains of the Laboratory Mouse, Oxford University Press, Oxford, pp. 12^403. Guinan, J.J., Jr., 1986. E¡ect of e¡erent neural activity on cochlear mechanics. Scand. Audiol. Suppl. 25, 53^62. Henry, K.R., 1982. Age-related changes in sensitivity of the postpubertal ear to acoustic trauma. Hear. Res. 8, 285^294. Henry, K.R., 1983. Ageing and audition. In: Willott, J.F. (Ed.), The Auditory Psychobiology of the Mouse. Charles C. Thomas, Spring¢eld, IL, pp. 470^493. Henry, K.R., Chole, R.A., 1980. Genotypic di¡erences in behavioral,
37
physiological and anatomical expressions of age-related hearing loss in the laboratory mouse. Audiology 19, 369^383. Horner, K.C., Lenoir, M., Bock, G.R., 1985. Distortion product otoacoustic emissions in hearing-impaired mutant mice. J. Acoust. Soc. Am. 78, 1603^1611. Hunter, K.P., Willott, J.F., 1987. Aging and the auditory brainstem response in mice with severe or minimal presbycusis. Hear. Res. 30, 207^218. Jimenez, A.M., Stagner, B.B., Martin, G.K., Lonsbury-Martin, B.L., 1997. Patterns of distortion-product otoacoustic emissions in aging C57BL/6J mice. Assoc. Res. Otolaryngol. Abstr. 20, 101. Johnson, K.R., Erway, L.C., Cook, S.A., Willott, J.F., Zheng, Q.Y., 1997. A major gene a¡ecting age-related hearing loss in C57BL/6J mice. Hear. Res. 114, 83^92. Johnsson, L.-G., Hawkins, J.E., 1972. Sensory and neural degeneration with aging, as seen in microdissection of the human inner ear. Ann. Otol. Rhinol. Otolaryngol. 81, 179^193. Johnsson, L.-G., Hawkins, J.E., 1979. Age-related degeneration of the inner ear. In: Han, S.S., Coons, D.H. (Eds.), Special Senses in Aging. Institute of Gerontology, Ann Arbor, MI, pp. 119^135. Keithley, E.M., Ryan, A.F., Feldman, M.L., 1992. Cochlear degeneration in aged rats of four strains. Hear. Res. 59, 171^178. Kemp, D.T., 1978. Stimulated acoustic emissions from within the human auditory system. J. Acoust. Soc. Am. 64, 1386^1391. Kemp, D.T., Brown, A.M., 1984. Ear canal acoustic and round window electrical correlates of 2f1 3f2 distortion generated in the cochlea. Hear. Res. 13, 39^46. Kim, D.O., 1980. Cochlear mechanics: Implications of electrophysiological and acoustical observations. Hear. Res. 2, 297^317. Kim, D.O., Molnar, C.E., Matthews, J.W., 1980. Cochlear mechanics: Nonlinear behavior behavior in two-tone responses as re£ected in cochlear-nerve-¢ber responses and in ear-canal sound pressure. J. Acoust. Soc. Am. 67, 1704^1721. Kossl, M., 1992. High frequency distortion products from the ears of two bat species Megaderma lyra and Carollia prespicillata. Hear. Res. 59, 156^164. Le Calvez, S., Avan, P., Gilain, L., Romand, R., 1998a. CD1 hearingimpaired mice. I: Distortion product otoacoustic emission levels, cochlear function and morphology. Hear. Res. 120, 37^50. Le Calvez, S., Guilhaume, A., Romand, R., Aran, J.M., Avan, P., 1998b. CD1 hearing-impaired mice. II. Group latencies and optimal f2/f1 ratios of distortion product otoacoustic emissions, and scanning electron microscopy. Hear. Res. 120, 51^61. Li, H.-S., 1992. In£uence of genotype and age on acute acoustic trauma and recovery in CBA/Ca and C57BL/6J mice. Acta Otolaryngol. 112, 965^967. Li, H.-S., Borg, E., 1991. Age-related loss of auditory sensitivity in two mouse genotypes. Acta Otolaryngol. 111, 827^834. Li, H.-S., Hultcrantz, M., 1994. Age-related degeneration of the organ of Corti in two genotypes of mice. Otorhinolaryngology 56, 61^ 67. Liberman, M.C., Puria, S., Guinan, J.J.Jr., 1996. The ipsilaterally evoked olivocochlear re£ex causes rapid adaptation of the 2f1 3f2 distortion product otoacoustic emission. J. Acoust. Soc. Am. 99, 3572^3584. Lonsbury-Martin, B.L., Martin, G.K., Probst, R., Coats, A.C., 1987. Acoustic distortion products in rabbit ear canal. I. Basic features and physiological vulnerability. Hear. Res. 28, 173^189. Lonsbury-Martin, B.L., Cutler, W.M., Martin, G.K., 1991. Evidence for the in£uence of aging on distortion-product otoacoustic emissions in humans. J. Acoust. Soc. Am. 89, 1749^1759. Martin, G.K., Lonsbury-Martin, B.L., Probst, R., Scheinin, S.A., Coats, A.C., 1987. Acoustic distortion products in rabbit ear canal. II. Sites of origin revealed by suppression contours and puretone exposures. Hear. Res. 28, 191^208.
HEARES 3239 19-7-99
38
K. Parham et al. / Hearing Research 134 (1999) 29^38
Mikaelian, D.O., 1979. Development and degeneration of hearing in the C57BL/6J mouse: Relation of electrophysiologic responses from the round window and cochlear nucleus to cochlear anatomy and behavioral responses. Laryngoscope 89, 1^15. Mills, D.M., Rubel, E.W., 1994. Variation of distortion product otoacoustic emissions with furosemide injection. Hear. Res. 77, 183^ 199. Mills, D.M., Norton, S.J., Rubel, E.W., 1993. Vulnerability and adaptation of distortion product otoacoustic emissions to endocochlear potential variation. J. Acoust. Soc. Am. 94, 2108^2122. Mountain, D.C., 1980. Changes in endolymphatic potential and crossed olivocochlear bundle stimulation alters cochlear mechanics. Science 210, 71^72. Norton, S.J., Rubel, E.W., 1990. Active and passive ADP components in mammalian and avian ears. In: Dallos, P., Geisler, C.D., Matthews, J.W., Ruggero, M.A., Steele, C.R. (Eds.), Mechanics and Biophysics of Hearing. Springer-Verlag, New York, pp. 219^226. Parham, K., 1997. Distortion product otoacoustic emissions in the C57BL/6J mouse model of age-related hearing loss. Hear. Res. 112, 216^234. Parham, K., Willott, D.O., 1988. Acoustic startle response in young and aging C57BL/6J and CBA/J mice. Behav. Neurosci. 102, 881^ 886. Parham, K., Sun, X., Kim, D.O., 1996. Examination of optimal stimulus parameters for distortion product otoacoustic emissions in mouse models of presbycusis. Assoc. Res. Otolaryngol. Abstr. 19, 26. Parham, K., Gerber, A., McKowen, A., Sun, X.-M., Kim, D.O., 1997. Distortion product otoacoustic emissions of the DBA/2J mouse with early-onset progressive hearing loss. Assoc. Res. Otolaryngol. Abstr. 20, 194. Patuzzi, R., Rajan, R., 1992. Additivity of threshold elevations produced by disruption of outer hair cell function. Hear. Res. 60, 165^177. Patuzzi, R., Yates, G.K., Johnstone, B.M., 1989. The origin of the low-frequency microphonic in the ¢rst cochlear turn of the guinea pig. Hear. Res. 39, 177^188. Schmiedt, R.A., 1986. Acoustic distortion in the ear canal, I. Cubic di¡erence tones: E¡ects of acute noise injury. J. Acoust. Soc. Am. 79, 1481^1490. Schrott, A., Puel, J.-L., Rebillard, G., 1991. Cochlear origin of 2f1 3f2 distortion products assessed by using 2 types of mutant mice. Hear. Res. 52, 245^254. Schuknecht, H.F., 1964. Further observations on the pathology of presbycusis. Arch. Otolaryngol. 80, 369^382. Schuknecht, H.F., 1974. Pathology of the Ear. Harvard University Press, Cambridge, MA. Schuknecht, H.F., Gacek, M.R., 1993. Cochlear pathology in presbycusis. Ann. Otol. Rhinol. Laryngol. 102, 1^16. Schulte, B.A., Schmiedt, R., 1992. Lateral wall Na, K-ATPase and endocochlear potentials decline with age in quiet-reared gerbils. Hear. Res. 61, 35^46. Shimada, A., Ebisu, M., Morita, T., Takeuchi, T., Umemura, T., 1998. Age-related changes in the cochlea and cochlear nuclei of dogs. J. Vet. Med. Sci. 60, 41^48. Shone, G., Altschuler, R.A., Miller, J.M., Nuttall, A.L., 1991. The e¡ect of noise exposure on the aging ear. Hear. Res. 56, 173^178. Siegel, J.H., Kim, D.O., 1982. E¡erent neural control of cochlear mechanics? Olivocochlear bundle stimulation a¡ects cochlear biomechanical nonlinearity. Hear. Res. 6, 171^182. Sjostrom, B., Anniko, M., 1990. Variability in genetically induced age-related impairment of auditory brainstem response thresholds. Acta Otolaryngol. 109, 353^360.
Smurzynski, J., Kim, D.O., 1992. Distortion-product and click-evoked otoacoustic emissions of normally hearing adults. Hear. Res. 58, 227^240. Spongr, V.P., Flood, D.G., Frisina, R.D., Salvi, R.J., 1997. Quantitative measures of hair cells in CBA and C57BL/6 mice throughout their life spans. J. Acoust. Soc. Am. 101, 3546^3553. Stover, L., Norton, S.J., 1993. The e¡ects of aging on otoacoustic emissions. J. Acoust. Soc. Am. 94, 2670^2681. Sun, J.C., Bohne, B.A., Harding, G.W., 1994. Is the older ear more susceptible to noise damage? Laryngoscope 104, 1251^1258. Sun, X., Parham, K., Kim, D.O., 1996. Distortion product otoacoustic emissions of an animal model of presbycusis, the CBA/J mouse. Assoc. Res. Otolaryngol. Abstr. 19, 25. Sun, X.-M., Parham, K., Kim, D.O., 1997. E¡ects of stimulus parameters on distortion product otoacoustic emissions in the C57BL/6J mouse with age-related hearing loss. Assoc. Res. Otolaryngol. Abstr. 20, 101. Walton, J.P., Frisina, R.D., O'Neill, W.E., 1998. Age-related alteration in processing of temporal sound features in the auditory midbrain of the CBA mouse. J. Neurosci. 18, 2764^2776. Wenngren, B.-I., Anniko, M., 1988. A frequency-speci¢c auditory brainstem response technique exempli¢ed in the determination of age-related auditory thresholds. Acta Otolaryngol. 106, 238^243. Whitehead, M.L., Lonsbury-Martin, B.L., Martin, G.K., 1992a. Evidence for two discrete source of 2f1 3f2 distortion-product otoacoustic emission in rabbit: I. Di¡erential dependence on stimulus parameters. J. Acoust. Soc. Am. 91, 1587^1607. Whitehead, M.L., Lonsbury-Martin, B.L., Martin, G.K., 1992b. Evidence for two discrete source of 2f1 3f2 distortion-product otoacoustic emission in rabbit: II. Di¡erential physiological vulnerability. J. Acoust. Soc. Am. 92, 2662^2682. Wiederhold, M.L., Mahoney, J.W., Kellogg, D.L., 1986. Acoustic overstimulation reduces 2f1 3f2 cochlear emissions at all levels in the cat. In: Allen, J.B., Hall, J.L., Hubbard, A., Neely, S.T., Tubis, A. (Eds.), Peripheral Auditory Mechanisms. Springer, Berlin, pp. 322^329. Willott, J.F., 1986. E¡ects of aging, hearing loss, and anatomical location on threshold of inferior colliculus neurons in C57BL/6 and CBA mice. J. Neurophysiol. 56, 391^480. Willott, J.F., 1991. Aging and the Auditory System: Anatomy, Physiology, and Psychophysics. Singlar Publishing, San Diego, CA. Willott, J.F., Erway, L.C., 1998. Genetics of age-related hearing loss in mice. IV. Cochlear pathology and hearing loss in 25 BXD recommbinant inbred mouse strains. Hear. Res. 119, 27^36. Willott, J.F., Jackson, L.M., Hunter, K.P., 1987. Morphometric study of the anteroventral cochlear nucleus of two mouse models of presbycusis. J. Comp. Neurol. 260, 472^480. Willott, J.F., Parham, K., Hunter, K.P., 1988. Response properties of inferior colliculus neurons in young and very old CBA/J mice. Hear. Res. 37, 1^14. Willott, J.F., Parham, K., Hunter, K.P., 1991. Comparison of the auditory sensitivity of neurons in the cochlear nucleus and inferior colliculus of young and aging C57BL/6J and CBA/J mice. Hear. Res. 53, 78^94. Willott, J.F., Carlson, S., Chen, H., 1994. Prepulse inhibition of the startle response in mice: relationship to hearing loss and auditory system plasticity. Behav. Neurosci. 108, 703^713. Zurek, P.M., Clark, W.W., Kim, D.O., 1982. The behavior of acoustic distortion products in the ear canals of chinchillas with normal or damaged ears. J. Acoust. Soc. Am. 72, 774^780. Zwicker, E., 1986. Suppression and (2f13f2) di¡erence tones in a nonlinear cochlear preprocessing model with active feedback. J. Acoust. Soc. Am. 80, 163^176.
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