**3.2.1 CAP threshold and growth with stimulus intensity**

By plotting CAP thresholds across a range of test frequencies, Hellstrom & Schmiedt (1990) compared CAP audibility curves of young and old gerbils and found a varying degree of frequency-specific threshold elevation in old gerbils. Compared to young gerbils, the interanimal variability of thresholds was much higher in old gerbils for frequencies above 3 kHz. Below 3 kHz, old gerbils showed an average of less than 20 dB threshold elevation, while the difference increased at higher frequencies to more than 30 dB. The growth of the peakto-peak CAP amplitude with increasing level of the tone pip was considerably reduced in old gerbils and a quantitative analysis confirmed that the slopes of the CAP input-output functions were significantly reduced for test frequencies between 1 and 8 kHz. While the elevated CAP thresholds in old gerbils reflect the elevated thresholds at the tip of the tuning curve in recordings from auditory nerve fibres (Schmiedt et al., 1990), the reduced slope of the CAP growth functions in old gerbils was not reflected in the rate-level functions of auditory nerve fibres (Hellstrom & Schmiedt, 1991). Given that the slopes of rate-level

The Mongolian Gerbil as a Model for

the Analysis of Peripheral and Central Age-Dependent Hearing Loss 71

predominantly in the apical turn and to a lesser degree in the extreme basal turn. This group of animals showed the least degree of threshold elevation (0-25 dB). Eight animals with a moderate degree of hair cell loss (8-14%) showed, on average, higher degrees of threshold shift (5-55dB). Outer hair cell loss was more pronounced at the apex, but was also present towards the base of the cochlea. In 2 gerbils, 41-54% of the hair cells, predominantly outer and to a lesser degree inner hair cells, were missing. The hair cell loss in this group was associated with more than 50 dB hearing loss. In addition to hair cell loss, a varying proportion of outer hair cells in the low-frequency (apical) region of old gerbils appeared grossly abnormal with a spherical shape and larger diameters. These cells were located between normally appearing outer hair cells. No such abnormalities were found in young animals. Although the degree of hair cell loss was associated with the degree of threshold shift in the 3 groups, the pattern of hair cell loss did not correlate with the frequencydependent CAP threshold shifts along the cochlea. Loss of outer hair cells at the apex was found without corresponding threshold shifts for frequencies below 3 kHz. Above 4 kHz, threshold shift was present without a loss of outer hair cells in the corresponding frequency region. These data demonstrate that cytocochleograms cannot predict the frequency-specific

CAP threshold shifts in old gerbils raised in a low noise environment.

affected by hearing loss and the position of cell pathology along the cochlea.

**3.6 Spiral ganglion cells and auditory dendrites** 

predominantly affected high frequencies.

**3.5 Pathology of non-sensory cells in the organ of Corti and Reissner's membrane**  Adams & Schulte (1997) expanded the analysis of cochlear pathology in old gerbils to the non-sensory cells of the organ of Corti and Reissner's membrane. In addition to the loss and pathology of hair cells, they observed pathological changes to pillar cells in regions where outer hair cells had been lost. Compared to young gerbils, where the cells forming Reissner's membrane appeared uniformly distributed, gerbils older than 2 years showed a formation of cell clusters mixed with regions of lower cell density. However, this rearrangement of cells in Reissner's membrane appeared to not be related to hearing loss. In summary, Adams & Schulte (1997) emphasised the discrepancy between the frequencies

Keithley et al. (1989) compared the density of spiral ganglion cells in young and old gerbils. The mean ganglion cell density averaged along the whole cochlea was 1106 cells/mm² for 4 gerbils with an age of 2 months. Compared to the mean of these young gerbils, the density decreased to 86% and 83% in 5 animals aged 24-30 months and in 3 animals aged 36-42 months respectively, though the difference between the young animals and the 2 groups of old animals was not significant in this sample. When they compared mean spiral ganglion cell density for separate half turns of the cochlea, a significant reduction that varied between 16 and 55% in the two groups of old gerbils with reference to the 2 month old animals was only found for the most basal position (80-90% from the apical end, corresponding to frequencies above 20 kHz). Overall, the loss of spiral ganglion cells was limited and

Based on a small sample that precluded statistical analysis, Suryadevara et al. (2001) suggested a slightly decreased number of auditory dendrites per inner hair cell in old gerbils. Their data in young gerbils showed a gradient of auditory nerve fibre dendrite

functions of auditory nerve fibres did not differ between young and old gerbils, Hellstrom & Schmiedt (1990, 1991) argued that a loss of auditory nerve fibres and spiral ganglion cells and a loss of synchrony of the auditory nerve fibre population response could result in reduced CAP amplitudes in old gerbils.

#### **3.2.2 CAP measure of cochlear frequency selectivity**

CAP measurements have also been conducted to compare cochlear frequency selectivity of young and old gerbils (Hellstrom & Schmiedt, 1996). A forward masking paradigm was used to determine masked CAP tuning curves at probe frequencies between 1 and 16 kHz. Briefly, a probe of a given frequency was presented 10-15 dB above the CAP threshold, eliciting a robust CAP response. The response to the probe was masked by a 60 ms tone burst that was presented 5 ms before the probe. The masked CAP tuning curve was obtained by plotting the masker level that just suppressed the response to the probe as a function of masker frequency. The masked CAP tuning curves share many characteristics with auditory nerve fibre tuning curves. The elevation of threshold at the tip of single fibre tuning curves in old gerbils (Schmiedt et al., 1990), especially at higher frequencies, was also evident in masked CAP tuning curves. In addition, the loss of frequency selectivity in auditory nerve fibres with characteristic frequencies above 4 kHz in old gerbils was paralleled by a corresponding loss of frequency selectivity in the masked CAP tuning curves (Hellstrom & Schmiedt, 1996).

#### **3.3 Distortion product otoacoustic emissions (DPOAE)**

Distortion product otoacoustic emissions (DPOAE) characterise the function of outer hair cells that are the central element of the "cochlear amplifier". They can be used to determine the sensitivity of the cochlea and to construct audiograms (Janssen et al., 2006). Eckrich et al. (2008) measured DPOAE audiograms of "laboratory" gerbils and gerbils that had been caught in the wild and bred for 6-7 generations in captivity. While thresholds of "wild" gerbils remained stable across age, thresholds of 15-28 month old, domesticated gerbils were increased at 2 kHz (6 dB), between 8 and 20 kHz (6-11 dB) and above 44 kHz (6-12 dB), when compared to the thresholds of 3 and 6 month old, domesticated gerbils. For the most basal test frequencies above 50 kHz, threshold elevation of more than 6 dB was present in the 9 and 12 month old, domesticated gerbils. Eckrich et al. (2008) suggested that elevated DPOAE thresholds in the older gerbils may have been caused by a loss of the endocochlear potential and/or a loss of outer hair cells.

#### **3.4 Age-dependent hair cell loss**

The loss of hair cells is one mechanism that causes hearing loss. Cytocochleograms are plots of the proportion of missing and abnormal hair cells as a function of the position along the cochlea. When the cochlear place-frequency map is known, frequency specific hearing loss can be directly correlated with hair cell loss. Tarnowski et al. (1991) performed such a comparison of cytocochleograms and CAP thresholds in 16 old gerbils raised in a low-noise environment. In their sample, they found a substantial inter-animal variation of threshold shift and hair cell loss and defined 3 groups based on the degree of hair cell loss. In 6 animals with minimal hair cell loss (5-8%), only outer hair cells were missing,

functions of auditory nerve fibres did not differ between young and old gerbils, Hellstrom & Schmiedt (1990, 1991) argued that a loss of auditory nerve fibres and spiral ganglion cells and a loss of synchrony of the auditory nerve fibre population response could result in

CAP measurements have also been conducted to compare cochlear frequency selectivity of young and old gerbils (Hellstrom & Schmiedt, 1996). A forward masking paradigm was used to determine masked CAP tuning curves at probe frequencies between 1 and 16 kHz. Briefly, a probe of a given frequency was presented 10-15 dB above the CAP threshold, eliciting a robust CAP response. The response to the probe was masked by a 60 ms tone burst that was presented 5 ms before the probe. The masked CAP tuning curve was obtained by plotting the masker level that just suppressed the response to the probe as a function of masker frequency. The masked CAP tuning curves share many characteristics with auditory nerve fibre tuning curves. The elevation of threshold at the tip of single fibre tuning curves in old gerbils (Schmiedt et al., 1990), especially at higher frequencies, was also evident in masked CAP tuning curves. In addition, the loss of frequency selectivity in auditory nerve fibres with characteristic frequencies above 4 kHz in old gerbils was paralleled by a corresponding loss of frequency selectivity in the masked CAP tuning curves

Distortion product otoacoustic emissions (DPOAE) characterise the function of outer hair cells that are the central element of the "cochlear amplifier". They can be used to determine the sensitivity of the cochlea and to construct audiograms (Janssen et al., 2006). Eckrich et al. (2008) measured DPOAE audiograms of "laboratory" gerbils and gerbils that had been caught in the wild and bred for 6-7 generations in captivity. While thresholds of "wild" gerbils remained stable across age, thresholds of 15-28 month old, domesticated gerbils were increased at 2 kHz (6 dB), between 8 and 20 kHz (6-11 dB) and above 44 kHz (6-12 dB), when compared to the thresholds of 3 and 6 month old, domesticated gerbils. For the most basal test frequencies above 50 kHz, threshold elevation of more than 6 dB was present in the 9 and 12 month old, domesticated gerbils. Eckrich et al. (2008) suggested that elevated DPOAE thresholds in the older gerbils may have been caused by a loss of the endocochlear

The loss of hair cells is one mechanism that causes hearing loss. Cytocochleograms are plots of the proportion of missing and abnormal hair cells as a function of the position along the cochlea. When the cochlear place-frequency map is known, frequency specific hearing loss can be directly correlated with hair cell loss. Tarnowski et al. (1991) performed such a comparison of cytocochleograms and CAP thresholds in 16 old gerbils raised in a low-noise environment. In their sample, they found a substantial inter-animal variation of threshold shift and hair cell loss and defined 3 groups based on the degree of hair cell loss. In 6 animals with minimal hair cell loss (5-8%), only outer hair cells were missing,

reduced CAP amplitudes in old gerbils.

(Hellstrom & Schmiedt, 1996).

potential and/or a loss of outer hair cells.

**3.4 Age-dependent hair cell loss** 

**3.2.2 CAP measure of cochlear frequency selectivity** 

**3.3 Distortion product otoacoustic emissions (DPOAE)** 

predominantly in the apical turn and to a lesser degree in the extreme basal turn. This group of animals showed the least degree of threshold elevation (0-25 dB). Eight animals with a moderate degree of hair cell loss (8-14%) showed, on average, higher degrees of threshold shift (5-55dB). Outer hair cell loss was more pronounced at the apex, but was also present towards the base of the cochlea. In 2 gerbils, 41-54% of the hair cells, predominantly outer and to a lesser degree inner hair cells, were missing. The hair cell loss in this group was associated with more than 50 dB hearing loss. In addition to hair cell loss, a varying proportion of outer hair cells in the low-frequency (apical) region of old gerbils appeared grossly abnormal with a spherical shape and larger diameters. These cells were located between normally appearing outer hair cells. No such abnormalities were found in young animals. Although the degree of hair cell loss was associated with the degree of threshold shift in the 3 groups, the pattern of hair cell loss did not correlate with the frequencydependent CAP threshold shifts along the cochlea. Loss of outer hair cells at the apex was found without corresponding threshold shifts for frequencies below 3 kHz. Above 4 kHz, threshold shift was present without a loss of outer hair cells in the corresponding frequency region. These data demonstrate that cytocochleograms cannot predict the frequency-specific CAP threshold shifts in old gerbils raised in a low noise environment.

### **3.5 Pathology of non-sensory cells in the organ of Corti and Reissner's membrane**

Adams & Schulte (1997) expanded the analysis of cochlear pathology in old gerbils to the non-sensory cells of the organ of Corti and Reissner's membrane. In addition to the loss and pathology of hair cells, they observed pathological changes to pillar cells in regions where outer hair cells had been lost. Compared to young gerbils, where the cells forming Reissner's membrane appeared uniformly distributed, gerbils older than 2 years showed a formation of cell clusters mixed with regions of lower cell density. However, this rearrangement of cells in Reissner's membrane appeared to not be related to hearing loss. In summary, Adams & Schulte (1997) emphasised the discrepancy between the frequencies affected by hearing loss and the position of cell pathology along the cochlea.
