**3.7.3 Changes in enzymes regulating potassium homeostasis**

Spicer et al. (1997) compared the Na,K-ATPase-immunoreactivity (an ion exchange enzyme that uses ATP to pump 3 sodium ions out of the cell in exchange for 2 potassium ions that are pumped into the cell) in the lateral wall and SV of young and old gerbils. Immunostaining in cochleae of old gerbils was more variable than in young gerbils. Old animals showed strial atrophy and no Na,K-ATPase immunoreactivity at the apex, best preservation of SV and immunoreactivity in the middle, and atrophy of SV and loss of immunoreactivity at the base of the cochlea. Immunoreactivity in type II, IV and V fibrocytes of old gerbils decreased less than expression in the adjacent SV, although complete SV degeneration was also associated with loss of immunoreactivity in fibrocytes. The observation that a loss of Na,K-ATPase immunoreactivity in fibrocytes of the spiral ligament appeared to lag behind the loss of staining in the SV supports the suggestion that changes in fibrocytes occur secondarily to alterations in the SV. Sakaguchi et al. (1998) found that age-dependent changes in the expression of the Na-K-Cl co-transporter closely paralleled those reported for the Na,K-ATPase.

Schulte & Schmiedt (1992) determined the Na,K-ATPase immunoreactive volume of the SV from immunostained cochlear sections. A plot of the EP as a function of the normalised SV

The Mongolian Gerbil as a Model for

ipsi- and contra-lateral IC.

et al., 1993a).

**4.1 The auditory brainstem response (ABR)** 

**4.1.1 Age-dependent changes of ABR thresholds** 

**4.1.2 Age-dependent changes of ABR growth functions** 

**4. The auditory brainstem nuclei** 

the Analysis of Peripheral and Central Age-Dependent Hearing Loss 75

An overview of the auditory pathway is summarised in Strutz (1991) and Schwartz (1991). The central processes of the auditory nerve fibres enter the brain through the internal auditory meatus. Each fibre bifurcates when it enters the cochlear nucleus and sends an ascending branch to the antero-ventral (AVCN) and a descending branch through the postero-ventral (PVCN) to the dorsal (DCN) cochlear nucleus. All auditory nerve fibres terminate in the cochlear nucleus. Neurons of the ventral cochlear nucleus (VCN) predominantly project to the ipsi- and contra-lateral nuclei of the superior olivary complex. The neurons of the DCN project primarily to the contra-lateral, and to a lesser degree, to the ipsi-lateral inferior colliculus (IC). The medial nucleus of the trapezoid body (MNTB) receives input from the contra-lateral VCN and projects primarily to the ipsi-lateral medial (MSO) and lateral (LSO) nuclei of the superior olive. MSO and LSO also receive input from the ipsi- and contra-lateral VCN. MSO neurons project almost exclusively to the ipsi-lateral IC and send collaterals to the dorsal nucleus of the lateral lemniscus. The LSO projects to the

The auditory brainstem response (ABR), recorded from needle electrodes placed behind the ear and the vertex, reflects the synchronised neural activity to the onset of a stimulus. It is less invasive than single-fibre and CAP recordings for evaluating hearing status. Short tonepips elicit a typical ABR waveform in gerbils with peaks occurring at characteristic latencies. They have been termed i (1-2 ms), ii-iii (2-3.5 ms) and iv (4-5 ms) and can be homologised with the human ABR waves I (generated by the auditory nerve), III (generated by the cochlear nucleus or the MNTB) and V (generated by the lateral lemniscus and IC; Boettcher

Age-related hearing loss in gerbils was first reported by Henry et al. (1980), showing 15-20 dB threshold elevation for frequencies between 1 and 32 kHz in 2 year old as compared to 3 month old gerbils. Mills et al. (1990) derived ABR thresholds for gerbils between 8 and 36 months of age that were raised in a low-noise environment. Thresholds in 3 year old gerbils varied over a wide range; some old animals showed no or only small threshold elevation compared to young controls, while some old gerbils had more than 50 dB hearing loss. Hearing loss was less than 10 dB in a group of 19 month old gerbils, increased to 10-20 dB at 2 years and further progressed with age. Mean threshold shift at 3 years was approximately 20 dB for the 1-4 kHz range and 25-30 dB for higher frequencies. Threshold shift determined by ABR and CAP measurements in 3 year old gerbils showed a good correspondence.

Boettcher et al. (1993a) compared wave ii-iii and wave iv ABR input-output functions of young and old gerbils. The plots of wave ii-iii and iv amplitude as a function of the tone pip level showed a reduction in old as compared to young gerbils that was not directly related to threshold. This was seen through the response amplitude of the best old gerbils with near normal thresholds being greatly reduced at high stimulus levels, especially for the lower test

volume showed a group of 4 old gerbils with an EP below 20 mV where the SV volume was reduced by more than 70%. In another group of 9 old gerbils, EP varied between 50 and 80 mV with an associated loss of the SV volume between 20% and 70%. Thus, a reduction of the SV volume expressing Na,K-ATPase by up to 70% was associated with only a small loss of the EP. Only when the loss of Na,K-ATPase expressing SV volume increased beyond 70% did the EP show an abrupt break down: the EP appeared tolerant to a relatively large loss of Na,K-ATPase. Consistent with a mean reduction of the Na,K-ATPase immunoreactive volume of SV, the activity of this enzyme was reduced in the lateral wall of old as compared to young gerbils (Gratton et al., 1995) and a low level of Na,K-ATPase activity was associated with a low EP (Gratton et al., 1997b).

Spicer & Schulte (1998) proposed a medial pathway for the recycling of potassium released by inner hair cells. In old gerbils, in contrast to the SV and the lateral wall, fibrocytes of the spiral limbus showed unaltered or upregulated Na,K-ATPase immunoreactivity. In addition, interdental cells remained immunoreactive in cochleae with SV atrophy. Based on these observations, Spicer & Schulte (1998) suggested a normal function of inner hair cells in old gerbils with strial atrophy (although the hearing status of the specimen they analysed was not known). Potassium released by inner hair cells can be recycled into the endolymph by the medial pathway via the remaining Na,K-ATPase immunoreactive limbal fibrocytes and interdental cells.
