**4. The auditory brainstem nuclei**

74 Hearing Loss

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

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

The data discussed above describe a wide range of age-dependent pathologies of the gerbil cochlea. In summary, they suggest that loss of EP due to pathology of the SV and the lateral wall are the main factors that contribute to the threshold shifts observed in auditory nerve fibres and the CAP in old gerbils. This pattern resembles the category of strial atrophy in

The loss of sensitivity was most pronounced at the tip of single-fibre tuning curves (Schmiedt et al., 1990) and in masked CAP tuning curves (Hellstrom & Schmiedt, 1996) of old gerbils and led to a decreased tip-to-tail ratio of the tuning curves. These changes are similar to the effects of a reduced EP on single-fibre tuning curves in cat (Sewell, 1984). Consequently, Hellstrom & Schmiedt (1996) proposed that "the quiet-aged gerbil can be used as a model for an intact hair-cell system coupled to a chronically lowered EP". This view was supported by a subsequent study where changes of cochlear function due to a reduction of the EP by chronic furosemide application to the round window in young gerbils resembled those found in quiet-aged, old gerbils (Lang et al., 2010). Schmiedt (1983) proposed the "dead battery theory" and reported that increasing the EP by current injection into scala media in an old animal with an initial EP of 41 mV was associated with increased

In summary, cochlear sensitivity in quiet-aged gerbils declines on average with a high degree of inter-animal variability. The loss of EP due to degeneration of the SV appears to be the main reason for decreased sensitivity in old gerbils, while loss of hair cells and auditory nerve fibres appear less important. Consequently, gerbils are a useful model of human strial

associated with a low EP (Gratton et al., 1997b).

**3.8 The gerbil as a model of strial or metabolic presbyacusis** 

CAP amplitude and a 20 dB reduction of CAP threshold.

and interdental cells.

humans (Schuknecht & Gacek, 1993).

or metabolic cochlear presbyacusis.

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 ipsi- and contra-lateral IC.
