**3.7.1 Age-dependent loss of the endocochlear potential**

Several studies in gerbils have shown that the EP, on average, declines with age and interanimal variability of the EP in old gerbils becomes much higher compared to young gerbils (Gratton et al., 1996, 1997a; Schmiedt, 1983, 1996; Schulte & Schmiedt, 1992). The EP in young gerbils (Schmiedt, 1983) was highest at the base, determined through the round window (92 mV), and slightly lower at more apical locations (76-81 mV); the reduction of the mean EP determined in 3 year old gerbils relative to the means obtained in young gerbils was more pronounced at the base (40 kHz region: 31 mV) and the apex (0.5 kHz region: 27 mV) as compared to the intermediate parts of the cochlea (2 kHz region: 19 mV; 16 kHz region: 23 mV). The loss of the EP and threshold shifts in old gerbils were not related to each other in a direct and simple way. The pattern of CAP threshold shift from low to high frequencies differed from the pattern of EP loss. In addition, the plots of CAP threshold shift as a function of EP shift (Schmiedt, 1983) demonstrate no correlation for young and 30 month old gerbils, despite a variation of the EP over a 40-60 mV range. Only the data from 3 year old gerbils indicated some correlation between CAP threshold and EP, although the scatter in the data was large. Overall, a linear regression analysis suggested that the variation of EP in 3 year old gerbils accounts for 31% of the variation in CAP thresholds (Schmiedt, 1983). The reduction of the mean EP was not associated with a mean loss of potassium concentration in the endolymph of old gerbils and the "effects of age are primarily on EP generation, and not on the chemical potential of Ke +" between endolymph and perilymph (Schmiedt, 1996).

#### **3.7.2 Histological changes in the stria vascularis and the spiral ligament**

The stria vascularis (SV) plays a central role in the generation of the EP (Wangemann, 2006). Age-dependent changes in the microvasculature that might lead to ischemia and affect SV

diameter within the osseous spiral lamina, with increasing diameter from the scala vestibuli to the scala tympani side. With decreasing endocochlear potential (EP) in old gerbils, this gradient disappeared due to fewer large diameter fibres found near scala tympani. In addition, the cross-sectional area of spiral ganglion cells decreased with decreasing EP. Thus, decreasing EP was associated with a loss or shrinkage of large diameter auditory

Rüttiger et al. (2007) found an age dependent reduction of BDNF mRNA expression in high frequency spiral ganglion cells. In contrast, BDNF protein expression was preserved in the cochlear ganglion cells of old gerbils but declined in their central and peripheral processes.

The sensitivity of the mechano-electrical transduction by hair cells in the mammalian cochlea depends on the endocochlear potential (EP) in scala media (Wangemann, 2006). The positive EP (80-100 mV) together with the negative intracellular potential of hair cells is the driving force (battery) of sensory transduction. The important contribution of the EP to the sensitivity of the cochlea was demonstrated in experiments, where a reduction of the endocochlear potential by the application of furosemide was associated with threshold shifts

Several studies in gerbils have shown that the EP, on average, declines with age and interanimal variability of the EP in old gerbils becomes much higher compared to young gerbils (Gratton et al., 1996, 1997a; Schmiedt, 1983, 1996; Schulte & Schmiedt, 1992). The EP in young gerbils (Schmiedt, 1983) was highest at the base, determined through the round window (92 mV), and slightly lower at more apical locations (76-81 mV); the reduction of the mean EP determined in 3 year old gerbils relative to the means obtained in young gerbils was more pronounced at the base (40 kHz region: 31 mV) and the apex (0.5 kHz region: 27 mV) as compared to the intermediate parts of the cochlea (2 kHz region: 19 mV; 16 kHz region: 23 mV). The loss of the EP and threshold shifts in old gerbils were not related to each other in a direct and simple way. The pattern of CAP threshold shift from low to high frequencies differed from the pattern of EP loss. In addition, the plots of CAP threshold shift as a function of EP shift (Schmiedt, 1983) demonstrate no correlation for young and 30 month old gerbils, despite a variation of the EP over a 40-60 mV range. Only the data from 3 year old gerbils indicated some correlation between CAP threshold and EP, although the scatter in the data was large. Overall, a linear regression analysis suggested that the variation of EP in 3 year old gerbils accounts for 31% of the variation in CAP thresholds (Schmiedt, 1983). The reduction of the mean EP was not associated with a mean loss of potassium concentration in the endolymph of old gerbils and the "effects of age are

+" between endolymph

**3.7 The endocochlear potential and pathology related to endolymph homeostasis** 

nerve fibre dendrites and a reduction of the size of spiral ganglion cells.

in single auditory nerve fibres in cat (Sewell, 1984).

**3.7.1 Age-dependent loss of the endocochlear potential** 

primarily on EP generation, and not on the chemical potential of Ke

**3.7.2 Histological changes in the stria vascularis and the spiral ligament** 

The stria vascularis (SV) plays a central role in the generation of the EP (Wangemann, 2006). Age-dependent changes in the microvasculature that might lead to ischemia and affect SV

and perilymph (Schmiedt, 1996).

function have been the focus of several studies (Gratton & Schulte, 1995; Gratton et al., 1996, 1997b; Sakaguchi et al., 1997a, 1997b; Thomopoulos et al., 1997). Gratton & Schulte (1995) described small regions at the apical and basal ends of the SV that were devoid of capillaries in gerbils as young as 5-10 months. With increasing age, loss of capillaries progressed from both ends towards the middle of the cochlea. Gerbils older than 33 months showed a normal pattern of strial vascularisation only in the mid-region of the cochlea. In the regions of capillary loss, strial atrophy was observed with missing marginal cells and "clumps of pigment" in remaining cells. Gratton et al. (1996) found a significant correlation between the proportion of the SV with normal vascularisation and the EP. However, due to the large inter-animal variability of both parameters, the correlation coefficient indicated that SV pathology explained only up to 37% of the EP variation.

In addition to loss of vascularisation and atrophy of the SV, different types of fibrocytes in the spiral ligament of old gerbils are also affected. Spicer & Schulte (2002) suggested that vacuolisation of type II fibrocytes in regions of old cochleae that show no strial atrophy can be regarded as an early event in the development of strial pathology. Regions with apoptotic or necrotic type II fibrocytes were associated with moderate degeneration of SV, while regions with a complete absence of type II fibrocytes showed advanced SV atrophy. Also, type IV and V fibrocytes showed vacuolisation in old gerbils, while type I fibrocytes did not. Thus, vacuolisation was found in Na,K-ATPase positive fibrocyte types II, IV and V, but not in negative type I fibrocytes. Unfortunately the hearing status was not known and could not be directly correlated with the degree of structural changes in these specimen. Based on their data, Spicer & Schulte (2002) put forward the hypothesis that, within the potassium recycling pathway, impaired secretion of potassium into the endolymph by strial marginal cells could reduce the flow of potassium towards the stria and lead to potassium accumulation and the development of vacuoles in Na,K-ATPase positive fibrocytes. They proposed that dysfunction of marginal cells is the first step leading to fibrocyte pathology and strial degeneration.
