Effects of furosemide applied chronically to the round window: a model of metabolic presbyacusis

Abstract

Hearing thresholds in elderly humans without a history of noise exposure commonly show a profile of a flat loss at low frequencies coupled with a loss that increases with frequency above approximately 2 kHz. This profile and the relatively robust distortion product otoacoustic emissions that are found in elderly subjects challenge the common belief that age-related hearing loss (presbyacusis) is based primarily on sensory-cell disorders. Here, we examine a model of presbyacusis wherein the endocochlear potential (EP) is reduced by means of furosemide applied chronically to one cochlea of a young gerbil. The model results in an EP that is reduced from 90 to approximately 60 mV, a value often seen in quiet-aged gerbils, with no concomitant loss of hair cells. Resulting measures of cochlear and neural function are quantitatively similar to those seen in aging gerbils and humans, e.g., a flat threshold loss at low frequencies with a high-frequency roll-off of approximately -8.4 dB/octave. The effect of the EP on neural thresholds can be parsimoniously explained by the known gain characteristics of the cochlear amplifier as a function of cochlear location: in the apex, amplification is limited to approximately 20 dB, whereas in the base, the gain can be as high as 60 dB. At high frequencies, amplification is directly proportional to the EP on an approximately 1 dB/mV basis. This model suggests that the primary factor in true age-related hearing loss is an energy-starved cochlear amplifier that results in a specific audiogram profile.

Fig. 1.

Neural (CAP) responses and DPOAEs in a gerbil infused with furosemide for 28 d. Top, CAP thresholds to tone pips as a function of frequency for the right ear (RE) treated with furosemide and left, untreated ear (LE). The curves plot the audibility curves of each ear. Control thresholds are within the normal range for the gerbil. EP values measured in the basal (T1; 16 kHz), middle (T2; 2 kHz), and upper (T3; 1 kHz) turns are shown for each cochlea. Note that the EP was shifted 40–50 mV in the apical turns, yet the corresponding neural shifts were only ∼15 dB. In the base, the EP shift was ∼70–80 mV and corresponded to a 60 dB shift of the neural response.Arrow marks the probe frequency used to obtain the tuning curve data. Middle, CAP tuning (solid line) and suppression (dashed lines) boundaries obtained with masking procedures from the furosemide-treated ear. The probe frequency was 8 kHz, at which neural thresholds were elevated by ∼25 dB. The tuning is still sharp, and the suppression boundaries indicate that two-tone suppression is still present, despite an EP reduced to 30 mV. Bottom, DPOAE amplitudes obtained in the same ears with low-level primaries fixed at 50 dB SPL (L₁ =L₂) and swept across frequency with a ratio off₁/f₂= 1.2. The resulting DPOAE amplitudes from the treated ear show only minor changes from control values, except at the highest frequencies.Dotted curve is acoustic noise floor.

Fig. 2.

Neural I/O functions plotting the CAP peak amplitude as a function of the intensity of acoustic probe tones at 2, 4, 8, and 16 kHz. Data are from the animal of Figure 1 and represent an average of 24 epochs at each intensity level. All furosemide-treated ears showed similar trends compared with control ears, i.e., reduced slopes with much-reduced maximum amplitudes.

Fig. 3.

Hair cell counts along the cochlear spiral. Frequency–distance map of the cochlear length is taken fromMüller (1996). Top, Counts obtained from the control cochlea show almost no loss. Blank region was caused by missed section during processing. Bottom, Hair cell loss in the treated ear was very minor and was within normal control bounds. These profiles are similar to the three other furosemide-treated cochleas processed for hair cell counts. There was no evidence that the furosemide at pump concentrations of between 1 and 10 mg/ml affected hair cell survival.

Fig. 4.

Neural threshold data from furosemide-treated and aged gerbils compared with their respective control groups. EP measured at the round window is shown for each curve.Top, CAP thresholds obtained from right ears of five gerbils after 7 d of 5 mg/ml furosemide treatment. Control means are from seven control ears; error bars are SEM.Bottom, Same as above but for a group of five aged ears. Controls are the same as the top panel. Note parallel shift of treated and aged ears at low frequencies coupled with an increasing loss at high frequencies.

Fig. 5.

Mean ± SEM DPOAE amplitudes from furosemide-treated and quiet-aged gerbils obtained in a similar manner to those in the bottom panel of Figure 1.Top, Mean DPOAE amplitudes from six control and 11 furosemide ears from gerbils. Middle, Mean DPOAEs from 10 young control ears and 38 gerbils aged between 36 and 45 months.Bottom, Comparison of mean DPOAE amplitudes from thetop and middle panels. Note the quantitative similarity of the amplitudes from the two groups. The DPOAE amplitudes are approximately flat across frequency and do not reflect typical neural threshold shifts shown in this figure and in Figures 1 and 7.

Fig. 6.

Scatter plots of CAP threshold shifts and EP shifts compared in the same cochlea at similar locations. CAP threshold shifts at 16 kHz are thus plotted with EP measures taken at turn 1 (T1) or the RW, whereas CAP shifts at 2 and 1 kHz are plotted with EP measures taken at turns 2 (T2) and 3 (T3), respectively. Thus, each pointrepresents a neural shift plotted against a corresponding EP shift in a given cochlea. Lines are not best-fits to data but are drawn either as an upper bound at 20 dB (dashed) or with a slope of 1 dB/mV (solid). Top, Data from 16 furosemide-treated animals; the contralateral, untreated cochlea served as individual control in each animal.Bottom, Data from 35 aged gerbils; control data were taken from a group of 10 young cochleas. The 16 kHz data in both groups correlate well with EP loss; however, the 1 and 2 kHz data shift asymptotically to 20 dB for EP reductions >20 mV. The similarity between the data strongly support the EP loss as being the defining factor in the presbyacusis of the quiet-aged gerbil.

Fig. 7.

Mean ± SEM CAP threshold shifts in aged and furosemide-treated gerbils compared with human audiograms. Hearing loss is relative to the respective control data for the gerbils.Top, Mean data obtained from three groups of aged gerbils are shown. An early cohort of 36-month-old gerbils (open squares), a second cohort tested 3 years after the first (open triangles), and a third cohort of very old gerbils tested at between ages 38 and 45 months (gray triangles). Mean data from 10 gerbils treated with 5 mg/ml furosemide for 7 d are also plotted (filled circles). Mean EP values obtained at the RW are shown for each group. Note the relatively flat loss at low frequencies coupled with an increasing loss at high frequencies. Dashed line is a best fit through the furosemide data at 4 kHz and below. Solid line is best fit through the furosemide data above 4 kHz and has a slope of −8.4 dB/octave. Break point between thedashed and solid lines is at 4.2 kHz. The increased loss at low frequencies for the 38-month-old gerbils most likely has origins in apical hair cell loss that is seen in these very old gerbils (see Results). Bottom, Audiograms from 123 elderly humans without a significant history of noise exposure (from Jerger et al., 1993). Dashed line is a best fit for female data at low frequencies. Solid line is drawn through the female data at high frequencies with the same slope as found from the furosemide model in top panel.Arrow indicates the break point in humans at 1.3 kHz.