Long-term treatment with aldosterone slows the progression of age-related hearing loss

Abstract

Age-related hearing loss (ARHL), clinically referred to as presbycusis, is one of the three most prevalent chronic medical conditions of our elderly, with the majority of persons over the age of 60 suffering from some degree of ARHL. The progressive loss of auditory sensitivity and perceptual capability results in significant declines in workplace productivity, quality of life, cognition and abilities to communicate effectively. Aldosterone is a mineralocorticoid hormone produced in the adrenal glands and plays a role in the maintenance of key ion pumps, including the Na-K(+)-Cl co-transporter 1 or NKCC1, which is involved in homeostatic maintenance of the endocochlear potential. Previously we reported that aldosterone (1 μM) increases NKCC1 protein expression in vitro and that this up-regulation of NKCC1 was not dose-dependent (dosing range from 1 nM to 100 μM). In the current study we measured behavioral and electrophysiological hearing function in middle-aged mice following long-term systemic treatment with aldosterone. We also confirmed that blood pressure remained stable during treatment and that NKCC1 protein expression was upregulated. Pre-pulse inhibition of the acoustic startle response was used as a functional measure of hearing, and the auditory brainstem response was used as an objective measure of peripheral sensitivity. Long-term treatment with aldosterone improved both behavioral and physiological measures of hearing (ABR thresholds). These results are the first to demonstrate a protective effect of aldosterone on age-related hearing loss and pave the way for translational drug development, using aldosterone as a key component to prevent or slow down the progression of ARHL.

Keywords: Aging; Aldosterone; Drug development; Hearing loss; Startle reflex.

Figure 1

General schematic of the study showing baseline testing, aldosterone administration, ABR assessment, blood pressure assessment, and all behavioral testing. Baseline testing was completed 2 weeks prior to the initiation of treatment via slow release, subcutaneous pellets. Behavioral testing commenced at 4 weeks and continued throughout study, while electrophysiological assessment was completed at baseline and 2 weeks after the last behavioral test.

Figure 2

Mean diastolic and systolic blood pressure measurements taken at baseline (middle-aged), at 2 months (ALD 60 day), and at 4 months (ALD 120 day) following aldosterone pellet implantation. No significant effect on blood pressure was found on diastolic or systolic blood pressure.

Figure 3

Mean startle input/output functions and comparison of maximum startle amplitudes for treated (1^st and 2^nd pellet) and untreated (baseline) mice. Panel A shows mean (SEM) startle amplitudes from the aldosterone treated group across the two test time points (2 and 4 months). Pane B and C show that no significant differences in startle amplitude to suprathreshold stimuli were found at any test time point between treated and control animals..

Figure 4

Noise burst pre-pulse inhibition at various pre-pulse intensities for treated (open squares) and untreated or control (filled squares) mice across the 4-month period. Panel A shows inhibition levels for a 20 dB pre-pulse, the 40 dB pre-pulse (Panel B) and 55 dB pre-pulse (Panel C). Significant increases in inhibition emerged in the treated group at the 6-week testing point, and continued throughout testing. A similar pattern was seen for the 55 dB pre-pulse shown in Panel C, with expected overall inhibition increases due to the increasing pre-pulse intensity. Significant difference in mean % PPI is indicated by the (*).

Figure 5

Example of PPI at 4 weeks of treatment for a pre-pulse having silent gap embedded 60 ms before the SES and gap durations which varied from 1 to 30 ms for treated (open squares) and untreated or control (filled squares) mice. Increases in gap duration results in a systematic increase in PPI. No significant effects of treatment were found for any gap duration at any test point.

Figure 6

The therapeutic effects of aldosterone treatment on auditory brainstem response metrics at 2 and 4 months following the initiation of treatment. (A) Wideband noise mean thresholds measured before treatment and at 2 and 4 months following treatment. Aldosterone treated mice showed stable thresholds, whereas control mice continued to display threshold increases characteristic of age-related hearing loss. (B) P1 amplitude shifts following 2 and 4 months of treatment show declines in the control group relative to the aldosterone-treated animals. (C) P4 amplitude shift following 2 and 4 months aldosterone treatment reveal significant preservation of amplitudes in the aldosterone-treated mice. Positive voltage shifts in B and C indicate greater ABR peak amplitudes. Error bars represent S.E.M., ANOVA results: *p<0.05; **p<0.01; ****p<0.001.

Figure 7

ALD inhibits age-related NKCC1 down regulation in the cochlear lateral wall. Lateral walls of cochlea from young adult (Y: 3–4 month), middle age (M: 22–24 month) and middle age following 4 months of ALD treatment (M- ALD). NKCC1 protein expressions were detected using western blots and the expressed levels are relative to β-actin expression, an equal loading control. The expression was determined quantitatively using densitometry (NIH Image J) as shown in the bar graph. The data are representative of the triplicates for each subject group. Results are means ±SD. One Way ANOVA indicated a significant main effect of treatment: F(2,6)=9.16, p=0.015; Bonferroni multiple comparison post-hoc test: young adult (Y) vs. middle age (M), t=3.81, df=2), p<0.05; middle age(M) vs. middle age ALD treatment (M, ALD), t=3.59, df=2, p<0.05 (*).