Noise-induced and age-related functional and structural cochlear alterations in Igf1-/+ mice

Abstract

[Background]: The physiological age-related decrease in circulating IGF-I levels have been related to cognitive and brain alterations. Therefore, IGF-I is considered a neuroprotective agent. Human IGF-I deficiency is a rare disease associated with poor growth rates, mental retardation and syndromic hearing loss (OMIM608747). Igf1-/- mice are dwarfs with poor survival rates and congenital profound deafness, which worsens with ageing. Our objective was to compare the susceptibility of Igf1+/- and Igf1+/+ mice to damage by using exposure to excessive noise at different ages. [Methods]: Animals. Igf1+/- and Igf1+/+ mice were maintained in MF1OlaHsd*129/Sv genetic bakground. Hearing. Auditory Brainstem Responses (ABR) was performed with a Tucker Davis Technologies workstation before (pre) and 3, 14 and 28 days after noise exposure. Noise exposure. Mice were exposed awake in a sound reverberant chamber to a violet swept sine noise enriched in high frequencies as reported5 at 105 dB SPL for 30 minutes. Cochlear morphology and inmunohistochemistry. Cresyl-violet or hematoxilin-eosin staining of 10 mm cochlear paraffin and frozen sections. Serial frozen sections (10 mm) were collected to detect neurofilament and synaptophysin. Hair-cell quantification. Decalcified cochleae were mid-sectioned exposing ~80% of the whole extent of the basilar membrane. The organ of Corti (OC) was dissected and phalloidin-stained, and its total length was divided into equidistant 5% sectors as reported6 using stereological software (CAST®). The number of inner (IHC) and outer (OHC) hair cells in systematically randomly sampled areas were determined, and cell density (cells/1000 mm2) was estimated for each sector. RT-qPCR. RNA expression levels of cochlear genes involved in synaptogenesis, inflammation and cell cycle were analyzed by real time quantitative PCR using probes from TaqMan®. Serum determinations. IGF-I levels were determined using a specific ELISA assay (OCTEIA Rat/Mouse IGF-I kit, IDS Ltd.). Statistical analysis. A mixed model procedure with ANOVA or Student t-test was carried out with SPSS v19.0 software or with RealTime StatMiner® software for RT-qPCR data. Post hoc multiple comparisons included Bonferroni and Tamhane tests. Data are expressed as mean±SEM. The results were considered significant at p<0.05. [Results]: Igf1+/+ and Igf1+/- mice show an age-dependent decrease in IGF-I serum levels, especially from 6 months of age on, which correlates with the increase in ABR thresholds. Noise-exposure experiments with 1 and 3 months-old mice did not reveal differences between genotypes, both genotypes were equally sensible to NIHL. However, 6 month-old Igf1+/- presented greater susceptibility to noise damage, with higher threshold shifts and a poorer recovery compared to noise-exposed Igf1+/+ mice. The cellular and molecular mechanisms underlying susceptibility to damage will be discussed. [Conclusion]: These data suggest that IGF-I moderate deficit enhances otic sensibility to damage. Therefore, IGF-I-based therapies could contribute to prevent or ameliorate age-related and noise-induced hearing loss.