Inner hair cell ribbon synapse plasticity might be molecular basis of temporary hearing threshold shifts in mice

Recent studies have reported that noise exposure at relatively low intensities can cause temporary threshold shifts (TTS) in hearing. However, the mechanism underlying the TTS is still on debate. Here, we report that an acoustic stimulation (100 dB SPL, white noise) induced TTS in mice, with the maximal ABR threshold elevations seen on the 4(th) day after noise exposure. On the other hand, there were no significant morphological changes in the cochlea. Further, there were paralleled changes of pre-synaptic ribbons in both the number and postsynaptic density (PSDs) during this noise exposure. The numbers of presynaptic ribbon, postsynaptic density (PSDs), and colocalized puncta correlated with the shifts of ABR thresholds. Moreover, a complete recovery of ABR thresholds and synaptic puncta was seen on the 14(th) day after the noise stimulations. Thus, our study may indicate that noise exposure can cause a decline in cochlear ribbon synapses and result in consequent hearing loss. The reduction of synaptic puncta appears reversible and may contribute to hearing restoration in mice after noise exposure.

Individual daytime noise exposure in different microenvironments

BACKGROUND

Numerous studies showed that chronic noise exposure modeled through noise mapping is associated with adverse health effects. However, knowledge about real individual noise exposure, emitted by several sources, is limited.

OBJECTIVES

To explain the variation in individual daytime noise exposure regarding different microenvironments, activities and individual characteristics.

MATERIALS AND METHODS

In a repeated measures study in Augsburg, Germany (March 2007-December 2008), 109 individuals participated in 305 individual noise measurements with a mean duration of 5.5h. Whereabouts and activities were recorded in a diary. One-minute averages of A-weighted equivalent continuous sound pressure levels (Leq) were determined. We used mixed additive models to elucidate the variation of Leq by diary-based information, baseline characteristics and time-invariant variables like long-term noise exposure.

RESULTS

Overall noise levels were highly variable (median: 64 dB(A); range: 37-105 dB(A)). Highest noise levels were measured in traffic during bicycling (69 dB(A); 49-97 dB(A)) and lowest while resting at home (54 dB(A); 37-94 dB(A)). Nearly all diary-based information as well as physical activity, sex and age-group had significant influences on individual noise. In an additional analysis restricted to times spent at the residences, long-term noise exposure did not improve the model fit.

CONCLUSIONS

Individual exposures to day-time noise were moderate to high and showed high variations in different microenvironments except when being in traffic. Individual noise levels were greatly determined by personal activities but also seemed to depend on environmental noise levels.

Childhood sensorineural hearing loss: effects of combined exposure with aging or noise exposure later in life

The aim of the study was to examine childhood high-frequency sensorineural hearing loss (HF-SNHL) and the effects of combined exposure with aging or noise exposure on HF hearing thresholds in adulthood. Population-based cohort study of 30,003 adults (mean age 40 years) underwent an audiometry and completed a hearing questionnaire. At age 7-13 years, the same people had participated in a longitudinal school hearing investigation, in which 283 participants were diagnosed with HF-SNHL [PTA 3-8 kHz ≥ 25 dB HL (mean 45 dB HL), worse hearing ear], and 29,720 participants had normal hearing thresholds. The effect of childhood HF-SNHL on adult hearing threshold was significantly moderated by age. Age stratified analyses showed that the difference in HF hearing thresholds between adults with and without childhood HF-SNHL was 33 dB (95 % CI 31-34) in young adults (n = 173, aged 20-39 years) and 37 dB (95 % CI 34-39) in middle-aged adults (n = 110, aged 40-56 years). The combined exposure of childhood HF-SNHL and noise exposure showed a simple additive effect. It appears to be a super-additive effect of childhood-onset HF-SNHL and aging on adult hearing thresholds. An explanation might be that already damaged hair cells are more susceptible to age-related degeneration. To exclude possible birth cohort effects, the finding should be confirmed by a study with several audiometries in adulthood.

Structural changes in the adult rat auditory system induced by brief postnatal noise exposure

In previous studies (Grécová et al., Eur J Neurosci 29:1921-1930, 2009; Bures et al., Eur J Neurosci 32:155-164, 2010), we demonstrated that after an early postnatal short noise exposure (8 min 125 dB, day 14) changes in the frequency tuning curves as well as changes in the coding of sound intensity are present in the inferior colliculus (IC) of adult rats. In this study, we analyze on the basis of the Golgi-Cox method the morphology of neurons in the IC, the medial geniculate body (MGB) and the auditory cortex (AC) of 3-month-old Long-Evans rats exposed to identical noise at postnatal day 14 and compare the results to littermate controls. In rats exposed to noise as pups, the mean total length of the neuronal tree was found to be larger in the external cortex and the central nucleus of the IC and in the ventral division of the MGB. In addition, the numerical density of dendritic spines was decreased on the branches of neurons in the ventral division of the MGB in noise-exposed animals. In the AC, the mean total length of the apical dendritic segments of pyramidal neurons was significantly shorter in noise-exposed rats, however, only slight differences with respect to controls were observed in the length of basal dendrites of pyramidal cells as well as in the neuronal trees of AC non-pyramidal neurons. The numerical density of dendritic spines on the branches of pyramidal AC neurons was lower in exposed rats than in controls. These findings demonstrate that early postnatal short noise exposure can induce permanent changes in the development of neurons in the central auditory system, which apparently represent morphological correlates of functional plasticity.

Genetic variations in protocadherin 15 and their interactions with noise exposure associated with noise-induced hearing loss in Chinese population

OBJECTIVES

The purpose of this study was to examine the associations between genetic variations in the Protocadherin 15 gene (PCDH15) and the risk to noise induced hearing loss (NIHL) in a Chinese population.

METHODS

A case-control study was conducted with 476 noise-sensitive workers (NIHL) and 475 noise-resistant workers (normal) matched for gender, years of noise exposure, and intensity of noise exposure. 13 tag single-nucleotide polymorphisms in PCDH15 were genotyped using nanofluidic dynamic arrays on the Fluidigm platform. Multiple logistic regression was used to analyze the associations of genetic variations of PCDH15 with NIHL adjusted by age, smoking/drinking status, and cumulative noise exposure and their interactions with noise exposure.

RESULTS

The allele frequency and genotypes of rs1104085 were significantly associated with the risk of NIHL(P=0.009 and 0.005 respectively ). The subjects carrying variant alleles (CT or CC) of rs11004085 had a decreased the risk for NIHL (adjusted odds ratio=0.587, 95% confidence interval 0.409-0.842) compared with subjects who had the wild-type (TT) homozygotes. The interactions were found between the SNPs of rs1100085, rs10825122, rs1930146, rs2384437, rs4540756, and rs2384375 and noise exposure.

CONCLUSIONS

Genetic variations of PCDH15 and their interactions with occupational noise exposure are associated with genetic susceptibility to NIHL and modify the risk of noise induced hearing loss.

Indoor noise exposure at home: a field study in the family of urban schoolchildren

This article aims at evaluating indoor noise levels at home and investigating the factors that may influence their variability. An 8-day noise measurement campaign was conducted in the homes of 44 schoolchildren attending the public primary schools of Besançon (France). The presence of the inhabitants in the dwelling and the noisy events occurring indoors and outdoors was daily collected using a time-location-activity diary (TLAD); 902 time periods were analyzed. The indoor noise level increased significantly with the outdoor noise level, along with the duration of the presence or level of activity of the inhabitants at home. However, this effect may vary according to the period of day and the day of the week. Moreover, a significant part of the day and evening indoor noise level variability was explained when considering the TLAD variables: 46% and 45% in the bedroom, 54% and 39% in the main room, respectively. Our results highlight the complexity of the indoor environment in the dwellings of children living in an urban area. Combining the inhabitant presence and indoor noise source descriptors with outdoor noise levels and other dwelling or inhabitant characteristics could improve large-scale epidemiological studies. However, additional efforts are still needed, particularly during the night period.