The effect of noise trauma following training exposures in the mouse

Intrauterine exposure to diesel exhaust diminishes adult ovarian reserve

OBJECTIVE

To analyze ovarian and uterine morphologic changes resulting from intrauterine and postnatal exposure to diesel exhaust.

DESIGN

Crossover study. Experimental groups: intrauterine and postnatal clean air exposure; intrauterine exposure to diesel only; postnatal exposure to diesel only; and intrauterine and postnatal exposure to diesel.

SETTING

Laboratory of Experimental Air Pollution.

ANIMAL(S)

Swiss mice.

INTERVENTION(S)

Mice exposed to diesel exhaust with doses that correspond to the daily average PM₂.₅ levels (fine particles in the ambient air 2.5 μm or less in size) reported by the World Health Organization.

MAIN OUTCOME MEASURE(S)

Morphometric analyses of the ovaries and uterus were performed to define the relative area occupied by follicles, corpus luteum, and stroma and the proportionate area of glands, epithelial layer, and stroma within the uterine endometrium.

RESULT(S)

A significant reduction in the proportion of primordial follicles was observed in intrauterine-exposed animals, those exposed during the postnatal period, and in animals exposed during both phases. Primary follicle proportion was reduced in animals exposed during pregnancy. No significant changes were detected in uterine morphology.

CONCLUSION(S)

Intrauterine exposure to acceptable levels of diesel exhaust compromises the reproductive potential of female mice, diminishing ovarian reserve when sexual maturity is achieved. This effect could increase the risk of premature menopause. The findings raise concern about current environmental guidelines for diesel exposure, warranting more careful examination of this issue in humans by regulatory authorities.

Vezatin, an integral membrane protein of adherens junctions, is required for the sound resilience of cochlear hair cells

Loud sound exposure is a significant cause of hearing loss worldwide. We asked whether a lack of vezatin, an ubiquitous adherens junction protein, could result in noise-induced hearing loss. Conditional mutant mice bearing non-functional vezatin alleles only in the sensory cells of the inner ear (hair cells) indeed exhibited irreversible hearing loss after only one minute exposure to a 105 dB broadband sound. In addition, mutant mice spontaneously underwent late onset progressive hearing loss and vestibular dysfunction related to substantial hair cell death. We establish that vezatin is an integral membrane protein with two adjacent transmembrane domains, and cytoplasmic N- and C-terminal regions. Late recruitment of vezatin at junctions between MDCKII cells indicates that the protein does not play a role in the formation of junctions, but rather participates in their stability. Moreover, we show that vezatin directly interacts with radixin in its actin-binding conformation. Accordingly, we provide evidence that vezatin associates with actin filaments at cell–cell junctions. Our results emphasize the overlooked role of the junctions between hair cells and their supporting cells in the auditory epithelium resilience to sound trauma.

Changes in Guinea pig cochlear hair cells after sound conditioning and noise exposure

Sound conditioning has reduced noise-induced hearing loss in experimental mammalian animals and in clinical observation. Forty guinea pigs were grouped as: A, control; B, conditioning noise exposure group; C, high level noise exposure group; and D, conditioning noise exposure followed by a high level noise exposure group. Auditory brainstem response thresholds were measured. The cochlear sensory epithelia surface was observed microscopically. Calmodulin, F-actin and heat shock protein 70 (HSP70) in hair cells were immunohistochemistrically stained. The intracellular free calcium was stained for confocal microscopy. The ABR threshold shift after noise exposure was higher in group C than D, and showed a quicker and better recovery in group D than C. Stereocilia loss and the disarrangement of outer hair cells were observed, with the greatest changes seen in group C, followed by groups D and B. The most intensive immunohistochemical intracellular expressions of calmodulin, F-actin, and HSP70 were found in group D, followed by groups C, B and A. The highest intensity of the fluorescent intracellular free Ca2+ staining in the isolated outer hair cells was observed in group C. The ABR and morphological studies confirmed the protective effect from noise trauma of sound conditioning. The protective mechanism of hair cells during sound conditioning was enforced through the increase of cellular cytoskeleton proteins and through the relieving of intracellular calcium overloading caused by the traumatic noise.

The influence of genetic variation in oxidative stress genes on human noise susceptibility

Noise induced hearing loss (NIHL) is a complex disease caused by an interaction between genetic and environmental factors. Damage in the cochlea as a result of noise exposure appears to be mediated by reactive oxygen species (ROS). To investigate whether genetic variation in the human protective antioxidant system is associated with high or low susceptibility to NIHL, genetic polymorphisms derived from genes involved in the oxidative stress response were analysed in the 10% most susceptible and 10% most resistant extremes of 1200 Swedish noise-exposed workers. The genetic polymorphisms included 2 deletion polymorphisms for the GSTM1 and GSTT1 gene, and 14 SNPs derived from the CAT, SOD, GPX, GSR and GSTP1 genes. No significant differences were found between susceptible and resistant groups, providing no support for a major role of genetic variation of antioxidant enzymes in the susceptibility to NIHL.

Temporary and permanent noise-induced changes in distortion product otoacoustic emissions in CBA/CaJ mice

A number of studies have shown that the ear can be protected from sound over-exposure, either by activating the cochlear efferent system, or by sound 'conditioning' in which the role of the efferent system is less certain. To study more definitively the molecular basis of deliberately induced cochlear protection from excessive sounds, it is advantageous to determine, for an inbred mouse strain, a range of noise exposure parameters that effectively alter cochlear function. As an initial step towards this goal, young CBA/CaJ mice were exposed to a 105-dB SPL octave-band noise (OBN), centered at 10 kHz, for various lengths of time consisting of 10 min, or 0.5, 1, 3, or 6 h. Distortion product otoacoustic emissions (DPOAEs) at the 2f1-f2 frequency, in response to equilevel primary tones of low to moderate levels, were used to quantify the damaging effects of these sound over-exposures on cochlear function. In addition, staining for acetylcholinesterase (AChE) activity to assess for noise-induced changes in the pattern of efferent-nerve innervation to the cochlea was also performed in a subset of mice that were exposed to the longest-lasting 6-h OBN. The 10-min OBN resulted in only temporary reductions in DPOAE levels, which recovered to pre-exposure values within 5 days. Increasing the exposure to 0.5 h resulted in permanent DPOAE losses that, for low primary-tone levels, were still present at 31 days post-exposure. Additionally, the 1-h and longer exposures caused permanent reductions in DPOAEs for all test levels, which were measurable at 31 days following exposure. Light-microscopic observations restricted to the 11-18-kHz frequency region of the organ of Corti, for a subset of mice exposed to the 6-h OBN, uncovered a significant loss of outer hair cells (OHCs). However, despite the OHC loss in this region, the AChE activity associated with the related pattern of efferent innervation remained largely intact.

Unilateral hearing losses alter loud sound-induced temporary threshold shifts and efferent effects in the normal-hearing ear

In animals with bilaterally normal hearing, olivocochlear pathways can protect the cochlea from the temporary shifts in hearing sensitivity (temporary threshold shifts; TTSs) caused by short-duration intense loud sounds. The crossed olivocochlear pathway provides protection during binaural loud sound, and uncrossed pathways protect when monaural or binaural loud sounds occur in noise backgrounds. Here I demonstrate that when there is a chronic unilateral hearing loss, effects of loud sounds, and efferent effects on loud sound, in the normal-hearing ear differ markedly from normal. Three categories of test animals with unilateral hearing loss were tested for effects at the normal-hearing ear. In all categories a monaural loud tone to the normal-hearing ear produced lower-than-normal TTSs, apparently because of a tonic re-setting of that ear's susceptibility to loud sound. Second, in the two test categories in which the hearing-loss ear was only partly damaged, binaural loud sound exacerbated TTSs in the normal-hearing ear because it caused threshold shifts that were a combination of "pure" TTSs and uncrossed efferent suppression of cochlear sensitivity. (In normal cats, this binaural tone results in crossed olivocochlear protection that reduces TTS.) Binaural loud sound did not produce such uncrossed efferent effects in the test category in which the nontest ear had suffered total hearing loss, suggesting that this uncrossed efferent effect required binaural input to the CNS. It is noteworthy that, in the absence of this uncrossed efferent suppression, the pure loud sound-alone induced TTSs after binaural exposure were low. Thus in the absence of any efferent effect, the normal-hearing cochlea had a reduced susceptibility to loud tone-induced damage. Finally, the results suggest that, with respect to cochlear actions at high sound levels, uncrossed and crossed efferent pathways may exert different effects at the one type of receptor cell.

Vulnerability to noise-induced hearing loss in 'middle-aged' and young adult mice: a dose-response approach in CBA, C57BL, and BALB inbred strains

Vulnerability of the cochlea to noise-induced permanent threshold shifts (NIPTS) was examined in young adult (1-2 months) and 'middle-aged' (5-7 months) CBA/CaJ, C57BL/6J, and BALB/cJ inbred mice. For each age and strain, a dose-response paradigm was applied, whereby groups of up to 12 animals were exposed to intense broadband noise (110 dB SPL) for varying durations. Exposure durations reliably associated with <10% and >90% probability of a criterion amount of NIPTS (determined 2 weeks post-exposure) were identified, and the minimum NIPTS exposure and the slope of the dose-response relation were then derived by numerical modeling. For all three strains, young adult mice were more susceptible to NIPTS than older adults; That is, a shorter exposure was able to cause NIPTS in the younger mice. Strain comparisons revealed that C57 mice were more susceptible than CBAs in the older age group only. At both ages examined, however, BALB mice were most susceptible to NIPTS. When animals with a similar amount of NIPTS were compared, outer hair cell loss in the cochlear base was more widespread in the younger animals. BALB mice appear particularly susceptible to noise-induced outer hair cell loss throughout life. Our data suggest that the mechanism or site of noise injury differs between young adults and older adults, and may depend on genetic background. The finding that both BALB and C57 mice, which show pronounced age-related hearing loss, are also especially vulnerable to noise supports the notion that genes associated with age-related hearing loss often act by rendering the cochlea susceptible to insults.

Sound conditioning reduces noise-induced permanent threshold shift in mice

The phenomenon of conditioning-related protection, whereby prior exposure to moderate-level, non-traumatic, sound protects the ear from subsequent traumatic exposure, has been documented in a number of mammalian species. To probe the molecular mechanisms underlying this effect, the mouse would be a useful model; however, a previous study reported no conditioning effects in this species (Fowler et al. , 1995). In our study, mice (CBA/CaJ) were exposed to a traumatic octave-band noise (8-16 kHz at 100 dB SPL for 2 h) with, or without, prior exposure to a sound-conditioning protocol consisting of exposure to the same noise band at lower sound pressure levels. Two conditioning protocols were investigated: one (81 dB SPL for 1 week) was analogous to those used in other conditioning studies in mammals; the second was significantly shorter (89 dB SPL for 15 min). Noise-induced permanent threshold shift (PTS) was assessed in a terminal experiment, after the traumatic exposure, via compound action potentials. Neither conditioning protocol elevated threshold, indeed both protocols increased amplitudes of distortion product otoacoustic emissions when animals were conditioned but not traumatized. Both conditioning exposures significantly reduced PTS from the subsequent traumatic exposure, compared to groups exposed without prior conditioning. Protective effects of 15-min conditioning were maximal when the condition-trauma interval was 24 h; protection disappeared when the traumatic exposure was presented 48 h after conditioning. These data are consistent with the view that protein synthesis is required for expression of the protective effect. The enhancement of distortion products in the condition-only state suggests that conditioning changes outer hair cell function.

Acoustic injury in mice: 129/SvEv is exceptionally resistant to noise-induced hearing loss

129/SvEv is an inbred mouse strain popular for use in genetic knockout studies. Here, we compare normal auditory function and vulnerability to acoustic injury in wild-type mice of the 129/SvEv vs. CBA/CaJ strains. Compound action potentials (CAPs) and distortion product otoacoustic emissions (DPOAEs) showed slightly higher thresholds for 129/SvEv re CBA/CaJ, especially at frequencies >20 kHz. Middle-ear motion (i.e. umbo velocity) was similar in the two strains; although frequencies >20 kHz could not be evaluated. Permanent threshold shift (PTS) and hair cell losses, measured 1 week after high-intensity exposure to an 8-16 kHz noise band, were smaller in129/SvEv at all exposure levels and durations from 97 dB SPLx2 h to 106 dB SPLx8 h. Furthermore, PTS growth with increasing exposure energy was slower in 129/SvEv (<2 dB/dB) than CBA/CaJ (9 dB/dB). These data suggest that the vulnerability differences lie in the inner ear, not the middle ear. Several 129/Sv substrains show age-related hearing loss (AHL): 129/SvEv has not yet been evaluated (Zheng, Q.Y., Johnson, K. R., Erway, L.C., 1999. Assessment of hearing in 80 inbred strains of mice by ABR threshold analyses. Hear. Res. 130, 94-107). Thus, although other strains with AHL, e.g. C57Bl/6J, show increased vulnerability to noise-induced hearing loss (NIHL), pairing of AHL and NIHL vulnerabilities may not be obligatory.

Heat stress and protection from permanent acoustic injury in mice

The inner ear can be permanently damaged by overexposure to high-level noise; however, damage can be decreased by previous exposure to moderate level, nontraumatic noise (). The mechanism of this "protective" effect is unclear, but a role for heat shock proteins has been suggested. The aim of the present study was to directly test protective effects of heat stress in the ear. For physiological experiments, CBA/CaJ mice were exposed to an intense octave band of noise (8-16 kHz) at 100 dB SPL for 2 hr, either with or without previous whole-body heat stress (rectal temperature to 41. 5 degrees C for 15 min). The interval between heat stress and sound exposure varied in different groups from 6 to 96 hr. One week later, inner ear function was assessed in each animal via comparison of compound action potential thresholds to mean values from unexposed controls. Permanent threshold shifts (PTSs) were approximately 40 dB in the group sound-exposed without previous heat stress. Heat-stressed animals were protected from acoustic injury: mean PTS in the group with 6 hr heat-stress-trauma interval was reduced to approximately 10 dB. This heat stress protection disappeared when the treatment-trauma interval surpassed 24 hr. A parallel set of quantitative PCR experiments measured heat-shock protein mRNA in the cochlea and showed 100- to 200-fold increase over control 30 min after heat treatment, with levels returning to baseline at 6 hr after treatment. Results are consistent with the idea that upregulation of heat shock proteins protects the ear from acoustic injury.

Sound-induced priming of the chinchilla auditory system

Exposure of the auditory system to either continuous or interrupted nontraumatic noises, often collectively referred to as priming exposures, has been shown, in a number of experimental paradigms, to reduce the susceptibility of the auditory system to noise-induced hearing and sensory cell loss from a subsequent traumatic exposure. Using auditory evoked potentials to obtain pure-tone thresholds and cochleograms to quantify sensory cell losses, the issue of priming-induced protective effects was examined in the chinchilla. Priming was accomplished with either a continuous noise or with a continuous noise followed by an interrupted noise. Trauma was induced by exposure to high-level impacts over a 5-day period that resulted in an asymptotic threshold shift. A comparison of the two groups of primed subjects with an unprimed control group showed that there were some statistically significant reductions in the asymptotic response of the primed groups to the traumatic exposure but no differences in permanent changes in thresholds among the three groups 30 days following the traumatic exposure. There were, however, some statistically significant, frequency-specific, reductions in outer hair cell loss in the primed groups. When conditioning was followed by the interrupted exposure that produced a threshold shift toughening effect, the conditioning protocol had no effect on the response of subjects to the interrupted exposure. There were also no differences in thresholds or sensory cell loss between the two primed groups 30 days post-trauma. Priming protocols may have different effects on the development of noise-induced trauma that are dependent on the nature of the traumatic stimulus, that is, long-term high-level impact noise exposure versus acute continuous noise exposure.

Noise-induced hearing loss in the noise-toughened auditory system

The auditory system, toughened by an interrupted noise exposure, has been shown in several reports to be less affected by (or protected from) a subsequent high-level noise exposure. Exposure to 115 dB peak SPL, 1 kHz narrow band (400 Hz) transients presented l/s, 6 h/day, to four groups of chinchillas produced a 10-28 dB toughening effect across the 0.5-8.0 kHz test frequency range. Following either a 30 day or an 18 h recovery period the animals were exposed to the same impulses but presented at 121 or 127 dB peak SPL for five uninterrupted days, thus producing an asymptotic threshold shift (ATS) condition. Comparisons between toughened and untoughened control subjects showed: (1) During the 121 dB exposure there was a statistically significant reduction of 10-25 dB in ATS across the entire test frequency range. Thirty days following the 121 dB exposure there were no significant differences in the postexposure permanent effects on thresholds and sensory cell loss. (2) During the 127 dB exposure only the group with the 30 day interval between the toughening and traumatic exposures showed a small (approximately 10 dB), statistically significant, frequency-specific (8 kHz), reduction in ATS. Thirty days following the 127 dB exposure a statistically significant protective effect on threshold was measured only at 16.0 kHz. However, both toughened groups showed less inner hair cell loss at and above 1.0 kHz, while only the group with the 18 h interval between the toughening and traumatic exposures showed less outer hair cell loss at and above 1.0 kHz. There were no systematic differences in the response of the toughened animals that could be attributed to the 30 day or 18 h post-toughening interval.

Acquired resistance to acoustic trauma by sound conditioning is primarily mediated by changes restricted to the cochlea, not by systemic responses

Hearing loss caused by intense sound exposure can be significantly reduced by pre-exposing subjects to moderate-level acoustic stimuli. This phenomenon occurs in a variety of mammals. We investigated whether sound conditioning provides acquired resistance to acoustic trauma through local mechanisms selectively in the conditioned ears or if systemic mechanisms are involved that would yield contralateral protection in unconditioned ears. Guinea pigs (group I) in which one external ear canal was occluded were exposed to conditioning sound (2-20 kHz, 85 dB SPL, 5 h/day, 10 days). After removing the occlusion, the animals were then subjected bilaterally to intense noise (2-20 kHz, 110 dB SPL, 5 h) 5 days after the last conditioning exposure. Animals without ear canal occlusion were also exposed to the intense sound without conditioning (group II) or following the same conditioning exposure (group III). Intense sound exposure caused significantly greater permanent ABR threshold shifts at all frequencies tested (4, 8, 12, 16 and 20 kHz) in group II than in group III. In group I, the occluded ears showed significantly greater threshold shifts at all frequencies compared to the unoccluded ears. The threshold shifts in the occluded ears in group I were identical to those observed in group II; and the shifts in unoccluded ears in group I were identical to those in group III. Protective effects provided by sound conditioning were almost the same in group III and in the unoccluded ears in group I. The extent of hair cell damage supported the physiological findings. These results indicate that acquired resistance to acoustic trauma provided by sound conditioning is restricted to the cochlea exposed to conditioning sound, suggesting that conditioning protection is mediated primarily by the changes that occur locally within the conditioned cochlea. This animal model, with unilateral external ear canal occlusion during sound conditioning, is useful for studies of the mechanisms of conditioning protection.

The medial cochlear efferent system does not appear to contribute to the development of acquired resistance to acoustic trauma

Noise-induced hearing loss (NIHL) was compared between sound conditioned and unconditioned guinea pigs, in which the left ear in both groups had been perfused with strychnine. Animals in the conditioned group were subjected to moderate sound (85 dB SPL broadband, 5 h/day, 10 days) and then exposed to intense sound (110 dB SPL broadband, 5 h). Unconditioned animals were exposed only to the intense sound. Following intense sound exposure, strychnine-treated ears showed greater NIHL than untreated ears in both unconditioned and conditioned animals, demonstrating the role of the medial efferents to reduce NIHL. Conditioned animals, however, showed smaller hearing loss and cochlear damage in both strychnine-treated and untreated ears compared to unconditioned animals; the protective effects given by conditioning were equivalent between the strychnine-treated and untreated ears. These results suggest that, although the medial efferent system acts to attenuate NIHL, it may not be necessary for the acquired resistance to NIHL provided by conditioning.

Conditioning the auditory system with continuous vs. interrupted noise of equal acoustic energy: is either exposure more protective?

The purpose of this study was to test the hypothesis that differences exist in the amount of protection provided by prior sound conditioning with continuous vs. interrupted, moderate-level noise. Differences were determined by monitoring the changes that occurred in cubic (2f1-f2) distortion product otoacoustic emission (DPOAE) amplitude growth functions subsequent to a traumatizing noise exposure (105 dB SPL, 1.0-2.0 kHz octave band noise presented 24 h per day for 3 days) in guinea pigs which had been conditioned with either continuous (89 dB SPL, 1.0-2.0 kHz octave band noise presented 24 h per day for 11 days) or interrupted noise (95 dB SPL, 1.0-2.0 kHz octave band noise presented on a 6-h 'on'/18-h 'off' schedule for 11 days) of equal acoustic energy. Results suggest that there are significant differences in the degree of protection provided by prior sound conditioning with the continuous and interrupted schedules of moderate-level noise used in this study. Specifically, the interrupted conditioning protocol afforded some degree of protection against the damaging effects of the traumatizing noise exposure, limited to frequencies above the noise exposure band. Conversely, there was a lack of any consistent and sizable protective effect found across the entire test frequency range for the continuous sound conditioning protocol.

The "toughening" phenomenon in rat's auditory organ

In audiological "toughening" or "conditioning" phenomenon prior exposure to moderate noise reduces the extent of hearing deterioration caused by the subsequent exposure to traumatic test noise known to cause inner ear damage. "Toughening" has been demonstrated in many mammalian laboratory animals such as guinea pig and chinchilla but not in rat or mouse. Our aim was to study the occurrence of this phenomenon in the rat. Ninety-one white male Wistar rats were divided into four groups: unexposed control group (U, n = 10), "conditioning" only (C, n = 32), "conditioning" plus test noise (C + T, n = 36) and test noise only (T, n = 13). Groups C and C + T were "conditioned" for 10 hours with 4.0 kHz OBN between 55 and 95 dB sound pressure levels (SPLs). After 10 hours rest groups C + T and T were exposed to the same noise at 105 dB SPL for 13 hours. The hearing thresholds were determined by auditory brainstem response audiometry (ABR) either immediately after or 3 weeks after the exposures. After that the animals were sacrificed. The cochleas were removed and perilymphatically fixed and further processed for quantitative cytocochleograms. Both the temporary (TTS) and the permanent threshold shifts (PTS) were smaller in animals which had been "conditioned" prior exposure to traumatic noise. Yet only 95 dB SPL "conditioning" gave statistically significant difference (p < 0.05) in PTS. From our results we conclude that "conditioning" effect seems to be present also in the rat. However to confirm this, further experiments are needed. The mechanisms behind "conditioning" are still unknown and also to clarify them, further efforts are needed.