Confocal microscopic analysis of the chinchilla organ of Corti following exposure to high-level impact noise.

To better understand the nature of mechanical changes following exposure to high-level impact noise, normal cochleas and cochleas from chinchillas exposed to either 125 or 131 dB SPL noise were stained with phalloidin for F-actin and examined using confocal microscopy. As seen in previous experiments, 125 dB exposures produced much more variable results than 131 dB exposures. Some cochleas were relatively unscathed by the exposure, whereas others showed damage to outer hair cells (OHCs) immediately after the exposure that included gross distortions of cell bodies and reduced F-actin in cuticular plates. Twenty-four hours later, there was also disorientation of actin filaments in supporting cells. After 30 days, Deiters cells were disarrayed and cups were separated from OHC neural poles. Exposure to noise at a level of 131 dB SPL produced less variable results than 125 dB exposure, and damage was generally more widespread and severe.

Quantitative measures of hair cell loss in CBA and C57BL/6 mice throughout their life spans.

The CBA mouse shows little evidence of hearing loss until late in life, whereas the C57BL/6 strain develops a severe and progressive, high-frequency sensorineural hearing loss beginning around 3-6 months of age. These functional differences have been linked to genetic differences in the amount of hair cell loss as a function of age; however, a precise quantitative description of the sensory cell loss is unavailable. The present study provides mean values of inner hair cell (IHC) and outer hair cell (OHC) loss for CBA and C57BL/6 mice at 1, 3, 8, 18, and 26 months of age. CBA mice showed little evidence of hair cell loss until 18 months of age. At 26 months of age, OHC losses in the apex and base of the cochlea were approximately 65% and 50%, respectively, and IHC losses were approximately 25% and 35%. By contrast, C57BL/6 mice showed approximately a 75% OHC and a 55% IHC loss in the base of the cochlea at 3 months of age. OHC and IHC losses increased rapidly with age along a base-to-apex gradient. By 26 months of age, more than 80% of the OHCs were missing throughout the entire cochlea; however, IHC losses ranged from 100% near the base of the cochlea to approximately 20% in the apex.

Synaptic loss in the central nucleus of the inferior colliculus correlates with sensorineural hearing loss in the C57BL/6 mouse model of presbycusis.

Between 3 and 25 months of age, light and electron microscopic features of principal neurons in the central nucleus of the inferior colliculus of the C57BL/6 mouse were quantitated. This mouse strain has a genetic defect producing progressive sensorineural hearing loss which starts during young adulthood (2 months of age) with high-frequency sounds. During the second year of life, hearing is severely impaired, progressively involving all frequencies. The hearing loss was documented in the present study by auditory brainstem recordings of the mice at various ages. The cochleas from many of the same animals showed massive loss of both inner and outer hair cells beginning at the base (high-frequency region) and progressing with age along the entire length to the apex (low-frequency region). In the inferior colliculi, there was a significant decrease in the size of principal neurons in the central nucleus. There was a dramatic decrease in the number of synapses of all morphologic types on principal neuronal somas. The percentage of somatic membrane covered by synapses decreased by 67%. A ventral (high frequency) to dorsal (low frequency) gradient of synaptic loss could not be identified within the central nucleus. These synaptic changes may be related to the equally dramatic physiologic changes which have been noted in the central nucleus of the inferior colliculus, in which response properties of neurons normally sensitive to high-frequency sounds become more sensitive to low-frequency sounds. The synaptic loss noted in this study may be due to more than the loss of primary afferent pathways. It may represent alterations of the complex synaptic circuitry related to the central deficits of presbycusis.

Effect of high-frequency interrupted noise exposures on evoked-potential thresholds, distortion-product otoacoustic emissions, and outer hair cell loss.

The effect of high-frequency interrupted noise exposures on evoked potential (EP) thresholds, distortion-product otoacoustic emissions (DPOAEs), and status of the outer hair cells was studied with the aim of understanding the correspondence among the three measures. Animal subjects were exposed to an octave band noise centered at 4 kHz at 85 dB SPL for 6 hr/day for 10 days. EP and DPOAE recordings were made before the exposure and on days 1, 2, 4, 6, 8, and 10 of exposure. A final set of measurements were made 5 days after the last exposure, following which the animals were sacrificed and their cochleas were examined using scanning electron microscopy. Both EPs and DPOAEs showed a worsening of auditory function after the first exposure and then showed a progressive recovery toward baseline. However, there was no consistent relationship between changes in EP thresholds and changes in DPOAEs nor were there any systematic changes in outer hair cells that corresponded with the changes in DPOAEs. Furthermore, EP thresholds often revealed considerable deficits in function while DPOAEs were normal.

Elevation of auditory thresholds by spontaneous cochlear oscillations.

The inner ear sometimes acts as a robust sound generator, continuously broadcasting sounds (spontaneous otoacoustic emissions) which can be intense enough to be heard by other individuals standing nearby. Paradoxically, most individuals are unaware of the sounds generated within their ears. Two hypotheses could explain this paradox: (1) the spontaneous emissions may not be transmitted to the central nervous system; or (2) the spontaneous emission produces a continuous, high rate of neural activity, which, like the natural pattern of spontaneous activity, is ignored by the central nervous system. Here we demonstrate that high-intensity spontaneous otoacoustic emission can vigorously activate auditory nerve fibres in mammals (Chinchilla laniger). This 'internal biological noise' creates a 'line busy' signal that significantly degrades a neuron's ability to respond to sound and results in a hearing loss completely different from that caused by damage to sensory cells.

Localization of F-actin and fodrin along the organ of Corti in the chinchilla.

The distribution of the two cytoskeletal proteins, filamentous actin (F-actin) and fodrin, was investigated along the organ of Corti of the chinchilla using laser scanning confocal fluorescence microscopy. High intensity labeling of F-actin was seen in outer and inner hair cells, including the stereocilia. High intensity staining was also seen for fodrin in outer and inner hair cells, but not in their stereocilia. Staining intensity of both proteins along the lateral cell wall of the outer hair cells appeared to be greater in the middle and basal cochlear turns than in the apical turn. Pillars and Deiters cells also exhibited high intensity labeling of F-actin. The lack of significant differences in the distribution of fodrin between outer and inner hair cells makes the role of this protein in the active processes still unclear. Comparison of the distribution of F-actin and fodrin in the chinchilla with those reported in the guinea pigs suggest possible species differences.

The relationship among distortion-product otoacoustic emissions, evoked potential thresholds, and outer hair cells following interrupted noise exposures.

Distortion-product otoacoustic emissions (DPOAEs) are gaining popularity as indicators of the status of the cochlea. The efficacy of DPOAEs as indicators of changes in thresholds and the status of outer hair cells (OHCs) were examined using an animal model. Monaural chinchillas were exposed to an octave band noise (OBN) centered at 0.5 kHz at 95 dB SPL for 6 hr/day for 10 days. DPOAEs and evoked potential thresholds were recorded before, during, and after the exposures. The animals were sacrificed 5 days after the last exposure, and the status of OHCs was assessed using scanning electron microscopy. Results indicate that both evoked potential thresholds and DPOAEs effectively track the temporary changes associated with interrupted noise exposures. However, DPOAEs often recovered to their baseline even when there was a threshold shift of > 25 dB. Furthermore, at 5 days postexposure, both evoked potential thresholds and DPOAEs were normal despite considerable OHC pathology. The findings suggest that normal DPOAEs may not guarantee normal cochlear status and, therefore, results of DPOAE measurements should be interpreted cautiously.

Anatomical effects of impact noise.

Four groups of binaural chinchillas were exposed to impact noise (B-duration = 200 ms) ranging from 119 dB to 137 dB peak equivalent SPL at repetition rates of 1/s or 4/s. The duration of exposure was adjusted so that each exposure consisted of equal acoustic energy. Animals were then sacrificed immediately, 24 h or 30 days after the exposure and their cochleas subjected to scanning electron microscopy. For exposures of 119 dB or greater, there appeared to be direct mechanical damage, including large clefts between the third row of outer hair cells and Deiters' cells and fracture of tight cell junctions at the reticular lamina. There was also a progressive increase in cochlear damage over the 30 days of recovery. The patterns of cochlear pathology are compared with hearing losses and cochleograms of chinchillas previously subjected to similar exposures and with results of studies using higher level impulse noise. The results are discussed in terms of 'critical level' for impact and impulse noise.

Changes in distortion product otoacoustic emissions and outer hair cells following interrupted noise exposures.

Changes in distortion product otoacoustic emissions (DPOAEs) were examined during and after interrupted noise exposures and compared to the condition of the outer hair cells (OHCs) and inner hair cells (IHCs) as assessed by scanning electron microscopy (SEM). Binaural, adult chinchillas were exposed to a 95 dB SPL, octave band noise centered at 0.5 kHz for 15 days using a 3 h on/9 h off schedule. DPOAEs were measured before, during and after the exposures. DPOAE amplitudes decreased significantly during the first few days of the interrupted noise exposures and then began to recover. At most frequencies, the emission amplitudes recovered completely to pre-exposure baseline values by five days after the last exposure. The results of the present study indicate that the changes in DPOAE amplitude paralleled the recovery in the amplitude and threshold of the compound action potentials as reported previously (Boettcher et al., 1992). Although the DPOAEs completely recovered, considerable OHC loss and stereocilia disarray was evident even four weeks after exposure.

The role of middle ear muscles in the development of resistance to noise induced hearing loss.

The role of middle ear muscles (MEMs) in the development of increased resistance to noise induced hearing loss (NIHL) was studied using monaural chinchillas. Animals with severed MEMs as well as those with intact MEMs were exposed to an octave band noise (OBN) centered at 0.5 kHz at 95 dB for six hours/day for ten consecutive days. Results indicated that animals with severed MEMs showed greater initial threshold shifts (TS) than the animals with intact MEMs. Both the groups showed a decrease in TS over the ten days of exposure. The subjects were given five days of recovery and then re-exposed to the same noise at 106 dB for 48 h. Permanent threshold shifts (PTS) in each group was compared against those in a control group exposed to the noise only at the higher level. Interestingly, both the 'conditioned' groups incurred substantially less PTS than the control group exposed only to the higher level.

Effect of low-frequency "conditioning" on hearing loss from high-frequency exposure.

Recent research has revealed that repeated exposures to a low-frequency noise results in a progressive reduction in threshold shifts (TS). This reduction in TS is not restricted to the exposure frequency, but can be observed at frequencies up to 3 or 4 octaves higher. Such "conditioning" exposures have also been observed to protect the auditory system against hearing loss from exposures to the same noise at higher levels. The aim of this study was to determine if "conditioning" using low-frequency exposures protects the auditory system against hearing loss from a high-frequency exposure. Monaural chinchillas were exposed to a 0.5-kHz octave band noise (OBN) at 95 dB SPL for 6 h a day. The animals were allowed to recover for 5 days, following which they were exposed to a 4-kHz OBN at 100 dB SPL for 48 h. Hearing thresholds determined using evoked potential recordings, indicated significantly greater permanent threshold shifts in this group of animals when compared to a control group exposed only to the 4-kHz OBN. These results were confirmed by histological examination which revealed greater hair cell loss in the experimental group.

Protection from noise induced hearing loss: is prolonged 'conditioning' necessary?

The effect of prior 'conditioning' noise exposures on the protection from subsequent higher level exposures was studied using four groups of chinchillas. The three experimental groups were 'conditioned' using a 0.5 kHz octave band noise at 95 dB SPL for 6 h a day. The first group was exposed to the noise once and allowed to recover for nine days prior to the second exposure. The second and third groups were exposed for ten and twenty days respectively. The first group showed only small reductions in threshold shift (TS) following the second exposure. The other two groups showed significant reductions in TS with repeated exposures. Following the last 'conditioning' exposure, all three experimental groups were allowed to recover for five days before exposing them to the same noise at 106 dB SPL for 48 h. Threshold shifts recorded following the 106 dB exposure were compared against those recorded in a control group exposed only to the higher level. Each of the three experimental groups developed significantly less permanent threshold shifts than the control group. However, there were no significant differences among the three experimental groups and the differences in hair cell losses were insignificant.

Physiological and histological changes associated with the reduction in threshold shift during interrupted noise exposure.

The compound action potential (AP) was recorded from one group of chinchillas exposed to interrupted noise (95 dB SPL, octave band centered at 500 Hz, 3 h on, 9 h off) for 15 days. A second group of chinchillas was exposed to the same interrupted noise for 1, 2 or 15 days and their cochleas were analyzed by scanning electron microscopy (SEM). During the first few days of the exposure, the AP threshold was elevated approximately 40 dB at the low-to-mid frequencies; however, the threshold shifts decreased with increasing exposure duration so that the threshold shift was only about 10 dB after the 15th day of exposure. The amplitude of the AP also recovered with exposure time. In contrast to the improvement in AP threshold, the number of missing hair cells increased and the condition of the stereocilia on inner and outer hair cells deteriorated between the first and 15th day of the exposure.

Effects of noise and salicylate on hair cell loss in the chinchilla cochlea.

Chinchillas were exposed to octave band noise, sodium salicylate (300 mg/kg per day intraperitoneally), or the combination of both agents for 15 days. The octave band noise exposure was centered at 500 Hz at an intensity of either 80 or 105 dB sound pressure level. The effects of the experimental treatments were evaluated by determining the number of missing hair cells after recovery as a function of location within the cochlea using a surface preparation technique. Average cochleograms were calculated for each of five experimental groups. Animals given salicylate alone showed little or no hair cell loss. Noise exposure at 80 dB resulted in a mild (less than 30%) outer hair cell loss in the apical turn of the cochlea, whereas exposure at 105 dB resulted in moderate (50%) outer hair cell loss (outer hair cell first row particularly) in the apical half of the cochlea, mild outer hair cell loss in the basal region of the cochlea, and a mild loss of inner hair cells. The amount of hair cell loss in the groups exposed to the combination of salicylates and noise was not significantly different from the corresponding groups exposed to noise alone. Statistical analysis of the data suggest that the combination of salicylate plus noise does not produce any greater hair cell loss than noise alone.

The effect of 'conditioning' on hearing loss from a high frequency traumatic exposure.

The role of high frequency, low level 'conditioning' exposures as moderators of hearing loss from subsequent exposure to the same noise at a higher level was studied using monaural chinchillas. All the animals in the experimental groups were exposed to an octave band noise centered at 4 kHz at 85 dB SPL for 6 h a day for 10 days. One of the experimental groups was allowed to recover for 5 days and the other was allowed to recover for 18 h, prior to the higher level exposure at 100 dB for 48 h. A third group exposed only to the higher level constituted the control group. A comparison of threshold shifts and hair cell loss after 4 weeks of recovery across the three groups revealed: (a) the 5-day recovery group incurred greater threshold shifts than the other two groups; the hair cell loss in this group was greater than in the 18-h recovery group, but the same as in the control group and (b) the 18-h recovery group incurred considerably less threshold shift as well as hair cell loss than the other two groups. The results were also compared with the results from similar exposures using low frequency noise which indicated that the base vs. apex differences in the cochlea appear to extend to the effects of 'conditioning' exposures.