Vlasta Spongr

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.

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.

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.

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 post exposure, 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.

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.

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.

Hair Cell Loss and Synaptic Loss in Inferior Colliculus of C57BL/6 MICE

The C57BL/6 mouse is an extremely popular animal model of presbycusis because of its relatively short life span and genetic pattern of high-frequency sensorineural hearing loss (SNHL) (Henry and Chole, 1980; Willott, 1984; Erway et al., 1993) that resembles the age-related hearing loss seen in humans (Nadol, 1993). Presbycusis and SNHL have traditionally been thought of as peripheral disorders that mainly result in the loss of sensitivity and frequency selectivity (Schmiedt and Schulte, 1992). However, recent studies suggest that peripheral pathologies can lead to functional and anatomical changes in the central nervous system (Hall, 1974, 1976; Wightman, 1982; Morest and Bohne, 1983; Willott, 1984; Salvi and Arehole, 1985; Arehole et al., 1987a; Robertson and Irvine, 1989; Salvi et al., 1990) that may contribute to some of the hearing deficits associated with SNHL and presbycusis.

Changes in distributions of fodrin and F‐actin in the cochlea after noise exposure as revealed by confocal microscopy

Changes in distributions of two cytoskeleton proteins, 240 kD alpha fodrin and filamentous actin, were studied in chinchillas exposed to 125 dB SPL impact noise and sacrificed immediately or 24 h after the exposure. To preserve the physiological integrity of the organ of Corti as much as possible, the surface preparations were studied using confocal fluorescence microscopy by serial, noninvasive optical sectioning. Immediately after noise exposure the damaged OHCs showed staining at the cuticular plates for both proteins, which varied in intensity. However, the lateral surfaces of the hair cells were not labeled. After 24 h, scars have replaced the missing cells and intense labeling of F‐actin was seen in the Deiter’s cells bodies and phalangeal processes. The results will be discussed in terms of role of fodrin and F‐actin in the pathology induced by noise exposure. [Work supported by NIDCD 1R01DC01237‐O1A1.]

Frequency differences in the development of protection against NIHL by low‐level “toughening” exposures

The aim of this study was to determine if low‐level intermittent exposures to a 0.5‐kHz OBN has a protective effect for future high‐level exposures (1) at the same frequency or (2) at a higher frequency (4‐kHz OBN). Four groups of monaural chinchillas were used as subjects. Thresholds were determined using evoked potentials recorded from a chronic electrode implanted in the inferior colliculus. In the first experiment, the experimental group was exposed to a low‐level noise (an OBN centered at 0.5 kHz at 95 dB for 6 h/day for 10 days). After a 5‐day recovery period when the thresholds returned to baseline, the animals were re‐exposed to the same noise at 106 dB for 48 h. A control group was exposed only to the higher‐level noise. The second experiment has an identical exposure schedule, except that the traumatic exposure involved a 4‐kHz OBN presented at 100 dB. The 10‐day “toughening” exposure reduced the amount of PTS in the low‐frequency experimental group by 20–30 dB over the range of 0.5–4 kHz. Conve...