Scar formation in the vestibular sensory epithelium after aminoglycoside toxicity

Hair cell degeneration and the repair process due to differing types of trauma have been studied extensively in the organ of Corti. It has been determined that, during scar formation, after differing types of trauma to the auditory sensory system, the reticular lamina is maintained with adherens junctions and tight junctions. We investigated the repair process within the vestibular epithelium. Hair cell degeneration was induced by the unilateral application of streptomycin to the inner ears of guinea pigs. Whole mount preparations of all five vestibular organs were processed and examined by fluorescence, light and electron microscopy. Scar formation was seen as early as 4 days post-treatment with streptomycin and was noted to coincide with hair cell degeneration. Neighboring supporting cells swelled and filled the space beneath the degenerating hair cell. Between three and five supporting cells participate in the reparative process. The distribution of cytokeratin is also altered during scar formation. The area once occupied by the hair cell becomes filled with cytokeratin-rich processes of supporting cells. It appears that differing numbers of supporting cells are involved in the reparative process within the vestibular sensory epithelium as compared to the auditory system. The reticular lamina remains intact at all times. This may possibly prevent mixing of fluids between different compartments in the inner ear and dysfunction of the vestibular sensory organs.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31340/1/0000250.pd

Scar formation after drug-induced cochlear insult

Structural and molecular changes in the guinea pig organ of Corti were studied using histochemistry and electron microscopy in the course of drug-induced hair cell degeneration. Actin filaments disappear from the cuticular plate and the stereocilia. An actin-rich bridge appears in the apical region of dying hair cells. Two supporting cells form a scar for a given hair cell. The supporting cells expand and invade the spaces of Nuel and then the region previously occupied by the hair cell. The scar region becomes cytokeratin-labeled. In this study, the apical domain of the hair cell is the last part of the cell to degenerate. Hair cell degeneration coincides temporally with scar formation. We define the resulting scar as a `type I' scar. The results provide preliminary information about the molecular composition of the type I scar and suggest a structural basis for the dynamics of scar formation.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29474/1/0000560.pd

Hereditary deafness occurring in cd/1 mice

Different strains of mice provide a valuable research tool for studying both hereditary and acquired forms of deafness. The cd/1 strain has been found to demonstrate hereditary cochlear pathology. The characteristics of hearing loss in cd/1 mice have not previously been reported. In this investigation auditory thresholds were obtained by measuring evoked brain stem responses in subjects of three different ages: 3 weeks, 10 weeks and 6 months. The results were compared with thresholds obtained from CBA/Ca mice (which have normal hearing) and C57BL/6 mice (which are known to have a genetically determined pre-senile progressive cochlear hearing loss). A significant hearing loss was observed which progressed from high to low frequencies, and with age. Extensive degeneration was observed throughout the organ of Corti. cd/1 mice may provide a useful model for studying genetically determined deafness.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28997/1/0000025.pd

The supporting-cell antigen: a receptor-like protein tyrosine phosphatase expressed in the sensory epithelia of the inner ear

After noise- or drug-induced hair-cell loss, the sensory epithelia of the avian inner ear can regenerate new hair cells. Few molecular markers are available for the supporting-cell precursors of the hair cells that regenerate, and little is known about the signaling mechanisms underlying this regenerative response. Hybridoma methodology was used to obtain a monoclonal antibody (mAb) that stains the apical surface of supporting cells in the sensory epithelia of the inner ear. The mAb recognizes the supporting-cell antigen (SCA), a protein that is also found on the apical surfaces of retinal Müller cells, renal tubule cells, and intestinal brush border cells. Expression screening and molecular cloning reveal that the SCA is a novel receptor-like protein tyrosine phosphatase (RPTP), sharing similarity with human density-enhanced phosphatase, an RPTP thought to have a role in the density-dependent arrest of cell growth. In response to hair-cell damage induced by noise in vivo or hair-cell loss caused by ototoxic drug treatment in vitro, some supporting cells show a dramatic decrease in SCA expression levels on their apical surface. This decrease occurs before supporting cells are known to first enter S-phase after trauma, indicating that it may be a primary rather than a secondary response to injury. These results indicate that the SCA is a signaling molecule that may influence the potential of nonsensory supporting cells to either proliferate or differentiate into hair cell

3-D analysis of F-actin in stereocilia of cochlear hair cells after loud noise exposure

Fluorescence microscopy can be a useful tool in the early detection of pathological changes in the stereocilia of outer hair cells which have undergone acoustic overstimulation. Fluorescent phalloidin, a highly specific F-actin stain, can be used to label F-actin in stereocilia. In this study, phalloidin label is used to determine quantitative changes of F-actin in the stereocilia of guinea pigs exposed to loud noise (117 dB; octave band noise, centered at 1 kHz; 4 h). Reliably determining three-dimensional (3-D) structural changes in stereocilia is a challenging problem in optical microscopy since stereocilia diameter is close to the optical resolution limit. In order to alleviate the problem, a computational 3-D microscopy technique is used (Avinash et al., 1992). Whole-mounts of the cochlear second and third turns were examined in a Leitz Orthoplan microscope through a Leitz Plan Apo objective lens (100 x ; 1.32 N.A.; 170/0.17). Images were acquired with a charge-coupled device camera where the focus was shifted in 0.2 [mu]m steps using a piezoelectric translator. Images were processed with the appropriate point spread function of the optical system. Analysis of control cochleas indicate that our technique can resolve single stereocilia and distinguish between various intensities of label along each stereocilia. In noise-exposed cochleas, our data show length and intensity changes in the phalloidin label. These results suggest that both depolymerization and polymerization of F-actin can occur in stereocilia of outer hair cells after acoustic overstimulation. Our findings demonstrate the applicability of computational 3-D microscopy to quantitative and qualitative analysis of stereocilia.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30805/1/0000463.pd

Long-Term Effects of Acoustic Trauma on Electrically Evoked Otoacoustic Emission

Electrically evoked otoacoustic emissions (EEOAEs) are sounds measured in the ear canal when alternating current (AC) stimulation is passed into the cochlea. These sounds are attributed to the motile responses of outer hair cells (OHCs). The EEOAE has characteristic amplitude, phase, and fine structure. Multicomponent analysis of the EEOAE shows short (SDC) and long delay components (LDC) that are thought to originate from OHCs near the AC stimulating site and from OHCs at more remote locations, respectively. We measured the effects of various loud noise exposures on the EEOAE and the cochlear whole-nerve action potential (CAP) in animals chronically implanted with a scala tympani electrode. Noise exposures that produced permanent (PTS) or temporary threshold shifts (TTS) were associated with frequency-specific changes in CAP thresholds, EEOAE fine structure, and reductions in the amplitude of the LDC. A frequent observation in this study was an increase in the overall EEOAE amplitude after the noise exposure. The increase was correlated with increased SDC amplitude. The SDC was present in animals chemically treated with ototoxic drugs and mechanical damage to the cochlea. The SDC was eliminated after disarticulation of the ossicular chain. The presence of EEOAE fine structure in the postexposure response is an indicator of TTS in advance of CAP recovery. The results suggest that the EEOAE might be used to differentiate the mechanisms associated with TTS and PTS.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/41387/1/10162_2005_Article_11.pd

Vestibular dysfunction in the adult CBA/CaJ mouse after lead and cadmium treatment

Objectives: The vestibular system allows the perception of position and motion and its dysfunction presents as motion impairment, vertigo and balance abnormalities, leading to debilitating psychological discomfort and difficulty performing daily tasks. Although declines and deficits in vestibular function have been noted in rats exposed to lead (Pb) and in humans exposed to Pb and cadmium (Cd), no studies have directly examined the pathological and pathophysiological effects upon the vestibular apparatus of the inner ear. Methods: Eighteen young adult mice were exposed through their drinking water (3 mM Pb, 300 µM Cd, or a control treatment) for 10 weeks. Before and after treatment, they underwent a vestibular assessment, consisting of a rotarod performance test and a novel head stability test to measure the vestibulocolic reflex. At the conclusion of the study, the utricles were analyzed immunohistologically for condition of hair cells and nerve fibers. Results: Increased levels of Pb exposure correlated with decreased head stability in space; no significant decline in performance on rotarod test was found. No damage to the hair cells or the nerve fibers of the utricle was observed in histology. Conclusions: The young adult CBA/CaJ mouse is able to tolerate occupationally‐relevant Pb and Cd exposure well, but the correlation between Pb exposure and reduced head stability suggests that Pb exposure causes a decline in vestibular function. © 2016 Wiley Periodicals, Inc. Environ Toxicol, 2016. © 2016 Wiley Periodicals, Inc. Environ Toxicol 32: 869–876, 2017.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/136249/1/tox22286_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/136249/2/tox22286.pd

Hearing dysfunction in heterozygous M itf Mi‐wh /+ mice, a model for W aardenburg syndrome type 2 and T ietz syndrome

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95364/1/pcmr12030.pd

Morphological and physiological effects of long duration infusion of strychnine into the organ of Corti

Acute strychnine administration has long been used as a method to eliminate the effects of efferent activity. It has been shown that long after termination of chronic strychnine infusion into the cochlea, the ear becomes more susceptible to acoustic trauma suggesting that chronic strychnine infusion results in long lasting or permanent disruption of efferent function. Much research has been directed towards the functional significance of the olivocochlear system. However, there is little information concerning the effect of long duration inactivation of the medial olivocochlear system in an awake behaving animal. This study was designed to determine the structural and functional consequences of inactivation of the efferents by chronic infusion of strychnine into the cochlear perilymph of guinea pigs for two weeks via an osmotic pump. Physiological evaluations showed that the strychnine infusion eliminated the efferent induced reduction of the cochlear whole-nerve action potential three weeks after cessation of strychnine infusion. Contralateral efferent function remained unaltered. Histological evaluation at the light and electron microscopic levels revealed disoriented efferent synapses under the outer hair cells.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47459/1/11068_2004_Article_241027.pd

Glial cell line-derived neurotrophic factor and chronic electrical stimulation prevent VIII cranial nerve degeneration following denervation

As with other cranial nerves and many CNS neurons, primary auditory neurons degenerate as a consequence of loss of input from their target cells, the inner hair cells (IHCs). Electrical stimulation (ES) of spiral ganglion cells (SGCs) has been shown to enhance their survival. Glial cell line-derived neurotrophic factor (GDNF) has also been shown to increase survival of SGCs following IHC loss. In this study, the combined effects of the GDNF transgene delivered by adenoviral vectors (Ad- GDNF ) and ES were tested on SGCs after first eliminating the IHCs. Animal groups received Ad- GDNF or ES or both. Ad- GDNF was inoculated into the cochlea of guinea pigs after deafening, to overexpress human GDNF . ES-treated animals were implanted with a cochlear implant electrode and chronically stimulated. A third group of animals received both Ad- GDNF and ES (GDNF/ES). Electrically evoked auditory brainstem responses were recorded from ES-treated animals at the start and end of the stimulation period. Animals were sacrificed 43 days after deafening and their ears prepared for evaluation of IHC survival and SGC counts. Treated ears exhibited significantly greater SGC survival than nontreated ears. The GDNF/ES combination provided significantly better preservation of SGC density than either treatment alone. Insofar as ES parameters were optimized for maximal protection (saturated effect), the further augmentation of the protection by GDNF suggests that the mechanisms of GDNF- and ES-mediated SGC protection are, at least in part, independent. We suggest that GDNF/ES combined treatment in cochlear implant recipients will improve auditory perception. These findings may have implications for the prevention and treatment of other neurodegenerative processes. J. Comp. Neurol. 454:350–360, 2002. © 2002 Wiley-Liss, Inc.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/34465/1/10480_ftp.pd