Functional recovery of hearing following ampa-induced reversible disruption of hair cell afferent synapses in the avian inner ear

Nondeterministic nature of sensorineural outcomes following noise trauma

Over 1.1 billion individuals are at risk for noise induced hearing loss yet there is no accepted therapy. A long history of research has demonstrated that excessive noise exposure will kill outer hair cells (OHCs). Such observations have fueled the notion that dead OHCs underlie hearing loss. Therefore, previous and current therapeutic approaches are based on preventing the loss of OHCs. However, the relationship between OHC loss and hearing loss is at best a modest correlation. This suggests that in addition to the death of OHCs, other mechanisms may regulate the type and degree of hearing loss. In the current study, we tested the hypothesis that permanent noise-induced-hearing loss is consequent to additional mechanisms beyond the noise dose and the death of OHCs. Hooded male rats were randomly divided into noise and control groups. Morphological and physiological assessments were conducted on both groups. The combined results suggest that beyond OHC loss, the surviving cochlear elements shape sensorineural outcomes, which can be nondeterministic. These findings provide the basis for individualized ototherapeutics that manipulate surviving cellular elements in order to bias cochlear function towards normal hearing even in the presence of dead OHCs.

Summary: The current findings provide the basis for individualized ototherapeutics that manipulate surviving cellular elements in order to bias cochlear function towards normal hearing even in the presence of dead cells.

[Pathophysiology of hearing loss : Classification and treatment options]

From the therapeutic perspective, the etiology and pathophysiology of hearing loss can be classified based on the extent of the primary cause. Hearing loss can have very different consequences for cell preservation in the organ of Corti and the spiral ganglion. These not only have implications for prosthetic therapy outcome, but may also influence the potential for future causal molecular therapies. Etiologies leading to deficits that are limited to one or a few molecules without having an effect on cell survival have the greatest potential for future causal therapy using molecular and cellular approaches. Preliminary success for molecular therapy was recently reported in animal experiments. Unfortunately, the incidence of these types of hearing loss is very low and in the future the therapy of hearing loss will therefore also require several different approaches. In addition to peripheral pathophysiology, hearing loss has consequences on the functioning of the brain, which can vary greatly due to individual adaptation to the situation without hearing. The authors therefore argue for individualization of the diagnostics and therapy that focus not only the symptom of hearing loss, but also the individual pathophysiology and consequences. Only with individualized therapy can the success of treating hearing disorders be significantly improved.

Recovery of otoacoustic emissions after high-level noise exposure in the American bullfrog

The American bullfrog (Rana catesbeiana) has an amphibian papilla (AP) that senses airborne, low-frequency sound and generates distortion product otoacoustic emissions (DPOAEs) similar to other vertebrate species. Although ranid frogs are typically found in noisy environments, the effects of noise on the AP have not been studied. First, we determined the noise levels that diminished DPOAE at 2f1-f2 using an f2 stimulus level at 80 dB SPL and that also produced morphological damage of the sensory epithelium. Second, we compared DPOAE (2f1-f2) responses with histopathologic changes occurring in bullfrogs after noise exposure. Consistent morphological damage, such as fragmented hair cells and missing bundles, as well as elimination of DPOAE responses were seen only after very high-level (>150 dB SPL) sound exposures. The morphological response of hair cells to noise differed along the mediolateral AP axis: medial hair cells were sensitive to noise and lateral hair cells were relatively insensitive to noise. Renewed or repaired hair cells were not observed until 9 days post-exposure. Following noise exposure, DPOAE responses disappeared within 24 h and then recovered to normal pre-exposure levels within 3-4 days. Our results suggest that DPOAEs in the bullfrog are sensitive to the initial period of hair cell damage. After noise-induced damage, the bullfrog AP has functional recovery mechanisms that do not depend on substantial hair cell regeneration or repair. Thus, the bullfrog auditory system might serve as an interesting model for investigation of ways to prevent noise damage.

Glutamate agonist causes irreversible degeneration of inner hair cells

Glutamate neurotoxicity in cochlear hair cells was investigated by administering the glutamate agonist alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) into the scala tympani of Mongolian gerbils. AMPA administration caused the formation of large number of vacuoles in the inner hair cells (IHCs) and dendritic terminals. The number of degenerated hair cells was counted using rhodamine-phalloidin and Hoechst 33342 staining. The administration of 50 microM AMPA caused reversible elevation of the auditory brainstem response threshold without loss of IHCs. In contrast, 200 microM AMPA induced a substantial elevation of the auditory brainstem response threshold with the characteristic disappearance of IHCs. As cochlear ischemia involves excessive glutamate release, these results suggest that an elevated glutamate level in the cochlea is responsible for the progressive IHC death related to ischemic injury.

Evoked cochlear potentials in the barn owl

Gross electrical responses to tone bursts were measured in adult barn owls, using a single-ended wire electrode placed onto the round window. Cochlear microphonic (CM) and compound action potential (CAP) responses were evaluated separately. Both potentials were physiologically vulnerable. Selective abolishment of neural responses at high frequencies confirmed that the CAP was of neural origin, while the CM remained unaffected. CAP latencies decreased with increasing stimulus frequency and CAP amplitudes were correlated with known variations in afferent fibre numbers from the different papillar regions. This suggests a local origin of the CAP along the tonotopic gradient within the basilar papilla. The audiograms derived from CAP and CM threshold responses both showed a broad frequency region of optimal sensitivity, very similar to behavioural and single-unit data, but shifted upward in absolute sensitivity. CAP thresholds rose above 8 kHz, while CM responses showed unchanged sensitivity up to 10 kHz.

Brain-derived neurotrophic factor treatment does not improve functional recovery after hair cell regeneration in the pigeon

CONCLUSIONS

Brain-derived neurotrophic factor (BDNF) supply to the inner ear does not improve the time course or the extent of functional recovery after hair cell regeneration. Specifically it does not improve the residual threshold elevation observed after the completion of spontaneous recovery.

OBJECTIVE

The avian inner ear is capable of hair cell regeneration and substantial functional recovery, but residual hearing deficits remain. We investigated whether functional recovery can be improved by intracochlear application of BDNF, which plays an important role in auditory ontogenesis and maintenance during adult life.

METHODS

Hair cells in adult pigeons were destroyed by local application of gentamicin. After 3 days either BDNF or control solution was administered to the scala tympani by implanted osmotic minipumps for 8 weeks. Auditory brain stem responses (ABR) to tone pips were used to assess recovery of hearing thresholds in both groups.

RESULTS

The application of gentamicin caused a frequency-dependent hearing loss that ranged from 24.8 dB SPL at low frequencies to 66.2 dB SPL at high frequencies. After day 10 substantial recovery was observed, but a significant threshold shift remained. The time course of recovery in the control and BDNF-treated groups was similar, without significant residual threshold differences in any frequency range.

Survival of partially differentiated mouse embryonic stem cells in the scala media of the guinea pig cochlea

The low regenerative capacity of the hair cells of the mammalian inner ear is a major obstacle for functional recovery following sensorineural hearing loss. A potential treatment is to replace damaged tissue by transplantation of stem cells. To test this approach, undifferentiated and partially differentiated mouse embryonic stem (ES) cells were delivered into the scala media of the deafened guinea pig cochlea. Transplanted cells survived in the scala media for a postoperative period of at least nine weeks, evidenced by histochemical and direct fluorescent detection of enhanced green fluorescent protein (EGFP). Transplanted cells were discovered near the spiral ligament and stria vascularis in the endolymph fluid of the scala media. In some cases, cells were observed close to the damaged organ of Corti structure. There was no evidence of significant immunological rejection of the implanted ES cells despite the absence of immunosuppression. Our surgical approach allowed efficient delivery of ES cells to the scala media while preserving the delicate structures of the cochlea. This is the first report of the survival of partially differentiated ES cells in the scala media of the mammalian cochlea, and it provides support for the potential of cell-based therapies for sensorineural hearing impairment.

Transient Ca2+-permeable AMPA receptors in postnatal rat primary auditory neurons

Fast excitatory transmission in the nervous system is mostly mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors whose subunit composition governs physiological characteristics such as ligand affinity and ion conductance properties. Here, we report that AMPA receptors at inner hair cell (IHC) synapses lack the GluR2 subunit and are transiently Ca2+-permeable before hearing onset as evidenced using agonist-induced Co2+ accumulation, Western blots and GluR2 confocal microscopy in the rat cochlea. AMPA (100 microM) induced Co2+ accumulation in primary auditory neurons until postnatal day (PND) 10. This accumulation was concentration-dependent, strengthened by cyclothiazide (50 microM) and blocked by GYKI 52466 (80 microM) and Joro spider toxin (1 microM). It was unaffected by D-AP5 (50 microM), and it could not be elicited by 56 mM K+ or 1 mM NMDA + 10 microM glycine. Western blots showed that GluR1 immunoreactivity, present in homogenates of immature cochleas, had disappeared by PND12. GluR2 immunoreactivity was not detected until PND10 and GluR3 and GluR4 immunoreactivities were detected at all the ages examined. Confocal microscopy confirmed that the GluR2 immunofluorescence was not located postsynaptically to IHCs before PND10. In conclusion, AMPA receptors on maturing primary auditory neurons differ from those on adult neurons. They are probably composed of GluR1, GluR3 and GluR4 subunits and have a high Ca2+ permeability. The postsynaptic expression of GluR2 subunits may be continuously regulated by the presynaptic activity allowing for variations in the Ca2+ permeability and physiological properties of the receptor.

Survival of neuronal tissue following xenograft implantation into the adult rat inner ear

The poor regenerative capacity of the spiral ganglion neurons of the mammalian inner ear has initiated research on how to assist the functional recovery of the injured auditory system. A possible treatment is to use a biological implant with a potential to establish central or peripheral synaptic contacts to develop into a functional auditory unit. The feasibility of this approach was tested by xenograft implantation of dorsal root ganglion (DRG) neurons from embryonic days 13 to 14, mouse expressing either LacZ or enhanced green fluorescent protein (EGFP) into the scala tympani of the adult rat inner ear. Transplanted DRG neurons survived in the scala tympani for a postoperative period ranging from 3 to 10 weeks, as verified by histochemical detection of LacZ, EGFP fluorescence and immunohistochemical labeling of the neuronal markers neurofilament and Thy 1.2. DRG neurons were found close to structures near the sensory epithelium (the organ of Corti) as well as adjacent to the spiral ganglion neurons with their peripheral dendrites. These results illustrate not only the survival of xenografted DRG neurons in the adult inner ear but also the feasibility of a neuronal transplantation strategy in the degenerated auditory system, thereby creating possibilities to replace spiral ganglion neurons.