Morphological correlates of functional recovery in the chicken inner ear after gentamycin treatment

Hair cell regeneration, reinnervation, and restoration of hearing thresholds in the avian hearing organ

Hearing starts, at the cellular level, with mechanoelectrical transduction by sensory hair cells. Sound information is then transmitted via afferent synaptic connections with auditory neurons. Frequency information is encoded by the location of hair cells along the cochlear duct. Loss of hair cells, synapses, or auditory neurons leads to permanent hearing loss in mammals. Birds, in contrast, regenerate auditory hair cells and functionally recover from hearing loss. Here, we characterized regeneration and reinnervation in sisomicin-deafened chickens and found that afferent neurons contact regenerated hair cells at the tips of basal projections. In contrast to development, synaptic specializations are established at these locations distant from the hair cells' bodies. The protrusions then contracted as regenerated hair cells matured and became functional 2 weeks post-deafening. We found that auditory thresholds recovered after 4-5 weeks. We interpret the regeneration-specific synaptic reestablishment as a location-preserving process that might be needed to maintain tonotopic fidelity.

Avian auditory hair cell regeneration is accompanied by JAK/STAT-dependent expression of immune-related genes in supporting cells

The avian hearing organ is the basilar papilla that, in sharp contrast to the mammalian cochlea, can regenerate sensory hair cells and thereby recover from deafness within weeks. The mechanisms that trigger, sustain and terminate the regenerative response in vivo are largely unknown. Here, we profile the changes in gene expression in the chicken basilar papilla after aminoglycoside antibiotic-induced hair cell loss using RNA-sequencing. We identified changes in gene expression of a group of immune-related genes and confirmed with single-cell RNA-sequencing that these changes occur in supporting cells. In situ hybridization was used to further validate these findings. We determined that the JAK/STAT signaling pathway is essential for upregulation of the damage-response genes in supporting cells during the second day after induction of hair cell loss. Four days after ototoxic damage, we identified newly regenerated, nascent auditory hair cells that express genes linked to termination of the JAK/STAT signaling response. The robust, transient expression of immune-related genes in supporting cells suggests a potential functional involvement of JAK/STAT signaling in sensory hair cell regeneration.

Effects of selective auditory-nerve damage on the behavioral audiogram and temporal integration in the budgerigar

Auditory-nerve fibers are lost steadily with age and as a possible consequence of noise-induced glutamate excitotoxicity. Auditory-nerve loss in the absence of other cochlear pathologies is thought to be undetectable with a pure-tone audiogram while degrading real-world speech perception (hidden hearing loss). Perceptual deficits remain unclear, however, due in part to the limited behavioral capacity of existing rodent models to discriminate complex sounds. The budgerigar is an avian vocal learner with human-like behavioral sensitivity to many simple and complex sounds and the capacity to mimic speech. Previous studies in this species show that intracochlear kainic-acid infusion reduces wave 1 of the auditory brainstem response by 40-70%, consistent with substantial excitotoxic auditory-nerve damage. The present study used operant-conditioning procedures in trained budgerigars to quantify kainic-acid effects on tone detection across frequency (0.25-8 kHz; the audiogram) and as a function of duration (20-160 ms; temporal integration). Tone thresholds in control animals were lowest from 1 to 4 kHz and decreased with increasing duration as in previous studies of the budgerigar. Behavioral results in kainic-acid-exposed animals were as sensitive as in controls, suggesting preservation of the audiogram and temporal integration despite auditory-nerve loss associated with up to 70% wave 1 reduction. Distortion-product otoacoustic emissions were also preserved in kainic-acid exposed animals, consistent with normal hair-cell function. These results highlight considerable perceptual resistance of tone-detection performance with selective auditory-nerve loss. Future behavioral studies in budgerigars with auditory-nerve damage can use complex speech-like stimuli to help clarify aspects of auditory perception impacted by this common cochlear pathology.

Oral administration of geranylgeranylacetone to protect vestibular hair cells

OBJECTIVE

We recently reported that the heat shock response played a major role in the protection of hair cells against stress. Oral administration of the heat shock inducer, geranylgeranylacetone (GGA) protected hair cells against intense noise. In our present study, we investigated the effect of GGA on vestibular hair cell death induced by an aminoglycoside.

METHODS

We used CBA/N mice aged 4-6 weeks. The mice were divided into two groups, GGA and control. Mice in the GGA group were fed a diet containing GGA (0.5%) for 4 weeks, and those in the control group were fed a standard diet. Immunohistochemical analyses for Hsp70 were performed in four animals. The utricles of the remaining animals were cultured in medium for 24h with neomycin to induce hair cell death. After fixation, the vestibular hair cells were immunohistochemically stained against calmodulin, and hair cell survival was evaluated.

RESULTS

The vestibular hair cells of mice in the GGA group expressed Hsp70. In addition, after exposure to neomycin, vestibular hair cell survival was higher in the GGA group than in the control group.

CONCLUSION

Our results demonstrated the oral administration of GGA induced the heat shock response in the vestibule and could protect sensory cells.

Lgr5+ cells regenerate hair cells via proliferation and direct transdifferentiation in damaged neonatal mouse utricle

Recruitment of endogenous progenitors is critical during tissue repair. The inner ear utricle requires mechanosensory hair cells (HCs) to detect linear acceleration. After damage, non-mammalian utricles regenerate HCs via both proliferation and direct transdifferentiation. In adult mammals, limited transdifferentiation from unidentified progenitors occurs to regenerate extrastriolar Type II HCs. Here we show that HC damage in neonatal mouse utricle activates the Wnt target gene Lgr5 in striolar supporting cells. Lineage tracing and time-lapse microscopy reveal that Lgr5+ cells transdifferentiate into HC-like cells in vitro. In contrast to adults, HC ablation in neonatal utricles in vivo recruits Lgr5+ cells to regenerate striolar HCs through mitotic and transdifferentiation pathways. Both Type I and II HCs are regenerated, and regenerated HCs display stereocilia and synapses. Lastly, stabilized ß-catenin in Lgr5+ cells enhances mitotic activity and HC regeneration. Thus Lgr5 marks Wnt-regulated, damage-activated HC progenitors and may help uncover factors driving mammalian HC regeneration.

Coenzyme Q10 protects hair cells against aminoglycoside

It is well known that the production of free radicals is associated with sensory cell death induced by an aminoglycoside. Many researchers have reported that antioxidant reagents protect sensory cells in the inner ear, and coenzyme Q10 (CoQ10) is an antioxidant that is consumed as a health food in many countries. The purpose of this study was to investigate the role of CoQ10 in mammalian vestibular hair cell death induced by aminoglycoside. Cultured utricles of CBA/CaN mice were divided into three groups (control group, neomycin group, and neomycin + CoQ10 group). In the neomycin group, utricles were cultured with neomycin (1 mM) to induce hair cell death. In the neomycin + CoQ10 group, utricles were cultured with neomycin and water-soluble CoQ10 (30–0.3 µM). Twenty-four hours after exposure to neomycin, the cultured tissues were fixed, and vestibular hair cells were labeled using an anti-calmodulin antibody. Significantly more hair cells survived in the neomycin + CoQ10 group than in the neomycin group. These data indicate that CoQ10 protects sensory hair cells against neomycin-induced death in the mammalian vestibular epithelium; therefore, CoQ10 may be useful as a protective drug in the inner ear.

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.

Return of function after hair cell regeneration

The ultimate goal of hair cell regeneration is to restore functional hearing. Because birds begin perceiving and producing song early in life, they provide a propitious model for studying not only whether regeneration of lost hair cells can return auditory sensitivity but also whether this regenerated periphery can restore complex auditory perception and production. They are the only animal where hair cell regeneration occurs naturally after hair cell loss and where the ability to correctly perceive and produce complex acoustic signals is critical to procreation and survival. The purpose of this review article is to survey the most recent literature on behavioral measures of auditory functional return in adult birds after hair cell regeneration. The first portion of the review summarizes the effect of ototoxic drug induced hair cell loss and regeneration on hearing loss and recovery for pure tones. The second portion reviews studies of complex, species-specific vocalization discrimination and recognition after hair cell regeneration. Finally, we discuss the relevance of temporary hearing loss and recovery through hair cell regeneration on complex call and song production. Hearing sensitivity is restored, except for the highest frequencies, after hair cell regeneration in birds, but there are enduring changes to complex auditory perception. These changes do not appear to provide any obstacle to future auditory or vocal learning. This article is part of a Special Issue entitled "Inner Ear Development and Regeneration".

Intratympanic injection of shRNA-expressing lentivirus causes gene silencing in the inner ear in chicken

The hair cells and their neural innervation in the avian inner ear can regenerate after injury. Identifying the genes involved in the regeneration and neuroplasticity of avian hair cell will enable us to experimentally induce new hair cell production and potentially harness this process for therapeutic replacement of hair cells in mammals and ultimately in humans suffering from sensorineural hearing loss. In this study, we developed a method for suppressing the expression level of genes in avian inner ear by intratympanic injection of shRNA-expressing lentivirus. The intratympanic injection approach is more convenient and presumably of less implication when compared with two existing methods, in which a nano-particles or gelfoam containing a recombinant virus is placed in the middle ear by surgery, or a recombinant virus is directly injected into the inner ear. Thus, we developed an easier method for identifying and characterizing molecules involved in the process of avian hair cell regeneration and re-innervation.

The challenge of hair cell regeneration

Sensory hair cells of the inner ear are responsible for translating auditory or vestibular stimuli into electrical energy that can be perceived by the nervous system. Although hair cells are exquisitely mechanically sensitive, they can be easily damaged by excessive stimulation by ototoxic drugs and by the effects of aging. In mammals, auditory hair cells are never replaced, such that cumulative damage to the ear causes progressive and permanent deafness. In contrast, non-mammalian vertebrates are capable of replacing lost hair cells, which has led to efforts to understand the molecular and cellular basis of regenerative responses in different vertebrate species. In this review, we describe recent progress in understanding the limits to hair cell regeneration in mammals and discuss the obstacles that currently exist for therapeutic approaches to hair cell replacement.

Activin potentiates proliferation in mature avian auditory sensory epithelium

Humans and other mammals are highly susceptible to permanent hearing and balance deficits due to an inability to regenerate sensory hair cells lost to inner ear trauma. In contrast, nonmammalian vertebrates, such as birds, robustly regenerate replacement hair cells and restore hearing and balance functions to near-normal levels. There is considerable interest in understanding the cellular mechanisms responsible for this difference in regenerative capacity. Here we report on involvement of the TGFbeta superfamily type II activin receptors, Acvr2a and Acvr2b, in regulating proliferation in mature avian auditory sensory epithelium. Cultured, posthatch avian auditory sensory epithelium treated with Acvr2a and Acvr2b inhibitors shows decreased proliferation of support cells, the cell type that gives rise to new hair cells. Conversely, addition of activin A, an Acvr2a/b ligand, potentiates support cell proliferation. Neither treatment (inhibitor or ligand) affected hair cell survival, suggesting a specific effect of Acvr2a/b signaling on support cell mitogenicity. Using immunocytochemistry, Acvr2a, Acvr2b, and downstream Smad effector proteins were differentially localized in avian and mammalian auditory sensory epithelia. Collectively, these data suggest that signaling through Acvr2a/b promotes support cell proliferation in mature avian auditory sensory epithelium and that this signaling pathway may be incomplete, or actively blocked, in the adult mammalian ear.

Effects of restricted basilar papillar lesions and hair cell regeneration on auditory forebrain frequency organization in adult European starlings

The frequency organization of neurons in the forebrain Field L complex (FLC) of adult starlings was investigated to determine the effects of hair cell (HC) destruction in the basal portion of the basilar papilla (BP) and of subsequent HC regeneration. Conventional microelectrode mapping techniques were used in normal starlings and in lesioned starlings either 2 d or 6-10 weeks after aminoglycoside treatment. Histological examination of the BP and recordings of auditory brainstem evoked responses confirmed massive loss of HCs in the basal portion of the BP and hearing losses at frequencies >2 kHz in starlings tested 2 d after aminoglycoside treatment. In these birds, all neurons in the region of the FLC in which characteristic frequencies (CFs) normally increase from 2 to 6 kHz had CF in the range of 2-4 kHz. The significantly elevated thresholds of responses in this region of altered tonotopic organization indicated that they were the residue of prelesion responses and did not reflect CNS plasticity. In the long-term recovery birds, there was histological evidence of substantial HC regeneration. The tonotopic organization of the high-frequency region of the FLC did not differ from that in normal starlings, but the mean threshold at CF in this frequency range was intermediate between the values in the normal and lesioned short-recovery groups. The recovery of normal tonotopicity indicates considerable stability of the topography of neuronal connections in the avian auditory system, but the residual loss of sensitivity suggests deficiencies in high-frequency HC function.

Regeneration of vestibular otolith afferents after ototoxic damage

Regeneration of receptor cells and subsequent functional recovery after damage in the auditory and vestibular systems of many vertebrates is well known. Spontaneous regeneration of mammalian hair cells does not occur. However, recent approaches provide hope for similar restoration of hearing and balance in humans after loss. Newly regenerated hair cells receive afferent terminal contacts, yet nothing is known about how reinnervation progresses or whether regenerated afferents finally develop normal termination fields. We hypothesized that neural regeneration in the vestibular otolith system would recapitulate the topographic phenotype of afferent innervation so characteristic of normal development. We used an ototoxic agent to produce complete vestibular receptor cell loss and epithelial denervation, and then quantitatively examined afferent regeneration at discrete periods up to 1 year in otolith maculas. Here, we report that bouton, dimorph, and calyx afferents all regenerate slowly at different time epochs, through a progressive temporal sequence. Furthermore, our data suggest that both the hair cells and their innervating afferents transdifferentiate from an early form into more advanced forms during regeneration. Finally, we show that regeneration remarkably recapitulates the topographic organization of afferent macular innervation, comparable with that developed through normative morphogenesis. However, we also show that regenerated terminal morphologies were significantly less complex than normal fibers. Whether these structural fiber changes lead to alterations in afferent responsiveness is unknown. If true, adaptive plasticity in the central neural processing of motion information would be necessitated, because it is known that many vestibular-related behaviors fully recover during regeneration.

Role of cochlear integrity in cochlear nucleus glucose metabolism and neuron number after cochlea removal in aging broiler chickens

In the chicken auditory system, cochlear nucleus (nucleus magnocellularis, NM) neurons receive their only excitatory input from the ipsilateral cochlea. Cochlea removal (CR) results in an immediate decrease in NM neuron electrical activity, followed by death of approximately 30% of NM neurons. Previous work showed a decrease in NM activity and subsequent loss of NM neurons in all chicks. Egg layer adults showed NM neuron loss after CR, while neuron number remained stable in broiler adults. This suggested that effects of CR on NM were age- and breed-dependent. We now know that most aging egg layer chickens maintain largely normal cochleae throughout adulthood. Some exhibit cochlear damage with age. The converse is true of broiler chickens. Most aging broiler chickens display cochlear degeneration, with some maintaining normal cochlear anatomy throughout adulthood. The presence of extensive cochlear damage may alter the effect of CR on NM, leading to the described differences. Here, we examine the effect of unilateral CR on NM glucose metabolism and neuron number in 2, 30, 39, and 52 week-old broiler chickens found to have normal cochleae. Chickens with damaged cochleae were excluded. Using 2-deoxyglucose uptake to evaluate bilateral NM glucose metabolism, we found significantly decreased uptake ipsilateral to CR at each age examined. Bilateral cell counts revealed significant neuron loss ipsilateral to CR at each age examined. This suggests that NM glucose metabolism decreases and subsequent neuron death occurs in aging broiler chickens when a normal cochlea is removed. The status of the cochlea must play a role in the effect of deafferentation on NM glucose metabolism and neuron survival. The effect of CR appears to be dependent upon neither age nor breed, but upon cochlear integrity instead.

Planar cell polarity and a potential role for a Wnt morphogen gradient in stereociliary bundle orientation in the mammalian inner ear

The planar cell polarity (PCP) pathway, a noncanonical Wnt signaling pathway, is crucial for embryonic development in all animals as it is responsible for the regulation of coordinated orientation of structures within the plane of the various epithelia. In the mammalian cochlea, one of the best examples of planar polarity in vertebrates, stereociliary bundles located on mechanosensory hair cells within the sensory epithelium are all uniformly polarized. Generation of this polarity is important for hair cell mechanotransduction and auditory perception as stereociliary bundles are only sensitive to vibrations in their single plane of polarization. We describe the two step developmental process that results in the generation of planar polarity in the mammalian inner ear. Furthermore, we review evidence for the role of Wnt signaling, and the possible generation of a Wnt gradient, in planar polarity.

Effects of Intratympanic Gentamicin on Vestibular Afferents and Hair Cells in the Chinchilla

Gentamicin is toxic to vestibular hair cells, but its effects on vestibular afferents have not been defined. We treated anesthetized chinchillas with one injection of gentamicin (26.7 mg/ml) into the middle ear and made extracellular recordings from afferents after 5–25 (early) or 90–115 days (late). The relative proportions of regular, intermediate, and irregular afferents did not change after treatment. The spontaneous firing rate of regular afferents was lower ( P < 0.001) on the treated side (early: 44.3 ± 16.3; late: 33.9 ± 13.2 spikes·s−1) than on the untreated side (54.9 ± 16.8 spikes·s−1). Spontaneous rates of irregular and intermediate afferents did not change. The majority of treated afferents did not measurably respond to tilt or rotation (82% in the early group, 76% in the late group). Those that did respond had abnormally low sensitivities ( P < 0.001). Treated canal units that responded to rotation had mean sensitivities only 5–7% of the values for untreated canal afferents. Treated otolith afferents had mean sensitivities 23–28% of the values for untreated otolith units. Sensitivity to externally applied galvanic currents was unaffected for all afferents. Intratympanic gentamicin treatment reduced the histological density of all hair cells by 57% ( P = 0.04). The density of hair cells with calyx endings was reduced by 99% ( P = 0.03), although some remaining hair cells had other features suggestive of type I morphology. Type II hair cell density was not significantly reduced. These findings suggest that a single intratympanic gentamicin injection causes partial damage and loss of vestibular hair cells, particularly type I hair cells or their calyceal afferent endings, does not damage the afferent spike initiation zones, and preserves enough hair cell synaptic activity to drive the spontaneous activity of vestibular afferents.

Selective vestibular ablation by intratympanic gentamicin in patients with unilateral active Ménière's disease: a prospective, double-blind, placebo-controlled, randomized clinical trial

OBJECTIVE

To establish the efficacy of intratympanic gentamicin treatment in patients with unilateral Ménière's disease.

MATERIAL AND METHODS

This was a prospective, double-blind, randomized clinical trial of intratympanic gentamicin versus intratympanic buffer solution (placebo) in patients with established active Ménière's disease in the affected ear. Outcome measures included the number of vertiginous spells, degree of sensorineural hearing loss, labyrinthine function and labyrinthine asymmetry.

RESULTS

Topical gentamicin provided a significant reduction in the number of vertiginous spells, although a "placebo effect" was also observed. Sensorineural hearing loss did not occur in the gentamicin group, although some deterioration occurred in the placebo group.

CONCLUSIONS

Intratympanic gentamicin is a safe and efficient treatment for the vertiginous spells associated with Ménière's disease. When applied early in the course of the disease, it may prevent some of the sensorineural hearing deterioration associated with it.

In vitro reconstitution of signal transmission from a hair cell to the growth cone of a chick vestibular ganglion cell

Signal transmission from a chick hair cell to the growth cone of a vestibular ganglion cell was examined by placing an acutely dissociated hair cell on the growth cone of a cultured vestibular ganglion cell. Electrical stimuli were applied to the hair cell while monitoring the intracellular Ca(2+) concentration ([Ca(2+)](i)) at the growth cone or recording whole-cell currents from the vestibular ganglion cell. Electrical stimulation of the hair cell induced [Ca(2+)](i) increases at the growth cone and inward currents in the vestibular ganglion cell. The [Ca(2+)](i) increase was blocked by 6-cyano-7-nitroquinoxaline (CNQX) (10 microM) but not by 2-amino-5-phosphonovaleric acid (APV; 50 microM). Glutamate (100 nM-300 microM) applied to the vestibular ganglion cell by the Y-tube method induced inward currents which were also antagonized by CNQX, but not by APV. These results indicate that the electrical stimulation of a hair cell induced glutamate or glutamate like agent release from the hair cell, which activated non-N-methyl-D-aspartate receptors at the growth cone of the vestibular ganglion cell, followed by action potentials and [Ca(2+)](i) elevation in the vestibular ganglion cell. This is the first demonstration of in vitro reconstitution of functional signal transmission from a hair cell to a vestibular ganglion cell.

Ultrastructural analysis of [3H]thymidine-labeled cells in the rat utricular macula

Ototoxic drugs stimulate cell proliferation in adult rat vestibular sensory epithelia, as does the infusion of transforming growth factor alpha (TGFalpha) plus insulin. We sought to determine whether new hair cells can be regenerated by means of a mitotic pathway. Previously, studies have shown that the nuclei of some newly generated cells are located in the lumenal half of the sensory epithelium, suggesting that some may be newly generated sensory hair cells. The aim of this study was to examine the ultrastructural characteristics of newly proliferated cells after TGFalpha stimulation and/or aminoglycoside damage in the utricular sensory epithelium of the adult rat. The cell proliferation marker tritiated-thymidine was infused, with or without TGFalpha plus insulin, into the inner ears of normal or aminoglycoside-damaged rats for 3 or 7 days by means of osmotic pumps. Autoradiographic techniques and light microscopy were used to identify cells synthesizing DNA. Sections with labeled cells were re-embedded, processed for transmission electron microscopy, and the ultrastructural characteristics of the labeled cells were examined. The following five classes of tritiated-thymidine labeled cells were identified in the sensory epithelium: (1) labeled cells with synaptic specializations that appeared to be newly generated hair cells, (2) labeled supporting cells, (3) labeled leukocytes, (4) labeled cells that we have classified as "active cells" in that they are relatively nondescript but contain massive numbers of polyribosomes, and (5) labeled degenerating hair cells. These findings suggest that new hair cells can be generated in situ by means of a mitotic mechanism in the vestibular sensory epithelium of adult mammals.

Vocal memory and learning in adult Bengalese Finches with regenerated hair cells

Critical learning periods are common in vertebrate development. In many birds, song learning is limited by a critical period; juveniles copy songs from adult birds by forming memories of those songs during a restricted developmental period and then using auditory feedback to practice their own vocalizations. Adult songs are stable over time regardless of exposure to other birds, but auditory feedback is required for the maintenance of stable adult song. A technique was developed to reversibly deafen Bengalese Finches by destruction and regeneration of inner ear auditory hair cells. With this approach, we asked two questions about the plasticity of song information stored in the adult brain. First, do adult birds store memories or "templates" of their songs that exist independent of auditory reinforcement? Such memories could be used to control vocal output by acting as fixed models of song to which ongoing vocalizations are matched. Second, can adult song learning, which does not normally occur in this species, be induced by removing and then restoring hearing? Studying changes in adult song behavior during hair cell loss and regeneration revealed two findings: (1) adult birds store memories or templates of their songs that exist independent of auditory input and can be used to restore normal vocal behavior when hearing is restored; (2) under experimental circumstances, adult birds can be induced to acquire song material from other birds. Results suggest that, in Bengalese Finches, the degree of behavioral and neural plasticity in juvenile and adult birds may be less distinct that previously thought.

Electrically evoked otoacoustic emissions from the chicken ear

The outer hair cell electromotile response is believed to underlie the sharp tuning and exquisite sensitivity of the mammalian inner ear, and contribute to the production of electrically evoked otoacoustic emissions (EEOAEs) and sound-evoked otoacoustic emissions (OAEs). Avian ears are also sharply tuned, extremely sensitive and generate spontaneous and sound-evoked OAEs, but avian hair cells do not exhibit somatic electromotility. However, stereocilia bundle movements have been observed in avian and amphibian hair cells suggesting that EEOAEs might arise from electrically evoked bundle movements. Here, we demonstrate for the first time that AC current applied to the round window of the chicken evokes EEOAE of up to 18 dB SPL. The EEOAE produces a bandpass response with maximum amplitude in the 1000-3000 Hz range; the response drops off rapidly above 4000 Hz and below 500 Hz. The impulse response to current pulses is characterized by a large peak sometimes followed by a damped oscillation with a frequency around 2000 Hz. EEOAEs decreased significantly after anoxia and paraformaldehyde damage of the cochlea. Kanamycin-induced hair cell loss also caused a significant reduction in EEOAE and distortion product OAE; these emissions showed only a small recovery at long recovery times, when most hair cells should have regenerated. These results suggest that the EEOAE has a biological origin in the cochlea, which could presumably involve electrically evoked stereocilia bundle movements.

Hair cell regeneration and recovery of auditory thresholds following aminoglycoside ototoxicity in Bengalese finches

Birds regenerate auditory hair cells when original hair cells are lost. Regenerated hair cells become innervated and restore hearing function. Functional recovery during hair cell regeneration is particularly interesting in animals that depend on hearing for vocal communication. Bengalese finches are songbirds that depend on auditory feedback for normal song learning and maintenance. We examined the structural and functional recovery of the Bengalese finch basilar papilla after aminoglycoside ototoxicity. Birds were treated with the ototoxic aminoglycoside, amikacin, daily for 1 week. Treatment resulted in hair cell loss across the basal half of the basilar papilla and corresponding high frequency hearing loss. Hair cell regeneration and recovery of auditory brainstem responses were compared in the same animals. Survival times following treatment were between 1 day and 12 weeks. Analysis of structural recovery at weekly intervals indicated that hair cells in the Bengalese finch papilla require a maximum of 1 week to regenerate and appear with immature morphology at the epithelial surface. An additional 6 days are required for adult-like morphology to develop. Repopulation of the damaged region was complete by 8 weeks. Recovery of auditory thresholds began 1 week after treatment and reached asymptote by 4 weeks. Slight residual threshold shifts at 2.0 kHz and above were observed up to 12 weeks after treatment. Direct comparison of structural and functional recovery indicates that auditory thresholds recover maximally before a full complement of hair cells has regenerated.

Changes in the avian cochlea after single high-dose gentamicin

PURPOSE

Define the time course of functional and anatomical damage and subsequent recovery (by regeneration) of hair cells in the chicken inner ear after a single high-dose of gentamicin.

MATERIALS AND METHODS

Broiler chicks were given a single intraperitoneal dose (200 mg/kg) of gentamicin (n = 39) or saline (n = 39). Functional status was evaluated with auditory brainstem response (ABR) thresholds before injection and before sacrifice at 2, 5, 9, 16, 21, 28, and 70 days postinjection. The cochleae were then examined with scanning electron microscopy (SEM) to assess the extent of damage along the cochlea and absolute hair cell numbers in the basal 15% of the cochlea (high-frequency region).

RESULTS

Considerable variability between animals was seen for both ABR and SEM changes. Damage was maximal at 5 days postinjection with an average ABR threshold shift of 12 dB (range -10 to 50 dB) and basal cochlear damage of 28% (range 12%-57%). Hair cell counts were significantly decreased in the basal 15% of the cochlea at 5 days. Hair cell regeneration resulted in rapid anatomical and functional recovery, but evidence of hair cell disorganization persisted at 70 days despite improved thresholds.

CONCLUSION

A single high dose of gentamicin produces a significant but variable anatomical and functional insult in the chick cochlea. Hair cell regeneration results in rapid but incomplete recovery.

Cellular studies of auditory hair cell regeneration in birds

A decade ago it was discovered that mature birds are able to regenerate hair cells, the receptors for auditory perception. This surprising finding generated hope in the field of auditory neuroscience that new hair cells someday may be coaxed to form in another class of warm-blooded vertebrates, mammals. We have made considerable progress toward understanding some cellular and molecular events that lead to hair cell regeneration in birds. This review discusses our current understanding of avian hair cell regeneration, with some comparisons to other vertebrate classes and other regenerative systems.

Central nervous system plasticity during hair cell loss and regeneration

Following cochlear ablation, auditory neurons in the central nervous system (CNS) undergo alterations in morphology and function, including neuronal cell death. The trigger for these CNS changes is the abrupt cessation of afferent input via eighth nerve fiber activity. Gentamicin can cause ototoxic damage to cochlear hair cells responsible for high frequency hearing, which seems likely to cause a frequency-specific loss of input into the CNS. In birds, these hair cells can regenerate, presumably restoring input into the CNS. This review summarizes current knowledge of how CNS auditory neurons respond to this transient, frequency-specific loss of cochlear function. A single systemic injection of a high dose of gentamicin results in the complete loss of high frequency hair cells by 5 days, followed by the regeneration of new hair cells. Both hair cell-specific functional measures and estimates of CNS afferent activity suggest that newly regenerated hair cells restore afferent input to brainstem auditory neurons. Frequency-specific neuronal cell death and shrinkage occur following gentamicin damage to hair cells, with an unexpected recovery of neuronal cell number at longer survival times. A newly-developed method for topical, unilateral gentamicin application will allow future studies to compare neuronal changes within a given animal.

Tonotopic changes in 2-deoxyglucose activity in chick cochlear nucleus during hair cell loss and regeneration

Following cochlear ablation, auditory neurons in the central nervous system (CNS) undergo alterations in morphology and function, including neuronal cell death. The trigger for these CNS changes is the abrupt cessation of eighth nerve fiber activity. Gentamicin can cause ototoxic damage to cochlear hair cells responsible for high frequency hearing. In birds, these hair cells can regenerate. Therefore, gentamicin causes a partial, yet reversible insult to the ear. It is not known how this partial hair cell damage affects excitatory input to the cochlear nucleus. We examined chick cochlear nucleus activity during hair cell loss and regeneration by measuring 2-deoxyglucose (2DG) uptake. Normal animals showed a rostral to caudal gradient of 2DG activity, with higher activity in caudal regions. When hair cells are damaged (2, 5 days), 2DG uptake is decreased in cochlear nucleus. When hair cells regenerate (9, 16, 28 days), 2DG uptake returns to control levels. This decrease and subsequent return of activity only occurs in the rostral, high frequency region of the cochlear nucleus. No changes are seen in the caudal, low frequency region. These results suggest that changes in activity of cochlear nucleus occur at a similar time course to anatomical changes in the cochlea.

Dynamic studies of ototoxicity in mature avian auditory epithelium

Hearing loss induced by ototoxicity is a worldwide problem despite the development of newer antibiotics and chemotherapy agents. The cellular mechanisms responsible for aminoglycoside-induced hearing loss are still poorly understood. We have developed two different methods of studying the dynamic cellular and subcellular changes in the chick auditory sensory epithelium that occur during hair cell death. The first study was performed in mature chicks after a single, high dose injection of gentamicin, which results in the rapid loss of all hair cells in the basal third of the cochlea. Chicks were sacrificed at discrete time points after drug treatment, and transmission electron microscopy was performed to study the ultrastructural changes in basal hair cells during the course of cell death. We noted various changes in the cell morphology including accumulation of cytoplasmic inclusion bodies, dispersion of the cytoplasmic polyribosomes, mitochondrial swelling, and cellular extrusion by 24 h after injection. The next two studies were performed using tissue cultures from mature avian auditory sensory epithelium. Cultured cells were labeled using vital fluorescent markers, and levels of intracellular calcium and reactive oxygen species within hair cells were studied following aminoglycoside exposure. We identified a dose-dependent increase in the levels of intracellular calcium, which was blocked by an inhibitor of voltage-gated calcium channels. We also found that levels of reactive oxygen species in hair cells greatly increased after exposure to gentamicin, and this response was blocked by two different antioxidants. These studies serve to identify key cellular and molecular changes in hair cells in response to ototoxic drugs. Further study of these processes may lead to a better understanding of how ototoxicity is induced and to potential preventative interventions.

Responses of auditory nerve fibers innervating regenerated hair cells after local application of gentamicin at the round window of the cochlea in the pigeon

Hair cells in the basilar papilla of birds have the capacity to regenerate after injury. There is also functional recovery of hearing after regeneration of the hair cells. The present study was undertaken to determine the effect of local aminoglycoside application on the physiology of auditory nerve fibers innervating regenerated hair cells. Collagen sponges loaded with gentamicin were placed at the round window of the cochlea in adult pigeons. The local application of gentamicin-loaded collagen sponges resulted in total hair cell loss over at least the basal 62% of the basilar papilla. According to the pigeon cochlear place-frequency map (Smolders, Ding-Pfennigdorff and Klinke, Hear. Res. 92 (1995) 151-169), frequencies above 0.3 kHz are represented in this area. Physiological data on single auditory nerve fibers were obtained 14 weeks after gentamicin treatment. The response properties showed the following characteristics when compared to control data: CF thresholds (CF = characteristic frequency) were elevated in units with CF above 0.15 kHz, sharpness of tuning (Q10dB) was reduced in units with CF above 0.38 kHz, low-frequency slopes of the tuning curves were reduced in units with CF above 0.25 kHz, high frequency slopes of the tuning curves were reduced in units with CF above 0.4 kHz, spontaneous firing rate was reduced in units with CF above 0.38 kHz, dynamic range of rate-intensity functions at CF was reduced in units with CF above 0.4 kHz and the slopes of these rate-intensity functions were elevated in units with CF above 0.4 kHz. Maximum discharge rate was the only parameter that remained unchanged in regenerated ears. The results show that the response properties of auditory nerve fibers which innervate areas of the papilla that were previously devoid of hair cells are poorer than the controls, but that action potential generation in the afferent fibers is unaffected. This suggests that despite structural regeneration of the basilar papilla, functional recovery of the auditory periphery is incomplete at the level of the hair cell or the hair cell-afferent synapse.

Recovery of the vestibulocolic reflex after aminoglycoside ototoxicity in domestic chickens

Avian auditory and vestibular hair cells regenerate after damage by ototoxic drugs, but until recently there was little evidence that regenerated vestibular hair cells function normally. In an earlier study we showed that the vestibuloocular reflex (VOR) is eliminated with aminoglycoside antibiotic treatment and recovers as hair cells regenerate. The VOR, which stabilizes the eye in the head, is an open-loop system that is thought to depend largely on regularly firing afferents. Recovery of the VOR is highly correlated with the regeneration of type I hair cells. In contrast, the vestibulocolic reflex (VCR), which stabilizes the head in space, is a closed-loop, negative-feedback system that seems to depend more on irregularly firing afferent input and is thought to be subserved by different circuitry than the VOR. We examined whether this different reflex also of vestibular origin would show similar recovery after hair cell regeneration. Lesions of the vestibular hair cells of 10-day-old chicks were created by a 5-day course of streptomycin sulfate. One day after completion of streptomycin treatment there was no measurable VCR gain, and total hair cell density was approximately 35% of that in untreated, age-matched controls. At 2 wk postlesion there was significant recovery of the VCR; at this time two subjects showed VCR gains within the range of control chicks. At 3 wk postlesion all subjects showed VCR gains and phase shifts within the normal range. These data show that the VCR recovers before the VOR. Unlike VOR gain, recovering VCR gain correlates equally well with the density of regenerating type I and type II vestibular hair cells, except at high frequencies. Several factors other than hair cell regeneration, such as length of stereocilia, reafferentation of hair cells, and compensation involving central neural pathways, may be involved in behavioral recovery. Our data suggest that one or more of these factors differentially affect the recovery of these two vestibular reflexes.

High-frequency auditory feedback is not required for adult song maintenance in Bengalese finches

Male Bengalese finches do not normally change their vocal patterns in adulthood; song is stereotyped and stable over time. Adult song maintenance requires auditory feedback. If adults are deafened, song will degrade within 1 week. We tested whether feedback of all sound frequencies is required for song maintenance. The avian basilar papilla is tonotopically organized; hair cells in the basal region encode high frequencies, and low frequencies are encoded in progressively apical regions. We restricted the spectral range of feedback available to a bird by killing either auditory hair cells encoding higher frequencies or those encoding both high and low frequencies and documented resultant changes in song. Birds were treated with either Amikacin alone to kill high-frequency hair cells or Amikacin and sound exposure to target hair cells across the entire papilla. During treatment, song was recorded from all birds weekly. After treatment and song recording, evoked-potential audiograms were evaluated on each bird, and papillas were evaluated by scanning electron microscopy. Results showed that hair cell damage over 46-63% of the basal papilla and the corresponding high-frequency hearing loss had no effect on song structure. In birds with hair cell damage extending further into the apical region of the papilla and corresponding low-frequency and high-frequency hearing loss, song degradation occurred within 1 week of beginning treatment and was comparable with degradation after surgical deafening. We conclude that either low-frequency spectral cues or temporal cues via feedback of the song amplitude envelope are sufficient for song maintenance in adult Bengalese finches.

Evidence for loss and recovery of chick brainstem auditory neurons during gentamicin-induced cochlear damage and regeneration

It is well documented that damage to the chick cochlea caused by acoustic overstimulation or ototoxic drugs is reversible. Second-order auditory neurons in nucleus magnocellularis (NM) are sensitive to changes in input from the cochlea. However, few experiments studying changes in NM during cochlear hair cell loss and regeneration have been reported. Chicks were given a single systemic dose of gentamicin, which results in maximal hair cell loss in the base of the cochlea after 5 days. Many new hair cells are present by 9 days. These new hair cells are mature but not completely recovered in organization by 70 days. We counted neurons in Nissl-stained sections of the brainstem within specific tonotopic regions of NM, comparing absolute cell number between gentamicin- and saline-treated animals at both short and long survival times. Our data suggest that neuronal number in rostral NM parallels hair cell number in the base of the cochlea. That is, after a single dose of gentamicin, we see a loss of both cochlear hair cells and NM neurons early, followed by a recovery of both cochlear hair cells and NM neurons later. These results suggest that neurons, like cochlear hair cells, can recover following gentamicin-induced damage.

Recovery of semicircular canal primary afferent activity in the pigeon after streptomycin ototoxicity

Recovery of semicircular canal primary afferent activity in the pigeon after streptomycin ototoxicity. J. Neurophysiol. 80: 3297-3311, 1998. The electrophysiological activity of horizontal semicircular canal primary afferents (HSCPA) was investigated in vivo in the barbiturate-anesthetized pigeon by means of extracellular single-fiber vestibular nerve action potential recordings. The spontaneous and driven discharges to pulse (step/trapezoid waveform, peak velocity = 120 degrees/s) and sum-of-sines (0.03, 0.09, 0.21, 0.39, 0.93, 1.83 Hz, peak velocity = 30 degrees/s for each frequency) rotations were measured both in normal control animals and a group of animals at 30, 40, 50, 60, 71, and 150 days postinjection sequence (PIS) of streptomycin sulfate. Prior to 30 days PIS, the activity in the nerve was not appropriately modulated during and after rotation. At 30 days PIS and thereafter, the responses resembled those observed in control animals but with systematic changes in parameters of fitted pulse responses and fitted Bode plots as days PIS increased. The return of parameters characterizing the neural dynamics of the semicircular canals were monotonic and could be best described by either linear or exponential functions. After 30 days PIS, the mechanical cupula-endolymph system, the function of which can be inferred from the cupula long time constant (tauL) following step velocity, did not change systematically (tauL = 6.92 +/- 3.96, 8.64 +/- 5.52, 8.35 +/- 4.21, 10.00 +/- 2.79, 9.05 +/- 3.67, 7.05 +/- 2.72; means +/- SD). However, the mean gain (G) of the HSCPA response to pulse rotation nearly doubled between 30 and 150 days PIS (from 1.31 +/- 0. 39 to 2.40 +/- 1.04) and returned linearly to control values (G = 2. 39 +/- 0.77) over this time period [G = 1.33 + 0.009(PIS-30), R2 = 0. 92, P < 0.05]. Meanwhile, neural adaptation as quantitated using a fractional operator, k, decayed exponentially (single exponential) to an asymptote. The time constant of this exponential was approximately 55 days [k = 0.034 + 0.33e-(PIS-30)/55.4, R2 = 0.99, P < 0.01]. Features of the spontaneous discharge previously shown to be correlated with k changed appropriately. That is, the coefficient of variation (CV) and frequency of firing (FF) decayed and grew asymptotically, respectively. These parameters also exhibited an exponential time course of return to control values from 30 to 150 days PIS [CV = 0.44 + 0.65e-(PIS-30)/21.5, R2 = 0.96, P < 0.01, and FF = 39.97 + 101.42(1 - e-(PIS-30)/32.6), R2 = 0.97, P < 0.01]. The trends of recovery for G, k, and tauL derived from analysis of the pulse response were confirmed by strong positive correlations with best fitted parameters obtained from analysis of the sum-of-sines frequency domain response of HSCPAs. There were statistically significant correlations (r = 0.90, P < 0.05 and r = 0.93, P < 0.05) between parameters (G, k) derived from pulse responses and those (G', k') from sum-of-sines responses, respectively. The cupula time constant based on sum-of-sines' data (tau'L) showed no statistically significant change between 30 and 150 days PIS (P > 0.05, analysis of variance). Thus the results in present study indicate that both the spontaneous discharge and the driven response to rotation of pigeon HSCPAs recovered their normal physiological status between 30 and 150 days PIS after hair cell death due to aminoglycoside ototoxicity. The recovery was systematic for the parameters chosen to be tested with the exception of the cupula long time constant, tauL. The mechanisms (changes in ciliary dynamics, changes in hair cell ionic currents, changes in bouton terminals, etc.) underlying these changes await further morphophysiological studies.

Effects of columella removal on inner ear morphology in the chick

We are currently removing the single middle ear bone (columella) in the domestic chick to introduce chemical agents directly into the inner ear. Since we are interested in the effect of these agents on neural structures within the avian basilar papilla (BP), we are concerned about any subtle changes that might result from the surgical procedure of columella removal alone. The purpose of this study was to use light and transmission electron microscopy to analyze morphological changes in the inner ear after columella removal. Fifteen-day-old chicks underwent a unilateral, bilateral or a sham removal of the columella. After columella removal, the oval window was either plugged with Gelfoam or Kimwipe (standard accepted procedure to prevent possible perilymph leak) or left uncovered. After a 5-day survival period, morphological changes were observed in the tegmentum vasculosum (TV) of all ears receiving a columella removal as compared to unoperated ears. Further, ears with Gelfoam plugging the oval window also had damage to the hair cells and support cells of the basilar papilla. In contrast, there were no observable differences in either auditory afferent or efferent nerve terminals on hair cells in the BP from any ears that had the columella removed compared to those from unoperated ears. These results suggest that columella removal alone may produce morphological changes to the TV within 5 days of surgery but not to structures within the BP. On the other hand, columella removal with a Gelfoam plug results in damage not only to the TV but also to cells within the basilar papilla during this same survival time. Despite damage to other structures within the inner ear, cochlear efferent and afferent terminals on surviving hair cells were unaffected by columella removal with or without plugging.

Round window administration of gentamicin: a new method for the study of ototoxicity of cochlear hair cells

Damage to inner ear sensory hair cells after systemic administration of ototoxic drugs has been documented in humans and animals. Birds have the ability to regenerate new hair cells to replace those damaged by drugs or noise. Unfortunately, the systemic administration of gentamicin damages both ears in a variable fashion with potentially confounding systemic drug effects. We developed a method of direct application of gentamicin to one cochlea of hatchling chickens, allowing the other ear to serve as a within-animal control. We tested variables including the vehicle for application, location of application, dosage, and duration of gentamicin exposure. After 5 or 28 days survival, the percent length damage to the cochlea and regeneration of hair cells was evaluated using scanning electron microscopy. Controls consisted of the opposite unexposed cochlea and additional animals which received saline instead of gentamicin. Excellent damage was achieved using gentamicin-soaked Gelfoam pledgets applied to the round window membrane. The percent length damage could be varied from 15 to 100% by changing the dosage of gentamicin, with exposures as short as 30 min. No damage was observed in control animals. Regeneration of hair cells was observed in both the base and apex by 28 days survival.

Hair cells and supporting cells share a common progenitor in the avian inner ear

Sensory organs of the vertebrate inner ear contain two major cell types: hair cells (HCs) and supporting cells (SCs). To study the lineage relationships between these two populations, replication-defective retroviral vectors encoding marker genes were delivered to the otic vesicle of the chicken embryo. The resulting labeled clones were analyzed in the hearing organ of the chicken, called the basilar papilla (BP), after cellular differentiation. BPs were allowed to develop for 2 weeks after delivery of the retrovirus, were removed, and were processed histochemically as whole mounts to identify clones of cells. Clusters of labeled cells were evident in the sensory epithelium, the nonsensory epithelium, and in adjacent tissues. Labeled cell types included HCs, two morphologically distinct types of SCs, homogene cells, border cells, hyaline cells, ganglion cells, and connective tissue cells. Each clone was sectioned and cell-type identification was performed on sensory clones expressing retrovirally transduced beta-galactosidase. Cell composition was determined for 41 sensory clones, most of which contained both HCs and SCs. Clones containing one HC and one SC were observed, suggesting that a common progenitor exists that can remain bipotential up to its final mitotic division. The possibility that these two cell types may also arise from a mitotic precursor during HC regeneration in the mature basilar papilla is consistent with their developmental history.

Hair cell regeneration after local application of gentamicin at the round window of the cochlea in the pigeon

Hair cells in the basilar papilla of birds have the capacity to regenerate after injury. Methods commonly used to induce cochlear damage are systemic application of ototoxic substances such as aminoglycoside antibiotics or loud sound. Both methods have disadvantages. The systemic application of antibiotics results in damage restricted to the basal 50% of the papilla and has severe side effects on the kidneys. Loud sound damages only small parts of the papilla and is restricted to the short hair cells. The present study was undertaken to determine the effect of local aminoglycoside application on the physiology and morphology of the avian basilar papilla. Collagen sponges loaded with gentamicin were placed at the round window of the cochlea in adult pigeons. The time course of hearing thresholds was determined from auditory brain stem responses elicited with pure tone bursts within a frequency range of 0.35-5.565 kHz. The condition of the basilar papilla was determined from scanning electron micrographs. Five days after application of the collagen sponges loaded with gentamicin severe hearing loss, except for the lowest frequency tested, was observed. Only at the apical 20% of the basilar papilla hair cells were left intact, all other hair cells were missing or damaged. At all frequencies there was little functional recovery until day 13 after implantation. At frequencies above 1 kHz functional recovery occurred at a rate of up to 4 dB/day until day 21, beyond that day recovery continued at a rate below 1 dB/day until day 48 at the 5.6 kHz. Below 1 kHz recovery occurred up to day 22, the recovery rate was below 2 dB/day. A residual hearing loss of about 15-25 dB remained at all frequencies, except for the lowest frequency tested. At day 20 new hair cells were seen on the basilar papilla. At day 48 the hair cells appeared to have recovered fully, except for the orientation of the hair cell bundles. The advantage of the local application of the aminoglycoside drug over systemic application is that it damages almost all hair cells in the basilar papilla and it has no toxic side effects. The damage is more extensive than with systemic application.

Regeneration of cochlear efferent nerve terminals after gentamycin damage

Chickens recover auditory function after hair cell loss caused by ototoxic drug damage or acoustic overstimulation, indicating that mechanisms exist to reestablish appropriate neuronal connections to regenerated hair cells. However, despite similar hair cell regeneration times, hearing recovery takes substantially longer after aminoglycoside than after sound damage. We have therefore begun examining damage and regeneration of efferent nerve terminals by immunolabeling whole-mount cochleae for differentially localized synaptic proteins and by visualizing the distribution of label with confocal microscopy. In undamaged cochleae, the synaptic proteins synapsin and syntaxin show similar distribution patterns corresponding to the large cup-like terminals on short hair cells. After gentamycin administration, these terminals are disrupted as hair cells are lost, leaving smaller, more numerous synapsin-reactive structures in the sensory epithelium. Syntaxin reactivity remains associated with the extruded hair cells, indicating that the presynaptic membrane is still attached to the postsynaptic site. In contrast, after sound damage, both synapsin and syntaxin reactivity are lost from the epithelium with extruded hair cells. As regenerated hair cells differentiate after gentamycin treatment, the synapsin labeling associated with cup-like efferent endings reappears but is not completely restored even after 60 d of recovery. Thus, efferent terminals are reestablished much more slowly than after sound damage (), consistent with the prolonged loss of hearing function. This in vivo model system allows comparison of axonal reconnection after either complete loss (sound damage) or partial disruption (gentamycin treatment) of axon terminals. Elucidating the differences in recovery between these injuries can provide insights into reinnervation mechanisms.

Sensory cells determine afferent terminal morphology in cross-innervated electroreceptor organs: implications for hair cells

Type I and type II hair cells of the vestibular system are innervated by afferents that form calyceal and bouton terminals, respectively. These cannot be experimentally cross-innervated in the inner ear to determine how they influence each other. However, analogous organs are accessible for transplantation and cross-innervation in the brown ghost electric fish. These fish possess three types of electroreceptor organs. Of these, the sensory receptors of the type I tuberous organ are S-100- and parvalbumin-positive with a calbindin-positive afferent that forms a large calyx around the organ. Neither the sensory receptors nor the afferents of the ampullary organs label with these antibodies, and the afferent branches form a single large bouton beneath each receptor cell. In controls, when cut ampullary afferents reinnervate transplanted ampullary organs, they have characteristic calbindin-negative terminals with large boutons. When type I tuberous afferents reinnervate ampullary organs, receptor cells remain S-100- and parvalbumin-negative, and the tuberous afferents still express calbindin. The nerve terminals, however, make large ampullary-like boutons on the receptor cells. These results suggest that (1) afferent terminal morphology is dictated by the receptor organ; (2) expression of calbindin by the afferent is not suppressed by innervation of the incorrect end organ; (3) ampullary organs generate ampullary receptor cells although innervated by tuberous afferents; and (4) ampullary receptor cells can be trophically supported by tuberous afferents.

Shaker-1 mutations reveal roles for myosin VIIA in both development and function of cochlear hair cells

The mouse shaker-1 locus, Myo7a, encodes myosin VIIA and mutations in the orthologous gene in humans cause Usher syndrome type 1B or non-syndromic deafness. Myo7a is expressed very early in sensory hair cell development in the inner ear. We describe the effects of three mutations on cochlear hair cell development and function. In the Myo7a816SB and Myo7a6J mutants, stereocilia grow and form rows of graded heights as normal, but the bundles become progressively more disorganised. Most of these mutants show no gross electrophysiological responses, but some did show evidence of hair cell depolarisation despite the disorganisation of their bundles. In contrast, the original shaker-1 mutants, Myo7ash1, had normal early development of stereocilia bundles, but still showed abnormal cochlear responses. These findings suggest that myosin VIIA is required for normal stereocilia bundle organisation and has a role in the function of cochlear hair cells.

Recent insights into regeneration of auditory and vestibular hair cells

Advances in hair cell regeneration are progressing at a rapid rate. This review will highlight and critique recent attempts to understand some of the cellular and molecular mechanisms underlying hair cell regeneration in non-mammalian vertebrates and efforts to induce regeneration in the mammalian inner ear sensory epithelium.

Recovery of hearing and vocal behavior after hair-cell regeneration

Postmitotic hair-cell regeneration in the inner ear of birds provides an opportunity to study the effect of renewed auditory input on auditory perception, vocal production, and vocal learning in a vertebrate. We used behavioral conditioning to test both perception and vocal production in a small Australian parrot, the budgerigar. Results show that both auditory perception and vocal production are disrupted when hair cells are damaged or lost but that these behaviors return to near normal over time. Precision in vocal production completely recovers well before recovery of full auditory function. These results may have particular relevance for understanding the relation between hearing loss and human speech production especially where there is consideration of an auditory prosthetic device. The present results show, at least for a bird, that even limited recovery of auditory input soon after deafening can support full recovery of vocal precision.

Expression of neurotrophins and Trk receptors in the developing, adult, and regenerating avian cochlea

We studied the expression of neurotrophins and their Trk receptors in the chicken cochlea. Based on in situ hybridization, brain-derived neurotrophic factor (BDNF) is the major neurotrophin there, in contrast to the mammalian cochlea, where neurotrophin-3 (NT-3) predominates. NT-3 mRNA labeling was weak and found only during a short time period in the early cochleas. During embryogenesis, BDNF mRNA was first seen in early differentiating hair cells. Afferent cochlear neurons expressed trkB mRNA from the early stages of gangliogenesis onward. In accordance, in vitro, BDNF promoted survival of dissociated neurons and stimulated neuritogenesis from ganglionic explants. High levels of BDNF mRNA in hair cells and trkB mRNA in cochlear neurons persisted in the mature cochlea. In addition, mRNA for the truncated TrkB receptor was expressed in nonneuronal cells, specifically in supporting cells, located adjacent to the site of BDNF synthesis and nerve endings. Following acoustic trauma, regenerated hair cells acquired BDNF mRNA expression at early stages of differentiation. Truncated trkB mRNA was lost from supporting cells that regenerated into hair cells. High levels of BDNF mRNA persisted in surviving hair cells and trkB mRNA in cochlear neurons after noise exposure. These results suggest that in the avian cochlea, peripheral target-derived BDNF contributes to the onset and maintenance of hearing function by supporting neuronal survival and regulating the (re)innervation process. Truncated TrkB receptors may regulate the BDNF concentration available to neurites, and they might have an important role during reinnervation.

Sound damage and gentamicin treatment produce different patterns of damage to the efferent innervation of the chick cochlea

Both sound exposure and gentamicin treatment cause damage to sensory hair cells in the peripheral chick auditory organ, the basilar papilla. This induces a regeneration response which replaces hair cells and restores auditory function. Since functional recovery requires the re-establishment of connections between regenerated hair cells and the central nervous system, we have investigated the effects of sound damage and gentamicin treatment on the neuronal elements within the cochlea. Whole-mount preparations of basilar papillae were labeled with phalloidin to label the actin cytoskeleton and antibodies to neurofilaments, choline acetyltransferase, and synapsin to label neurons; and examined by confocal laser scanning microscopy. When chicks are treated with gentamicin or exposed to acoustic overstimulation, the transverse nerve fibers show no changes from normal cochleae assayed in parallel. Efferent nerve terminals, however, disappear from areas depleted of hair cells following acoustic trauma. In contrast, efferent nerve endings are still present in the areas of hair cell loss following gentamicin treatment, although their morphological appearance is greatly altered. These differences in the response of efferent nerve terminals to sound exposure versus gentamicin treatment may account, at least in part, for the discrepancies reported in the time of recovery of auditory function.

Ionic currents in regenerating avian vestibular hair cells

By applying the conventional whole-cell patch-clamp technique in combination with the slice procedure, we have investigated the properties of avian semicircular canal hair cells in situ. Passive and active electrical properties of hair cells from control animals have been compared with those of regenerating hair cells following streptomycin treatment (that killed almost all hair cells). Regenerating type II hair cells showed patterns of responses qualitatively similar to those of normal hair cells. However, parameters reflecting the total number of ionic channels and the surface area of type II hair cells changed during recovery-suggesting that new hair cells came from smaller precursors which (with time) reacquired the same electrophysiological properties as normal hair cells. Finally, we have investigated the ionic properties of a small sample of type 1 hair cells. Ionic currents of regenerating type I hair cells did not show, at least in the temporal window considered (up to 10 weeks from the end of the streptomycin treatment), the typical ionic currents of normal type I hair cells, but expressed instead ionic currents resembling those of type II hair cells. The possibility that regenerating type I hair cells can transdifferentiate from type II hair cells is therefore suggested.

Cellular interactions as a response to injury in the organ of Corti in culture

We discovered and described ultrastructurally the intricate relationships between the sensory cells and their supporting cells in cultures of the organ of Corti following laser beam irradiation. Injury was performed using a 440 nm nitrogen-dye pulse laser aimed at the cuticular plates of inner hair cells. Laser injury is compared with mechanical injury inflicted on the hair cell region by a pulled-glass pipette. Regardless of the type of injury, but depending on its severity, the surviving hair cells may: (1) lose their stereocilia but subsist at the surface of the organ; (2) retain contact with the reticular lamina but be overgrown by the processes of the supporting cells; or (3) become sequestered from the reticular lamina and internalized among the supporting cells, where they either remain dedifferentiated or regrow an apical process which regains contact with the surface of the organ. All supporting cells, including pillar and Deiters cells take part in wrapping their respective inner or outer hair cells. The supporting cells not only cover the injured sensory cells, but also invert their villi toward the maimed cuticular plates and release an extracellular matrix around them. We suggest that the supporting cells play a protective and trophic role in the recovery of injured hair cells.

Quantification of the process of hair cell loss and recovery in the chinchilla crista ampullaris after gentamicin treatment

The degree of ototoxic drug sensitivity and hair cell repair was determined in the chinchilla horizontal crista ampullaris after intraotic administration of gentamicin. Histological evaluation was made of 22 cristae ampullaris from one normal and six post-treatment (PT) animal groups killed at 1, 4, 7, 14, 28, and 56 days. New hair cell production was quantified, using the dissector technique. Transmission electron microscopy was used to investigate the ultrastructural characteristics of the hair cells in the regenerated epithelium. At 1 day PT, type I and II hair cells presented cytoplasmic vacuolization, swollen nerve calyces and 20% of type I and 18% of type II hair cells were lost. At 4 days PT, 95% of type I hair cells and 14% of type II hair cells had disappeared. In addition, most of the type II hair cells showed clumping of nuclear material. Nerve fibers were not found in the sensory epithelium, but were still observed below the basal lamina. Supporting cells appeared unaffected, maintaining their location in the crista. At 1 and 4 days PT, the damage to hair cells was more pronounced in the central region of the crista ampullaris. The degree of ototoxic damage at 7 days was similar to that of 14 days: no type I hair cells were present and most of the type II hair cells had disappeared; supporting cell nuclei began to occupy the apical part of the sensory epithelium and most of the nerve fibers had retracted. Quantitatively, 87 and 93% of type II hair cells were lost at 7 and 14 days PT, respectively. Initial signs of hair cell recovery began at 28 days PT; immature type II-like hair cells appeared, supporting cell nuclei began to align at the base of the sensory epithelium and nerve fibers penetrating the basal lamina were observed. No type I hair cells were found, but 40% of the normal number of type II hair cells were present. Hair cells appeared to regenerate in the peripheral areas of the cristae ampullaris first. At 56 days PT, an increase in the number of mature type II hair cells was present, supporting cells were aligned at the base of the epithelium, and more nerve fibers appeared to penetrate the basal lamina to the sensory epithelium. Although type I hair cells were absent from the epithelium 55% of the normal number of type II hair cells were present. At this time, more regenerated hair cells were located in the center of the cristae ampullaris as compared to the periphery. At the transmission electron microscopic level, type II hair cells at different stages of maturation were observed. Some exhibited mature stereocilia, a cuticular plate, and terminal endings with synaptic specialization opposing these hair cells. In conclusion, type I hair cells were more sensitive than type II hair cells to gentamicin intoxication (as they disappeared as early as 4 days PT). After 56 days PT, the number of type II hair cells reached 55% of normal. No type I hair cells had regenerated at this time. These results demonstrate quantitatively the differential ototoxic sensitivity and regenerative capacity of hair cells.

Discharge properties of pigeon single auditory nerve fibers after recovery from severe acoustic trauma

The time course of recovery of compound action potential (CAP) thresholds was observed in individual adult pigeons after severe acoustic trauma. Each bird had electrodes implanted on the round window of both ears. One ear was exposed to a tone of 0.7 kHz at 136-142 dB SPL for 1 hr under general anesthesia. Recovery of CAP audiograms was monitored twice a week after trauma. Single unit recordings from auditory nerve fibers were made after 3 weeks and after 4 or more months of the exposure. The CAP was abolished immediately after overstimulation in all animals. Based on the temporal patterns of functional recovery of the CAP three groups of animals were identified. The first group was characterized by fast functional recovery starting immediately after trauma followed by a return to pre-exposure values within 3 weeks. In the second group, slow functional recovery of threshold started 1-2 weeks after trauma followed by a return to pre-exposure values by 4-5 weeks. A mean residual hearing loss of 26.3 dB at 2 kHz remained. The third group consisted of animals that did not recover after trauma. Three weeks after the exposure, tuning curves of single auditory nerve fibers were very broad and sometimes irregular in shape. Their thresholds hovered around 120 dB SPL. Spontaneous firing rate and driven rate were much reduced. Four or more months after exposure, the thresholds and sharpness of tuning of many single units were almost completely recovered. Spontaneous firing rate and driven rate were comparable to those of control animals. In the slow recovery group neuronal tuning properties showed less recovery, especially at frequencies above the exposure frequency. Thresholds and sharpness of tuning were normal at frequencies below the exposure frequency, but were much poorer at frequencies above the exposure. Spontaneous firing rate was much reduced in fibers with high characteristic frequencies. In fast recovering animals, the papilla was repopulated with hair cells after 4 months. In slow recovering animals, short (abneural) hair cells were still missing over large parts of the papilla after 4 months of recovery. Residual short (abneural) hair cell loss was largest at two areas, one more basal and the other more apical to the characteristic place of the traumatizing frequency. The results show that, in adult birds, functional recovery from severe damage to both short (abneural) and tall (neural) hair cells occurs. However, the onset of recovery is delayed and the time course is slower than after destruction of short (abneural) hair cells alone. Also, recovery is incomplete, both functionally and morphologically. There is residual permanent hearing loss, and regeneration of short (abneural) hair cells is incomplete.