The effects of exposure to intense sound on the DC endocochlear potential in the chick

Hair Cell Regeneration: From Animals to Humans

Cochlear hair cells convert sound into electrical signals that are relayed via the spiral ganglion neurons to the central auditory pathway. Hair cells are vulnerable to damage caused by excessive noise, aging, and ototoxic agents. Non-mammals can regenerate lost hair cells by mitotic regeneration and direct transdifferentiation of surrounding supporting cells. However, in mature mammals, damaged hair cells are not replaced, resulting in permanent hearing loss. Recent studies have uncovered mechanisms by which sensory organs in non-mammals and the neonatal mammalian cochlea regenerate hair cells, and outlined possible mechanisms why this ability declines rapidly with age in mammals. Here, we review similarities and differences between avian, zebrafish, and mammalian hair cell regeneration. Moreover, we discuss advances and limitations of hair cell regeneration in the mature cochlea and their potential applications to human hearing loss.

Differences in molecular mechanisms of K+ clearance in the auditory sensory epithelium of birds and mammals

Mechanoelectrical transduction in the vertebrate inner ear is a highly conserved mechanism depending on K+ influx into hair cells. Here, we investigated the molecular underpinnings of subsequent K+ recycling in the chicken basilar papilla and compared it with those in the mammalian auditory sensory epithelium. Like mammals, the avian auditory hair cell uses KCNQ4, KCNMA1, and KCNMB1 as K+ efflux systems. Expression of KCNQ1 and KCNE1 suggests an additional efflux apparatus in avian hair cells. Marked differences were observed for K+ clearance. In mammals, KCC3, KCC4, Kir4.1, and CLC-K are present in supporting cells. Of these proteins, only CLC-K is expressed in avian supporting cells. Instead, they possess NKCC1 to move K+ across the membrane. This expression pattern suggests an avian clearance mechanism reminiscent of the well-established K+ uptake apparatus present in inner ear secretory cells. Altogether, tetrapod hair cells show similar mechanisms and supporting cells distinct molecular underpinnings of K+ recycling.

Molecular bases of K+ secretory cells in the inner ear: shared and distinct features between birds and mammals

In the cochlea, mammals maintain a uniquely high endolymphatic potential (EP), which is not observed in other vertebrate groups. However, a high [K⁺] is always present in the inner ear endolymph. Here, we show that Kir4.1, which is required in the mammalian stria vascularis to generate the highly positive EP, is absent in the functionally equivalent avian tegmentum vasculosum. In contrast, the molecular repertoire required for K⁺ secretion, specifically NKCC1, KCNQ1, KCNE1, BSND and CLC-K, is shared between the tegmentum vasculosum, the vestibular dark cells and the marginal cells of the stria vascularis. We further show that in barn owls, the tegmentum vasculosum is enlarged and a higher EP (~+34 mV) maintained, compared to other birds. Our data suggest that both the tegmentum vasculosum and the stratified stria vascularis evolved from an ancestral vestibular epithelium that already featured the major cell types of the auditory epithelia. Genetic recruitment of Kir4.1 specifically to strial melanocytes was then a crucial step in mammalian evolution enabling an increase in the cochlear EP. An increased EP may be related to high-frequency hearing, as this is a hallmark of barn owls among birds and mammals among amniotes.

WDR1 expression in normal and noise-damaged Sprague-Dawley rat cochleae

WD40 repeat protein 1 (WDR1) has been suggested as a protective mechanism or a sign of regeneration in avian cochlea. However, its role in mammalian cochlea has yet to be determined. Hence, we investigated WDR1 expression in sound-overstimulated Sprague-Dawley rats. Rats were divided into three groups (the permanent and temporary threshold shift [PTS and TTS] groups and the control group) according to the extent of noise exposure and euthanized immediately, 3, or 7 days after noise exposure for cochlear harvest. Immunocytochemistry localized WDR1 to outer hair cells, Deiter's cells, outer sulcus cells, and Reissner's membrane in the control group, and the PTS and TTS groups exhibited stronger WDR1 expression in the same cochlear regions than the controls. Moreover, WDR1 expression in these noise-exposed groups was extended to inner hair cells and basal cells of the stria vascularis. The expression of WDR1 in the PTS and TTS groups showed differences in intensity and shifts of localization, based on exposure length and recovery duration. Contrary to the avian cochlea, hair cell regeneration does not naturally occur in the acoustically damaged mammalian cochlea. Therefore, elevated WDR1 expression after acoustic overstimulation in the current experiments may provide a mechanism for protection against noise exposure.

WDR1 presence in the songbird basilar papilla

WD40 repeat 1 protein (WDR1) was first reported in the acoustically injured chicken inner ear, and bioinformatics revealed that WDR1 has numerous WD40 repeats, important for protein-protein interactions. It has significant homology to actin interacting protein 1 (Aip1) in several lower species such as yeast, roundworm, fruitfly and frog. Several studies have shown that Aip1 binds cofilin/actin depolymerizing factor, and that these interactions are pivotal for actin disassembly via actin filament severing and actin monomer capping. However, the role of WDR1 in auditory function has yet to be determined. WDR1 is typically restricted to hair cells of the normal avian basilar papilla, but is redistributed towards supporting cells after acoustic overstimulation, suggesting that WDR1 may be involved in inner ear response to noise stress. One aim of the present study was to resolve the question as to whether stress factors, other than intense sound, could induce changes in WDR1 presence in the affected avian inner ear. Several techniques were used to assess WDR1 presence in the inner ears of songbird strains, including Belgian Waterslager (BW) canary, an avian strain with degenerative hearing loss thought to have a genetic basis. Reverse transcription, followed by polymerase chain reactions with WDR1-specific primers, confirmed WDR1 presence in the basilar papillae of adult BW, non-BW canaries, and zebra finches. Confocal microscopy examinations, following immunocytochemistry with anti-WDR1 antibody, localized WDR1 to the hair cell cytoplasm along the avian sensory epithelium. In addition, little, if any, staining by anti-WDR1 antibody was observed among supporting cells in the chicken or songbird ear. The present observations confirm and extend the early findings of WDR1 localization in hair cells, but not in supporting cells, in the normal avian basilar papilla. However, unlike supporting cells in the acoustically damaged chicken basilar papilla, the inner ear of the BW canary showed little, if any, WDR1 up-regulation in supporting cells. This may be due to the fact that the BW canary already has established hearing loss and/or to the possibility that the mechanism(s) involved in BW hearing loss may not be related to WDR1.

The effects of intense sound exposure on phase locking in the chick (Gallus domesticus) cochlear nerve

Little is known about changes that occur to phase locking in the auditory nerve following exposure to intense and damaging levels of sound. The present study evaluated synchronization in the discharge patterns of cochlear nerve units collected from two groups of young chicks (Gallus domesticus), one shortly after removal from an exposure to a 120-dB, 900-Hz pure tone for 48 h and the other from a group of non-exposed control animals. Spontaneous activity, the characteristic frequency (CF), CF threshold and a phase-locked peri-stimulus time histogram were obtained for every unit in each group. Vector strength and temporal dispersion were calculated from these peri-stimulus time histograms, and plotted against the unit's CF. All parameters of unit responses were then compared between control and exposed units. The results in exposed units revealed that CF thresholds were elevated by 30-35 dB whereas spontaneous activity declined by 24%. In both control and exposed units a high degree of synchronization was observed in the low frequencies. The level of synchronization above approximately 0.5 kHz then systematically declined. The vector strengths in units recorded shortly after removal from the exposure were identical to those seen in control chicks. The deterioration in discharge activity of exposed units, seen in CF threshold and spontaneous activity, contrasted with the total absence of any overstimulation effect on synchronization. This suggested that synchronization arises from mechanisms unscathed by the acoustic trauma induced by the exposure.

Breed-dependent susceptibility to acute sound exposure in young chickens

Commercially available chickens fall into two categories: egg layers and broilers. Durham et al. (Hear. Res. 166 (2002) 82-95) showed that despite similar noisy living environments, cochleae of most adult broilers show extensive damage, while cochleae of adult egg layers are largely normal. This finding suggests that egg layers and broilers differ in their susceptibility to noise damage. Here, we evaluate breed differences in susceptibility to acoustic trauma. Young egg layers and broilers (10-17 weeks) were exposed to a 1500Hz pure tone (120dB SPL; 24h) and killed 24 or 72h later. Cochleae were prepared for scanning electron microscopy and photomicrographs of the cochlear surface were used to determine location and severity of damage. Cochleae were grouped based upon damage severity (moderate or severe). While location and area of damage were similar between both breeds at each recovery time, cochlear damage at 72h was more extensive than at 24h. We found no quantitative breed differences within either damage category or recovery time. However, more egg layers (25/27) than broilers (16/32) displayed severely damaged cochleae. Our findings conflict with those reported by Durham et al. (2002). Our results identify a breed-dependent difference in susceptibility to acute sound exposure, with young egg layers displaying increased sensitivity.

Three-dimensional culture of newborn rat utricle using an extracellular matrix promotes formation of a cyst

The vestibule is the end organ devoted to sensing of head movements in space. To function properly, its mechano-receptors require the presence of a unique apical extracellular medium, the endolymph. Numerous studies have elucidated the mechanisms involved in the production and homeostasis of this unique medium and the responses of sensory cells to stimulation. However, anatomical constraints have prevented direct and simultaneous studies of their relationships. The aim of this study was the development of an in vitro model that would allow concomitant investigations on maturation and physiological properties of both the hair cells and their endolymphatic compartment. A three-dimensional (3D) culture of newborn rat utricles using an extracellular matrix sustaining 3D cellular growth was developed during 3, 6, or 10 days in vitro (DIV). Using morphological and electrophysiological techniques, we describe the de novo formation of a cyst. It was composed of the sensory epithelium and non-sensory cells-canalar, dark and intermediate cells-that polarized so that their apical surface faced its lumen. During the time of culture, the utricular potential (UP) was steady (-1.1+/-5.0 mV) in oxygenated condition, while in anoxia, the UP significantly decreased to -8.4+/-1.0 mV at 8 DIV. Over the same period, the K+ concentration in the cyst increased up to 86.1+/-33.9 mM (versus 5.6+/-1.5 mM in the bath). These observations indicated that the mechanisms generating the UP and the K-secretory activity were functional at this stage. Concomitantly, the hair cells acquired mature and functional properties: the type 1 and type 2 phenotypes, a mean resting membrane potential of -68.1+/-4.6 mV and typical electrophysiological responses. This preparation provides a powerful means to simultaneous access the hair cells and their endolymphatic compartment, with the possibility to use multi-technical approaches to investigate their interdependent relationships.

Loss and recovery of sound-evoked otoacoustic emissions in young chicks following acoustic trauma

Young and adult chickens exhibit substantial inner-ear damage and post-exposure deterioration in cochlear nerve activity following exposure to intense sound. Both the structural and functional losses largely recover in both age groups within 2-4 weeks after exposure. However, some aspects of acoustic trauma differ between the young and adult chicken ear. Overstimulation in the young chick causes considerable post-exposure loss and then recovery of the steady-state endocochlear potential, while in the adult animal there is little post-exposure effect on this potential. Moreover, in adults there is post-exposure loss but little recovery in the distortion product otoacoustic emission (DPOAE). The present study explores the possibility of an age difference in the effects of overstimulation on the DPOAE by examining these emissions in young chicks following exposure to an intense pure tone. Chicks exposed to intense sound were formed into groups at 0 and 12 days of recovery, and these were complemented by two additional groups of age-matched controls. The cubic difference tone emission (the 2f(1)-f(2) DPOAE component) was measured at 9 levels for 13 frequencies in all groups. Shortly after the exposure, the DPOAE reliably declined with the maximum loss at or above the exposure tone frequency. The exposed chicks examined 12 days after exposure showed complete recovery of the DPOAE. It would appear that 12 days of recovery sufficiently repaired inner ear damage to completely restore DPOAE production. This result is different from that in adult chicken and may be related to the greater severity of acoustic damage in the adult ear, a reduced susceptibility of the young ear to acoustic trauma, or the ability of the young animal to more successfully repair the inner ear.

Spatial tuning curves along the chick basilar papilla in normal and sound-exposed ears

Intense sound exposure destroys chick short hair cells and damages the tectorial membrane. Within a few days postexposure, signs of repair appear resulting in nearly complete structural recovery of the inner ear. Tectorial membrane repair, however, is incomplete, leaving a permanent defect on the sensory surface. The consequences of this defect on cochlear function, and particularly frequency analysis, are unclear. The present study organizes the sound-induced discharge activity of cochlear nerve units to describe the distribution of neural activity along the tonotopic axis of the basilar papilla. The distribution of this activity is compared in 12-day postexposed and age-matched control groups. Spontaneous activity, tuning curves, and rate-intensity functions were measured in each unit. Discharge activity at 60 frequency and intensity combinations was identified in the tuning curves of hundreds of units. Activity at each of these criterion frequency/intensity combinations was plotted against the unit's characteristic frequency to construct spatial tuning curves (STCs). The STCs depict tone-driven cochlear nerve activity along the length of the papilla. Tuning sharpness, low- and high- frequency slopes, and the maximum response were quantified for each STC. The sharpness of tuning increased with increasing criterion frequency. However, within a frequency, increasing sound intensity yielded more broadly tuned STCs. Also, the high-frequency slope was consistently steeper than the low-frequency slope. The STCs of exposed ears exhibited slightly less frequency selectivity than control ears across all frequencies and larger maximum responses for STCs with criterion frequencies spanning the tectorial membrane defect. When rate-intensity types were segregated, differences were observed in the STCs between saturating and sloping-up units. We propose that STC shape may be determined by global mechanical events, as well as localized tuning and nonlinear processes associated with individual hair cells. The results indicated that 12 days after intense sound exposure, global and local contributions to spatially distributed neural activity are restored.

Hair cell regeneration: winging our way towards a sound future

The discovery of hair cell regeneration in the inner ear of birds provides new optimism that there may be a treatment for hearing and balance disorders. In this review we describe the process of hair cell regeneration in birds; including restoration of function, recovery of perception and what is currently known about molecular events, such as growth factors and signalling systems. We examine some of the key recent findings in both birds and mammals.

WDR1 colocalizes with ADF and actin in the normal and noise-damaged chick cochlea

Auditory hair cells of birds, unlike hair cells in the mammalian organ of Corti, can regenerate following sound-induced loss. We have identified several genes that are upregulated following such an insult. One gene, WDR1, encodes the vertebrate homologue of actin-interacting protein 1, which interacts with actin depolymerization factor (ADF) to enhance the rate of actin filament cleavage. We examined WDR1 expression in the developing, mature, and noise-damaged chick cochlea by in situ hybridization and immunocytochemistry. In the mature cochlea, WDR1 mRNA was detected in hair cells, homogene cells, and cuboidal cells, all of which contain high levels of F-actin. In the developing inner ear, WDR1 mRNA was detected in homogene cells and cuboidal cells by embryonic day 7, in the undifferentiated sensory epithelium by day 9, and in hair cells at embryonic day 16. We also demonstrated colocalization of WDR1, ADF, and F-actin in all three cell types in the normal and noise-damaged cochlea. Immediately after acoustic overstimulation, WDR1 mRNA was seen in supporting cells. These cells contribute to the structural integrity of the basilar papilla, the maintenance of the ionic barrier at the reticular lamina, and the generation of new hair cells. These results indicate that one of the immediate responses of the supporting cell after noise exposure is to induce WDR1 gene expression and thus to increase the rate of actin filament turnover. These results suggest that WDR1 may play a role either in restoring cytoskeletal integrity in supporting cells or in a cell signaling pathway required for regeneration.

Long-term changes in off-lesion endocochlear potential after induction of localized lesions in the lateral wall

Localized lesions were produced in various turns of the guinea pig cochlea by means of a photochemical reaction between systemically administered rose bengal dye and green light illumination. The endocochlear potential (EP) was measured at various off-lesion sites, and a morphological examination was performed. In a previous study, this same investigation was done at 3 days, at which time all sites apical to the lesion showed significant EP depression, and damage to the stria vascularis at the lesion was ongoing. In the present 2-week study, the apical EP values were not different from the basal values, and all experimental values were essentially the same as the EP values found in control animals. Morphological examination revealed that the previously damaged structures were greatly repaired. Localized damage and early apical EP depression followed by damage repair and eventual EP recovery could account for the clinical course of certain cases of idiopathic sudden hearing loss involving low-tone deafness.

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.

Neither endocochlear potential nor tegmentum vasculosum are affected in hearing impaired belgian waterslager canaries

We previously showed that the Belgian Waterslager canary strain is affected by a hereditary hearing loss that is associated with a reduced number of hair cells and hair cell pathologies in the basilar papilla. Since hair cell pathologies were also present in the sacculus, Weisleder et al. (1994) suggested that these birds are afflicted by Scheibe's like dysplasia, a cochleo-saccular defect. In mammals, cochleo-saccular defects are characterized primarily by the lack of an endocochlear potential and abnormalities in the Stria vascularis which only secondarily lead to hair cell loss (Steel and Bock, 1983; Steel, 1994; 1995). Here we report the endocochlear potential of six ears from three non-Belgian Waterslager canaries and three ears of two Belgian Waterslager canaries to decide if Waterslager canaries are affected by a cochleo-saccular or by a neuroepithelial defect. The mean endocochlear potential was 17.6+/-2. 5 mV in the non-Waterslager canaries and 20.3+/-0.6 mV in Waterslager canaries. In addition, and consistent with the presence of a normal endocochlear potential, light microscopy of the tegmentum vasculosum provided no evidence for pathology. These data show that Belgian Waterslager canaries are affected by a neuroepithelial rather than a cochleo-saccular inner ear defect.

Tip-link integrity on chick tall hair cell stereocilia following intense sound exposure

Hair bundle tip links have been implicated in the process of hair cell transduction, and previous studies have shown that acoustic overstimulation or exposure to low calcium can disrupt them. Severed tip links would thus be expected to cause a loss in hair cell function. This study investigates the presence of tip links on chick tall hair cells at three exposure durations and three recovery durations. After 4, 24, or 48 h of exposure, and 24, 96, and 288 h of recovery, the basilar papilla was harvested and prepared for scanning electron microscopy. Photomicrographs of hair bundles from sound-exposed and age-matched control ears were obtained in regions of the papilla adjacent to the 'patch' lesion. The percentage of tip links present on these hair bundles was determined from the photomicrographs. After 4, 24, or 48 h of exposure, an average of 49%, 41.1% and 52% of the observed sensory hairs exhibited links. This was significantly lower than that seen in the control ears (71.2%). There also was a reliable recovery of tip links between 24 and 48 h of exposure. The recovery continued and by 24 h post exposure, tip links were present on 61.3% of the sensory hairs. At subsequent recovery intervals, the mean number of tip links on sound-exposed tall hair cells was statistically the same as seen on control cells. The results indicated a predictable loss in the number of tip links during the exposure and their restoration within a relatively short time after the exposure. This structural damage to the tall hair cell, and its recovery, could account for some of the loss and recovery of function in the auditory periphery of these sound-damaged chicks.

A gene upregulated in the acoustically damaged chick basilar papilla encodes a novel WD40 repeat protein

The chick WDR1 gene is expressed at higher levels in the chick basilar papilla after acoustic overstimulation. The 3.3-kb WDR1 cDNA encodes a novel 67-kDa protein containing nine WD40 repeats, motifs that mediate protein-protein interactions. The predicted WDR1 protein has high sequence identity to WD40-repeat proteins in budding yeast (Saccharomyces cerevisiae), two slime molds (Dictyostelium discoideum and Physarum polycephalum), and the roundworm (Caenorhabditis elegans). The yeast and P. polycephalum proteins bind actin, suggesting that the novel chick protein may be an actin-binding protein. Sequence database comparisons identified mouse and human cDNAs with high sequence identity to the chick WDR1 cDNA. The mouse Wdr1 and human WDR1 proteins showed 95% sequence identity to each other and 86% identity to the chick WDR1 protein. Northern blot analysis of total RNA from the chick basilar papilla after noise trauma revealed increased levels of a 3.1-kb transcript in the lesioned area. The WDR1 gene was mapped to human chromosome 4, between 22 and 24 cM from the telomere of 4p.

Hair cell loss and regeneration after severe acoustic overstimulation in the adult pigeon

The extent of hair cell regeneration following acoustic overstimulation severe enough to destroy tall hair cells, was determined in adult pigeons. BrdU (5-bromo-2'-deoxyuridine) was used as a proliferation marker. Recovery of hearing thresholds in each individual animal was measured over a period of up to 16 weeks after trauma. In ears with loss of both short and tall hair cells, little or no functional recovery occurred. In ears with less damage, where significant functional recovery did occur, there were always a few rows of surviving hair cells left at the neural edge of the basilar papilla. In the region of hair cell loss, numerous BrdU labeled cells were found. However, only a small minority of these cells were regenerated hair cells, the majority being monolayer cells. Irrespective of the extent of the region of hair cell loss, regenerated hair cells were observed predominantly in a narrow strip at the transition from the abneural area of total hair cell loss and the neural area of hair cell survival. With increasing damage this strip moved progressively towards the neural edge of the papilla. No regeneration of hair cells was observed in the abneural region of total hair cell loss, even up to 16 weeks after trauma. The results indicate that there is a gradient in the destructive effect of loud sound across the width of the basilar papilla, from most detrimental at the abneural edge to least detrimental at the neural edge. Both tall and short hair cells can regenerate after sound trauma. Whether they do regenerate or not depends on the degree of damage to the area of the papilla where they normally reside. Regeneration of new hair cells occurs only in a narrow longitudinal band, which moves from abneural into the neural direction with increasing damage. In the area neural to this band, hair cells survive the overstimulation. In the area abneural to this band, sound damage is so severe, that no regeneration of hair cells occurs. As a consequence morphological and functional deficits persist.

Two-tone rate suppression boundaries of cochlear ganglion neurons in chickens following acoustic trauma

The purpose of the present study was to examine the effects of acoustic trauma and hair cell loss and regeneration on the two-tone rate suppression (TTRS) boundaries of cochlear ganglion neurons in chickens. Chickens were exposed for 48 hours to a 525-Hz, 120-dB SPL tone which destroyed the hair cells and tectorial membrane in a crescent-shaped patch along the abneural side of the basilar papilla. Afterwards, TTRS boundaries were recorded from cochlear ganglion neurons at 0-1, 5, 14, and 28 days postexposure. Acoustic trauma reduced the percentage of neurons with TTRS boundaries below CF (TTRSb) (52.6% to 8.2%) and above CF (TTRSa) (88.4% to 46.6%). In addition, the exposure reduced TTRS boundary slopes, elevated best suppression threshold (BST), and increased the frequency separation between the tips of the TTRS boundaries and CF. All the TTRS measures started to recover by 5 days postexposure and by 14 days and 28 days postexposure, most measures had recovered to normal levels. However, the BST, TTRS slopes, and the frequency separation of TTRSb boundaries from CF were still slightly abnormal near the exposure frequency. In addition, the percentage of neurons with TTRS below CF was reduced significantly. The partial recovery of TTRS boundaries is presumably due to the regeneration of hair cells and the lower honeycomb layer of the tectorial membrane. The residual TTRS deficits observed 28 days postexposure were most closely associated with the missing upper fibrous layer of the tectorial membrane.

Steady state EP is not responsible for hearing loss in adult chickens following acoustic trauma

The steady state DC endocochlear potential (EP) in young chicks shows a large decrease after acoustic overstimulation followed by a rapid recovery that parallels the recovery of threshold (Poje et al., Hear. Res. 82 (1995) 197-204). These results raise a question as to whether or not the EP could account for the hearing loss and make a significant contribution to the recovery of the threshold. In contrast to results in young chicks, we show that acoustic overstimulation, which causes extensive hair cell damage, does not cause a decrease in the steady state EP in adult chickens. However, there is a significant reduction in the negative EP seen during anoxia which persists even after 4 weeks of recovery. Thus, our results indicate that the steady state EP cannot account for the hearing loss observed in adult chickens.

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.

Regeneration after tall hair cell damage following severe acoustic trauma in adult pigeons: correlation between cochlear morphology, compound action potential responses and single fiber properties in single animals

The time course of recovery of compound action potential (CAP) thresholds was observed in individual adult pigeons after severe acoustic trauma. Pigeons were overstimulated with a tone of 0.7 kHz and 136-142 dB SPL presented to one ear for 1 h under general anesthesia. Recovery of CAP audiograms was monitored at regular intervals after trauma. A new semi-stereotaxic approach to the peripheral part of the auditory nerve was developed. This permitted activity from single auditory nerve fibers to be recorded over a wide range of characteristic frequencies (CFs), including high CFs, without having to open the inner ear. Single unit recordings were made after three weeks and after 4 or more months of recovery. The time course of recovery, the single unit properties, and the morphological status of the basilar papilla were correlated. The CAP was abolished in all animals after overstimulation. Three groups of animals were identified according to the functional recovery of the CAP thresholds recorded at regular intervals with implanted electrodes: Group 1: Fast functional recovery starting immediately after trauma, followed by recovery to pre-exposure values within 3 weeks. Group 2: Slow functional recovery of threshold starting 1-2 weeks after trauma and ending 4-5 weeks after trauma. A mean residual hearing loss of 26.3 dB at 2 kHz remained. Group 3: No recovery of CAP thresholds up to 8 months after trauma. Three weeks after trauma, very few responsive neurons were found in groups 2 and 3. Tuning curves were very broad and sometimes irregular in shape. Thresholds were very high, around 120 dB SPL. Spontaneous firing rate was much reduced, especially in neurons with high CFs. After 4 or more months of recovery, the response properties of single units in group 1 had only partially recovered. Thresholds and sharpness of tuning of many single units were normal: however, in general they were still poorer than in control animals. Spontaneous firing rate was comparable to control animals. Neurons from animals in group 2 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 CFs. The basilar papilla in animals without recovery showed total loss of the sensory epithelium. The basal lamina of the basilar membrane, however, remained intact and was covered with cuboidal cells. 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 functional recovery from severe damage to both short (abneural) and tall (neural) hair cells occurs in adult birds. However, the onset of recovery is delayed and the time course is slower than after destruction of short (abneural) hair cells alone. Furthermore recovery is incomplete, both functionally and morphologically. There are residual permanent hearing losses and regeneration of short (abneural) hair cells is incomplete.

Tuning, spontaneous activity and tonotopic map in chicken cochlear ganglion neurons following sound-induced hair cell loss and regeneration

Adult chickens were exposed for 48 h to a 525 Hz, 120 dB SPL tone that destroyed the hair cells and tectorial membrane in a crescent-shaped patch along the abneural edge of the basilar papilla. Single-unit recordings were obtained from cochlear ganglion neurons 0-1, 5, 14 and 28 days post-exposure to determine what effect the cochlear lesion had on neural discharge patterns and if the discharge patterns fully recovered. Immediately after exposure, the tuning curves were extremely broad and CF thresholds were elevated by 30-40 dB. In addition, the average spontaneous rate and percentage of neurons with interspike interval histograms with preferred intervals were greatly reduced. Tuning curves and spontaneous activity started to recover by 5 days post-exposure; however, some W-shaped tuning curves with two distinct tips and a hypersensitive tail were observed at this time. W-shaped tuning curves disappeared and spontaneous activity recovered to normal levels 14-28 days post-exposure. However, the CF thresholds of the most sensitive neurons were still slightly elevated, tuning curve slopes below CF were shallower than normal, and thresholds in the low-frequency tail of the tuning curves were often hypersensitive. These functional deficits were most closely associated with residual damage to the upper fibrous layer of the tectorial membrane. To determine if the cochlear frequency-place map was altered by the cochlear lesion, four physiologically characterized neurons were labeled with biocytin at 5 days post-exposure. The CFs of the labeled neurons were consistent with the normal frequency-place map (Chen et al. (1994) Hearing Research 81, 130-136) indicating that the tonotopic map was not altered.

Effects of kanamycin ototoxicity and hair cell regeneration on the DC endocochlear potential in adult chickens

High doses of aminoglycoside antibiotics cause massive damage to the avian basilar papilla. The resulting functional loss could conceivably arise from the reduction in the DC endocochlear potential (EP) due to impairment of the tegmentum vasculosum (TV) or to shunting of current through the damaged sensory epithelium. To test this hypothesis, the EP was measured in adult chickens after destroying hair cells in the basal half of the cochlea with a high dose (400 mg/kg per day for 10 days) of kanamycin (KM). KM treatment caused an increase in the steady-state EP from +18.1 to +23.3 mV and a decrease in the magnitude of the negative EP from -42.0 to -19.2 mV. The EP showed almost no change between 1 and 2 days and 1 week post-KM treatment. After 4 weeks of recovery, most hair cells had regenerated; however, the steady-state EP was still elevated by 13% and the negative EP was depressed by 37%. These results suggest that functional loss as shown by the large reduction in cochlear microphonic (CM) and the elevated thresholds of compound action potential (CAP) following KM treatment is not due to a reduction in the EP but may arise from functional deficits in the hair cells and/or the auditory nerve.