Cochlear Integrity Research Articles

Auditory outcomes after scala vestibuli array insertion are similar to those after scala tympani insertion 1year after cochlear implantation.

In cochlear implantation, a scala vestibuli (SV) insertion of an electrode array is a rare occurrence and is reported to be linked to poor hearing outcomes. Using the same electrode array, the auditory performance of patients with a complete SV location was compared with that of patients having a complete scala tympani (ST) location 1year after implantation. Thirty-three patients were included in this retrospective case-control study (SV, n = 12; ST, n = 21). The matching criteria were electrode array type, age at implantation, and duration of severe or profound deafness. The array location was analyzed using 3D reconstruction of postoperative CT scans. Postoperative audiological evaluation of the implanted ear was performed using pure-tone audiometry, speech recognition of monosyllabic words in quiet, and words and sentences in noise. On the preoperative CT scan, six patients in the SV group presented with both round window (RW) and ST ossification, three with RW ossification alone, and three with no RW ossification. Auditory performance did not differ between SV and ST groups 1year after cochlear implantation. Speech recognition of words was 49 ± 7.6% and 56 ± 5.0% in quiet and 75 ± 9.5% and 66 ± 6.0% in noise in SV and ST groups, respectively. ST insertion is the gold standard that allows the three cochlear scalae to preserve scalar cochlear integrity. However, 1year after implantation, a planned or unexpected SV insertion is not detrimental to hearing outcomes, providing similar auditory performance in quiet and noise to ST insertion.

Galectin-3 protects auditory function in female mice

Sex differences in the development of sensorineural hearing loss have been recognized in various inner ear disorders, but the molecular basis for such differences is poorly understood. Autosomal genes have been shown to cause sex differences in disease susceptibility, but many genes exerting sex-dependent effects on auditory function remain to be identified. Galectin-3 (Gal-3), a protein encoded by the autosomal gene Lgals3, is a member of the β-galactoside-binding protein family, and has been linked to multiple biological processes, including immune responses, apoptosis, and cell adhesion. Here, we investigated auditory function and hair cell integrity in Gal-3 knockout (KO, Lgals3−/−) and wild-type (WT, Lgals3+/+) mice from age 1 to 6 months. KO mice show a more rapid age-related increase in ABR thresholds compared to WT mice. Noticeably, the threshold deterioration in female KO mice is significantly greater than in the male KO and WT mice. The ABR threshold elevation manifests over a broad frequency range in female KO mice, whereas the threshold elevations are confined to high frequencies in the male KO and WT mice. Moreover, DPOAE input/output functions reveal a similar pattern of auditory dysfunction, with the female KO mice displaying a significantly greater reduction in DPOAE amplitudes than male KO mice and WT mice of both sexes. Finally, age-related outer hair cell loss is greater for female KO mice compared to male KO mice and WT mice of both sexes. Together, these results indicate that Gal-3 deficiency exacerbates age-related cochlear degeneration and auditory dysfunction in female mice. Our study identifies Gal-3 as a sex-dependent molecule for maintaining female cochlear integrity.

Multi-Frequency Intraoperative Monitoring of Hearing Preservation during Cochlear Implantation.

(1) Background: Current indications for cochlear implants (CIs) have expanded to include patients with appreciable low-frequency hearing. However, longitudinal results indicate that only one-third of these recipients retain full hearing preservation. In another words, the remaining two-thirds lose this facility either partially or fully. This points to the need to better understand the impact of cochlear implantation on cochlear integrity. Intracochlear electrocochleography (ECochG) involves the recording of electrical potentials generated in the inner ear in response to acoustic stimuli, and previous studies have shown that these potentials give an indication of residual inner ear function. Aim of the research: The aim is to monitor intracochlear ECochG during CI surgery and gain a better understanding of how the implant impacted inner ear function. A newly developed SPL Chirp was used for stimulation. (2) Methods: Intracochlear ECochG signals were measured in a subject with residual preoperative low-frequency hearing, while an electrode array was introduced into the cochlea and was continued until the round window was sealed. Afterwards, surgical events were reviewed with the surgeon; preoperative and postoperative radiological data and hearing thresholds were also evaluated. (3) Conclusions: Real-time intraoperative monitoring, with multifrequency evaluation and video recording, has the potential to allow surgeons and audiologists to continuously assess cochlear function. ECochG monitoring may be a useful tool during cochlear implantation to gain frequency-specific information on the status of the patient’s hearing, assisting surgeons to lower hearing trauma during the operation.

Assessment of cochlear electrophysiology in typically developing children and children with auditory processing disorder

ObjectiveChildren with auditory processing disorder (APD) are reported to have abnormal auditory brainstem responses (ABR) but little is understood about their cochlear integrity. Poor cochlear integrity can affect neural responses. In this study, cochlear and auditory brainstem integrity was investigated in children with APD. MethodTwenty children with APD, sixteen typically developing children and twenty adults participated in this study. Click evoked electrocochleography (ECochG) and ABRs were recorded from all the participants. Cochlear responses were analyzed using a) latency and amplitude of summating potential; action potential, b) transmission time between summating potential and action potential, c) summating potential/action potential amplitude ratio and d) action potential latency difference to condensation and rarefaction polarity. Amplitude in the ABR components was examined. ResultsChildren with APD showed similar cochlear function to the typically developing children. There were no significant differences in wave I amplitude between children with APD and typically developing children. However, wave V amplitude was significantly reduced in children with APD compared to typically developing children. ConclusionIn the absence of any functional differences in the cochlea, children with APD can show poor amplitude in the later components of the ABR. The ABR anomalies observed in children with APD arise due to poor neural processing, possibly after the first auditory synapse.

Association Between Nonoptimal Blood Pressure and Cochlear Function.

The association between hearing loss and risk factors for cardiovascular disease, including high blood pressure (BP), has been evaluated in numerous studies. However, data from population- and laboratory-based studies remain inconclusive. Furthermore, most prior work has focused on the effects of BP level on behavioral hearing sensitivity. In this study, we investigated cochlear integrity using distortion product otoacoustic emissions (DPOAEs) in persons with subtle elevation in BP levels (nonoptimal BP) hypothesizing that nonoptimal BP would be associated with poorer cochlear function. Sixty individuals [55% male, mean age = 31.82 (SD = 11.17) years] took part in the study. The authors measured pure-tone audiometric thresholds from 0.25 to 16 kHz and computed four pure-tone averages (PTAs) for the following frequency combinations (in kHz): PTA0.25, 0.5, 0.75, PTA1, 1.5, 2, 3, PTA4, 6, 8, and PTA10, 12.5, 16. DPOAEs at the frequency 2f1-f2 were recorded for L1/L2 = 65/55 dB SPL using an f2/f1 ratio of 1.22. BP was measured, and subjects were categorized as having either optimal BP (systolic/diastolic <120 and <80 mm Hg) or nonoptimal BP (systolic ≥120 or diastolic ≥80 mm Hg or use of antihypertensives). Between-group differences in behavioral thresholds and DPOAE levels were evaluated using 95% confidence intervals. Pearson product-moment correlations were run to assess the relationships between: (1) thresholds (all four PTAs) and BP level and (2) DPOAE [at low (f2 ≤ 2 kHz), mid (f2 > 2 kHz and ≤10 kHz), and high (f2 > 10 kHz) frequency bins] and BP level. Linear mixed-effects models were constructed to account for the effects of BP status, stimulus frequency, age and sex on thresholds, and DPOAE amplitudes. Significant positive correlations between diastolic BP and all four PTAs and systolic BP and PTA0.25, 0.5, 0.75 and PTA4, 6, 8 were observed. There was not a significant effect of BP status on hearing thresholds from 0.5 to 16 kHz after adjustment for age, sex, and frequency. Correlations between diastolic and systolic BP and DPOAE levels were statistically significant at the high frequencies and for the relationship between diastolic BP and DPOAE level at the mid frequencies. Averaged across frequency, the nonoptimal BP group had DPOAE levels 1.50 dB lower (poorer) than the optimal BP group and differences were statistically significant (p = 0.03). Initial findings suggest significant correlations between diastolic BP and behavioral thresholds and diastolic BP and mid-frequency DPOAE levels. However, adjusted models indicate other factors are more important drivers of impaired auditory function. Contrary to our hypothesis, we found that subtle BP elevation was not associated with poorer hearing sensitivity or cochlear dysfunction. We consider explanations for the null results. Greater elevation in BP (i.e., hypertension itself) may be associated with more pronounced effects on cochlear function, warranting further investigation. This study suggests that OAEs may be a viable tool to characterize the relationship between cardiometabolic risk factors (and in particular, stage 2 hypertension) and hearing health.

Redox activation of excitatory pathways in auditory neurons as mechanism of age-related hearing loss

Age-related hearing (ARHL) loss affects a large part of the human population with a major impact on our aging societies. Yet, underlying mechanisms are not understood, and no validated therapy or prevention exists. NADPH oxidases (NOX), are important sources of reactive oxygen species (ROS) in the cochlea and might therefore be involved in the pathogenesis of ARHL. Here we investigate ARHL in a mouse model. Wild type mice showed early loss of hearing and cochlear integrity, while animals deficient in the NOX subunit p22phox remained unaffected up to six months. Genes of the excitatory pathway were down-regulated in p22phox-deficient auditory neurons. Our results demonstrate that NOX activity leads to upregulation of genes of the excitatory pathway, to excitotoxic cochlear damage, and ultimately to ARHL. In the absence of functional NOXs, aging mice conserve hearing and cochlear morphology. Our study offers new insights into pathomechanisms and future therapeutic targets of ARHL.

Sphingosine 1-phosphate-mediated activation of ezrin-radixin-moesin proteins contributes to cytoskeletal remodeling and changes of membrane properties in epithelial otic vesicle progenitors

Hearing loss is among the most prevalent sensory impairments in humans. Cochlear implantable devices represent the current therapies for hearing loss but have various shortcomings. ERM (ezrin- radixin -moesin) are a family of adaptor proteins that link plasma membrane with actin cytoskeleton, playing a crucial role in cell morphology and in the formation of membrane protrusions. Recently, bioactive sphingolipids have emerged as regulators of ERM proteins. Sphingosine 1-phosphate (S1P) is a pleiotropic sphingolipid which regulates fundamental cellular functions such as proliferation, survival, migration as well as processes such as development and inflammation mainly via ligation to its specific receptors S1PR (S1P1–5). Experimental findings demonstrate a key role for S1P signaling axis in the maintenance of auditory function. Preservation of cellular junctions is a fundamental function both for S1P and ERM proteins, crucial for the maintenance of cochlear integrity. In the present work, S1P was found to activate ERM in a S1P2-dependent manner in murine auditory epithelial progenitors US/VOT-E36. S1P-induced ERM activation potently contributed to actin cytoskeletal remodeling and to the appearance of ionic currents and membrane passive properties changes typical of more differentiated cells. Moreover, PKC and Akt activation was found to mediate S1P-induced ERM phosphorylation. The obtained findings contribute to demonstrate the role of S1P signaling pathway in inner ear biology and to disclose potential innovative therapeutical approaches in the field of hearing loss prevention and treatment.

Air and Bone Conduction Frequency-specific Auditory Brainstem Response in Children with Agenesis of the External Auditory Canal.

Introduction The tone-evoked auditory brainstem responses (tone-ABR) enable the differential diagnosis in the evaluation of children until 12 months of age, including those with external and/or middle ear malformations. The use of auditory stimuli with frequency specificity by air and bone conduction allows characterization of hearing profile. Objective The objective of our study was to compare the results obtained in tone-ABR by air and bone conduction in children until 12 months, with agenesis of the external auditory canal. Method The study was cross-sectional, observational, individual, and contemporary. We conducted the research with tone-ABR by air and bone conduction in the frequencies of 500 Hz and 2000 Hz in 32 children, 23 boys, from one to 12 months old, with agenesis of the external auditory canal. Results The tone-ABR thresholds were significantly elevated for air conduction in the frequencies of 500 Hz and 2000 Hz, while the thresholds of bone conduction had normal values in both ears. We found no statistically significant difference between genders and ears for most of the comparisons. Conclusion The thresholds obtained by bone conduction did not alter the thresholds in children with conductive hearing loss. However, the conductive hearing loss alter all thresholds by air conduction. The tone-ABR by bone conduction is an important tool for assessing cochlear integrity in children with agenesis of the external auditory canal under 12 months.

Role of Endogenous Bone Marrow Stem Cells Mobilization in Repair of Damaged Inner Ear in Rats.

Background and ObjectivesThe utilization of the stem cells is widely used in the last few years in different fields of medicine, either by external transplantation or endogenous mobilization, most of these studies still experimental on animals; few were tried on human as in the spinal cord injury or myocardial infarction. As regard its use in the inner ear, stem cell transplantation was examined in many previous studies, while the mobilization idea is a new method to be experimented in inner ear hair cell regeneration. The present work assessed the possibility of mobilizing endogenous bone marrow derived stem cells (SCs) in rats using granulocyte colony stimulating factor (G-CSF) to induce regeneration and repair to experimentally damaged inner ear hair cells by Amikacin injection.MethodsThe study included thirty adult Sprague Dawley male rats. Experimental induction of inner ear damage was done by repeated intratympanic injection of amikacin sulfate. Mobilization of bone marrow SCs was provoked by subcutaneous injection of GCSF. Cochlear integrity, induction of hearing loss and functional recovery of sensory hearing loss were assessed using Distortion Product Otoacoustic Emission (DPOAEs). The morphological alteration and recovery of the organ of Corti was assessed histologically using the light and scanning electron microscopes.ResultsAfter six month duration, there was improvement in 50% of the sensorineural DPOAE results. Functional recovery coincided with the repair of structural components of organ of Corti.ConclusionsSCs mobilization by G-CSF is a promising alternative method for replacement therapy in sensorineural hearing loss.

Endogenous protection against oxidative stress caused by cisplatin: role of superoxide dismutase

Conclusion: The administration of cisplatin induces the activation of superoxide dismutase (SOD) as a response to oxidative stress in the cochleae of Sprague-Dawley rats and this activation is proportional to the activation of the intrinsic pathway of apoptosis. Objectives: To determine the role of the antioxidant endogenous mechanism in the preservation of cochlear integrity and function in an experimental model of cisplatin ototoxicity. Methods: Sixteen Sprague-Dawley rats were studied at 7 days after intraperitoneal injection of CDDP (n = 8) or 10 ml/kg NaCl 0.9% w/v in the control group (n = 8) by means of auditory steady-state responses. These findings were compared with the expression of SOD and caspase-3/7 and caspase-9 activities. Results: Groups receiving cisplatin showed increased auditory thresholds after injection of cisplatin and control groups maintained normal hearing. Measurements of caspase-3/7 and caspase-9 showed a significant increase in cisplatin-treated rats. A significantly increased activity of total SOD in whole cochlear extracts was observed in animals from the CDDP groups vs control animals. Likewise, differences between CDDP groups were also statistically significant.

Ototoxicity and otoprotection in the inner ear of guinea pigs using gentamicin and amikacin: ultrastructural and functional aspects

SummaryOtotoxicity is still a challenge to medicine. The discovery of self-protecting endogenous mechanisms of the outer hair cells associated with their functional and ultra-structural assessment methods has opened new horizons in the understanding and controlling of these mechanisms. Aim: this paper aimed at establishing whether or not underdoses of gentamicin could protect the inner ear against the harmful effects of amikacin, based on these protection mechanisms and determine if the otoacoustic emission amplitudes could be associated with the level of hair cell integrity. Materials and Methods: Experimental study. We used 31 guinea pigs. They were injected with saline solution, gentamicin and amikacin, alone and in combinations -intramuscular injections - during 12, 30 and 42 days. The otoacoustic emissions were recorded in the beginning and at the end of the experiment, comparing it with the cochlear integrity study carried out by electron microscopy. Results: gentamicin underdoses did not protect the inner ear against amikacin toxicity; the reduction in otoacoustic emissions was strongly associated with an increase in hair cell lesions. Conclusion: these findings help understand inner ear otoprotection and ototoxicity. Establishing the correlation between the emissions amplitude an cell integrity plays an important role in the follow up of hair cell damage, with possible monitoring of ototoxicity caused by drugs in humans.

Cochlear ablation in adult ferrets results in changes in insulin-like growth factor-1 and synaptophysin immunostaining in the cochlear nucleus

Afferent activity modulates synaptic plasticity as well as the levels of activity-dependent molecules such as growth factors. Disruption of this activity due to deafferentation has been shown to result in an altered trophic support and consequently in changes in neuronal excitability and synaptic transmission. In the present study, to test whether lack of cochlear integrity results in changes in insulin-growth factor-1 (IGF-1) and synaptophysin immunostaining in the cochlear nucleus, the first relay structure in the auditory pathway, unilateral cochlear ablations were performed in adult ferrets. Changes in IGF-1 and synaptophysin immunostaining were assessed in the anteroventral (AVCN), posteroventral (PVCN) and dorsal cochlear nucleus (DCN) at 1, 20 and 90 days after deafferentation. An increase in IGF-1 immunostaining within AVCN, PVCN and DCN was observed ipsilaterally at all survival times after cochlear ablation when compared with the contralateral side and unoperated animals. This increase was accompanied by a significant ipsilateral increase in the mean gray level of synaptophysin immunostaining as well as a decrease in the area of synaptophysin immunostaining at 1 and 20 days after the ablation in AVCN, PVCN and DCN compared with the contralateral side and control animals. These changes in synaptophysin immunostaining were no longer present 90 days after cochlear ablation. The present results provide evidence of a persistent upregulation in IGF-1 and a transitory upregulation in synaptophysin levels in the cochlear nucleus that may reflect neuroprotective mechanisms following the loss of trophic support from spiral ganglion neurons.

Origin of sound‐evoked EMG responses in human masseter muscles

Sound is a natural stimulus for both cochlear and saccular receptors. At high intensities it evokes in active masseter muscles of healthy subjects two overlapping reflexes: p11/n15 and p16/n21 waves, whose origin has not yet been demonstrated. Our purpose was to test which receptor in the inner ear is responsible for these reflexes. We compared masseter EMG responses induced in normal subjects (n = 9) by loud clicks (70-100 dB normal hearing level (NHL), 0.1 ms, 3 Hz) to those evoked in subjects with a selective lesion of the cochlea (n = 5), of the vestibule (n = 1) or with mixed cochlear-vestibular failure (n = 5). In controls, 100 dB clicks induced bilaterally, in the unrectified mean EMG (unrEMG), a clear p11 wave followed by a less clear n15 wave and a subsequent n21 wave. Lowering the intensity to 70 dB clicks abolished the p11/n15 wave, while a p16 wave appeared. Rectified mean EMG (rectEMG) showed, at all intensities, an inhibitory deflection corresponding to the p16/n21 wave in the unrEMG. Compared to controls, all deaf subjects had a normal p11 wave, together with more prominent n15 wave; however, the p16/n21 waves, and their corresponding inhibition in the rectEMG, were absent. The vestibular patient had bilaterally clear p11 waves only when 100 dB clicks were delivered bilaterally or to the unaffected ear. Stimulation of the affected ear induced only p16/n21 waves. Data from mixed patients were consistent with those of deaf and vestibular patients. We conclude that click-induced masseter p11/n15 waves are vestibular dependent, while p16/n21 waves depend on cochlear integrity.

Residual hearing in cochlear implant patients

Preservation of residual hearing should be a desirable outcome of implant surgery. Prevention of neural degeneration due to loss of residual hair cells, together with the continuous progress in cochlear implant technology should be able to preserve cochlear integrity as well as possible. The degree of hearing preservation may vary depending on surgical approach, maximum insertion depth and other factors not uniformly considered to date. The aim of this retrospective case controlled study is to evaluate residual hearing after cochlear implant surgery. In particular, we analyzed data obtained with use of two different kinds of electrode arrays, with and without rigid introductor (stylet). We report the results on 37 patients with measurable preoperative hearing thresholds, mean age of 28 years (5-70 years), having the following implants: seven Advanced Bionics, four Med-El, 24 Cochlear, two MXM; 19 of them were performed using the stylet and the other 18 without it. A minimally invasive surgical approach was performed with a short retroauricular incision and a 1.2 mm cochleostomy. A complete electrode array insertion was obtained in all patients. Responses to pure-tone stimuli were measured for each ear in pre-implantation conditions and 3-12 months after surgery. After implantation 14 patients (38%) showed no hearing threshold variation, 29 (78%) maintained an appreciable hearing threshold level in the implanted ear, 8 (22%) had a total loss of residual hearing. Median increases of threshold levels were, in all 37 studied patients, 5, 10, 10 and 5 dB HL, respectively, for 125, 250, 500 and 1 kHz. For the 18 patients having implants without the stylet median increases of threshold levels were 0, 10, 5 and 7 dB HL; in the stylet group, they were 10, 5, 5 and 10 dB HL. On a comparison between the stylet and the non-stylet group, no significant differences in mean hearing threshold worsening were found. Data seem to suggest that cochlear function is less sensitive to mechanical trauma during implant surgery than was thought. Besides, electrode array stiffness seems not to influence preservation of cochlear residual functional integrity. Finally, the authors hypothesize a direct spiral ganglion activation under strong mechanical stimulation.

The value of otoacoustic emissions in the investigation of noise damage

Otoacoustic emissions (OAEs) are sounds generated by a healthy cochlea, as a by-product of the outer hair cell (OHC) activity, and therefore they can provide valuable information on OHC function. Hence, OAE recording techniques can be used for assessment of noise damage, as the most extensive noise-induced changes occur in the cochlea, involving the OHCs, which seem to be particularly vulnerable to noise. OAE recording techniques are simple, non-invasive, and they provide objective information and they are available in routine clinical practice. OAEs are characterized by a high sensitivity in detecting subtle changes of the OHCs and this property makes them very valuable in identifying early noise induced change, even before any notable shift in the audiometric thresholds. Although there is a high inter-individual variability, OAEs display remarkable intra-individual stability and, therefore, they are useful in intra-individual monitoring of subtle changes in cochlear integrity following noise exposure. I...

Impact of electrode insertion depth on intracochlear trauma

To assess the effect of cochlear implant (CI) insertion depth and surgical technique on intracochlear trauma. Twenty-one fresh human temporal bones were implanted with CI electrodes and underwent histologic processing and evaluation. Specimens were grouped into 3 categories: 1) soft implantation technique and standard electrode; 2) soft implantation technique and flexible prototype array; 3) forceful implantations and standard electrode. Based on the grading system (1 to 4), 2 numeric values were calculated indicating the overall severity of cochlear damage (trauma indices). Mean trauma index values were 13.8, 36.3, and 59.2 for group 1, 2, and 3, respectively. Differences in cochlear trauma (trauma index) were nonsignificant between specimens in groups 1 and 2 but were significant between groups 1 and 3. This study gives evidence that intracochlear trauma increases with deep insertions. Thus, in cases where cochlear integrity might be important, limited insertions should be achieved.

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 ∼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.

Effect of cochlear integrity on cochlear nucleus neuron glucose metabolism in aged adult broiler chickens

Abrupt removal of excitatory input is devastating to post-synaptic neurons in normally functioning sensory systems. In both mammalian and avian auditory systems, abrupt temporary or permanent experimental deafferentation stimulates a cascade of changes in central auditory structures that can result in neuron death. Effects of naturally occurring progressive deafferentation on central auditory structure and function have not been fully described. Extensive naturally occurring cochlear damage is found in some aged chickens, despite their regenerative capacity, providing the opportunity to examine the effects of this type of deafferentation on the avian cochlear nucleus (nucleus magnocellularis, NM).Previous evaluation of NM oxidative metabolism using cytochrome oxidase histochemistry revealed that naturally occurring cochlear damage results in down-regulated metabolism in corresponding regions of NM. It is unknown how progressive hair cell damage and loss affects NM glucose uptake. Here, NM glucose metabolism is assessed using 2-deoxyglucose uptake as a marker for metabolic activity in the presence of normal, mildly damaged, severely damaged, and totally damaged cochlear hair cells. Results indicate that while severe and total cochlear damage significantly decrease NM oxidative metabolism, only total damage results in significantly decreased NM glucose metabolism. Results are discussed in the context of functional reorganization and trophic support.

Contributions of age, cochlear integrity, and auditory environment to avian cochlear nucleus metabolism

Most commercially raised broiler chickens display progressive cochlear degeneration with age [Hear. Res. 166 (2002) 82]. Recent work examining the effects of age and cochlear degeneration on avian cochlear nucleus (nucleus magnocellularis, NM) metabolism showed that changes in metabolic activity occur with age and cochlear damage [Hear. Res. 175 (2003) 101]. The auditory environment also differed between facilities housing young and adult birds. The relative contributions of age, cochlear degeneration, and auditory environment to these changes in NM metabolism are unknown. Using cytochrome oxidase (CO) histochemistry, NM neuron metabolism is examined in several age groups of birds under varying conditions. When normal cochlear integrity and auditory environment are held constant, CO staining is significantly decreased in adult vs. young birds. When age and auditory environment are held constant, CO staining is significantly decreased in birds with damaged vs. normal cochleae. When age and normal cochlear integrity are held constant, CO staining is significantly decreased in birds living in a quiet vs. noisy environment. All factors examined cause changes in CO staining, which is indicative of NM metabolic activity. Results are discussed in the context of mitochondrial aging, afferent regulation, and auditory deprivation and enrichment.

Time course and quantification of changes in cochlear integrity observed in commercially raised broiler chickens

Extensive cochlear hair cell damage and loss occurs in aged broiler chickens. We describe the time course and several characteristics of this decline in cochlear integrity in 19-, 30-, 38- and 66-week-old commercially raised broiler chickens. The 19-week-old group is normal and serves as a baseline for comparison. Generally, cochlear damage increases in severity and percent length of the cochlea with age. Hair cell density increases from the base to the apex. Density is similar across the groups in regions of the cochlea that sustained little or no damage, and decreases in regions of extreme damage. Counts of normal and abnormal hair cells are inversely related. A subset of 66-week-old birds has higher density measurements and increased hair cell counts in the apical region of the cochlea. The progressive damage found in these commercially raised birds is described in the context of both the effects of age and noise exposure on the auditory system. Two additional groups of birds were raised at the University of Kansas Medical Center in a quieter environment to determine the cause of the damage seen in age-matched commercially raised birds. These cochleae are largely normal; a small number displayed damage. This suggests that noise exposure exacerbates naturally occurring cochlear degradation.