Etiology of distinct membrane excitability in pre- and posthearing auditory neurons relies on activity of Cl- channel TMEM16A.

The developmental rehearsal for the debut of hearing is marked by massive changes in the membrane properties of hair cells (HCs) and spiral ganglion neurons (SGNs). Whereas the underlying mechanisms for the developing HC transition to mature stage are understood in detail, the maturation of SGNs from hyperexcitable prehearing to quiescent posthearing neurons with broad dynamic range is unknown. Here, we demonstrated using pharmacological approaches, caged-Ca(2+) photolysis, and gramicidin patch recordings that the prehearing SGN uses Ca(2+)-activated Cl(-) conductance to depolarize the resting membrane potential and to prime the neurons in a hyperexcitable state. Immunostaining of the cochlea preparation revealed the identity and expression of the Ca(2+)-activated Cl(-) channel transmembrane member 16A (TMEM16A) in SGNs. Moreover, null deletion of TMEM16A reduced the Ca(2+)-activated Cl(-) currents and action potential firing in SGNs. To determine whether Cl(-) ions and TMEM16A are involved in the transition between pre- and posthearing features of SGNs we measured the intracellular Cl(-) concentration [Cl(-)]i in SGNs. Surprisingly, [Cl(-)]i in SGNs from prehearing mice was ∼90 mM, which was significantly higher than posthearing neurons, ∼20 mM, demonstrating discernible altered roles of Cl(-) channels in the developing neuron. The switch in [Cl(-)]i stems from delayed expression of the development of intracellular Cl(-) regulating mechanisms. Because the Cl(-) channel is the only active ion-selective conductance with a reversal potential that lies within the dynamic range of SGN action potentials, developmental alteration of [Cl(-)]i, and hence the equilibrium potential for Cl(-) (ECl), transforms pre- to posthearing phenotype.

Functional significance of K+ channel ß-subunit KCNE3 in auditory neurons.

The KCNE3 ß-subunit interacts with and regulates the voltage-dependent gating, kinetics, and pharmacology of a variety of Kv channels in neurons. Because a single neuron may express multiple KCNE3 partners, it is impossible to predict the overall functional relevance of the single transmembrane domain peptide on the pore-forming K(+) channel subunits with which it associates. In the inner ear, the role of KCNE3 is undefined, despite its association with Meniere disease and tinnitus. To gain insights on the functional significance of KCNE3 in auditory neurons, we examined the properties of spiral ganglion neurons (SGNs) in Kcne3 null mutant neurons relative to their age-matched controls. We demonstrate that null deletion of Kcne3 abolishes characteristic wide variations in the resting membrane potentials of SGNs and yields age-dependent alterations in action potential and firing properties of neurons along the contour of the cochlear axis, in comparison with age-matched wild-type neurons. The properties of basal SGNs were markedly altered in Kcne3(-/-) mice compared with the wild-type controls; these include reduced action potential latency, amplitude, and increased firing frequency. Analyses of the underlying conductance demonstrate that null mutation of Kcne3 results in enhanced outward K(+) currents, which is sufficient to explain the ensuing membrane potential changes. Additionally, we have demonstrated that KCNE3 may regulate the activity of Kv4.2 channels in SGNs. Finally, there were developmentally mediated compensatory changes that occurred such that, by 8 weeks after birth, the electrical properties of the null mutant neurons were virtually indistinguishable from the wild-type neurons, suggesting that ion channel remodeling in auditory neurons progresses beyond hearing onset.

Spermine synthase deficiency leads to deafness and a profound sensitivity to alpha-difluoromethylornithine.

Male gyro (Gy) mice, which have an X chromosomal deletion inactivating the SpmS and Phex genes, were found to be profoundly hearing impaired. This defect was due to alteration in polyamine content due to the absence of spermine synthase, the product of the SpmS gene. It was reversed by breeding the Gy strain with CAG/SpmS mice, a transgenic line that ubiquitously expresses spermine synthase under the control of a composite cytomegalovirus-IE enhancer/chicken beta-actin promoter. There was an almost complete loss of the endocochlear potential in the Gy mice, which parallels the hearing deficiency, and this was also reversed by the production of spermine from the spermine synthase transgene. Gy mice showed a striking toxic response to treatment with the ornithine decarboxylase inhibitor alpha-difluoromethylornithine (DFMO). Within 2-3 days of exposure to DFMO in the drinking water, the Gy mice suffered a catastrophic loss of motor function resulting in death within 5 days. This effect was due to an inability to maintain normal balance and was also prevented by the transgenic expression of spermine synthase. DFMO treatment of control mice or Gy-CAG/SpmS had no effect on balance. The loss of balance in Gy mice treated with DFMO was due to inhibition of polyamine synthesis because it was prevented by administration of putrescine. Our results are consistent with a critical role for polyamines in regulation of Kir channels that maintain the endocochlear potential and emphasize the importance of normal spermidine:spermine ratio in the hearing and balance functions of the inner ear.

Conservation of hearing by simultaneous mutation of Na,K-ATPase and NKCC1.

Although drug-induced and age-related hearing losses are frequent otologic problems affecting millions of people, their underlying mechanisms remain uncertain. The inner ear is exclusively endowed with a positive endocochlear potential (EP) that serves as the main driving force for the generation of receptor potential in hair cells to confer hearing. Deterioration of EP leads to hearing loss or deafness. The generation of EP relies on the activity of many ion transporters to establish active potassium (K(+)) cycling within the inner ear, including K(+) channels, the Na-K-2Cl co-transporter (NKCC1), and the alpha(1) and alpha(2) isoforms of Na,K-ATPase. We show that heterozygous deletion of either NKCC1, alpha(1)-Na,K-ATPase, or alpha(2)-Na,K-ATPase independently results in progressive, age-dependent hearing loss with minimal alteration in cochlear morphology. Double heterozygote deletion of NKCC1 with alpha(1)-Na,K-ATPase also shows a progressive, though delayed, age-dependent hearing loss. Remarkably, double heterozygote deletion of NKCC1 with alpha(2)-Na,K-ATPase demonstrates a striking preservation of hearing threshold both initially and with age. Measurements of the EP confirm the anticipated drop in potential for genotypes that demonstrate age-dependent hearing loss. The EP generated by the NKCC1 + alpha(2)-Na,K-ATPase double heterozygote, however, is maintained at a level comparable to that of the control condition, suggesting a potential advantage in this combination of ion transporter modification. These observations provide insight into the detailed mechanisms of EP generation, and results of combination-knockout experiments may have important implications in the future treatment of drug-induced and age-related hearing losses.

Functional consequences of polyamine synthesis inhibition by L-alpha-difluoromethylornithine (DFMO): cellular mechanisms for DFMO-mediated ototoxicity.

L-Alpha-difluoromethylornithine (DFMO) is a chemopreventive agent for colon cancer in clinical trials. Yet, the drug produces an across-frequency elevation of the hearing threshold, suggesting that DFMO may affect a common trait along the cochlear spiral. The mechanism for the ototoxic effects of DFMO remains uncertain. The cochlear duct is exclusively endowed with endocochlear potential (EP). EP is a requisite for normal sound transduction, as it provides the electromotive force that determines the magnitude of the receptor potential of hair cells. EP is generated by the high throughput of K(+) across cells of the stria vascularis, conferred partly by the activity of Kir4.1 channels. Here, we show that the ototoxicity of DFMO may be mediated by alteration of the inward rectification of Kir4.1 channels, resulting in a marked reduction in EP. These findings are surprising given that the present model for EP generation asserts that Kir4.1 confers the outflow of K(+) in the stria vascularis. We have proposed an alternative model. These findings should also enable the rational design of new pharmaceuticals devoid of the untoward effect of DFMO.

Intratympanic steroid treatment: a review.

OBJECTIVE: To review published literature regarding the use of intratympanic steroids in the treatment of Meniere's disease and sudden sensorineural hearing loss and to make recommendations regarding their use based on the literature review. DATA SOURCES: Literature review from 1996 to 2003, PubMed, Medline Plus, and Web of Science. STUDY SELECTION: Retrospective case series and uncontrolled prospective cohort studies were the only types of studies available for review. CONCLUSION: On the basis of the available literature, a weak recommendation is made to use intratympanic steroid treatment of sudden hearing loss if oral steroid therapy fails or is contraindicated. The available studies regarding intratympanic steroid treatment of Meniere's disease and tinnitus are inadequate to answer the question of the efficacy of this treatment for these conditions. Higher quality studies are needed.

Difluoromethylornithine-induced reversible hearing loss across a wide frequency range.

OBJECTIVES: Alpha-difluoromethylornithine (DFMO) is an antineoplastic agent that causes reversible hearing loss (HL) by an unknown mechanism. Previous neonatal gerbil studies have identified a dosing regimen of 1 g/kg per day of DFMO given for 3 weeks that results in reversible HL on click-evoked auditory brainstem response (ABR). The objectives of this study are 1) to measure HL and recovery at several frequencies in neonatal gerbils after DFMO therapy at two different dosing regimens and 2) to identify any effects of DFMO on cochlear histology. STUDY DESIGN: Prospective, nonrandomized, experimental design with placebo controls. METHODS: ABR to tone pips at 2, 4, 8, 16, and 32 kHz were recorded on 62 21-day-old Mongolian gerbils after daily subcutaneous injections of DFMO (group A at 1 g/kg, group B at 750 mg/kg) or saline from day 3 to day 20 after birth. Thirty-seven animals were retested after a 3-week drug-free recovery period. Twenty-seven animals were killed for analysis of cochlear histology. RESULTS: Animals that were administered DFMO demonstrated higher ABR thresholds across all five frequencies, with mean threshold differences of 21 to 29 dB in group A and 11 to 17 dB in group B as compared with controls. Higher thresholds were demonstrated at higher frequencies. Fewer side effects were noted at the lower dose. After a 3-week drug-free recovery period, auditory thresholds returned to pretreatment levels. No significant cochlear structural abnormalities were identified under light microscopy. CONCLUSION: An 18-day regimen of 750 mg/kg per day of DFMO given subcutaneously in neonatal gerbils causes minimal side effects with broad-frequency HL that, after 3 weeks of recovery, is fully reversible.

The gender gap in a surgical subspecialty: analysis of career and lifestyle factors.

BACKGROUND: Although the percentage of women in surgical subspecialties is increasing, little is known about the experiences of these women compared with their male counterparts. OBJECTIVE: To identify career and lifestyle factors that distinguish female otolaryngologists. DESIGN, SETTING, AND PARTICIPANTS: Otolaryngologists were asked to respond to a confidential 119-item questionnaire. The instrument was sent to all 502 female members of the American Academy of Otolaryngology-Head and Neck Surgery who had finished their residency training and were practicing medicine. For response comparison, the survey was mailed to 2 male otolaryngologists who were matched to each female survey recipient for years since completion of training, geographic region, and practice type. RESULTS: Of the 673 respondents (52.6% response rate), women were more likely to be divorced or separated (P =.001) and have fewer children (P <.001). In contrast to men, women reduced their work hours in conjunction with having more children (P <.001). Controlling for professional hours and hours spent in the operating room per week, type of practice, and years since completion of residency, women earned 15% to 20% less per year than men (P <.001). Men relied more on their spouse or partner for household responsibilities and child care (P <.001), and 34.3% of the women (compared with 7.1% of the men) spent 21 to 40 h/wk on household management (P <.001). CONCLUSION: Although male and female otolaryngologists receive equal training opportunities, women earn less money for performing similar jobs and have increased family responsibilities, which may effect their career advancement.

Neonatal middle ear effusion predicts chronic otitis media with effusion.

HYPOTHESIS AND AIMS: The specific aims of the research are to determine whether newborn ears with persistent middle ear effusion at age 30 to 48 hours are more likely to develop chronic otitis media with effusion over the first year of life when compared with ears without persistent middle ear effusion. The hypothesis is that neonates with middle ear effusion persisting to 30 to 48 hours are more likely to develop chronic otitis media with effusion. STUDY DESIGN: Prospective, case-control design. Loupe-magnified pneumatic otoscopy performed at the time of newborn hearing screening determined presence or absence of effusion. Infants enrolled in the study returned for outpatient examinations. SETTING: University medical center well-baby nursery and out-patient audiology clinic. SUBJECTS: From 454 neonates, 14 experimental subjects with neonatal middle ear effusions and 15 control subjects free of neonatal effusion were recruited for the study and followed-up for 1 year. INTERVENTIONS: Outpatient study tests included transient-evoked otoacoustic emissions, tympanometry, pneumatic otoscopy, and visual reinforcement audiometry (starting at age 6 months), at 3, 6, 9, and 12 months of age. Experimental (neonatal effusion) infants were followed-up starting at age 1 month. Infants found at any follow-up examination to have effusion on otoscopy were followed-up and tested 1 month later. MAIN OUTCOME MEASURES: Chronic otitis media with effusion defined as hypomobile or immobile tympanic membrane on pneumatic otoscopy in one or both ears for three consecutive monthly examinations. Hearing loss defined as greater than 25-dB hearing loss visual reinforcement audiometry thresholds. RESULTS: Eight experimental infants (58%) and three control infants (20%) developed chronic otitis media with effusion (p < 0.04). The average number of effusions was 1.27 for control and 4.14 for experimental infants (average number of effusions for each group at 3-, 6-, 9-, and 12-month visits). Warbled tone and speech visual reinforcement audiometry thresholds averaged 3 dB worse in the experimental group, but these differences were not statistically significant. For the control group, mean visual reinforcement audiometry thresholds never exceeded 25 dB hearing loss. For the experimental group, mean visual reinforcement audiometry thresholds exceeded 25 dB hearing loss at 1,000, 2,000, and 4,000 Hz at 9 months. CONCLUSIONS: A majority of infants with persistent neonatal middle ear effusion found by pneumatic otoscopy at 30 to 48 hours will develop chronic otitis media with effusion during the first year of life. However, chronic otitis media with effusion is common in all infants (20% of controls), a time during which infants are examined and tested frequently.

The otolaryngologist's role in management of hearing loss in infancy and childhood.

This article reviews the role of the otolaryngologist-head and neck surgeon-in the diagnosis and treatment of hearing loss in infants and young children. The otolaryngologist is well-versed in the anatomy, physiology, and pathophysiology of the auditory system, as well as the craniofacial syndromes that can involve the head and neck in combination with deafness. In this paper, the various causes of congenital hearing loss are described, as well as the steps required for proper diagnosis. Finally, surgeries used by otolaryngologists to treat childhood hearing loss, their indications, and outcomes, are discussed.

Neonatal Hearing Screening with Otoscopy, Auditory Brain Stem Response, and Otoacoustic Emissions.

A study was performed to investigate the relationship between external and middle ear factors and hearing screening results by auditory brain stem response (ABR) and transient evoked otoacoustic emissions (EOAEs). The ears of 200 well newborns aged 5 hours to 48 hours underwent screening by ABR and EOAEs, followed by otoscopic examination. The pass rates for ABR and EOAE screening were 88.5% and 79%, respectively. On otoscopic examination, 13% (53 of 400) ears had occluding vernix obscuring the view of the tympanic membrane. Cleaning of vernix was attempted in ears that failed ABR or EOAE screening. Seventeen ears that failed ABR were cleaned, and 12 (71%) of them passed repeat ABR. Thirty-three ears that failed EOAE screening were cleaned, and 22 (67%) of them passed repeat emissions testing. Cleaning vernix increased the pass rates for ABR and EOAE screening to 91.5% and 84%, respectively. Decreased tympanic membrane mobility was found in 9% of ears that could be evaluated otoscopically. Increased failure rates for both ABR and EOAE screening were found in infant ears with decreased tympanic membrane mobility, but significance testing could not be performed because of inadequate sample size. Prevalence of occluding external canal vernix and middle ear effusion as a function of increasing infant age were studied. Implications for newborn hearing screening are discussed.