Influence of genotype and age on acute acoustic trauma and recovery in CBA/Ca and C57BL/6J mice

Role of Oxidative Stress in Sensorineural Hearing Loss

Hearing is essential for communication, and its loss can cause a serious disruption to one's social life. Hearing loss is also recognized as a major risk factor for dementia; therefore, addressing hearing loss is a pressing global issue. Sensorineural hearing loss, the predominant type of hearing loss, is mainly due to damage to the inner ear along with a variety of pathologies including ischemia, noise, trauma, aging, and ototoxic drugs. In addition to genetic factors, oxidative stress has been identified as a common mechanism underlying several cochlear pathologies. The cochlea, which plays a major role in auditory function, requires high-energy metabolism and is, therefore, highly susceptible to oxidative stress, particularly in the mitochondria. Based on these pathological findings, the potential of antioxidants for the treatment of hearing loss has been demonstrated in several animal studies. However, results from human studies are insufficient, and future clinical trials are required. This review discusses the relationship between sensorineural hearing loss and reactive oxidative species (ROS), with particular emphasis on age-related hearing loss, noise-induced hearing loss, and ischemia-reperfusion injury. Based on these mechanisms, the current status and future perspectives of ROS-targeted therapy for sensorineural hearing loss are described.

Cochlear zinc signaling dysregulation is associated with noise-induced hearing loss, and zinc chelation enhances cochlear recovery

Exposure to loud noise triggers sensory organ damage and degeneration that, in turn, leads to hearing loss. Despite the troublesome impact of noise-induced hearing loss (NIHL) in individuals and societies, treatment strategies that protect and restore hearing are few and insufficient. As such, identification and mechanistic understanding of the signaling pathways involved in NIHL are required. Biological zinc is mostly bound to proteins, where it plays major structural or catalytic roles; however, there is also a pool of unbound, mobile (labile) zinc. Labile zinc is mostly found in vesicles in secretory tissues, where it is released and plays a critical signaling role. In the brain, labile zinc fine-tunes neurotransmission and sensory processing. However, injury-induced dysregulation of labile zinc signaling contributes to neurodegeneration. Here, we tested whether zinc dysregulation occurs and contributes to NIHL in mice. We found that ZnT3, the vesicular zinc transporter responsible for loading zinc into vesicles, is expressed in cochlear hair cells and the spiral limbus, with labile zinc also present in the same areas. Soon after noise trauma, ZnT3 and zinc levels are significantly increased, and their subcellular localization is vastly altered. Disruption of zinc signaling, either via ZnT3 deletion or pharmacological zinc chelation, mitigated NIHL, as evidenced by enhanced auditory brainstem responses, distortion product otoacoustic emissions, and number of hair cell synapses. These data reveal that noise-induced zinc dysregulation is associated with cochlear dysfunction and recovery after NIHL, and point to zinc chelation as a potential treatment for mitigating NIHL.

Experimental animal models of drug-induced sensorineural hearing loss: a narrative review

Objective

This narrative review describes experimental animal models of sensorineural hearing loss (SNHL) caused by ototoxic agents.

Background

SNHL primarily results from damage to the sensory organ within the inner ear or the vestibulocochlear nerve (cranial nerve VIII). The main etiology of SNHL includes genetic diseases, presbycusis, ototoxic agents, infection, and noise exposure. Animal models with functional and anatomic damage to the sensory organ within the inner ear or the vestibulocochlear nerve mimicking the damage seen in humans are employed to explore the mechanism and potential treatment of SNHL. These animal models of SNHL are commonly established using ototoxic agents.

Methods

A literature search of PubMed, Embase, and Web of Science was performed for research articles on hearing loss and ototoxic agents in animal models of hearing loss.

Conclusions

Common ototoxic medications such as aminoglycoside antibiotics (AABs) and platinum antitumor drugs are extensively used to induce SNHL in experimental animals. The effect of ototoxic agents in vivo is influenced by the chemical mechanisms of the ototoxic agents, the species of animal, routes of administration of the ototoxic agents, and the dosage of ototoxic agents. Animal models of drug-induced SNHL contribute to understanding the hearing mechanism and reveal the function of different parts of the auditory system in humans.

Genome-wide association study identifies 7q11.22 and 7q36.3 associated with noise-induced hearing loss among Chinese population

Noise-induced hearing loss (NIHL) seriously affects the life quality of humans and causes huge economic losses to society. To identify novel genetic loci involved in NIHL, we conducted a genome-wide association study (GWAS) for this symptom in Chinese populations. GWAS scan was performed in 89 NIHL subjects (cases) and 209 subjects with normal hearing who have been exposed to a similar noise environment (controls), followed by a replication study consisting of 53 cases and 360 controls. We identified that four candidate pathways were nominally significantly associated with NIHL, including the Erbb, Wnt, hedgehog and intraflagellar transport pathways. In addition, two novel index single-nucleotide polymorphisms, rs35075890 in the intron of AUTS2 gene at 7q11.22 (combined P = 1.3 × 10^-6 ) and rs10081191 in the intron of PTPRN2 gene at 7q36.3 (combined P = 2.1 × 10^-6 ), were significantly associated with NIHL. Furthermore, the expression quantitative trait loci analyses revealed that in brain tissues, the genotypes of rs35075890 are significantly associated with the expression levels of AUTS2, and the genotypes of rs10081191 are significantly associated with the expressions of PTPRN2 and WDR60. In conclusion, our findings highlight two novel loci at 7q11.22 and 7q36.3 conferring susceptibility to NIHL.

Mouse methods and models for studies in hearing

Laboratory mice have become the dominant animal model for hearing research. The mouse cochlea operates according to standard "mammalian" principles, uses the same cochlear cell types, and exhibits the same types of injury as found in other mammals. The typical mouse lifespan is less than 3 years, yet the age-associated pathologies that may be found are quite similar to longer-lived mammals. All Schuknecht's types of presbycusis have been identified in existing mouse lines, some favoring hair cell loss while others favor strial degeneration. Although noise exposure generally affects the mouse cochlea in a manner similar to other mammals, mice appear more prone to permanent alterations to hair cells or the organ of Corti than to hair cell loss. Therapeutic compounds may be applied systemically or locally through the tympanic membrane or onto (or through) the round window membrane. The thinness of the mouse cochlear capsule and annular ligament may promote drug entry from the middle ear, although an extremely active middle ear lining may quickly remove most drugs. Preclinical testing of any therapeutic will always require tests in multiple animal models. Mice constitute one model providing supporting evidence for any therapeutic, while genetically engineered mice can test hypotheses about mechanisms.

[The genetic aspects of occupational hearing impairment]

This article was designed to be the overview of the current literature publications concerning the identification of the genetic markers of susceptibility to the noise-induced loss of hearing. The analysis of these data has demonstrated that the major gene polymorphisms associated with the development of this pathological condition are localized in the genes encoding for the antioxidant systems, potassium homeostasis, and adhesion molecules as well as in the genes involved in intercellular coupling, the mechanisms underlying the cellular response to stress, activation and regulation of heat shock proteins, and signaling function of the immune system. It is concluded that the further investigations into the genetic aspects of the full-genome sequencing techniques and the search for genomic associations could greatly contribute to the development of personalized medicine and the reduction of risks of occupational noise-induced sensorineural impairment of hearing.

Current insights in noise-induced hearing loss: a literature review of the underlying mechanism, pathophysiology, asymmetry, and management options

BACKGROUND

Noise-induced hearing loss is one of the most common forms of sensorineural hearing loss, is a major health problem, is largely preventable and is probably more widespread than revealed by conventional pure tone threshold testing. Noise-induced damage to the cochlea is traditionally considered to be associated with symmetrical mild to moderate hearing loss with associated tinnitus; however, there is a significant number of patients with asymmetrical thresholds and, depending on the exposure, severe to profound hearing loss as well.

MAIN BODY

Recent epidemiology and animal studies have provided further insight into the pathophysiology, clinical findings, social and economic impacts of noise-induced hearing loss. Furthermore, it is recently shown that acoustic trauma is associated with vestibular dysfunction, with associated dizziness that is not always measurable with current techniques. Deliberation of the prevalence, treatment and prevention of noise-induced hearing loss is important and timely. Currently, prevention and protection are the first lines of defence, although promising protective effects are emerging from multiple different pharmaceutical agents, such as steroids, antioxidants and neurotrophins.

CONCLUSION

This review provides a comprehensive update on the pathophysiology, investigations, prevalence of asymmetry, associated symptoms, and current strategies on the prevention and treatment of noise-induced hearing loss.

Application of Mouse Models to Research in Hearing and Balance

Laboratory mice (Mus musculus) have become the major model species for inner ear research. The major uses of mice include gene discovery, characterization, and confirmation. Every application of mice is founded on assumptions about what mice represent and how the information gained may be generalized. A host of successes support the continued use of mice to understand hearing and balance. Depending on the research question, however, some mouse models and research designs will be more appropriate than others. Here, we recount some of the history and successes of the use of mice in hearing and vestibular studies and offer guidelines to those considering how to apply mouse models.

Molecular Structure and Regulation of P2X Receptors With a Special Emphasis on the Role of P2X2 in the Auditory System

The P2X purinergic receptors are cation-selective channels gated by extracellular adenosine 5'-triphosphate (ATP). These purinergic receptors are found in virtually all mammalian cell types and facilitate a number of important physiological processes. Within the past few years, the characterization of crystal structures of the zebrafish P2X4 receptor in its closed and open states has provided critical insights into the mechanisms of ligand binding and channel activation. Understanding of this gating mechanism has facilitated to design and interpret new modeling and structure-function experiments to better elucidate how different agonists and antagonists can affect the receptor with differing levels of potency. This review summarizes the current knowledge on the structure, activation, allosteric modulators, function, and location of the different P2X receptors. Moreover, an emphasis on the P2X2 receptors has been placed in respect to its role in the auditory system. In particular, the discovery of three missense mutations in P2X2 receptors could become important areas of study in the field of gene therapy to treat progressive and noise-induced hearing loss. J. Cell. Physiol. 231: 1656-1670, 2016. © 2015 Wiley Periodicals, Inc.

FVB/NJ mice demonstrate a youthful sensitivity to noise-induced hearing loss and provide a useful genetic model for the study of neural hearing loss

The hybrid mouse diversity panel (HMDP), a panel of 100 strains, has been employed in genome wide association studies (GWAS) to study complex traits in mice. Hearing is a complex trait and the CBA/CaJ mouse strain is a widely used model for age-related hearing loss (ARHI) and noise induced hearing loss (NIHL). The CBA/CaJ strain's youthful sensitivity to noise and limited age-related loss led us to attempt to identify additional strains segregating a similar phenotype for our panel. FVB/NJ is part of the HMDP and has been previously described as having a similar ARHI phenotype to CBA/CaJ. For these reasons, we have studied the FVB/NJ mouse for ARHI and NIHL phenotypes in hopes of incorporating its phenotype into HMDP studies. We demonstrate that FVB/NJ exhibits ARHI at an earlier age than CBA/CaJ and young FVB/NJ mice are vulnerable to NIHL up until 10 to 12 weeks. This suggests that FVB/NJ may be used as an additional genetic model for neural forms of progressive hearing loss and for the study of youthful sensitivity to noise.

Genetic background affects human glial fibrillary acidic protein promoter activity

The human glial fibrillary acidic protein (hGFAP) promoter has been used to generate numerous transgenic mouse lines, which has facilitated the analysis of astrocyte function in health and disease. Here, we evaluated the expression levels of various hGFAP transgenes at different ages in the two most commonly used inbred mouse strains, FVB/N (FVB) and C57BL/6N (B6N). In general, transgenic mice maintained on the B6N background displayed weaker transgene expression compared with transgenic FVB mice. Higher level of transgene expression in B6N mice could be regained by crossbreeding to FVB wild type mice. However, the endogenous murine GFAP expression was equivalent in both strains. In addition, we found that endogenous GFAP expression was increased in transgenic mice in comparison to wild type mice. The activities of the hGFAP transgenes were not age-dependently regulated. Our data highlight the importance of proper expression analysis when non-homologous recombination transgenesis is used.

A brief history of hair cell regeneration research and speculations on the future

Millions of people worldwide suffer from hearing and balance disorders caused by loss of the sensory hair cells that convert sound vibrations and head movements into electrical signals that are conveyed to the brain. In mammals, the great majority of hair cells are produced during embryogenesis. Hair cells that are lost after birth are virtually irreplaceable, leading to permanent disability. Other vertebrates, such as fish and amphibians, produce hair cells throughout life. However, hair cell replacement after damage to the mature inner ear was either not investigated or assumed to be impossible until studies in the late 1980s proved this to be false. Adult birds were shown to regenerate lost hair cells in the auditory sensory epithelium after noise- and ototoxic drug-induced damage. Since then, the field of hair cell regeneration has continued to investigate the capacity of the auditory and vestibular epithelia in vertebrates (fishes, birds, reptiles, and mammals) to regenerate hair cells and to recover function, the molecular mechanisms governing these regenerative capabilities, and the prospect of designing biologically-based treatments for hearing loss and balance disorders. Here, we review the major findings of the field during the past 25 years and speculate how future inner ear repair may one day be achieved.

[The diversity of aging models]

Most of the signalling pathways involved in aging regulation have been recently found well conserved at various levels throughout the evolution. Taking this into account, a diversity of model organisms, including worms, rodents, and lemurs as well, allows to address different questions: how to understand the interactions between genetic and environmental factors while challenging theories of aging, to preserve hearing integrity, to fight against senescence of neural stem cells, or to explore brain fitness from gene expression to cognitive and social behavior? Here are the main issues that can be considered, stressing the complementarities of the models. The differentiation of aging physiological aspects from those induced by age-related pathologies will also be specified. By emphasizing recent ability of technologies to promote new aging insights, we discuss towards a better understanding of mechanisms governing aging.

Plasticity of serotonergic innervation of the inferior colliculus in mice following acoustic trauma

Acoustic trauma often results in permanent damage to the cochlea, triggering changes in processing within central auditory structures such as the inferior colliculus (IC). The serotonergic neuromodulatory system, present in the IC, is responsive to chronic changes in the activity of sensory systems. The current study investigated whether the density of serotonergic innervation in the IC is changed following acoustic trauma. The trauma stimulus consisted of an 8 kHz pure tone presented at a level of 113 dB SPL for six consecutive hours to anesthetized CBA/J mice. Following a minimum recovery period of three weeks, serotonergic fibers were visualized via histochemical techniques targeting the serotonin reuptake transporter (SERT) and quantified using stereologic probes. SERT-positive fiber densities were then compared between the traumatized and protected hemispheres of unilaterally traumatized subjects and those of controls. A significant effect of acoustic trauma was found between the hemispheres of unilaterally traumatized subjects such that the IC contralateral to the ear of exposure contained a lower density of SERT-positive fibers than the IC ipsilateral to acoustic trauma. No significant difference in density was found between the hemispheres of control subjects. Additional dimensions of variability in serotonergic fibers were seen among subdivisions of the IC and with age. The central IC had a slightly but significantly lowered density of serotonergic fibers than other subdivisions of the IC, and serotonergic fibers also declined with age. Overall, the results indicate that acoustic trauma is capable of producing modest but significant decreases in the density of serotonergic fibers innervating the IC.

Protection by low-dose kanamycin against noise-induced hearing loss in mice: dependence on dosing regimen and genetic background

We recently demonstrated that sub-chronic low-dose kanamycin (KM, 300 mg/kg sc, 2×/day, 10 days) dramatically reduces permanent noise-induced hearing loss (NIHL) and hair cell loss in 1 month old CBA/J mice (Fernandez et al., 2010, J. Assoc. Res. Otolaryngol. 11, 235-244). Protection by KM remained for at least 48 h after the last dose, and appeared to involve a cumulative effect of multiple doses as part of a preconditioning process. The first month of life lies within the early 'sensitive period' for both cochlear noise and ototoxic injury in mice, and CBA/J mice appear exquisitely vulnerable to noise during this period (Ohlemiller et al., 2011; Hearing Res. 272, 13-20). From our initial data, we could not rule out 1) that less rigorous treatment protocols than the intensive one we applied may be equally-or more-protective; 2) that protection by KM is tightly linked to processes unique to the sensitive period for noise or ototoxins; or 3) that protection by KM is exclusive to CBA/J mice. The present experiments address these questions by varying the number and timing of fixed doses (300 mg/kg sc) of KM, as well as the age at treatment in CBA/J mice. We also tested for protection in young C57BL/6J (B6) mice. We find that nearly complete protection against at least 2 h of intense (110 dB SPL) broadband noise can be observed in CBA/J mice at least for ages up to 1 year. Reducing dosing frequency to as little as once every other day (a four-fold decrease in dosing frequency) appeared as protective as twice per day. However, reducing the number of doses to just 1 or 2, followed by noise 24 or 48 h later greatly reduced protection. Notably, hearing thresholds and hair cells in young B6 mice appeared completely unprotected by the same regimen that dramatically protects CBA/J mice. We conclude that protective effects of KM against NIHL in CBA/J mice can be engaged by a wide range of dosing regimens, and are not exclusive to the sensitive period for noise or ototoxins. While we cannot presently judge the generality of protection across genetic backgrounds, it appears not to be universal, since B6 showed no benefit. Classical genetic approaches based on CBA/J × B6 crosses may reveal loci critical to protective cascades engaged by kanamycin and perhaps other preconditioners. Their human analogs may partly determine who is at elevated risk of acquired hearing loss.

A corticosteroid-responsive transcription factor, promyelocytic leukemia zinc finger protein, mediates protection of the cochlea from acoustic trauma

Animals can be induced to resist cochlear damage associated with acoustic trauma by exposure to a variety of "conditioning" stimuli, including restraint stress, moderate level sound, heat stress, hypoxia, and corticosteroids. Here we identify in mice a corticosteroid-responsive transcription factor, PLZF (promyelocytic leukemia zinc finger protein), which mediates conditioned protection of the cochlea from acoustic trauma. PLZF mRNA levels in the cochlea are increased following conditioning stimuli, including restraint stress, dexamethasone administration, and moderate-to-high level acoustic stimulation. Heterozygous mutant (luxoid.Zbtb16(LU)/J) mice deficient in PLZF have hearing and responses to acoustic trauma similar to their wild type littermates but are unable to generate conditioning-induced protection from acoustic trauma. PLZF immunoreactivity is present in the spiral ganglion, lateral wall of the cochlea, and organ of Corti, all targets for acoustic trauma. PLZF is also present in the brain and PLZF mRNA in brain is elevated following conditioning stimuli. The identification of a transcription factor that mediates conditioned protection from trauma provides a tool for understanding the protective action of corticosteroids, which are widely used in treating acute hearing loss, and has relevance to understanding the role of corticosteroids in trauma protection.

Divergence of noise vulnerability in cochleae of young CBA/J and CBA/CaJ mice

CBA/CaJ and CBA/J inbred mouse strains appear relatively resistant to age- and noise-related cochlear pathology, and constitute the predominant 'good hearing' control strains in mouse studies of hearing and deafness. These strains have often been treated as nearly equivalent in their hearing characteristics, and have even been mixed in some studies. Nevertheless, we recently showed that their trajectories with regard to age-associated cochlear pathology diverge after one year of age (Ohlemiller et al., 2010a). We also recently reported that they show quite different susceptibility to cochlear noise injury during the 'sensitive period' of heightened vulnerability to noise common to many models, CBA/J being far more vulnerable than CBA/CaJ (Fernandez et al., 2010 J. Assoc. Res. Otolaryngol. 11:235-244). Here we explore this relation in a side-by-side comparison of the effect of varying noise exposure duration in young (6 week) and older (6 month) CBA/J and CBA/CaJ mice, and in F1 hybrids formed from these. Both the extent of permanent noise-induced threshold shifts (NIPTS) and the probability of a defined NIPTS were determined as exposure to intense broadband noise (4-45 kHz, 110 dB SPL) increased by factors of two from 7 s to 4 h. At 6 months of age the two strains appeared very similar by both measures. At 6 weeks of age, however, both the extent and probability of NIPTS grew much more rapidly with noise duration in CBA/J than in CBA/CaJ. The 'threshold' exposure duration for NIPTS was <1.0 min in CBA/J versus >4.0 min in CBA/CaJ. F1 hybrid mice showed both NIPTS and hair cell loss similar to that in CBA/J. This suggests that dominant-acting alleles at unknown loci distinguish CBA/J from CBA/CaJ. These loci have novel effects on hearing phenotype, as they come into play only during the sensitive period, and may encode factors that demarcate this period. Since the cochlear cells whose fragility defines the early window appear to be hair cells, these loci may principally impact the mechanical or metabolic resiliency of hair cells or the organ of Corti.

A major effect QTL on chromosome 18 for noise injury to the mouse cochlear lateral wall

We recently demonstrated a striking difference among inbred mouse strains in the effects of a single noise exposure, whereby CBA/J and CBA/CaJ (CBA) mice show moderate reversible reduction in the endocochlear potential (EP) while C57BL/6J (B6) mice do not (Ohlemiller, K.K., Gagnon, P.M., 2007. Genetic dependence of cochlear cells and structures injured by noise. Hear. Res. 224, 34-50). Acute EP reduction in CBA was reliably associated with characteristic pathology of the spiral ligament and stria vascularis, both immediately after noise and 8weeks later. Analysis of B6xCBA F1 hybrid mice indicated that EP reduction and its anatomic correlates are co-inherited in an autosomal dominant manner. Further analysis of N2 mice resulting from the backcross of F1 hybrids to B6 mice led us to suggest that the EP reduction phenotype principally reflects the influence of a small number of quantitative trait loci (QTLs). Here we report the results of QTL mapping of the EP reduction phenotype in CBA/J using 106 N2 mice from a (CBAxB6)xB6 backcross. Correlation of acute post-noise EP with 135 markers distributed throughout the genome revealed a single major effect QTL on chromosome 18 (12.5 cM, LOD 3.57) (Nirep, for noise-induced reduction in EP QTL), and two marginally significant QTLs on chromosomes 5 and 16 (LOD 1.43 and 1.73, respectively). Our results underscore that fact that different cochlear structures may possess different susceptibilities to noise through the influence of non-overlapping genes. While Nirep and similar-acting QTLs do not appear to influence the extent of permanent hearing loss from a single noise exposure, they could reduce the homeostatic 'reserve' of the lateral wall in protracted or continual exposures, and thereby influence long term threshold stability.

Analysis of gene polymorphisms associated with K ion circulation in the inner ear of patients susceptible and resistant to noise-induced hearing loss

Noise-induced hearing loss (NIHL) is one of the leading occupational health risks in industrialized countries. It results from an interaction between environmental and genetic factors, however the nature of the genetic factors contributing to NIHL has not yet been clarified. Here, we investigated whether genetic variations in 10 genes putatively involved in the potassium recycling pathway in the inner ear may influence susceptibility to noise. 99 SNPs were genotyped in Polish noise-exposed workers, categorized into susceptible and resistant subjects. The most interesting results were obtained for KCNE1 and KCNQ4 as we replicated associations that were previously reported in a Swedish sample set, hence confirming that they are NIHL susceptibility genes. Additionally we report significant associations in GJB1, GJB2, GJB4, KCNJ10 and KCNQ1, however due to the lack of replication in the Swedish sample set, these results should be seen as suggestive.

Genome-wide screening for genetic loci associated with noise-induced hearing loss

Noise-induced hearing loss (NIHL) is one of the more common sources of environmentally induced hearing loss in adults. In a mouse model, Castaneous (CAST/Ei) is an inbred strain that is resistant to NIHL, while the C57BL/6J strain is susceptible. We have used the genome-tagged mice (GTM) library of congenic strains, carrying defined segments of the CAST/Ei genome introgressed onto the C57BL/6J background, to search for loci modifying the noise-induced damage seen in the C57BL/6J strain. NIHL was induced by exposing 6-8-week old mice to 108 dB SPL intensity noise. We tested the hearing of each mouse strain up to 23 days after noise exposure using auditory brainstem response (ABR). This study identifies a number of genetic loci that modify the initial response to damaging noise, as well as long-term recovery. The data suggest that multiple alleles within the CAST/Ei genome modify the pathogenesis of NIHL and that screening congenic libraries for loci that underlie traits of interest can be easily carried out in a high-throughput fashion.

Continuous glucose monitoring in infants of very low birth weight

OBJECTIVES

To evaluate the feasibility and efficacy of a continuous glucose monitoring system (CGMS) in a population of infants of very low birth weight (VLBW).

STUDY DESIGN

Infants weighing <or=1,500 g and of <or=32 weeks of gestation were recruited within 24 h of delivery. A subcutaneous sensor connected to a CGMS was inserted and maintained for 7 days or until dysfunction. Therapeutic management followed the usual standard protocols.

RESULTS

38 patients (21 male) were included over 17 months. Their mean gestational age was 27.5 +/- 2.0 weeks and their mean birth weight was 958.3 +/- 205.5 g. Their perinatal histories and complications during admission were unremarkable for extremely premature babies. Continuous monitoring lasted an average of 7.84 +/- 1.99 days per patient. Hyperglycaemia was detected in 22 (57.90%) patients and it lasted a mean of 20.33 +/- 30.13 h, while 14 (36.8%) presented with hypoglycaemia for a mean of 2.45 +/- 2.3 h.

CONCLUSIONS

The CGMS gave a safe and useful estimate of glucose levels in VLBW infants, revealing abnormal glucose levels at a much higher rate than expected by usual sampling. However, it was not able to provide real-time glucose concentration data. CGMS may be very useful in providing information on the role of hyper- and hypoglycaemia on short- and long-term outcomes in VLBW infants.

Genetic dependence of cochlear cells and structures injured by noise

The acute and permanent effects of a single damaging noise exposure were compared in CBA/J, C57BL/6 (B6), and closely related strains of mice. Two hours of broadband noise (4-45 kHz) at 110 dB SPL led to temporary reduction in the endocochlear potential (EP) of CBA/J and CBA/CaJ (CBA) mice and acute cellular changes in cochlear stria vascularis and spiral ligament. For the same exposure, B6 mice showed no EP reduction and little of the pathology seen in CBA. Eight weeks after exposure, all mice showed a normal EP, but only CBA mice showed injury and cell loss in cochlear lateral wall, despite the fact that B6 sustained larger permanent threshold shifts. Examination of noise injury in B6 congenics carrying alternate alleles of genes encoding otocadherin (Cdh23), agouti protein, and tyrosinase (albinism) indicated that none of these loci can account for the strain differences observed. Examination of CBA x B6 F1 mice and N2 backcross mice to B6 further indicated that susceptibility to noise-related EP reduction and associated cell pathology are inherited in an autosomal dominant manner, and are established by one or a few large effect quantitative trait loci. Findings support a common genetic basis for an entire constellation of noise-related cochlear pathologies in cochlear lateral wall and spiral limbus. Even within species, cellular targets of acute and permanent cochlear noise injury may vary with genetic makeup.

The contribution of genes involved in potassium-recycling in the inner ear to noise-induced hearing loss

Noise-induced hearing loss (NIHL) is one of the most important occupational diseases and, after presbyacusis, the most frequent cause of hearing loss. NIHL is a complex disease caused by an interaction between environmental and genetic factors. The various environmental factors involved in NIHL have been relatively extensively studied. On the other hand, little research has been performed on the genetic factors responsible for NIHL. To test whether the variation in genes involved in coupling of cells and potassium recycling in the inner ear might partly explain the variability in susceptibility to noise, we performed a case-control association study using 35 SNPs selected in 10 candidate genes on a total of 218 samples selected from a population of 1,261 Swedish male noise-exposed workers. We have obtained significant differences between susceptible and resistant individuals for the allele, genotype, and haplotype frequencies for three SNPs of the KCNE1 gene, and for the allele frequencies for one SNP of KCNQ1 and one SNP of KCNQ4. Patch-clamp experiments in high K+-concentrations using a Chinese hamster ovary (CHO) cell model were performed to investigate the possibility that the KCNE1-p.85N variant (NT_011512.10:g.21483550G>A; NP_00210.2:p.Asp85Asn) was causative for high noise susceptibility. The normalized current density generated by KCNQ1/KCNE1-p.85N channels, thus containing the susceptibility variant, differed significantly from that from wild-type channels. Furthermore, the midpoint potential of KCNQ1/KCNE1-p.85N channels (i.e., the voltage at which 50% of the channels are open) differed from that of wild-type channels. Further genetic and physiological studies will be necessary to confirm these findings.

Contributions of mouse models to understanding of age- and noise-related hearing loss

Once an oddity, mice have become the most widely used hearing research model. Their value for research in noise-induced hearing loss (NIHL) stems from their high vulnerability to noise and reduced variance of results, made possible by genetic standardization. To research in age-related hearing loss (ARHL), they offer economies of small size and a short lifespan, both of which reduce housing costs. Inbred mouse strains show a wide range of noise sensitivities and rates of hearing loss with age. These can be studied using classical genetic analysis, as well as hypothesis-driven experiments utilizing genetic engineering. Through such investigations, presently 3 loci have been identified to date that contribute to NIHL, 10 that promote ARHL, and at least 6 loci that promote both. The types of genes involved implicate homeostatic and protective mechanisms as key to the appearance of either type of pathology and support a causal link between injury and some apparent ARHL. While the majority of mouse ARHL models examined most closely resemble sensory ARHL, recent work has identified mice possessing the essential characteristics of neural and strial ARHL. Using these models, it should be possible to identify genes and alleles that promote the major forms of ARHL and their combinations.

The effect of an age-related hearing loss gene (Ahl) on noise-induced hearing loss and cochlear damage from low-frequency noise

Inbred C57BL/6J mice carry two copies of an age-related hearing loss gene (Ahl). It has been shown that these mice begin losing high-frequency hearing at two months. Several functional studies have reported that the Ahl gene renders mice more susceptible to noise-induced hearing loss (NIHL) than strains which do not carry this gene [e.g., Hear. Res. 93 (1996) 181; Hear. Res. 155 (2001) 82; J. Assoc. Res. Otolaryngol. 2 (2001) 233]. Johnson et al. [Hear. Res. 114 (1997) 83] developed a congenic B6.CAST-+Ahl mouse which carries the wild-type allele from Mus musculus castaneus at the Ahl locus. Five each of young C57BL/6J males and females, and B6.CAST-+Ahl males were exposed to a 4-kHz octave band of noise at 108 dB SPL for 4 h. Non-noise-exposed mice of the same strains and age served as controls. The noise-exposed mice were functionally tested for ABR thresholds and DPOAE levels pre-exposure and three times post-exposure: 0 days to determine the magnitude of temporary threshold shift (TTS); 6 days to determine rate of recovery; and 20 days to determine the magnitude of permanent threshold shift (PTS). At 20 days post-exposure, the animals underwent cardiac perfusion to fix their cochleae. The isolated cochleae were embedded in plastic and dissected into flat preparations. By phase-contrast microscopy, each cochlea was evaluated from apex to base to quantify the losses of hair cells, nerve fibers and stria vascularis and to localize stereocilia damage. Functional data from each mouse were aligned with the cytocochleogram using the frequency-place map of Ou et al. [Hear. Res. 145 (2000) 111; Hear. Res. 145 (2000) 123]. Sizable variation in the magnitude of TTS, PTS and hair-cell loss was found among mice of the same genetic strain. The congenic B6.CAST-+Ahl male mice had significantly less TTS immediately post-exposure than C57BL/6J males or females but not less PTS or hair-cell losses at 20 days post-exposure. These results indicate that, at one month of age, mice carrying two copies of the Ahl gene have an increased susceptibility to TTS from a low-frequency noise before they have any indication of age-related hearing or hair-cell loss. However, this appeared not to be the case for PTS. The Ahl gene appears to play a role in susceptibility to NIHL but, other genes as well as systemic and local factors must also be involved.

Distortion product otoacoustic emissions show exceptional resistance to noise exposure in MOLF/Ei mice

Baseline distortion-product otoacoustic emissions (DPOAEs) at several primary-tone levels were compared between naive 2- to 3-month old inbred CBA/CaJ (CBA) and wild-derived MOLF/Ei (MOLF) mice. Only minor DPOAE differences were noted between the two strains and these differences were not systematic across frequency or test levels. These emission findings were consistent with earlier results on auditory brainstem response thresholds reported by others [Zheng et al., Hear. Res. 130 (1999) 94-107] thus suggesting that both CBA and MOLF strains have normal hearing. Subsequent episodes of over-exposure to a 105-dB SPL, octave-band noise centered at 10 kHz for 8 h revealed that MOLF DPOAEs were exceptionally resistant to the adverse aftereffects of excessive noise exposure as compared to CBA mice. Unlike the noise-exposure resistant inbred 129/SvEvTac strain, which has reduced baseline DPOAE levels especially at high frequencies, MOLF mice have normal DPOAEs making the interpretation of noise-exposure effects more straightforward.

Protection against Acoustic Trauma by Forward and Backward Sound Conditioning

The purpose of the present study was to determine if short-term sound conditioning provides protection when delivered either before (forward sound conditioning) or after (backward sound conditioning) a traumatic exposure in the guinea pig. Two different sound conditioning paradigms were studied (1 kHz, 81 dB SPL, 24 h; 6.3 kHz, 78 dB SPL, 24 h). The 1-kHz forward sound conditioning paradigm (81 dB SPL, 24 h) protected distortion product otoacoustic emissions (DPOAEs) against a short-duration acoustic trauma (2.7 kHz, 103 dB SPL, 5 min) compared to the group exposed to the acoustic trauma alone. The 1-kHz forward sound conditioning paradigm (81 dB SPL, 24 h) also protected both the auditory brainstem response (ABR) thresholds and DPOAEs against a longer-duration acoustic trauma (2.7 kHz, 103 dB SPL, 30 min). The group exposed to the acoustic trauma alone showed ABR threshold shifts between 15 and 24 dB, and DPOAE amplitude shifts between 11 and 24 dB, while the group with 1-kHz forward sound conditioning showed statistically significant protection at all ABR frequencies and at all DPOAE frequencies. The 1-kHz backward sound conditioning paradigm protected against acoustic trauma (2.7 kHz, 103 dB SPL, 30 min). The ABR thresholds were protected at 1, 2 and 4 kHz, and DPOAEs at all frequencies (except 8 kHz) when compared to the group exposed only to the acoustic trauma. The 6.3-kHz forward sound conditioning paradigm protected against acoustic trauma (5.5 kHz, 109 dB SPL, 30 min) at 6.3, 8 and 10 kHz. The 6.3-kHz backward sound conditioning paradigm showed no protection against acoustic trauma at any DPOAE frequency. Taken together, these findings are important for understanding how the auditory system can be modulated by acoustic stimulation and highlights the importance of the acoustic environment during the recovery process of the auditory system.

Cellular correlates of progressive hearing loss in 129S6/SvEv mice

Several strains of mice hear well initially but show progressive sensorineural hearing loss. Affected cochlear cell types include all those known to be affected in human age-related hearing loss (ARHL), or presbycusis. Thus these mice have been offered as models of human ARHL. At present, however, few mouse ARHL models are sufficiently well described to serve as the basis for specific hypotheses about human ARHL. We examined 1-month-old and 15-month-old 129S6/SvEv (129S6) mice and compared them with BALB/cJ and CBA/J mice. Age-related elevation of compound action potential thresholds was interpreted in the light of endocochlear potentials and changes in hair cells, afferent neurons, fibrocytes in spiral limbus and ligament, and supporting cells within the organ of Corti. Aging in 129S6 mice was associated with high-frequency hearing loss. Four components of age-related cochlear degeneration emerged from quantitative analyses, including 1) basal loss of outer hair cells; 2) basal loss of type IV fibrocytes in the spiral ligament; 3) apical loss of fibrocytes in spiral limbus, and 4) anomalies of supporting cells in the cochlear base. Although neuronal loss was not consistently found, two mice showed loss of afferent dendrites and cell bodies in the cochlear apex without inner hair cell loss. Despite multifaceted degeneration, hearing loss in 129S6 mice appears to be best explained by degenerative changes in outer hair cells and in the organ of Corti, conforming to human sensory ARHL. Age-related changes in the apical spiral limbus may promote pathology of the medial organ of Corti and eventual loss of afferent neurons, with possible implications for human neural ARHL.

Susceptibility to acoustic trauma in young and aged gerbils

The effect of age on susceptibility to noise-induced hearing loss (NIHL), the effect of gender on the interaction of age-related hearing loss (ARHL) and NIHL, and the relative contributions of ARHL and NIHL to total hearing loss are poorly understood. The issues are difficult to resolve empirically in human subjects because of lack of control over extrinsic variables and for ethical reasons. Accordingly, these issues were examined in a well-studied animal model of both ARHL and NIHL, the Mongolian gerbil. Animals were exposed to an intense tone (3.5 kHz, 113 dB SPL, 1 h) either as young adults (6-8 months) or near the end of the average lifespan of the species (34-38 months). Hearing thresholds were determined with the auditory brainstem response (ABR). ARHL was approximately 5-10 dB, with slightly more observed in males at 16 kHz (p<0.05). NIHL of approximately 15-20 dB was similar for the young and old groups, suggesting no differences in susceptibility as a function of age. There were no gender differences in NIHL. The relative contributions of ARHL and NIHL to total hearing loss in aged, noise-exposed gerbils were predicted by an addition of ARHL and NIHL in dB, similar to an international standard on hearing loss allocation, ISO-1999 [Determination of Occupational Noise Exposure and Estimation of Noise-Induced Hearing Impairment (1990)]. Previous evaluations of ISO-1999 using the gerbil animal model concluded that addition of ARHL and NIHL in dB overpredicts total hearing loss. However, in these studies, ARHL was large and nearly equal to NIHL. In the current study, where ARHL was much less than NIHL, addition of the two factors in dB, as recommended by ISO-1999, results in fairly accurate predictions of total hearing loss.

Endocochlear potentials and compound action potential recovery: functions in the C57BL/6J mouse

The C57BL/6J mouse suffers from cochlear degeneration beginning at an early age and has been used as a model of age-related hearing loss (presbyacusis). Here, the endocochlear potential (EP) and compound action potential (CAP) responses were determined in one-, four-, 12- and 24-month-old C57BL/6J mice. CAP measures included thresholds to tone pips, input/output (I/O) functions, and recovery functions to conditioning tones. EP values among the four age groups did not differ significantly (P>0.05) in either the basal or apical turns. CAP thresholds were increased significantly by 10 to 30 dB in the four-month group compared to the one-month controls at 11.3, 16, 20, and 22.6 kHz. CAP I/O functions were shallower in the four-month group compared to controls at all frequencies. In the 12- and 24-month-old mice, CAP responses were absent, despite normal EP values in these animals. Recovery functions after conditioning tones were obtained at 8, 16, 20 and 22.6 kHz; the functions had fast and slow components at all frequencies tested in both the one- and four-month-old groups. The corresponding recovery curves were identical for both age groups, even with significant threshold shifts in the older group. The two component recovery curves provide the first physiological evidence that different spontaneous rate (SR) classes of auditory neurons exist in the C57BL/6J mouse. Moreover, the unchanged recovery functions in the older group suggest that there was no loss of activity of the low-SR fiber population with age under conditions where the EP remains stable, in contrast to the gerbil model of presbyacusis where there is a loss of low-SR fiber activity and EP does decline with age.

Plastic changes in the central auditory system after hearing loss, restoration of function, and during learning

Traditionally the auditory system was considered a hard-wired sensory system; this view has been challenged in recent years in light of the plasticity of other sensory systems, particularly the visual and somatosensory systems. Practical experience in clinical audiology together with the use of prosthetic devices, such as cochlear implants, contributed significantly to the present view on the plasticity of the central auditory system, which was originally based on data obtained in animal experiments. The loss of auditory receptors, the hair cells, results in profound changes in the structure and function of the central auditory system, typically demonstrated by a reorganization of the projection maps in the auditory cortex. These plastic changes occur not only as a consequence of mechanical lesions of the cochlea or biochemical lesions of the hair cells by ototoxic drugs, but also as a consequence of the loss of hair cells in connection with aging or noise exposure. In light of the aging world population and the increasing amount of noise in the modern world, understanding the plasticity of the central auditory system has its practical consequences and urgency. In most of these situations, a common denominator of central plastic changes is a deterioration of inhibition in the subcortical auditory nuclei and the auditory cortex. In addition to the processes that are elicited by decreased or lost receptor function, the function of nerve cells in the adult central auditory system may dynamically change in the process of learning. A better understanding of the plastic changes in the central auditory system after sensory deafferentation, sensory stimulation, and learning may contribute significantly to improvement in the rehabilitation of damaged or lost auditory function and consequently to improved speech processing and production.

Vulnerability to noise-induced hearing loss in 'middle-aged' and young adult mice: a dose-response approach in CBA, C57BL, and BALB inbred strains

Vulnerability of the cochlea to noise-induced permanent threshold shifts (NIPTS) was examined in young adult (1-2 months) and 'middle-aged' (5-7 months) CBA/CaJ, C57BL/6J, and BALB/cJ inbred mice. For each age and strain, a dose-response paradigm was applied, whereby groups of up to 12 animals were exposed to intense broadband noise (110 dB SPL) for varying durations. Exposure durations reliably associated with <10% and >90% probability of a criterion amount of NIPTS (determined 2 weeks post-exposure) were identified, and the minimum NIPTS exposure and the slope of the dose-response relation were then derived by numerical modeling. For all three strains, young adult mice were more susceptible to NIPTS than older adults; That is, a shorter exposure was able to cause NIPTS in the younger mice. Strain comparisons revealed that C57 mice were more susceptible than CBAs in the older age group only. At both ages examined, however, BALB mice were most susceptible to NIPTS. When animals with a similar amount of NIPTS were compared, outer hair cell loss in the cochlear base was more widespread in the younger animals. BALB mice appear particularly susceptible to noise-induced outer hair cell loss throughout life. Our data suggest that the mechanism or site of noise injury differs between young adults and older adults, and may depend on genetic background. The finding that both BALB and C57 mice, which show pronounced age-related hearing loss, are also especially vulnerable to noise supports the notion that genes associated with age-related hearing loss often act by rendering the cochlea susceptible to insults.

Noise damage in the C57BL/CBA mouse cochlea

The present study was designed to determine the response to noise of the auditory system of a genetically well-defined laboratory mouse in preparation for examining the effect of noise on mice with specific genetic mutations. The mice were C57BL/CBA F1 hybrids. Eight mice served as non-noise-exposed controls and 39 mice were exposed for 1-24 h to an octave band of noise with a center frequency of 2, 4 or 8 kHz and a sound pressure level of 100-120 dB. Auditory brainstem response thresholds were measured pre-exposure and several times post-exposure (i.e., 0-27 days) to determine the magnitude of the temporary threshold shift (TTS) and permanent threshold shift (PTS). After fixation by cardiac perfusion, the cochleas from each mouse were embedded in plastic, dissected into quarter turns of the cochlear duct and analyzed quantitatively. Immediately post-exposure, all mice had sizable TTSs at the tested frequencies (i.e., 3-50 kHz). At this time, two mice were killed. Thresholds of the other 37 mice recovered somewhat in the first 4 days post-exposure. One mouse fully recovered from its TTS; 10 mice were left with PTSs at all frequencies; 26 mice recovered at some frequencies but not others. Most mice with PTSs for 30-50 kHz had focal losses of inner and outer hair cells in the basal 20% of the organ of Corti, often with degeneration of adjacent myelinated nerve fibers in the osseous spiral lamina. On the other hand, mice with PTSs for the lower frequencies (i.e., 3-20 kHz) had stereocilia disarray without significant hair cell losses in the second and first turns. Considerable variability was found in the magnitude of hair cell losses in those mice that received identical noise exposures, despite their genetic homogeneity.

Distortion product otoacoustic emissions in the CBA/J mouse model of presbycusis

CBA mice do not exhibit age-related loss of auditory sensitivity or cochlear pathology until relatively late in life. Therefore, this strain is believed to be an excellent animal model for the examination of the effects of age on the cochlea. To evaluate the effects of age on outer hair cell function, 2f1-f2 distortion product otoacoustic emissions (DPOAEs) were measured for f2 between 8 and 16 kHz in CBA/J mice between 1 and 25 months of age. CBA mice exhibited mild age-related changes in DPOAE level and detection threshold at 17 months of age, and changes of 20-40 dB by 25 months of age. The DPOAE level decreased and detection threshold increased with age in a frequency-dependent manner, starting at high frequencies and eventually extending to low frequencies. The range of frequencies in which notches were observed in the DPOAE input/output (I/O) functions extended toward lower frequencies by 17 months of age. Notches were absent in the I/O functions of 25-month-old mice. The present results for a frequency range of 8-16 kHz suggest that age has modest effects on outer hair cell function in CBA mice.

Survival-fixation of the cochlea: a technique for following time-dependent degeneration and repair in noise-exposed chinchillas

To minimize problems with data interpretation due to interanimal variation in susceptibility to noise, we developed a survival-fixation paradigm which involves fixing one cochlea of an experimental chinchilla at one post-exposure time and fixing the second cochlea as much as 14-24 days later. This paradigm is analytically effective because there is a high correlation in the magnitude and pattern of damage in the left and right cochleas of binaurally exposed animals. Thus, each experimental animal provides two snapshots in the degeneration and repair continua in which it can be certain that both cochleas sustained equivalent amounts of damage during the exposure. Using this technique, the time course of degeneration of different structures and cells in the organ of Corti can be determined and primary damage can be distinguished from secondary effects. The present paper discusses the issues which had to be addressed to develop this technique and provides preliminary results from chinchillas exposed to a traumatic noise.

Quantitative measure of genetic differences in susceptibility to noise-induced hearing loss in two strains of mice

The CBA/CaJ (CB) and C57BL/6J (B6) inbred strains of mice were exposed for 1 h to noise intensities between 98 and 119 dB SPL. Previous studies indicated that the B6 mice exhibited permanent threshold shifts (PTS) after 1h exposure to 110 dB, whereas the CB mice did not exhibit any PTS. These differences in susceptibility to noise-induced hearing loss (NIHL) appear to be due to a gene for age-related hearing loss (AHL). The current study was designed to determine dose-response curves for NIHL over the ranges of intensities of noise that would characterize the B6 and CB inbred strains of mice. Because of the considerable differences in sensitivity to NIHL, the noise exposures for the two strains overlapped only at 110 and 113 dB. Nevertheless, the two strains exhibited two different dose-response curves, offset and with different slopes. We postulate that the B6 strain of mice exhibits a more linear increase for PTS from 98-113 dB, consistent with incremental effects on some metabolic physiological mechanism(s); the abrupt transition in NIHL between 113 and 116 dB for the CB mice is consistent with an ototraumatic structural injury.

Age-related alteration in processing of temporal sound features in the auditory midbrain of the CBA mouse

The perception of complex sounds, such as speech and animal vocalizations, requires the central auditory system to analyze rapid, ongoing fluctuations in sound frequency and intensity. A decline in temporal acuity has been identified as one component of age-related hearing loss. The detection of short, silent gaps is thought to reflect an important fundamental dimension of temporal resolution. In this study we compared the neural response elicited by silent gaps imbedded in noise of single neurons in the inferior colliculus (IC) of young and old CBA mice. IC neurons were classified by their temporal discharge patterns. Phasic units, which accounted for the majority of response types encountered, tended to have the shortest minimal gap thresholds (MGTs), regardless of age. We report three age-related changes in neural processing of silent gaps. First, although the shortest MGTs (1-2 msec) were observed in phasic units from both young and old animals, the number of neurons exhibiting the shortest MGTs was much lower in old mice, regardless of the presentation level. Second, in the majority of phasic units, recovery of response to the stimulus after the silent gap was of a lower magnitude and much slower in units from old mice. Finally, the neuronal map representing response latency versus best frequency was found to be altered in the old IC. These results demonstrate a central auditory system correlate for age-related decline in temporal processing at the level of the auditory midbrain.

Effects of noise on inferior colliculus evoked potentials and cochlear anatomy in young and aged chinchillas

Like many aging humans, the aging chinchilla tends to lose high-frequency sensitivity at a faster rate than low-frequency sensitivity. This feature, combined with its excellent low-frequency hearing, makes the chinchilla attractive as an animal model for studying the relationship between noise-induced hearing loss (NIHL) and age-related hearing loss (AHL). In the present study, we examined susceptibility to noise in 15 aged (10-15 years old) and 15 young chinchillas. Two levels of noise were used, with the aim of determining whether age-related differences exist in the magnitude and rate of recovery from temporary threshold shifts produced by a moderate-level (95 dB) noise exposure, or in susceptibility to permanent threshold shifts and cochlear damage caused by a high-level (106 dB) noise exposure. Thresholds and response amplitudes at 0.5, 1, 2, 4, 8 and 16 kHz were determined from evoked potentials recorded from the inferior colliculus. Cochlear histology was performed on animals exposed to high-level noise. The results suggest that older animals are equally vulnerable to moderate-level noise, but may be slightly more vulnerable to high-level noise. For moderate-level exposures, there appears to be a simple additive relationship (in dB) between AHL and NIHL. For high-level exposures, the relationship may be more complex.

Quantitative measures of hair cell loss in CBA and C57BL/6 mice throughout their life spans

The CBA mouse shows little evidence of hearing loss until late in life, whereas the C57BL/6 strain develops a severe and progressive, high-frequency sensorineural hearing loss beginning around 3-6 months of age. These functional differences have been linked to genetic differences in the amount of hair cell loss as a function of age; however, a precise quantitative description of the sensory cell loss is unavailable. The present study provides mean values of inner hair cell (IHC) and outer hair cell (OHC) loss for CBA and C57BL/6 mice at 1, 3, 8, 18, and 26 months of age. CBA mice showed little evidence of hair cell loss until 18 months of age. At 26 months of age, OHC losses in the apex and base of the cochlea were approximately 65% and 50%, respectively, and IHC losses were approximately 25% and 35%. By contrast, C57BL/6 mice showed approximately a 75% OHC and a 55% IHC loss in the base of the cochlea at 3 months of age. OHC and IHC losses increased rapidly with age along a base-to-apex gradient. By 26 months of age, more than 80% of the OHCs were missing throughout the entire cochlea; however, IHC losses ranged from 100% near the base of the cochlea to approximately 20% in the apex.

Auditory evoked potentials in aged gerbils: responses elicited by noises separated by a silent gap

The compound action potential (CAP) and the auditory brainstem response (ABR; waves ii and iv) were recorded in young (4-8 month) and aged (33-37 month) gerbils using a paradigm similar to that used in some psychophysical studies of gap detection (a pair of identical low-pass noises separated by a silent gap). Response amplitudes were analyzed in terms of absolute amplitudes and the 'amplitude ratio' (the amplitude of the response to the second noise of a pair divided by that to the first). Response latencies were analyzed in terms of the absolute latencies as well as the 'latency shift' (the latency of the response to the second noise minus that to the first). Response amplitudes were much smaller in the aged subjects for both the first and second stimuli of a pair. There were minimal changes in amplitude ratios across age for both the CAP and ABR. Absolute latencies were similar between groups for the first stimulus of a pair, but latencies to wave iv were much longer for the aged subjects when the gap was short. Thus, the latency shift for the aged group was much longer for wave iv in the aged compared to the young group, but were similar between groups for the CAP or wave ii of the ABR. The results suggest that there may be changes in coding of temporal information in the auditory brainstem of aged gerbils which are not a direct result of abnormal temporal processing in the auditory periphery.

Genetics of age-related hearing loss in mice. III. Susceptibility of inbred and F1 hybrid strains to noise-induced hearing loss

Some humans and mice are genetically predisposed to age-related hearing loss (AHL), and others are variously susceptible to noise-induced hearing loss (NIHL). The inbred C57BL/6J (B6) mice exhibit AHL at an early age, whereas the inbred CBA/CaJ (CB) mice do not. The B6 mice are much more susceptible to NIHL than are the CB mice (Shone et al., 1991; Li, 1992a). The B6 mice possess an Ahl gene which maps to chromosome 10 (Erway et al., 1995). This study was designed, using these two inbred strains plus two F1 hybrid strains of mice, to begin to test the hypothesis that the Ahl genotypes may influence the susceptibility to NIHL. These strains of mice (with putative genotypes) are: inbred CB (+/+) and B6 (Ahl/Ahl); hybrid CBB6F1 (+/Ahl) and B6D2F1 (Ahl/Ahl; D2 represents inbred DBA/2J). Twenty-four mice of each of these four strains were exposed to noise (110 dB for 0, 1 or 2 h) and tested for auditory-evoked brainstem response (ABR) thresholds. The CB and CBB6F1 strains of mice did not differ significantly from each other, exhibiting mostly temporary threshold shifts. The B6 and B6D2F1 strains of mice did not differ significantly from each other, but did exhibit permanent threshold shifts. These results support the hypothesis that genetic predisposition to AHL may be revealed at a younger age by NIHL. This suggests that it may be possible to use the NIHL to distinguish segregating genotypes (+/Ahl vs. Ahl/Ahl) among backcross progeny and thereby to identify and map single genes for AHL.

Sensorineural hearing loss alters recovery from short-term adaptation in the C57BL/6 mouse

Several strains of laboratory mouse (Mus musculus) have a pattern of hearing loss which resembles that found in humans. The C57BL/6 strain of mouse has a genetic defect that results in degeneration of the organ of Corti, originating in the basal, high-frequency region and then proceeding apically over time. The end result is a severe-to-profound sensorineural hearing loss (SNHL) by 14 months of age. In contrast, auditory function of the CBA strain remains normal through its early life span then slowly declines later in life, much like that typified by human presbycusis. The purpose of the present study was to compare ABR (peak 5) forward masking recovery functions in young, normal-hearing CBA and C57BL/6 mice to hearing-impaired C57BL/6 mice. ABR audiograms were obtained prior to collecting the tone-on-tone forward masking data. Masking was defined as a 50% reduction in the P5 component of the ABR, elicited and masked by 12 kHz tone bursts, using masker/probe time delays from 0 to 100 ms. Time constants were computed from an exponential model fit to the recovery functions (masker level vs. time delay). In hearing-impaired animals there was a significant increase in recovery from short-term adaptation as measured by the time constants, as well as a significant latency shift in the P5 component. The effects of SNHL on the recovery of the P5 component from short-term adaptation was comparable to that reported behaviorally for human hearing-impaired listeners and physiologically from the inferior colliculus (IC) of chinchillas suffering permanent threshold shifts.

Influence of age on noise-induced permanent threshold shifts in CBA/Ca and C57BL/6J mice

Two inbred strains of mice, CBA/Ca (showing a moderate hearing loss with onset late in life) and C57BL/6J (undergoing spontaneous auditory degeneration with onset during young adulthood), were exposed to a broad-band noise of 120 dB SPL (2-7 kHz) for 5 min at 1,2,3,6 or 12 (only for CBA) months of age. Permanent threshold shifts (PTS) were determined by recording auditory brainstem response (ABR) 1 month after exposure. C57 mice were more severely affected by acoustic trauma than age-matched CBA mice. With increasing age, susceptibility to PTS decreased in CBA mice but remained constant in C57 mice. Results indicate that the auditory system of CBA mice undergoes a progressive resistance to noise damage, whereas the persistent high susceptibility to acoustic trauma in C57 mice may be related to their genetic predisposition to rapid auditory degeneration.