ABR/DPOAE publications

Animal studies indicate that a combination of kanamycin (KM) and noise produces a synergistic effect, whereby the threshold shift from the combination is greater than the sum of the shifts caused by either agent alone. Most such studies have focused on adult animals, and it has remained unclear whether younger, presumably more susceptible, animals show an even greater synergistic effect. The present study tested the hypothesis that young CBA/J mice receiving a low dose of KM (300 mg/kg, 2x/day, s.c.) from 20 to 30 days post-gestational age followed by brief noise exposure (110 dB SPL; 4-45 kHz, 30 s) would show greater noise-induced permanent threshold shifts (NIPTS) than mice receiving either treatment alone. Noise exposure produced 30-40 dB of NIPTS and moderate hair cell loss in young saline-treated mice. KM alone at this dose had no effect on thresholds. Surprisingly, mice receiving KM plus noise were protected from NIPTS, showing ABR thresholds not significantly different from unexposed controls. Mice receiving KM prior to noise exposure also showed significantly less outer hair cell loss than saline-treated mice. Additional experiments indicated protection by KM when the noise was applied either 24 or 48 h after the last KM injection. Our results demonstrate a powerful protective effect of sub-chronic low-dose kanamycin against NIPTS in young CBA/J mice. Repeated kanamycin exposure may establish a preconditioned protective state, the molecular bases of which remain to be determined.

Insulin-like growth factor-I (IGF-I) belongs to the family of insulin-related peptides that fulfils a key role during the late development of the nervous system. Human IGF1 mutations cause profound deafness, poor growth and mental retardation. Accordingly, Igf1(-/-) null mice are dwarfs that have low survival rates, cochlear alterations and severe sensorineural deafness. Presbycusis (age-related hearing loss) is a common disorder associated with aging that causes social and cognitive problems. Aging is also associated with a decrease in circulating IGF-I levels and this reduction has been related to cognitive and brain alterations, although there is no information as yet regarding the relationship between presbycusis and IGF-I biodisponibility. Here we present a longitudinal study of wild type Igf1(+/+) and null Igf1(-/-) mice from 2 to 12 months of age comparing the temporal progression of several parameters: hearing, brain morphology, cochlear cytoarchitecture, insulin-related factors and IGF gene expression and IGF-I serum levels. Complementary invasive and non-invasive techniques were used, including auditory brainstem-evoked response (ABR) recordings and in vivo MRI brain imaging. Igf1(-/-) null mice presented profound deafness at all the ages studied, without any obvious worsening of hearing parameters with aging. Igf1(+/+) wild type mice suffered significant age-related hearing loss, their auditory thresholds and peak I latencies augmenting as they aged, in parallel with a decrease in the circulating levels of IGF-I. Accordingly, there was an age-related spiral ganglion degeneration in wild type mice that was not evident in the Igf1 null mice. However, the Igf1(-/-) null mice in turn developed a prematurely aged stria vascularis reminiscent of the diabetic strial phenotype. Our data indicate that IGF-I is required for the correct development and maintenance of hearing, supporting the idea that IGF-I-based therapies could contribute to prevent or ameliorate age-related hearing loss.

Background: Noise-induced hearing loss (NIHL) is one of the most common occupational injuries in the United States. It would be extremely valuable if a safe, inexpensive compound could be identified which protects worker hearing from noise. In a series of experiments, Kopke has shown that the compound N-acetyl-L-cysteine (L-NAC) can protect the hearing of chinchillas from the effects of a single exposure to noise. L-NAC is used in clinical medicine and is very safe. Although L-NAC was reported to be promising, it has not been successful in other studies (Kramer et al., 2006; Hamernik et al., 2008). The present study was undertaken to determine if L-NAC could protect C57BL/6J (B6) mice from the permanent effects of noise. Method: Two groups of five B6 mice were injected with either 300 or 600 mg/kg L-NAC approximately 1 hr prior to a 104 dB broadband noise exposure and again immediately after the exposure. A control group (N = 7) was exposed to the same noise level but injected with vehicle (sterile saline). Auditory brainstem response measurements were made at 4, 8, 16 and 32 kHz one week prior to and 12 days after exposure. Conclusions: There were no statistically significant differences in ABR threshold shifts between the mice receiving L-NAC and the control mice. This indicates that L-NAC was not effective in preventing permanent threshold shift in this mouse model of NIHL.

Our knowledge of age-related changes in auditory processing in the central auditory system is limited, unlike the changes in the peripheral hearing organs which are more extensively studied. This study aims to further understanding of temporal processing in aging using non-invasive electrophysiological measurements in a rat model system. Amplitude modulation following responses (AMFRs) were assessed using sinusoidally amplitude modulated (SAM) tones presented to aged (92- to 95-weeks old) and young (9- to 12-weeks old) Fischer-344 rats. The modulation frequency and sound level were systematically varied, and the SAM stimuli were also presented simultaneously with wideband background noise at various levels. The overall shapes and cutoff frequencies of the AMFR temporal modulation transfer functions (tMTFs) were similar between young and aged animals. The fast Fourier transform (FFT) amplitudes of the aged animals were similar to the young in the 181-512 Hz modulation frequency range, but were significantly lower at most modulation frequencies above and below. There were no significant age-related differences in the nature of growth or FFT amplitudes with change in sound level at 256 and 1024 Hz modulation frequencies. The AMFR amplitudes were also not correlated with the ABR wave I or wave III amplitudes elicited for broadband click stimuli presented at the same sound level suggesting that sustained AMFR provide complementary information to phasic ABR responses. The FFT amplitudes varied significantly between young and aged animals for SAM stimuli in the presence of background noise, depending on the modulation frequency used and signal to noise ratio. The results show that the representation of temporally modulated stimuli is similar between young and aged animals in quiet listening conditions, but diverges substantially with the addition of background noise. This is consistent with a decrease in inhibition causing altered temporal processing with age.

Acoustic communication involves both the generation and the detection of a signal. In the coqui frog ( Eleutherodactylus coqui ), it is known that the spectral contents of its calls systematically change with altitude above sea level. Here, distortion product otoacoustic emissions are used to assess the frequency range over which the inner ear is sensitive. It is found that both the spectral contents of the calls and the inner-ear sensitivity change in a similar fashion along an altitudinal gradient. As a result, the call frequencies and the auditory tuning are closely matched at all altitudes. We suggest that the animal's body size determines the frequency particulars of the call apparatus and the inner ear.

The role of innate immunity and macrophage recruitment to the inner ear after hair cell injury is a subject where little is known. In this paper, we demonstrate recruitment of monocytes and macrophages to the inner ear after kanamycin. We also examined the effect of fractalkine receptor (CX3CR1) deletion in kanamycin ototoxicity. We observed more functional and structural damage in CX3CR1 null mice compared to wild-type and heterozygous littermates. In order to determine if increased susceptibility to kanamycin resulted from CX3CR1 deletion from cochlear leukocytes, we created bone marrow chimeras by transplanting CX3CR1-null bone marrow into wild-type mice whose native bone marrow was ablated by lethal irradiation. These mice were then treated with kanamycin sulfate. Auditory brainstem responses (ABR), hair cell counts, and numbers of macrophages recruited to the cochlea were recorded in irradiated mice that received either wild-type, CX3CR1 heterozygous, or CX3CR1 knockout bone marrow. A strong correlation was present between numbers of macrophages and hair cell death in recipients transplanted with CX3CR1 null marrow. No correlation between macrophage number and hair cell loss was present in mice transplanted with wild-type or CX3CR1 heterozygous marrow. We suggest that CX3CR1 plays a role in modulating the detrimental effects of cochlear macrophages after kanamycin ototoxicity. Our data point to the possibility that CX3CR1-deficient cochlear macrophages exacerbate kanamycin ototoxicity while CX3CR1-expressing monocytes do not.

Overexposure to intense sound can cause temporary or permanent hearing loss. Postexposure recovery of threshold sensitivity has been assumed to indicate reversal of damage to delicate mechano-sensory and neural structures of the inner ear and no persistent or delayed consequences for auditory function. Here, we show, using cochlear functional assays and confocal imaging of the inner ear in mouse, that acoustic overexposures causing moderate, but completely reversible, threshold elevation leave cochlear sensory cells intact, but cause acute loss of afferent nerve terminals and delayed degeneration of the cochlear nerve. Results suggest that noise-induced damage to the ear has progressive consequences that are considerably more widespread than are revealed by conventional threshold testing. This primary neurodegeneration should add to difficulties hearing in noisy environments, and could contribute to tinnitus, hyperacusis, and other perceptual anomalies commonly associated with inner ear damage.

Traditionally, the calyx of Held synapse is viewed as a highly reliable relay in the sound localization circuit of the auditory brainstem, with every presynaptic action potential triggering a postsynaptic action potential in vivo . However, this view is at odds with slice recordings that report large short-term depression (STD). To investigate the reliability and precision of this synapse, we compared slice and in vivo recordings from medial nucleus of the trapezoid body neurons of young adult mice. We show that the extracellularly recorded complex waveform can be used to estimate both presynaptic release and postsynaptic excitability. Whereas under standard slice conditions the synapse underwent large STD, both extracellular and whole-cell recordings indicated that in vivo the size of the EPSPs was independent of recent history. The estimated quantal content was typically <20 in vivo , much lower than in the resting synapse under standard slice conditions. However, due to the large quantal size and summation of EPSPs, the safety factor of this synapse was generally still sufficiently large and postsynaptic failures were observed only infrequently in vivo . When present, failures were typically due to stochastic fluctuations in EPSP size or postsynaptic spike depression. In vivo , the calyx of Held synapse thus functions as a tonic synapse. The price it pays for its low release probability is an increase in jitter and synaptic latency and occasional postsynaptic failures.

Deafness is the most common sensory disorder in humans and the aetiology of genetic deafness is complex. Mouse mutants have been crucial in identifying genes involved in hearing. However, many deafness genes remain unidentified. Using N ‐ethyl N −nitrosourea (ENU) mutagenesis to generate new mouse models of deafness, we identified a novel semi‐dominant mouse mutant, Cloth‐ears ( Clth ). Cloth‐ears mice show reduced acoustic startle response and mild hearing loss from ∼30 days old. Auditory‐evoked brainstem response (ABR) and distortion product otoacoustic emission (DPOAE) analyses indicate that the peripheral neural auditory pathway is impaired in Cloth‐ears mice, but that cochlear function is normal. In addition, both Clth/Clth and Clth/+ mice display paroxysmal tremor episodes with behavioural arrest. Clth/Clth mice also show a milder continuous tremor during movement and rest. Longitudinal phenotypic analysis showed that Clth/+ and Clth/Clth mice also have complex defects in behaviour, growth, neurological and motor function. Positional cloning of Cloth‐ears identified a point mutation in the neuronal voltage‐gated sodium channel α‐subunit gene, Scn8a , causing an aspartic acid to valine (D981V) change six amino acids downstream of the sixth transmembrane segment of the second domain (D2S6). Complementation testing with a known Scn8a mouse mutant confirmed that this mutation is responsible for the Cloth‐ears phenotype. Our findings suggest a novel role for Scn8a in peripheral neural hearing loss and paroxysmal motor dysfunction.

The strength of the medial olivocochlear (OC) reflex is routinely assayed by measuring suppression of ipsilateral responses such as otoacoustic emissions (OAEs) by a brief contralateral noise, e.g., (Berlin, C.I., Hood, L.J., Cecola, P., Jackson, D.F., Szabo, P. 1993. Does type I afferent dysfunction reveal itself through lack of efferent suppression. Hear. Res. 65, 40-50). Here, we show in anesthetized guinea pigs, that the magnitude of OC-mediated suppression of ipsilateral cochlear responses (i.e., compound actions potentials (CAPs), distortion product (DP) OAEs and round-window noise) slowly builds over 2-3 min during a sustained contralateral noise. The magnitude of this build-up suppression was largest at low ipsilateral stimulus intensities, as seen for suppression measured at contra-noise onset. However, as a function of stimulus frequency, build-up suppression magnitude was complementary to onset suppression, i.e., largest at the lowest and highest frequencies tested. Both build-up and onset suppression were eliminated by cutting the OC bundle. In contrast to "slow effects" of shock-evoked medial OC activity (Sridhar, T.S., Liberman, M.C., Brown, M.C., Sewell, W.F. 1995. A novel cholinergic "slow effect" of efferent stimulation on cochlear potentials in the guinea pig. J. Neurosci. 15, 3667-3678), which are mediated by slow intracellular changes in Ca concentration in OHCs, build-up effects of contralateral noise are immediately extinguished upon OC bundle transection and are likely mediated by central modulation of the response rates in MOC fibers due to the sustained noise. Results suggest that conventional tests of OC reflex strength may underestimate its magnitude in noisy environments.

The present study aimed to identify morphological correlates of environment‐induced changes at excitatory synapses of the primary auditory cortex (A1). We used the Golgi‐Cox stain technique to compare pyramidal cells dendritic properties of Sprague‐Dawley rats exposed to different environmental manipulations. Sholl analysis, dendritic length measures, and spine density counts were used to monitor the effects of sensory deafness and an auditory version of environmental enrichment (EE). We found that deafness decreased apical dendritic length leaving basal dendritic length unchanged, whereas EE selectively increased basal dendritic length without changing apical dendritic length. On the contrary, deafness decreased while EE increased spine density in both basal and apical dendrites of A1 Layer 2/3 (LII/III) neurons. To determine whether stress contributed to the observed morphological changes in A1, we studied neural morphology in a restraint‐induced model that lacked behaviorally relevant acoustic cues. We found that stress selectively decreased apical dendritic length in the auditory but not in the visual primary cortex. Similar to the acoustic manipulation, stress‐induced changes in dendritic length possessed a layer‐specific pattern displaying LII/III neurons from stressed animals with normal apical dendrites but shorter basal dendrites, while infragranular neurons (Layers V and VI) displayed shorter apical dendrites but normal basal dendrites. The same treatment did not induce similar changes in the visual cortex, demonstrating that the auditory cortex is an exquisitely sensitive target of neocortical plasticity, and that prolonged exposure to different acoustic as well as emotional environmental manipulation may produce specific changes in dendritic shape and spine density. Synapse 64:97–110, 2010. © 2009 Wiley‐Liss, Inc.

Atoh1 is a basic helix–loop–helix transcription factor necessary for the specification of inner ear hair cells and central auditory system neurons derived from the rhombic lip. We used the Cre–loxP system and two Cre -driver lines ( Egr2 Cre and Hoxb1 Cre ) to delete Atoh1 from different regions of the cochlear nucleus (CN) and accessory auditory nuclei (AAN). Adult Atoh1 -conditional knock-out mice ( Atoh1 CKO ) are behaviorally deaf, have diminished auditory brainstem evoked responses, and have disrupted CN and AAN morphology and connectivity. In addition, Egr2; Atoh1 CKO mice lose spiral ganglion neurons in the cochlea and AAN neurons during the first 3 d of life, revealing a novel critical period in the development of these neurons. These new mouse models of predominantly central deafness illuminate the importance of the CN for support of a subset of peripheral and central auditory neurons.

Auditory neuropathy is a hearing disorder characterized by normal function of outer hair cells, evidenced by intact cochlear microphonic (CM) potentials and otoacoustic emissions (OAEs), with absent or severely dys-synchronized auditory brainstem responses (ABRs). To determine if selective lesions of inner hair cells (IHCs) and auditory nerve fibers (ANFs) can account for these primary clinical features of auditory neuropathy, we measured physiological responses from chinchillas with large lesions of ANFs (about 85%) and IHCs (45% loss in the apical half of the cochlea; 73% in the basal half). Distortion product OAEs and CM potentials were significantly enhanced, whereas summating potentials and compound action potentials (CAPs) were significantly reduced. CAP threshold was elevated by 7.5dB, but response synchrony was well preserved down to threshold levels of stimulation. Similarly, ABR threshold was elevated by 5.6dB, but all waves were present and well synchronized down to threshold levels in all animals. Thus, large lesions of IHCs and ANFs reduced response amplitudes but did not abolish or severely dys-synchronize CAPs or ABRs. Pathologies other than or in addition to ANF and IHC loss are likely to account for the evoked potential dys-synchrony that is a clinical hallmark of auditory neuropathy in humans.

Background Auditory brainstem responses (ABRs) are used to study auditory acuity in animal-based medical research. ABRs are evoked by acoustic stimuli, and consist of an electrical signal resulting from summated activity in the auditory nerve and brainstem nuclei. ABR analysis determines the sound intensity at which a neural response first appears (hearing threshold). Traditionally, threshold has been assessed by visual estimation of a series of ABRs evoked by different sound intensities. Here we develop an automated threshold detection method that eliminates the variability and subjectivity associated with visual estimation. Results The automated method is a robust computational procedure that detects the sound level at which the peak amplitude of the evoked ABR signal first exceeds four times the standard deviation of the baseline noise. Implementation of the procedure was achieved by evoking ABRs in response to click and tone stimuli, under normal and experimental conditions (adult stem cell transplantation into cochlea). Automated detection revealed that the threshold shift from pre- to post-surgery hearing levels was similar in mice receiving stem cell transplantation or sham injection for click and tone stimuli. Visual estimation by independent observers corroborated these results but revealed variability in ABR threshold shifts and significance levels for stem cell-transplanted and sham-injected animals. Conclusion In summary, the automated detection method avoids the subjectivity of visual analysis and offers a rapid, easily accessible http://axograph.com/source/abr.html approach to measure hearing threshold levels in auditory brainstem response.

Hearing relies on Ca 2+ influx-triggered exocytosis in cochlear inner hair cells (IHCs). Here we studied the role of the Ca 2+ channel subunit Ca V β 2 in hearing. Of the Ca V β 1–4 mRNAs, IHCs predominantly contained Ca V β 2 . Hearing was severely impaired in mice lacking Ca V β 2 in extracardiac tissues ( Ca V β 2 −/− ). This involved deficits in cochlear amplification and sound encoding. Otoacoustic emissions were reduced or absent in Ca V β 2 −/− mice, which showed strongly elevated auditory thresholds in single neuron recordings and auditory brainstem response measurements. Ca V β 2 −/− IHCs showed greatly reduced exocytosis (by 68%). This was mostly attributable to a decreased number of membrane-standing Ca V 1.3 channels. Confocal Ca 2+ imaging revealed presynaptic Ca 2+ microdomains albeit with much lower amplitudes, indicating synaptic clustering of fewer Ca V 1.3 channels. The coupling of the remaining Ca 2+ influx to IHC exocytosis appeared unaffected. Extracellular recordings of sound-evoked spiking in the cochlear nucleus and auditory nerve revealed reduced spike rates in the Ca V β 2 −/− mice. Still, sizable onset and adapted spike rates were found during suprathreshold stimulation in Ca V β 2 −/− mice. This indicated that residual synaptic sound encoding occurred, although the number of presynaptic Ca V 1.3 channels and exocytosis were reduced to one-third. The normal developmental upregulation, clustering, and gating of large-conductance Ca 2+ activated potassium channels in IHCs were impaired in the absence of Ca V β 2 . Moreover, we found the developmental efferent innervation to persist in Ca V β 2 -deficient IHCs. In summary, Ca V β 2 has an essential role in regulating the abundance and properties of Ca V 1.3 channels in IHCs and, thereby, is critical for IHC development and synaptic encoding of sound.

Mutations in the CLRN1 gene cause Usher syndrome type 3 (USH3), a human disease characterized by progressive blindness and deafness. Clarin 1, the protein product of CLRN1, is a four-transmembrane protein predicted to be associated with ribbon synapses of photoreceptors and cochlear hair cells, and recently demonstrated to be associated with the cytoskeleton. To study Clrn1, we created a Clrn1 knockout (KO) mouse and characterized the histological and functional consequences of Clrn1 deletion in the retina and cochlea. Clrn1 KO mice do not develop a retinal degeneration phenotype, but exhibit progressive loss of sensory hair cells in the cochlea and deterioration of the organ of Corti by 4 months. Hair cell stereocilia in KO animals were longer and disorganized by 4 months, and some Clrn1 KO mice exhibited circling behavior by 5-6 months of age. Clrn1 mRNA expression was localized in the retina using in situ hybridization (ISH), laser capture microdissection (LCM), and RT-PCR. Retinal Clrn1 transcripts were found throughout development and adulthood by RT-PCR, although expression peaked at P7 and declined to undetectable levels in adult retina by ISH. LCM localized Clrn1 transcripts to the retinas inner nuclear layer, and WT levels of retinal Clrn1 expression were observed in photoreceptor-less retinas. Examination of Clrn1 KO mice suggests that CLRN1 is unnecessary in the murine retina but essential for normal cochlear development and function. This may reflect a redundancy in the mouse retina not present in human retina. In contrast to mouse KO models of USH1 and USH2, our data indicate that Clrn1 expression in the retina is restricted to the Müller glia. This is a novel finding, as most retinal degeneration associated proteins are expressed in photoreceptors, not in glia. If CLRN1 expression in humans is comparable to the expression pattern observed in mice, this is the first report of an inner retinal protein that, when mutated, causes retinal degeneration.

Background: Infant crying is an important cue for mothers to respond adequately. Inappropriate response to infant crying can hinder social development in infants. In rodents, the pup-mother interaction largely depends on pup's calls. Mouse pups emit high frequency to ultrasonic vocalization (2-90 kHz) to communicate with their dam for maternal care. However, little is known about how the maternal response to infant crying or pup calls affects social development over the long term. Methodology/principal findings: Here we used mice lacking acid-sensing ion channel 3 (Asic3(-/-)) to create a hearing deficit to probe the effect of caregiver hearing on maternal care and adolescent social development. Female Asic3(-/-) mice showed elevated hearing thresholds for low to ultrasonic frequency (4-32 kHz) on auditory brain stem response, which thus hindered their response to their pups' wriggling calls and ultrasonic vocalization, as well as their retrieval of pups. In adolescence, pups reared by Asic3(-/-) mice showed a social deficit in juvenile social behaviors as compared with those reared by wild-type or heterozygous dams. The social-deficit phenotype in juvenile mice reared by Asic3(-/-) mice was associated with the reduced serotonin transmission of the brain. However, Asic3(-/-) pups cross-fostered to wild-type dams showed rescued social deficit. Conclusions/significance: Inadequate response to pups' calls as a result of ASIC3-dependent hearing loss confers maternal deficits in caregivers and social development deficits in their young.

Complexins (CPXs I-IV) presumably act as regulators of the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complex, but their function in the intact mammalian nervous system is not well established. Here, we explored the role of CPXs in the mouse auditory system. Hearing was impaired in CPX I knock-out mice but normal in knock-out mice for CPXs II, III, IV, and III/IV as measured by auditory brainstem responses. Complexins were not detectable in cochlear hair cells but CPX I was expressed in spiral ganglion neurons (SGNs) that give rise to the auditory nerve. Ca(2+)-dependent exocytosis of inner hair cells and sound encoding by SGNs were unaffected in CPX I knock-out mice. In the absence of CPX I, the resting release probability in the endbulb of Held synapses of the auditory nerve fibers with bushy cells in the cochlear nucleus was reduced. As predicted by computational modeling, bushy cells had decreased spike rates at sound onset as well as longer and more variable first spike latencies explaining the abnormal auditory brainstem responses. In addition, we found synaptic transmission to outlast the stimulus at many endbulb of Held synapses in vitro and in vivo, suggesting impaired synchronization of release to stimulus offset. Although sound encoding in the cochlea proceeds in the absence of complexins, CPX I is required for faithful processing of sound onset and offset in the cochlear nucleus.