Chemical neuroprotection in the cochlea: The modulation of dopamine release from lateral olivocochlear efferents

Causal relationship between psychiatric disorders and sensorineural hearing loss: A bidirectional two-sample mendelian randomization analysis

OBJECTIVE

This study employed bidirectional two-sample Mendelian randomization (MR) to investigate the causal links between psychiatric disorders and sensorineural hearing loss (SNHL).

METHODS

Instrumental variables were chosen from genome-wide association studies of schizophrenia (SCH, N = 127,906), bipolar disorder (BD, N = 51,710), major depressive disorder (MDD, N = 500,199), and SNHL (N = 212,544). In the univariable MR analysis, the inverse-variance weighted method (IVW) was conducted as the primary analysis, complemented by various sensitivity analyses to ensure result robustness.

RESULTS

SCH exhibited a decreased the risk of SNHL (OR = 0.949, P = 0.005), whereas BD showed an increased incidence of SNHL (OR = 1.145, P = 0.005). No causal association was found for MDD on SNHL (OR = 1.088, P = 0.246). Multivariable MR validated these results. In the reverse direction, genetically predicted SNHL was linked to a decreased risk of SCH with suggestive significance (OR = 0.912, P = 0.023). No reverse causal relationships were observed for SNHL influencing BD or MDD. These findings remained consistent across various MR methods and sensitivity analyses.

CONCLUSION

This study demonstrated that the causal relationships between diverse psychiatric disorders with SNHL were heterogeneous. Specifically, SCH was inversely associated with SNHL susceptibility, and similarly, a reduced risk of SNHL was observed in schizophrenia patients. In contrast, BD exhibited an increased incidence of SNHL, although SNHL did not influence the prevalence of BD. No causal association between MDD and SNHL was found.

The continued importance of comparative auditory research to modern scientific discovery

A rich history of comparative research in the auditory field has afforded a synthetic view of sound information processing by ears and brains. Some organisms have proven to be powerful models for human hearing due to fundamental similarities (e.g., well-matched hearing ranges), while others feature intriguing differences (e.g., atympanic ears) that invite further study. Work across diverse "non-traditional" organisms, from small mammals to avians to amphibians and beyond, continues to propel auditory science forward, netting a variety of biomedical and technological advances along the way. In this brief review, limited primarily to tetrapod vertebrates, we discuss the continued importance of comparative studies in hearing research from the periphery to central nervous system with a focus on outstanding questions such as mechanisms for sound capture, peripheral and central processing of directional/spatial information, and non-canonical auditory processing, including efferent and hormonal effects.

Animal-to-Human Translation Difficulties and Problems With Proposed Coding-in-Noise Deficits in Noise-Induced Synaptopathy and Hidden Hearing Loss

Noise induced synaptopathy (NIS) and hidden hearing loss (NIHHL) have been hot topic in hearing research since a massive synaptic loss was identified in CBA mice after a brief noise exposure that did not cause permanent threshold shift (PTS) in 2009. Based upon the amount of synaptic loss and the bias of it to synapses with a group of auditory nerve fibers (ANFs) with low spontaneous rate (LSR), coding-in-noise deficit (CIND) has been speculated as the major difficult of hearing in subjects with NIS and NIHHL. This speculation is based upon the idea that the coding of sound at high level against background noise relies mainly on the LSR ANFs. However, the translation from animal data to humans for NIS remains to be justified due to the difference in noise exposure between laboratory animals and human subjects in real life, the lack of morphological data and reliable functional methods to quantify or estimate the loss of the afferent synapses by noise. Moreover, there is no clear, robust data revealing the CIND even in animals with the synaptic loss but no PTS. In humans, both positive and negative reports are available. The difficulty in verifying CINDs has led a re-examination of the hypothesis that CIND is the major deficit associated with NIS and NIHHL, and the theoretical basis of this idea on the role of LSR ANFs. This review summarized the current status of research in NIS and NIHHL, with focus on the translational difficulty from animal data to human clinicals, the technical difficulties in quantifying NIS in humans, and the problems with the SR theory on signal coding. Temporal fluctuation profile model was discussed as a potential alternative for signal coding at high sound level against background noise, in association with the mechanisms of efferent control on the cochlea gain.

Hearing Loss in Neurological Disorders

Sensorineural hearing loss (SNHL) affects approximately 466 million people worldwide, which is projected to reach 900 million by 2050. Its histological characteristics are lesions in cochlear hair cells, supporting cells, and auditory nerve endings. Neurological disorders cover a wide range of diseases affecting the nervous system, including Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), autism spectrum disorder (ASD), etc. Many studies have revealed that neurological disorders manifest with hearing loss, in addition to typical nervous symptoms. The prevalence, manifestations, and neuropathological mechanisms underlying vary among different diseases. In this review, we discuss the relevant literature, from clinical trials to research mice models, to provide an overview of auditory dysfunctions in the most common neurological disorders, particularly those associated with hearing loss, and to explain their underlying pathological and molecular mechanisms.

Diverse identities and sites of action of cochlear neurotransmitters

Accurate encoding of acoustic stimuli requires temporally precise responses to sound integrated with cellular mechanisms that encode the complexity of stimuli over varying timescales and orders of magnitude of intensity. Sound in mammals is initially encoded in the cochlea, the peripheral hearing organ, which contains functionally specialized cells (including hair cells, afferent and efferent neurons, and a multitude of supporting cells) to allow faithful acoustic perception. To accomplish the demanding physiological requirements of hearing, the cochlea has developed synaptic arrangements that operate over different timescales, with varied strengths, and with the ability to adjust function in dynamic hearing conditions. Multiple neurotransmitters interact to support the precision and complexity of hearing. Here, we review the location of release, action, and function of neurotransmitters in the mammalian cochlea with an emphasis on recent work describing the complexity of signaling.

The Effect of Parkinson's Disease on Otoacoustic Emissions and Efferent Suppression of Transient Evoked Otoacoustic Emissions

Purpose Several studies have demonstrated increased auditory thresholds in patients with Parkinson's disease (PD) based on subjective tonal audiometry. However, the pathophysiological mechanisms underlying auditory dysfunction in PD remain elusive. The primary aim of this study was to investigate cochlear and olivocochlear function in PD using objective measurements and to assess the effect of dopaminergic medication on auditory function. Method Eighteen patients with PD and 18 gender- and age-matched healthy controls (HCs) were included. Patients with PD participated in medication on and off conditions. Linear mixed models were used to determine the effect of PD on tonal audiometry, transient evoked and distortion product otoacoustic emissions (OAEs), and efferent suppression (ES). Results Tonal audiometry revealed normal auditory thresholds in patients with PD for their age across all frequencies. OAE signal amplitudes demonstrated a significant interaction effect between group (PD vs. HC) and frequency, indicating decreased OAEs at low frequencies and increased OAEs at high frequencies in patients with PD. No significant differences were found between patients with PD and HCs regarding ES. In addition, no significant effect of medication status was found on auditory measurements in patients with PD. Conclusions Altered OAEs support the hypothesis of cochlear alterations in PD. No evidence was found for the involvement of the medial olivocochlear system. Altogether, OAEs may provide an objective early indicator of auditory alterations in PD and should complement subjective tonal audiometry when assessing and monitoring auditory function in PD.

Chronic Oral Selegiline Treatment Mitigates Age-Related Hearing Loss in BALB/c Mice

Age-related hearing loss (ARHL), a sensorineural hearing loss of multifactorial origin, increases its prevalence in aging societies. Besides hearing aids and cochlear implants, there is no FDA approved efficient pharmacotherapy to either cure or prevent ARHL. We hypothesized that selegiline, an antiparkinsonian drug, could be a promising candidate for the treatment due to its complex neuroprotective, antioxidant, antiapoptotic, and dopaminergic neurotransmission enhancing effects. We monitored by repeated Auditory Brainstem Response (ABR) measurements the effect of chronic per os selegiline administration on the hearing function in BALB/c and DBA/2J mice, which strains exhibit moderate and rapid progressive high frequency hearing loss, respectively. The treatments were started at 1 month of age and lasted until almost a year and 5 months of age, respectively. In BALB/c mice, 4 mg/kg selegiline significantly mitigated the progression of ARHL at higher frequencies. Used in a wide dose range (0.15-45 mg/kg), selegiline had no effect in DBA/2J mice. Our results suggest that selegiline can partially preserve the hearing in certain forms of ARHL by alleviating its development. It might also be otoprotective in other mammals or humans.

Lateralization of cochlear dysfunction as a specific biomarker of Parkinson's disease

In the last decade, animal studies highlighted the sensitivity of hearing function to lack of specific cochlear dopamine receptors, while several studies on humans reported association between hearing loss and Parkinson's disease, partially recovered after levodopa administration in de novo patients. Taken together, these observations suggest investigating the possible use of cochlear function outcome variables, particularly, otoacoustic emissions, as sensitive biomarkers of Parkinson's disease. Any lateralization of hearing dysfunction correlated with Parkinson's disease lateralization would (i) further confirm their association and (ii) provide a disease-specific differential outcome variable. Differential indicators are particularly useful for diagnostic purposes, because their effectiveness is not limited by physiological inter-subject fluctuations of the outcome variable. Recent advances in the acquisition and analysis techniques of otoacoustic emissions suggest using them for evaluating differential cochlear damage in the two ears. In this study, we quantitatively evaluated hearing function in a population of subjects with Parkinson's disease, to investigate the occurrence of hearing loss, and, particularly, whether hearing dysfunction shows lateralization correlated with motor symptoms. Pure tone audiometry and distortion product otoacoustic emissions were used as outcome variables in 80 patients (mean age 65 ± 9 years) and 41 controls (mean age 64 ± 10 years). An advanced customized acquisition and analysis system was developed and used for otoacoustic testing, which guarantees response stability independent of probe insertion depth, and has the sensitivity necessary to accurately assess the low levels of otoacoustic response typical of elderly subjects. To our knowledge, this is the first study introducing the distinction between ipsilateral and contralateral ear, with respect to the body side more affected by Parkinson's disease motor symptoms. Significant asymmetry was found in the auditory function, as both otoacoustic responses and audiometric hearing levels were worse in the ipsilateral ear. Significantly worse hearing function was also observed in patients with Parkinson's disease compared to controls, confirming previous studies. Several pathophysiological mechanisms may be hypothesized to explain asymmetric cochlear damage in Parkinson's disease, including the impairment of dopamine release and the involvement of extra-dopaminergic circuits, with the cholinergic pathway as a likely candidate. The observed asymmetry in the audiological response of patients with Parkinson's disease suggests that lateralization of hearing dysfunction could represent a specific non-motor signature of the disease. The possible diagnostic use of cochlear dysfunction asymmetry as a specific biomarker of Parkinson's disease deserves further investigation, needing a more precise quantitative assessment, which would require a larger sample size.

The identification of dual protective agents against cisplatin-induced oto- and nephrotoxicity using the zebrafish model

Dose-limiting toxicities for cisplatin administration, including ototoxicity and nephrotoxicity, impact the clinical utility of this effective chemotherapy agent and lead to lifelong complications, particularly in pediatric cancer survivors. Using a two-pronged drug screen employing the zebrafish lateral line as an in vivo readout for ototoxicity and kidney cell-based nephrotoxicity assay, we screened 1280 compounds and identified 22 that were both oto- and nephroprotective. Of these, dopamine and L-mimosine, a plant-based amino acid active in the dopamine pathway, were further investigated. Dopamine and L-mimosine protected the hair cells in the zebrafish otic vesicle from cisplatin-induced damage and preserved zebrafish larval glomerular filtration. Importantly, these compounds did not abrogate the cytotoxic effects of cisplatin on human cancer cells. This study provides insights into the mechanisms underlying cisplatin-induced oto- and nephrotoxicity and compelling preclinical evidence for the potential utility of dopamine and L-mimosine in the safer administration of cisplatin.

Future Perspectives on the Relevance of Auditory Markers in Prodromal Parkinson's Disease

Research on auditory processing in Parkinson's disease (PD) has recently made substantial progress. At present, evidence has been found for altered auditory processing in the clinical stage of PD. The auditory alterations in PD have been demonstrated with low-cost and non-invasive assessments that are already used in routine clinical practice. Since auditory alterations have been reported early in disease progression, it would be highly relevant to investigate whether auditory markers could be provided in the prodromal stage of PD. In addition, auditory alterations in early stage PD might be modulated by dopaminergic medication. Therefore, the aim of this review is (1) to summarize the literature on auditory processing in PD with a specific focus on the early disease stages, (2) to give future perspectives on which audiological and electrophysiological measurements could be useful in the prodromal stage of PD and (3) to assess the effect of dopaminergic medication on potential auditory markers in the prodromal stage of PD.

Pre-exposure to Lower-Level Noise Mitigates Cochlear Synaptic Loss Induced by High-Level Noise

The auditory sensory organs appear to be less damaged by exposure to high-level noise that is presented after exposure to non-traumatizing low-level noise. This phenomenon is known as the toughening or conditioning effect. Functionally, it is manifested by a reduced threshold shift, and morphologically by a reduced hair cell loss. However, it remains unclear whether prior exposure to toughening noise can mitigate the synaptic loss induced by exposure to damaging noise. Since the cochlear afferent synapse between the inner hair cells and primary auditory neurons has been identified as a novel site involved in noise-induced cochlear damage, we were interested in assessing whether this synapse can be toughened. In the present study, the synaptic loss was induced by a damaging noise exposure (106 dB SPL) and compared across Guinea pigs who had and had not been previously exposed to a toughening noise (85 dB SPL). Results revealed that the toughening noise heavily reduced the synaptic loss observed 1 day after exposure to the damaging noise. Although it was significant, the protective effect of the toughening noise on permanent synaptic loss was much smaller. Compared with cases in the control group without noise exposure, coding deficits were seen in both toughened groups, as reflected in the compound action potential (CAP) by signals with amplitude modulation. In general, the pre-exposure to the toughening noise resulted in a significantly reduced synaptic loss by the high-level noise. However, this morphological protection was not accompanied by a robust functional benefit.

Auditory Dysfunction in Parkinson's Disease

PD is a progressive and complex neurological disorder with heterogeneous symptomatology. PD is characterized by classical motor features of parkinsonism and nonmotor symptoms and involves extensive regions of the nervous system, various neurotransmitters, and protein aggregates. Extensive evidence supports auditory dysfunction as an additional nonmotor feature of PD. Studies indicate a broad range of auditory impairments in PD, from the peripheral hearing system to the auditory brainstem and cortical areas. For instance, research demonstrates a higher occurrence of hearing loss in early-onset PD and evidence of abnormal auditory evoked potentials, event-related potentials, and habituation to novel stimuli. Electrophysiological data, such as auditory P3a, also is suggested as a sensitive measure of illness duration and severity. Improvement in auditory responses following dopaminergic therapies also indicates the presence of similar neurotransmitters (i.e., glutamate and dopamine) in the auditory system and basal ganglia. Nonetheless, hearing impairments in PD have received little attention in clinical practice so far. This review summarizes evidence of peripheral and central auditory impairments in PD and provides conclusions and directions for future empirical and clinical research. © 2020 International Parkinson and Movement Disorder Society.

A comparison of auditory and vestibular dysfunction in Parkinson's disease and Multiple System Atrophy

INTRODUCTION

Vertigo and disequilibrium are common symptoms in idiopathic Parkinson's disease (PD) and in Multiple System Atrophy (MSA). Hearing loss has been recently recognized as an additional non-motor feature in PD. The aim of this study is to evaluate audio-vestibular function in patients affected by PD and MSA.

METHODS

Fifteen patients with PD, 16 patients with MSA and 20 age-matched healthy controls (HC) were enrolled. Audio-vestibular examination included pure-tone audiometry (PTA), vestibular bed-side examination, video Head Impulse Test (vHIT), and cervical Vestibular-Evoked Myogenic Potentials (cVEMPs).

RESULTS

PD and MSA patients showed worse PTA thresholds compared to HC at high frequencies. MSA patients showed worse PTA thresholds at 125 Hz compared to HC. In patients with PD, a direct correlation between disease duration and PTA thresholds was found at 2000 Hz and 4000 Hz. In patients with MSA, disease duration was directly related to PTA thresholds at 125 Hz and 250 Hz. Among PD patients, cVEMPs were absent bilaterally in 46.7% and unilaterally in 13.3% of the subjects. Among MSA patients, cVEMPs were absent bilaterally in 26.7% and unilaterally in 40% of the subjects; p13 latency was significantly increased in PD patients as compared to HC. A significant inverse relationship was found between disease duration and cVEMP amplitude in MSA patients.

CONCLUSION

We found that high-frequency hearing loss and cVEMP abnormalities are frequent features of both MSA and PD, suggesting that an audio-vestibular dysfunction may be present in these patients even in the absence of self-reported auditory or vestibular symptoms.

Systematic Audiological Assessment of Auditory Functioning in Patients With Parkinson's Disease

Purpose Alterations in primary auditory functioning have been reported in patients with Parkinson's disease (PD). Despite the current findings, the pathophysiological mechanisms underlying these alterations remain unclear, and the effect of dopaminergic medication on auditory functioning in PD has been explored insufficiently. Therefore, this study aimed to systematically investigate primary auditory functioning in patients with PD by using both subjective and objective audiological measurements. Method In this case-control study, 25 patients with PD and 25 age-, gender-, and education-matched healthy controls underwent an audiological test battery consisting of tonal audiometry, short increment sensitivity index, otoacoustic emissions (OAEs), and speech audiometry. Patients with PD were tested in the on- and off-medication states. Results Increased OAE amplitudes were found when patients with PD were tested without dopaminergic medication. In addition, speech audiometry in silence and multitalker babble noise demonstrated higher phoneme scores for patients with PD in the off-medication condition. The results showed no differences in auditory functioning between patients with PD in the on-medication condition and healthy controls. No effect of disease stage or motor score was evident. Conclusions This study provides evidence for a top-down involvement in auditory processing in PD at both central and peripheral levels. Most important, the increase in OAE amplitude in the off-medication condition in PD is hypothesized to be linked to a dysfunction of the olivocochlear efferent system, which is known to have an inhibitory effect on outer hair cell functioning. Future studies may clarify whether OAEs may facilitate an early diagnosis of PD.

Use of the guinea pig in studies on the development and prevention of acquired sensorineural hearing loss, with an emphasis on noise

Guinea pigs have been used in diverse studies to better understand acquired hearing loss induced by noise and ototoxic drugs. The guinea pig has its best hearing at slightly higher frequencies relative to humans, but its hearing is more similar to humans than the rat or mouse. Like other rodents, it is more vulnerable to noise injury than the human or nonhuman primate models. There is a wealth of information on auditory function and vulnerability of the inner ear to diverse insults in the guinea pig. With respect to the assessment of potential otoprotective agents, guinea pigs are also docile animals that are relatively easy to dose via systemic injections or gavage. Of interest, the cochlea and the round window are easily accessible, notably for direct cochlear therapy, as in the chinchilla, making the guinea pig a most relevant and suitable model for hearing. This article reviews the use of the guinea pig in basic auditory research, provides detailed discussion of its use in studies on noise injury and other injuries leading to acquired sensorineural hearing loss, and lists some therapeutics assessed in these laboratory animal models to prevent acquired sensorineural hearing loss.

Association of Behavior With Noise-Induced Hearing Loss Among Attendees of an Outdoor Music Festival: A Secondary Analysis of a Randomized Clinical Trial

Importance

To date, factors associated with noise-induced hearing loss at music festivals have not yet been analyzed in a single comprehensive data set. In addition, little is known about the hearing loss-associated behavior of music festival attendees.

Objectives

To assess which factors are associated with the occurrence of a temporary threshold shift (TTS) after music exposure and to investigate the behavior of music festival attendees.

Design, Setting, and Participants

This prospective post hoc analysis gathered data from a randomized, single-blind clinical trial conducted on September 5, 2015, at an outdoor music festival in Amsterdam, the Netherlands. Adult volunteers with normal hearing were recruited via social media from August 26 through September 3, 2015. Intention to use earplugs was an exclusion criterion. Of 86 volunteers assessed, 51 were included. This post hoc analysis was performed from October 3, 2016, through February 27, 2017.

Interventions

Music festival visit for 4.5 hours.

Main Outcomes and Measures

The primary outcome was a TTS on a standard audiogram for the frequencies 3.0- and 4.0-kHz. Multivariable linear regression was performed to determine which factors are associated with a TTS. A questionnaire on behavior, hearing, and tinnitus was distributed to the participants before and after the festival visit.

Results

A total of 51 participants were included (18 men [35%] and 33 women [65%]) with a mean (SD) age of 27 (6) years. Mean (SD) threshold change across 3.0 and 4.0 kHz was 5.4 (5.7) dB for the right ear and 4.0 (6.1) dB for the left ear. Earplug use (absolute difference in the left ear, -6.0 dB [95% CI, -8.7 to -3.2 dB]; in the right ear, -6.4 dB [95% CI, -8.8 to -4.1 dB]), quantity of alcohol use (absolute difference per unit in the left ear, 1.1 dB [95% CI, 0.5 to 1.7 dB]; in the right ear, 0.7 dB [95% CI, 0.1 to 1.4 dB]), drug use (absolute difference in the right ear, 6.0 dB [95% CI, 0.9 to 11.1 dB]), and male sex (absolute difference in the right ear, 4.1 dB [95% CI, 0.3 to 5.9 dB]) were independently associated with hearing loss, with earplug use being the most important factor. Unprotected participants reported significantly worse subjective hearing performance and tinnitus after the festival visit than did participants using earplugs (Cramer V, 0.62 [95% CI, 0.47-0.79] and 0.39 [95% CI, 0.16-0.62], respectively). In the earplug group, the perceived loudness (r = -0.72; 95% CI, -1.00 to -0.43) and appreciation (r = 0.53; 95% CI, 0.29 to 0.78) of music and speech perception (r = 0.21; 95% CI, 0.09 to 0.35) were correlated with the duration of earplug use.

Conclusions and Relevance

The present study identified nonuse of earplugs, use of alcohol and drugs, and male sex as associated with a TTS at an outdoor music festival. Physicians should consider these factors to raise awareness about the combined risk of attending music festivals without using earplugs while consuming alcohol and/or drugs. The intention to use earplugs was correlated with the loudness and appreciation of music with earplugs, which may advocate for the use of personalized earplugs.

Trial Registration

trialregister.nl Identifier: NTR5401.

Remission of Recurrent Cochlear Hydrops Associated With Bromocriptine Treatment for Macroprolactinoma

Objective Current recommendations for cochlear hydrops treatment include systemic glucocorticoids and diuretics. Cochlear cells express dopamine receptors, although their role is unknown in the pathophysiology of cochlear hydrops. Case Description We report the case of remission of recurrent right-sided cochlear hydrops in a young male patient treated with bromocriptine due to pituitary macroprolactinoma. Transient improvement was observed after oral steroid and diuretic treatment, but cochlear hydrops recurred until the dose of bromocriptine was increased to 10 mg daily. Conclusion Bromocriptine may stimulate dopamine receptors in cochlear cells with potential therapeutic role in patients with cochlear hydrops. There are no widely accepted and effective treatments for endolymphatic hydrops, and identifying potential new and efficacious therapeutics is of high relevance.

Targeted single-cell electroporation loading of Ca2+ indicators in the mature hemicochlea preparation

Ca²⁺ is an important intracellular messenger and regulator in both physiological and pathophysiological mechanisms in the hearing organ. Investigation of cellular Ca²⁺ homeostasis in the mature cochlea is hampered by the special anatomy and high vulnerability of the organ. A quick, straightforward and reliable Ca²⁺ imaging method with high spatial and temporal resolution in the mature organ of Corti is missing. Cell cultures or isolated cells do not preserve the special microenvironment and intercellular communication, while cochlear explants are excised from only a restricted portion of the organ of Corti and usually from neonatal pre-hearing murines. The hemicochlea, prepared from hearing mice allows tonotopic experimental approach on the radial perspective in the basal, middle and apical turns of the organ. We used the preparation recently for functional imaging in supporting cells of the organ of Corti after bulk loading of the Ca²⁺ indicator. However, bulk loading takes long time, is variable and non-selective, and causes the accumulation of the indicator in the extracellular space. In this study we show the improved labeling of supporting cells of the organ of Corti by targeted single-cell electroporation in mature mouse hemicochlea. Single-cell electroporation proved to be a reliable way of reducing the duration and variability of loading and allowed subcellular Ca²⁺ imaging by increasing the signal-to-noise ratio, while cell viability was retained during the experiments. We demonstrated the applicability of the method by measuring the effect of purinergic, TRPA1, TRPV1 and ACh receptor stimulation on intracellular Ca²⁺ concentration at the cellular and subcellular level. In agreement with previous results, ATP evoked reversible and repeatable Ca²⁺ transients in Deiters', Hensen's and Claudius' cells. TRPA1 and TRPV1 stimulation by AITC and capsaicin, respectively, failed to induce any Ca²⁺ response in the supporting cells, except in a single Hensen's cell in which AITC evoked transients with smaller amplitude. AITC also caused the displacement of the tissue. Carbachol, agonist of ACh receptors induced Ca²⁺ transients in about a third of Deiters' and fifth of Hensen's cells. Here we have presented a fast and cell-specific indicator loading method allowing subcellular functional Ca²⁺ imaging in supporting cells of the organ of Corti in the mature hemicochlea preparation, thus providing a straightforward tool for deciphering the poorly understood regulation of Ca²⁺ homeostasis in these cells.

Developmental signature, synaptic connectivity and neurotransmission are conserved between vertebrate hair cells and tunicate coronal cells

In tunicates, the coronal organ represents a sentinel checking particle entrance into the pharynx. The organ differentiates from an anterior embryonic area considered a proto-placode. For their embryonic origin, morphological features and function, coronal sensory cells have been hypothesized to be homologues to vertebrate hair cells. However, vertebrate hair cells derive from a posterior placode. This contradicts one of the principle historical criteria for homology, similarity of position, which could be taken as evidence against coronal cells/hair cells homology. In the tunicates Ciona intestinalis and C. robusta, we found that the coronal organ expresses genes (Atoh, Notch, Delta-like, Hairy-b, and Musashi) characterizing vertebrate neural and hair cell development. Moreover, coronal cells exhibit a complex synaptic connectivity pattern, and express neurotransmitters (Glu, ACh, GABA, 5-HT, and catecholamines), or enzymes for their synthetic machinery, involved in hair cell activity. Lastly, coronal cells express the Trpa gene, which encodes an ion channel expressed in hair cells. These data lead us to hypothesize a model in which competence to make secondary mechanoreceptors was initially broadly distributed through placode territories, but has become confined to different placodes during the evolution of the vertebrate and tunicate lineages.

Connectivity and ultrastructure of dopaminergic innervation of the inner ear and auditory efferent system of a vocal fish

Dopamine (DA) is a conserved modulator of vertebrate neural circuitry, yet our knowledge of its role in peripheral auditory processing is limited to mammals. The present study combines immunohistochemistry, neural tract tracing, and electron microscopy to investigate the origin and synaptic characteristics of DA fibers innervating the inner ear and the hindbrain auditory efferent nucleus in the plainfin midshipman, a vocal fish that relies upon the detection of mate calls for reproductive success. We identify a DA cell group in the diencephalon as a common source for innervation of both the hindbrain auditory efferent nucleus and saccule, the main hearing endorgan of the inner ear. We show that DA terminals in the saccule contain vesicles but transmitter release appears paracrine in nature, due to the apparent lack of synaptic contacts. In contrast, in the hindbrain, DA terminals form traditional synaptic contacts with auditory efferent neuronal cell bodies and dendrites, as well as unlabeled axon terminals, which, in turn, form inhibitory-like synapses on auditory efferent somata. Our results suggest a distinct functional role for brain-derived DA in the direct and indirect modulation of the peripheral auditory system of a vocal nonmammalian vertebrate.

Ablation of the auditory cortex results in changes in the expression of neurotransmission-related mRNAs in the cochlea

The auditory cortex (AC) dynamically regulates responses of the Organ of Corti to sound through descending connections to both the medial (MOC) and lateral (LOC) olivocochlear efferent systems. We have recently provided evidence that AC has a reinforcement role in the responses to sound of the auditory brainstem nuclei. In a molecular level, we have shown that descending inputs from AC are needed to regulate the expression of molecules involved in outer hair cell (OHC) electromotility control, such as prestin and the α10 nicotinic acetylcholine receptor (nAchR). In this report, we show that descending connections from AC to olivocochlear neurons are necessary to regulate the expression of molecules involved in cochlear afferent signaling. RT-qPCR was performed in rats at 1, 7 and 15 days after unilateral ablation of the AC, and analyzed the time course changes in gene transcripts involved in neurotransmission at the first auditory synapse. This included the glutamate metabolism enzyme glutamate decarboxylase 1 (glud1) and AMPA glutamate receptor subunits GluA2-4. In addition, gene transcripts involved in efferent regulation of type I spiral ganglion neuron (SGN) excitability mediated by LOC, such as the α7 nAchR, the D2 dopamine receptor, and the α1, and γ2 GABAA receptor subunits, were also investigated. Unilateral AC ablation induced up-regulation of GluA3 receptor subunit transcripts, whereas both GluA2 and GluA4 mRNA receptors were down-regulated already at 1 day after the ablation. Unilateral removal of the AC also resulted in up-regulation of the transcripts for α7 nAchR subunit, D2 dopamine receptor, and α1 GABAA receptor subunit at 1 day after the ablation. Fifteen days after the injury, AC ablations induced an up-regulation of glud1 transcripts.

Noise Stress Induces an Epidermal Growth Factor Receptor/Xeroderma Pigmentosum-A Response in the Auditory Nerve

In response to toxic stressors, cancer cells defend themselves by mobilizing one or more epidermal growth factor receptor (EGFR) cascades that employ xeroderma pigmentosum-A (XPA) to repair damaged genes. Recent experiments discovered that neurons within the auditory nerve exhibit basal levels of EGFR+XPA co-expression. This finding implied that auditory neurons in particular or neurons in general have the capacity to mobilize an EGFR+XPA defense. Therefore, the current study tested the hypothesis that noise stress would alter the expression pattern of EGFR/XPA within the auditory nerve. Design-based stereology was used to quantify the proportion of neurons that expressed EGFR, XPA, and EGFR+XPA with and without noise stress. The results revealed an intricate neuronal response that is suggestive of alterations to both co-expression and individual expression of EGFR and XPA. In both the apical and middle cochlear coils, the noise stress depleted EGFR+XPA expression. Furthermore, there was a reduction in the proportion of neurons that expressed XPA-alone in the middle coils. However, the noise stress caused a significant increase in the proportion of neurons that expressed EGFR-alone in the middle coils. The basal cochlear coils failed to mobilize a significant response to the noise stress. These results suggest that EGFR and XPA might be part of the molecular defense repertoire of the auditory nerve.

Stochastic Resonance Controlled Upregulation of Internal Noise after Hearing Loss as a Putative Cause of Tinnitus-Related Neuronal Hyperactivity

Subjective tinnitus is generally assumed to be a consequence of hearing loss. In animal studies it has been demonstrated that acoustic trauma induced cochlear damage can lead to behavioral signs of tinnitus. In addition it was shown that noise trauma may lead to deafferentation of cochlear inner hair cells (IHC) even in the absence of elevated hearing thresholds, and it seems conceivable that such hidden hearing loss may be sufficient to cause tinnitus. Numerous studies have indicated that tinnitus is correlated with pathologically increased spontaneous firing rates and hyperactivity of neurons along the auditory pathway. It has been proposed that this hyperactivity is the consequence of a mechanism aiming to compensate for reduced input to the auditory system by increasing central neuronal gain, a mechanism referred to as homeostatic plasticity (HP), thereby maintaining mean firing rates over longer timescales for stabilization of neuronal processing. Here we propose an alternative, new interpretation of tinnitus-related development of neuronal hyperactivity in terms of information theory. In particular, we suggest that stochastic resonance (SR) plays a key role in both short- and long-term plasticity within the auditory system and that SR is the primary cause of neuronal hyperactivity and tinnitus. We argue that following hearing loss, SR serves to lift signals above the increased neuronal thresholds, thereby partly compensating for the hearing loss. In our model, the increased amount of internal noise-which is crucial for SR to work-corresponds to neuronal hyperactivity which subsequently causes neuronal plasticity along the auditory pathway and finally may lead to the development of a phantom percept, i.e., subjective tinnitus. We demonstrate the plausibility of our hypothesis using a computational model and provide exemplary findings in human patients that are consistent with that model. Finally we discuss the observed asymmetry in human tinnitus pitch distribution as a consequence of asymmetry of the distribution of auditory nerve type I fibers along the cochlea in the context of our model.

Dopamine Modulates the Activity of Sensory Hair Cells

The senses of hearing and balance are subject to modulation by efferent signaling, including the release of dopamine (DA). How DA influences the activity of the auditory and vestibular systems and its site of action are not well understood. Here we show that dopaminergic efferent fibers innervate the acousticolateralis epithelium of the zebrafish during development but do not directly form synapses with hair cells. However, a member of the D1-like receptor family, D1b, tightly localizes to ribbon synapses in inner ear and lateral-line hair cells. To assess modulation of hair-cell activity, we reversibly activated or inhibited D1-like receptors (D1Rs) in lateral-line hair cells. In extracellular recordings from hair cells, we observed that D1R agonist SKF-38393 increased microphonic potentials, whereas D1R antagonist SCH-23390 decreased microphonic potentials. Using ratiometric calcium imaging, we found that increased D1R activity resulted in larger calcium transients in hair cells. The increase of intracellular calcium requires Cav1.3a channels, as a Cav1 calcium channel antagonist, isradipine, blocked the increase in calcium transients elicited by the agonist SKF-38393. Collectively, our results suggest that DA is released in a paracrine fashion and acts at ribbon synapses, likely enhancing the activity of presynaptic Cav1.3a channels and thereby increasing neurotransmission.

SIGNIFICANCE STATEMENT

The neurotransmitter dopamine acts in a paracrine fashion (diffusion over a short distance) in several tissues and bodily organs, influencing and regulating their activity. The cellular target and mechanism of the action of dopamine in mechanosensory organs, such as the inner ear and lateral-line organ, is not clearly understood. Here we demonstrate that dopamine receptors are present in sensory hair cells at synaptic sites that are required for signaling to the brain. When nearby neurons release dopamine, activation of the dopamine receptors increases the activity of these mechanosensitive cells. The mechanism of dopamine activation requires voltage-gated calcium channels that are also present at hair-cell synapses.

Neuroanatomical Evidence for Catecholamines as Modulators of Audition and Acoustic Behavior in a Vocal Teleost

The plainfin midshipman fish (Porichthys notatus) is a well-studied model to understand the neural and endocrine mechanisms underlying vocal-acoustic communication across vertebrates. It is well established that steroid hormones such as estrogen drive seasonal peripheral auditory plasticity in female Porichthys in order to better encode the male's advertisement call. However, little is known of the neural substrates that underlie the motivation and coordinated behavioral response to auditory social signals. Catecholamines, which include dopamine and noradrenaline, are good candidates for this function, as they are thought to modulate the salience of and reinforce appropriate behavior to socially relevant stimuli. This chapter summarizes our recent studies which aimed to characterize catecholamine innervation in the central and peripheral auditory system of Porichthys as well as test the hypotheses that innervation of the auditory system is seasonally plastic and catecholaminergic neurons are activated in response to conspecific vocalizations. Of particular significance is the discovery of direct dopaminergic innervation of the saccule, the main hearing end organ, by neurons in the diencephalon, which also robustly innervate the cholinergic auditory efferent nucleus in the hindbrain. Seasonal changes in dopamine innervation in both these areas appear dependent on reproductive state in females and may ultimately function to modulate the sensitivity of the peripheral auditory system as an adaptation to the seasonally changing soundscape. Diencephalic dopaminergic neurons are indeed active in response to exposure to midshipman vocalizations and are in a perfect position to integrate the detection and appropriate motor response to conspecific acoustic signals for successful reproduction.

Vulnerability to acoustic trauma in the normal hearing ear with contralateral hearing loss

OBJECTIVES

We undertook an animal study to investigate the functional and histological changes that occur in the normal hearing ear of following acoustic trauma.

METHODS

As an animal model of unilateral hearing loss, the right ears of CBA mice were deafened by cochlear destruction at 6 weeks of age (SSD group). The control groups included mice that underwent a sham surgery, and mice that were exposed to noise binaurally and monaurally (by plugging the right ear completely). At 10 weeks of age, all mice were exposed to acoustic trauma (110 dB sound pressure level for 1 hour) that induced a transient threshold shift (TTS). Changes in the hearing thresholds of the left ear were assessed over the next 4 weeks by measuring the auditory brainstem responses (ABRs) and distortion product otoacoustic emissions (DPOAEs).

RESULTS

Following the noise exposure, the SSD group showed a permanent threshold shift (PTS) of about 10 dB, whereas the other groups showed full recovery from the TTS. The threshold of the DPOAEs of the left ears were increased after noise exposure but returned to normal in all groups, with no significant differences among the groups. Histological evaluation showed no apparent cellular loss or apoptosis in the left ears of all groups, including the SSD group.

CONCLUSIONS

These results suggest that normal hearing ears are more vulnerable to acoustic trauma following contralateral unilateral cochlear ablation. This increased vulnerability may be due to damaged neural structures.

Catecholaminergic connectivity to the inner ear, central auditory, and vocal motor circuitry in the plainfin midshipman fish porichthys notatus

Although the neuroanatomical distribution of catecholaminergic (CA) neurons has been well documented across all vertebrate classes, few studies have examined CA connectivity to physiologically and anatomically identified neural circuitry that controls behavior. The goal of this study was to characterize CA distribution in the brain and inner ear of the plainfin midshipman fish (Porichthys notatus) with particular emphasis on their relationship with anatomically labeled circuitry that both produces and encodes social acoustic signals in this species. Neurobiotin labeling of the main auditory end organ, the saccule, combined with tyrosine hydroxylase immunofluorescence (TH-ir) revealed a strong CA innervation of both the peripheral and central auditory system. Diencephalic TH-ir neurons in the periventricular posterior tuberculum, known to be dopaminergic, send ascending projections to the ventral telencephalon and prominent descending projections to vocal-acoustic integration sites, notably the hindbrain octavolateralis efferent nucleus, as well as onto the base of hair cells in the saccule via nerve VIII. Neurobiotin backfills of the vocal nerve in combination with TH-ir revealed CA terminals on all components of the vocal pattern generator, which appears to largely originate from local TH-ir neurons but may include input from diencephalic projections as well. This study provides strong neuroanatomical evidence that catecholamines are important modulators of both auditory and vocal circuitry and acoustic-driven social behavior in midshipman fish. This demonstration of TH-ir terminals in the main end organ of hearing in a nonmammalian vertebrate suggests a conserved and important anatomical and functional role for dopamine in normal audition.

Disruption of lateral olivocochlear neurons with a dopaminergic neurotoxin depresses spontaneous auditory nerve activity

Neurons of the lateral olivocochlear (LOC) system project from the auditory brainstem to the cochlea, where they synapse on radial dendrites of auditory nerve fibers. Selective LOC disruption depresses sound-evoked auditory nerve activity in the guinea pig, but enhances it in the mouse. Here, LOC disruption depressed spontaneous auditory nerve activity in the guinea pig. Recordings from single auditory nerve fibers revealed a significantly reduced proportion of fibers with the highest spontaneous firing rates (SRs) and an increased proportion of neurons with lower SRs. Ensemble activity, estimated using round window noise, also decreased after LOC disruption. Decreased spontaneous activity after LOC disruption may be a consequence of reduced tonic release of excitatory transmitters from the LOC terminals in guinea pigs.

Regulation of dopamine D2 receptors in the guinea pig cochlea

CONCLUSION

Our study demonstrates that the regulation of D2 receptors may be frequency specific. The reduction in cochlear microphonics (CM) and distortion product otoacoustic emission (DPOAE) amplitudes after perfusion with a D2 antagonist suggests that this receptor plays a role in the regulation of cochlear hair cell activation.

OBJECTIVES

Dopaminergic terminals are subject to negative feedback from dopamine D2 receptors. In the present study we investigated whether the regulation of dopamine D2 receptor is frequency specific and evaluated changes in CM in guinea pig cochlea.

METHODS

A total of 30 male guinea pigs were randomly assigned to 3 groups and perfused with artificial perilymph (AP), AP containing ethanol (0.1%), or a D2 antagonist (L741626) for 2 h. In each group, compound action potentials (CAPs) evoked by a 1, 2, 4, 8, 16 or 24 kHz tone pip, CM evoked by 4 kHz tone bursts, and DPOAEs were measured before and 2 h after perilymphatic perfusion.

RESULTS

Perfusion with the D2 antagonist resulted in increased CAP thresholds compared with the other two groups at high frequencies (4, 8, 16, 24 kHz, p < 0.05); however, no significant increase was observed at low frequencies (1, 2 kHz, p > 0.05). There was a significant reduction in DPOAEs and CM amplitudes after the 2 h perfusion with the D2 antagonist. A CM input/output (I/O) function curve plotted with the stimulating level as input and the CM relative amplitude as output indicated obvious nonlinearity after the 2 h perfusion in all three groups.

Physiological characterization of vestibular efferent brainstem neurons using a transgenic mouse model

The functional role of efferent innervation of the vestibular end-organs in the inner ear remains elusive. This study provides the first physiological characterization of the cholinergic vestibular efferent (VE) neurons in the brainstem by utilizing a transgenic mouse model, expressing eGFP under a choline-acetyltransferase (ChAT)-locus spanning promoter in combination with targeted patch clamp recordings. The intrinsic electrical properties of the eGFP-positive VE neurons were compared to the properties of the lateral olivocochlear (LOC) brainstem neurons, which gives rise to efferent innervation of the cochlea. Both VE and the LOC neurons were marked by their negative resting membrane potential <-75 mV and their passive responses in the hyperpolarizing range. In contrast, the response properties of VE and LOC neurons differed significantly in the depolarizing range. When injected with positive currents, VE neurons fired action potentials faithfully to the onset of depolarization followed by sparse firing with long inter-spike intervals. This response gave rise to a low response gain. The LOC neurons, conversely, responded with a characteristic delayed tonic firing upon depolarizing stimuli, giving rise to higher response gain than the VE neurons. Depolarization triggered large TEA insensitive outward currents with fast inactivation kinetics, indicating A-type potassium currents, in both the inner ear-projecting neuronal types. Immunohistochemistry confirmed expression of Kv4.3 and 4.2 ion channel subunits in both the VE and LOC neurons. The difference in spiking responses to depolarization is related to a two-fold impact of these transient outward currents on somatic integration in the LOC neurons compared to in VE neurons. It is speculated that the physiological properties of the VE neurons might be compatible with a wide-spread control over motion and gravity sensation in the inner ear, providing likewise feed-back amplification of abrupt and strong phasic signals from the semi-circular canals and of tonic signals from the gravito-sensitive macular organs.

Protective effect of rasagiline in aminoglycoside ototoxicity

Sensorineural hearing losses (SNHLs; e.g., ototoxicant- and noise-induced hearing loss or presbycusis) are among the most frequent sensory deficits, but they lack effective drug therapies. The majority of recent therapeutic approaches focused on the trials of antioxidants and reactive oxygen species (ROS) scavengers in SNHLs. The rationale for these studies was the prominent role of disturbed redox homeostasis and the consequent ROS elevation. Although the antioxidant therapies in several animal studies seemed to be promising, clinical trials have failed to fulfill expectations. We investigated the potential of rasagiline, an FDA-approved monomanine oxidase type B inhibitor (MAO-B) inhibitor type anti-parkinsonian drug, as an otoprotectant. We showed a dose-dependent alleviation of the kanamycin-induced threshold shifts measured by auditory brainstem response (ABR) in an ototoxicant aminoglycoside antibiotic-based hearing loss model in mice. This effect proved to be statistically significant at a 6-mg/kg (s.c.) dose. The most prominent effect appeared at 16kHz, which is the hearing sensitivity optimum for mice. The neuroprotective, antiapoptotic and antioxidant effects of rasagiline in animal models, all targeting a specific mechanism of aminoglycoside injury, may explain this otoprotection. The dopaminergic neurotransmission enhancer effect of rasagiline might also contribute to the protection. Dopamine (DA), released from lateral olivocochlear (LOC) fibers, was shown to exert a protective action against excitotoxicity, a pathological factor in the aminoglycoside-induced SNHL. We have shown that rasagiline enhanced the electric stimulation-evoked release of DA from an acute mouse cochlea preparation in a dose-dependent manner. Using inhibitors of voltage-gated Na(+)-, Ca(2+) channels and DA transporters, we revealed that rasagiline potentiated the action potential-evoked release of DA by inhibiting the reuptake. The complex, multifactorial pathomechanism of SNHLs most likely requires drugs acting on multiple targets for effective therapy. Rasagiline, with its multi-target action and favorable adverse effects profile, might be a good candidate for a clinical trial testing the otoprotective indication.

'Ecstasy' enhances noise-induced hearing loss

'Ecstasy' or 3,4-methylenedioxy-N-methamphetamine (MDMA) is an amphetamine abused for its euphoric, empathogenic, hallucinatory, and stimulant effects. It is also used to treat certain psychiatric disorders. Common settings for Ecstasy use are nightclubs and "rave" parties where participants consume MDMA and dance to loud music. One concern with the club setting is that exposure to loud sounds can cause permanent sensorineural hearing loss. Another concern is that consumption of MDMA may enhance such hearing loss. Whereas this latter possibility has not been investigated, this study tested the hypothesis that MDMA enhances noise-induced hearing loss (NIHL) by exposing rats to either MDMA, noise trauma, both MDMA and noise, or neither treatment. MDMA was given in a binge pattern of 5 mg/kg per intraperitoneal injections every 2 h for a total of four injections to animals in the two MDMA-treated groups (MDMA-only and Noise + MDMA). Saline injections were given to the animals in the two non-MDMA groups (Control and Noise-only). Following the final injection, noise trauma was induced by a 10 kHz tone at 120 dB SPL for 1 h to animals in the two noise trauma-treated groups (Noise-only and Noise + MDMA). Hearing loss was assessed by the auditory brainstem response (ABR) and cochlear histology. Results showed that MDMA enhanced NIHL compared to Noise-only and that MDMA alone caused no hearing loss. This implies that "clubbers" and "rave-goers" are exacerbating the amount of NIHL when they consume MDMA and listen to loud sounds. In contrast to earlier reports, the present study found that MDMA by itself caused no changes in the click-evoked ABR's wave latencies or amplitudes.

In vivo electrochemical monitoring of the change of cochlear perilymph ascorbate during salicylate-induced tinnitus

As one of the most important neurochemicals in biological systems, ascorbate plays vital roles in many physiological and pathological processes. In order to understand the roles of ascorbate in the pathological process of tinnitus, this study demonstrates an in vivo method for real time monitoring of the changes of ascorbate level in the cochlear perilymph of guinea pigs during the acute period of tinnitus induced by local microinfusion of salicylate with carbon fiber microelectrodes (CFMEs) modified with multiwalled carbon nanotubes (MWNTs). To accomplish in vivo electrochemical monitoring of ascorbate in the microenvironment of the cochlear perilymph, the MWNT-modified CFME is used as working electrode, a microsized Ag/AgCl is used as reference electrode, and Pt wire is used as counter electrode. Three electrodes are combined together around a capillary to form integrated capillary-electrodes. The integrated capillary-electrode is carefully implanted into the cochlear perilymph of guinea pigs and used both for externally microinfusing of salicylate into the cochlear perilymph and for real time monitoring of the change of ascorbate levels. The in vivo voltammetric method based on the integrated capillary-electrodes possesses a high selectivity and a good linearity for ascorbate determination in the cochlear perilymph of guinea pigs. With such a method, the basal level of cochlear perilymph ascorbate is determined to be 45.0 ± 5.1 μM (n = 6). The microinfusion of 10 mM salicylate (1 μL/min, 5 min) into the cochlear decreases the ascorbate level to 28 ± 10% of the basal level (n = 6) with a statistical significance (P < 0.05), implying that the decrease in ascorbate level in the cochlear may be associated with salicylate-induced tinnitus. This study essentially offers a new method for in vivo monitoring of the cochlear perilymph ascorbate following the salicylate-induced tinnitus and can thus be useful for investigation on chemical essences involved in tinnitus.

Dopaminergic signaling in the cochlea: receptor expression patterns and deletion phenotypes

Pharmacological studies suggest that dopamine release from lateral olivocochlear efferent neurons suppresses spontaneous and sound-evoked activity in cochlear nerve fibers and helps control noise-induced excitotoxicity; however, the literature on cochlear expression and localization of dopamine receptors is contradictory. To better characterize cochlear dopaminergic signaling, we studied receptor localization using immunohistochemistry or reverse transcriptase PCR and assessed histopathology, cochlear responses and olivocochlear function in mice with targeted deletion of each of the five receptor subtypes. In normal ears, D1, D2, and D5 receptors were detected in microdissected immature (postnatal days 10-13) spiral ganglion cells and outer hair cells but not inner hair cells. D4 was detected in spiral ganglion cells only. In whole cochlea samples from adults, transcripts for D1, D2, D4, and D5 were present, whereas D3 mRNA was never detected. D1 and D2 immunolabeling was localized to cochlear nerve fibers, near the first nodes of Ranvier (D2) and in the inner spiral bundle region (D1 and D2) where presynaptic olivocochlear terminals are found. No other receptor labeling was consistent. Cochlear function was normal in D3, D4, and D5 knock-outs. D1 and D2 knock-outs showed slight, but significant enhancement and suppression, respectively, of cochlear responses, both in the neural output [auditory brainstem response (ABR) wave 1] and in outer hair cell function [distortion product otoacoustic emissions (DPOAEs)]. Vulnerability to acoustic injury was significantly increased in D2, D4 and D5 lines: D1 could not be tested, and no differences were seen in D3 mutants, consistent with a lack of receptor expression. The increased vulnerability in D2 knock-outs was seen in DPOAEs, suggesting a role for dopamine in the outer hair cell area. In D4 and D5 knock-outs, the increased noise vulnerability was seen only in ABRs, consistent with a role for dopaminergic signaling in minimizing neural damage.