Transient peripheral vestibular hypofunction measured with vestibular short-latency evoked potentials following noise exposure in rats.

Exposure to 120 dB sound pressure level (SPL) band-limited noise results in delayed onset latency and reduced vestibular short-latency evoked potential (VsEP) responses. These changes are still present 4 wk after noise overstimulation. Noise-induced hearing loss (NIHL) has been shown to vary in extent and duration based on the noise intensity. This study investigated whether noise-induced peripheral vestibular hypofunction (NPVH) would also decrease in extent and/or duration with less intense noise exposure. In the present study, rats were exposed to a less intense noise (110 dB SPL) but for the same duration (6 h) and frequency range (500-4,000 Hz) as used in previous studies. The VsEP was assessed 1, 3, 7, 14, 21, and 28 days after noise exposure. In contrast to 120 dB SPL noise exposure, the 110 dB SPL noise exposures produced smaller deficits in VsEP responses that fully recovered in 62% (13/21) of animals within 1 wk. These findings suggest that NPVH, a loss or attenuation of VsEP responses with a requirement for elevated stimulus intensity to elicit measurable responses, is similar to NIHL, that is, lower sound levels produce a smaller or transient deficit. These results show that it will be important to determine the extent and duration of vestibular hypofunction for different noise exposure conditions and their impact on balance.NEW & NOTEWORTHY This is the first study to show a temporary noise-induced peripheral vestibular hypofunction that recovers following exposure to continuous noise.

The rat as a model for studying noise injury and otoprotection.

A major challenge for those studying noise-induced injury pre-clinically is the selection of an animal model. Noise injury models are particularly relevant in an age when people are constantly bombarded by loud noise due to occupation and/or recreation. The rat has been widely used for noise-related morphological, physiological, biochemical, and molecular assessment. Noise exposure resulting in a temporary (TTS) or permanent threshold shift (PTS) yields trauma in peripheral and central auditory related pathways. While the precise nature of noise-related injuries continues to be delineated, both PTS and TTS (with or without hidden hearing loss) result in homeostatic changes implicated in conditions such as tinnitus and hyperacusis. Compared to mice, rats generally tolerate exposure to loud sounds reasonably well, often without exhibiting other physical non-inner ear related symptoms such as death, loss of consciousness, or seizures [Skradski, Clark, Jiang, White, Fu, and Ptacek (2001). Neuron 31, 537-544; Faingold (2002). Hear. Res. 168, 223-237; Firstova, Abaimov, Surina, Poletaeva, Fedotova, and Kovalev (2012). Bull Exp. Biol. Med. 154, 196-198; De Sarro, Russo, Citraro, and Meldrum (2017). Epilepsy Behav. 71, 165-173]. This ability of the rat to thrive following noise exposure permits study of long-term effects. Like the mouse, the rat also offers a well-characterized genome allowing genetic manipulations (i.e., knock-out, viral-based gene expression modulation, and optogenetics). Rat models of noise-related injury also provide valuable information for understanding mechanistic changes to identify therapeutic targets for treatment. This article provides a framework for selection of the rat as a model for noise injury studies.

Vestibular short-latency evoked potential abolished by low-frequency noise exposure in rats.

The vestibular system plays a critical role in detection of head movements and is essential for normal postural control. Because of their anatomical proximity to the cochlea, the otolith organs are selectively exposed to sound pressure and are at risk for noise overstimulation. Clinical reports suggest a link between noise exposure and balance problems, but the structural and physiological basis for this linkage is not well understood. The goal of this study was to determine the effects of low-frequency noise (LFN) on the otolith organs by correlating changes in vestibular short-latency evoked potentials (VsEPs) with changes in saccular afferent endings following noise exposure. LFN exposure transiently abolished the VsEP and reduced the number of stained calyces within the sacculus. Although some recovery of the VsEP waveform could be observed within 3 days after noise, at 3 wk recovery was only partial in most animals, consistent with a reduced number of afferents with calyceal endings. These data show that a single intense noise exposure is capable of causing a vestibular deficit that appears to mirror the synaptic deficit associated with hidden hearing loss after noise-induced cochlear injury. NEW & NOTEWORTHY This is the first study to explore the effects of low-frequency high-intensity noise on vestibular short-latency evoked potential (VsEP) responses, which shows a linkage between attenuated noise-induced VsEPs and pathological changes to otolith organ afferents. This finding suggests a potential limitation of the VsEP for evaluation of vestibular dysfunction, since the VsEP measurement may assess the activity of a specific class rather than all afferents.

Macrophage migration inhibitory factor acts as a neurotrophin in the developing inner ear.

This study is the first to demonstrate that macrophage migration inhibitory factor (MIF), an immune system 'inflammatory' cytokine that is released by the developing otocyst, plays a role in regulating early innervation of the mouse and chick inner ear. We demonstrate that MIF is a major bioactive component of the previously uncharacterized otocyst-derived factor, which directs initial neurite outgrowth from the statoacoustic ganglion (SAG) to the developing inner ear. Recombinant MIF acts as a neurotrophin in promoting both SAG directional neurite outgrowth and neuronal survival and is expressed in both the developing and mature inner ear of chick and mouse. A MIF receptor, CD74, is found on both embryonic SAG neurons and adult mouse spiral ganglion neurons. Mif knockout mice are hearing impaired and demonstrate altered innervation to the organ of Corti, as well as fewer sensory hair cells. Furthermore, mouse embryonic stem cells become neuron-like when exposed to picomolar levels of MIF, suggesting the general importance of this cytokine in neural development.

Wavelet transform of single-trial vestibular short-latency evoked potential reveals temporary reduction in signal detectability and temporal precision following noise exposure.

The vestibular short-latency evoked potential (VsEP) reflects the activity of irregular vestibular afferents and their target neurons in the brain stem. Attenuation of trial-averaged VsEP waveforms is widely accepted as an indicator of vestibular dysfunction, however, more quantitative analyses of VsEP waveforms could reveal underlying neural properties of VsEP waveforms. Here, we present a time-frequency analysis of the VsEP with a wavelet transform on a single-trial basis, which allows us to examine trial-by-trial variability in the strength of VsEP waves as well as their temporal coherence across trials. Using this method, we examined changes in the VsEP following 110 dB SPL noise exposure in rats. We found detectability of head jerks based on the power of wavelet transform coefficients was significantly reduced 1 day after noise exposure but recovered nearly to pre-exposure level in 3 - 7 days and completely by 28 days after exposure. Temporal coherence of VsEP waves across trials was also significantly reduced on 1 day after exposure but recovered with a similar time course. Additionally, we found a significant reduction in the number of calretinin-positive calyces in the sacculi collected 28 days after noise exposure. Furthermore, the number of calretinin-positive calyces was significantly correlated with the degree of reduction in temporal coherence and/or signal detectability of the smallest-amplitude jerks. This new analysis of the VsEP provides more quantitative descriptions of noise-induced changes as well as new insights into potential mechanisms underlying noise-induced vestibular dysfunction. Significance Statement: Our study presents a new method of VsEP quantification using wavelet transform on a single-trial basis. It also describes a novel approach to determine the stimulus threshold of the VsEP based on signal-detection theory and Rayleigh statistics. The present analysis could also be applied to analysis of auditory brain stem response (ABR). Thus, it has the potential to provide new insights into the physiological properties that underlie peripheral vestibular and auditory dysfunction.

Nestin-expressing cells in the developing, mature and noise-exposed cochlear epithelium.

The auditory sensory epithelium in non-mammalian vertebrates can replace lost hair cells by transdifferentiation of supporting cells, but this regenerative ability is lost in the mammalian cochlea. Future cell-based treatment of hearing loss may depend on stem cell transplantation or on transdifferentiation of endogenous cells in the cochlea. For both approaches, identification of cells with stem cell features within the mature cochlea may be useful. Here we use a Nestin-ß-gal mouse to examine the presence of Nestin positive cells in the mature auditory epithelium, and determine how overstimulation of the ear impacts these cells. Nestin positive cells were found in the apical turn of the cochlea lateral to the outer hair cell area. This pattern of expression persisted into mature age. The area of Nestin positive cells was increased after the noise lesion. This increase in area coincided with an increase in expression of the Nestin mRNA. The data suggest that cells with potential stem cell features remain in the mature mammalian cochlea, restricted to the apical turn, and that an additional set of signals is necessary to trigger their contribution to cell replacement therapy in the ear. As such, this population of cells could serve to generate cochlear stem cells for research and potential therapy, and may be a target for treatments based on induced transdifferentiation of endogenous cochlear cells.

Balance beam crossing times are slower after noise exposure in rats.

Introduction: The vestibular system integrates signals related to vision, head position, gravity, motion, and body position to provide stability during motion through the environment. Disruption in any of these systems can reduce agility and lead to changes in ability to safely navigate one's environment. Causes of vestibular decline are diverse; however, excessive noise exposure can lead to otolith organ dysfunction. Specifically, 120 decibel (dB) sound pressure level (SPL) 1.5 kHz-centered 3-octave band noise (1.5 kHz 3OBN) causes peripheral vestibular dysfunction in rats, measured by vestibular short-latency evoked potential (VsEP) and reduced calretinin-immunolabeling of calyx-only afferent terminals in the striolar region of the saccule. The present study examined the functional impact of this noise exposure condition, examining changes in motor performance after noise exposure with a balance beam crossing task. Methods: Balance beam crossing time in rats was assessed for 19 weeks before and 5 weeks after noise exposure. Balance beam crossings were scored to assess proficiency in the task. When animals were proficient, they received a single exposure to 120 dB SPL 3-octave band noise. Results: During the initial training phase slower crossing times and higher scores, including multiple failures were observed. This was followed by a period of significant improvement leading to proficiency, characterized by fast and stable crossing times and consistently low scores. After noise exposure, crossing times were significantly elevated from baseline for 4-weeks. A total of 5 weeks after noise exposure, crossing times improved, and though still trending higher than baseline, they were no longer significantly different from baseline. Discussion: These findings show that the noise-induced peripheral vestibular changes we previously observed at cellular and electro-physiological levels also have an impact at a functional level. It has been previously shown that imbalance is associated with slower walking speed in older adults and aged rats. These findings in noise-exposed rats may have implications for people who experience noisy environments and for seniors with a history of noise exposure who also experience balance disorders and may be at increased fall risk.

Noise overstimulation of young adult UMHET4 mice accelerates age-related hearing loss.

Many factors contribute to hearing loss commonly found in older adults. There can be natural aging of cellular elements, hearing loss previously induced by environmental factors such as noise or ototoxic drugs as well as genetic and epigenetic influences. Even when noise overstimulation does not immediately cause permanent hearing loss it has recently been shown to increase later age-related hearing loss (ARHL). The present study further investigated this condition in the UMHET4 mouse model by comparing a small arms fire (SAF)-like impulse noise exposure that has the greatest immediate effect in more apical cochlear regions to a broadband noise (BBN) exposure that has the greatest immediate effect in more basal cochlear regions. Both noise exposures were given at levels that only induced temporary auditory brainstem response (ABR) threshold shifts (TS). Mice were noise exposed at 5 months of age followed by ABR assessment at 6, 12, 18, 21, and 24 months of age. Mice that received the SAF-like impulse noise had accelerated age-related TS at 4 kHz that appeared at 12 months of age (significantly increased compared to no-noise controls). This increased TS at 4 kHz continued at 18 and 21 months but was no longer significantly greater at 24 months of age. The SAF-like impulse noise also induced a significantly greater mean TS at 48 kHz, first appearing at 18 months of age and continuing to be significantly greater than controls at 21 and 24 months. The BBN induced a different pace and pattern of enhanced age-related ABR TS. The mean TS for the BBN group first became significantly greater than controls at 18 months of age and only at 48 kHz. It remained significantly greater than controls at 21 months but was no longer significantly greater at 24 months of age. Results, therefore, show different influences on ARHL for the two different noise exposure conditions. Noise-induced enhancement appears to provide more an acceleration than overall total increase in ARHL.

Rapamycin Added to Diet in Late Mid-Life Delays Age-Related Hearing Loss in UMHET4 Mice.

Our previous study demonstrated rapamycin added to diet at 4 months of age had significantly less age-related outer hair cell loss in the basal half of the cochlea at 22 months of age compared to mice without rapamycin. The present study tested adding rapamycin to diet later in life, at 14 months of age, and added a longitudinal assessment of auditory brain stem response (ABR). The present study used UMHET4 mice, a 4 way cross in which all grandparental strains lack the Cdh23753A allele that predisposes to early onset, progressive hearing loss. UMHET4 mice typically have normal hearing until 16-17 months, then exhibit threshold shifts at low frequencies/apical cochlea and later in more basal high frequency regions. ABR thresholds at 4, 12, 24, and 48 kHz were assessed at 12, 18, and 24 months of age and compared to baseline ABR thresholds acquired at 5 months of age to determine threshold shifts (TS). There was no TS at 12 months of age at any frequency tested. At 18 months of age mice with rapamycin added to diet at 14 months had a significantly lower mean TS at 4 and 12 kHz compared to mice on control diet with no significant difference at 24 and 48 kHz. At 24 months of age, the mean 4 kHz TS in rapamycin diet group was no longer significantly lower than the control diet group, while the 12 kHz mean remained significantly lower. Mean TS at 24 and 48 kHz in the rapamycin diet group became significantly lower than in the control diet group at 24 months. Hair cell counts at 24 months showed large loss in the apical half of most rapamycin and control diet mice cochleae with no significant difference between groups. There was only mild outer hair cell loss in the basal half of rapamycin and control diet mice cochleae with no significant difference between groups. The results show that a later life addition of rapamycin can decrease age-related hearing loss in the mouse model, however, it also suggests that this decrease is a delay/deceleration rather than a complete prevention.

The intrinsic electrophysiological properties of neurons derived from mouse embryonic stem cells overexpressing neurogenin-1.

A mouse embryonic stem (ES) cell line containing an inducible transgene for the proneural gene Neurog1 has been used to generate glutamatergic neurons at a high efficiency. The present study used in vitro electrophysiology to establish the timeline for acquiring a functional neuronal phenotype in Neurog1-induced cells exhibiting a neuronal morphology. TTX-sensitive action potentials could be evoked from over 80% of the cells after only 4.5 days in vitro (DIV). These cells uniformly showed rapidly adapting responses to current injection, firing one to three action potentials at the onset of the stimulus. In the absence of Neurog1, a limited number of ES cells adopted a neuronal morphology, but these cells displayed slow calcium depolarizations rather than sodium-based spikes. Voltage-gated Na(+), K(+), and Ca(2+) currents were present in nearly all induced cells as early as 4.5 DIV. The voltage-dependent properties of these currents changed little from 4 to 12 DIV with half-activation voltage varying by <10 mV for any current type throughout the culture period. This study demonstrates that forced expression of proneural genes can induce ES cells to quickly acquire a functional neuronal phenotype with mature electrophysiological properties. Transient overexpression of Neurog1 may be used in neural repair strategies that require the rapid induction of functional neurons from pluripotent stem cells.

Exposure to Intense Noise Causes Vestibular Loss.

INTRODUCTION: The vestibular system is essential for normal postural control and balance. Because of their proximity to the cochlea, the otolith organs are vulnerable to noise. We previously showed that head jerks that evoke vestibular nerve activity were no longer capable of inducing a response after noise overstimulation. The present study adds a greater range of jerk intensities to determine if the response was abolished or required more intense stimulation (threshold shift). MATERIALS AND METHODS: Vestibular short-latency evoked potential (VsEP) measurements were taken before noise exposure and compared to repeated measurements taken at specific time points for 28 days after noise exposure. Calretinin was used to identify changes in calyx-only afferents in the sacculus. RESULTS: Results showed that more intense jerk stimuli could generate a VsEP, although it was severely attenuated relative to prenoise values. When the VsEP was evaluated 4 weeks after noise exposure, partial recovery was observed. CONCLUSION: These data suggest that noise overstimulation, such as can occur in the military, could introduce an increased risk of imbalance that should be evaluated before returning a subject to situations that require normal agility and motion. Moreover, although there is recovery with time, some dysfunction persists for extended periods.

Effects of Noise Exposure on the Vestibular System: A Systematic Review.

Despite our understanding of the impact of noise-induced damage to the auditory system, much less is known about the impact of noise exposure on the vestibular system. In this article, we review the anatomical, physiological, and functional evidence for noise-induced damage to peripheral and central vestibular structures. Morphological studies in several animal models have demonstrated cellular damage throughout the peripheral vestibular system and particularly in the otolith organs; however, there is a paucity of data on the effect of noise exposure on human vestibular end organs. Physiological studies have corroborated morphological studies by demonstrating disruption across vestibular pathways with otolith-mediated pathways impacted more than semicircular canal-mediated pathways. Similar to the temporary threshold shifts observed in the auditory system, physiological studies in animals have suggested a capacity for recovery following noise-induced vestibular damage. Human studies have demonstrated that diminished sacculo-collic responses are related to the severity of noise-induced hearing loss, and dose-dependent vestibular deficits following noise exposure have been corroborated in animal models. Further work is needed to better understand the physiological and functional consequences of noise-induced vestibular impairment in animals and humans.

Glutamatergic neuronal differentiation of mouse embryonic stem cells after transient expression of neurogenin 1 and treatment with BDNF and GDNF: in vitro and in vivo studies.

Differentiation of the pluripotent neuroepithelium into neurons and glia is accomplished by the interaction of growth factors and cell-type restricted transcription factors. One approach to obtaining a particular neuronal phenotype is by recapitulating the expression of these factors in embryonic stem (ES) cells. Toward the eventual goal of auditory nerve replacement, the aim of the current investigation was to generate auditory nerve-like glutamatergic neurons from ES cells. Transient expression of Neurog1 promoted widespread neuronal differentiation in vitro; when supplemented with brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF), 75% of ES cell-derived neurons attained a glutamatergic phenotype after 5 d in vitro. Mouse ES cells were also placed into deafened guinea pig cochleae and Neurog1 expression was induced for 48 h followed by 26 d of BDNF/GDNF infusion. In vivo differentiation resulted in 50-75% of ES cells bearing markers of early neurons, and a majority of these cells had a glutamatergic phenotype. This is the first study to report a high percentage of ES cell differentiation into a glutamatergic phenotype and sets the stage for cell replacement of auditory nerve.

Small Arms Fire-like noise: Effects on Hearing Loss, Gap Detection and the Influence of Preventive Treatment.

A noise-induced loss of inner hair cell (IHC) - auditory nerve synaptic connections has been suggested as a factor that can trigger the progression of maladaptive plastic changes leading to noise-induced tinnitus. The present study used a military relevant small arms fire (SAF)-like noise (50 biphasic impulses over 2.5 min at 152 dB SPL given unilaterally to the right ear) to induce loss (∼1/3) of IHC synaptic ribbons (associated with synapse loss) in rat cochleae with only minor (less than 10%) loss of outer hair cells. Approximately half of the noise-exposed rats showed poorer Gap Detection post-noise, a behavioral indication suggesting the presence of tinnitus. There was significantly greater loss of IHC ribbons in noise-exposed rats with reduced Gap Detection compared to noise-exposed rats retaining normal Gap Detection. We have previously shown systemic administration of piribedil, memantine, and/or ACEMg significantly reduced loss of IHC ribbons induced by a 3 h 4 kHz octave band 117 dB (SPL) noise. The present study examined if this treatment would also reduce ribbon loss from the SAF-like noise exposure and if this would prevent the reduced Gap Detection. As in the previous study, piribedil, memantine, and ACEMg treatment significantly reduced the noise-induced loss of ribbons, such that it was no longer significantly different from normal. However, it did not prevent development of the reduced Gap Detection indication of tinnitus in all treated noise-exposed rats, reducing the incidence but not reaching significance.

Stem cell transplantation for auditory nerve replacement.

The successful function of cochlear prostheses depends on activation of auditory nerve. The survival of auditory nerve neurons, however, can vary widely in candidates for cochlear implants and influence implant efficacy. Stem cells offer the potential for improving the function of cochlear prostheses and increasing the candidate pool by replacing lost auditory nerve. The first phase of studies for stem cell replacement of auditory nerve has examined the in vitro survival and differentiation as well as in vivo differentiation and survival of exogenous embryonic and tissue stem cells placed into scala tympani and/or modiolus. These studies are reviewed and new results on in vivo placement of B-5 mouse embryonic stem cells into scala tympani of the guinea pig cochleae with differentiation into a glutamatergic neuronal phenotype are presented. Research on the integration and connections of stem cell derived neurons in the cochlea is described. Finally, an alternative approach is considered, based on the use of endogenous progenitors rather than exogenous stem cells, with a review of promising findings that have identified stem cell-like progenitors in cochlear and vestibular tissues to provide the potential for auditory nerve replacement.

Age-related auditory pathology in the CBA/J mouse.

Commercially obtained aged male CBA/J mice presented a complex pattern of hearing loss and morphological changes. A significant threshold shift in auditory brainstem responses (ABR) occurred at 3 months of age at 4 kHz without apparent loss of hair cells, rising slowly at later ages accompanied by loss of apical hair cells. A delayed high-frequency deficit started at 24 kHz around the age of 12 months. At 20-26 months, threshold shifts at 12 and 24 kHz and the accompanying hair cell loss at the base of the cochlea were highly variable with some animals appearing almost normal and others showing large deficits. Spiral ganglion cells degenerated by 18 months in all regions of the cochlea, with cell density reduced by approximately 25%. There was no degeneration of the stria vascularis and the endocochlear potential remained stable from 3 to 25 months of age regardless of whether the animals had normal or highly elevated ABR thresholds. The slow high-frequency hearing loss combined with a modest reduction of ganglion cell density and an unchanged endocochlear potential suggest sensorineural presbycusis. The superimposed early hearing loss at low frequencies, which is not seen in animals bred in-house, may complicate the use of these animals as a presbycusis model.

Changes in glycine immunoreactivity in the rat superior olivary complex following deafness.

The balance between inhibitory and excitatory amino acid neurotransmitters contributes to the control of normal functioning of the auditory brainstem. Changes in the level of neuronal activity within the auditory brainstem pathways influence the balance between inhibition and excitation. Activity-dependent plasticity in the auditory pathways can be studied by creating a large decrease in activity through peripheral deafening. Deafness-related decreases in GABA have previously been shown in the inferior colliculus. However, glycine is a more prevalent inhibitory transmitter in the mature superior olivary complex (SOC). The present study therefore examined if there were deafness-related changes in glycine in the SOC using postembedding immunocytochemistry. Animals were bilaterally deafened by an intrascalar injection of neomycin. Five nuclei in the SOC, the lateral superior olive (LSO), superior paraolivary nucleus (SPoN), and the medial, lateral, and ventral nuclei of the trapezoid body (MNTB, LNTB, and VNTB) were examined 14 days following the deafening and compared to normal hearing age-matched controls. The LSO and SPoN were divided into high and low frequency regions. The number of glycine immunoreactive puncta on the somata of principal cells showed significant decreases in all regions assessed, with changes ranging from 50% in the VNTB to 23% in the LSO.

Deafness associated changes in expression of two-pore domain potassium channels in the rat cochlear nucleus.

Two-pore domain potassium channels (K(2PD)+) play an important role in setting resting membrane potential by regulating background leakage of potassium ions, which in turn controls neuronal excitability. To determine whether these channels contribute to activity-dependent plasticity following deafness, we used quantitative real-time PCR to examine the expression of 10 K(2PD)+ subunits in the rat cochlear nucleus at 3 days, 3 weeks and 3 months after bilateral cochlear ablation. There was a large sustained decrease in the expression of TASK-5, a subunit that is predominantly expressed in auditory brain stem neurons, and in the TASK-1 subunit which is highly expressed in several types of cochlear nucleus neurons. TWIK-1 and THIK-2 also showed significant decreases in expression that were maintained across all time points. TWIK-2, TREK-1 and TREK-2 showed no significant change in expression at 3 days but showed large decreases at 3 weeks and 3 months following deafness. TRAAK and TASK-3 subunits showed significant decreases at 3 days and 3 weeks following deafness, but these differences were no longer significant at 3 months. Dramatic changes in expression of K(2PD)+ subunits suggest these channels may play a role in deafness-associated changes in the excitability of cochlear nucleus neurons.

Treatment with Piribedil and Memantine Reduces Noise-Induced Loss of Inner Hair Cell Synaptic Ribbons.

Noise overstimulation can induce loss of synaptic ribbons associated with loss of Inner Hair Cell - Auditory Nerve synaptic connections. This study examined if systemic administration of Piribedil, a dopamine agonist that reduces the sound evoked auditory nerve compound action potential and/or Memantine, an NMDA receptor open channel blocker, would reduce noise-induced loss of Inner Hair Cell ribbons. Rats received systemic Memantine and/or Piribedil for 3 days before and 3 days after a 3 hour 4 kHz octave band noise at 117 dB (SPL). At 21 days following the noise there was a 26% and 38% loss of synaptic ribbons in regions 5.5 and 6.5 mm from apex, respectively, elevations in 4-, 8- and 20 kHz tonal ABR thresholds and reduced dynamic output at higher intensities of stimulation. Combined treatment with Piribedil and Memantine produced a significant reduction in the noise-induced loss of ribbons in both regions and changes in ABR sensitivity and dynamic responsiveness. Piribedil alone gave significant reduction in only the 5.5 mm region and Memantine alone did not reach significance in either region. Results identify treatments that could prevent the hearing loss and hearing disorders that result from noise-induced loss of Inner Hair Cell - Auditory Nerve synaptic connections.