Differential olivocochlear projections from lateral versus medial zones of the superior olivary complex

Surface electrical stimulation of the auditory cortex preserves efferent medial olivocochlear neurons and reduces cochlear traits of age-related hearing loss

The auditory cortex is the source of descending connections providing contextual feedback for auditory signal processing at almost all levels of the lemniscal auditory pathway. Such feedback is essential for cognitive processing. It is likely that corticofugal pathways are degraded with aging, becoming important players in age-related hearing loss and, by extension, in cognitive decline. We are testing the hypothesis that surface, epidural stimulation of the auditory cortex during aging may regulate the activity of corticofugal pathways, resulting in modulation of central and peripheral traits of auditory aging. Increased auditory thresholds during ongoing age-related hearing loss in the rat are attenuated after two weeks of epidural stimulation with direct current applied to the surface of the auditory cortex for two weeks in alternate days (Fernández del Campo et al., 2024). Here we report that the same cortical electrical stimulation protocol induces structural and cytochemical changes in the aging cochlea and auditory brainstem, which may underlie recovery of age-degraded auditory sensitivity. Specifically, we found that in 18 month-old rats after two weeks of cortical electrical stimulation there is, relative to age-matched non-stimulated rats: a) a larger number of choline acetyltransferase immunoreactive neuronal cell body profiles in the ventral nucleus of the trapezoid body, originating the medial olivocochlear system.; b) a reduction of age-related dystrophic changes in the stria vascularis; c) diminished immunoreactivity for the pro-inflammatory cytokine TNFα in the stria vascularis and spiral ligament. d) diminished immunoreactivity for Iba1 and changes in the morphology of Iba1 immunoreactive cells in the lateral wall, suggesting reduced activation of macrophage/microglia; d) Increased immunoreactivity levels for calretinin in spiral ganglion neurons, suggesting excitability modulation by corticofugal stimulation. Altogether, these findings support that non-invasive neuromodulation of the auditory cortex during aging preserves the cochlear efferent system and ameliorates cochlear aging traits, including stria vascularis dystrophy, dysregulated inflammation and altered excitability in primary auditory neurons.

The human OPA1delTTAG mutation induces adult onset and progressive auditory neuropathy in mice

Dominant optic atrophy (DOA) is one of the most prevalent forms of hereditary optic neuropathies and is mainly caused by heterozygous variants in OPA1, encoding a mitochondrial dynamin-related large GTPase. The clinical spectrum of DOA has been extended to a wide variety of syndromic presentations, called DOAplus, including deafness as the main secondary symptom associated to vision impairment. To date, the pathophysiological mechanisms underlying the deafness in DOA remain unknown. To gain insights into the process leading to hearing impairment, we have analyzed the Opa1delTTAG mouse model that recapitulates the DOAplus syndrome through complementary approaches combining morpho-physiology, biochemistry, and cellular and molecular biology. We found that Opa1delTTAG mutation leads an adult-onset progressive auditory neuropathy in mice, as attested by the auditory brainstem response threshold shift over time. However, the mutant mice harbored larger otoacoustic emissions in comparison to wild-type littermates, whereas the endocochlear potential, which is a proxy for the functional state of the stria vascularis, was comparable between both genotypes. Ultrastructural examination of the mutant mice revealed a selective loss of sensory inner hair cells, together with a progressive degeneration of the axons and myelin sheaths of the afferent terminals of the spiral ganglion neurons, supporting an auditory neuropathy spectrum disorder (ANSD). Molecular assessment of cochlea demonstrated a reduction of Opa1 mRNA level by greater than 40%, supporting haploinsufficiency as the disease mechanism. In addition, we evidenced an early increase in Sirtuin 3 level and in Beclin1 activity, and subsequently an age-related mtDNA depletion, increased oxidative stress, mitophagy as well as an impaired autophagic flux. Together, these results support a novel role for OPA1 in the maintenance of inner hair cells and auditory neural structures, addressing new challenges for the exploration and treatment of OPA1-linked ANSD in patients.

The Cholinergic Lateral Line Efferent Synapse: Structural, Functional and Molecular Similarities With Those of the Cochlea

Vertebrate hair cell (HC) systems are innervated by efferent fibers that modulate their response to external stimuli. In mammals, the best studied efferent-HC synapse, the cholinergic medial olivocochlear (MOC) efferent system, makes direct synaptic contacts with HCs. The net effect of MOC activity is to hyperpolarize HCs through the activation of α9α10 nicotinic cholinergic receptors (nAChRs) and the subsequent activation of Ca²⁺-dependent SK2 potassium channels. A serious obstacle in research on many mammalian sensory systems in their native context is that their constituent neurons are difficult to access even in newborn animals, hampering circuit observation, mapping, or controlled manipulation. By contrast, fishes and amphibians have a superficial and accessible mechanosensory system, the lateral line (LL), which circumvents many of these problems. LL responsiveness is modulated by efferent neurons which aid to distinguish between external and self-generated stimuli. One component of the LL efferent system is cholinergic and its activation inhibits LL afferent activity, similar to what has been described for MOC efferents. The zebrafish (Danio rerio) has emerged as a powerful model system for studying human hearing and balance disorders, since LL HC are structurally and functionally analogous to cochlear HCs, but are optically and pharmacologically accessible within an intact specimen. Complementing mammalian studies, zebrafish have been used to gain significant insights into many facets of HC biology, including mechanotransduction and synaptic physiology as well as mechanisms of both hereditary and acquired HC dysfunction. With the rise of the zebrafish LL as a model in which to study auditory system function and disease, there has been an increased interest in studying its efferent system and evaluate the similarity between mammalian and piscine efferent synapses. Advances derived from studies in zebrafish include understanding the effect of the LL efferent system on HC and afferent activity, and revealing that an α9-containing nAChR, functionally coupled to SK channels, operates at the LL efferent synapse. In this review, we discuss the tools and findings of these recent investigations into zebrafish efferent-HC synapse, their commonalities with the mammalian counterpart and discuss several emerging areas for future studies.

Olivocochlear Changes Associated With Aging Predominantly Affect the Medial Olivocochlear System

Age-related hearing loss (ARHL) is a public health problem that has been associated with negative health outcomes ranging from increased frailty to an elevated risk of developing dementia. Significant gaps remain in our knowledge of the underlying central neural mechanisms, especially those related to the efferent auditory pathways. Thus, the aim of this study was to quantify and compare age-related alterations in the cholinergic olivocochlear efferent auditory neurons. We assessed, in young-adult and aged CBA mice, the number of cholinergic olivocochlear neurons, auditory brainstem response (ABR) thresholds in silence and in presence of background noise, and the expression of excitatory and inhibitory proteins in the ventral nucleus of the trapezoid body (VNTB) and in the lateral superior olive (LSO). In association with aging, we found a significant decrease in the number of medial olivocochlear (MOC) cholinergic neurons together with changes in the ratio of excitatory and inhibitory proteins in the VNTB. Furthermore, in old mice we identified a correlation between the number of MOC neurons and ABR thresholds in the presence of background noise. In contrast, the alterations observed in the lateral olivocochlear (LOC) system were less significant. The decrease in the number of LOC cells associated with aging was 2.7-fold lower than in MOC and in the absence of changes in the expression of excitatory and inhibitory proteins in the LSO. These differences suggest that aging alters the medial and lateral olivocochlear efferent pathways in a differential manner and that the changes observed may account for some of the symptoms seen in ARHL.

Distinct forms of synaptic plasticity during ascending vs descending control of medial olivocochlear efferent neurons

Activity in each brain region is shaped by the convergence of ascending and descending axonal pathways, and the balance and characteristics of these determine the neural output. The medial olivocochlear (MOC) efferent system is part of a reflex arc that critically controls auditory sensitivity. Multiple central pathways contact MOC neurons, raising the question of how a reflex arc could be engaged by diverse inputs. We examined functional properties of synapses onto brainstem MOC neurons from ascending (ventral cochlear nucleus, VCN) and descending (inferior colliculus, IC) sources in mice using an optogenetic approach. We found that these pathways exhibited opposing forms of short-term plasticity, with the VCN input showing depression and the IC input showing marked facilitation. By using a conductance-clamp approach, we found that combinations of facilitating and depressing inputs enabled firing of MOC neurons over a surprisingly wide dynamic range, suggesting an essential role for descending signaling to a brainstem nucleus.

Characterization of Developmental Changes in Spontaneous Electrical Activity of Medial Superior Olivary Neurons Before Hearing Onset With a Combination of Injectable and Volatile Anesthesia

In this work the impact of two widely used anesthetics on the electrical activity of auditory brainstem neurons was studied during postnatal development. Spontaneous electrical activity in neonate rats of either sex was analyzed through a ventral craniotomy in mechanically ventilated pups to carry out patch clamp and multi-electrode electrophysiology recordings in the medial region of the superior olivary complex (SOC) between birth (postnatal day 0, P0) and P12. Recordings were obtained in pups anesthetized with the injectable mix of ketamine/xylazine (K/X mix), with the volatile anesthetic isoflurane (ISO), or in pups anesthetized with K/X mix that were also exposed to ISO. The results of patch clamp recordings demonstrate for the first time that olivary and periolivary neurons in the medial region of the SOC fire bursts of action potentials. The results of multielectrode recordings suggest that the firing pattern of single units recorded in K/X mix is similar to that recorded in ISO anesthetized rat pups. Taken together, the results of this study provide a framework to use injectable and volatile anesthetics for future studies to obtain functional information on the activity of medial superior olivary neurons in vivo.

Projections from the ventral nucleus of the lateral lemniscus to the cochlea in the mouse

Auditory efferents originate in the central auditory system and project to the cochlea. Although the specific anatomy of the olivocochlear (OC) efferents can vary between species, two types of auditory efferents have been identified based upon the general location of their cell bodies and their distinctly different axon terminations in the organ of Corti. In the mouse, the relatively small somata of the lateral (LOC) efferents reside in the lateral superior olive (LSO), have unmyelinated axons, and terminate around ipsilateral inner hair cells (IHCs), primarily against the afferent processes of type I auditory nerve fibers. In contrast, the larger somata of the medial (MOC) efferents are distributed in the ventral nucleus of the trapezoid body (VNTB), have myelinated axons, and terminate bilaterally against the base of multiple outer hair cells (OHCs). Using in vivo retrograde cell body marking, anterograde axon tracing, immunohistochemistry, and electron microscopy, we have identified a group of efferent neurons in mouse, whose cell bodies reside in the ventral nucleus of the lateral lemniscus (VNLL). By virtue of their location, we call them dorsal efferent (DE) neurons. Labeled DE cells were immuno-negative for tyrosine hydroxylase, glycine, and GABA, but immuno-positive for choline acetyltransferase. Morphologically, DEs resembled LOC efferents by their small somata, unmyelinated axons, and ipsilateral projection to IHCs. These three classes of efferent neurons all project axons directly to the cochlea and exhibit cholinergic staining characteristics. The challenge is to discover the contributions of this new population of neurons to auditory efferent function.

Short-Term Test-Retest Reliability of Contralateral Suppression of Click-Evoked Otoacoustic Emissions in Normal-Hearing Subjects

Purpose The objective of the current study was to investigate the short-term test-retest reliability of contralateral suppression (CS) of click-evoked otoacoustic emissions (CEOAEs) using commercially available otoacoustic emission equipment. Method Twenty-three young normal-hearing subjects were tested. An otoscopic evaluation, admittance measures, pure-tone audiometry, measurements of CEOAEs without and with contralateral acoustic stimulation (CAS) to determine CS were performed at baseline (n = 23), an immediate retest without and with refitting of the probe (only CS of CEOAEs; n = 11), and a retest after 1 week (n = 23) were performed. Test-retest reliability parameters were determined on CEOAE response amplitudes without and with CAS, and on raw and normalized CS indices between baseline and the other test moments. Results Repeated-measures analysis of variance indicated no random or systematic changes in CEOAE response amplitudes without and with CAS, and in raw and normalized CS indices between the test moments. Moderate-to-high intraclass correlation coefficients with mostly high significant between-subjects variability between baseline and each consecutive test moment were found for CEOAE response amplitude without and with CAS, and for the raw and normalized CS indices. Other reliability parameters deteriorated between CEOAE response amplitudes with CAS as compared to without CAS, between baseline and retest with probe refitting, and after 1 week, as well as for frequency-specific raw and normalized CS indices as compared to global CS indices. Conclusions There was considerable variability in raw and normalized CS indices as measured using CEOAEs with CAS using commercially available otoacoustic emission equipment. More research is needed to optimize the measurement of CS of CEOAEs and to reduce influencing factors, as well as to make generalization of test-retest reliability data possible.

Conditioned attenuation of dolphin monaural and binaural auditory evoked potentials after preferential stimulation of one ear

Previous studies have demonstrated that some species of odontocetes can be conditioned to reduce hearing sensitivity when warned of an impending intense sound; however, the underlying mechanisms remain poorly understood. In the present study, conditioned hearing attenuation was elicited in two bottlenose dolphins by pairing a 10-kHz tone (the conditioned stimulus) with a more intense tone (the unconditioned stimulus) at 28 kHz. Testing was performed in air, with sounds presented via contact transducers. Hearing was assessed via noninvasive measurement of monaural auditory nerve responses (ANR) and binaural auditory brainstem responses (ABR). ABRs/ANRs were measured in response to 40-kHz tone bursts, over 2 to 3-s time intervals before and after the conditioned and unconditioned stimuli. Results showed reductions in ABR/ANR amplitude and increases in latency after pairing the warning and more intense tones. Monaural ANRs from the left and right ears were attenuated by similar amounts when the warning and more intense sounds were preferentially applied to the right ear. The data support a neural mechanism operating at the level of the cochlea and/or auditory nerve and suggest the involvement of neural projections that can affect the contralateral ear.

Hair cell maturation is differentially regulated along the tonotopic axis of the mammalian cochlea

Key points

Outer hair cells (OHCs) enhance the sensitivity and the frequency tuning of the mammalian cochlea.

Similar to the primary sensory receptor, the inner hair cells (IHCs), the mature functional characteristics of OHCs are acquired before hearing onset.

We found that OHCs, like IHCs, fire spontaneous Ca²⁺‐induced action potentials (APs) during immature stages of development, which are driven by Ca_V1.3 Ca²⁺ channels.

We also showed that the development of low‐ and high‐frequency hair cells is differentially regulated during pre‐hearing stages, with the former cells being more strongly dependent on experience‐independent Ca²⁺ action potential activity.

Abstract

Sound amplification within the mammalian cochlea depends upon specialized hair cells, the outer hair cells (OHCs), which possess both sensory and motile capabilities. In various altricial rodents, OHCs become functionally competent from around postnatal day 7 (P7), before the primary sensory inner hair cells (IHCs), which become competent at about the onset of hearing (P12). The mechanisms responsible for the maturation of OHCs and their synaptic specialization remain poorly understood. We report that spontaneous Ca²⁺ activity in the immature cochlea, which is generated by Ca_V1.3 Ca²⁺ channels, differentially regulates the maturation of hair cells along the cochlea. Under near‐physiological recording conditions we found that, similar to IHCs, immature OHCs elicited spontaneous Ca²⁺ action potentials (APs), but only during the first few postnatal days. Genetic ablation of these APs in vivo, using Ca_V1.3^−/− mice, prevented the normal developmental acquisition of mature‐like basolateral membrane currents in low‐frequency (apical) hair cells, such as I_K,n (carried by KCNQ4 channels), I_SK2 and I_ACh (α9α10nAChRs) in OHCs and I_K,n and I_K,f (BK channels) in IHCs. Electromotility and prestin expression in OHCs were normal in Ca_V1.3^−/− mice. The maturation of high‐frequency (basal) hair cells was also affected in Ca_V1.3^−/− mice, but to a much lesser extent than apical cells. However, a characteristic feature in Ca_V1.3^−/− mice was the reduced hair cell size irrespective of their cochlear location. We conclude that the development of low‐ and high‐frequency hair cells is differentially regulated during development, with apical cells being more strongly dependent on experience‐independent Ca²⁺ APs.

Outer hair cells (OHCs) enhance the sensitivity and the frequency tuning of the mammalian cochlea.

Similar to the primary sensory receptor, the inner hair cells (IHCs), the mature functional characteristics of OHCs are acquired before hearing onset.

We found that OHCs, like IHCs, fire spontaneous Ca²⁺‐induced action potentials (APs) during immature stages of development, which are driven by Ca_V1.3 Ca²⁺ channels.

We also showed that the development of low‐ and high‐frequency hair cells is differentially regulated during pre‐hearing stages, with the former cells being more strongly dependent on experience‐independent Ca²⁺ action potential activity.

Synaptic Inhibition of Medial Olivocochlear Efferent Neurons by Neurons of the Medial Nucleus of the Trapezoid Body

Medial olivocochlear (MOC) efferent neurons in the brainstem comprise the final stage of descending control of the mammalian peripheral auditory system through axon projections to the cochlea. MOC activity adjusts cochlear gain and frequency tuning, and protects the ear from acoustic trauma. The neuronal pathways that activate and modulate the MOC somata in the brainstem to drive these cochlear effects are poorly understood. Evidence suggests that MOC neurons are primarily excited by sound stimuli in a three-neuron activation loop from the auditory nerve via an intermediate neuron in the cochlear nucleus. Anatomical studies suggest that MOC neurons receive diverse synaptic inputs, but the functional effect of additional synaptic influences on MOC neuron responses is unknown. Here we use patch-clamp electrophysiological recordings from identified MOC neurons in brainstem slices from mice of either sex to demonstrate that in addition to excitatory glutamatergic synapses, MOC neurons receive inhibitory GABAergic and glycinergic synaptic inputs. These synapses are activated by electrical stimulation of axons near the medial nucleus of the trapezoid body (MNTB). Focal glutamate uncaging confirms MNTB neurons as a source of inhibitory synapses onto MOC neurons. MNTB neurons inhibit MOC action potentials, but this effect depresses with repeat activation. This work identifies a new pathway of connectivity between brainstem auditory neurons and indicates that MOC neurons are both excited and inhibited by sound stimuli received at the same ear. The pathway depression suggests that the effect of MNTB inhibition of MOC neurons diminishes over the course of a sustained sound.SIGNIFICANCE STATEMENT Medial olivocochlear (MOC) neurons are the final stage of descending control of the mammalian auditory system and exert influence on cochlear mechanics to modulate perception of acoustic stimuli. The brainstem pathways that drive MOC function are poorly understood. Here we show for the first time that MOC neurons are inhibited by neurons of the MNTB, which may suppress the effects of MOC activity on the cochlea.

Cochlear Efferent Innervation Is Sparse in Humans and Decreases with Age

The mammalian cochlea is innervated by two cholinergic feedback systems called the medial olivocochlear (MOC) and lateral olivocochlear (LOC) pathways, which send control signals from the brainstem back to the outer hair cells and auditory-nerve fibers, respectively. Despite countless studies of the cochlear projections of these efferent fibers in animal models, comparable data for humans are almost completely lacking. Here, we immunostained the cochlear sensory epithelium from 23 normal-aging humans (14 males and 9 females), 0-86 years of age, with cholinergic markers to quantify the normal density of MOC and LOC projections, and the degree of age-related degeneration. In younger ears, the MOC density peaks in mid-cochlear regions and falls off both apically and basally, whereas the LOC innervation peaks near the apex. In older ears, MOC density decreases dramatically, whereas the LOC density does not. The loss of MOC feedback may contribute to the age-related decrease in word recognition in noise; however, even at its peak, the MOC density is lower than in other mammals, suggesting the MOC pathway is less important for human hearing.SIGNIFICANCE STATEMENT The cochlear epithelium and its sensory innervation are modulated by the olivocochlear (OC) efferent pathway. Although the medial OC (MOC) reflex has been extensively studied in humans, via contralateral sound suppression, the cochlear projections of these cholinergic neurons have not been described in humans. Here, we use immunostaining to quantify the MOC projections to outer hair cells and lateral OC (LOC) projections to the inner hair cell area in humans 0-89 years of age. We show age-related loss of MOC, but not LOC, innervation, which likely contributes to hearing impairments, and a relative paucity of MOC terminals at all ages, which may account for the relative weakness of the human MOC reflex and the difficulty in demonstrating a robust functional role in human experiments.

Targets of olivocochlear collaterals in cochlear nucleus of rat and guinea pig

Descending auditory pathways can modify afferent auditory input en route to cortex. One component of these pathways is the olivocochlear system which originates in brainstem and terminates in cochlea. Medial olivocochlear (MOC) neurons also project collaterals to cochlear nucleus and make synaptic contacts with dendrites of multipolar neurons. Two broadly distinct populations of multipolar cells exist: T-stellate and D-stellate neurons, thought to project to inferior colliculus and contralateral cochlear nucleus, respectively. It is unclear which of these neurons receive direct MOC collateral input due to conflicting results between in vivo and in vitro studies. This study used anatomical techniques to identify which multipolar cell population receives synaptic innervation from MOC collaterals. The retrograde tracer Fluorogold was injected into inferior colliculus or cochlear nucleus to label T-stellate and D-stellate neurons, respectively. Axonal branches of MOC neurons were labeled by biocytin injections at the floor of the fourth ventricle. Fluorogold injections resulted in labeled cochlear nucleus multipolar neurons. Biocytin abundantly labeled MOC collaterals which entered cochlear nucleus. Microscopic analysis revealed that MOC collaterals made some putative synaptic contacts with the retrogradely labeled neurons but many more putative contacts were observed on unidentified neural targets. This suggest that both T- and D-stellate neurons receive synaptic innervation from the MOC collaterals on their somata and proximal dendrites. The prevalence of these contacts cannot be stated with certainty because of technical limitations, but the possibility exists that the collaterals may also make contacts with neurons not projecting to inferior colliculus or the contralateral cochlear nucleus.

Does Contralateral Inhibition of Transient Evoked Otoacoustic Emissions Suggest Sex or Ear Laterality Effects?

PURPOSE

The purpose of this study was to examine contralateral inhibition of transient evoked otoacoustic emissions (TEOAEs) in young adults with normal hearing as a function of ear and sex.

METHOD

Young female (n = 50) and male (n = 50) adults with normal hearing participated. TEOAEs were measured bilaterally with 80 dB peSPL nonlinear clicks and 60 dB peSPL linear clicks with and without a contralateral broadband noise elicitor at 65 dB SPL. Absolute TEOAE inhibition and normalized TEOAE inhibition (i.e., percentage of inhibition) were examined.

RESULTS

With both 80 and 60 dB peSPL evoking stimuli, there were significant main effects of ear and sex (p < .05). TEOAE levels were larger in women and in the right ear. There were no statistically significant main effects of ear and sex on absolute TEOAE inhibition (p > .05). Significant main effects of ear and sex were, however, found with normalized TEOAE inhibition (p < .05; greater in men and in the left ear). Statistically significant negative correlations and significant linear predictive relations were found between TEOAE levels and normalized TEOAE inhibitions in both ears (p < .001). There is no evidence of the same with absolute inhibition of TEOAEs (p > .05).

CONCLUSIONS

If one considers that efferent inhibition of TEOAEs is an absolute quantity, the significant effect of ear and sex on normalized inhibition and the negative association and linear predictive relationship between TEOAE level and inhibition can be viewed as spurious effects. As such, contralateral inhibition of TEOAEs does not suggest sex or ear laterality effects.

Olivocochlear Efferents in Animals and Humans: From Anatomy to Clinical Relevance

Olivocochlear efferents allow the central auditory system to adjust the functioning of the inner ear during active and passive listening. While many aspects of efferent anatomy, physiology and function are well established, others remain controversial. This article reviews the current knowledge on olivocochlear efferents, with emphasis on human medial efferents. The review covers (1) the anatomy and physiology of olivocochlear efferents in animals; (2) the methods used for investigating this auditory feedback system in humans, their limitations and best practices; (3) the characteristics of medial-olivocochlear efferents in humans, with a critical analysis of some discrepancies across human studies and between animal and human studies; (4) the possible roles of olivocochlear efferents in hearing, discussing the evidence in favor and against their role in facilitating the detection of signals in noise and in protecting the auditory system from excessive acoustic stimulation; and (5) the emerging association between abnormal olivocochlear efferent function and several health conditions. Finally, we summarize some open issues and introduce promising approaches for investigating the roles of efferents in human hearing using cochlear implants.

Conditioned attenuation of auditory brainstem responses in dolphins warned of an intense noise exposure: Temporal and spectral patterns

Conditioned reductions in hearing sensitivity were elicited in two bottlenose dolphins by pairing a 10-kHz tone (the conditioned stimulus) with a more intense tone (the unconditioned stimulus) at 20, 40, or 80 kHz. Hearing was assessed via noninvasive measurement of auditory brainstem responses (ABRs) to 20 - to 133-kHz tone bursts presented at randomized intervals from 1 to 3 ms. ABRs within each trial were obtained by averaging the instantaneous electroencephalogram, time-locked to tone burst onsets, over 2- to 3-s time intervals. In initial testing, ABR amplitudes were reduced (relative to baseline values) in one dolphin after the conditioned stimulus, but before the unconditioned stimulus, demonstrating conditioned hearing attenuation. In subsequent testing with both dolphins, ABRs were attenuated throughout the entire 31-s trial. Maximum ABR threshold shifts occurred at and above the unconditioned stimulus frequency and were above 40 dB for some conditions. The results (1) confirm that dolphins can be conditioned to reduce hearing sensitivity when warned of an impending noise exposure, (2) show that hearing attenuation occurs within the cochlea or auditory nerve, and (3) support the hypothesis that toothed whales can "self-mitigate" some effects of noise if warned of an impending exposure.

Functional Interplay Between the Putative Measures of Rostral and Caudal Efferent Regulation of Speech Perception in Noise

Efferent modulation has been demonstrated to be very important for speech perception, especially in the presence of noise. We examined the functional relationship between two efferent systems: the rostral and caudal efferent pathways and their individual influences on speech perception in noise. Earlier studies have shown that these two efferent mechanisms were correlated with speech perception in noise. However, previously, these mechanisms were studied in isolation, and their functional relationship with each other was not investigated. We used a correlational design to study the relationship if any, between these two mechanisms in young and old normal hearing individuals. We recorded context-dependent brainstem encoding as an index of rostral efferent function and contralateral suppression of otoacoustic emissions as an index of caudal efferent function in groups with good and poor speech perception in noise. These efferent mechanisms were analysed for their relationship with each other and with speech perception in noise. We found that the two efferent mechanisms did not show any functional relationship. Interestingly, both the efferent mechanisms correlated with speech perception in noise and they even emerged as significant predictors. Based on the data, we posit that the two efferent mechanisms function relatively independently but with a common goal of fine-tuning the afferent input and refining auditory perception in degraded listening conditions.

The mammalian olivocochlear system--a legacy of non-cerebellar research in the Mugnaini lab

Although the major emphasis of Enrico Mugnaini's research has been on investigations of the cerebellum, a significant amount of work over a relatively short span of time was also done in his lab on a number of other brain systems. These centered on sensory systems. One of these extra-cerebellar systems that he embraced was the auditory system. Portions of the cochlear nucleus, the first synaptic relay station along the central auditory pathways, possess a cerebellar-like circuitry and neurochemistry, and this no doubt lured Enrico into the auditory field. As new tools became available to pursue neuroanatomical research in general, which included a novel antibody to glutamic acid decarboxylase (GAD), Enrico's lab soon branched out into investigating many other brain structures beyond the cerebellum, with an overall goal of producing a map illustrating GAD expression in the brain. In collaboration with long-term colleagues, one of these many non-cerebellar regions he took an interest in was an efferent pathway originating in the superior olive and projecting to the cochlea, the peripheral end organ for hearing. There was a need for a more complete neurochemical map of this olivocochlear efferent system, and armed with new antibodies and well-established tract tracing tools, together we set out to further explore this system. This short review describes the work done with Enrico on the olivocochlear system of rodents, and also continues the story beyond Enrico's lab to reveal how the work done in his lab fits into the larger scheme of current, ongoing research into the olivocochlear system.

Effects of pulsatile electrical stimulation of the round window on central hyperactivity after cochlear trauma in guinea pig

Partial hearing loss induced by acoustic trauma has been shown in animal models to result in an increased spontaneous firing rate in central auditory structures. This so-called hyperactivity has been suggested to be involved in the generation of tinnitus, a phantom auditory sensation. Although there is no universal cure for tinnitus, electrical stimulation of the cochlea, as achieved by a cochlear implant, can result in significant reduction of the tinnitus percept. However, the mechanism by which this tinnitus suppression occurs is as yet unknown and furthermore cochlear implantation may not be an optimal treatment option for tinnitus sufferers who are not profoundly deaf. A better understanding of the mechanism of tinnitus suppression by electrical stimulation of the cochlea, may lead to the development of more specialised devices for those for whom a cochlear implant is not appropriate. This study aimed to investigate the effects of electrical stimulation in the form of brief biphasic shocks delivered to the round window of the cochlea on the spontaneous firing rates of hyperactive inferior colliculus neurons following acoustic trauma in guinea pigs. Effects during the stimulation itself included both inhibition and excitation but spontaneous firing was suppressed for up to hundreds of ms after the cessation of the shock train in all sampled hyperactive neurons. Pharmacological block of olivocochlear efferent action on outer hair cells did not eliminate the prolonged suppression observed in inferior colliculus neurons, and it is therefore likely that activation of the afferent pathways is responsible for the central effects observed.

Auditory system of fruit flies

The fruit fly, Drosophila melanogaster, is an invaluable model for auditory research. Advantages of using the fruit fly include its stereotyped behavior in response to a particular sound, and the availability of molecular-genetic tools to manipulate gene expression and cellular activity. Although the receiver type in fruit flies differs from that in mammals, the auditory systems of mammals and fruit flies are strikingly similar with regard to the level of development, transduction mechanism, mechanical amplification, and central projections. These similarities strongly support the use of the fruit fly to study the general principles of acoustic information processing. In this review, we introduce acoustic communication and discuss recent advances in our understanding on hearing in fruit flies. This article is part of a Special Issue entitled <Annual Reviews 2016>.

Short-term plasticity and modulation of synaptic transmission at mammalian inhibitory cholinergic olivocochlear synapses

The organ of Corti, the mammalian sensory epithelium of the inner ear, has two types of mechanoreceptor cells, inner hair cells (IHCs) and outer hair cells (OHCs). In this sensory epithelium, vibrations produced by sound waves are transformed into electrical signals. When depolarized by incoming sounds, IHCs release glutamate and activate auditory nerve fibers innervating them and OHCs, by virtue of their electromotile property, increase the amplification and fine tuning of sound signals. The medial olivocochlear (MOC) system, an efferent feedback system, inhibits OHC activity and thereby reduces the sensitivity and sharp tuning of cochlear afferent fibers. During neonatal development, IHCs fire Ca²⁺ action potentials which evoke glutamate release promoting activity in the immature auditory system in the absence of sensory stimuli. During this period, MOC fibers also innervate IHCs and are thought to modulate their firing rate. Both the MOC-OHC and the MOC-IHC synapses are cholinergic, fast and inhibitory and mediated by the α9α10 nicotinic cholinergic receptor (nAChR) coupled to the activation of calcium-activated potassium channels that hyperpolarize the hair cells. In this review we discuss the biophysical, functional and molecular data which demonstrate that at the synapses between MOC efferent fibers and cochlear hair cells, modulation of transmitter release as well as short term synaptic plasticity mechanisms, operating both at the presynaptic terminal and at the postsynaptic hair-cell, determine the efficacy of these synapses and shape the hair cell response pattern.

Efferent feedback minimizes cochlear neuropathy from moderate noise exposure

Although protective effects of the cochlea's efferent feedback pathways have been well documented, prior work has focused on hair cell damage and cochlear threshold elevation and, correspondingly, on the high sound pressure levels (>100 dB SPL) necessary to produce them. Here we explore the noise-induced loss of cochlear neurons that occurs with lower-intensity exposures and in the absence of permanent threshold shifts. Using confocal microscopy to count synapses between hair cells and cochlear nerve fibers, and using measurement of auditory brainstem responses and otoacoustic emissions to assess cochlear presynaptic and postsynaptic function, we compare the damage from a weeklong exposure to moderate-level noise (84 dB SPL) in mice with varying degrees of cochlear de-efferentation induced by surgical lesion to the olivocochlear pathway. Such exposure causes minimal acute threshold shifts and no chronic shifts in mice with normal efferent feedback. In de-efferented animals, there was up to 40% loss of cochlear nerve synapses and a corresponding decline in the amplitude of the auditory brainstem response. Quantitative analysis of the de-efferentation in inner versus outer hair cell areas suggested that outer hair cell efferents are the most important in minimizing this neuropathy, presumably by virtue of their sound-evoked feedback reduction of cochlear amplification. The moderate nature of this acoustic overexposure suggests that cochlear neurons are at risk even in everyday acoustic environments, so the need for cochlear protection is plausible as a driving force in the design of this feedback pathway.

Progress in cochlear physiology after Békésy

In the fifty years since Békésy was awarded the Nobel Prize, cochlear physiology has blossomed. Many topics that are now current are things Békésy could not have imagined. In this review we start by describing progress in understanding the origin of cochlear gross potentials, particularly the cochlear microphonic, an area in which Békésy had extensive experience. We then review progress in areas of cochlear physiology that were mostly unknown to Békésy, including: (1) stereocilia mechano-electrical transduction, force production, and response amplification, (2) outer hair cell (OHC) somatic motility and its molecular basis in prestin, (3) cochlear amplification and related micromechanics, including the evidence that prestin is the main motor for cochlear amplification, (4) the influence of the tectorial membrane, (5) cochlear micromechanics and the mechanical drives to inner hair cell stereocilia, (6) otoacoustic emissions, and (7) olivocochlear efferents and their influence on cochlear physiology. We then return to a subject that Békésy knew well: cochlear fluids and standing currents, as well as our present understanding of energy dependence on the lateral wall of the cochlea. Finally, we touch on cochlear pathologies including noise damage and aging, with an emphasis on where the field might go in the future.

The efferent medial olivocochlear-hair cell synapse

Amplification of incoming sounds in the inner ear is modulated by an efferent pathway which travels back from the brain all the way to the cochlea. The medial olivocochlear system makes synaptic contacts with hair cells, where the neurotransmitter acetylcholine is released. Synaptic transmission is mediated by a unique nicotinic cholinergic receptor composed of α9 and α10 subunits, which is highly Ca2+ permeable and is coupled to a Ca2+-activated SK potassium channel. Thus, hyperpolarization of hair cells follows efferent fiber activation. In this work we review the literature that has enlightened our knowledge concerning the intimacies of this synapse.

Changes in amplitude and phase of distortion-product otoacoustic emission fine-structure and separated components during efferent activation

Medial olivocochlear (MOC) efferent fibers synapse directly on the outer hair cells (OHCs). Efferent activation evoked by contralateral acoustic stimulation (CAS) will affect OHC amplification and subsequent measures of distortion-product otoacoustic emissions (DPOAEs). The aim of this study was to investigate measures of total and separated DPOAEs during efferent activation. Efferent activation produces both suppression and enhancement of the total DPOAE level. Level enhancements occurred near fine-structure minima and were associated with consistent MOC evoked upward shifts in DPOAE fine-structure frequency. Examination of the phase of the separated components revealed that frequency shifts stemmed from increasing phase leads of the reflection component during CAS, while the generator component phase was nearly invariant. Separation of the two DPOAE components responsible for the fine-structure revealed more consistent reduction of the levels of both components. Using vector subtraction (which takes into account both level and phase) to estimate the changes in the unseparated DPOAE provided consistent evidence of DPOAE suppression. Including phase information provided a more sensitive, valid and consistent estimate of CAS function even if one does not know the position of the DPOAE in the fine-structure.

Release and elementary mechanisms of nitric oxide in hair cells

The enzyme nitric oxide (NO) synthase, that produces the signaling molecule NO, has been identified in several cell types in the inner ear. However, it is unclear whether a measurable quantity of NO is released in the inner ear to confer specific functions. Indeed, the functional significance of NO and the elementary cellular mechanism thereof are most uncertain. Here, we demonstrate that the sensory epithelia of the frog saccule release NO and explore its release mechanisms by using self-referencing NO-selective electrodes. Additionally, we investigated the functional effects of NO on electrical properties of hair cells and determined their underlying cellular mechanism. We show detectable amounts of NO are released by hair cells (>50 nM). Furthermore, a hair-cell efferent modulator acetylcholine produces at least a threefold increase in NO release. NO not only attenuated the baseline membrane oscillations but it also increased the magnitude of current required to generate the characteristic membrane potential oscillations. This resulted in a rightward shift in the frequency-current relationship and altered the excitability of hair cells. Our data suggest that these effects ensue because NO reduces whole cell Ca(2+) current and drastically decreases the open probability of single-channel events of the L-type and non L-type Ca(2+) channels in hair cells, an effect that is mediated through direct nitrosylation of the channel and activation of protein kinase G. Finally, NO increases the magnitude of Ca(2+)-activated K(+) currents via direct NO nitrosylation. We conclude that NO-mediated inhibition serves as a component of efferent nerve modulation of hair cells.

Lack of nAChR activity depresses cochlear maturation and up-regulates GABA system components: temporal profiling of gene expression in alpha9 null mice

Background

It has previously been shown that deletion of chrna9, the gene encoding the α9 nicotinic acetylcholine receptor (nAChR) subunit, results in abnormal synaptic terminal structure. Additionally, all nAChR-mediated cochlear activity is lost, as characterized by a failure of the descending efferent system to suppress cochlear responses to sound. In an effort to characterize the molecular mechanisms underlying the structural and functional consequences following loss of α9 subunit expression, we performed whole-transcriptome gene expression analyses on cochleae of wild type and α9 knockout (α9^−/−) mice during postnatal days spanning critical periods of synapse formation and maturation.

Principal Findings

Data revealed that loss of α9 receptor subunit expression leads to an up-regulation of genes involved in synaptic transmission and ion channel activity. Unexpectedly, loss of α9 receptor subunit expression also resulted in an increased expression of genes encoding GABA receptor subunits and the GABA synthetic enzyme, glutamic acid decarboxylase. These data suggest the existence of a previously unrecognized association between the nicotinic cholinergic and GABAergic systems in the cochlea. Computational analyses have highlighted differential expression of several gene sets upon loss of nicotinic cholinergic activity in the cochlea. Time-series analysis of whole transcriptome patterns, represented as self-organizing maps, revealed a disparate pattern of gene expression between α9^−/− and wild type cochleae at the onset of hearing (P13), with knockout samples resembling immature postnatal ages.

Conclusions

We have taken a systems biology approach to provide insight into molecular programs influenced by the loss of nicotinic receptor-based cholinergic activity in the cochlea and to identify candidate genes that may be involved in nicotinic cholinergic synapse formation, stabilization or function within the inner ear. Additionally, our data indicate a change in the GABAergic system upon loss of α9 nicotinic receptor subunit within the cochlea.

F1 (CBA×C57) mice show superior hearing in old age relative to their parental strains: hybrid vigor or a new animal model for "golden ears"?

Age-related hearing loss - presbycusis - is the most common communication problem and third most prevalent chronic medical disorder of the aged. The CBA and C57BL/6 mouse strains are useful for studying features of presbycusis. The CBA loses its hearing slowly, like most humans. Because the C57 develops a rapid, high frequency hearing loss by middle age, it has an "old" ear but a relatively young brain, a model that helps separate peripheral (cochlear) from central (brain) etiologies. This field of sensory neuroscience lacks a good mouse model for the 5-10% of aged humans with normal cochlear sensitivity, but who have trouble perceiving speech in background noise. We hypothesized that F1 (CBA×C57) hybrids would have better hearing than either parental strain. Measurements of peripheral auditory sensitivity supported this hypothesis, however, a rapid decline in the auditory efferent feedback system, did not. Therefore, F1s might be an optimal model for studying cases where the peripheral hearing is quite good in old age; thereby allowing isolation of central auditory changes due to brain neurodegeneration.

Dynamics of real time DPOAE contralateral suppression in chinchillas and humans Dinámica de la supresión contralateral de las DPOAE en tiempo real en chinchillas y humanos

The dynamics of contralateral acoustic suppression were studied using real time (millisecond resolution) distortion product otoacoustic emissions (DPOAEs) in chinchillas and humans. Latency of DPOAE suppression onset is 26 ms in chinchillas and 45 ms in humans. After onset, suppression builds over time before tending to plateau, reflecting a temporal integration process with a time constant of 100 ms (chinchillas). In chinchillas, suppression persists for 40 ms even when elicited by stimuli as short as 5 ms. With stimuli >40 ms, offset and onset latencies are similar and duration of suppression equals that of the contralateral stimulus. A comparison of DPOAE suppression onset latency with neural latency data from the pathways involved suggests the following timing scheme: stimulus onset to activity in (ventral) cochlear nucleus, 4ms (15% of delay); transfer to olivocochlear efferents, 9 ms (35%); efferent conduction to presynaptic OHC site, 4ms (15%); synaptic and mechanical events at OHCs, 9 ms (35% of delay).

The nicotinic receptor of cochlear hair cells: a possible pharmacotherapeutic target?

Mechanosensory hair cells of the organ of Corti transmit information regarding sound to the central nervous system by way of peripheral afferent neurons. In return, the central nervous system provides feedback and modulates the afferent stream of information through efferent neurons. The medial olivocochlear efferent system makes direct synaptic contacts with outer hair cells and inhibits amplification brought about by the active mechanical process inherent to these cells. This feedback system offers the potential to improve the detection of signals in background noise, to selectively attend to particular signals, and to protect the periphery from damage caused by overly loud sounds. Acetylcholine released at the synapse between efferent terminals and outer hair cells activates a peculiar nicotinic cholinergic receptor subtype, the alpha9alpha10 receptor. At present no pharmacotherapeutic approaches have been designed that target this cholinergic receptor to treat pathologies of the auditory system. The potential use of alpha9alpha10 selective drugs in conditions such as noise-induced hearing loss, tinnitus and auditory processing disorders is discussed.

Otoacoustic emissions and effects of contralateral white noise stimulation on transient evoked otoacoustic emissions in diabetic children

OBJECTIVE

In this study, our aim was to determine presence of dysfunction in the efferent auditory system of children with type-I diabetes mellitus (DM) presenting no evidence of symptomatic neuropathy.

METHODS

Thirty children with type-I DM (DM group) and 31 age matched healthy children (control group) with normal hearing and middle ear function were entered to the study. Distortion product otoacoustic emissions (DPOAE), transiently evoked otoacoustic emissions (TEOAE), and spontaneous otoacoustic emissions (SOAE) measurements were performed. Then, the TEOAE recording was repeated while a continuous broadband white noise (bandwidth: 50-8000 Hz) presented at 40 dB SL was delivered to the contralateral ear for efferent auditory system suppression.

RESULTS

We found that contralateral stimulation (CS) with white noise resulted in significantly more pronounced suppression of the TEOAE response amplitude in healthy controls compared to DM group at 2000 and 4000 Hz frequencies. Further, a relatively higher percentage of the controls had suppression in at least three frequencies compared to DM group. SOAE prevalence was found to be higher in the DM group.

CONCLUSIONS

Our findings suggest presence of a dysfunction in medial olivocochlear efferent system in diabetic children. This may be regarded as an early central manifestation of diabetic neuropathy.

Human medial olivocochlear reflex: effects as functions of contralateral, ipsilateral, and bilateral elicitor bandwidths

Animal studies have led to the view that the acoustic medial olivocochlear (MOC) efferent reflex provides sharply tuned frequency-specific feedback that inhibits cochlear amplification. To determine if MOC activation is indeed narrow band, we measured the MOC effects in humans elicited by 60-dB sound pressure level (SPL) contralateral, ipsilateral, and bilateral noise bands as a function of noise bandwidth from 1/2 to 6.7 octaves. MOC effects were quantified by the change in stimulus frequency otoacoustic emissions from 40 dB SPL probe tones near 0.5, 1, and 4 kHz. In a second experiment, the noise bands were centered 2 octaves below probe frequencies near 1 and 4 kHz. In all cases, the MOC effects increased as elicitor bandwidth increased, with the effect saturating at about 4 octaves. Generally, the MOC effects increased as the probe frequency decreased, opposite expectations based on MOC innervation density in the cochlea. Bilateral-elicitor effects were always the largest. The ratio of ipsilateral/contralateral effects depended on elicitor bandwidth; the ratio was large for narrow-band probe-centered elicitors and approximately unity for wide-band elicitors. In another experiment, the MOC effects from increasing elicitor bandwidths were calculated from measurements of the MOC effects from adjacent half-octave noise bands. The predicted bandwidth function agreed well with the measured bandwidth function for contralateral elicitors, but overestimated it for ipsilateral and bilateral elicitors. Overall, the results indicate that (1) the MOC reflexes integrate excitation from almost the entire cochlear length, (2) as elicitor bandwidth is increased, the excitation from newly stimulated cochlear regions more than overcomes the reduced excitation at frequencies in the center of the elicitor band, and (3) contralateral, ipsilateral, and bilateral elicitors show MOC reflex spatial summation over most of the cochlea, but ipsilateral spatial summation is less additive than contralateral.

Slow build-up of cochlear suppression during sustained contralateral noise: central modulation of olivocochlear efferents?

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.

Amplitude modulation of DPOAEs by acoustic stimulation of the contralateral ear

CONCLUSION

Otoacoustic emissions generated by outer hair cells (OHCs) are influenced by stimulation of the contralateral ear via a neural pathway involving the olivo-cochlear efferent system. This is often referred to as a contralateral 'suppression reflex', but we suggest that such a term is inappropriate since distortion product otoacoustic emissions (DPOAEs) can be both enhanced and suppressed, and there is continuous modulation with no threshold effects.

OBJECTIVE

To characterize the continuous amplitude modulation of DPOAEs by contralateral sound stimulation.

MATERIALS AND METHODS

In an animal model (chinchilla), DPOAEs were recorded in real time from one ear during presentation of acoustic stimuli to the opposite ear.

RESULTS

DPOAE amplitude is suppressed by an increase in contralateral stimulation, and enhanced by a decrease in same, i.e. the emissions are continuously modulated by activity in the opposite ear. The input-output function shows a linear relationship to this system over a 40-50 dB range of contralateral stimulus levels. After a neural delay time of approximately 25 ms, DPOAE amplitude closely follows contralateral amplitude signals up to modulation frequencies of approximately 20 Hz. Thus, stimuli to one ear continually modulate the OHC system (and therefore the biomechanical amplification) of the contralateral cochlea.

Contralateral suppression of distortion product otoacoustic emissions and the middle-ear muscle reflex in human ears

Distortion product otoacoustic emissions (DPOAEs) were measured in the absence and presence of contralateral noise at five levels--below, equal to, and above the middle-ear muscle (MEM) reflex threshold. The resultant changes in DPOAE level and phase were dependent on stimulus frequency and noise level. Both low-level noise, believed to elicit the medial olivocochlear (MOC) reflex, and high-level noise, thought to activate both MOC and MEM reflexes, significantly decreased the DPOAE level. However, the shift from sole MOC effect to mixed MOC and MEM effects was not as dramatic as we thought. While low-level noise resulted in a minimum DPOAE phase change, high-level noise caused a substantial phase lead for 1 and 2kHz. With increasing frequency, phase lag became more notable. The present study suggests the following: (1) DPOAE contralateral suppression by low-level sound most likely does not involve the effect of the MEM reflex and signal crossover; and (2) combined analysis of DPOAE level and phase changes warrants further investigations to overcome the difficulty in separating the effects of MOC efferents and MEM contraction. The results also imply that OAE measurement has the potential for being used to investigate the effect of the MEM reflex on sound transmission.