Medial olivocochlear reflex interneurons are located in the posteroventral cochlear nucleus: a kainic acid lesion study in guinea pigs

Fast inhibition slows and desynchronizes mouse auditory efferent neuron activity

The encoding of acoustic stimuli requires precise neuron timing. Auditory neurons in the cochlear nucleus (CN) and brainstem are well-suited for accurate analysis of fast acoustic signals, given their physiological specializations of fast membrane time constants, fast axonal conduction, and reliable synaptic transmission. The medial olivocochlear (MOC) neurons that provide efferent inhibition of the cochlea reside in the ventral brainstem and participate in these fast neural circuits. However, their modulation of cochlear function occurs over time scales of a slower nature. This suggests the presence of mechanisms that restrict MOC inhibition of cochlear function. To determine how monaural excitatory and inhibitory synaptic inputs integrate to affect the timing of MOC neuron activity, we developed a novel in vitro slice preparation ('wedge-slice'). The wedge-slice maintains the ascending auditory nerve root, the entire CN and projecting axons, while preserving the ability to perform visually guided patch-clamp electrophysiology recordings from genetically identified MOC neurons. The 'in vivo-like' timing of the wedge-slice demonstrates that the inhibitory pathway accelerates relative to the excitatory pathway when the ascending circuit is intact, and the CN portion of the inhibitory circuit is precise enough to compensate for reduced precision in later synapses. When combined with machine learning PSC analysis and computational modeling, we demonstrate a larger suppression of MOC neuron activity when the inhibition occurs with in vivo-like timing. This delay of MOC activity may ensure that the MOC system is only engaged by sustained background sounds, preventing a maladaptive hyper-suppression of cochlear activity.

Abnormal outer hair cell efferent innervation in Hoxb1-dependent sensorineural hearing loss

Autosomal recessive mutation of HOXB1 and Hoxb1 causes sensorineural hearing loss in patients and mice, respectively, characterized by the presence of higher auditory thresholds; however, the origin of the defects along the auditory pathway is still unknown. In this study, we assessed whether the abnormal auditory threshold and malformation of the sensory auditory cells, the outer hair cells, described in Hoxb1null mutants depend on the absence of efferent motor innervation, or alternatively, is due to altered sensory auditory components. By using a whole series of conditional mutant mice, which inactivate Hoxb1 in either rhombomere 4-derived sensory cochlear neurons or efferent motor neurons, we found that the hearing phenotype is mainly reproduced when efferent motor neurons are specifically affected. Our data strongly suggest that the interactions between olivocochlear motor neurons and outer hair cells during a critical postnatal period are crucial for both hair cell survival and the establishment of the cochlear amplification of sound.

Auditory brainstem mechanisms likely compensate for self-imposed peripheral inhibition

Feedback networks in the brain regulate downstream auditory function as peripheral as the cochlea. However, the upstream neural consequences of this peripheral regulation are less understood. For instance, the medial olivocochlear reflex (MOCR) in the brainstem causes putative attenuation of responses generated in the cochlea and cortex, but those generated in the brainstem are perplexingly unaffected. Based on known neural circuitry, we hypothesized that the inhibition of peripheral input is compensated for by positive feedback in the brainstem over time. We predicted that the inhibition could be captured at the brainstem with shorter (1.5 s) than previously employed long duration (240 s) stimuli where this inhibition is likely compensated for. Results from 16 normal-hearing human listeners support our hypothesis in that when the MOCR is activated, there is a robust reduction of responses generated at the periphery, brainstem, and cortex for short-duration stimuli. Such inhibition at the brainstem, however, diminishes for long-duration stimuli suggesting some compensatory mechanisms at play. Our findings provide a novel non-invasive window into potential gain compensation mechanisms in the brainstem that may have implications for auditory disorders such as tinnitus. Our methodology will be useful in the evaluation of efferent function in individuals with hearing loss.

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.

Synaptic plasticity of inhibitory synapses onto medial olivocochlear efferent neurons

Abstract

The descending auditory system modulates the ascending system at every level. The final descending, or efferent, stage comprises lateral olivocochlear and medial olivocochlear (MOC) neurons. MOC somata in the ventral brainstem project axons to the cochlea to synapse onto outer hair cells (OHC), inhibiting OHC‐mediated cochlear amplification. MOC suppression of OHC function is implicated in cochlear gain control with changing sound intensity, detection of salient stimuli, attention and protection against acoustic trauma. Thus, sound excites MOC neurons to provide negative feedback of the cochlea. Sound also inhibits MOC neurons via medial nucleus of the trapezoid body (MNTB) neurons. However, MNTB–MOC synapses exhibit short‐term depression, suggesting reduced MNTB–MOC inhibition during sustained stimuli. Further, due to high rates of both baseline and sound‐evoked activity in MNTB neurons in vivo, MNTB–MOC synapses may be tonically depressed. To probe this, we characterized short‐term plasticity of MNTB–MOC synapses in mouse brain slices. We mimicked in vivo‐like temperature and extracellular calcium conditions, and in vivo‐like activity patterns of fast synaptic activation rates, sustained activation and prior tonic activity. Synaptic depression was sensitive to extracellular calcium concentration and temperature. During rapid MNTB axon stimulation, postsynaptic currents in MOC neurons summated but with concurrent depression, resulting in smaller, sustained currents, suggesting tonic inhibition of MOC neurons during rapid circuit activity. Low levels of baseline MNTB activity did not significantly reduce responses to subsequent rapid activity that mimics sound stimulation, indicating that, in vivo, MNTB inhibition of MOC neurons persists despite tonic synaptic depression.

Key points

Inhibitory synapses from the medial nucleus of the trapezoid body (MNTB) onto medial olivocochlear (MOC) neurons exhibit short‐term plasticity that is sensitive to calcium and temperature, with enhanced synaptic depression occurring at higher calcium concentrations and at room temperature.

High rates of background synaptic activity that mimic the upper limits of spontaneous MNTB activity cause tonic synaptic depression of MNTB–MOC synapses that limits further synaptic inhibition.

High rates of activity at MNTB–MOC synapses cause synaptic summation with concurrent depression to yield a response with an initial large amplitude that decays to a tonic inhibition.

Olivocochlear projections contribute to superior intensity coding in cochlear nucleus small cells

Understanding communication signals, especially in noisy environments, is crucial to social interactions. Yet, as we age, acoustic signals can be disrupted by cochlear damage and the subsequent auditory nerve fibre degeneration. The most vulnerable medium- and high-threshold-auditory nerve fibres innervate various cell types in the cochlear nucleus, among which the small cells are unique in receiving this input exclusively. Furthermore, small cells project to medial olivocochlear (MOC) neurons, which in turn send branched collaterals back into the small cell cap. Here, we use single-unit recordings to characterise small cell firing characteristics and demonstrate superior intensity coding in this cell class. We show converse effects when activating/blocking the MOC system, demonstrating that small-cell unique coding properties are facilitated by direct cholinergic input from the MOC system. Small cells also maintain tone-level coding in the presence of background noise. Finally, small cells precisely code low-frequency modulation more accurately than other ventral cochlear nucleus cell types, demonstrating accurate envelope coding that may be important for vocalisation processing. These results highlight the small cell olivocochlear circuit as a key player in signal processing in noisy environments, which may be selectively degraded in ageing or after noise insult. KEY POINTS: Cochlear nucleus small cells receive input from low/medium spontaneous rate auditory nerve fibres and medial olivocochlear neurons. Electrical stimulation of medial olivocochlear neurons in the ventral nucleus of the trapezoid body and blocking cholinergic input to small cells using atropine demonstrates an excitatory cholinergic input to small cells, which increases responses to suprathreshold sound. Unique inputs to small cells produce superior sound intensity coding. This coding of intensity is preserved in the presence of background noise, an effect exclusive to this cell type in the cochlear nucleus. These results suggest that small cells serve an essential function in the ascending auditory system, which may be relevant to disorders such as hidden hearing loss.

Expression Patterns of the Neuropeptide Urocortin 3 and Its Receptor CRFR2 in the Mouse Central Auditory System

Sensory systems have to be malleable to context-dependent modulations occurring over different time scales, in order to serve their evolutionary function of informing about the external world while also eliciting survival-promoting behaviors. Stress is a major context-dependent signal that can have fast and delayed effects on sensory systems, especially on the auditory system. Urocortin 3 (UCN3) is a member of the corticotropin-releasing factor family. As a neuropeptide, UCN3 regulates synaptic activity much faster than the classic steroid hormones of the hypothalamic-pituitary-adrenal axis. Moreover, due to the lack of synaptic re-uptake mechanisms, UCN3 can have more long-lasting and far-reaching effects. To date, a modest number of studies have reported the presence of UCN3 or its receptor CRFR2 in the auditory system, particularly in the cochlea and the superior olivary complex, and have highlighted the importance of this stress neuropeptide for protecting auditory function. However, a comprehensive map of all neurons synthesizing UCN3 or CRFR2 within the auditory pathway is lacking. Here, we utilize two reporter mouse lines to elucidate the expression patterns of UCN3 and CRFR2 in the auditory system. Additional immunolabelling enables further characterization of the neurons that synthesize UCN3 or CRFR2. Surprisingly, our results indicate that within the auditory system, UCN3 is expressed predominantly in principal cells, whereas CRFR2 expression is strongest in non-principal, presumably multisensory, cell types. Based on the presence or absence of overlap between UCN3 and CRFR2 labeling, our data suggest unusual modes of neuromodulation by UCN3, involving volume transmission and autocrine signaling.

Effects of Multisession Anodal Electrical Stimulation of the Auditory Cortex on Temporary Noise-Induced Hearing Loss in the Rat

The protective effect of the efferent system against acoustic trauma (AT) has been shown by several experimental approaches, including damage to one ear, sectioning of the olivocochlear bundle (OCB) in the floor of the IV ventricle, and knock-in mice overexpressing outer hair cell (OHC) cholinergic receptors, among others. Such effects have been related to changes in the regulation of the cholinergic efferent system and in cochlear amplification, which ultimately reverse upon protective hearing suppression. In addition to well-known circuits of the brainstem, the descending corticofugal pathway also regulates efferent neurons of the olivary complex. In this study, we applied our recently developed experimental paradigm of multiple sessions of electrical stimulation (ES) to activate the efferent system in combination with noise overstimulation. ABR thresholds increased 1 and 2 days after AT (8-16 kHz bandpass noise at 107 dB for 90 min) recovering at AT + 14 days. However, after multiple sessions of epidural anodal stimulation, no changes in thresholds were observed following AT. Although an inflammatory response was also observed 1 day after AT in both groups, the counts of reactive macrophages in both experimental conditions suggest decreased inflammation in the epidural stimulation group. Quantitative immunocytochemistry for choline acetyltransferase (ChAT) showed a significant decrease in the size and optical density of the efferent terminals 1 day after AT and a rebound at 14 days, suggesting depletion of the terminals followed by a long-term compensatory response. Such a synthesis recovery was significantly higher upon cortical stimulation. No significant correlation was found between ChAT optical density and size of the buttons in sham controls (SC) and ES/AT + 1day animals; however, significant negative correlations were shown in all other experimental conditions. Therefore, our comparative analysis suggests that cochleotopic cholinergic neurotransmission is also better preserved after multisession epidural stimulation.

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.

The medial olivocochlear reflex strength is modulated during a visual working memory task

Top-down modulation of sensory responses to distracting stimuli by selective attention has been proposed as an important mechanism by which our brain can maintain relevant information during working memory tasks. Previous works in visual working memory (VWM) have reported modulation of neural responses to distracting sounds at different levels of the central auditory pathways. Whether these modulations occur also at the level of the auditory receptor is unknown. Here, we hypothesize that cochlear responses to irrelevant auditory stimuli can be modulated by the medial olivocochlear system during VWM. Twenty-one subjects (13 males, mean age 25.3 yr) with normal hearing performed a visual change detection task with different VWM load conditions (high load = 4 visual objects; low load = 2 visual objects). Auditory stimuli were presented as distractors and allowed the measurement of distortion product otoacoustic emissions (DPOAEs) and scalp auditory evoked potentials. In addition, the medial olivocochlear reflex strength was evaluated by adding contralateral acoustic stimulation. We found larger contralateral acoustic suppression of DPOAEs during the visual working memory period (n = 21) compared with control experiments (n = 10), in which individuals were passively exposed to the same experimental conditions. These results show that during the visual working memory period there is a modulation of the medial olivocochlear reflex strength, suggesting a possible common mechanism for top-down filtering of auditory responses during cognitive processes.NEW & NOTEWORTHY The auditory efferent system has been proposed to function as a biological filter of cochlear responses during selective attention. Here, we recorded electroencephalographic activity and otoacoustic emissions in response to auditory distractors during a visual working memory task in humans. We found that the olivocochlear efferent activity is modulated during the visual working memory period suggesting a common mechanism for suppressing cochlear responses during selective attention and working memory.

In Vitro Wedge Slice Preparation for Mimicking In Vivo Neuronal Circuit Connectivity

In vitro slice electrophysiology techniques measure single-cell activity with precise electrical and temporal resolution. Brain slices must be relatively thin to properly visualize and access neurons for patch-clamping or imaging, and in vitro examination of brain circuitry is limited to only what is physically present in the acute slice. To maintain the benefits of in vitro slice experimentation while preserving a larger portion of presynaptic nuclei, we developed a novel slice preparation. This "wedge slice" was designed for patch-clamp electrophysiology recordings to characterize the diverse monaural, sound-driven inputs to medial olivocochlear (MOC) neurons in the brainstem. These neurons receive their primary afferent excitatory and inhibitory inputs from neurons activated by stimuli in the contralateral ear and corresponding cochlear nucleus (CN). An asymmetrical brain slice was designed which is thickest in the rostro-caudal domain at the lateral edge of one hemisphere and then thins towards the lateral edge of the opposite hemisphere. This slice contains, on the thick side, the auditory nerve root conveying information about auditory stimuli to the brain, the intrinsic CN circuitry, and both the disynaptic excitatory and trisynaptic inhibitory afferent pathways that converge on contralateral MOC neurons. Recording is performed from MOC neurons on the thin side of the slice, where they are visualized using DIC optics for typical patch-clamp experiments. Direct stimulation of the auditory nerve is performed as it enters the auditory brainstem, allowing for intrinsic CN circuit activity and synaptic plasticity to occur at synapses upstream of MOC neurons. With this technique, one can mimic in vivo circuit activation as closely as possible within the slice. This wedge slice preparation is applicable to other brain circuits where circuit analyses would benefit from preservation of upstream connectivity and long-range inputs, in combination with the technical advantages of in vitro slice physiology.

Chemical Effects of Kainic Acid Injection into the Rat Superior Olivary Region

Kainic acid injections have been used to destroy neuron somata in particular regions without damaging fiber tracts. We injected a solution of kainic acid into the region of the rat superior olivary complex in an effort to destroy its cholinergic projections to the cochlea and cochlear nucleus, which derive especially from the lateral superior olivary nucleus and ventral nucleus of the trapezoid body. In the lateral superior olivary nucleus, there were relatively small but fairly consistent decreases of choline acetyltransferase (ChAT) activity, larger decreases of acetylcholinesterase (AChE) activity, and consistent decreases of malate dehydrogenase activity, as a marker for oxidative metabolism. Other superior olivary regions were less affected by the kainic acid injections, but most showed overall significant decreases of AChE activity. Our results suggest that the cholinergic neurons giving rise to the centrifugal pathways to the cochlea and cochlear nucleus are more resistant to the effects of kainic acid than are those that receive major ascending input from the cochlear nucleus and project to higher levels of the auditory system. Comparison with published anatomical studies suggests that this resistance to the effects of kainic acid is related to relatively little glutamatergic input to the somata and proximal dendrites of these neurons. We also found a consistent approximately 16 % decrease of ChAT activity in the injected-side facial nerve root, which is most easily explained as a small effect of kainic acid on the facial nerve fibers passing through the injection site.

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.

Glutamatergic Projections to the Cochlear Nucleus are Redistributed in Tinnitus

Tinnitus alters auditory-somatosensory plasticity in the cochlear nucleus (CN). Correspondingly, bimodal auditory-somatosensory stimulation treatment attenuates tinnitus, both in animals and humans (Marks et al., 2018). Therefore, we hypothesized that tinnitus is associated with altered somatosensory innervation of the CN. Here, we studied the expression of vesicular glutamate transporters 1 and 2 (VGLUT1 and VGLUT2) in the CN, which reveals glutamatergic projections from the cochlea as well as somatosensory systems to this brainstem auditory center. Guinea pigs were unilaterally exposed to narrowband noise and behaviorally tested for tinnitus using gap-prepulse inhibition of the acoustic startle. Following physiological and behavioral measures, brain sections were immunohistochemically stained for VGLUT1 or VGLUT2. Puncta density was determined for each region of the ipsilateral and contralateral CN. Tinnitus was associated with an ipsilateral upregulation of VGLUT2 puncta density in the granule cell domain (GCD) and anteroventral CN (AVCN). Furthermore, there was a tinnitus-associated interaural asymmetry for VGLUT1 expression in the AVCN and deep layer of the dorsal CN (DCN3), due to contralateral downregulation of VGLUT1 expression. These tinnitus-related glutamatergic imbalances were reversed upon bimodal stimulation treatment. Tinnitus-associated ipsilateral upregulation of VGLUT2-positive projections likely derives from somatosensory projections to the GCD and AVCN. This upregulation may underlie the neurophysiological hallmarks of tinnitus in the CN. Reversing the increased ipsilateral glutamatergic innervation in the CN is likely a key mechanism in treating tinnitus.

GC-B Deficient Mice With Axon Bifurcation Loss Exhibit Compromised Auditory Processing

Sensory axon T-like branching (bifurcation) in neurons from dorsal root ganglia and cranial sensory ganglia depends on the molecular signaling cascade involving the secreted factor C-type natriuretic peptide, the natriuretic peptide receptor guanylyl cyclase B (GC-B; also known as Npr2) and cGMP-dependent protein kinase I (cGKI, also known as PKGI). The bifurcation of cranial nerves is suggested to be important for information processing by second-order neurons in the hindbrain or spinal cord. Indeed, mice with a spontaneous GC-B loss of function mutation (Npr2^cn/cn ) display an impaired bifurcation of auditory nerve (AN) fibers. However, these mice did not show any obvious sign of impaired basal hearing. Here, we demonstrate that mice with a targeted inactivation of the GC-B gene (Npr2 ^lacZ/lacZ , GC-B KO mice) show an elevation of audiometric thresholds. In the inner ear, the cochlear hair cells in GC-B KO mice were nevertheless similar to those from wild type mice, justified by the typical expression of functionally relevant marker proteins. However, efferent cholinergic feedback to inner and outer hair cells was reduced in GC-B KO mice, linked to very likely reduced rapid efferent feedback. Sound-evoked AN responses of GC-B KO mice were elevated, a feature that is known to occur when the efferent axo-dendritic feedback on AN is compromised. Furthermore, late sound-evoked brainstem responses were significantly delayed in GC-B KO mice. This delay in sound response was accompanied by a weaker sensitivity of the auditory steady state response to amplitude-modulated sound stimuli. Finally, the acoustic startle response (ASR) - one of the fastest auditory responses - and the prepulse inhibition of the ASR indicated significant changes in temporal precision of auditory processing. These findings suggest that GC-B-controlled axon bifurcation of spiral ganglion neurons is important for proper activation of second-order neurons in the hindbrain and is a prerequisite for proper temporal auditory processing likely by establishing accurate efferent top-down control circuits. These data hypothesize that the bifurcation pattern of cranial nerves is important to shape spatial and temporal information processing for sensory feedback control.

Click-Evoked Auditory Efferent Activity: Rate and Level Effects

There currently are no standardized protocols to evaluate auditory efferent function in humans. Typical tests use broadband noise to activate the efferents, but only test the contralateral efferent pathway, risk activating the middle ear muscle reflex (MEMR), and are laborious for clinical use. In an attempt to develop a clinical test of bilateral auditory efferent function, we have designed a method that uses clicks to evoke efferent activity, obtain click-evoked otoacoustic emissions (CEOAEs), and monitor MEMR. This allows for near-simultaneous estimation of cochlear and efferent function. In the present study, we manipulated click level (60, 70, and 80 dB peak-equivalent sound pressure level [peSPL]) and rate (40, 50, and 62.5 Hz) to identify an optimal rate-level combination that evokes measurable efferent modulation of CEOAEs. Our findings (n = 58) demonstrate that almost all click levels and rates used caused significant inhibition of CEOAEs, with a significant interaction between level and rate effects. Predictably, bilateral activation produced greater inhibition compared to stimulating the efferents only in the ipsilateral or contralateral ear. In examining the click rate-level effects during bilateral activation in greater detail, we observed a 1-dB inhibition of CEOAE level for each 10-dB increase in click level, with rate held constant at 62.5 Hz. Similarly, a 10-Hz increase in rate produced a 0.74-dB reduction in CEOAE level, with click level held constant at 80 dB peSPL. The effect size (Cohen's d) was small for either monaural condition and medium for bilateral, faster-rate, and higher-level conditions. We were also able to reliably extract CEOAEs from efferent eliciting clicks. We conclude that clicks can indeed be profitably employed to simultaneously evaluate cochlear health using CEOAEs as well as their efferent modulation. Furthermore, using bilateral clicks allows the evaluation of both the crossed and uncrossed elements of the auditory efferent nervous system, while yielding larger, more discernible, inhibition of the CEOAEs relative to either ipsilateral or contralateral condition.

Increased medial olivocochlear reflex strength in normal-hearing, noise-exposed humans

Research suggests that college-aged adults are vulnerable to tinnitus and hearing loss due to exposure to traumatic levels of noise on a regular basis. Recent human studies have associated exposure to high noise exposure background (NEB, i.e., routine noise exposure) with the reduced cochlear output and impaired speech processing ability in subjects with clinically normal hearing sensitivity. While the relationship between NEB and the functions of the auditory afferent neurons are studied in the literature, little is known about the effects of NEB on functioning of the auditory efferent system. The objective of the present study was to investigate the relationship between medial olivocochlear reflex (MOCR) strength and NEB in subjects with clinically normal hearing sensitivity. It was hypothesized that subjects with high NEB would exhibit reduced afferent input to the MOCR circuit which would subsequently lead to reduced strength of the MOCR. In normal-hearing listeners, the study examined (1) the association between NEB and baseline click-evoked otoacoustic emissions (CEOAEs) and (2) the association between NEB and MOCR strength. The MOCR was measured using CEOAEs evoked by 60 dB pSPL linear clicks in a contralateral acoustic stimulation (CAS)-off and CAS-on (a broadband noise at 60 dB SPL) condition. Participants with at least 6 dB signal-to-noise ratio (SNR) in the CAS-off and CAS-on conditions were included for analysis. A normalized CEOAE inhibition index was calculated to express MOCR strength in a percentage value. NEB was estimated using a validated questionnaire. The results showed that NEB was not associated with the baseline CEOAE amplitude (r = -0.112, p = 0.586). Contrary to the hypothesis, MOCR strength was positively correlated with NEB (r = 0.557, p = 0.003). NEB remained a significant predictor of MOCR strength (β = 2.98, t(19) = 3.474, p = 0.003) after the unstandardized coefficient was adjusted to control for effects of smoking, sound level tolerance (SLT) and tinnitus. These data provide evidence that MOCR strength is associated with NEB. The functional significance of increased MOCR strength is discussed.

Strain-specific differences in the development of neuronal excitability in the mouse ventral nucleus of the trapezoid body

This investigation compared the development of neuronal excitability in the ventral nucleus of the trapezoid body (VNTB) between two strains of mice with differing progression rates for age-related hearing loss. In contrast to CBA/Ca (CBA) mice, the C57BL/6J (C57) strain are subject to hearing loss from a younger age and are more prone to damage from sound over-exposure. Higher firing rates in the medial olivocochlear system (MOC) are associated with protection from loud sounds and these cells are located in the VNTB. We postulated that reduced neuronal firing of the MOC in C57 mice could contribute to hearing loss in this strain by reducing efferent protection. Whole cell patch clamp was used to compare the electrical properties of VNTB neurons from the two strains initially in two age groups: before and after hearing onset at ∼ P9 and ∼P16, respectively. Prior to hearing onset VNTB neurons electrophysiological properties were identical in both strains, but started to diverge after hearing onset. One week after hearing onset VNTB neurons of C57 mice had larger amplitude action potentials but in contrast to CBA mice, their waveform failed to accelerate with increasing age, consistent with the faster inactivation of voltage-gated potassium currents in C57 VNTB neurons. The lower frequency action potential firing of C57 VNTB neurons at P16 was maintained to P28, indicating that this change was not a developmental delay. We conclude that C57 VNTB neurons fire at lower frequencies than in the CBA strain, supporting the hypothesis that reduced MOC firing could contribute to the greater hearing loss of the C57 strain.

Cellular and Molecular Underpinnings of Neuronal Assembly in the Central Auditory System during Mouse Development

During development, the organization of the auditory system into distinct functional subcircuits depends on the spatially and temporally ordered sequence of neuronal specification, differentiation, migration and connectivity. Regional patterning along the antero-posterior axis and neuronal subtype specification along the dorso-ventral axis intersect to determine proper neuronal fate and assembly of rhombomere-specific auditory subcircuits. By taking advantage of the increasing number of transgenic mouse lines, recent studies have expanded the knowledge of developmental mechanisms involved in the formation and refinement of the auditory system. Here, we summarize several findings dealing with the molecular and cellular mechanisms that underlie the assembly of central auditory subcircuits during mouse development, focusing primarily on the rhombomeric and dorso-ventral origin of auditory nuclei and their associated molecular genetic pathways.

The Corticofugal Effects of Auditory Cortex Microstimulation on Auditory Nerve and Superior Olivary Complex Responses Are Mediated via Alpha-9 Nicotinic Receptor Subunit

Background and Objective

The auditory efferent system is a complex network of descending pathways, which mainly originate in the primary auditory cortex and are directed to several auditory subcortical nuclei. These descending pathways are connected to olivocochlear neurons, which in turn make synapses with auditory nerve neurons and outer hair cells (OHC) of the cochlea. The olivocochlear function can be studied using contralateral acoustic stimulation, which suppresses auditory nerve and cochlear responses. In the present work, we tested the proposal that the corticofugal effects that modulate the strength of the olivocochlear reflex on auditory nerve responses are produced through cholinergic synapses between medial olivocochlear (MOC) neurons and OHCs via alpha-9/10 nicotinic receptors.

Methods

We used wild type (WT) and alpha-9 nicotinic receptor knock-out (KO) mice, which lack cholinergic transmission between MOC neurons and OHC, to record auditory cortex evoked potentials and to evaluate the consequences of auditory cortex electrical microstimulation in the effects produced by contralateral acoustic stimulation on auditory brainstem responses (ABR).

Results

Auditory cortex evoked potentials at 15 kHz were similar in WT and KO mice. We found that auditory cortex microstimulation produces an enhancement of contralateral noise suppression of ABR waves I and III in WT mice but not in KO mice. On the other hand, corticofugal modulations of wave V amplitudes were significant in both genotypes.

Conclusion

These findings show that the corticofugal modulation of contralateral acoustic suppressions of auditory nerve (ABR wave I) and superior olivary complex (ABR wave III) responses are mediated through MOC synapses.

Corticofugal modulation of peripheral auditory responses

The auditory efferent system originates in the auditory cortex and projects to the medial geniculate body (MGB), inferior colliculus (IC), cochlear nucleus (CN) and superior olivary complex (SOC) reaching the cochlea through olivocochlear (OC) fibers. This unique neuronal network is organized in several afferent-efferent feedback loops including: the (i) colliculo-thalamic-cortico-collicular; (ii) cortico-(collicular)-OC; and (iii) cortico-(collicular)-CN pathways. Recent experiments demonstrate that blocking ongoing auditory-cortex activity with pharmacological and physical methods modulates the amplitude of cochlear potentials. In addition, auditory-cortex microstimulation independently modulates cochlear sensitivity and the strength of the OC reflex. In this mini-review, anatomical and physiological evidence supporting the presence of a functional efferent network from the auditory cortex to the cochlear receptor is presented. Special emphasis is given to the corticofugal effects on initial auditory processing, that is, on CN, auditory nerve and cochlear responses. A working model of three parallel pathways from the auditory cortex to the cochlea and auditory nerve is proposed.

Stronger efferent suppression of cochlear neural potentials by contralateral acoustic stimulation in awake than in anesthetized chinchilla

There are two types of sensory cells in the mammalian cochlea, inner hair cells, which make synaptic contact with auditory-nerve afferent fibers, and outer hair cells that are innervated by crossed and uncrossed medial olivocochlear (MOC) efferent fibers. Contralateral acoustic stimulation activates the uncrossed efferent MOC fibers reducing cochlear neural responses, thus modifying the input to the central auditory system. The chinchilla, among all studied mammals, displays the lowest percentage of uncrossed MOC fibers raising questions about the strength and frequency distribution of the contralateral-sound effect in this species. On the other hand, MOC effects on cochlear sensitivity have been mainly studied in anesthetized animals and since the MOC-neuron activity depends on the level of anesthesia, it is important to assess the influence of anesthesia in the strength of efferent effects. Seven adult chinchillas (Chinchilla laniger) were chronically implanted with round-window electrodes in both cochleae. We compared the effect of contralateral sound in awake and anesthetized condition. Compound action potentials (CAP) and cochlear microphonics (CM) were measured in the ipsilateral cochlea in response to tones in absence and presence of contralateral sound. Control measurements performed after middle-ear muscles section in one animal discarded any possible middle-ear reflex activation. Contralateral sound produced CAP amplitude reductions in all chinchillas, with suppression effects greater by about 1–3 dB in awake than in anesthetized animals. In contrast, CM amplitude increases of up to 1.9 dB were found in only three awake chinchillas. In both conditions the strongest efferent effects were produced by contralateral tones at frequencies equal or close to those of ipsilateral tones. Contralateral CAP suppressions for 1–6 kHz ipsilateral tones corresponded to a span of uncrossed MOC fiber innervation reaching at least the central third of the chinchilla cochlea.

The olivocochlear reflex strength and cochlear sensitivity are independently modulated by auditory cortex microstimulation

In mammals, efferent projections to the cochlear receptor are constituted by olivocochlear (OC) fibers that originate in the superior olivary complex. Medial and lateral OC neurons make synapses with outer hair cells and with auditory nerve fibers, respectively. In addition to the OC system, there are also descending projections from the auditory cortex that are directed towards the thalamus, inferior colliculus, cochlear nucleus, and superior olivary complex. Olivocochlear function can be assessed by measuring a brainstem reflex mediated by auditory nerve fibers, cochlear nucleus neurons, and OC fibers. Although it is known that the OC reflex is activated by contralateral acoustic stimulation and produces a suppression of cochlear responses, the influence of cortical descending pathways in the OC reflex is largely unknown. Here, we used auditory cortex electrical microstimulation in chinchillas to study a possible cortical modulation of cochlear and auditory nerve responses to tones in the absence and presence of contralateral noise. We found that cortical microstimulation produces two different peripheral modulations: (i) changes in cochlear sensitivity evidenced by amplitude modulation of cochlear microphonics and auditory nerve compound action potentials and (ii) enhancement or suppression of the OC reflex strength as measured by auditory nerve responses, which depended on the intersubject variability of the OC reflex. Moreover, both corticofugal effects were not correlated, suggesting the presence of two functionally different efferent pathways. These results demonstrate that auditory cortex electrical microstimulation independently modulates the OC reflex strength and cochlear sensitivity.

Increased contralateral suppression of otoacoustic emissions indicates a hyperresponsive medial olivocochlear system in humans with tinnitus and hyperacusis

Atypical medial olivocochlear (MOC) feedback from brain stem to cochlea has been proposed to play a role in tinnitus, but even well-constructed tests of this idea have yielded inconsistent results. In the present study, it was hypothesized that low sound tolerance (mild to moderate hyperacusis), which can accompany tinnitus or occur on its own, might contribute to the inconsistency. Sound-level tolerance (SLT) was assessed in subjects (all men) with clinically normal or near-normal thresholds to form threshold-, age-, and sex-matched groups: 1) no tinnitus/high SLT, 2) no tinnitus/low SLT, 3) tinnitus/high SLT, and 4) tinnitus/low SLT. MOC function was measured from the ear canal as the change in magnitude of distortion-product otoacoustic emissions (DPOAE) elicited by broadband noise presented to the contralateral ear. The noise reduced DPOAE magnitude in all groups ("contralateral suppression"), but significantly more reduction occurred in groups with tinnitus and/or low SLT, indicating hyperresponsiveness of the MOC system compared with the group with no tinnitus/high SLT. The results suggest hyperresponsiveness of the interneurons of the MOC system residing in the cochlear nucleus and/or MOC neurons themselves. The present data, combined with previous human and animal data, indicate that neural pathways involving every major division of the cochlear nucleus manifest hyperactivity and/or hyperresponsiveness in tinnitus and/or low SLT. The overactivation may develop in each pathway separately. However, a more parsimonious hypothesis is that top-down neuromodulation is the driving force behind ubiquitous overactivation of the auditory brain stem and may correspond to attentional spotlighting on the auditory domain in tinnitus and hyperacusis.

Hox genes and region-specific sensorimotor circuit formation in the hindbrain and spinal cord

Homeobox (Hox) genes were originally discovered in the fruit fly Drosophila, where they function through a conserved homeodomain as transcriptional regulators to control embryonic morphogenesis. In vertebrates, 39 Hox genes have been identified and like their Drosophila counterparts they are organized within chromosomal clusters. Hox genes interact with various cofactors, such as the TALE homeodomain proteins, in recognition of consensus sequences within regulatory elements of their target genes. In vertebrates, Hox genes display spatially restricted patterns of expression within the developing hindbrain and spinal cord, and are considered crucial determinants of segmental identity and cell specification along the anterioposterior and dorsoventral axes of the embryo. Here, we review their later roles in the assembly of neuronal circuitry, in stereotypic neuronal migration, axon pathfinding, and topographic connectivity. Importantly, we will put some emphasis on how their early-segmented expression patterns can influence the formation of complex vital hindbrain and spinal cord circuitries.

Identification of inputs to olivocochlear neurons using transneuronal labeling with pseudorabies virus (PRV)

Olivocochlear (OC) neurons respond to sound and provide descending input that controls processing in the cochlea. The identities of neurons in the pathways providing inputs to OC neurons are incompletely understood. To explore these pathways, the retrograde transneuronal tracer pseudorabies virus (Bartha strain, expressing green fluorescent protein) was used to label OC neurons and their inputs in guinea pigs. Labeling of OC neurons began 1 day after injection into the cochlea. On day 2 (and for longer survival times), transneuronal labeling spread to the cochlear nucleus, inferior colliculus, and other brainstem areas. There was a correlation between the numbers of these transneuronally labeled neurons and the number of labeled medial (M) OC neurons, suggesting that the spread of labeling proceeds mainly via synapses on MOC neurons. In the cochlear nucleus, the transneuronally labeled neurons were multipolar cells including the subtype known as planar cells. In the central nucleus of the inferior colliculus, transneuronally labeled neurons were of two principal types: neurons with disc-shaped dendritic fields and neurons with dendrites in a stellate pattern. Transneuronal labeling was also observed in pyramidal cells in the auditory cortex and in centers not typically associated with the auditory pathway such as the pontine reticular formation, subcoerulean nucleus, and the pontine dorsal raphe. These data provide information on the identity of neurons providing input to OC neurons, which are located in auditory as well as non-auditory centers.

Ultrastructure of spines and associated terminals on brainstem neurons controlling auditory input

Spines are unique cellular appendages that isolate synaptic input to neurons and play a role in synaptic plasticity. Using the electron microscope, we studied spines and their associated synaptic terminals on three groups of brainstem neurons: tensor tympani motoneurons, stapedius motoneurons, and medial olivocochlear neurons, all of which exert reflexive control of processes in the auditory periphery. These spines are generally simple in shape; they are infrequent and found on the somata as well as the dendrites. Spines do not differ in volume among the three groups of neurons. In all cases, the spines are associated with a synaptic terminal that engulfs the spine rather than abuts its head. The positions of the synapses are variable, and some are found at a distance from the spine, suggesting that the isolation of synaptic input is of diminished importance for these spines. Each group of neurons receives three common types of synaptic terminals. The type of terminal associated with spines of the motoneurons contains pleomorphic vesicles, whereas the type associated with spines of olivocochlear neurons contains large round vesicles. Thus, spine-associated terminals in the motoneurons appear to be associated with inhibitory processes but in olivocochlear neurons they are associated with excitatory processes.

Assembly of the auditory circuitry by a Hox genetic network in the mouse brainstem

Rhombomeres (r) contribute to brainstem auditory nuclei during development. Hox genes are determinants of rhombomere-derived fate and neuronal connectivity. Little is known about the contribution of individual rhombomeres and their associated Hox codes to auditory sensorimotor circuitry. Here, we show that r4 contributes to functionally linked sensory and motor components, including the ventral nucleus of lateral lemniscus, posterior ventral cochlear nuclei (VCN), and motor olivocochlear neurons. Assembly of the r4-derived auditory components is involved in sound perception and depends on regulatory interactions between Hoxb1 and Hoxb2. Indeed, in Hoxb1 and Hoxb2 mutant mice the transmission of low-level auditory stimuli is lost, resulting in hearing impairments. On the other hand, Hoxa2 regulates the Rig1 axon guidance receptor and controls contralateral projections from the anterior VCN to the medial nucleus of the trapezoid body, a circuit involved in sound localization. Thus, individual rhombomeres and their associated Hox codes control the assembly of distinct functionally segregated sub-circuits in the developing auditory brainstem.

Sound perception and sound localization are controlled by two distinct circuits in the central nervous system. However, the cellular and molecular determinants underlying their development are poorly understood. Here, we show that a spatially restricted region of the brainstem, the rhombomere 4, and two members of the Hox gene family, Hoxb1 and Hoxb2, are directly implicated in the development of the circuit leading to sound perception and sound amplification. In the absence of Hoxb1 and Hoxb2 function, we found severe morphological defects in the hair cell population implicated in transducing the acoustic signal, leading ultimately to severe hearing impairments in adult mutant mice. In contrast, the expression in the cochlear nucleus of another Hox member, Hoxa2, regulates the guidance receptor Rig1 and contralateral connectivity in the sound localization circuit. Some of the auditory dysfunctions described in our mouse models resemble pathological hearing conditions in humans, in which patients have an elevated hearing threshold sensitivity, as recorded in audiograms. Thus, this study provides mechanistic insight into the genetic and functional regulation of Hox genes during development and assembly of the auditory system.

Planar multipolar cells in the cochlear nucleus project to medial olivocochlear neurons in mouse

Medial olivocochlear (MOC) neurons originate in the superior olivary complex and project to the cochlea, where they act to reduce the effects of noise masking and protect the cochlea from damage. MOC neurons respond to sound via a reflex pathway; however, in this pathway the cochlear nucleus cell type that provides input to MOC neurons is not known. We investigated whether multipolar cells of the ventral cochlear nucleus have projections to MOC neurons by labeling them with injections into the dorsal cochlear nucleus. The projections of one type of labeled multipolar cell, planar neurons, were traced into the ventral nucleus of the trapezoid body, where they were observed terminating on MOC neurons (labeled in some cases by a second cochlear injection of FluoroGold). These terminations formed what appear to be excitatory synapses, i.e., containing small, round vesicles and prominent postsynaptic densities. These data suggest that cochlear nucleus planar multipolar neurons drive the MOC neuron's response to sound.

Auditory cortex basal activity modulates cochlear responses in chinchillas

BACKGROUND

The auditory efferent system has unique neuroanatomical pathways that connect the cerebral cortex with sensory receptor cells. Pyramidal neurons located in layers V and VI of the primary auditory cortex constitute descending projections to the thalamus, inferior colliculus, and even directly to the superior olivary complex and to the cochlear nucleus. Efferent pathways are connected to the cochlear receptor by the olivocochlear system, which innervates outer hair cells and auditory nerve fibers. The functional role of the cortico-olivocochlear efferent system remains debated. We hypothesized that auditory cortex basal activity modulates cochlear and auditory-nerve afferent responses through the efferent system.

METHODOLOGY/PRINCIPAL FINDINGS

Cochlear microphonics (CM), auditory-nerve compound action potentials (CAP) and auditory cortex evoked potentials (ACEP) were recorded in twenty anesthetized chinchillas, before, during and after auditory cortex deactivation by two methods: lidocaine microinjections or cortical cooling with cryoloops. Auditory cortex deactivation induced a transient reduction in ACEP amplitudes in fifteen animals (deactivation experiments) and a permanent reduction in five chinchillas (lesion experiments). We found significant changes in the amplitude of CM in both types of experiments, being the most common effect a CM decrease found in fifteen animals. Concomitantly to CM amplitude changes, we found CAP increases in seven chinchillas and CAP reductions in thirteen animals. Although ACEP amplitudes were completely recovered after ninety minutes in deactivation experiments, only partial recovery was observed in the magnitudes of cochlear responses.

CONCLUSIONS/SIGNIFICANCE

These results show that blocking ongoing auditory cortex activity modulates CM and CAP responses, demonstrating that cortico-olivocochlear circuits regulate auditory nerve and cochlear responses through a basal efferent tone. The diversity of the obtained effects suggests that there are at least two functional pathways from the auditory cortex to the cochlea.

Frequency tuning of the contralateral medial olivocochlear reflex in humans

Activation of the medial olivocochlear (MOC) efferents attenuates cochlear gain and reduces the amplitudes of mechanical, electrical, and neural cochlear outputs. The functional roles of the MOC efferents are not fully understood, especially in humans, despite postulations that they are involved in protection against acoustic trauma, facilitation of transient-sound perception, etc. Delineating the frequency tuning properties of the MOC efferents would provide critical evidence to support or refute these postulated functional roles. By utilizing spontaneous otoacoustic emissions (SOAEs), a cochlear measure sensitive to MOC modulation, we systematically demonstrate in humans that the contralateral MOC reflex is tuned to a fixed frequency band between 500 and 1,000 Hz independent of SOAE frequency. Our results question the role of the MOC reflex in protection against acoustic trauma or facilitation of transient-sound perception.

The multiple functions of T stellate/multipolar/chopper cells in the ventral cochlear nucleus

Acoustic information is brought to the brain by auditory nerve fibers, all of which terminate in the cochlear nuclei, and is passed up the auditory pathway through the principal cells of the cochlear nuclei. A population of neurons variously known as T stellate, type I multipolar, planar multipolar, or chopper cells forms one of the major ascending auditory pathways through the brainstem. T Stellate cells are sharply tuned; as a population they encode the spectrum of sounds. In these neurons, phasic excitation from the auditory nerve is made more tonic by feedforward excitation, coactivation of inhibitory with excitatory inputs, relatively large excitatory currents through NMDA receptors, and relatively little synaptic depression. The mechanisms that make firing tonic also obscure the fine structure of sounds that is represented in the excitatory inputs from the auditory nerve and account for the characteristic chopping response patterns with which T stellate cells respond to tones. In contrast with other principal cells of the ventral cochlear nucleus (VCN), T stellate cells lack a low-voltage-activated potassium conductance and are therefore sensitive to small, steady, neuromodulating currents. The presence of cholinergic, serotonergic and noradrenergic receptors allows the excitability of these cells to be modulated by medial olivocochlear efferent neurons and by neuronal circuits associated with arousal. T Stellate cells deliver acoustic information to the ipsilateral dorsal cochlear nucleus (DCN), ventral nucleus of the trapezoid body (VNTB), periolivary regions around the lateral superior olivary nucleus (LSO), and to the contralateral ventral lemniscal nuclei (VNLL) and inferior colliculus (IC). It is likely that T stellate cells participate in feedback loops through both medial and lateral olivocochlear efferent neurons and they may be a source of ipsilateral excitation of the LSO.

Auditory brainstem circuits that mediate the middle ear muscle reflex

The middle ear muscle (MEM) reflex is one of two major descending systems to the auditory periphery. There are two middle ear muscles (MEMs): the stapedius and the tensor tympani. In man, the stapedius contracts in response to intense low frequency acoustic stimuli, exerting forces perpendicular to the stapes superstructure, increasing middle ear impedance and attenuating the intensity of sound energy reaching the inner ear (cochlea). The tensor tympani is believed to contract in response to self-generated noise (chewing, swallowing) and non-auditory stimuli. The MEM reflex pathways begin with sound presented to the ear. Transduction of sound occurs in the cochlea, resulting in an action potential that is transmitted along the auditory nerve to the cochlear nucleus in the brainstem (the first relay station for all ascending sound information originating in the ear). Unknown interneurons in the ventral cochlear nucleus project either directly or indirectly to MEM motoneurons located elsewhere in the brainstem. Motoneurons provide efferent innervation to the MEMs. Although the ascending and descending limbs of these reflex pathways have been well characterized, the identity of the reflex interneurons is not known, as are the source of modulatory inputs to these pathways. The aim of this article is to (a) provide an overview of MEM reflex anatomy and physiology, (b) present new data on MEM reflex anatomy and physiology from our laboratory and others, and (c) describe the clinical implications of our research.

Ventral cochlear nucleus responses to contralateral sound are mediated by commissural and olivocochlear pathways

In the normal guinea pig, contralateral sound inhibits more than a third of ventral cochlear nucleus (VCN) neurons but excites <4% of these neurons. However, unilateral conductive hearing loss (CHL) and cochlear ablation (CA) result in a major enhancement of contralateral excitation. The response properties of the contralateral excitation produced by CHL and CA are similar, suggesting similar pathways are involved for both types of hearing loss. Here we used the neurotoxin melittin to test the hypothesis that this "compensatory" contralateral excitation is mediated either by direct glutamatergic CN-commissural projections or by cholinergic neurons of the olivocochlear bundle (OCB) that send collaterals to the VCN. Unit responses were recorded from the left VCN of anesthetized, unilaterally deafened guinea pigs (CHL via ossicular disruption, or CA via mechanical destruction). Neural responses were obtained with 16-channel electrodes to enable simultaneous data collection from a large number of single- and multiunits in response to ipsi- and contralateral tone burst and noise stimuli. Lesions of each pathway had differential effects on the contralateral excitation. We conclude that contralateral excitation has a fast and a slow component. The fast excitation is likely mediated by glutamatergic neurons located in medial regions of VCN that send their commissural axons to the other CN via the dorsal/intermediate acoustic striae. The slow component is likely mediated by the OCB collateral projections to the CN. Commissural neurons that leave the CN via the trapezoid body are an additional source of fast, contralateral excitation.

Dendrites of medial olivocochlear neurons in mouse

Stains for acetylcholinesterase (AChE) and retrograde labeling with Fluorogold (FG) were used to study olivocochlear neurons and their dendritic patterns in mice. The two methods gave similar results for location and number of somata. The total number of medial olivocochlear (MOC) neurons in the ventral nucleus of the trapezoid body (VNTB) is about 170 per side. An additional dozen large olivocochlear neurons are located in the dorsal periolivary nucleus (DPO). Dendrites of all of these neurons are long and extend in all directions from the cell bodies, a pattern that contrasts with the sharp frequency tuning of their responses. For VNTB neurons, there were greater numbers of dendrites directed medially than laterally and those directed medially were longer (on average, 25-50% longer). Dendrite extensions were most pronounced for neurons located in the rostral portion of the VNTB. When each dendrite from a single neuron was represented as a vector, and all the vectors summed, the result was also skewed toward the medial direction. DPO neurons, however, had more symmetric dendrites that projected into more dorsal parts of the trapezoid body, suggesting that this small group of olivocochlear neurons has very different physiological properties. Dendrites of both types of neurons were somewhat elongated rostrally, about 20% longer than those directed caudally. These results can be interpreted as extensions of dendrites of olivocochlear neurons toward their synaptic inputs: medially to meet crossing fibers from the cochlear nucleus that are part of the MOC reflex pathway, and rostrally to meet descending inputs from higher centers.

Olivocochlear efferents: anatomy, physiology, function, and the measurement of efferent effects in humans

This review covers the basic anatomy and physiology of the olivocochlear reflexes and the use of otoacoustic emissions (OAEs) in humans to monitor the effects of one group, the medial olivocochlear (MOC) efferents. MOC fibers synapse on outer hair cells (OHCs), and activation of these fibers inhibits basilar membrane responses to low-level sounds. This MOC-induced decrease in the gain of the cochlear amplifier is reflected in changes in OAEs. Any OAE can be used to monitor MOC effects on the cochlear amplifier. Each OAE type has its own advantages and disadvantages. The most straightforward technique for monitoring MOC effects is to elicit MOC activity with an elicitor sound contralateral to the OAE test ear. MOC effects can also be monitored using an ipsilateral elicitor of MOC activity, but the ipsilateral elicitor brings additional problems caused by suppression and cochlear slow intrinsic effects. To measure MOC effects accurately, one must ensure that there are no middle-ear-muscle contractions. Although standard clinical middle-ear-muscle tests are not adequate for this, adequate tests can usually be done with OAE-measuring instruments. An additional complication is that most probe sounds also elicit MOC activity, although this does not prevent the probe from showing MOC effects elicited by contralateral sound. A variety of data indicate that MOC efferents help to reduce acoustic trauma and lessen the masking of transients by background noise; for instance, they aid in speech comprehension in noise. However, much remains to be learned about the role of efferents in auditory function. Monitoring MOC effects in humans using OAEs should continue to provide valuable insights into the role of MOC efferents and may also provide clinical benefits.

Ultrastructure of synaptic input to medial olivocochlear neurons

Medial olivocochlear (MOC) neurons project from the brain to the cochlea to form the efferent limb of the MOC reflex. To study synaptic inputs to MOC neurons, we retrogradely labeled these neurons using horseradish peroxidase injections into the cochlea. Labeled neurons were identified in the ventral nucleus of the trapezoid body and documented with the light microscope before being studied with serial-section electron microscopy. MOC somata and dendrites were innervated by three different types of synapses, distinguished as either having: 1) large, round synaptic vesicles and forming asymmetric contacts; 2) small, round vesicles plus a few dense core vesicles and forming asymmetric contacts; or 3) pleomorphic vesicles and forming symmetric contacts. The first two types were the most frequent on somata. Acetylcholinesterase-stained material confirmed that the type containing large, round vesicles is most common on dendrites. We kept track of the synaptic terminals in serial sections and compiled them into three-dimensional swellings. Swellings with large, round vesicles formed up to seven synapses per swelling, were largest in size, and sometimes formed complex arrangements engulfing spines of MOC neurons. Swellings with small, round vesicles formed up to four synapses per swelling. The morphology of this type of synapse, and the moderate sizes of the swellings forming it, suggests that it originates from posteroventral cochlear nucleus stellate/multipolar neurons. This input may thus provide the sound-evoked input to MOC neurons that causes their reflexive response to sound.

The time course of recovery from suppression and facilitation from single units in the mammalian cochlear nucleus

The responses to two identical, consecutive pure tone stimuli with varying inter-stimulus intervals (delta ts) were measured for 89 neurons in the cochlear nucleus of the anaesthetised guinea pig. We observed two main effects; either a decrease (suppression) or an increase (facilitation) in response to the second tone followed by an exponential recovery. Response behaviour correlated with the unit type; primary-like, primary-like with notch and transient-chopper units showed a recovery from suppression that was very similar to that already reported in the auditory nerve. For chopper units the strength of the adaptation was correlated with the units regularity of spike discharge; sustained chopper (CS) units showed less suppression than transient choppers. Onset units showed complete suppression at short delta ts. Pause/Build (PB) units responded with increased activity to the second tone. In contrast to previous studies in the cochlear nucleus the recovery from suppression or facilitation was well described by a single exponential function, enabling us to define a recovery time constant and a maximum suppression/facilitation. There appeared to be a hierarchy in the time constant of recovery with PB and CS units showing the longest recovery times and onset units showing the shortest.

Central auditory pathways mediating the rat middle ear muscle reflexes

The middle ear muscle (MEM) reflexes function to protect the inner ear from intense acoustic stimuli and to reduce acoustic masking. Sound presented to the same side or to the opposite side activates the MEM reflex on both sides. The ascending limbs of these pathways must be the auditory nerve fibers originating in the cochlea and terminating in the cochlear nucleus, the first relay station for all ascending auditory information. The descending limbs project from the motoneurons in the brainstem to the MEMs on both sides, causing their contraction. Although the ascending and descending pathways are well described, the cochlear nucleus interneurons that mediate these reflex pathways have not been identified. In order to localize the MEM reflex interneurons, we developed a physiologically based reflex assay in the rat that can be used to determine the integrity of the reflex pathways after experimental manipulations. This assay monitored the change in tone levels and distortion product otoacoustic emissions within the ear canal in one ear during the presentation of a reflex-eliciting sound stimulus in the contralateral ear. Preliminary findings using surgical transection and focal lesioning of the auditory brainstem to interrupt the MEM reflexes suggest that MEM reflex interneurons are located in the ventral cochlear nucleus.

Responses of ventral cochlear nucleus neurons to contralateral sound after conductive hearing loss

Conductive hearing loss (CHL) is an attenuation of signals stimulating the cochlea, without damage to the auditory end organ. It can cause central auditory processing deficits that outlast the CHL itself. Measures of oxidative metabolism show a decrease in activity of nuclei receiving input originating at the affected ear but, surprisingly, an increase in the activity of second-order neurons of the opposite ear. In normal hearing animals, contralateral sound produces an inhibitory response to broadband noise in approximately one third of ventral cochlear nucleus (VCN) neurons. Excitatory responses also occur but are very rare. We looked for changes in the binaural properties of neurons in the VCN of guinea pigs at intervals immediately, 1 day, 1 wk, and 2 wk after the induction of a unilateral CHL by ossicular disruption. CHL was always induced in the ear ipsilateral to the VCN from which recordings were made. The main observations were as follows: 1) ipsilateral excitatory thresholds were raised by at least 40 dB; 2) contralateral inhibitory responses showed a small but statistically significant immediate decrease followed by an increase, returning to normal by 14 days; and 3) there was a large increase in the proportion of units with excitatory responses to contralateral BBN. The increase was immediate and lasting. The latencies of the excitatory responses were at least 6 ms, consistent with activation by a path involving several synapses and inconsistent with cross talk. The latencies and rate-level functions of contralateral excitation were similar to those seen occasionally in normal hearing animals, suggesting an upregulation of an existing pathway. In conclusion, contralateral excitatory inputs to the VCN exist, which are not normally effective, and can compensate rapidly for large changes in afferent input.