GABAergic synapses between auditory efferent neurons and type II spiral ganglion afferent neurons in the mouse cochlea

Cochlear outer hair cells (OHCs) are electromotile and are implicated in mechanisms of amplification of responses to sound that enhance sound sensitivity and frequency tuning. They send information to the brain through glutamatergic synapses onto a small subpopulation of neurons of the ascending auditory nerve, the type II spiral ganglion neurons (SGNs). The OHC synapses onto type II SGNs are sparse and weak, suggesting that type II SGNs respond primarily to loud and possibly damaging levels of sound. OHCs also receive innervation from the brain through the medial olivocochlear (MOC) efferent neurons. MOC neurons are cholinergic yet exert an inhibitory effect on auditory function as they are coupled to alpha9/alpha10 nicotinic acetylcholine receptors (nAChRs) on OHCs, which leads to calcium influx that gates SK potassium channels. The net hyperpolarization exerted by this efferent synapse reduces OHC activity-evoked electromotility and is implicated in cochlear gain control, protection against acoustic trauma, and attention. MOC neurons also label for markers of gamma-aminobutyric acid (GABA) and GABA synthesis. GABAB autoreceptor (GABABR) activation by GABA released from MOC terminals has been demonstrated to reduce ACh release, confirming important negative feedback roles for GABA. However, the full complement of GABAergic activity in the cochlea is not currently understood, including the mechanisms that regulate GABA release from MOC axon terminals, whether GABA diffuses from MOC axon terminals to other postsynaptic cells, and the location and function of GABAA receptors (GABAARs). Previous electron microscopy studies suggest that MOC neurons form contacts onto several other cell types in the cochlea, but whether these contacts form functional synapses, and what neurotransmitters are employed, are unknown. Here we use immunohistochemistry, optical neurotransmitter imaging and patch-clamp electrophysiology from hair cells, afferent dendrites, and efferent axons to demonstrate that in addition to presynaptic GABABR autoreceptor activation, MOC efferent axon terminals release GABA onto type II SGN afferent dendrites with postsynaptic activity mediated by GABAARs. This synapse may have multiple roles including developmental regulation of cochlear innervation, fine tuning of OHC activity, or providing feedback to the brain about MOC and OHC activity.

Genetic tools for studying cochlear inhibition

Efferent feedback to the mammalian cochlea includes cholinergic medial olivocochlear neurons (MOCs) that release ACh to hyperpolarize and shunt the voltage change that drives electromotility of outer hair cells (OHCs). Via brainstem connectivity, MOCs are activated by sound in a frequency- and intensity-dependent manner, thereby reducing the amplification of cochlear vibration provided by OHC electromotility. Among other roles, this efferent feedback protects the cochlea from acoustic trauma. Lesion studies, as well as a variety of genetic mouse models, support the hypothesis of efferent protection from acoustic trauma. Genetic knockout and gain-of-function knockin of the unique α9α10-containing nicotinic acetylcholine receptor (nAChR) in hair cells show that acoustic protection correlates with the efficacy of cholinergic inhibition of OHCs. This protective effect was replicated by viral transduction of the gain-of-function α9L9'T nAChR into α9-knockout mice. Continued progress with "efferent gene therapy" will require a reliable method for visualizing nAChR expression in cochlear hair cells. To that end, mice expressing HA-tagged α9 or α10 nAChRs were generated using CRISPR technology. This progress will facilitate continued study of the hair cell nAChR as a therapeutic target to prevent hearing loss and potentially to ameliorate associated pathologies such as hyperacusis.

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.

Auditory Deprivation during Development Alters Efferent Neural Feedback and Perception

Auditory experience plays a critical role in hearing development. Developmental auditory deprivation because of otitis media, a common childhood disease, produces long-standing changes in the central auditory system, even after the middle ear pathology is resolved. The effects of sound deprivation because of otitis media have been mostly studied in the ascending auditory system but remain to be examined in the descending pathway that runs from the auditory cortex to the cochlea via the brainstem. Alterations in the efferent neural system could be important because the descending olivocochlear pathway influences the neural representation of transient sounds in noise in the afferent auditory system and is thought to be involved in auditory learning. Here, we show that the inhibitory strength of the medial olivocochlear efferents is weaker in children with a documented history of otitis media relative to controls; both boys and girls were included in the study. In addition, children with otitis media history required a higher signal-to-noise ratio on a sentence-in-noise recognition task than controls to achieve the same criterion performance level. Poorer speech-in-noise recognition, a hallmark of impaired central auditory processing, was related to efferent inhibition, and could not be attributed to the middle ear or cochlear mechanics.SIGNIFICANCE STATEMENT Otitis media is the second most common reason children go to the doctor. Previously, degraded auditory experience because of otitis media has been associated with reorganized ascending neural pathways, even after middle ear pathology resolved. Here, we show that altered afferent auditory input because of otitis media during childhood is also associated with long-lasting reduced descending neural pathway function and poorer speech-in-noise recognition. These novel, efferent findings may be important for the detection and treatment of childhood otitis media.

Experience-dependent flexibility in a molecularly diverse central-to-peripheral auditory feedback system

Brainstem olivocochlear neurons (OCNs) modulate the earliest stages of auditory processing through feedback projections to the cochlea and have been shown to influence hearing and protect the ear from sound-induced damage. Here, we used single-nucleus sequencing, anatomical reconstructions, and electrophysiology to characterize murine OCNs during postnatal development, in mature animals, and after sound exposure. We identified markers for known medial (MOC) and lateral (LOC) OCN subtypes, and show that they express distinct cohorts of physiologically relevant genes that change over development. In addition, we discovered a neuropeptide-enriched LOC subtype that produces Neuropeptide Y along with other neurotransmitters. Throughout the cochlea, both LOC subtypes extend arborizations over wide frequency domains. Moreover, LOC neuropeptide expression is strongly upregulated days after acoustic trauma, potentially providing a sustained protective signal to the cochlea. OCNs are therefore poised to have diffuse, dynamic effects on early auditory processing over timescales ranging from milliseconds to days.

Molecular and functional changes to postsynaptic cholinergic signaling in the vestibular sensory organs of aging C57BL/6 mice

Cholinergic circuits in the central nervous system are vulnerable to age-related functional decline, but it is not known if aging impacts cholinergic signaling in the vestibular sensory organs, which are critically important to balance maintenance and visual gaze stability. We have previously shown cholinergic neurotransmission between vestibular efferent terminals and type II mechanosensory hair cells requires the alpha9 (Chrna9) nicotinic receptor subunit. Homozygous knockout of the alpha9 subunit causes vestibulo-ocular reflex adaptation deficits that mirror those observed in aged mice. This prompted examination of cholinergic signaling in the vestibular sensory organs of aged mice. We confirmed older (>24 months) mice had impaired performance in a balance beam task compared to young (3-4 months) adult mice. While there was no qualitative loss of cholinergic axon varicosities in the crista ampullaris of old mice, qPCR analysis revealed reduced expression of nicotinic receptor subunit genes Chrna1, Chrna9, and Chrna10 in the cristae of old relative to young mice. Functionally, single-cell patch clamp recordings taken from type II vestibular hair cells exposed to acetylcholine show reduced conductance through alpha9/10 subunit-containing nicotinic receptors in older mice, despite preserved passive membrane properties and voltage-activated conductances. These findings suggest that cholinergic signaling in the peripheral vestibular sensory organs is vulnerable to aging processes, manifesting in dynamic molecular and functional age-related changes. Given the importance of these organs to our everyday activities, and the dramatic increase in fall incidence in the elderly, further investigation into the mechanisms of altered peripheral vestibular function in older humans is warranted.

Organisation of enkephalin inputs and outputs of the central nucleus of the amygdala in mice

The central nucleus of the amygdala (CeA) is a key hub integrating sensory inputs and modulating behavioural outputs. The CeA is a complex structure with discrete subdivisions, high peptidergic heterogeneity and broad CNS afferent and efferent projections. While several neuropeptide systems within the CeA have been examined in detail, less is known about CeA preproenkephalin (ppENK) cells. Here, we used a recently developed transgenic Penk-Cre mouse line to advance our understanding of the efferent and afferent connectivity of ppENK in the CeA. First, to determine the fidelity of Cre expression in Penk-Cre transgenic mice, we conducted RNAscope in the CeA of Penk-Cre mice. Our analysis revealed that 96.6% of CeA Cre+ neurons co-expressed pENK mRNA, and 99.7% of CeA pENK+ neurons co-expressed Cre mRNA, indicating faithful recapitulation of Cre expression in CeA ppENK-expressing cells, supporting the fidelity of the Penk-Cre reporter mouse. Anterograde tracing of CeA^Penk cells showed strong efferent projections to the extended amygdala, midbrain and hindbrain PBN and NTS. Retrograde tracing of Penk afferents to the CeA were more restricted, with primary innervation originating within the amygdala complex and bed nucleus of the stria terminalis, and minor innervation from the parabrachial nucleus and nucleus of the solitary tract. Together, our data provide a comprehensive map of ENKergic efferent and afferent connectivity of the CeA in Penk-Cre mice. Further, we highlight both the utility and limitations of the Penk-Cre mice to study the function of CeA, PBN and NTS ppENK cells.

The ins and outs of the caudal nucleus of the solitary tract: An overview of cellular populations and anatomical connections

The body and brain are in constant two-way communication. Driving this communication is a region in the lower brainstem: the dorsal vagal complex. Within the dorsal vagal complex, the caudal nucleus of the solitary tract (cNTS) is a major first stop for incoming information from the body to the brain carried by the vagus nerve. The anatomy of this region makes it ideally positioned to respond to signals of change in both emotional and bodily states. In turn, the cNTS controls the activity of regions throughout the brain that are involved in the control of both behaviour and physiology. This review is intended to help anyone with an interest in the cNTS. First, I provide an overview of the architecture of the cNTS and outline the wide range of neurotransmitters expressed in subsets of neurons in the cNTS. Next, in detail, I discuss the known inputs and outputs of the cNTS and briefly highlight what is known regarding the neurochemical makeup and function of those connections. Then, I discuss one group of cNTS neurons: glucagon-like peptide-1 (GLP-1)-expressing neurons. GLP-1 neurons serve as a good example of a group of cNTS neurons, which receive input from varied sources and have the ability to modulate both behaviour and physiology. Finally, I consider what we might learn about other cNTS neurons from our study of GLP-1 neurons and why it is important to remember that the manipulation of molecularly defined subsets of cNTS neurons is likely to affect physiology and behaviours beyond those monitored in individual experiments.

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.

Rapid Enhancement of Subcortical Neural Responses to Sine-Wave Speech

The efferent auditory nervous system may be a potent force in shaping how the brain responds to behaviorally significant sounds. Previous human experiments using the frequency following response (FFR) have shown efferent-induced modulation of subcortical auditory function online and over short- and long-term time scales; however, a contemporary understanding of FFR generation presents new questions about whether previous effects were constrained solely to the auditory subcortex. The present experiment used sine-wave speech (SWS), an acoustically-sparse stimulus in which dynamic pure tones represent speech formant contours, to evoke FFR_SWS. Due to the higher stimulus frequencies used in SWS, this approach biased neural responses toward brainstem generators and allowed for three stimuli (/bɔ/, /bu/, and /bo/) to be used to evoke FFR_SWSbefore and after listeners in a training group were made aware that they were hearing a degraded speech stimulus. All SWS stimuli were rapidly perceived as speech when presented with a SWS carrier phrase, and average token identification reached ceiling performance during a perceptual training phase. Compared to a control group which remained naïve throughout the experiment, training group FFR_SWS amplitudes were enhanced post-training for each stimulus. Further, linear support vector machine classification of training group FFR_SWS significantly improved post-training compared to the control group, indicating that training-induced neural enhancements were sufficient to bolster machine learning classification accuracy. These results suggest that the efferent auditory system may rapidly modulate auditory brainstem representation of sounds depending on their context and perception as non-speech or speech.

Deletion of Oncomodulin Gives Rise to Early Progressive Cochlear Dysfunction in C57 and CBA Mice

Ca²⁺ signaling is a major contributor to sensory hair cell function in the cochlea. Oncomodulin (OCM) is a Ca²⁺ binding protein (CaBP) preferentially expressed in outer hair cells (OHCs) of the cochlea and few other specialized cell types. Here, we expand on our previous reports and show that OCM delays hearing loss in mice of two different genetic backgrounds: CBA/CaJ and C57Bl/6J. In both backgrounds, genetic disruption of Ocm leads to early progressive hearing loss as measured by auditory brainstem response (ABR) and distortion product otoacoustic emission (DPOAE). In both strains, loss of Ocm reduced hearing across lifetime (hearing span) by more than 50% relative to wild type (WT). Even though the two WT strains have very different hearing spans, OCM plays a considerable and similar role within their genetic environment to regulate hearing function. The accelerated age-related hearing loss (ARHL) of the Ocm KO illustrates the importance of Ca²⁺ signaling in maintaining hearing health. Manipulation of OCM and Ca²⁺ signaling may reveal important clues to the systems of function/dysfunction that lead to ARHL.

The early development and physiology of Xenopus laevis tadpole lateral line system

Xenopus laevis has a lateral line mechanosensory system throughout its full life cycle, and a previous study on prefeeding stage tadpoles revealed that it may play a role in motor responses to both water suction and water jets. Here, we investigated the physiology of the anterior lateral line system in newly hatched tadpoles and the motor outputs induced by its activation in response to brief suction stimuli. High-speed videoing showed tadpoles tended to turn and swim away when strong suction was applied close to the head. The lateral line neuromasts were revealed by using DASPEI staining, and their inactivation with neomycin eliminated tadpole motor responses to suction. In immobilized preparations, suction or electrically stimulating the anterior lateral line nerve reliably initiated swimming but the motor nerve discharges implicating turning was observed only occasionally. The same stimulation applied during ongoing fictive swimming produced a halting response. The anterior lateral line nerve showed spontaneous afferent discharges at rest and increased activity during stimulation. Efferent activities were only recorded during tadpole fictive swimming and were largely synchronous with the ipsilateral motor nerve discharges. Finally, calcium imaging identified neurons with fluorescence increase time-locked with suction stimulation in the hindbrain and midbrain. A cluster of neurons at the entry point of the anterior lateral line nerve in the dorsolateral hindbrain had the shortest latency in their responses, supporting their potential sensory interneuron identity. Future studies need to reveal how the lateral line sensory information is processed by the central circuit to determine tadpole motor behavior.NEW & NOTEWORTHY We studied Xenopus tadpole motor responses to anterior lateral line stimulation using high-speed videos, electrophysiology and calcium imaging. Activating the lateral line reliably started swimming. At high stimulation intensities, turning was observed behaviorally but suitable motor nerve discharges were seen only occasionally in immobilized tadpoles. Suction applied during swimming produced a halting response. We analyzed afferent and efferent activities of the tadpole anterior lateral line nerve and located sensory interneurons using calcium imaging.

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.

Neural Circuits of Interoception

The present paper considers recent progress in our understanding of the afferent/ascending neural pathways and neural circuits of interoception. Of particular note is the extensive role of rostral neural systems, including cortical systems, in the recognition of internal body states, and the reciprocal role of efferent/descending systems in the regulation of those states. Together these reciprocal interacting networks entail interoceptive circuits that play an important role in a broad range of functions beyond the homeostatic maintenance of physiological steady-states. These include the regulation of behavioral, cognitive, and affective processes across conscious and nonconscious levels of processing. We highlight recent advances and knowledge gaps that are important for accelerating progress in the study of interoception.

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.

Behavioral Approaches to Study Top-Down Influences on Active Listening

The massive network of descending corticofugal projections has been long-recognized by anatomists, but their functional contributions to sound processing and auditory-guided behaviors remain a mystery. Most efforts to characterize the auditory corticofugal system have been inductive; wherein function is inferred from a few studies employing a wide range of methods to manipulate varying limbs of the descending system in a variety of species and preparations. An alternative approach, which we focus on here, is to first establish auditory-guided behaviors that reflect the contribution of top-down influences on auditory perception. To this end, we postulate that auditory corticofugal systems may contribute to active listening behaviors in which the timing of bottom-up sound cues can be predicted from top-down signals arising from cross-modal cues, temporal integration, or self-initiated movements. Here, we describe a behavioral framework for investigating how auditory perceptual performance is enhanced when subjects can anticipate the timing of upcoming target sounds. Our first paradigm, studied both in human subjects and mice, reports species-specific differences in visually cued expectation of sound onset in a signal-in-noise detection task. A second paradigm performed in mice reveals the benefits of temporal regularity as a perceptual grouping cue when detecting repeating target tones in complex background noise. A final behavioral approach demonstrates significant improvements in frequency discrimination threshold and perceptual sensitivity when auditory targets are presented at a predictable temporal interval following motor self-initiation of the trial. Collectively, these three behavioral approaches identify paradigms to study top-down influences on sound perception that are amenable to head-fixed preparations in genetically tractable animals, where it is possible to monitor and manipulate particular nodes of the descending auditory pathway with unparalleled precision.

Medial olivocochlear reflex effects on amplitude growth functions of long- and short-latency components of click-evoked otoacoustic emissions in humans

Functional outcomes of medial olivocochlear reflex (MOCR) activation, such as improved hearing in background noise and protection from noise damage, involve moderate to high sound levels. Previous noninvasive measurements of MOCR in humans focused primarily on otoacoustic emissions (OAEs) evoked at low sound levels. Interpreting MOCR effects on OAEs at higher levels is complicated by the possibility of the middle-ear muscle reflex and by components of OAEs arising from different locations along the length of the cochlear spiral. We overcame these issues by presenting click stimuli at a very slow rate and by time-frequency windowing the resulting click-evoked (CE)OAEs into short-latency (SL) and long-latency (LL) components. We characterized the effects of MOCR on CEOAE components using multiple measures to more comprehensively assess these effects throughout much of the dynamic range of hearing. These measures included CEOAE amplitude attenuation, equivalent input attenuation, phase, and slope of growth functions. Results show that MOCR effects are smaller on SL components than LL components, consistent with SL components being generated slightly basal of the characteristic frequency region. Amplitude attenuation measures showed the largest effects at the lowest stimulus levels, but slope change and equivalent input attenuation measures did not decrease at higher stimulus levels. These latter measures are less commonly reported and may provide insight into the variability in listening performance and noise susceptibility seen across individuals.NEW & NOTEWORTHY The auditory efferent system, operating at moderate to high sound levels, may improve hearing in background noise and provide protection from noise damage. We used otoacoustic emissions to measure these efferent effects across a wide range of sound levels and identified level-dependent and independent effects. Previous reports have focused on level-dependent measures. The level-independent effects identified here may provide new insights into the functional relevance of auditory efferent activity in humans.

Electric Eels Wield a Functional Venom Analogue

In this paper, I draw an analogy between the use of electricity by electric eels (Electrophorus electricus) to paralyze prey muscles and the use of venoms that paralyze prey by disrupting the neuromuscular junction. The eel’s strategy depends on the recently discovered ability of eels to activate prey motor neuron efferents with high-voltage pulses. Usually, eels use high voltage to cause brief, whole-body tetanus, thus preventing escape while swallowing prey whole. However, when eels struggle with large prey, or with prey held precariously, they often curl to bring their tail to the opposite side. This more than doubles the strength of the electric field within shocked prey, ensuring maximal stimulation of motor neuron efferents. Eels then deliver repeated volleys of high-voltage pulses at a rate of approximately 100 Hz. This causes muscle fatigue that attenuates prey movement, thus preventing both escape and defense while the eel manipulates and swallows the helpless animal. Presumably, the evolution of enough electrical power to remotely activate ion channels in prey efferents sets the stage for the selection of eel behaviors that functionally “poison” prey muscles.

Unraveling the Molecular Players at the Cholinergic Efferent Synapse of the Zebrafish Lateral Line

The lateral line (LL) is a sensory system that allows fish and amphibians to detect water currents. LL responsiveness is modulated by efferent neurons that aid in distinguishing between external and self-generated stimuli, maintaining sensitivity to relevant cues. One component of the efferent system is cholinergic, the activation of which inhibits afferent activity. LL hair cells (HCs) share structural, functional, and molecular similarities with those of the cochlea, making them a popular model for studying human hearing and balance disorders. Because of these commonalities, one could propose that the receptor at the LL efferent synapse is a α9α10 nicotinic acetylcholine receptor (nAChR). However, the identities of the molecular players underlying ACh-mediated inhibition in the LL remain unknown. Surprisingly, through the analysis of single-cell expression studies and in situ hybridization, we describe that α9, but not the α10, subunits are enriched in zebrafish HCs. Moreover, the heterologous expression of zebrafish α9 subunits indicates that homomeric receptors are functional and exhibit robust ACh-gated currents blocked by α-bungarotoxin and strychnine. In addition, in vivo Ca²⁺ imaging on mechanically stimulated zebrafish LL HCs show that ACh elicits a decrease in evoked Ca²⁺ signals, regardless of HC polarity. This effect is blocked by both α-bungarotoxin and apamin, indicating coupling of ACh-mediated effects to small-conductance Ca²⁺-activated potassium (SKs) channels. Our results indicate that an α9-containing (α9*) nAChR operates at the zebrafish LL efferent synapse. Moreover, the activation of α9* nAChRs most likely leads to LL HC hyperpolarization served by SK channels.SIGNIFICANCE STATEMENT The fish lateral line (LL) mechanosensory system shares structural, functional, and molecular similarities with those of the mammalian cochlea. Thus, it has become an accessible model for studying human hearing and balance disorders. However, the molecular players serving efferent control of LL hair cell (HC) activity have not been identified. Here we demonstrate that, different from the hearing organ of vertebrate species, a nicotinic acetylcholine receptor composed only of α9 subunits operates at the LL efferent synapse. Activation of α9-containing receptors leads to LL HC hyperpolarization because of the opening of small-conductance Ca²⁺-activated potassium channels. These results will further aid in the interpretation of data obtained from LL HCs as a model for cochlear HCs.

Activation of GABAB receptors results in excitatory modulation of calyx terminals in rat semicircular canal cristae

Previous studies have found GABA in vestibular end organs. However, existence of GABA receptors or possible GABAergic effects on vestibular nerve afferents has not been investigated. The current study was conducted to determine whether activation of GABAB receptors affects calyx afferent terminals in the central region of the cristae of semicircular canals. We used patch-clamp recording in postnatal day 13-18 (P13-P18) Sprague-Dawley rats of either sex. Application of GABAB receptor agonist baclofen inhibited voltage-sensitive potassium currents. This effect was blocked by selective GABAB receptor antagonist CGP 35348. Application of antagonists of small (SK)- and large-conductance potassium (BK) channels almost completely blocked the effects of baclofen. The remaining baclofen effect was blocked by cadmium chloride, suggesting that it could be due to inhibition of voltage-gated calcium channels. Furthermore, baclofen had no effect in the absence of calcium in the extracellular fluid. Inhibition of potassium currents by GABAB activation resulted in an excitatory effect on calyx terminal action potential firing. While in the control condition calyces could only fire a single action potential during step depolarizations, in the presence of baclofen they fired continuously during steps and a few even showed repetitive discharge. We also found a decrease in threshold for action potential generation and a decrease in first-spike latency during step depolarization. These results provide the first evidence for the presence of GABAB receptors on calyx terminals, showing that their activation results in an excitatory effect and that GABA inputs could be used to modulate calyx response properties.NEW & NOTEWORTHY Using in vitro whole cell patch-clamp recordings from calyx terminals in the vestibular end organs, we show that activation of GABAB receptors result in an excitatory effect, with decreased spike-frequency adaptation and shortened first-spike latencies. Our results suggest that these effects are mediated through inhibition of calcium-sensitive potassium channels.

Nicotine evoked efferent transmitter release onto immature cochlear inner hair cells

Olivocochlear neurons make temporary cholinergic synapses on inner hair cells of the rodent cochlea in the first 2 to 3 wk after birth. Repetitive stimulation of these efferent neurons causes facilitation of evoked release and increased spontaneous release that continues for seconds to minutes. Presynaptic nicotinic acetylcholine receptors (nAChRs) are known to modulate neurotransmitter release from brain neurons. The present study explores the hypothesis that presynaptic nAChRs help to increase spontaneous release from efferent terminals on cochlear hair cells. Direct application of nicotine (which does not activate the hair cells' α9α10-containing nAChRs) produces sustained efferent transmitter release, implicating presynaptic nAChRs in this response. The effect of nicotine was reduced by application of ryanodine that reduces release of calcium from intraterminal stores.NEW & NOTEWORTHY Sensory organs exhibit spontaneous activity before the onset of response to external stimuli. Such activity in the cochlea is subject to modulation by cholinergic efferent neurons that directly inhibit sensory hair cells (inner hair cells). Those efferent neurons are themselves subject to various modulatory mechanisms. One such mechanism is positive feedback by released acetylcholine onto presynaptic nicotinic acetylcholine receptors causing further release of acetylcholine.

Efferent synaptic transmission at the vestibular type II hair cell synapse

In the vestibular peripheral organs, type I and type II hair cells (HCs) transmit incoming signals via glutamatergic quantal transmission onto afferent nerve fibers. Additionally, type I HCs transmit via "non-quantal" transmission to calyx afferent fibers, by accumulation of glutamate and potassium in the synaptic cleft. Vestibular efferent inputs originating in the brainstem contact type II HCs and vestibular afferents. Here, synaptic inputs to type II HCs were characterized by using electrical and optogenetic stimulation of efferent fibers combined with in vitro whole cell patch-clamp recording from type II HCs in the rodent vestibular crista. Properties of efferent synaptic currents in type II HCs were similar to those found in cochlear HCs and mediated by activation of α9-containing nicotinic acetylcholine receptors (nAChRs) and small-conductance calcium-activated potassium (SK) channels. While efferents showed a low probability of release at low frequencies of stimulation, repetitive stimulation resulted in facilitation and increased probability of release. Notably, the membrane potential of type II HCs during optogenetic stimulation of efferents showed a strong hyperpolarization in response to single pulses and was further enhanced by repetitive stimulation. Such efferent-mediated inhibition of type II HCs can provide a mechanism to adjust the contribution of signals from type I and type II HCs to vestibular nerve fibers, with a shift of the response to be more like that of calyx-only afferents with faster non-quantal responses.NEW & NOTEWORTHY Type II vestibular hair cells (HCs) receive inputs from efferent neurons in the brain stem. We used in vitro optogenetic and electrical stimulation of vestibular efferent fibers to study their synaptic inputs to type II HCs. Stimulation of efferents inhibited type II HCs, similar to efferent effects on cochlear HCs. We propose that efferent inputs adjust the contribution of signals from type I and II HCs to vestibular nerve fibers.

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.

Efferent Inputs Are Required for Normal Function of Vestibular Nerve Afferents

A group of vestibular afferent nerve fibers with irregular-firing resting discharges are thought to play a prominent role in responses to fast head movements and vestibular plasticity. We show that, in C57BL/6 mice (either sex, 4-5 weeks old), normal activity in the efferent vestibular pathway is required for function of these irregular afferents. Thermal inhibition of efferent fibers results in a profound inhibition of irregular afferents' resting discharges, rendering them inadequate for signaling head movements. In this way, efferent inputs adjust the contribution of the peripheral irregular afferent pathway that plays a critical role in peripheral vestibular signaling and plasticity.SIGNIFICANCE STATEMENT Vestibular end organs in the inner ear receive efferent inputs from the brainstem. Previously, electrical stimulation of efferents was linked to an increase in resting discharges of afferents and a decrease in their sensitivities. Here, we show that localized thermal inhibition of unmyelinated efferents results in a significant decrease in the activity of afferent nerve fibers, particularly those with irregular resting discharges implicated in responses to fast head movements and vestibular compensation. Thus, by upregulating and downregulating of afferent firing, particularly irregular afferents, efferents adjust neural activity sensitive to rapid head movements. These findings support the notion that peripheral vestibular end organs are not passive transducers of head movements and their sensory signal transmission is modulated by efferent inputs.

Vagal innervation of the stomach reassessed: brain-gut connectome uses smart terminals

Brain-gut neural communications have long been considered limited because of conspicuous numerical mismatches. The vagus, the parasympathetic nerve connecting brain and gut, contains thousands of axons, whereas the gastrointestinal (GI) tract contains millions of intrinsic neurons in local plexuses. The numerical paradox was initially recognized in terms of efferent projections, but the number of afferents, which comprise the majority (≈ 80%) of neurites in the vagus, is also relatively small. The present survey of recent morphological observations suggests that vagal terminals, and more generally autonomic and visceral afferent arbors in the stomach as well as throughout the gut, elaborate arbors that are extensive, regionally specialized, polymorphic, polytopic, and polymodal, commonly with multiplicities of receptors and binding sites-smart terminals. The morphological specializations and dynamic tuning of one-to-many efferent projections and many-to-one convergences of contacts onto afferents create a complex architecture capable of extensive peripheral integration in the brain-gut connectome and offset many of the disparities between axon and target numbers. Appreciating this complex architecture can help in the design of therapies for GI disorders.

Forebrain Dopamine System Regulates Inner Ear Auditory Sensitivity to Socially Relevant Acoustic Signals

Dopamine is integral to attentional and motivational processes, but studies are largely restricted to the central nervous system. In mammals [1, 2] and fishes [3, 4], central dopaminergic neurons project to the inner ear and could modulate acoustic signals at the earliest stages of processing. Studies in rodents show dopamine inhibits cochlear afferent neurons and protects against noise-induced acoustic injury [5-10]. However, other functions for inner ear dopamine have not been investigated, and the effect of dopamine on peripheral auditory processing in non-mammalians remains unknown [11, 12]. Insights could be gained by studies conducted in the context of intraspecific acoustic communication. We present evidence from a vocal fish linking reproductive-state-dependent changes in auditory sensitivity with seasonal changes in the dopaminergic efferent system in the saccule, their primary organ of hearing. Plainfin midshipman (Porichthys notatus) migrate from deep-water winter habitats to the intertidal zone in the summer to breed. Nesting males produce nocturnal vocalizations to attract females [13]. Both sexes undergo seasonal enhancement of hearing sensitivity at the level of the hair cell [14-16], increasing the likelihood of detecting conspecific signals [17, 18]. Importantly, reproductive females concurrently have reduced dopaminergic input to the saccule [19]. Here, we show that dopamine decreases saccule auditory sensitivity via a D2-like receptor. Saccule D2a receptor expression is reduced in the summer and correlates with sensitivity within and across seasons. We propose that reproductive-state-dependent changes to the dopaminergic efferent system provide a release of inhibition in the saccule, enhancing peripheral encoding of social-acoustic signals.

Parallel pathways for sound processing and functional connectivity among layer 5 and 6 auditory corticofugal neurons

Cortical layers (L) 5 and 6 are populated by intermingled cell-types with distinct inputs and downstream targets. Here, we made optogenetically guided recordings from L5 corticofugal (CF) and L6 corticothalamic (CT) neurons in the auditory cortex of awake mice to discern differences in sensory processing and underlying patterns of functional connectivity. Whereas L5 CF neurons showed broad stimulus selectivity with sluggish response latencies and extended temporal non-linearities, L6 CTs exhibited sparse selectivity and rapid temporal processing. L5 CF spikes lagged behind neighboring units and imposed weak feedforward excitation within the local column. By contrast, L6 CT spikes drove robust and sustained activity, particularly in local fast-spiking interneurons. Our findings underscore a duality among sub-cortical projection neurons, where L5 CF units are canonical broadcast neurons that integrate sensory inputs for transmission to distributed downstream targets, while L6 CT neurons are positioned to regulate thalamocortical response gain and selectivity.

A paracrine role for white thermogenic adipocytes in innervation: an evidence-based hypothesis

White adipose tissue (WAT) stores energy and also plays an important endocrine role in producing adipokines for communication with the peripheral and central nervous system. WAT consists of the major lipogenic unilocular adipocytes and the minor populations of beige and brite multilocular adipocytes. These multilocular adipocytes express thermogenic genes and have phenotypic similarity with thermogenic brown adipose tissue. According to a current paradigm, multilocular adipocytes have a thermogenic function in WAT. In this mini review, we discuss data revealing heterogeneity among multilocular cell subsets in WAT and their functions beyond thermogenesis. We propose a hypothetical neuroendocrine role for multilocular adipocytes subsets in the formation of adaptive sensory-sympathetic circuits between the central nervous system and adipose tissue, which activate lipolysis and thermogenesis in WAT in high energy demand situations.

Modified Origins of Cortical Projections to the Superior Colliculus in the Deaf: Dispersion of Auditory Efferents

Following the loss of a sensory modality, such as deafness or blindness, crossmodal plasticity is commonly identified in regions of the cerebrum that normally process the deprived modality. It has been hypothesized that significant changes in the patterns of cortical afferent and efferent projections may underlie these functional crossmodal changes. However, studies of thalamocortical and corticocortical connections have refuted this hypothesis, instead revealing a profound resilience of cortical afferent projections following deafness and blindness. This report is the first study of cortical outputs following sensory deprivation, characterizing cortical projections to the superior colliculus in mature cats (N = 5, 3 female) with perinatal-onset deafness. The superior colliculus was exposed to a retrograde pathway tracer, and subsequently labeled cells throughout the cerebrum were identified and quantified. Overall, the percentage of cortical projections arising from auditory cortex was substantially increased, not decreased, in early-deaf cats compared with intact animals. Furthermore, the distribution of labeled cortical neurons was no longer localized to a particular cortical subregion of auditory cortex but dispersed across auditory cortical regions. Collectively, these results demonstrate that, although patterns of cortical afferents are stable following perinatal deafness, the patterns of cortical efferents to the superior colliculus are highly mutable.SIGNIFICANCE STATEMENT When a sense is lost, the remaining senses are functionally enhanced through compensatory crossmodal plasticity. In deafness, brain regions that normally process sound contribute to enhanced visual and somatosensory perception. We demonstrate that hearing loss alters connectivity between sensory cortex and the superior colliculus, a midbrain region that integrates sensory representations to guide orientation behavior. Contrasting expectation, the proportion of projections from auditory cortex increased in deaf animals compared with normal hearing, with a broad distribution across auditory fields. This is the first description of changes in cortical efferents following sensory loss and provides support for models predicting an inability to form a coherent, multisensory percept of the environment following periods of abnormal development.

Assessment of Ipsilateral Efferent Effects in Human via ECochG

Development of electrophysiological means to assess the medial olivocochlear (MOC) system in humans is important to further our understanding of the function of that system and for the refinement and validation of psychoacoustical and otoacoustic emission methods which are thought to probe the MOC. Based on measurements in anesthetized animals it has been hypothesized that the MOC-reflex (MOCR) can enhance the response to signals in noise, and several lines of evidence support such a role in humans. A difficulty in these studies is the isolation of efferent effects. Efferent activation can be triggered by acoustic stimulation of the contralateral or ipsilateral ear, but ipsilateral stimulation is thought to be more effective. However, ipsilateral stimulation complicates interpretation of effects since these sounds can affect the perception of other ipsilateral sounds by mechanisms not involving olivocochlear efferents. We assessed the ipsilaterally evoked MOCR in human using a transtympanic procedure to record mass-potentials from the cochlear promontory or the niche of the round window. Averaged compound action potential (CAP) responses to masked probe tones of 4 kHz with and without a precursor (designed to activate the MOCR but not the stapedius reflex) were extracted with a polarity alternating paradigm. The masker was either a simultaneous narrow band noise masker or a short (20-ms) tonal ON- or OFF-frequency forward masker. The subjects were screened for normal hearing (audiogram, tympanogram, threshold stapedius reflex) and psychoacoustically tested for the presence of a precursor effect. We observed a clear reduction of CAP amplitude by the precursor, for different masking conditions. Even without an MOCR, this is expected because the precursor will affect the response to subsequent stimuli via neural adaptation. To determine whether the precursor also activated the efferent system, we measured the CAP over a range of masker levels, with or without precursor, and for different types of masker. The results show CAP reduction consistent with the type of gain reduction caused by the MOCR. These results generally support psychoacoustical paradigms designed to probe the efferent system as indeed activating the MOCR system, but not all observations are consistent with this mechanism.

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.

T Cell Trafficking through Lymphatic Vessels

T cell migration within and between peripheral tissues and secondary lymphoid organs is essential for proper functioning of adaptive immunity. While active T cell migration within a tissue is fairly slow, blood vessels and lymphatic vessels (LVs) serve as speedy highways that enable T cells to travel rapidly over long distances. The molecular and cellular mechanisms of T cell migration out of blood vessels have been intensively studied over the past 30 years. By contrast, less is known about T cell trafficking through the lymphatic vasculature. This migratory process occurs in one manner within lymph nodes (LNs), where recirculating T cells continuously exit into efferent lymphatics to return to the blood circulation. In another manner, T cell trafficking through lymphatics also occurs in peripheral tissues, where T cells exit the tissue by means of afferent lymphatics, to migrate to draining LNs and back into blood. In this review, we highlight how the anatomy of the lymphatic vasculature supports T cell trafficking and review current knowledge regarding the molecular and cellular requirements of T cell migration through LVs. Finally, we summarize and discuss recent insights regarding the presumed relevance of T cell trafficking through afferent lymphatics.

Nicotinic acetylcholine receptors regulate vestibular afferent gain and activation timing

Little is known about the function of the cholinergic efferents innervating peripheral vestibular hair cells. We measured vestibular sensory evoked potentials (VsEPs) in α9 knockout (KO) mice, α10 KO mice, α7 KO mice, α9/10 and α7/9 double KO mice, and wild-type (WT) controls. We also studied the morphology and ultrastructure of efferent terminals on vestibular hair cells in α9, α10, and α9/10 KOs. Both type I and type ll vestibular hair cells express the α9 and α10 subunits. The efferent boutons on vestibular cells in α9, α10, and α9/10 KOs appeared normal, but a quantitative analysis was not performed. Mean VsEP thresholds were significantly elevated in α9 and α9/10 KO animals. Some α9 and α9/10 KO animals, however, had normal or near-normal thresholds, whereas others were greatly affected. Despite individual variability in threshold responses, latencies were consistently shortened. The double α7/9 KO resulted in decreased variance by normalizing waveforms and latencies. The phenotypes of the α7 and α10 single KOs were identical. Both α7 and α10 KO mice evidenced normal thresholds, decreased activation latencies, and larger amplitudes compared with WT mice. The data suggest a complex interaction of nicotinic acetylcholine receptors (nAChRs) in regulating vestibular afferent gain and activation timing. Although the α9/10 heteromeric nAChR is an important component of vestibular efferent activity, other peripheral or central nAChRs involving the α7 subunit or α10 subunit and α9 homomeric receptors are also important. J. Comp. Neurol. 525:1216-1233, 2017. © 2016 Wiley Periodicals, Inc.

Maturation of suprathreshold auditory nerve activity involves cochlear CGRP-receptor complex formation

In adult animals, the neuropeptide calcitonin gene‐related peptide (CGRP) is contained in cochlear efferent fibers projecting out to the cochlea, and contributes to increased suprathreshold sound‐evoked activity in the adult auditory nerve. Similarly, CGRP applied to the lateral‐line organ (hair cell organ) increases afferent nerve activity in adult frogs (post‐metamorphic day 30), yet this increase is developmentally delayed from post‐metamorphic day 4–30. In this study, we discovered that there was also a developmental delay in increased suprathreshold sound‐evoked activity auditory nerve between juvenile and adult mice similar to what had been observed previously in frog. Moreover, juvenile mice with a targeted deletion of the αCGRP gene [CGRP null (−/−)] did not show a similar developmental increase in nerve activity, suggesting CGRP signaling is involved. This developmental delay is not due to a delay in CGRP expression, but instead is due to a delay in receptor formation. We observed that the increase in sound‐evoked nerve activity is correlated with increased formation of cochlear CGRP receptors, which require three complexed proteins (CLR, RAMP1, RCP) to be functional. CGRP receptor formation in the cochlea was incomplete at 1 month of age (juvenile), but complete by 3 months (adult), which corresponded to the onset of suprathreshold enhancement of sound‐evoked activity in wild‐type animals. Taken together, these data support a model for cochlear function that is enhanced by maturation of CGRP receptor complexes.

Recording and labeling at a site along the cochlea shows alignment of medial olivocochlear and auditory nerve tonotopic mappings

Medial olivocochlear (MOC) neurons provide an efferent innervation to outer hair cells (OHCs) of the cochlea, but their tonotopic mapping is incompletely known. In the present study of anesthetized guinea pigs, the MOC mapping was investigated using in vivo, extracellular recording, and labeling at a site along the cochlear course of the axons. The MOC axons enter the cochlea at its base and spiral apically, successively turning out to innervate OHCs according to their characteristic frequencies (CFs). Recordings made at a site in the cochlear basal turn yielded a distribution of MOC CFs with an upper limit, or "edge," due to usually absent higher-CF axons that presumably innervate more basal locations. The CFs at the edge, normalized across preparations, were equal to the CFs of the auditory nerve fibers (ANFs) at the recording sites (near 16 kHz). Corresponding anatomical data from extracellular injections showed spiraling MOC axons giving rise to an edge of labeling at the position of a narrow band of labeled ANFs. Overall, the edges of the MOC CFs and labeling, with their correspondences to ANFs, suggest similar tonotopic mappings of these efferent and afferent fibers, at least in the cochlear basal turn. They also suggest that MOC axons miss much of the position of the more basally located cochlear amplifier appropriate for their CF; instead, the MOC innervation may be optimized for protection from damage by acoustic overstimulation.

Hyperactivity in the medial olivocochlear efferent system is a common feature of tinnitus and hyperacusis in humans

Tinnitus and hyperacusis are common, burdensome sources of morbidity with a high rate of co-occurrence. Knudson et al. (J Neurophysiol 112: 3197-3208, 2014) demonstrated that efferent suppression of cochlear activity by the medial olivocochlear system is enhanced in individuals with tinnitus and/or hyperacusis. Their findings stress that atypical activity in the efferent auditory pathway may represent a shared substrate, as well as a potential therapeutic target, in tinnitus and hyperacusis.

Effect of human auditory efferent feedback on cochlear gain and compression

The mammalian auditory system includes a brainstem-mediated efferent pathway from the superior olivary complex by way of the medial olivocochlear system, which reduces the cochlear response to sound (Warr and Guinan, 1979; Liberman et al., 1996). The human medial olivocochlear response has an onset delay of between 25 and 40 ms and rise and decay constants in the region of 280 and 160 ms, respectively (Backus and Guinan, 2006). Physiological studies with nonhuman mammals indicate that onset and decay characteristics of efferent activation are dependent on the temporal and level characteristics of the auditory stimulus (Bacon and Smith, 1991; Guinan and Stankovic, 1996). This study uses a novel psychoacoustical masking technique using a precursor sound to obtain a measure of the efferent effect in humans. This technique avoids confounds currently associated with other psychoacoustical measures. Both temporal and level dependency of the efferent effect was measured, providing a comprehensive measure of the effect of human auditory efferents on cochlear gain and compression. Results indicate that a precursor (>20 dB SPL) induced efferent activation, resulting in a decrease in both maximum gain and maximum compression, with linearization of the compressive function for input sound levels between 50 and 70 dB SPL. Estimated gain decreased as precursor level increased, and increased as the silent interval between the precursor and combined masker-signal stimulus increased, consistent with a decay of the efferent effect. Human auditory efferent activation linearizes the cochlear response for mid-level sounds while reducing maximum gain.

Loss of α-calcitonin gene-related peptide (αCGRP) reduces the efficacy of the Vestibulo-ocular Reflex (VOR)

The neuroactive peptide calcitonin-gene related peptide (CGRP) is known to act at efferent synapses and their targets in hair cell organs, including the cochlea and lateral line. CGRP is also expressed in vestibular efferent neurons as well as a number of central vestibular neurons. Although CGRP-null (-/-) mice demonstrate a significant reduction in cochlear nerve sound-evoked activity compared with wild-type mice, it is unknown whether and how the loss of CGRP influence vestibular system function. Vestibular function was assessed by quantifying the vestibulo-ocular reflex (VOR) in alert mice. The loss of CGRP in (-/-) mice was associated with a reduction of the VOR gain of ≈50% without a concomitant change in phase. Using immunohistochemistry, we confirmed that, although CGRP staining was absent in the vestibular end-organs of null (-/-) mice, cholinergic staining appeared normal, suggesting that the overall gross development of vestibular efferent innervation was unaltered. We further confirmed that the observed deficit in vestibular function of null (-/-) mice was not the result of nontargeted effects at the level of the extraocular motor neurons and/or their innervation of extraocular muscles. Analysis of the relationship between vestibular quick phase amplitude and peak velocity revealed that extraocular motor function was unchanged, and immunohistochemistry revealed no abnormalities in motor endplates. Together, our findings show that the neurotransmitter CGRP plays a key role in ensuring VOR efficacy.

Localization of kainate receptors in inner and outer hair cell synapses

Glutamate plays a role in hair cell afferent transmission, but the receptors that mediate neurotransmission between outer hair cells (OHCs) and type II ganglion neurons are not well defined. A previous study using in situ hybridization showed that several kainate-type glutamate receptor (KAR) subunits are expressed in cochlear ganglion neurons. To determine whether KARs are expressed in hair cell synapses, we performed X-gal staining on mice expressing lacZ driven by the GluK5 promoter, and immunolabeling of glutamate receptors in whole-mount mammalian cochleae. X-gal staining revealed GluK5 expression in both type I and type II ganglion neurons and OHCs in adults. OHCs showed X-gal reactivity throughout maturation from postnatal day 4 (P4) to 1.5 months. Immunoreactivity for GluK5 in IHC afferent synapses appeared to be postsynaptic, similar to GluA2 (GluR2; AMPA-type glutamate receptor (AMPAR) subunit), while GluK2 may be on both sides of the synapses. In OHC afferent synapses, immunoreactivity for GluK2 and GluK5 was found, although GluK2 was only in those synapses bearing ribbons. GluA2 was not detected in adult OHC afferent synapses. Interestingly, GluK1, GluK2 and GluK5 were also detected in OHC efferent synapses, forming several active zones in each synaptic area. At P8, GluA2 and all KAR subunits except GluK4 were detected in OHC afferent synapses in the apical turn, and GluA2, GluK1, GluK3 decreased dramatically in the basal turn. These results indicate that AMPARs and KARs (GluK2/GluK5) are localized to IHC afferent synapses, while only KARs (GluK2/GluK5) are localized to OHC afferent synapses in adults. Glutamate spillover near OHCs may act on KARs in OHC efferent terminals to modulate transmission of acoustic information and OHC electromotility.

Mechanisms underlying the neuronal-based symptoms of allergy

Persons with allergies present with symptoms that often are the result of alterations in the nervous system. Neuronally based symptoms depend on the organ in which the allergic reaction occurs but can include red itchy eyes, sneezing, nasal congestion, rhinorrhea, coughing, bronchoconstriction, airway mucus secretion, dysphagia, altered gastrointestinal motility, and itchy swollen skin. These symptoms occur because mediators released during an allergic reaction can interact with sensory nerves, change processing in the central nervous system, and alter transmission in sympathetic, parasympathetic, and enteric autonomic nerves. In addition, evidence supports the idea that in some subjects this neuromodulation is, for reasons poorly understood, upregulated such that the same degree of nerve stimulus causes a larger effect than seen in healthy subjects. There are distinctions in the mechanisms and nerve types involved in allergen-induced neuromodulation among different organ systems, but general principles have emerged. The products of activated mast cells, other inflammatory cells, and resident cells can overtly stimulate nerve endings, cause long-lasting changes in neuronal excitability, increase synaptic efficacy, and also change gene expression in nerves, resulting in phenotypically altered neurons. A better understanding of these processes might lead to novel therapeutic strategies aimed at limiting the suffering of those with allergies.

Medial olivocochlear-induced transient-evoked otoacoustic emission amplitude shifts in individual subjects

Activation of the medial olivocochlear reflex (MOCR) can be assessed indirectly using transient-evoked otoacoustic emissions (TEOAEs). The change in TEOAE amplitudes when the MOCR is activated (medial olivocochlear (MOC) shift) has most often been quantified as the mean value in groups of subjects. The usefulness of MOC shift measurements may be increased by the ability to quantify significant shifts in individuals. This study used statistical resampling to quantify significant MOC shifts in 16 subjects. TEOAEs were obtained using transient stimuli containing energy from 1 to 10 kHz. A nonlinear paradigm was used to extract TEOAEs. Transient stimuli were presented at 30 dB sensation level (SL) with suppressor stimuli presented 12 dB higher. Contralateral white noise, used to activate the MOCR, was presented at 30 dB SL and was interleaved on and off in 30-s intervals during a 7-min recording period. Confounding factors of middle ear muscle reflex and slow amplitude drifts were accounted for. TEOAEs were analyzed in 11 1/3-octave frequency bands. The statistical significance of each individual MOC shift was determined using a bootstrap procedure. The minimum detectable MOC shifts ranged from 0.10 to 3.25 dB and were highly dependent on signal-to-noise ratio at each frequency. Subjects exhibited a wide range of magnitudes of significant MOC shifts in the 1.0-3.2-kHz region (median = 1.94 dB, range = 0.34-6.51 dB). There was considerable overlap between the magnitudes of significant and nonsignificant shifts. While most subjects had significant MOC shifts in one or more frequency bands below 4 kHz, few had significant shifts in all of these bands. Above 4 kHz, few significant shifts were seen, but this may have been due to lower signal-to-noise ratios. The specific frequency bands containing significant shifts were variable across individuals. Further work is needed to determine the clinical usefulness of examining MOC shifts in individuals.

Alpha-9 nicotinic acetylcholine receptor immunoreactivity in the rodent vestibular labyrinth

Vestibular tissues (cristae ampullares, macular otolithic organs, and Scarpa's ganglia) in chinchilla, rat, and guinea pig were examined for immunoreactivity to the alpha9 nicotinic acetylcholine receptor (nAChR) subunit. The alpha9 antibody was generated against a conserved peptide present in the intracellular loop of the predicted protein sequence of the guinea pig alpha9 nAChR subunit. In the vestibular periphery, staining was observed in calyces around type I hair cells, at the synaptic pole of type II hair cells, and in varying levels in Scarpa's ganglion cells. Ganglion cells were also triply labeled to detect alpha9, calretinin, and peripherin. Calretinin labels calyx-only afferents. Peripherin labels bouton-only afferents. Dimorphic afferents, which have both calyx and bouton endings, are not labeled by calretinin or peripherin. In these experiments, alpha9 was expressed in both calyx and dimorphic afferents. A subpopulation of small ganglion cells did not contain the alpha9 nAChR but did stain for peripherin. We surmise that these are bouton-only afferents. Bouton (regularly discharging) afferents also show efferent responses, although they are qualitatively different from those in irregularly discharging (calyx and dimorphic) afferents, much slower and longer lasting. Thus, regular afferents are probably more affected via a muscarinic cholinergic or a peptidergic mechanism, with a much smaller superimposed fast nicotinic-type response. This latter response could be due to one of the other nicotinic receptors that have been described in studies from other laboratories.

CaV1.3 channels are essential for development and presynaptic activity of cochlear inner hair cells

Cochlear inner hair cells (IHCs) release neurotransmitter onto afferent auditory nerve fibers in response to sound stimulation. During early development, afferent synaptic transmission is triggered by spontaneous Ca2+ spikes of IHCs, which are under efferent cholinergic control. Around the onset of hearing, large-conductance Ca2+-activated K+ channels are acquired, and Ca2+ spikes as well as the cholinergic innervation are lost. Here, we performed patch-clamp measurements in IHCs of mice lacking the CaV1.3 channel (CaV1.3-/-) to investigate the role of this prevailing voltage-gated Ca2+ channel in IHC development and synaptic function. The small Ca2+ current remaining in IHCs from 3-week-old CaV1.3-/- mice was mainly mediated by L-type Ca2+ channels, because it was sensitive to dihydropyridines but resistant to inhibitors of non-L-type Ca2+ channels such as omega-conotoxins GVIA and MVIIC and SNX-482. Depolarization induced only marginal exocytosis in CaV1.3-/- IHC, which was solely mediated by L-type Ca2+ channels, whereas robust exocytic responses were elicited by photolysis of caged Ca2+. Secretion triggered by short depolarizations was reduced proportionally to the Ca2+ current, suggesting that the coupling of the remaining channels to exocytosis was unchanged. CaV1.3-/- IHCs lacked the Ca2+ action potentials and displayed a complex developmental failure. Most strikingly, we observed a continued presence of efferent cholinergic synaptic transmission and a lack of functional large-conductance Ca2+-activated K+ channels up to 4 weeks after birth. We conclude that CaV1.3 channels are essential for normal hair cell development and synaptic transmission.

Calcitonin gene-related peptide suppresses hair cell responses to mechanical stimulation in the Xenopus lateral line organ

The presence of calcitonin gene-related peptide (CGRP) in the efferent fibers of virtually every hair cell organ studied suggests it may serve some fundamental but heretofore unknown role in control of hair cell function. We examined the effects of CGRP on spontaneous and stimulus-evoked discharge patterns in an in vitro preparation of the lateral line organ of Xenopus laevis. Discharge patterns were determined by sinusoidally displacing the cupula with a glass micropipette driven with a piezoelectric device while recording afferent fiber activity. All afferent fibers had characteristic frequencies of 16-32 Hz. Responses synchronized to cupular displacements as small as 20 nm. CGRP suppressed responses of the lateral line organ to displacement while increasing spontaneous discharge rate. In the presence of CGRP, stimulus-response curves were shifted 10 dB toward higher displacement levels. The suppression of stimulus-evoked responses suggests a function for CGRP as an efferent neurotransmitter that is similar to that of cholinergic efferent transmission in other hair cell organs. The 10 dB shift toward larger displacements makes it comparable in magnitude with the effects of electrical stimulation of efferents in the mammalian cochlea. This suggests a significant role for CGRP in efferent modulation of the output of this mechanosensory organ.

Long-term effects of sectioning the olivocochlear bundle in neonatal cats

The olivocochlear bundle (OCB) was cut in neonatal cats to evaluate its role in the development of normal cochlear function. Approximately 1 year after deefferentation, acute auditory nerve fiber (ANF) recordings were made from lesioned animals, lesion shams, and normal controls. The degree of deefferentation was quantified via light microscopic evaluation of the density of OCB fascicles in the tunnel of Corti, and selected cases were analyzed via electron microscopy. In the most successful cases, the deefferentation was virtually complete. ANFs from successfully lesioned animals exhibited significant pathophysiology compared with normals and with other animals in which the surgery failed to interrupt the OCB. Thresholds at the characteristic frequency (CF), the frequency at which ANFs are most sensitive, were elevated across the CF range, with maximal effects for CFs in the 10 kHz region. Frequency threshold or tuning curves displayed reduction of tip-to-tail ratios (the difference between CF and low-frequency "tail" thresholds) and decreased sharpness of tuning. These pathological changes are generally associated with outer hair cell (OHC) damage. However, light microscopic histological analysis showed minimal hair cell loss and no significant differences between normal and deefferented groups. Spontaneous discharge rates (SRs) were lower than normal; however, those fibers with the highest SRs remained more sensitive than those with lower SRs. Findings suggest that the interaction between OC efferents and OHCs early in development may be critical for full expression of active mechanical processes.

Relationship between the development of outer hair cell electromotility and efferent innervation: a study in cultured organ of corti of neonatal gerbils

Outer hair cell (OHC) electromotility, which powers the cochlear amplifier, develops at a later stage of hearing ontogeny. There has been speculation whether efferents play a necessary role in directing or achieving OHC maturation in mammals. In this study, we examine whether the development of OHC motility depends on the establishment of efferent innervation of the cells' synaptic pole by measuring electromotility of OHCs grown in cultures, deprived of efferent innervation. Tissue cultures of the organ of Corti were prepared from the cochleas of newborn gerbils. Solitary OHCs were obtained from 4- to 15-d-old cultures by enzymatic digestion and mechanical trituration. Length changes evoked by transcellular electrical stimulation were detected and measured with a photodiode sensor. Results show that OHCs develop electromotility between 6 and 13 d in culture without the presence of efferent innervation. The timetable for the onset of OHC electromotility is comparable with that in vivo. This demonstrates that the ontogeny of OHC electromotility is an intrinsic process that does not require the influence of efferent innervation.

Acetylcholine, outer hair cell electromotility, and the cochlear amplifier

The dominant efferent innervation of the cochlea terminates on outer hair cells (OHCs), with acetylcholine (ACh) being its principal neurotransmitter. OHCs respond with a somatic shape change to alterations in their membrane potential, and this electromotile response is believed to provide mechanical feedback to the basilar membrane. We examine the effects of ACh on electromotile responses in isolated OHCs and attempt to deduce the mechanism of ACh action. Axial electromotile amplitude and cell compliance increase in the presence of the ligand. This response occurs with a significantly greater latency than membrane current and potential changes attributable to ACh and is contemporaneous with Ca2+ release from intracellular stores. It is likely that increased axial compliance largely accounts for the increase in motility. The mechanical responses are probably related to a recently demonstrated slow efferent effect. The implications of the present findings related to commonly assumed efferent behavior in vivo are considered.