Role of alpha9 nicotinic ACh receptor subunits in the development and function of cochlear efferent innervation

Wireless electrocochleography in awake chinchillas: A model to study crossmodal modulations at the peripheral level

The discovery and development of electrocochleography (ECochG) in animal models has been fundamental for its implementation in clinical audiology and neurotology. In our laboratory, the use of round-window ECochG recordings in chinchillas has allowed a better understanding of auditory efferent functioning. In previous works, we gave evidence of the corticofugal modulation of auditory-nerve and cochlear responses during visual attention and working memory. However, whether these cognitive top-down mechanisms to the most peripheral structures of the auditory pathway are also active during audiovisual crossmodal stimulation is unknown. Here, we introduce a new technique, wireless ECochG to record compound-action potentials of the auditory nerve (CAP), cochlear microphonics (CM), and round-window noise (RWN) in awake chinchillas during a paradigm of crossmodal (visual and auditory) stimulation. We compared ECochG data obtained from four awake chinchillas recorded with a wireless ECochG system with wired ECochG recordings from six anesthetized animals. Although ECochG experiments with the wireless system had a lower signal-to-noise ratio than wired recordings, their quality was sufficient to compare ECochG potentials in awake crossmodal conditions. We found non-significant differences in CAP and CM amplitudes in response to audiovisual stimulation compared to auditory stimulation alone (clicks and tones). On the other hand, spontaneous auditory-nerve activity (RWN) was modulated by visual crossmodal stimulation, suggesting that visual crossmodal simulation can modulate spontaneous but not evoked auditory-nerve activity. However, given the limited sample of 10 animals (4 wireless and 6 wired), these results should be interpreted cautiously. Future experiments are required to substantiate these conclusions. In addition, we introduce the use of wireless ECochG in animal models as a useful tool for translational research.

Alpha9alpha10 knockout mice show altered physiological and behavioral responses to signals in masking noise

Medial olivocochlear (MOC) efferents modulate outer hair cell motility through specialized nicotinic acetylcholine receptors to support encoding of signals in noise. Transgenic mice lacking the alpha9 subunits of these receptors (α9KOs) have normal hearing in quiet and noise, but lack classic cochlear suppression effects and show abnormal temporal, spectral, and spatial processing. Mice deficient for both the alpha9 and alpha10 receptor subunits (α9α10KOs) may exhibit more severe MOC-related phenotypes. Like α9KOs, α9α10KOs have normal auditory brainstem response (ABR) thresholds and weak MOC reflexes. Here, we further characterized auditory function in α9α10KO mice. Wild-type (WT) and α9α10KO mice had similar ABR thresholds and acoustic startle response amplitudes in quiet and noise, and similar frequency and intensity difference sensitivity. α9α10KO mice had larger ABR Wave I amplitudes than WTs in quiet and noise. Other ABR metrics of hearing-in-noise function yielded conflicting findings regarding α9α10KO susceptibility to masking effects. α9α10KO mice also had larger startle amplitudes in tone backgrounds than WTs. Overall, α9α10KO mice had grossly normal auditory function in quiet and noise, although their larger ABR amplitudes and hyperreactive startles suggest some auditory processing abnormalities. These findings contribute to the growing literature showing mixed effects of MOC dysfunction on hearing.

Auxiliary protein and chaperone regulation of neuronal nicotinic receptor subtype expression and function

Neuronal nicotinic acetylcholine receptors (nAChRs) are a family of pentameric, ligand-gated ion channels that are located on the surface of neurons and non-neuronal cells and have multiple physiological and pathophysiological functions. In order to reach the cell surface, many nAChR subtypes require the help of chaperone and/or auxiliary/accessory proteins for their assembly, trafficking, pharmacological modulation, and normal functioning in vivo. The use of powerful genome-wide cDNA screening has led to the identification and characterisation of the molecules and mechanisms that participate in the assembly and trafficking of receptor subtypes, including chaperone and auxiliary or accessory proteins. The aim of this review is to describe the latest findings concerning nAChR chaperones and auxiliary proteins and pharmacological chaperones, and how some of them control receptor biogenesis or regulate channel activation and pharmacology. Some auxiliary proteins are subtype selective, some regulate various subtypes, and some not only modulate nAChRs but also target other receptors and signalling pathways. We also discuss how changes in auxiliary proteins may be involved in nAChR dysfunctions.

Pharmacological profiles and anti-inflammatory activity of pCN-diEPP and mCN-diEPP, new alpha9alpha10 nicotinic receptor ligands

Pain due to inflammation can be reduced by targeting the noncanonical nicotinic receptors (NCNR) in cells of the immune system that regulate the synthesis and release of pro- and anti-inflammatory cytokines. Although NCNR do not generate ion channel currents, the pharmacology of ion-channel forms of the receptors can predict drugs which may be effective regulators of the cholinergic anti-inflammatory system (CAS). Agonists of α7 type receptors have been definitively associated with CAS. Receptors containing α9 and α10 subunits have also been implicated. We have recently characterized two small molecules, pCN-diEPP and mCN-diEPP, as selective α9α10 agonists and antagonists, respectively. We used these drugs, along with nicotine, an α7 agonist and α9α10 antagonist, to probe the mixed populations of receptors that are formed when α7, α9, and α10 are all expressed together in Xenopus oocytes. We also evaluated the effects of the CN-diEPP compounds on regulating the ATP-induced release of interleukin-1β from monocytic THP-1 cells, which express NCNR. The compounds successfully identified separate populations of receptors when all three subunits were co-expressed, including a potential population of homomeric α10 receptors. The α9α10 agonist pCN-diEPP was the more effective regulator of interleukin-1β release in THP-1 cells. pCN-diEPP was also fully effective in a mouse model of inflammatory pain, while mCN-diEPP had only partial effects, requiring a higher dosage. The analgetic effects of pCN-diEPP and mCN-diEPP were retained in α7 knockout mice. Taken together, our results suggest that drugs that selectively activate α9α10 receptors may useful to reduce inflammatory pain through the CAS.

Alpha9alpha10 knockout mice show altered physiological and behavioral responses to signals in masking noise

Medial olivocochlear (MOC) efferents modulate outer hair cell motility through specialized nicotinic acetylcholine receptors to support encoding of signals in noise. Transgenic mice lacking the alpha9 subunits of these receptors (α9KOs) have normal hearing in quiet and noise, but lack classic cochlear suppression effects and show abnormal temporal, spectral, and spatial processing. Mice deficient for both the alpha9 and alpha10 receptor subunits (α9α10KOs) may exhibit more severe MOC-related phenotypes. Like α9KOs, α9α10KOs have normal auditory brainstem response (ABR) thresholds and weak MOC reflexes. Here, we further characterized auditory function in α9α10KO mice. Wildtype and α9α10KO mice had similar ABR thresholds and acoustic startle response (ASR) amplitudes in quiet and noise, and similar frequency and intensity difference sensitivity. α9α10KO mice had larger ABR Wave I amplitudes than wildtypes in quiet and noise, but the noise:quiet amplitude ratio suggested α9α10KOs were more susceptible to masking effects for some stimuli. α9α10KO mice also had larger startle amplitudes in tone backgrounds than wildtypes. Overall, α9α10KO mice had grossly normal auditory function in quiet and noise, though their larger ABR amplitudes and hyperreactive startles suggest some auditory processing abnormalities. These findings contribute to the growing literature showing mixed effects of MOC dysfunction on hearing.

Engineering olivocochlear inhibition to reduce acoustic trauma

Efferent brain-stem neurons release acetylcholine to desensitize cochlear hair cells and can protect the inner ear from acoustic trauma. That protection is absent from knockout mice lacking efferent inhibition and is stronger in mice with a gain-of-function point mutation of the hair cell-specific nicotinic acetylcholine receptor. The present work uses viral transduction of gain-of-function receptors to restore acoustic prophylaxis to the knockout mice. Widespread postsynaptic expression of the transgene was visualized in excised tissue with a fluorophore-conjugated peptide toxin that binds selectively to hair cell acetylcholine receptors. Viral transduction into efferent knockout mice reduced the temporary hearing loss measured 1 day post acoustic trauma. The acoustic evoked-response waveform (auditory brain-stem response) recovered more rapidly in treated mice than in control mice. Thus, both cochlear amplification by outer hair cells (threshold shift) and afferent signaling (evoked-response amplitude) in knockout mice were protected by viral transduction of hair cell acetylcholine receptors. Gene therapy to strengthen efferent cochlear feedback could be complementary to existing and future therapies to prevent hearing loss, including ear coverings, hearing aids, single-gene repair, or small-molecule therapies.

The α9α10 acetylcholine receptor: a non-neuronal nicotinic receptor

Within the superfamily of pentameric ligand-gated ion channels, cholinergic nicotinic receptors (nAChRs) were classically identified to mediate synaptic transmission in the nervous system and the neuromuscular junction. The α9 and α10 nAChR subunits were the last ones to be identified. Surprisingly, they do not fall into the dichotomic neuronal/muscle classification of nAChRs. They assemble into heteropentamers with a well-established function as canonical ion channels in inner ear hair cells, where they mediate central nervous system control of auditory and vestibular sensory processing. The present review includes expression, pharmacological, structure-function, molecular evolution and pathophysiological studies, that define receptors composed from α9 and α10 subunits as distant and distinct members within the nAChR family. Thus, although α9 and α10 were initially included within the neuronal subdivision of nAChR subunits, they form a distinct clade within the phylogeny of nAChRs. Following the classification of nAChR subunits based on their main synaptic site of action, α9 and α10 should receive a name in their own right.

Nicotinic acetylcholine receptor subtype expression, function, and pharmacology: Therapeutic potential of α-conotoxins

The pentameric nicotinic acetylcholine receptors (nAChRs) are typically classed as muscle- or neuronal-type, however, the latter has also been reported in non-neuronal cells. Given their broad distribution, nAChRs mediate numerous physiological and pathological processes including synaptic transmission, presynaptic modulation of transmitter release, neuropathic pain, inflammation, and cancer. There are 17 different nAChR subunits and combinations of these subunits produce subtypes with diverse pharmacological properties. The expression and role of some nAChR subtypes have been extensively deciphered with the aid of knock-out models. Many nAChR subtypes expressed in heterologous systems are selectively targeted by the disulfide-rich α-conotoxins. α-Conotoxins are small peptides isolated from the venom of cone snails, and a number of them have potential pharmaceutical value.

Expression of Chrna9 is regulated by Tbx3 in undifferentiated pluripotent stem cells

It was reported that nicotinic acetylcholine receptor (nAChR)-mediated signaling pathways affect the proliferation and differentiation of pluripotent stem cells. However, detail expression profiles of nAChR genes were unrevealed in these cells. In this study, we comprehensively investigated the gene expression of α subunit of nAChRs (Chrna) during differentiation and induction of pluripotent stem cells. Mouse embryonic stem (ES) cells expressed multiple Chrna genes (Chrna3-5, 7 and 9) in undifferentiated status. Among them, Chrna9 was markedly down-regulated upon the differentiation into mesenchymal cell lineage. In mouse tissues and cells, Chrna9 was mainly expressed in testes, ES cells and embryonal F9 teratocarcinoma stem cells. Expression of Chrna9 gene was acutely reduced during differentiation of ES and F9 cells within 24 h. In contrast, Chrna9 expression was increased in induced pluripotent stem cells established from mouse embryonic fibroblast. It was shown by the reporter assays that T element-like sequence in the promoter region of Chrna9 gene is important for its activities in ES cells. Chrna9 was markedly reduced by siRNA-mediated knockdown of Tbx3, a pluripotency-related transcription factor of the T-box gene family. These results indicate that Chrna9 is a nAChR gene that are transcriptionally regulated by Tbx3 in undifferentiated pluripotent cells.

Detection of West Nile Virus Envelope Protein in Brain Tissue with an Immunohistochemical Assay

Immunohistochemistry is a valuable tool for probing not only scientific questions but also clinical diagnoses. It provides power from localization of a protein within the milieu of a tissue section that may reflect positioning within or beyond the boundaries of a cell that is representative of the tissue at a discrete moment in time. The method can be applied broadly, including to tissues under normal, developmental, chemically, or genetically altered conditions and disease states.Disease manifesting from West Nile virus infection ranges from acute, systemic febrile symptoms to compromise of central nervous system function. Immunohistochemistry has been used to assess WNV infection in the nervous system in postmortem and experimental conditions, despite the lack of understanding of the precise route of viral entry. In addition to imprecise knowledge of initial viral entry into cells and whether entry is even the same between cell types, the fact that spontaneous viral mutations and environmental pressures from climate change may alter the prevalence of the disease state across geographical and climatological boundaries highlights the need for continued assessment of infection. Immunohistochemistry is a useful way to assess these aspects of WNV infection with the aim being to better understand the organs and cell types that are compromised by WNV infection. This chapter outlines how this can be carried out on brain tissue, but the procedures discussed can also be applied more broadly on tissue outside of the central nervous system.

RgIA4 Prevention of Acute Oxaliplatin-Induced Cold Allodynia Requires α9-Containing Nicotinic Acetylcholine Receptors and CD3+ T-Cells

Chemotherapy-induced neuropathic pain is a debilitating and dose-limiting side effect. Oxaliplatin is a third-generation platinum and antineoplastic compound that is commonly used to treat colorectal cancer and commonly yields neuropathic side effects. Available drugs such as duloxetine provide only modest benefits against oxaliplatin-induced neuropathy. A particularly disruptive symptom of oxaliplatin is painful cold sensitivity, known as cold allodynia. Previous studies of the Conus regius peptide, RgIA, and its analogs have demonstrated relief from oxaliplatin-induced cold allodynia, yielding improvement that persists even after treatment cessation. Moreover, underlying inflammatory and neuronal protection were shown at the cellular level in chronic constriction nerve injury models, consistent with disease-modifying effects. Despite these promising preclinical outcomes, the underlying molecular mechanism of action of RgIA4 remains an area of active investigation. This study aimed to determine the necessity of the α9 nAChR subunit and potential T-cell mechanisms in RgIA4 efficacy against acute oxaliplatin-induced cold allodynia. A single dose of oxaliplatin (10 mg/kg) was utilized followed by four daily doses of RgIA4. Subcutaneous administration of RgIA4 (40 µg/kg) prevented cold allodynia in wildtype mice but not in mice lacking the α9 nAChR-encoding gene, chrna9. RgIA4 also failed to reverse allodynia in mice depleted of CD3⁺ T-cells. In wildtype mice treated with oxaliplatin, quantitated circulating T-cells remained unaffected by RgIA4. Together, these results show that RgIA4 requires both chrna9 and CD3⁺ T-cells to exert its protective effects against acute cold-allodynia produced by oxaliplatin.

Corticofugal and Brainstem Functions Associated With Medial Olivocochlear Cholinergic Transmission

Cholinergic transmission is essential for survival and reproduction, as it is involved in several physiological responses. In the auditory system, both ascending and descending auditory pathways are modulated by cholinergic transmission, affecting the perception of sounds. The auditory efferent system is a neuronal network comprised of several feedback loops, including corticofugal and brainstem pathways to the cochlear receptor. The auditory efferent system's -final and mandatory synapses that connect the brain with the cochlear receptor- involve medial olivocochlear neurons and outer hair cells. A unique cholinergic transmission mediates these synapses through α9/α10 nicotinic receptors. To study this receptor, it was generated a strain of mice carrying a null mutation of the Chrna9 gene (α9-KO mice), lacking cholinergic transmission between medial olivocochlear neurons and outer hair cells, providing a unique opportunity to study the role of medial olivocochlear cholinergic transmission in auditory and cognitive functions. In this article, we review behavioral and physiological studies carried out to research auditory efferent function in the context of audition, cognition, and hearing impairments. Auditory studies have shown that hearing thresholds in the α9-KO mice are normal, while more complex auditory functions, such as frequency selectivity and sound localization, are altered. The corticofugal pathways have been studied in α9-KO mice using behavioral tasks, evidencing a reduced capacity to suppress auditory distractors during visual selective attention. Finally, we discuss the evolutionary role of the auditory efferent system detecting vocalizations in noise and its role in auditory disorders, such as the prevention of age-related hearing loss.

Vestibulo-Ocular Reflex Short-Term Adaptation Is Halved After Compensation for Unilateral Labyrinthectomy

Several prior studies, including those from this laboratory, have suggested that vestibulo-ocular reflex (VOR) adaptation and compensation are two neurologically related mechanisms. We therefore hypothesised that adaptation would be affected by compensation, depending on the amount of overlap between these two mechanisms. To better understand this overlap, we examined the effect of gain-increase (gain = eye velocity/head velocity) adaptation training on the VOR in compensated mice since both adaptation and compensation mechanisms are presumably driving the gain to increase. We tested 11 cba129 controls and 6 α9-knockout mice, which have a compromised efferent vestibular system (EVS) known to affect both adaptation and compensation mechanisms. Baseline VOR gains across frequencies (0.2 to 10 Hz) and velocities (20 to 100°/s) were measured on day 28 after unilateral labyrinthectomy (UL) and post-adaptation gains were measured after gain-increase training on day 31 post-UL. Our findings showed that after chronic compensation gain-increase adaptation, as a percentage of baseline, in both strains of mice (~14%), was about half compared to their previously reported healthy, non-operated counterparts (~32%). Surprisingly, there was no difference in gain-increase adaptation between control and α9-knockout mice. These data support the notion that adaptation and compensation are separate but overlapping processes. They also suggest that half of the original adaptation capacity remained in chronically compensated mice, regardless of EVS compromise associated with α9-knockout mice, and strongly suggest VOR adaptation training is a viable treatment strategy for vestibular rehabilitation therapy and, importantly, augments the compensatory process.

Selective Agonists and Antagonists of α9 Versus α7 Nicotinic Acetylcholine Receptors

Nicotinic acetylcholine receptors containing α9 subunits are essential for the auditory function and have been implicated, along with α7-containing nicotinic receptors, as potential targets for the treatment of inflammatory and neuropathic pain. The study of α9-containing receptors has been hampered by the lack of selective agonists. The only α9-selective antagonists previously identified are peptide conotoxins. Curiously, the activity of α7 and α9 receptors as modulators of inflammatory pain appears to not rely strictly on ion channel activation, which led to the identification of α7 "silent agonists" and phosphocholine as an "unconventional agonist" for α9 containing receptors. The parallel testing of the α7 silent agonist p-CF₃-diEPP and phosphocholine led to the discovery that p-CF₃-diEPP was an α9 agonist. In this report, we compared the activity of α7 and α9 with a family of structurally related compounds, most of which were previously shown to be α7 partial or silent agonists. We identify several potent α9-selective agonists as well as numerous potent and selective α9 antagonists and describe the structural basis for these activities. Several of these compounds have previously been shown to be effective in animal models of inflammatory pain, an activity that was assumed to be due to α7 silent agonism but may, in fact, be due to α9 activity. The α9-selective conotoxin antagonists have also been shown to reduce pain in similar models. Our identification of these new α9 agonists and antagonists may prove to be invaluable for defining an optimal approach for treating pain, allowing for reduced use of opioid drugs.

The α9α10 nicotinic acetylcholine receptor: a compelling drug target for hearing loss?

INTRODUCTION

Hearing loss is a major health problem, impacting education, communication, interpersonal relationships, and mental health. Drugs that prevent or restore hearing are lacking and hence novel drug targets are sought. There is the possibility of targeting the α9α10 nicotinic acetylcholine receptor (nAChR) in the prevention of noise-induced, hidden hearing loss and presbycusis. This receptor mediates synaptic transmission between medial olivocochlear efferent fibers and cochlear outer hair cells. This target is key since enhanced olivocochlear activity prevents noise-induced hearing loss and delays presbycusis.

AREAS COVERED

The work examines the α9α10 nicotinic acetylcholine receptor (nAChR), its role in noise-induced, hidden hearing loss and presbycusis and the possibility of targeting. Data has been searched in Pubmed, the World Report on Hearing from the World Health Organization and the Global Burden of Disease Study 2019.

EXPERT OPINION

The design of positive allosteric modulators of α9α10 nAChRs is proposed because of the advantage of reinforcing the medial olivocochlear (MOC)-hair cell endogenous neurotransmission without directly stimulating the target receptors, therefore avoiding receptor desensitization and reduced efficacy. The time is right for the discovery and development of α9α10 nAChRs targeting agents and high throughput screening assays will support this.

α9-Containing Nicotinic Receptors in Cancer

Neuronal nicotinic acetylcholine receptors containing the α9 or the α9 and α10 subunits are expressed in various extra-neuronal tissues. Moreover, most cancer cells and tissues highly express α9-containing receptors, and a number of studies have shown that they are powerful regulators of responses that stimulate cancer processes such as proliferation, inhibition of apoptosis, and metastasis. It has also emerged that their modulation is a promising target for drug development. The aim of this review is to summarize recent data showing the involvement of these receptors in controlling the downstream signaling cascades involved in the promotion of cancer.

Loss of α-9 Nicotinic Acetylcholine Receptor Subunit Predominantly Results in Impaired Postural Stability Rather Than Gaze Stability

The functional role of the mammalian efferent vestibular system (EVS) is not fully understood. One proposal is that the mammalian EVS plays a role in the long-term calibration of central vestibular pathways, for example during development. Here to test this possibility, we studied vestibular function in mice lacking a functional α9 subunit of the nicotinic acetylcholine receptor (nAChR) gene family, which mediates efferent activation of the vestibular periphery. We focused on an α9 (−/−) model with a deletion in exons 1 and 2. First, we quantified gaze stability by testing vestibulo-ocular reflex (VOR, 0.2–3 Hz) responses of both α9 (−/−) mouse models in dark and light conditions. VOR gains and phases were comparable for both α9 (−/−) mutants and wild-type controls. Second, we confirmed the lack of an effect from the α9 (−/−) mutation on central visuo-motor pathways/eye movement pathways via analyses of the optokinetic reflex (OKR) and quick phases of the VOR. We found no differences between α9 (−/−) mutants and wild-type controls. Third and finally, we investigated postural abilities during instrumented rotarod and balance beam tasks. Head movements were quantified using a 6D microelectromechanical systems (MEMS) module fixed to the mouse’s head. Compared to wild-type controls, we found head movements were strikingly altered in α9 (−/−) mice, most notably in the pitch axis. We confirmed these later results in another α9 (−/−) model, with a deletion in the exon 4 region. Overall, we conclude that the absence of the α9 subunit of nAChRs predominately results in an impairment of posture rather than gaze.

Functional and ultrastructural analysis of reafferent mechanosensation in larval zebrafish

All animals need to differentiate between exafferent stimuli, which are caused by the environment, and reafferent stimuli, which are caused by their own movement. In the case of mechanosensation in aquatic animals, the exafferent inputs are water vibrations in the animal's proximity, which need to be distinguishable from the reafferent inputs arising from fluid drag due to locomotion. Both of these inputs are detected by the lateral line, a collection of mechanosensory organs distributed along the surface of the body. In this study, we characterize in detail how hair cells-the receptor cells of the lateral line-in zebrafish larvae discriminate between such reafferent and exafferent signals. Using dye labeling of the lateral line nerve, we visualize two parallel descending inputs that can influence lateral line sensitivity. We combine functional imaging with ultra-structural EM circuit reconstruction to show that cholinergic signals originating from the hindbrain transmit efference copies (copies of the motor command that cancel out self-generated reafferent stimulation during locomotion) and that dopaminergic signals from the hypothalamus may have a role in threshold modulation, both in response to locomotion and salient stimuli. We further gain direct mechanistic insight into the core components of this circuit by loss-of-function perturbations using targeted ablations and gene knockouts. We propose that this simple circuit is the core implementation of mechanosensory reafferent suppression in these young animals and that it might form the first instantiation of state-dependent modulation found at later stages in development.

Characterizing the Access of Cholinergic Antagonists to Efferent Synapses in the Inner Ear

Stimulation of cholinergic efferent neurons innervating the inner ear has profound, well-characterized effects on vestibular and auditory physiology, after activating distinct ACh receptors (AChRs) on afferents and hair cells in peripheral endorgans. Efferent-mediated fast and slow excitation of vestibular afferents are mediated by α4β2*-containing nicotinic AChRs (nAChRs) and muscarinic AChRs (mAChRs), respectively. On the auditory side, efferent-mediated suppression of distortion product otoacoustic emissions (DPOAEs) is mediated by α9α10nAChRs. Previous characterization of these synaptic mechanisms utilized cholinergic drugs, that when systemically administered, also reach the CNS, which may limit their utility in probing efferent function without also considering central effects. Use of peripherally-acting cholinergic drugs with local application strategies may be useful, but this approach has remained relatively unexplored. Using multiple administration routes, we performed a combination of vestibular afferent and DPOAE recordings during efferent stimulation in mouse and turtle to determine whether charged mAChR or α9α10nAChR antagonists, with little CNS entry, can still engage efferent synaptic targets in the inner ear. The charged mAChR antagonists glycopyrrolate and methscopolamine blocked efferent-mediated slow excitation of mouse vestibular afferents following intraperitoneal, middle ear, or direct perilymphatic administration. Both mAChR antagonists were effective when delivered to the middle ear, contralateral to the side of afferent recordings, suggesting they gain vascular access after first entering the perilymphatic compartment. In contrast, charged α9α10nAChR antagonists blocked efferent-mediated suppression of DPOAEs only upon direct perilymphatic application, but failed to reach efferent synapses when systemically administered. These data show that efferent mechanisms are viable targets for further characterizing drug access in the inner ear.

Efferent neurons control hearing sensitivity and protect hearing from noise through the regulation of gap junctions between cochlear supporting cells

It is critical for hearing that the descending cochlear efferent system provides a negative feedback to hair cells to regulate hearing sensitivity and protect hearing from noise. The medial olivocochlear (MOC) efferent nerves project to outer hair cells (OHCs) to regulate OHC electromotility, which is an active cochlear amplifier and can increase hearing sensitivity. Here, we report that the MOC efferent nerves also could innervate supporting cells (SCs) in the vicinity of OHCs to regulate hearing sensitivity. MOC nerve fibers are cholinergic, and acetylcholine (ACh) is a primary neurotransmitter. Immunofluorescent staining showed that MOC nerve endings, presynaptic vesicular acetylcholine transporters (VAChTs), and postsynaptic ACh receptors were visible at SCs and in the SC area. Application of ACh in SCs could evoke a typical inward current and reduce gap junctions (GJs) between them, which consequently enhanced the direct effect of ACh on OHCs to shift but not eliminate OHC electromotility. This indirect, GJ-mediated inhibition had a long-lasting influence. In vivo experiments further demonstrated that deficiency of this GJ-mediated efferent pathway decreased the regulation of active cochlear amplification and compromised the protection against noise. In particular, distortion product otoacoustic emission (DPOAE) showed a delayed reduction after noise exposure. Our findings reveal a new pathway for the MOC efferent system via innervating SCs to control active cochlear amplification and hearing sensitivity. These data also suggest that this SC GJ-mediated efferent pathway may play a critical role in long-term efferent inhibition and is required for protection of hearing from noise trauma.NEW & NOTEWORTHY The cochlear efferent system provides a negative feedback to control hair cell activity and hearing sensitivity and plays a critical role in noise protection. We reveal a new efferent control pathway in which medial olivocochlear efferent fibers have innervations with cochlear supporting cells to control their gap junctions, therefore regulating outer hair cell electromotility and hearing sensitivity. This supporting cell gap junction-mediated efferent control pathway is required for the protection of hearing from noise.

The Hair Cell α9α10 Nicotinic Acetylcholine Receptor: Odd Cousin in an Old Family

Nicotinic acetylcholine receptors (nAChRs) are a subfamily of pentameric ligand-gated ion channels with members identified in most eumetazoan clades. In vertebrates, they are divided into three subgroups, according to their main tissue of expression: neuronal, muscle and hair cell nAChRs. Each receptor subtype is composed of different subunits, encoded by paralogous genes. The latest to be identified are the α9 and α10 subunits, expressed in the mechanosensory hair cells of the inner ear and the lateral line, where they mediate efferent modulation. α9α10 nAChRs are the most divergent amongst all nicotinic receptors, showing marked differences in their degree of sequence conservation, their expression pattern, their subunit co-assembly rules and, most importantly, their functional properties. Here, we review recent advances in the understanding of the structure and evolution of nAChRs. We discuss the functional consequences of sequence divergence and conservation, with special emphasis on the hair cell α9α10 receptor, a seemingly distant cousin of neuronal and muscle nicotinic receptors. Finally, we highlight potential links between the evolution of the octavolateral system and the extreme divergence of vertebrate α9α10 receptors.

Cy3-RgIA-5727 Labels and Inhibits α9-Containing nAChRs of Cochlear Hair Cells

Efferent cholinergic neurons inhibit sensory hair cells of the vertebrate inner ear through the combined action of calcium-permeable α9α10-containing nicotinic acetylcholine receptors (nAChRs) and associated calcium-dependent potassium channels. The venom of cone snails is a rich repository of bioactive peptides, many with channel blocking activities. The conopeptide analog, RgIA-5474, is a specific and potent antagonist of α9α10-containing nAChRs. We added an alkyl functional group to the N-terminus of the RgIA-5474, to enable click chemistry addition of the fluorescent cyanine dye, Cy3. The resulting peptide, Cy3-RgIA-5727, potently blocked mouse α9α10 nAChRs expressed in Xenopus oocytes (IC₅₀ 23 pM), with 290-fold less activity on α7 nAChRs and 40,000-fold less activity on all other tested nAChR subtypes. The tight binding of Cy3-RgIA-5727 provided robust visualization of hair cell nAChRs juxtaposed to cholinergic efferent terminals in excised, unfixed cochlear tissue from mice. Presumptive postsynaptic sites on outer hair cells (OHCs) were labeled, but absent from inner hair cells (IHCs) and from OHCs in cochlear tissue from α9-null mice and in cochlear tissue pre-incubated with non-Cy3-conjugated RgIA-5474. In cochlear tissue from younger (postnatal day 10) mice, Cy3-RgIA-5727 also labeled IHCs, corresponding to transient efferent innervation at that age. Cy3 puncta in Kölliker’s organ remained in the α9-null tissue. Pre-exposure with non-Cy3-conjugated RgIA-5474 or bovine serum albumin reduced this non-specific labeling to variable extents in different preparations. Cy3-RgIA-5727 and RgIA-5474 blocked the native hair cell nAChRs, within the constraints of application to the excised cochlear tissue. Cy3-RgIA-5727 or RgIA-5474 block of efferent synaptic currents in young IHCs was not relieved after 50 min washing, so effectively irreversible.

Prenatal Exposure to Tobacco and Alcohol Alters Development of the Neonatal Auditory System

Prenatal exposures to alcohol (PAE) and tobacco (PTE) are known to produce adverse neonatal and childhood outcomes including damage to the developing auditory system. Knowledge of the timing, extent, and combinations of these exposures on effects on the developing system is limited. As part of the physiological measurements from the Safe Passage Study, Auditory Brainstem Responses (ABRs) and Transient Otoacoustic Emissions (TEOAEs) were acquired on infants at birth and one-month of age. Research sites were in South Africa and the Northern Plains of the U.S. Prenatal information on alcohol and tobacco exposure was gathered prospectively on mother/infant dyads. Cluster analysis was used to characterize three levels of PAE and three levels of PTE. Repeated-measures ANOVAs were conducted for newborn and one-month-old infants for ABR peak latencies and amplitudes and TEOAE levels and signal-to-noise ratios. Analyses controlled for hours of life at test, gestational age at birth, sex, site, and other exposure. Significant main effects of PTE included reduced newborn ABR latencies from both ears. PTE also resulted in a significant reduction of ABR peak amplitudes elicited in infants at 1-month of age. PAE led to a reduction of TEOAE amplitude for 1-month-old infants but only in the left ear. Results indicate that PAE and PTE lead to early disruption of peripheral, brainstem, and cortical development and neuronal pathways of the auditory system, including the olivocochlear pathway.

Spatial organization and transitions of spontaneous neuronal activities in the developing sensory cortex

The sensory cortex underlies our ability to perceive and interact with the external world. Sensory perceptions are controlled by specialized neuronal circuits established through fine-tuning, which relies largely on neuronal activity during the development. Spontaneous neuronal activity is an essential driving force of neuronal circuit refinement. At early developmental stages, sensory cortices display spontaneous activities originating from the periphery and characterized by correlated firing arranged spatially according to the modality. The firing patterns are reorganized over time and become sparse, which is typical for the mature brain. This review focuses mainly on rodent sensory cortices. First, the features of the spontaneous activities during early postnatal stages are described. Then, the developmental changes in the spatial organization of the spontaneous activities and the transition mechanisms involved are discussed. The identification of the principles controlling the spatial organization of spontaneous activities in the developing sensory cortex is essential to understand the self-organization process of neuronal circuits.

Dose-Dependent Pattern of Cochlear Synaptic Degeneration in C57BL/6J Mice Induced by Repeated Noise Exposure

It is widely accepted that even a single acute noise exposure at moderate intensity that induces temporary threshold shift (TTS) can result in permanent loss of ribbon synapses between inner hair cells and afferents. However, effects of repeated or chronic noise exposures on the cochlear synapses especially medial olivocochlear (MOC) efferent synapses remain elusive. Based on a weeklong repeated exposure model of bandwidth noise over 2-20 kHz for 2 hours at seven intensities (88 to 106 dB SPL with 3 dB increment per gradient) on C57BL/6J mice, we attempted to explore the dose-response mechanism of prolonged noise-induced audiological dysfunction and cochlear synaptic degeneration. In our results, mice repeatedly exposed to relatively low-intensity noise (88, 91, and 94 dB SPL) showed few changes on auditory brainstem response (ABR), ribbon synapses, or MOC efferent synapses. Notably, repeated moderate-intensity noise exposures (97 and 100 dB SPL) not only caused hearing threshold shifts and the inner hair cell ribbon synaptopathy but also impaired MOC efferent synapses, which might contribute to complex patterns of damages on cochlear function and morphology. However, repeated high-intensity (103 and 106 dB SPL) noise exposures induced PTSs mainly accompanied by damages on cochlear amplifier function of outer hair cells and the inner hair cell ribbon synaptopathy, rather than the MOC efferent synaptic degeneration. Moreover, we observed a frequency-dependent vulnerability of the repeated acoustic trauma-induced cochlear synaptic degeneration. This study provides a sight into the hypothesis that noise-induced cochlear synaptic degeneration involves both afferent (ribbon synapses) and efferent (MOC terminals) pathology. The pattern of dose-dependent pathological changes induced by repeated noise exposure at various intensities provides a possible explanation for the complicated cochlear synaptic degeneration in humans. The underlying mechanisms remain to be studied in the future.

Cell-type identity of the avian cochlea

In contrast to mammals, birds recover naturally from acquired hearing loss, which makes them an ideal model for inner ear regeneration research. Here, we present a validated single-cell RNA sequencing resource of the avian cochlea. We describe specific markers for three distinct types of sensory hair cells, including a previously unknown subgroup, which we call superior tall hair cells. We identify markers for the supporting cells associated with tall hair cells, which represent the facultative stem cells of the avian inner ear. Likewise, we present markers for supporting cells that are located below the short cochlear hair cells. We further infer spatial expression gradients of hair cell genes along the tonotopic axis of the cochlea. This resource advances neurobiology, comparative biology, and regenerative medicine by providing a basis for comparative studies with non-regenerating mammalian cochleae and for longitudinal studies of the regenerating avian cochlea.

Olivocochlear efferent effects on perception and behavior

The role of the mammalian auditory olivocochlear efferent system in hearing has long been the subject of debate. Its ability to protect against damaging noise exposure is clear, but whether or not this is the primary function of a system that evolved in the absence of industrial noise remains controversial. Here we review the behavioral consequences of olivocochlear activation and diminished olivocochlear function. Attempts to demonstrate a role for hearing in noise have yielded conflicting results in both animal and human studies. A role in selective attention to sounds in the presence of distractors, or attention to visual stimuli in the presence of competing auditory stimuli, has been established in animal models, but again behavioral studies in humans remain equivocal. Auditory processing deficits occur in models of congenital olivocochlear dysfunction, but these deficits likely reflect abnormal central auditory development rather than direct effects of olivocochlear feedback. Additional proposed roles in age-related hearing loss, tinnitus, hyperacusis, and binaural or spatial hearing, are intriguing, but require additional study. These behavioral studies almost exclusively focus on medial olivocochlear effects, and many relied on lesioning techniques that can have unspecific effects. The consequences of lateral olivocochlear and of corticofugal pathway activation for perception remain unknown. As new tools for targeted manipulation of olivocochlear neurons emerge, there is potential for a transformation of our understanding of the role of the olivocochlear system in behavior across species.

Neonicotinoid Insecticides: Molecular Targets, Resistance, and Toxicity

Neonicotinoids have been used to protect crops and animals from insect pests since the 1990s, but there are concerns regarding their adverse effects on nontarget organisms, notably on bees. Enhanced resistance to neonicotinoids in pests is becoming well documented. We address the current understanding of neonicotinoid target site interactions, selectivity, and metabolism not only in pests but also in beneficial insects such as bees. The findings are relevant to the management of both neonicotinoids and the new generation of pesticides targeting insect nicotinic acetylcholine receptors.

Long-term potentiation of glycinergic synapses by semi-natural stimulation patterns during tonotopic map refinement

Before the onset of hearing, cochlea-generated patterns of spontaneous spike activity drive the maturation of central auditory circuits. In the glycinergic sound localization pathway from the medial nucleus of the trapezoid body (MNTB) to the lateral superior olive (LSO) this spontaneous activity guides the strengthening and silencing of synapses which underlies tonotopic map refinement. However, the mechanisms by which patterned activity regulates synaptic refinement in the MNTB-LSO pathway are still poorly understood. To address this question, we recorded from LSO neurons in slices from prehearing mice while stimulating MNTB afferents with stimulation patterns that mimicked those present in vivo. We found that these semi-natural stimulation patterns reliably elicited a novel form of long-term potentiation (LTP) of MNTB-LSO synapses. Stimulation patterns that lacked the characteristic high-frequency (200 Hz) component of prehearing spike activity failed to elicit potentiation. LTP was calcium dependent, required the activation of both g-protein coupled GABA_B and metabotropic glutamate receptors and involved an increase in postsynaptic glycine receptor-mediated currents. Our results provide a possible mechanism linking spontaneous spike bursts to tonotopic map refinement and further highlight the importance of the co-release of GABA and glutamate from immature glycinergic MNTB terminals.

Hair cell α9α10 nicotinic acetylcholine receptor functional expression regulated by ligand binding and deafness gene products

Auditory hair cells receive olivocochlear efferent innervation, which refines tonotopic mapping, improves sound discrimination, and mitigates acoustic trauma. The olivocochlear synapse involves α9α10 nicotinic acetylcholine receptors (nAChRs), which assemble in hair cells only coincident with cholinergic innervation and do not express in recombinant mammalian cell lines. Here, genome-wide screening determined that assembly and surface expression of α9α10 require ligand binding. Ion channel function additionally demands an auxiliary subunit, which can be transmembrane inner ear (TMIE) or TMEM132e. Both of these single-pass transmembrane proteins are enriched in hair cells and underlie nonsyndromic human deafness. Inner hair cells from TMIE mutant mice show altered postsynaptic α9α10 function and retain α9α10-mediated transmission beyond the second postnatal week associated with abnormally persistent cholinergic innervation. Collectively, this study provides a mechanism to link cholinergic input with α9α10 assembly, identifies unexpected functions for human deafness genes TMIE/TMEM132e, and enables drug discovery for this elusive nAChR implicated in prevalent auditory disorders.

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.

Impact of Key Nicotinic AChR Subunits on Post-Stroke Pneumococcal Pneumonia

Pneumonia is the most frequent severe medical complication after stroke. An overactivation of the cholinergic signaling after stroke contributes to immunosuppression and the development of spontaneous pneumonia caused by Gram-negative pathogens. The α7 nicotinic acetylcholine receptor (α7nAChR) has already been identified as an important mediator of the anti-inflammatory pathway after stroke. However, whether the α2, α5 and α9/10 nAChR expressed in the lung also play a role in suppression of pulmonary innate immunity after stroke is unknown. In the present study, we investigate the impact of various nAChRs on aspiration-induced pneumonia after stroke. Therefore, α2, α5, α7 and α9/10 nAChR knockout (KO) mice and wild type (WT) littermates were infected with Streptococcus pneumoniae (S. pneumoniae) three days after middle cerebral artery occlusion (MCAo). One day after infection pathogen clearance, cellularity in lung and spleen, cytokine secretion in bronchoalveolar lavage (BAL) and alveolar-capillary barrier were investigated. Here, we found that deficiency of various nAChRs does not contribute to an enhanced clearance of a Gram-positive pathogen causing post-stroke pneumonia in mice. In conclusion, these findings suggest that a single nAChR is not sufficient to mediate the impaired pulmonary defense against S. pneumoniae after experimental stroke.

Nicotinic Acetylcholine Receptors of PC12 Cells

Nicotinic acetylcholine receptors (nAChRs) have gained much attention in the scientific community since they play a significant role in multiple physiological and pathophysiological processes. Multiple approaches to study the receptors exist, with characterization of the receptors' functionality at a single cellular level using cell culturing being one of them. Derived from an adrenal medulla tumor, PC12 cells express nicotinic receptor subunits and form functional nicotinic receptors. Thus, the cells offer a convenient environment to address questions related to the functionality of the receptors. The review summarizes the findings on nicotinic receptors' expression and functions which were conducted using PC12 cells. Specific focus is given to α3-containing receptors as well as α7 receptor. Critical evaluation of findings is provided alongside insights into what can still be learned about nAChRs, using PC12 cells.

Acute nicotine administration stimulates ciliary activity via α3β4 nAChR in the mouse trachea

Mucociliary clearance, the continuous removal of mucus-trapped particles by cilia-driven directed transport of the airway lining fluid, is the primary innate defense mechanism of the airways. It is potently activated by acetylcholine (ACh) addressing muscarinic receptors with a currently less defined role of nicotinic ACh receptors (nAChR). We here set out to determine their contribution in driving ciliary activity in an explanted mouse trachea preparation utilizing selected agonists and antagonists and nAChR-subunit deficient mice. Nicotine (100 µM) induced an increase in ciliary beat frequency, accompanied by a sharp, but not long lasting increase in particle transport speed (PTS) on the mucosal surface showing marked desensitization within the next 30 min. Nicotine-induced PTS acceleration was sensitive to the general nAChR inhibitors mecamylamine and d-tubocurarine as well as to the α3β4-nAChR antagonist α-conotoxin AulB, but not to other antagonists primarily addressing α3β2-nAChR or α4-, α7- and α9-containing nAChR. Agonists at α3β*-nAChR (epibatidine, cytisine), but not cotinine mimicked the effect. Tracheas from mice with genetic deletion of nAChR subunits α5, α7, α9, α10, α9/10, and β2 retained full PTS response to nicotine, whereas this was entirely lost in tracheas from mice lacking the β4-subunit. Collectively, our data show that nicotinic stimulation of α3β4-nAChR acutely increases PTS to the same extent as the established strong activator ATP. In view of the marked desensitization observed in the present setting, the physiological relevance of these receptors in adapting mucociliary clearance to rapidly changing endogenous or environmental stimuli remains open.

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.

New advances regarding adaptation of the vestibulo-ocular reflex

This is a review summarizing the development of vestibulo-ocular reflex (VOR) adaptation behavior with relevance to rehabilitation over the last 10 years and examines VOR adaptation using head-on-body rotations, specifically the influence of training target contrast, position and velocity error signal, active vs. passive head rotations, and sinusoidal vs. head impulse rotations. This review discusses optimization of the single VOR adaptation training session, consolidation between repeated training sessions, and dynamic incremental VOR adaptation. Also considered are the effects of aging and the roles of the efferent vestibular system, cerebellum, and otoliths on angular VOR adaptation. Finally, this review examines VOR adaptation findings in studies using whole body rotations.

Strengthening of the Efferent Olivocochlear System Leads to Synaptic Dysfunction and Tonotopy Disruption of a Central Auditory Nucleus

The auditory system in many mammals is immature at birth but precisely organized in adults. Spontaneous activity in the inner ear plays a critical role in guiding this maturation process. This is shaped by an efferent pathway that descends from the brainstem and makes transient direct synaptic contacts with inner hair cells. In this work, we used an α9 cholinergic nicotinic receptor knock-in mouse model (of either sex) with enhanced medial efferent activity (Chrna9L9'T, L9'T) to further understand the role of the olivocochlear system in the correct establishment of auditory circuits. Wave III of auditory brainstem responses (which represents synchronized activity of synapses within the superior olivary complex) was smaller in L9'T mice, suggesting a central dysfunction. The mechanism underlying this functional alteration was analyzed in brain slices containing the medial nucleus of the trapezoid body (MNTB), where neurons are topographically organized along a mediolateral (ML) axis. The topographic organization of MNTB physiological properties observed in wildtype (WT) was abolished in L9'T mice. Additionally, electrophysiological recordings in slices indicated MNTB synaptic alterations. In vivo multielectrode recordings showed that the overall level of MNTB activity was reduced in the L9'T The present results indicate that the transient cochlear efferent innervation to inner hair cells during the critical period before the onset of hearing is involved in the refinement of topographic maps as well as in setting the properties of synaptic transmission at a central auditory nucleus.SIGNIFICANCE STATEMENT Cochlear inner hair cells of altricial mammals display spontaneous electrical activity before hearing onset. The pattern and firing rate of these cells are crucial for the correct maturation of the central auditory pathway. A descending efferent innervation from the CNS contacts the hair cells during this developmental window. The present work shows that genetic enhancement of efferent function disrupts the orderly topographic distribution of biophysical and synaptic properties in the auditory brainstem and causes severe synaptic dysfunction. This work adds to the notion that the transient efferent innervation to the cochlea is necessary for the correct establishment of the central auditory circuitry.

Efferent Inhibition of the Cochlea

Cholinergic efferent neurons originating in the brainstem innervate the acoustico-lateralis organs (inner ear, lateral line) of vertebrates. These release acetylcholine (ACh) to inhibit hair cells through activation of calcium-dependent potassium channels. In the mammalian cochlea, ACh shunts and suppresses outer hair cell (OHC) electromotility, reducing the essential amplification of basilar membrane motion. Consequently, medial olivocochlear neurons that inhibit OHCs reduce the sensitivity and frequency selectivity of afferent neurons driven by cochlear vibration of inner hair cells (IHCs). The cholinergic synapse on hair cells involves an unusual ionotropic ACh receptor, and a near-membrane postsynaptic cistern. Lateral olivocochlear (LOC) neurons modulate type I afferents by still-to-be-defined synaptic mechanisms. Olivocochlear neurons can be activated by a reflex arc that includes the auditory nerve and projections from the cochlear nucleus. They are also subject to modulation by higher-order central auditory interneurons. Through its actions on cochlear hair cells, afferent neurons, and higher centers, the olivocochlear system protects against age-related and noise-induced hearing loss, improves signal coding in noise under certain conditions, modulates selective attention to sensory stimuli, and influences sound localization.

OHC-TRECK: A Novel System Using a Mouse Model for Investigation of the Molecular Mechanisms Associated with Outer Hair Cell Death in the Inner Ear

Outer hair cells (OHCs) are responsible for the amplification of sound, and the death of these cells leads to hearing loss. Although the mechanisms for sound amplification and OHC death have been well investigated, the effects on the cochlea after OHC death are poorly understood. To study the consequences of OHC death, we established an OHC knockout system using a novel mouse model, Prestin-hDTR, which uses the prestin promoter to express the human diphtheria toxin (DT) receptor gene (hDTR). Administration of DT to adult Prestin-hDTR mice results in the depletion of almost all OHCs without significant damage to other cochlear and vestibular cells, suggesting that this system is an effective tool for the analysis of how other cells in the cochlea and vestibula are affected after OHC death. To evaluate the changes in the cochlea after OHC death, we performed differential gene expression analysis between the untreated and DT-treated groups of wild-type and Prestin-hDTR mice. This analysis revealed that genes associated with inflammatory/immune responses were significantly upregulated. Moreover, we found that several genes linked to hearing loss were strongly downregulated by OHC death. Together, these results suggest that this OHC knockout system is a useful tool to identify biomarkers associated with OHC death.

Deletion of nicotinic acetylcholine receptor alpha9 in mice resulted in altered bone structure

Alterations in bone strength and structure were found in knockout (KO) mouse strains with deletion of several acetylcholine receptors. Interestingly, the expression of the nicotinic acetylcholine receptors (nAChR) subunit α10 was down-regulated in osteogenic differentiated mesenchymal stem cells of patients with osteoporosis whereas the expression of subunit α9 was not altered. Since nAChR subunits α9 and α10 are often combined in a functional receptor, we analyzed here the bone of adult female KO mice with single deletion of either nAChR alpha9 (α9KO) or alpha10 (α10KO). Biomechanical testing showed a significant decrease of bending stiffness and maximal breaking force in α9KO compared to their corresponding wild type mice. Furthermore, an increase in trabecular pattern factor (Tb.Pf) and structure model index (SMI) was detected by μCT in α9KO indicating reduced bone mass. On the mRNA level a decrease of Collagen 1α1 and Connexin-43 was measured by real-time RT-PCR in α9KO while no alteration of osteoclast markers was detected in either mouse strain. Using electron microcopy we observed an increase in the number of osteocytes that showed signs of degeneration and cell death in the α9KO compared to their wild type mice, while α10KO showed no differences. In conclusion, we demonstrate alterations in bone strength, structure and bio-marker expression in α9KO mice which imply the induction of osteocyte degeneration. Thus, our data suggest that nAChR containing the α9 subunit might be involved in the homeostasis of osteocytes and therefore in bone mass regulation.

Effects of Non-traumatic Noise and Conductive Hearing Loss on Auditory System Function

The effects of traumatic noise-exposure and deafening on auditory system function have received a great deal of attention. However, lower levels of noise as well as temporary conductive hearing loss also have consequences on auditory physiology and hearing. Here we review how abnormal acoustic experience at early ages affects the ascending and descending auditory pathways, as well as hearing behavior.

Talking back: Development of the olivocochlear efferent system

Developing sensory systems must coordinate the growth of neural circuitry spanning from receptors in the peripheral nervous system (PNS) to multilayered networks within the central nervous system (CNS). This breadth presents particular challenges, as nascent processes must navigate across the CNS-PNS boundary and coalesce into a tightly intermingled wiring pattern, thereby enabling reliable integration from the PNS to the CNS and back. In the auditory system, feedforward spiral ganglion neurons (SGNs) from the periphery collect sound information via tonotopically organized connections in the cochlea and transmit this information to the brainstem for processing via the VIII cranial nerve. In turn, feedback olivocochlear neurons (OCNs) housed in the auditory brainstem send projections into the periphery, also through the VIII nerve. OCNs are motor neuron-like efferent cells that influence auditory processing within the cochlea and protect against noise damage in adult animals. These aligned feedforward and feedback systems develop in parallel, with SGN central axons reaching the developing auditory brainstem around the same time that the OCN axons extend out toward the developing inner ear. Recent findings have begun to unravel the genetic and molecular mechanisms that guide OCN development, from their origins in a generic pool of motor neuron precursors to their specialized roles as modulators of cochlear activity. One recurrent theme is the importance of efferent-afferent interactions, as afferent SGNs guide OCNs to their final locations within the sensory epithelium, and efferent OCNs shape the activity of the developing auditory system. This article is categorized under: Nervous System Development > Vertebrates: Regional Development.

Enhancement of the Medial Olivocochlear System Prevents Hidden Hearing Loss

Cochlear synaptopathy produced by exposure to noise levels that cause only transient auditory threshold elevations is a condition that affects many people and is believed to contribute to poor speech discrimination in noisy environments. These functional deficits in hearing, without changes in sensitivity, have been called hidden hearing loss (HHL). It has been proposed that activity of the medial olivocochlear (MOC) system can ameliorate acoustic trauma effects. Here we explore the role of the MOC system in HHL by comparing the performance of two different mouse models: an α9 nicotinic receptor subunit knock-out (KO; Chrna9 KO), which lacks cholinergic transmission between efferent neurons and hair cells; and a gain-of-function knock-in (KI; Chrna9L9'T KI) carrying an α9 point mutation that leads to enhanced cholinergic activity. Animals of either sex were exposed to sound pressure levels that in wild-type produced transient cochlear threshold shifts and a decrease in neural response amplitudes, together with the loss of ribbon synapses, which is indicative of cochlear synaptopathy. Moreover, a reduction in the number of efferent contacts to outer hair cells was observed. In Chrna9 KO ears, noise exposure produced permanent auditory threshold elevations together with cochlear synaptopathy. In contrast, the Chrna9L9'T KI was completely resistant to the same acoustic exposure protocol. These results show a positive correlation between the degree of HHL prevention and the level of cholinergic activity. Notably, enhancement of the MOC feedback promoted new afferent synapse formation, suggesting that it can trigger cellular and molecular mechanisms to protect and/or repair the inner ear sensory epithelium.SIGNIFICANCE STATEMENT Noise overexposure is a major cause of a variety of perceptual disabilities, including speech-in-noise difficulties, tinnitus, and hyperacusis. Here we show that exposure to noise levels that do not cause permanent threshold elevations or hair cell death can produce a loss of cochlear nerve synapses to inner hair cells as well as degeneration of medial olivocochlear (MOC) terminals contacting the outer hair cells. Enhancement of the MOC reflex can prevent both types of neuropathy, highlighting the potential use of drugs that increase α9α10 nicotinic cholinergic receptor activity as a pharmacotherapeutic strategy to avoid hidden hearing loss.

Prestin Contributes to Membrane Compartmentalization and Is Required for Normal Innervation of Outer Hair Cells

Outer hair cells (OHC) act as amplifiers and their function is modified by medial olivocochlear (MOC) efferents. The unique OHC motor protein, prestin, provides the molecular basis for somatic electromotility, which is required for sensitivity and frequency selectivity, the hallmarks of mammalian hearing. Prestin proteins are the major component of the lateral membrane of mature OHCs, which separates apical and basal domains. To investigate the contribution of prestin to this unique arrangement, we compared the distribution of membrane proteins in OHCs of wildtype (WT) and prestin-knockout (KO) mice. In WT, the apical protein PMCA2 was exclusively localized to the hair bundles, while it was also found at the lateral membrane in KOs. Similarly, a basal protein KCNQ4 did not coalesce at the base of OHCs but was widely dispersed in mice lacking prestin. Since the expression levels of PMCA2 and KCNQ4 remained unchanged in KOs, the data indicate that prestin is required for the normal distribution of apical and basal membrane proteins in OHCs. Since OHC synapses predominate in the basal subnuclear region, we also examined the synaptic architecture in prestin-KO mice. Although neurite densities were not affected, MOC efferent terminals in prestin-KO mice were no longer constrained to the basal pole as in WT. This trend was evident as early as at postnatal day 12. Furthermore, terminals were often enlarged and frequently appeared as singlets when compared to the multiple clusters of individual terminals in WT. This abnormality in MOC synaptic morphology in prestin-KO mice is similar to defects in mice lacking MOC pathway proteins such as α9/α10 nicotinic acetylcholine receptors and BK channels, indicating a role for prestin in the proper establishment of MOC synapses. To investigate the contribution of prestin’s electromotility, we also examined OHCs from a mouse model that expresses non-functional prestin (499-prestin). We found no changes in PMCA2 localization and MOC synaptic morphology in OHCs from 499-prestin mice. Taken together, these results indicate that prestin, independent of its motile function, plays an important structural role in membrane compartmentalization, which is required for the formation of normal efferent-OHC synapses in mature OHCs.

Cigarette smoke exposure effects on the brainstem expression of nicotinic acetylcholine receptors (nAChRs), and on cardiac, respiratory and sleep physiologies

Cigarette smoking during pregnancy is the largest modifiable risk factor for adverse outcomes in the infant. Investigations have focused on the psychoactive component of cigarettes, nicotine. One proposed mechanism leading to adverse effects is the interaction between nicotine and its nicotinic acetylcholine receptors (nAChRs). Much data has been generated over the past three decades on the effects of cigarette smoke exposure (CSE) on the expression of the nAChRs in the brainstem and physiological parameters related to cardiac, respiration and sleep, in the offspring of smoking mothers and animal models of nicotine exposure. This review summarises this data and discusses the main findings, highlighting that findings in animal models closely correlate with those from human studies, and that the major brainstem sites where the expression level for the nAChRs are consistently affected include those that play vital roles in cardiorespiration (hypoglossal nucleus, dorsal motor nucleus of the vagus, nucleus of the solitary tract), chemosensation (nucleus of the solitary tract, arcuate nucleus) and arousal (rostral mesopontine sites such as the locus coeruleus and nucleus pontis oralis). These findings provide evidence for the adverse effects of CSE during and after pregnancy to the infant and the need to continue with the health campaign advising against CSE.

A Gain-of-Function Mutation in the α9 Nicotinic Acetylcholine Receptor Alters Medial Olivocochlear Efferent Short-Term Synaptic Plasticity

Gain control of the auditory system operates at multiple levels. Cholinergic medial olivocochlear (MOC) fibers originate in the brainstem and make synaptic contacts at the base of the outer hair cells (OHCs), the final targets of several feedback loops from the periphery and higher-processing centers. Efferent activation inhibits OHC active amplification within the mammalian cochlea, through the activation of a calcium-permeable α9α10 ionotropic cholinergic nicotinic receptor (nAChR), functionally coupled to calcium activated SK2 potassium channels. Correct operation of this feedback requires careful matching of acoustic input with the strength of cochlear inhibition (Galambos, 1956; Wiederhold and Kiang, 1970; Gifford and Guinan, 1987), which is driven by the rate of MOC activity and short-term facilitation at the MOC-OHC synapse (Ballestero et al., 2011; Katz and Elgoyhen, 2014). The present work shows (in mice of either sex) that a mutation in the α9α10 nAChR with increased duration of channel gating (Taranda et al., 2009) greatly elongates hair cell-evoked IPSCs and Ca²⁺ signals. Interestingly, MOC-OHC synapses of L9'T mice presented reduced quantum content and increased presynaptic facilitation. These phenotypic changes lead to enhanced and sustained synaptic responses and OHC hyperpolarization upon high-frequency stimulation of MOC terminals. At the cochlear physiology level these changes were matched by a longer time course of efferent MOC suppression. This indicates that the properties of the MOC-OHC synapse directly determine the efficacy of the MOC feedback to the cochlea being a main player in the "gain control" of the auditory periphery.SIGNIFICANCE STATEMENT Plasticity can involve reciprocal signaling across chemical synapses. An opportunity to study this phenomenon occurs in the mammalian cochlea whose sensitivity is regulated by efferent olivocochlear neurons. These release acetylcholine to inhibit sensory hair cells. A point mutation in the hair cell's acetylcholine receptor that leads to increased gating of the receptor greatly elongates IPSCs. Interestingly, efferent terminals from mutant mice present a reduced resting release probability. However, upon high-frequency stimulation transmitter release facilitates strongly to produce stronger and far longer-lasting inhibition of cochlear function. Thus, central neuronal feedback on cochlear hair cells provides an opportunity to define plasticity mechanisms in cholinergic synapses other than the highly studied neuromuscular junction.

Effects of cochlear synaptopathy on middle-ear muscle reflexes in unanesthetized mice

Cochlear synaptopathy, i.e. the loss of auditory-nerve connections with cochlear hair cells, is seen in aging, noise damage, and other types of acquired sensorineural hearing loss. Because the subset of auditory-nerve fibers with high thresholds and low spontaneous rates (SRs) is disproportionately affected, audiometric thresholds are relatively insensitive to this primary neural degeneration. Although suprathreshold amplitudes of wave I of the auditory brainstem response (ABR) are attenuated in synaptopathic mice, there is not yet a robust diagnostic in humans. The middle-ear muscle reflex (MEMR) might be a sensitive metric (Valero et al., 2016), because low-SR fibers may be important drivers of the MEMR (Liberman and Kiang, 1984; Kobler et al., 1992). Here, to test the hypothesis that narrowband reflex elicitors can identify synaptopathic cochlear regions, we measured reflex growth functions in unanesthetized mice with varying degrees of noise-induced synaptopathy and in unexposed controls. To separate effects of the MEMR from those of the medial olivocochlear reflex, the other sound-evoked cochlear feedback loop, we used a mutant mouse strain with deletion of the acetylcholine receptor required for olivocochlear function. We demonstrate that the MEMR is normal when activated from non-synaptopathic cochlear regions, is greatly weakened in synaptopathic regions, and is a more sensitive indicator of moderate synaptopathy than the suprathreshold amplitude of ABR wave I.