Onset of cholinergic efferent synaptic function in sensory hair cells of the rat cochlea

In vivo AAV9-Myo7a gene rescue restores hearing and cholinergic efferent innervation in inner hair cells

In the mammalian cochlea, sensory hair cells are crucial for the transduction of acoustic stimuli into electrical signals, which are then relayed to the central auditory pathway via spiral ganglion neuron (SGN) afferent dendrites. The SGN output is directly modulated by inhibitory cholinergic axodendritic synapses from the efferent fibers originating in the superior olivary complex. When the adult cochlea is subjected to noxious stimuli or aging, the efferent system undergoes major rewiring, such that it reestablishes direct axosomatic contacts with the inner hair cells (IHCs), which occur only transiently during prehearing stages of development. The trigger, origin, and degree of efferent plasticity in the cochlea remains largely unknown. Using functional and morphological approaches, we demonstrate that efferent plasticity in the adult cochlea occurs as a direct consequence of mechanoelectrical transducer current dysfunction. We also show that, different from prehearing stages of development, the lateral olivocochlear - but not the medial olivocochlear - efferent fibers are those that form the axosomatic synapses with the IHCs. The study also demonstrates that in vivo restoration of IHC function using AAV-Myo7a rescue reestablishes the synaptic profile of adult IHCs and improves hearing, highlighting the potential of using gene-replacement therapy for progressive hearing loss.

Conductance properties of the α9α10 nicotinic acetylcholine receptor of neonatal mouse inner and outer hair cells

In the developing cochlea, just before the onset of hearing on postnatal day 12, the medial olivocochlear efferent axons in synaptic contact with the inner hair cells (IHCs) start withdrawing and new efferent synaptic connections are formed on the outer hair cells (OHCs), thereby progressing towards the adult pattern of medial olivocochlear efferent innervation. The synapses are inhibitory, calcium influx through the α9α10 nicotinic acetylcholine receptors (nAChRs) driving opening of calcium-dependent potassium channels. The nAChRs appear to function similarly in IHCs and OHCs, although with probable kinetic differences. Our aim was to assess their functional similarity in the neonatal mouse cochlea by making whole-cell recordings from both hair cell types between postnatal day 7 and 10 when nAChRs are expressed. ACh was applied to voltage-clamped hair cells by pressure-ejection from a pipette. The cells were dialysed with a Cs+-based solution designed to eliminate calcium-dependent potassium currents. There were differences in amplitude, voltage-sensitivity and reversal potential of the nAChR currents between IHCs and OHCs. There was also some indication that IHC nAChRs have slower activation and desensitization kinetics, although the relatively slow ACh application limited interpretation of this result. These differences, particularly concerning the reversal potential, might indicate the presence of different auxiliary protein subunits of the α9α10 receptor in neonatal IHCs and OHCs.

Virally-Mediated Enhancement of Efferent Inhibition Reduces Acoustic Trauma in Wild Type Murine Cochleas

Noise-induced hearing loss (NIHL) poses an emerging global health problem with only ear protection or sound avoidance as preventive strategies. In addition, however, the cochlea receives some protection from medial olivocochlear (MOC) efferent neurons, providing a potential target for therapeutic enhancement. Cholinergic efferents release ACh (Acetylycholine) to hyperpolarize and shunt the outer hair cells (OHCs), reducing sound-evoked activation. The (α9)2(α10)3 nicotinic ACh receptor (nAChR) on the OHCs mediates this effect. Transgenic knock-in mice with a gain-of-function nAChR (α9L9'T) suffer less NIHL. α9 knockout mice are more vulnerable to NIHL but can be rescued by viral transduction of the α9L9'T subunit. In this study, an HA-tagged gain-of-function α9 isoform was expressed in wildtype mice in an attempt to reduce NIHL. Synaptic integration of the virally-expressed nAChR subunit was confirmed by HA-immunopuncta in the postsynaptic membrane of OHCs. After noise exposure, α9L9'T-HA injected mice had less hearing loss (auditory brainstem response (ABR) thresholds and threshold shifts) than did control mice. ABRs of α9L9'T-HA injected mice also had larger wave1 amplitudes and better recovery of wave one amplitudes post noise exposure. Thus, virally-expressed α9L9'T combines effectively with native α9 and α10 subunits to mitigate NIHL in wildtype cochleas.

Co-release of GABA and ACh from medial olivocochlear neurons fine tunes cochlear efferent inhibition

During development, inner hair cells (IHCs) in the mammalian cochlea are unresponsive to acoustic stimuli but instead exhibit spontaneous activity. During this same period, neurons originating from the medial olivocochlear complex (MOC) transiently innervate IHCs, regulating their firing pattern which is crucial for the correct development of the auditory pathway. Although the MOC-IHC is a cholinergic synapse, previous evidence indicates the widespread presence of gamma-aminobutyric acid (GABA) signaling markers, including presynaptic GABAB receptors (GABABR). In this study, we explore the source of GABA by optogenetically activating either cholinergic or GABAergic fibers. The optogenetic stimulation of MOC terminals from GAD;ChR2-eYFP and ChAT;ChR2-eYFP mice evoked synaptic currents in IHCs that were blocked by α-bungarotoxin. This suggests that GABAergic fibers release ACh and activate α9α10 nicotinic acetylcholine receptors (nAChRs). Additionally, MOC cholinergic fibers release not only ACh but also GABA, as the effect of GABA on ACh response amplitude was prevented by applying the GABAB-R blocker (CGP 36216). Using optical neurotransmitter detection and calcium imaging techniques, we examined the extent of GABAergic modulation at the single synapse level. Our findings suggest heterogeneity in GABA modulation, as only 15 out of 31 recorded synaptic sites were modulated by applying the GABABR specific antagonist, CGP (100-200 μM). In conclusion, we provide compelling evidence that GABA and ACh are co-released from at least a subset of MOC terminals. In this circuit, GABA functions as a negative feedback mechanism, locally regulating the extent of cholinergic inhibition at certain efferent-IHC synapses during an immature stage.

Priming central sound processing circuits through induction of spontaneous activity in the cochlea before hearing onset

Sensory systems experience a period of intrinsically generated neural activity before maturation is complete and sensory transduction occurs. Here we review evidence describing the mechanisms and functions of this 'spontaneous' activity in the auditory system. Both ex vivo and in vivo studies indicate that this correlated activity is initiated by non-sensory supporting cells within the developing cochlea, which induce depolarization and burst firing of groups of nearby hair cells in the sensory epithelium, activity that is conveyed to auditory neurons that will later process similar sound features. This stereotyped neural burst firing promotes cellular maturation, synaptic refinement, acoustic sensitivity, and establishment of sound-responsive domains in the brain. While sensitive to perturbation, the developing auditory system exhibits remarkable homeostatic mechanisms to preserve periodic burst firing in deaf mice. Preservation of this early spontaneous activity in the context of deafness may enhance the efficacy of later interventions to restore hearing.

Cholinergic modulation in the vertebrate auditory pathway

Acetylcholine (ACh) is a prevalent neurotransmitter throughout the nervous system. In the brain, ACh is widely regarded as a potent neuromodulator. In neurons, ACh signals are conferred through a variety of receptors that influence a broad range of neurophysiological phenomena such as transmitter release or membrane excitability. In sensory circuitry, ACh modifies neural responses to stimuli and coordinates the activity of neurons across multiple levels of processing. These factors enable individual neurons or entire circuits to rapidly adapt to the dynamics of complex sensory stimuli, underscoring an essential role for ACh in sensory processing. In the auditory system, histological evidence shows that acetylcholine receptors (AChRs) are expressed at virtually every level of the ascending auditory pathway. Despite its apparent ubiquity in auditory circuitry, investigation of the roles of this cholinergic network has been mainly focused on the inner ear or forebrain structures, while less attention has been directed at regions between the cochlear nuclei and midbrain. In this review, we highlight what is known about cholinergic function throughout the auditory system from the ear to the cortex, but with a particular emphasis on brainstem and midbrain auditory centers. We will focus on receptor expression, mechanisms of modulation, and the functional implications of ACh for sound processing, with the broad goal of providing an overview of a newly emerging view of impactful cholinergic modulation throughout the auditory pathway.

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.

SK Current, Expressed During the Development and Regeneration of Chick Hair Cells, Contributes to the Patterning of Spontaneous Action Potentials

Chick hair cells display calcium (Ca2+)-sensitive spontaneous action potentials during development and regeneration. The role of this activity is unclear but thought to be involved in establishing proper synaptic connections and tonotopic maps, both of which are instrumental to normal hearing. Using an electrophysiological approach, this work investigated the functional expression of Ca2+-sensitive potassium [IK(Ca)] currents and their role in spontaneous electrical activity in the developing and regenerating hair cells (HCs) in the chick basilar papilla. The main IK(Ca) in developing and regenerating chick HCs is an SK current, based on its sensitivity to apamin. Analysis of the functional expression of SK current showed that most dramatic changes occurred between E8 and E16. Specifically, there is a developmental downregulation of the SK current after E16. The SK current gating was very sensitive to the availability of intracellular Ca2+ but showed very little sensitivity to T-type voltage-gated Ca2+ channels, which are one of the hallmarks of developing and regenerating hair cells. Additionally, apamin reduced the frequency of spontaneous electrical activity in HCs, suggesting that SK current participates in patterning the spontaneous electrical activity of HCs.

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.

Spontaneous activity in developing thalamic and cortical sensory networks

Developing sensory circuits exhibit different patterns of spontaneous activity, patterns that are related to the construction and refinement of functional networks. During the development of different sensory modalities, spontaneous activity originates in the immature peripheral sensory structures and in the higher-order central structures, such as the thalamus and cortex. Certainly, the perinatal thalamus exhibits spontaneous calcium waves, a pattern of activity that is fundamental for the formation of sensory maps and for circuit plasticity. Here, we review our current understanding of the maturation of early (including embryonic) patterns of spontaneous activity and their influence on the assembly of thalamic and cortical sensory networks. Overall, the data currently available suggest similarities between the developmental trajectory of brain activity in experimental models and humans, which in the future may help to improve the early diagnosis of developmental disorders.

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.

Diverse identities and sites of action of cochlear neurotransmitters

Accurate encoding of acoustic stimuli requires temporally precise responses to sound integrated with cellular mechanisms that encode the complexity of stimuli over varying timescales and orders of magnitude of intensity. Sound in mammals is initially encoded in the cochlea, the peripheral hearing organ, which contains functionally specialized cells (including hair cells, afferent and efferent neurons, and a multitude of supporting cells) to allow faithful acoustic perception. To accomplish the demanding physiological requirements of hearing, the cochlea has developed synaptic arrangements that operate over different timescales, with varied strengths, and with the ability to adjust function in dynamic hearing conditions. Multiple neurotransmitters interact to support the precision and complexity of hearing. Here, we review the location of release, action, and function of neurotransmitters in the mammalian cochlea with an emphasis on recent work describing the complexity of signaling.

Sex difference in the efferent inner hair cell synapses of the aging murine cochlea

Efferent innervation of the inner hair cells changes over time. At an early age in mice, inner hair cells receive efferent feedback, which helps fine-tune tonotopic maps in the brainstem. In adulthood, inner hair cell efferent innervation wanes but increases again in older animals. It is not clear, however, whether age-related inner hair cell efferents increase along the entire range of the cochlear frequencies, or if this increase is restricted to a particular frequency-region, and whether this phenomenon occurs in both sexes. Age-related hearing loss, presbycusis, affects men and women differently. In mice, this difference is also strain specific. In aging black six mice, the auditory brainstem response thresholds increase in females earlier than in males. Here, we study age-related increase of the inner hair cell efferent innervation throughout the cochlea before hearing onset, in one month old and in ten months old and older male and female black six mice. We collected confocal images of immunostained inner hair cell efferents and quantified the labeled terminals in the entire cochlea using a machine learning algorithm. The overall number of the inner hair cell efferents in both sexes did not change significantly between age-groups. The distribution of the inner hair cell efferent innervation did not differ across frequencies in the cochlea. However, in females, inner hair cells received on average up to four times more efferent innervation than in males per each of the frequency regions tested. Sex differences were also found in the oldest age-group tested (≥ 10 months) where on average inner hair cells received six times more efferents in females than in males of matching age. Our findings emphasize the importance of including both sexes in sensorineural hearing loss research.

Characterization of HA-tagged α9 and α10 nAChRs in the mouse cochlea

Neurons of the medial olivary complex inhibit cochlear hair cells through the activation of α9α10-containing nicotinic acetylcholine receptors (nAChRs). Efforts to study the localization of these proteins have been hampered by the absence of reliable antibodies. To overcome this obstacle, CRISPR-Cas9 gene editing was used to generate mice in which a hemagglutinin tag (HA) was attached to the C-terminus of either α9 or α10 proteins. Immunodetection of the HA tag on either subunit in the organ of Corti of adult mice revealed immunopuncta clustered at the synaptic pole of outer hair cells. These puncta were juxtaposed to immunolabeled presynaptic efferent terminals. HA immunopuncta also occurred in inner hair cells of pre-hearing (P7) but not in adult mice. These immunolabeling patterns were similar for both homozygous and heterozygous mice. All HA-tagged genotypes had auditory brainstem responses not significantly different from those of wild type littermates. The activation of efferent neurons in heterozygous mice evoked biphasic postsynaptic currents not significantly different from those of wild type hair cells. However, efferent synaptic responses were significantly smaller and less frequent in the homozygous mice. We show that HA-tagged nAChRs introduced in the mouse by a CRISPR knock-in are regulated and expressed like the native protein, and in the heterozygous condition mediate normal synaptic function. The animals thus generated have clear advantages for localization studies.

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.

Functional Postnatal Maturation of the Medial Olivocochlear Efferent-Outer Hair Cell Synapse

The organ of Corti, the auditory mammalian sensory epithelium, contains two types of mechanotransducer cells, inner hair cells (IHCs) and outer hair cells (OHCs). IHCs are involved in conveying acoustic stimuli to the CNS, while OHCs are implicated in the fine tuning and amplification of sounds. OHCs are innervated by medial olivocochlear (MOC) cholinergic efferent fibers. The functional characteristics of the MOC-OHC synapse during maturation were assessed by electrophysiological and pharmacological methods in mouse organs of Corti at postnatal day 11 (P11)-P13, hearing onset in altricial rodents, and at P20-P22 when the OHCs are morphologically and functionally mature. Synaptic currents were recorded in whole-cell voltage-clamped OHCs while electrically stimulating the MOC fibers. A progressive increase in the number of functional MOC-OHC synapses, as well as in their strength and efficacy, was observed between P11-13 and P20-22. At hearing onset, the MOC-OHC synapse presented facilitation during MOC fibers high-frequency stimulation that disappeared at mature stages. In addition, important changes were found in the VGCC that are coupled to transmitter release. Ca²⁺ flowing in through L-type VGCCs contribute to trigger ACh release together with P/Q- and R-type VGCCs at P11-P13, but not at P20-P22. Interestingly, N-type VGCCs were found to be involved in this process at P20-P22, but not at hearing onset. Moreover, the degree of compartmentalization of calcium channels with respect to BK channels and presynaptic release components significantly increased from P11-P13 to P20-P22. These results suggest that the MOC-OHC synapse is immature at the onset of hearing.SIGNIFICANCE STATEMENT The functional expression of both VGCCs and BK channels, as well as their localization with respect to the presynaptic components involved in transmitter release, are key elements in determining synaptic efficacy. In this work, we show dynamic changes in the expression of VGCCs and Ca²⁺-dependent BK K⁺ channels coupled to ACh release at the MOC-OHC synapse and their shift in compartmentalization during postnatal maturation. These processes most likely set the short-term plasticity pattern and reliability of the MOC-OHC synapse on high-frequency activity.

A Review of the Cholinergic System and Therapeutic Approaches to Treat Brain Disorders

Since its identification over a hundred years ago, the neurotransmitter acetylcholine (ACh) has proven to play an essential role in supporting many diverse functions. Some well-characterized functions include: chemical transmission at the neuromuscular junction; autonomic function in the peripheral nervous system; and, sustained attention, sleep/wake regulation, and learning and memory within the central nervous system. Within the brain, major cholinergic projection pathways from the basal forebrain and the brainstem support these centrally mediated processes, and dysregulation of the cholinergic system is implicated in cognitive decline associated with aging and dementias including Alzheimer's disease. ACh exerts its effects by binding to two different membrane-bound receptor classes: (1) G‑protein coupled muscarinic acetylcholine receptors (mAChRs), and (2) ligand-gated nicotinic acetylcholine receptors (nAChRs). These receptor systems are described in detail within this chapter along with discussion on the successes and failures of synthetic ligands designed to selectively target receptor subtypes for treating brain disorders. New molecular approaches and advances in our understanding of the target biology combined with opportunities to re-purpose existing cholinergic drugs for new indications continue to highlight the exciting opportunities for modulating this system for therapeutic purposes.

Spontaneous and Acetylcholine Evoked Calcium Transients in the Developing Mouse Utricle

Spontaneous calcium transients are present during early postnatal development in the mouse retina and cochlea, and play an important role in maturation of the sensory organs and neural circuits in the central nervous system (CNS). It is not known whether similar calcium transients occur during postnatal development in the vestibular sensory organs. Here we demonstrate spontaneous intracellular calcium transients in sensory hair cells (HCs) and supporting cells (SCs) in the murine utricular macula during the first two postnatal weeks. Calcium transients were monitored using a genetically encoded calcium indicator, GCaMP5G (G5), at 100 ms-frame⁻¹ in excised utricle sensory epithelia, including HCs, SCs, and neurons. The reporter line expressed G5 and tdTomato (tdT) in a Gad2-Cre dependent manner within a subset of utricular HCs, SCs and neurons. Kinetics of the G5 reporter limited temporal resolution to calcium events lasting longer than 200 ms. Spontaneous calcium transients lasting 1-2 s were observed in the expressing population of HCs at birth and slower spontaneous transients lasting 10-30 s appeared in SCs by P3. Beginning at P5, calcium transients could be modulated by application of the efferent neurotransmitter acetylcholine (ACh). In mature mice, calcium transients in the utricular macula occurred spontaneously, had a duration 1-2 s, and could be modulated by the exogenous application of acetylcholine (ACh) or muscarine. Long-lasting calcium transients evoked by ACh in mature mice were blocked by atropine, consistent with previous reports describing the role of muscarinic receptors expressed in calyx bearing afferents in efferent control of vestibular sensation. Large spontaneous and ACh evoked transients were reversibly blocked by the inositol trisphosphate receptor (IP₃R) antagonist aminoethoxydiphenyl borate (2-APB). Results demonstrate long-lasting calcium transients are present in the utricular macula during the first postnatal week, and that responses to ACh mature over this same time period.

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.

Developmental Synaptic Changes at the Transient Olivocochlear-Inner Hair Cell Synapse

In the mature mammalian cochlea, inner hair cells (IHCs) are mainly innervated by afferent fibers that convey sound information to the CNS. During postnatal development, however, medial olivocochlear (MOC) efferent fibers transiently innervate the IHCs. The MOC-IHC synapse, functional from postnatal day 0 (P0) to hearing onset (P12), undergoes dramatic changes in the sensitivity to acetylcholine (ACh) and in the expression of key postsynaptic proteins. To evaluate whether there are associated changes in the properties of ACh release during this period, we used a cochlear preparation from mice of either sex at P4, P6-P7, and P9-P11 and monitored transmitter release from MOC terminals in voltage-clamped IHCs in the whole-cell configuration. The quantum content increased 5.6× from P4 to P9-P11 due to increases in the size and replenishment rate of the readily releasable pool of synaptic vesicles without changes in their probability of release or quantum size. This strengthening in transmission was accompanied by changes in short-term plasticity properties, which switched from facilitation at P4 to depression at P9-P11. We have previously shown that at P9-P11, ACh release is supported by P/Q- and N-type voltage-gated calcium channels (VGCCs) and negatively regulated by BK potassium channels activated by Ca2+ influx through L-type VGCCs. We now show that at P4 and P6-P7, release is mediated by P/Q-, R- and L-type VGCCs. Interestingly, L-type VGCCs have a dual role: they both support release and fuel BK channels, suggesting that at immature stages presynaptic proteins involved in release are less compartmentalized.SIGNIFICANCE STATEMENT During postnatal development before the onset of hearing, cochlear inner hair cells (IHCs) present spontaneous Ca2+ action potentials that release glutamate at the first auditory synapse in the absence of sound stimulation. The IHC Ca2+ action potential frequency pattern, which is crucial for the correct establishment and function of the auditory system, is regulated by the efferent medial olivocochlear (MOC) system that transiently innervates IHCs during this period. We show here that developmental changes in synaptic strength and synaptic plasticity properties at the MOC-IHC synapse upon MOC fiber activation at different frequencies might be crucial for tightly shaping the pattern of afferent activity during this critical period.

Spiking Pattern of the Mouse Developing Inner Hair Cells Is Mostly Invariant Along the Tonotopic Axis

During development, the sensory cells of the cochlea, the inner hair cells (IHCs), fire spontaneous calcium action potentials. This activity at the pre-hearing stage allows the IHCs to autonomously excite the auditory nerve fibers and hence, represents an efficient mechanism to shape the tonotopic organization along the ascending auditory pathway. Using calcium imaging, we show that the activity in the developing cochlea consists of calcium waves that propagate across the supporting and sensory cells. Both basal and apical IHCs were characterized by similar spontaneous calcium transients interspaced with silent periods, consistent with bursts of action potentials recorded in patch-clamp. In addition, adjacent auditory hair cells tend to have a synchronized [Ca²⁺]_i activity, irrespective of their location along the base-to-apex gradient of the cochlea. Finally, we show that the mechanical ablation of the inner phalangeal cells (IPCs), a class of supporting cells, reduces the synchronized [Ca²⁺]_i activity between neighboring sensory cells. These findings support the hypothesis that the tonotopic map refinement in higher auditory centers would depend on the synchronization of a discrete number of auditory sensory cells.

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.

Sensory Hair Cells: An Introduction to Structure and Physiology

Sensory hair cells are specialized secondary sensory cells that mediate our senses of hearing, balance, linear acceleration, and angular acceleration (head rotation). In addition, hair cells in fish and amphibians mediate sensitivity to water movement through the lateral line system, and closely related electroreceptive cells mediate sensitivity to low-voltage electric fields in the aquatic environment of many fish species and several species of amphibian. Sensory hair cells share many structural and functional features across all vertebrate groups, while at the same time they are specialized for employment in a wide variety of sensory tasks. The complexity of hair cell structure is large, and the diversity of hair cell applications in sensory systems exceeds that seen for most, if not all, sensory cell types. The intent of this review is to summarize the more significant structural features and some of the more interesting and important physiological mechanisms that have been elucidated thus far. Outside vertebrates, hair cells are only known to exist in the coronal organ of tunicates. Electrical resonance, electromotility, and their exquisite mechanical sensitivity all contribute to the attractiveness of hair cells as a research subject.

Voltage-Gated Calcium Influx Modifies Cholinergic Inhibition of Inner Hair Cells in the Immature Rat Cochlea

Until postnatal day (P) 12, inner hair cells of the rat cochlea are invested with both afferent and efferent synaptic connections. With the onset of hearing at P12, the efferent synapses disappear, and afferent (ribbon) synapses operate with greater efficiency. This change coincides with increased expression of voltage-gated potassium channels, the loss of calcium-dependent electrogenesis, and the onset of graded receptor potentials driven by sound. The transient efferent synapses include near-membrane postsynaptic cisterns thought to regulate calcium influx through the hair cell's α9-containing and α10-containing nicotinic acetylcholine receptors. This influx activates small-conductance Ca²⁺-activated K⁺ (SK) channels. Serial-section electron microscopy of inner hair cells from two 9-d-old (male) rat pups revealed many postsynaptic efferent cisterns and presynaptic afferent ribbons whose average minimal separation in five cells ranged from 1.1 to 1.7 μm. Efferent synaptic function was studied in rat pups (age, 7-9 d) of either sex. The duration of these SK channel-mediated IPSCs was increased by enhanced calcium influx through L-type voltage-gated channels, combined with ryanodine-sensitive release from internal stores-presumably the near-membrane postsynaptic cistern. These data support the possibility that inner hair cell calcium electrogenesis modulates the efficacy of efferent inhibition during the maturation of inner hair cell synapses.SIGNIFICANCE STATEMENT Strict calcium buffering is essential for cellular function. This problem is especially acute for compact hair cells where increasing cytoplasmic calcium promotes the opposing functions of closely adjoining afferent and efferent synapses. The near-membrane postsynaptic cistern at efferent synapses segregates synaptic calcium signals by acting as a dynamic calcium store. The hair cell serves as an informative model for synapses with postsynaptic cisterns (C synapses) found in central neurons.

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.

ACh-induced hyperpolarization and decreased resistance in mammalian type II vestibular hair cells

In the mammalian vestibular periphery, electrical activation of the efferent vestibular system (EVS) has two effects on afferent activity: 1) it increases background afferent discharge and 2) decreases afferent sensitivity to rotational stimuli. Although the cellular mechanisms underlying these two contrasting afferent responses remain obscure, we postulated that the reduction in afferent sensitivity was attributed, in part, to the activation of α9- containing nicotinic acetylcholine (ACh) receptors (α9*nAChRs) and small-conductance potassium channels (SK) in vestibular type II hair cells, as demonstrated in the peripheral vestibular system of other vertebrates. To test this hypothesis, we examined the effects of the predominant EVS neurotransmitter ACh on vestibular type II hair cells from wild-type (wt) and α9-subunit nAChR knockout (α9^-/-) mice. Immunostaining for choline acetyltransferase revealed there were no obvious gross morphological differences in the peripheral EVS innervation among any of these strains. ACh application onto wt type II hair cells, at resting potentials, produced a fast inward current followed by a slower outward current, resulting in membrane hyperpolarization and decreased membrane resistance. Hyperpolarization and decreased resistance were due to gating of SK channels. Consistent with activation of α9*nAChRs and SK channels, these ACh-sensitive currents were antagonized by the α9*nAChR blocker strychnine and SK blockers apamin and tamapin. Type II hair cells from α9^-/- mice, however, failed to respond to ACh at all. These results confirm the critical importance of α9nAChRs in efferent modulation of mammalian type II vestibular hair cells. Application of exogenous ACh reduces electrical impedance, thereby decreasing type II hair cell sensitivity. NEW & NOTEWORTHY Expression of α9 nicotinic subunit was crucial for fast cholinergic modulation of mammalian vestibular type II hair cells. These findings show a multifaceted efferent mechanism for altering hair cell membrane potential and decreasing membrane resistance that should reduce sensitivity to hair bundle displacements.

Mice Lacking the Alpha9 Subunit of the Nicotinic Acetylcholine Receptor Exhibit Deficits in Frequency Difference Limens and Sound Localization

Sound processing in the cochlea is modulated by cholinergic efferent axons arising from medial olivocochlear neurons in the brainstem. These axons contact outer hair cells in the mature cochlea and inner hair cells during development and activate nicotinic acetylcholine receptors composed of α9 and α10 subunits. The α9 subunit is necessary for mediating the effects of acetylcholine on hair cells as genetic deletion of the α9 subunit results in functional cholinergic de-efferentation of the cochlea. Cholinergic modulation of spontaneous cochlear activity before hearing onset is important for the maturation of central auditory circuits. In α9KO mice, the developmental refinement of inhibitory afferents to the lateral superior olive is disturbed, resulting in decreased tonotopic organization of this sound localization nucleus. In this study, we used behavioral tests to investigate whether the circuit anomalies in α9KO mice correlate with sound localization or sound frequency processing. Using a conditioned lick suppression task to measure sound localization, we found that three out of four α9KO mice showed impaired minimum audible angles. Using a prepulse inhibition of the acoustic startle response paradigm, we found that the ability of α9KO mice to detect sound frequency changes was impaired, whereas their ability to detect sound intensity changes was not. These results demonstrate that cholinergic, nicotinic α9 subunit mediated transmission in the developing cochlear plays an important role in the maturation of hearing.

Inhibition of K+ outward currents by linopirdine in the cochlear outer hair cells of circling mice within the first postnatal week

Inhibition of K⁺ outward currents by linopirdine in the outer hair cells (OHCs) of circling mice (homozygous (cir/cir) mice), an animal model for human deafness (DFNB6 type), was investigated using a whole cell patch clamp technique. Littermate heterozygous (+/cir) and ICR mice of the same age (postnatal day (P) 0 –P6) were used as controls. Voltage steps from –100 mV to 40 mV elicited small inward currents (–100 mV~–70 mV) and slow rising K⁺ outward currents (–60 mV ~40 mV) which activated near –50 mV in all OHCs tested. Linopirdine, a known blocker of K⁺ currents activated at negative potentials (I_K,n), did cause inhibition at varying degree (severe, moderate, mild) in K⁺ outward currents of heterozygous (+/cir) or homozygous (cir/cir) mice OHCs in the concentration range between 1 and 100 µM, while it was apparent only in one ICR mice OHC out of nine OHCs at 100 µM. Although the half inhibition concentrations in heterozygous (+/cir) or homozygous (cir/cir) mice OHCs were close to those reported in I_K,n, biophysical and pharmacological properties of K⁺ outward currents, such as the activation close to –50 mV, small inward currents evoked by hyperpolarizing steps and TEA sensitivity, were not in line with I_K,n reported in other tissues. Our results show that the delayed rectifier type K⁺ outward currents, which are not similar to I_K,n with respect to biophysical and pharmacological properties, are inhibited by linopirdine in the developing (P0~P6) homozygous (cir/cir) or heterozygous (+/cir) mice OHCs.

Perinatal nicotine exposure impairs the maturation of glutamatergic inputs in the auditory brainstem

KEY POINTS

Chronic perinatal nicotine exposure causes abnormal auditory brainstem responses and auditory processing deficits in children and animal models. The effect of perinatal nicotine exposure on synaptic maturation in the auditory brainstem was investigated in granule cells in the ventral nucleus of the lateral lemniscus, which receive a single calyx-like input from the cochlear nucleus. Perinatal nicotine exposure caused a massive reduction in the amplitude of the excitatory input current. This caused a profound decrease in the number and temporal precision of spikes in these neurons. Perinatal nicotine exposure delayed the developmental downregulation of functional nicotinic acetylcholine receptors on these neurons.

ABSTRACT

Maternal smoking causes chronic nicotine exposure during early development and results in auditory processing deficits including delayed speech development and learning difficulties. Using a mouse model of chronic, perinatal nicotine exposure we explored to what extent synaptic inputs to granule cells in the ventral nucleus of the lateral lemniscus are affected by developmental nicotine treatment. These neurons receive one large calyx-like input from octopus cells in the cochlear nucleus and play a role in sound pattern analysis, including speech sounds. In addition, they exhibit high levels of α7 nicotinic acetylcholine receptors, especially during early development. Our whole-cell patch-clamp experiments show that perinatal nicotine exposure causes a profound reduction in synaptic input amplitude. In contrast, the number of inputs innervating each neuron and synaptic release properties of this calyx-like synapse remained unaltered. Spike number and spiking precision in response to synaptic stimulation were greatly diminished, especially for later stimuli during a stimulus train. Moreover, chronic nicotine exposure delayed the developmental downregulation of functional nicotinic acetylcholine receptors on these neurons, indicating a direct action of nicotine in this brain area. This presumably direct effect of perinatal nicotine exposure on synaptic maturation in the auditory brainstem might be one of the underlying causes for auditory processing difficulties in children of heavy smoking mothers.

Carbaryl-induced ototoxicity in rat postnatal cochlear organotypic cultures

Carbaryl, a widely used carbamate-based insecticide, is a potent anticholinesterase known to induce delayed neurotoxicity following chronic exposure. However, its potential toxic effects on the cochlea, the sensory organ for hearing that contains cholinergic efferent neurons and acetylcholine receptors on the hair cells (HC) and spiral ganglion neurons has heretofore not been evaluated. To assess ototoxic potential of carbaryl, cochlear organotypic cultures from postnatal day 3 rats were treated with doses of carbaryl ranging from 50 to 500 μM for 48 h up to 96 h. Carbaryl damaged both the sensory HC and spiral ganglion neurons in a dose- and duration-dependent manner. HC and neuronal damage was observed at carbaryl concentrations as low as 50 μM after 96-h treatment and 100 μM after 48-h treatment. Hair cell was greatest in the high frequency basal region of the cochlea and progressively decreased towards the apex consistent with the majority of ototoxic drugs. In contrast, damage to the spiral ganglion neurons was of similar magnitude in the basal and apical regions of the cochlea. Carbaryl damage was characterized by soma shrinkage, nuclear condensation and fragmentation, and blebbing, morphological features of programmed cell death. Carbaryl upregulated the expression of executioner caspase-3 in HC and spiral ganglion neurons indicating that cellular damage occurred primarily by caspase-mediated apoptosis. These results suggest that chronic exposure to carbaryl and other carbamate anticholinesterases may be ototoxic. © 2016 Wiley Periodicals, Inc. Environ Toxicol 32: 956-969, 2017.

GluA2-Containing AMPA Receptors Distinguish Ribbon-Associated from Ribbonless Afferent Contacts on Rat Cochlear Hair Cells

Mechanosensory hair cells release glutamate at ribbon synapses to excite postsynaptic afferent neurons, via AMPA-type ionotropic glutamate receptors (AMPARs). However, type II afferent neurons contacting outer hair cells in the mammalian cochlea were thought to differ in this respect, failing to show GluA immunolabeling and with many “ribbonless” afferent contacts. Here it is shown that antibodies to the AMPAR subunit GluA2 labeled afferent contacts below inner and outer hair cells in the rat cochlea, and that synaptic currents in type II afferents had AMPAR-specific pharmacology. Only half the postsynaptic densities of type II afferents that labeled for PSD-95, Shank, or Homer were associated with GluA2 immunopuncta or presynaptic ribbons, the “empty slots” corresponding to ribbonless contacts described previously. These results extend the universality of AMPAergic transmission by hair cells, and support the existence of silent afferent contacts.

Assessment of the expression and role of the α1-nAChR subunit in efferent cholinergic function during the development of the mammalian cochlea

Hair cell (HC) activity in the mammalian cochlea is modulated by cholinergic efferent inputs from the brainstem. These inhibitory inputs are mediated by calcium-permeable nicotinic acetylcholine receptors (nAChRs) containing α9- and α10-subunits and by subsequent activation of calcium-dependent potassium channels. Intriguingly, mRNAs of α1- and γ-nAChRs, subunits of the "muscle-type" nAChR have also been found in developing HCs (Cai T, Jen HI, Kang H, Klisch TJ, Zoghbi HY, Groves AK. J Neurosci 35: 5870-5883, 2015; Scheffer D, Sage C, Plazas PV, Huang M, Wedemeyer C, Zhang DS, Chen ZY, Elgoyhen AB, Corey DP, Pingault V. J Neurochem 103: 2651-2664, 2007; Sinkkonen ST, Chai R, Jan TA, Hartman BH, Laske RD, Gahlen F, Sinkkonen W, Cheng AG, Oshima K, Heller S. Sci Rep 1: 26, 2011) prompting proposals that another type of nAChR is present and may be critical during early synaptic development. Mouse genetics, histochemistry, pharmacology, and whole cell recording approaches were combined to test the role of α1-nAChR subunit in HC efferent synapse formation and cholinergic function. The onset of α1-mRNA expression in mouse HCs was found to coincide with the onset of the ACh response and efferent synaptic function. However, in mouse inner hair cells (IHCs) no response to the muscle-type nAChR agonists (±)-anatoxin A, (±)-epibatidine, (-)-nicotine, or 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP) was detected, arguing against the presence of an independent functional α1-containing muscle-type nAChR in IHCs. In α1-deficient mice, no obvious change of IHC efferent innervation was detected at embryonic day 18, contrary to the hyperinnervation observed at the neuromuscular junction. Additionally, ACh response and efferent synaptic activity were detectable in α1-deficient IHCs, suggesting that α1 is not necessary for assembly and membrane targeting of nAChRs or for efferent synapse formation in IHCs.

Re-Emergent Inhibition of Cochlear Inner Hair Cells in a Mouse Model of Hearing Loss

Hearing loss among the elderly correlates with diminished social, mental, and physical health. Age-related cochlear cell death does occur, but growing anatomical evidence suggests that synaptic rearrangements on sensory hair cells also contribute to auditory functional decline. Here we present voltage-clamp recordings from inner hair cells of the C57BL/6J mouse model of age-related hearing loss, which reveal that cholinergic synaptic inputs re-emerge during aging. These efferents are functionally inhibitory, using the same ionic mechanisms as do efferent contacts present transiently before the developmental onset of hearing. The strength of efferent inhibition of inner hair cells increases with hearing threshold elevation. These data indicate that the aged cochlea regains features of the developing cochlea and that efferent inhibition of the primary receptors of the auditory system re-emerges with hearing impairment.

SIGNIFICANCE STATEMENT

Synaptic changes in the auditory periphery are increasingly recognized as important factors in hearing loss. To date, anatomical work has described the loss of afferent contacts from cochlear hair cells. However, relatively little is known about the efferent innervation of the cochlea during hearing loss. We performed intracellular recordings from mouse inner hair cells across the lifespan and show that efferent innervation of inner hair cells arises in parallel with the loss of afferent contacts and elevated hearing threshold during aging. These efferent neurons inhibit inner hair cells, raising the possibility that they play a role in the progression of age-related hearing loss.

Adenomatous Polyposis Coli Protein Deletion in Efferent Olivocochlear Neurons Perturbs Afferent Synaptic Maturation and Reduces the Dynamic Range of Hearing

Normal hearing requires proper differentiation of afferent ribbon synapses between inner hair cells (IHCs) and spiral ganglion neurons (SGNs) that carry acoustic information to the brain. Within individual IHCs, presynaptic ribbons show a size gradient with larger ribbons on the modiolar face and smaller ribbons on the pillar face. This structural gradient is associated with a gradient of spontaneous rates and threshold sensitivity, which is essential for a wide dynamic range of hearing. Despite their importance for hearing, mechanisms that direct ribbon differentiation are poorly defined. We recently identified adenomatous polyposis coli protein (APC) as a key regulator of interneuronal synapse maturation. Here, we show that APC is required for ribbon size heterogeneity and normal cochlear function. Compared with wild-type littermates, APC conditional knock-out (cKO) mice exhibit decreased auditory brainstem responses. The IHC ribbon size gradient is also perturbed. Whereas the normal-developing IHCs display ribbon size gradients before hearing onset, ribbon sizes are aberrant in APC cKOs from neonatal ages on. Reporter expression studies show that the CaMKII-Cre used to delete the floxed APC gene is present in efferent olivocochlear (OC) neurons, not IHCs or SGNs. APC loss led to increased volumes and numbers of OC inhibitory dopaminergic boutons on neonatal SGN fibers. Our findings identify APC in efferent OC neurons as essential for regulating ribbon heterogeneity, dopaminergic terminal differentiation, and cochlear sensitivity. This APC effect on auditory epithelial cell synapses resembles interneuronal and nerve-muscle synapses, thereby defining a global role for APC in synaptic maturation in diverse cell types.

SIGNIFICANCE STATEMENT

This study identifies novel molecules and cellular interactions that are essential for the proper maturation of afferent ribbon synapses in sensory cells of the inner ear, and for normal hearing.

Spontaneous activity in the developing auditory system

Spontaneous electrical activity is a common feature of sensory systems during early development. This sensory-independent neuronal activity has been implicated in promoting their survival and maturation, as well as growth and refinement of their projections to yield circuits that can rapidly extract information about the external world. Periodic bursts of action potentials occur in auditory neurons of mammals before hearing onset. This activity is induced by inner hair cells (IHCs) within the developing cochlea, which establish functional connections with spiral ganglion neurons (SGNs) several weeks before they are capable of detecting external sounds. During this pre-hearing period, IHCs fire periodic bursts of Ca(2+) action potentials that excite SGNs, triggering brief but intense periods of activity that pass through auditory centers of the brain. Although spontaneous activity requires input from IHCs, there is ongoing debate about whether IHCs are intrinsically active and their firing periodically interrupted by external inhibitory input (IHC-inhibition model), or are intrinsically silent and their firing periodically promoted by an external excitatory stimulus (IHC-excitation model). There is accumulating evidence that inner supporting cells in Kölliker's organ spontaneously release ATP during this time, which can induce bursts of Ca(2+) spikes in IHCs that recapitulate many features of auditory neuron activity observed in vivo. Nevertheless, the role of supporting cells in this process remains to be established in vivo. A greater understanding of the molecular mechanisms responsible for generating IHC activity in the developing cochlea will help reveal how these events contribute to the maturation of nascent auditory circuits.

Activation of BK and SK channels by efferent synapses on outer hair cells in high-frequency regions of the rodent cochlea

Cholinergic neurons of the brainstem olivary complex project to and inhibit outer hair cells (OHCs), refining acoustic sensitivity of the mammalian cochlea. In all vertebrate hair cells studied to date, cholinergic inhibition results from the combined action of ionotropic acetylcholine receptors and associated calcium-activated potassium channels. Although inhibition was thought to involve exclusively small conductance (SK potassium channels), recent findings have shown that BK channels also contribute to inhibition in basal, high-frequency OHCs after the onset of hearing. Here we show that the waveform of randomly timed IPSCs (evoked by high extracellular potassium) in high-frequency OHCs is altered by blockade of either SK or BK channels, with BK channels supporting faster synaptic waveforms and SK channels supporting slower synaptic waveforms. Consistent with these findings, IPSCs recorded from high-frequency OHCs that express BK channels are briefer than IPSCs recorded from low-frequency (apical) OHCs that do not express BK channels and from immature high-frequency OHCs before the developmental onset of BK channel expression. Likewise, OHCs of BKα(-/-) mice lacking the pore-forming α-subunit of BK channels have longer IPSCs than do the OHCs of BKα(+/+) littermates. Furthermore, serial reconstruction of electron micrographs showed that postsynaptic cisterns of BKα(-/-) OHCs were smaller than those of BKα(+/+) OHCs, and immunofluorescent quantification showed that efferent presynaptic terminals of BKα(-/-) OHCs were smaller than those of BKα(+/+) OHCs. Together, these findings indicate that BK channels contribute to postsynaptic function, and influence the structural maturation of efferent-OHC synapses.

Efferent innervation of turtle semicircular canal cristae: comparisons with bird and mouse

In the vestibular periphery of nearly every vertebrate, cholinergic vestibular efferent neurons give rise to numerous presynaptic varicosities that target hair cells and afferent processes in the sensory neuroepithelium. Although pharmacological studies have described the postsynaptic actions of vestibular efferent stimulation in several species, characterization of efferent innervation patterns and the relative distribution of efferent varicosities among hair cells and afferents are also integral to understanding how efferent synapses operate. Vestibular efferent markers, however, have not been well characterized in the turtle, one of the animal models used by our laboratory. Here we sought to identify reliable efferent neuronal markers in the vestibular periphery of turtle, to use these markers to understand how efferent synapses are organized, and to compare efferent neuronal labeling patterns in turtle with two other amniotes using some of the same markers. Efferent fibers and varicosities were visualized in the semicircular canal of red-eared turtles (Trachemys scripta elegans), zebra finches (Taeniopygia guttata), and mice (Mus musculus) utilizing fluorescent immunohistochemistry with antibodies against choline acetyltransferase (ChAT). Vestibular hair cells and afferents were counterstained using antibodies to myosin VIIa and calretinin. In all species, ChAT labeled a population of small diameter fibers giving rise to numerous spherical varicosities abutting type II hair cells and afferent processes. That these ChAT-positive varicosities represent presynaptic release sites were demonstrated by colabeling with antibodies against the synaptic vesicle proteins synapsin I, SV2, or syntaxin and the neuropeptide calcitonin gene-related peptide. Comparisons of efferent innervation patterns among the three species are discussed.

Calcium-Induced calcium release during action potential firing in developing inner hair cells

In the mature auditory system, inner hair cells (IHCs) convert sound-induced vibrations into electrical signals that are relayed to the central nervous system via auditory afferents. Before the cochlea can respond to normal sound levels, developing IHCs fire calcium-based action potentials that disappear close to the onset of hearing. Action potential firing triggers transmitter release from the immature IHC that in turn generates experience-independent firing in auditory neurons. These early signaling events are thought to be essential for the organization and development of the auditory system and hair cells. A critical component of the action potential is the rise in intracellular calcium that activates both small conductance potassium channels essential during membrane repolarization, and triggers transmitter release from the cell. Whether this calcium signal is generated by calcium influx or requires calcium-induced calcium release (CICR) is not yet known. IHCs can generate CICR, but to date its physiological role has remained unclear. Here, we used high and low concentrations of ryanodine to block or enhance CICR to determine whether calcium release from intracellular stores affected action potential waveform, interspike interval, or changes in membrane capacitance during development of mouse IHCs. Blocking CICR resulted in mixed action potential waveforms with both brief and prolonged oscillations in membrane potential and intracellular calcium. This mixed behavior is captured well by our mathematical model of IHC electrical activity. We perform two-parameter bifurcation analysis of the model that predicts the dependence of IHCs firing patterns on the level of activation of two parameters, the SK2 channels activation and CICR rate. Our data show that CICR forms an important component of the calcium signal that shapes action potentials and regulates firing patterns, but is not involved directly in triggering exocytosis. These data provide important insights into the calcium signaling mechanisms involved in early developmental processes.

Molecular interaction of α-conotoxin RgIA with the rat α9α10 nicotinic acetylcholine receptor

The α9α10 nicotinic acetylcholine receptor (nAChR) was first identified in the auditory system, where it mediates synaptic transmission between efferent olivocochlear cholinergic fibers and cochlea hair cells. This receptor gained further attention due to its potential role in chronic pain and breast and lung cancers. We previously showed that α-conotoxin (α-CTx) RgIA, one of the few α9α10 selective ligands identified to date, is 300-fold less potent on human versus rat α9α10 nAChR. This species difference was conferred by only one residue in the (-), rather than (+), binding region of the α9 subunit. In light of this unexpected discovery, we sought to determine other interacting residues with α-CTx RgIA. A previous molecular modeling study, based on the structure of the homologous molluscan acetylcholine-binding protein, predicted that RgIA interacts with three residues on the α9(+) face and two residues on the α10(-) face of the α9α10 nAChR. However, mutations of these residues had little or no effect on toxin block of the α9α10 nAChR. In contrast, mutations of homologous residues in the opposing nAChR subunits (α10 Ε197, P200 and α9 T61, D121) resulted in 19- to 1700-fold loss of toxin activity. Based on the crystal structure of the extracellular domain (ECD) of human α9 nAChR, we modeled the rat α9α10 ECD and its complexes with α-CTx RgIA and acetylcholine. Our data support the interaction of α-CTx RgIA at the α10/α9 rather than the α9/α10 nAChR subunit interface, and may facilitate the development of selective ligands with therapeutic potential.

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

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

A 'calcium capacitor' shapes cholinergic inhibition of cochlear hair cells

Efferent cholinergic neurons project from the brainstem to inhibit sensory hair cells of the vertebrate inner ear. This inhibitory synapse combines the activity of an unusual class of ionotropic cholinergic receptor with that of nearby calcium-dependent potassium channels to shunt and hyperpolarize the hair cell. Postsynaptic calcium signalling is constrained by a thin near-membrane cistern that is co-extensive with the efferent terminal contacts. The postsynaptic cistern may play an essential role in calcium homeostasis, serving as sink or source, depending on ongoing activity and the degree of buffer saturation. Release of calcium from postsynaptic stores leads to a process of retrograde facilitation via the synthesis of nitric oxide in the hair cell. Activity-dependent synaptic modification may contribute to changes in hair cell innervation that occur during development, and in the aged or damaged cochlea.

Cholinergic efferent synaptic transmission regulates the maturation of auditory hair cell ribbon synapses

Spontaneous electrical activity generated by developing sensory cells and neurons is crucial for the maturation of neural circuits. The full maturation of mammalian auditory inner hair cells (IHCs) depends on patterns of spontaneous action potentials during a ‘critical period’ of development. The intrinsic spiking activity of IHCs can be modulated by inhibitory input from cholinergic efferent fibres descending from the brainstem, which transiently innervate immature IHCs. However, it remains unknown whether this transient efferent input to developing IHCs is required for their functional maturation. We used a mouse model that lacks the α9-nicotinic acetylcholine receptor subunit (α9nAChR) in IHCs and another lacking synaptotagmin-2 in the efferent terminals to remove or reduce efferent input to IHCs, respectively. We found that the efferent system is required for the developmental linearization of the Ca²⁺-sensitivity of vesicle fusion at IHC ribbon synapses, without affecting their general cell development. This provides the first direct evidence that the efferent system, by modulating IHC electrical activity, is required for the maturation of the IHC synaptic machinery. The central control of sensory cell development is unique among sensory systems.

In vivo generation of immature inner hair cells in neonatal mouse cochleae by ectopic Atoh1 expression

Regeneration of auditory hair cells (HCs) is a promising approach to restore hearing. Recent studies have demonstrated that induced pluripotent stem cells/embryonic stem cells or supporting cells (SCs) adjacent to HCs can be converted to adopt the HC fate. However, little is known about whether new HCs are characteristic of outer or inner HCs. Here, we showed in vivo conversion of 2 subtypes of SCs, inner border cells (IBs) and inner phalangeal cells (IPhs), to the inner HC (IHC) fate. This was achieved by ectopically activating Atoh1, a transcription factor necessary for HC fate, in IBs/IPhs at birth. Atoh1+ IBs/IPhs first turned on Pou4f3, another HC transcription factor, before expressing 8 HC markers. The conversion rate gradually increased from ∼2.4% at 1 week of age to ∼17.8% in adult. Interestingly, new HCs exhibited IHC characteristics such as straight line–shaped stereociliary bundles, expression of Fgf8 and otoferlin, and presence of larger outward currents than those of outer HCs. However, new HCs lacked the terminal differentiation IHC marker vGlut3, exhibited reduced density of presynaptic Cbtp2 puncta that had little postsynaptic GluR2 specialization, and displayed immature IHC outward currents. Our results demonstrate that the conversion rate of IBs/IPhs in vivo by Atoh1 ectopic expression into the IHC fate was higher and faster and the conversion was more complete than that of the 2 other SC subtypes underneath the outer HCs; however, these new IHCs are arrested before terminal differentiation. Thus, IBs/IPhs are good candidates to regenerate IHCs in vivo.

Ultrastructure of cisternal synapses on outer hair cells of the mouse cochlea

C (cisternal) synapses with a near membrane postsynaptic cistern are found on motor neurons and other central neurons, where their functional role is unknown. Similarly structured cisternal synapses mediate cholinergic inhibition of cochlear hair cells via α9α10-containing ionotropic receptors and associated calcium-activated (SK2) potassium channels, providing the opportunity to examine the ultrastructure of genetically altered cisternal synapses. Serial section electron microscopy was used to examine efferent synapses of outer hair cells (OHCs) in mice with diminished or enhanced cholinergic inhibition. The contact area of efferent terminals, the appositional area of the postsynaptic cistern, the distance of the cistern from the plasma membrane, and the average width of the cisternal lumen were recorded. The synaptic cisterns of wild-type OHCs were closely aligned (14-nm separation) with the hair cell membrane and coextensive with the micrometers-long synaptic terminals. The cisternal lumen averaged 18 nm so that the cisternal volume was approximately 30% larger than that of the cytoplasmic space between the cistern and the plasma membrane. Synaptic ultrastructure of α9L9'T knockin OHCs (acetylcholine receptor gain of function) were like those of wild-type littermates except that cisternal volumes were significantly larger. OHCs of SK2 knockout mice had few small efferent terminals. Synaptic cisterns were present, but smaller than those of wild-type littermates. Taken together, these data suggest that the cistern serves as a sink or buffer to isolate synaptic calcium signals.

Spatiotemporal pattern of action potential firing in developing inner hair cells of the mouse cochlea

Inner hair cells (IHCs) are the primary transducer for sound encoding in the cochlea. In contrast to the graded receptor potential of adult IHCs, immature hair cells fire spontaneous calcium action potentials during the first postnatal week. This spiking activity has been proposed to shape the tonotopic map along the ascending auditory pathway. Using perforated patch-clamp recordings, we show that developing IHCs fire spontaneous bursts of action potentials and that this pattern is indistinguishable along the basoapical gradient of the developing cochlea. In both apical and basal IHCs, the spiking behavior undergoes developmental changes, where the bursts of action potential tend to occur at a regular time interval and have a similar length toward the end of the first postnatal week. Although disruption of purinergic signaling does not interfere with the action potential firing pattern, pharmacological ablation of the α9α10 nicotinic receptor elicits an increase in the discharge rate. We therefore suggest that in addition to carrying place information to the ascending auditory nuclei, the IHCs firing pattern controlled by the α9α10 receptor conveys a temporal signature of the cochlear development.

Alternative splice isoforms of small conductance calcium-activated SK2 channels differ in molecular interactions and surface levels

Small conductance Ca²⁺-sensitive potassium (SK2) channels are voltage-independent, Ca²⁺-activated ion channels that conduct potassium cations and thereby modulate the intrinsic excitability and synaptic transmission of neurons and sensory hair cells. In the cochlea, SK2 channels are functionally coupled to the highly Ca²⁺ permeant α9/10-nicotinic acetylcholine receptors (nAChRs) at olivocochlear postsynaptic sites. SK2 activation leads to outer hair cell hyperpolarization and frequency-selective suppression of afferent sound transmission. These inhibitory responses are essential for normal regulation of sound sensitivity, frequency selectivity, and suppression of background noise. However, little is known about the molecular interactions of these key functional channels. Here we show that SK2 channels co-precipitate with α9/10-nAChRs and with the actin-binding protein α-actinin-1. SK2 alternative splicing, resulting in a 3 amino acid insertion in the intracellular 3′ terminus, modulates these interactions. Further, relative abundance of the SK2 splice variants changes during developmental stages of synapse maturation in both the avian cochlea and the mammalian forebrain. Using heterologous cell expression to separately study the 2 distinct isoforms, we show that the variants differ in protein interactions and surface expression levels, and that Ca²⁺ and Ca²⁺-bound calmodulin differentially regulate their protein interactions. Our findings suggest that the SK2 isoforms may be distinctly modulated by activity-induced Ca²⁺ influx. Alternative splicing of SK2 may serve as a novel mechanism to differentially regulate the maturation and function of olivocochlear and neuronal synapses.