Cochlear inner hair cells (IHCs) release neurotransmitter onto afferent auditory nerve fibers in response to sound stimulation. During early development, synaptic transmission is triggered by spontaneous Ca2+ spikes which are modulated by an efferent cholinergic innervation to IHCs. This synapse is inhibitory and mediated by the α9α10 nicotinic cholinergic receptor (nAChR). After the onset of hearing, large-conductance Ca2+-activated K+ channels are acquired and both the spiking activity and the efferent innervation disappear from IHCs. In this work, we studied the developmental changes in the membrane properties of cochlear IHCs from α10 nAChR gene (Chrna10) “knockout” mice. Electrophysiological properties of IHCs were studied by whole-cell recordings in acutely excised apical turns of the organ of Corti from developing mice. Neither the spiking activity nor the developmental functional expression of voltage-gated and/or calcium-sensitive K+ channels is altered in the absence of the α10 nAChR subunit. The present results show that the α10 nAChR subunit is not essential for the correct establishment of the intrinsic electrical properties of IHCs during development.
calcium spikes; voltage-gated K+ channels; calcium-sensitive K+ channels; cholinergic; development; auditory
Mechanosensory hair cells of the organ of Corti transmit information regarding sound to the central nervous system by way of peripheral afferent neurons. In return, the central nervous system provides feedback and modulates the afferent stream of information through efferent neurons. The medial olivocochlear efferent system makes direct synaptic contacts with outer hair cells and inhibits amplification brought about by the active mechanical process inherent to these cells. This feedback system offers the potential to improve the detection of signals in background noise, to selectively attend to particular signals, and to protect the periphery from damage caused by overly loud sounds. Acetylcholine released at the synapse between efferent terminals and outer hair cells activates a peculiar nicotinic cholinergic receptor subtype, the α9α10 receptor. At present no pharmacotherapeutic approaches have been designed that target this cholinergic receptor to treat pathologies of the auditory system. The potential use of α9α10 selective drugs in conditions such as noise-induced hearing loss, tinnitus and auditory processing disorders is discussed.
nicotinic cholinergic receptors; noise trauma; cochlea; tinnitus; efferent feedback
Efferent inhibition of cochlear hair cells is mediated by α9α10 nicotinic cholinergic receptors (nAChRs) functionally coupled to calcium-activated, small conductance (SK2) potassium channels. Before the onset of hearing, efferent fibers transiently make functional cholinergic synapses with inner hair cells (IHCs). The retraction of these fibers after the onset of hearing correlates with the cessation of transcription of the Chrna10 (but not the Chrna9) gene in IHCs. To further analyze this developmental change, we generated a transgenic mice whose IHCs constitutively express α10 into adulthood by expressing the α10 cDNA under the control of the Pou4f3 gene promoter. In situ hybridization showed that the α10 mRNA is expressed in IHCs of 8-week-old transgenic mice, but not in wild-type mice. Moreover, this mRNA is translated into a functional protein, since IHCs from P8-P10 α10 transgenic mice backcrossed to a Chrna10−/− background (whose IHCs have no cholinergic function) displayed normal synaptic and acetylcholine (ACh)-evoked currents in patch-clamp recordings. Thus, the α10 transgene restored nAChR function. However, in the α10 transgenic mice, no synaptic or ACh-evoked currents were observed in P16-18 IHCs, indicating developmental down-regulation of functional nAChRs after the onset of hearing, as normally observed in wild-type mice. The lack of functional ACh currents correlated with the lack of SK2 currents. These results indicate that multiple features of the efferent postsynaptic complex to IHCs, in addition to the nAChR subunits, are down-regulated in synchrony after the onset of hearing, leading to lack of responses to ACh.
nicotinic cholinergic receptors; efferent medial olivocochlear; SK2 channel; acetylcholine; transgenic mice
The synapse between olivocochlear (OC) neurons and cochlear mechanosensory hair cells is cholinergic, fast, and inhibitory. The inhibitory sign of this cholinergic synapse is accounted for by the activation of Ca2+-permeable postsynaptic α9α10 nicotinic receptors coupled to the opening of hyperpolarizing Ca2+-activated small-conductance type 2 (SK2)K+ channels. Acetylcholine (ACh) release at this synapse is supported by both P/Q- and N-type voltage-gated calcium channels (VGCCs). Although the OC synapse is cholinergic, an abundant OC GABA innervation is present along the mammalian cochlea. The role of this neurotransmitter at the OC efferent innervation, however, is for the most part unknown. We show that GABA fails to evoke fast postsynaptic inhibitory currents in apical developing inner and outer hair cells. However, electrical stimulation of OC efferent fibers activates presynaptic GABAB(1a,2) receptors [GABAB(1a,2)Rs] that downregulate the amount of ACh released at the OC–hair cell synapse, by inhibiting P/Q-type VGCCs. We confirmed the expression of GABABRs at OC terminals contacting the hair cells by coimmunostaining for GFP and synaptophysin in transgenic mice expressing GABAB1–GFP fusion proteins. Moreover, coimmunostaining with antibodies against the GABA synthetic enzyme glutamic acid decarboxylase and synaptophysin support the idea that GABA is directly synthesized at OC terminals contacting the hair cells during development. Thus, we demonstrate for the first time a physiological role for GABA in cochlear synaptic function. In addition, our data suggest that the GABAB1a isoform selectively inhibits release at efferent cholinergic synapses.
In the mammalian auditory system, the synapse between efferent olivocochlear (OC) neurons and sensory cochlear hair cells is cholinergic, fast and inhibitory. This efferent synapse is mediated by the nicotinic α9α10 receptor coupled to the activation of SK2 Ca2+-activated K+ channels that hyperpolarize the cell. So far, the ion channels that support and/or modulate neurotransmitter release from the OC terminals remain unknown. To identify these channels, we used an isolated mouse cochlear preparation and monitored transmitter release from the efferent synaptic terminals in inner hair cells (IHCs) voltage-clamped in the whole-cell recording configuration. Acetylcholine (ACh) release was evoked by electrically stimulating the efferent fibers that make axosomatic contacts with IHCs before the onset of hearing. Using the specific antagonists for P/Q-and N-type voltage-gated calcium channels (VGCCs), ω-agatoxin IVA and ω-conotoxin GVIA, respectively, we show that Ca2+ entering through both types of VGCCs support the release process at this synapse. Interestingly, we found that Ca2+ entering through the dihydropiridine-sensitive L-type VGCCs exerts a negative control on transmitter release. Moreover, using immunostaining techniques combined with electrophysiology and pharmacology, we show that BK Ca2+-activated K+ channels are transiently expressed at the OC efferent terminals contacting IHCs and that their activity modulates the release process at this synapse. The effects of dihydropiridines combined with iberiotoxin, a specific BK channel antagonist, strongly suggest that L-type VGCCs negatively regulate the release of ACh by fueling BK channels which are known to curtail the duration of the terminal action potential in several types of neurons.
Synaptic transmission; Calcium channels; BK channels; cochlea; hair cells; efferent
The α9α10 nicotinic acetylcholine receptors (nAChRs) mediates efferent inhibition of hair cell function within the auditory sensory organ. Gating of the nAChRs leads to activation of calcium-dependent potassium channels to hyperpolarize the hair cell. In efferent system, main calcium providers to SK channel are nAChR and synaptic cistern, which contribution to efferent inhibition is different between avian and mammalian species. Calcium permeation is more effective in nAChRs of mammalian cochlea than avian cochlea, and mammalian calcium permeability of nAChRs is about 3 times more than avian hair cell. Thus, mammalian nAChRs is a main component of efferent inhibition in cochlear hair cell system.
Nicotinic receptor; Hair cell; Cochlea; Acetylcholine; Calcium
Cochlear hair cells express SK2, a small-conductance Ca2+-activated K+ channel thought to act in concert with Ca2+-permeable nicotinic acetylcholine receptors (nAChRs) α9 and α10 in mediating suppressive effects of the olivocochlear efferent innervation. To probe the in vivo role of SK2 channels in hearing, we examined gene expression, cochlear function, efferent suppression and noise vulnerability in mice overexpressing SK2 channels. Cochlear thresholds, as measured by auditory brainstem responses and otoacoustic emissions were normal in overexpressers, as was overall cochlear morphology and the size, number and distribution of efferent terminals on outer hair cells. Cochlear expression levels of SK2 channels were elevated 8-fold, without striking changes in other SK channels or in the α9/α10 nAChRs. Shock-evoked efferent suppression of cochlear responses was significantly enhanced in overexpresser mice, as seen previously in α9 overexpresser mice (Maison et al. 2002); however, in contrast to α9 overexpressers, SK2 overexpressers were not protected from acoustic injury. Results suggest that efferent-mediated cochlear protection is mediated by other downstream effects of ACh-mediated Ca2+ entry, different from those involving SK2-mediated hyperpolarization and the associated reduction in outer hair cell electromotility.
Hair cell; Olivocochlear
Outer hair cells (OHCs) amplify the sound-evoked motion of the basilar membrane to enhance acoustic sensitivity and frequency selectivity. Medial olivocochlear (MOC) efferents inhibit OHCs to reduce the sound-evoked response of cochlear afferent neurons. OHC inhibition occurs through the activation of postsynaptic α9α10 nicotinic receptors tightly coupled to calcium-dependent SK2 channels that hyperpolarize the hair cell. MOC neurons are cholinergic but a number of other neurotransmitters and neuromodulators have been proposed to participate in efferent transmission, with emerging evidence for both pre- and postsynaptic effects. Cochlear inhibition in vivo is maximized by repetitive activation of the efferents, reflecting facilitation and summation of transmitter release onto outer hair cells. This review summarizes recent studies on cellular and molecular mechanisms of cholinergic inhibition and the regulation of those molecular components, in particular the involvement of intracellular calcium. Facilitation at the efferent synapse is compared in a variety of animals, as well as other possible mechanisms of modulation of ACh release. These results suggest that short-term plasticity contributes to effective cholinergic inhibition of hair cells.
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.
hair cell; development; cochlea; calcium current; exocytosis; efferent system
Immunostaining mouse cochleas for tyrosine hydroxylase (TH) and dopamine β-hydroxylase suggests that there is a rich adrenergic innervation throughout the auditory nerve trunk and a small dopaminergic innervation of the sensory cell areas. Surgical cuts in the brainstem confirm these dopaminergic fibers as part of the olivocochlear efferent bundle. Within the sensory epithelium, TH-positive terminals are seen only in the inner hair cell area, where they intermingle with other olivocochlear terminals expressing cholinergic markers (vesicular acetylcholine transporter; VAT). Double immunostaining suggests little colocalization of TH and VAT; quantification of terminal volumes suggests that TH-positive fibers constitute only 10–20% of the efferent innervation of the inner hair cell area. Immunostaining of mouse brainstem revealed a small population of TH-positive cells in and around the lateral superior olive. Consistent with cochlear projections, double staining for the cholinergic marker acetylcholinesterase suggested that TH-positive somata are not cholinergic and vice versa. All observations are consistent with the view that a small dopaminergic subgroup of lateral olivocochlear neurons 1) projects to the inner hair cell area, 2) is distinct from the larger cholinergic group projecting there, and 3) may correspond to lateral olivocochlear “shell” neurons described by others (Warr et al.  Hear. Res 108:89–111).
tyrosine hydroxylase; colocalization; hearing; feedback control; olivocochlear
Age-related hearing loss (presbycusis) is a major health concern for the elderly. Loss of spiral ganglion neurons (SGNs), the primary sensory relay of the auditory system, is associated consistently with presbycusis. The causative molecular events responsible for age-related loss of SGNs are unknown. Recent reports directly link age-related neuronal loss in cerebral cortex with the loss of high-affinity nicotine acetylcholine receptors (nAChRs). In cochlea, cholinergic synapses are made by olivocochlear efferent fibers on the outer hair cells that express α9 nAChR subunits and on the peripheral projections of SGNs that express α2, α4 –7, and β2–3 nAChR subunits. A significantly decreased expression of the β2 nAChR subunit in SGNs was found specifically in mice susceptible to presbycusis. Furthermore, mice lacking the β2 nAChR subunit (β2−/−), but not mice lacking the α5 nAChR subunit (α5−/−), have dramatic hearing loss and significant reduction in the number of SGNs. Our findings clearly established a requirement for β2 nAChR subunit in the maintenance of SGNs during aging.
presbycusis; aging; hearing loss; nicotinic receptors; spiral ganglion neurons; neurodegeneration
Amplification of incoming sounds in the inner ear is modulated by an efferent pathway which travels back from the brain all the way to the cochlea. The medial olivocochlear system makes synaptic contacts with hair cells, where the neurotransmitter acetylcholine is released. Synaptic transmission is mediated by a unique nicotinic cholinergic receptor composed of α9 and α10 subunits, which is highly Ca2+ permeable and is coupled to a Ca2+-activated SK potassium channel. Thus, hyperpolarization of hair cells follows efferent fiber activation. In this work we review the literature that has enlightened our knowledge concerning the intimacies of this synapse.
hair cells; nicotinic receptors; alpha9alpha10 nicotinic receptors; synaptic plasticity; prestin; cochlea; SK channels
Efferent innervation of the cochlea undergoes extensive modification early in development, but it is unclear if efferent synapses are modified by age, hearing loss, or both. Structural alterations in the cochlea affecting information transfer from the auditory periphery to the brain may contribute to age-related hearing deficits. We investigated changes to efferent innervation in the vicinity of inner hair cells (IHC) in young and old C57BL/6 mice using transmission electron microscopy to reveal increased efferent innervation of IHCs in older animals. Efferent contacts on IHCs contained focal pre-synaptic accumulations of small vesicles. Synaptic vesicle size and shape were heterogeneous. Postsynaptic cisterns were occasionally observed. Increased IHC efferent innervation was associated with a smaller number of afferent synapses per IHC, increased outer hair cell loss, and elevated auditory brainstem response thresholds. Efferent axons also formed synapses on afferent dendrites but with a reduced prevalence in older animals. Age-related reduction of afferent activity may engage signaling pathways that support the return to an immature state of efferent innervation of the cochlea.
Cochlear innervation; age-related hearing loss; olivocochlear; efferent; neural plasticity
It has previously been shown that deletion of chrna9, the gene encoding the α9 nicotinic acetylcholine receptor (nAChR) subunit, results in abnormal synaptic terminal structure. Additionally, all nAChR-mediated cochlear activity is lost, as characterized by a failure of the descending efferent system to suppress cochlear responses to sound. In an effort to characterize the molecular mechanisms underlying the structural and functional consequences following loss of α9 subunit expression, we performed whole-transcriptome gene expression analyses on cochleae of wild type and α9 knockout (α9−/−) mice during postnatal days spanning critical periods of synapse formation and maturation.
Data revealed that loss of α9 receptor subunit expression leads to an up-regulation of genes involved in synaptic transmission and ion channel activity. Unexpectedly, loss of α9 receptor subunit expression also resulted in an increased expression of genes encoding GABA receptor subunits and the GABA synthetic enzyme, glutamic acid decarboxylase. These data suggest the existence of a previously unrecognized association between the nicotinic cholinergic and GABAergic systems in the cochlea. Computational analyses have highlighted differential expression of several gene sets upon loss of nicotinic cholinergic activity in the cochlea. Time-series analysis of whole transcriptome patterns, represented as self-organizing maps, revealed a disparate pattern of gene expression between α9−/− and wild type cochleae at the onset of hearing (P13), with knockout samples resembling immature postnatal ages.
We have taken a systems biology approach to provide insight into molecular programs influenced by the loss of nicotinic receptor-based cholinergic activity in the cochlea and to identify candidate genes that may be involved in nicotinic cholinergic synapse formation, stabilization or function within the inner ear. Additionally, our data indicate a change in the GABAergic system upon loss of α9 nicotinic receptor subunit within the cochlea.
The mammalian cochlea has two types of sensory cells; inner hair cells, which receive auditory-nerve afferent innervation, and outer hair cells, innervated by efferent axons of the medial olivocochlear (MOC) system. The role of the MOC system in hearing is still controversial. Recently, by recording cochlear potentials in behaving chinchillas, we suggested that one of the possible functions of the efferent system is to reduce cochlear sensitivity during attention to other sensory modalities (Delano et al. in J Neurosci 27:4146–4153, 2007). However, in spite of these compelling results, the physiological effects of electrical MOC activation on cochlear potentials have not been described in detail in chinchillas. The main objective of the present work was to describe these efferent effects in the chinchilla, comparing them with those in other species and in behavioral experiments. We activated the MOC efferent axons in chinchillas with sectioned middle-ear muscles by applying current pulses at the fourth-ventricle floor. Auditory-nerve compound action potentials (CAP) and cochlear microphonics (CM) were acquired in response to clicks and tones of several frequencies, using a round-window electrode. Electrical efferent stimulation produced CAP amplitude suppressions reaching up to 11 dB. They were higher for low to moderate sound levels. Additionally, CM amplitude increments were found, the largest (≤ 2.5 dB) for low intensity tones. CAP suppression was present at all stimulus frequencies, but was greatest for 2 kHz. CM increments were highest for low-frequency tones, and almost absent at high frequencies. We conclude that the effect obtained in chinchilla is similar to but smaller than that observed in cats, and that the effects seen in awake chinchillas, albeit different in magnitude, are consistent with the activation of efferent fibers.
auditory efferent; olivocochlear; chinchilla; electric stimulation; cochlear potentials
In the developing mammalian cochlea, the sensory hair cells receive efferent innervation originating in the superior olivary complex. This input is mediated by α9/α10 nicotinic acetylcholine receptors (nAChRs) and is inhibitory due to the subsequent activation of calcium-dependent SK2 potassium channels. We examined the acquisition of this cholinergic efferent input using whole-cell voltage-clamp recordings from inner hair cells (IHCs) in acutely excised apical turns of the rat cochlea from embryonic day 21 to postnatal day 8 (P8). Responses to 1 mM acetylcholine (ACh) were detected from P0 on in almost every IHC. The ACh-activated current amplitude increased with age and demonstrated the same pharmacology as α9-containing nAChRs. Interestingly, at P0, the ACh response was not coupled to SK2 channels, so that the initial cholinergic response was excitatory and could trigger action potentials in IHCs. Coupling to SK current was detected earliest at P1 in a subset of IHCs and by P3 in every IHC studied. Clustered nAChRs and SK2 channels were found on IHCs from P1 on using Alexa Fluor 488 conjugated α-bungarotoxin and SK2 immunohistochemistry. The number of nAChRs clusters increased with age to 16 per IHC at P8. Cholinergic efferent synaptic currents first appeared in a subset of IHCs at P1 and by P3 in every IHC studied, contemporaneously with ACh-evoked SK currents, suggesting that SK2 channels may be necessary at onset of synaptic function. An analogous pattern of development was observed for the efferent synapses that form later (P6–P8) on outer hair cells in the basal cochlea.
onset of synaptic function; sensory hair cell; efferent synapse; synaptogenesis; α9/α10 nicotinic acetylcholine receptor
Cochlear inner hair cells (IHCs) are temporarily innervated by efferent cholinergic fibers prior to the onset of hearing. During low-frequency firing, these efferent synapses have a relatively low probability of transmitter release but facilitate strongly with repetitive stimulation. A retrograde signal from the hair cell to the efferent terminal contributes to this facilitation. When IHCs were treated with the ryanodine receptor agonist, cyclic adenosine phosphoribose (cADPR), release probability of the efferent terminal rose. This effect was quantified by computing the quantum content from a train of 100 suprathreshold stimuli to the efferent fibers. Quantum content was sevenfold higher when IHCs were treated with 100 μM cADPR (applied in the recording pipette). Since cADPR is membrane impermeant, this result implies that an extracellular messenger travels from the hair cell to the efferent terminal. cADPR is presumed to generate this messenger by increasing cytoplasmic calcium. Consistent with this presumption, voltage-gated calcium flux into the IHC also caused retrograde facilitation of efferent transmission. Retrograde facilitation was observed in IHCs of a vesicular glutamate transporter (VGlut3) null mouse and for wild-type rat hair cells subject to wide-spectrum glutamate receptor blockade, demonstrating that glutamate was unlikely to be the extracellular messenger. Rather, bath application of nitric oxide (NO) donors caused an increase in potassium-evoked efferent transmitter release while the NO scavenger carboxy-PTIO was able to prevent retrograde facilitation produced by cADPR or IHC depolarization. Thus, hair cell activity can drive retrograde facilitation of efferent input via calcium-dependent production of NO.
nitric oxide; hair cell; cochlea; efferent inhibition
Cholinergic efferent fibers modify hair cell responses to mechanical stimulation. It is hypothesized that calcium entering the hair cell through a nicotinic receptor activates a small-conductance (SK), calcium-activated potassium channel to hyperpolarize the hair cell. The calcium signal may be amplified by calcium-induced calcium release from the synaptic cisternae. Pharmacological tests of these ideas in the intact cochlea have been technically difficult because of the complex and fragile structure of the mammalian inner ear. We turned to the Xenopus laevis lateral line organ, whose simplicity and accessibility make it a model for understanding hair cell organ function in a relatively intact system. Drugs were applied to the inner surface of the skin while monitoring the effects of efferent stimulation on afferent fiber discharge rate. Efferent effects were blocked by antagonists of SK channels including apamin (EC50 = 0.5 μM) and dequalinium (EC50 = 12 μM). The effect of apamin was not enhanced by co-administration of phenylmethylsulfonyl fluoride, a proteolysis inhibitor. Efferent effects were attenuated by ryanodine, an agent that can interfere with calcium-induced calcium release, although relatively high (mM) concentrations of ryanodine were required. Fluorescent cationic styryl dyes, 4-di-2-asp and fm 1– 43, blocked efferent effects, although it was not possible to observe specific entry of the dye into the base of hair cells. These pharmacological findings in the Xenopus lateral line organ support the hypothesis that effects of efferent stimulation are mediated by calcium entry through the nicotinic receptor via activation of SK channels and suggest the generality of this mechanism in meditating cholinergic efferent effects.
Outer hair cells are the specialized sensory cells that empower the mammalian hearing organ, the cochlea, with its remarkable sensitivity and frequency selectivity. Sound-evoked receptor potentials in outer hair cells are shaped by both voltage-gated K+ channels that control the membrane potential and also ligand-gated K+ channels involved in the cholinergic efferent modulation of the membrane potential. The objectives of this study were to investigate the tonotopic contribution of BK channels to voltage- and ligand-gated currents in mature outer hair cells from the rat cochlea.
Findings In this work we used patch clamp electrophysiology and immunofluorescence in tonotopically defined segments of the rat cochlea to determine the contribution of BK channels to voltage- and ligand-gated currents in outer hair cells. Although voltage and ligand-gated currents have been investigated previously in hair cells from the rat cochlea, little is known about their tonotopic distribution or potential contribution to efferent inhibition. We found that apical (low frequency) outer hair cells had no BK channel immunoreactivity and little or no BK current. In marked contrast, basal (high frequency) outer hair cells had abundant BK channel immunoreactivity and BK currents contributed significantly to both voltage-gated and ACh-evoked K+ currents.
Our findings suggest that basal (high frequency) outer hair cells may employ an alternative mechanism of efferent inhibition mediated by BK channels instead of SK2 channels. Thus, efferent synapses may use different mechanisms of action both developmentally and tonotopically to support high frequency audition. High frequency audition has required various functional specializations of the mammalian cochlea, and as shown in our work, may include the utilization of BK channels at efferent synapses. This mechanism of efferent inhibition may be related to the unique acetylcholine receptors that have evolved in mammalian hair cells compared to those of other vertebrates.
Although protective effects of the cochlea’s efferent feedback pathways have been well documented, prior work has focused on hair cell damage and cochlear threshold elevation and, correspondingly, on the high sound pressure levels (> 100 dB SPL) necessary to produce them. Here we explore the noise-induced loss of cochlear neurons that occurs with lower intensity exposures and in the absence of permanent threshold shifts. Using confocal microscopy to count synapses between hair cells and cochlear nerve fibers, and using measurement of auditory brainstem responses and otoacoustic emissions to assess cochlear pre- and post-synaptic function, we compare the damage from a weeklong exposure to moderate-level noise (84 dB SPL) in mice with varying degrees of cochlear de-efferentation induced by surgical lesion to the olivocochlear pathway. Such exposure causes minimal acute threshold shift and no chronic shifts in mice with normal efferent feedback. In de-efferented animals, there was up to 40% loss of cochlear nerve synapses and a corresponding decline in the amplitude of the auditory brainstem response. Quantitative analysis of the de-efferentation in inner vs. outer hair cell areas suggested that outer hair cell efferents are most important in minimizing this neuropathy, presumably by virtue of their sound-evoked feedback reduction of cochlear amplification. The moderate nature of this acoustic overexposure suggests that cochlear neurons are at risk even in everyday acoustic environments, and, thus, that the need for cochlear protection is plausible as a driving force in the design of this feedback pathway.
Auditory neuropathy; olivocochlear; hair cells; cochlea; noise; acoustic injury; feedback
To further understand the roles and origins of γ-aminobutyric acid (GABA) and calcitonin gene-related peptide (CGRP) in the efferent innervation of the cochlea, we first produced in the mouse an immunocytochemical map of the efferent terminals that contain acetylcho-line (ACh), CGRP, and GABA. Olivocochlear (OC) terminals in inner and outer hair cell (IHC and OHC) regions were analyzed quantitatively along the cochlear spiral via light-microscopic observation of cochlear wholemounts immunostained with antibodies to glutamic acid decarboxylase (GAD), vesicular acetylcholine transporter (VAT), or the peptide CGRP. Further immunochemical characterization was performed in mice with chronic OC transection at the floor of the fourth ventricle to distinguish crossed from uncrossed contributions and, indirectly, the contributions of lateral versus medial components of the OC system. The results in mouse showed that (1) there are prominent GABAergic, cholinergic, and CGRPergic innervations in the OHC and IHC regions, (2) GABA and CGRP are extensively colocalized with ACh in all OC terminals in the IHC and OHC areas, (3) the longitudinal gradient of OC innervation peaks roughly at the 10-kHz region in the OHC area and is more uniform along the cochlear spiral in the IHC area, (4) in contrast to other mammalian species there is no radial gradient of OC innervation of the OHCs, and (5) all OHC efferent terminals arise from the medial OC system and terminals in the IHC area arise from the lateral OC system.
cochlea; efferents; hair cells; acetylcholine; γ-aminobutyric acid; calcitonin gene-related peptide
The overproduction of TNF and other cytokines can cause the pathophysiology of numerous diseases. Controlling cytokine synthesis and release is critical for preventing unrestrained inflammation and maintaining health. Recent studies identified an efferent vagus nerve-based mechanism termed “the cholinergic anti-inflammatory pathway” that controls cytokine production and inflammation. Here we review current advances related to the role of this pathway in neuro-immune interactions that prevent excessive inflammation. Experimental evidence indicates that vagus nerve cholinergic anti-inflammatory signaling requires alpha7 nicotinic acetylcholine receptors expressed on non-neuronal cytokine producing cells. Alpha7 nicotinic acetylcholine receptor agonists inhibit cytokine release and protect animals in a variety of experimental lethal inflammatory models. Knowledge related to the cholinergic anti-inflammatory pathway can be exploited in therapeutic approaches directed towards counteracting abnormal chronic and hyper-activated inflammatory responses.
cholinergic anti-inflammatory pathway; vagus nerve; TNF; inflammation; acetylcholine; α7 nicotinic acetylcholine receptor; muscarinic acetylcholine receptors
Acetylcholine (ACh) is the major neurotransmitter released from vestibular efferent terminals onto hair cells and afferents. Previous studies indicate that the two classes of acetylcholine receptors, nicotinic (nAChRs) and muscarinic receptors (mAChRs), are expressed by vestibular hair cells (VHCs). To identify if both classes of receptors are present in VHCs, whole cell, voltage-clamp- and current-clamp- patch recordings were performed on isolated pigeon vestibular type I and type II HCs during the application of the cholinergic agonists, acetylcholine and carbachol, and the cholinergic antagonists, d-tubocurarine and atropine. By applying in different combinations, these compounds were used to selectively activate either nAChRs or mAChRs. The effects of nAChR and mAChR activation on HC currents and zero electrode current potential (Vz) were monitored. It was found that presumed mAChR activation decreased both inward and outward currents in both type I and type II HCs, resulting in either a depolarization or hyperpolarization. Conversely, nAChR activation mainly increased both inward and outward currents in type II HCs, resulting in a hyperpolarization of their Vz. nAChR activation also increased outward currents in type I HCs resulting in either a depolarization or hyperpolarization of their Vz. The decrease of inward and outward currents and the depolarization of the Vz in type I pigeon HCs by activation of mAChRs represents a new finding. Ion channel candidates in pigeon vestibular HCs that might underlie the modulation of the macroscopic ionic currents and Vz by different AChR activation are discussed.
pigeon; acetylcholine; muscarinic receptor; nicotinic receptor; vestibular; hair cells
The efferent synaptic specialization of hair cells includes a near-membrane synaptic cistern, whose presence suggests a role for internal calcium stores in cholinergic inhibition. Calcium release channels from internal stores include ‘ryanodine receptors’, whose participation is usually demonstrated by sensitivity to the eponymous plant alkaloid, ryanodine. However, use of this and other store-active compounds on hair cells could be confounded by the unusual pharmacology of the α9α10-containing hair cell nicotinic cholinergic receptor (nAChR), which has been shown to be antagonized by a broad spectrum of compounds. Surprisingly, we found that ryanodine, rather than antagonizing, is a positive modulator of the α9α10 nAChR expressed in Xenopus oocytes, the first such compound to be found. The effect of ryanodine was to increase the apparent affinity and efficacy for acetylcholine (ACh). Correspondingly, ACh-evoked currents through the isolated cholinergic receptors of inner hair cells in excised mouse cochleas were approximately doubled by 200 μM ryanodine, a concentration that inhibits gating of the ryanodine receptor itself. This unusual positive modulation was not unique to the mammalian receptor. The response to ACh of chicken ‘short’ hair cells likewise was enhanced in the presence of 100 μM ryanodine. This facilitatory effect on current through the AChR could enhance brief (∼1 s) activation of associated calcium-dependent K+ (SK) channels in both chicken short hair cells and rat outer hair cells. This novel effect of ryanodine provides new opportunities for the design of compounds that potentiate α9α10-mediated responses and for potential inner ear therapeutics based on this interaction.
ryanodine; nicotinic receptors; α9α10 receptors; ion channels; acetylcholine; efferent system
The olivocochlear efferent system is both cholinergic and GABAergic and innervates sensory cells and sensory neurons of the inner ear. Cholinergic effects on cochlear sensory cells are well characterized, both in vivo and in vitro; however, the robust GABAergic innervation is poorly understood. To explore the functional roles of GABA in the inner ear, we characterized the cochlear phenotype of seven mouse lines with targeted deletion of a GABAA receptor subunit (α1, α2, α5, α6, β2, β3, or δ). Four of the lines (α1, α2, α6, and δ) were normal: there was no cochlear histopathology, and cochlear responses suggested normal function of hair cells, afferent fibers, and efferent feedback. The other three lines (α5, β2, and β3) showed threshold elevations indicative of outer hair cell dysfunction. α5 and β2 lines also showed decreased effects of efferent bundle activation, associated with decreased density of efferent terminals on outer hair cells: although the onset of this degeneration was later in α5 (>6 weeks) than β2 (<6 weeks), both lines shows normal efferent development (up to 3 weeks). Two lines (β2 and β3) showed signs of neuropathy, either decreased density of afferent innervation (β3) or decreased neural responses without concomitant attenuation of hair cell responses (β2). One of the lines (β2) showed a clear sexual dimorphism in cochlear phenotype. Results suggest that the GABAergic component of the olivocochlear system contributes to the long-term maintenance of hair cells and neurons in the inner ear.
efferent; hair cell; olivocochlear; auditory; feedback; knock-out mice