Alpha 9: an acetylcholine receptor with novel pharmacological properties expressed in rat cochlear hair cells

Nicotinic acetylcholine receptors regulate vestibular afferent gain and activation timing

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

Human polymorphisms in nicotinic receptors: a functional analysis in iPS-derived dopaminergic neurons

Tobacco smoking is a public health problem, with ∼5 million deaths per year, representing a heavy burden for many countries. No effective therapeutic strategies are currently available for nicotine addiction, and it is therefore crucial to understand the etiological and pathophysiological factors contributing to this addiction. The neuronal α5 nicotinic acetylcholine receptor (nAChR) subunit is critically involved in nicotine dependence. In particular, the human polymorphism α5D398N corresponds to the strongest correlation with nicotine dependence risk found to date in occidental populations, according to meta-analysis of genome-wide association studies. To understand the specific contribution of this subunit in the context of nicotine addiction, an efficient screening system for native human nAChRs is needed. We have differentiated human induced pluripotent stem (iPS) cells into midbrain dopaminergic (DA) neurons and obtained a comprehensive characterization of these neurons by quantitative RT-PCR. The functional properties of nAChRs expressed in these human DA neurons, with or without the polymorphism in the α5 subunit, were studied with the patch-clamp electrophysiological technique. Our results in human DA neurons carrying the polymorphism in the α5 subunit showed an increase in EC₅₀, indicating that, in the presence of the polymorphism, more nicotine or acetylcholine chloride is necessary to obtain the same effect. This human cell culturing system can now be used in drug discovery approaches to screen for compounds that interact specifically with human native and polymorphic nAChRs.-Deflorio, C., Blanchard, S., Carisì, M. C., Bohl, D., Maskos, U. Human polymorphisms in nicotinic receptors: a functional analysis in iPS-derived dopaminergic neurons.

The mammalian olivocochlear system--a legacy of non-cerebellar research in the Mugnaini lab

Although the major emphasis of Enrico Mugnaini's research has been on investigations of the cerebellum, a significant amount of work over a relatively short span of time was also done in his lab on a number of other brain systems. These centered on sensory systems. One of these extra-cerebellar systems that he embraced was the auditory system. Portions of the cochlear nucleus, the first synaptic relay station along the central auditory pathways, possess a cerebellar-like circuitry and neurochemistry, and this no doubt lured Enrico into the auditory field. As new tools became available to pursue neuroanatomical research in general, which included a novel antibody to glutamic acid decarboxylase (GAD), Enrico's lab soon branched out into investigating many other brain structures beyond the cerebellum, with an overall goal of producing a map illustrating GAD expression in the brain. In collaboration with long-term colleagues, one of these many non-cerebellar regions he took an interest in was an efferent pathway originating in the superior olive and projecting to the cochlea, the peripheral end organ for hearing. There was a need for a more complete neurochemical map of this olivocochlear efferent system, and armed with new antibodies and well-established tract tracing tools, together we set out to further explore this system. This short review describes the work done with Enrico on the olivocochlear system of rodents, and also continues the story beyond Enrico's lab to reveal how the work done in his lab fits into the larger scheme of current, ongoing research into the olivocochlear system.

Phosphocholine - an agonist of metabotropic but not of ionotropic functions of α9-containing nicotinic acetylcholine receptors

We demonstrated previously that phosphocholine and phosphocholine-modified macromolecules efficiently inhibit ATP-dependent release of interleukin-1β from human and murine monocytes by a mechanism involving nicotinic acetylcholine receptors (nAChR). Interleukin-1β is a potent pro-inflammatory cytokine of innate immunity that plays pivotal roles in host defence. Control of interleukin-1β release is vital as excessively high systemic levels cause life threatening inflammatory diseases. In spite of its structural similarity to acetylcholine, there are no other reports on interactions of phosphocholine with nAChR. In this study, we demonstrate that phosphocholine inhibits ion-channel function of ATP receptor P2X7 in monocytic cells via nAChR containing α9 and α10 subunits. In stark contrast to choline, phosphocholine does not evoke ion current responses in Xenopus laevis oocytes, which heterologously express functional homomeric nAChR composed of α9 subunits or heteromeric receptors containing α9 and α10 subunits. Preincubation of these oocytes with phosphocholine, however, attenuated choline-induced ion current changes, suggesting that phosphocholine may act as a silent agonist. We conclude that phophocholine activates immuno-modulatory nAChR expressed by monocytes but does not stimulate canonical ionotropic receptor functions.

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

Background and Objective

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

Methods

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

Results

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

Conclusion

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

Differential Effects of Quercetin and Quercetin Glycosides on Human α7 Nicotinic Acetylcholine Receptor-Mediated Ion Currents

Quercetin is a flavonoid usually found in fruits and vegetables. Aside from its antioxidative effects, quercetin, like other flavonoids, has a various neuropharmacological actions. Quercetin-3-O-rhamnoside (Rham1), quercetin-3-O-rutinoside (Rutin), and quercetin-3-(2(G)-rhamnosylrutinoside (Rham2) are mono-, di-, and tri-glycosylated forms of quercetin, respectively. In a previous study, we showed that quercetin can enhance α7 nicotinic acetylcholine receptor (α7 nAChR)-mediated ion currents. However, the role of the carbohydrates attached to quercetin in the regulation of α7 nAChR channel activity has not been determined. In the present study, we investigated the effects of quercetin glycosides on the acetylcholine induced peak inward current (I_ACh) in Xenopus oocytes expressing the α7 nAChR. I_ACh was measured with a two-electrode voltage clamp technique. In oocytes injected with α7 nAChR copy RNA, quercetin enhanced I_ACh, whereas quercetin glycosides inhibited I_ACh. Quercetin glycosides mediated an inhibition of I_ACh, which increased when they were pre-applied and the inhibitory effects were concentration dependent. The order of I_ACh inhibition by quercetin glycosides was Rutin≥Rham1>Rham2. Quercetin glycosides-mediated I_ACh enhancement was not affected by ACh concentration and appeared voltage-independent. Furthermore, quercetin-mediated I_ACh inhibition can be attenuated when quercetin is co-applied with Rham1 and Rutin, indicating that quercetin glycosides could interfere with quercetin-mediated α7 nAChR regulation and that the number of carbohydrates in the quercetin glycoside plays a key role in the interruption of quercetin action. These results show that quercetin and quercetin glycosides regulate the α7 nAChR in a differential manner.

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.

Genetic Correction of Induced Pluripotent Stem Cells From a Deaf Patient With MYO7A Mutation Results in Morphologic and Functional Recovery of the Derived Hair Cell-Like Cells

The genetic correction of induced pluripotent stem cells (iPSCs) induced from somatic cells of patients with sensorineural hearing loss (caused by hereditary factors) is a promising method for its treatment. The correction of gene mutations in iPSCs could restore the normal function of cells and provide a rich source of cells for transplantation. In the present study, iPSCs were generated from a deaf patient with compound heterozygous MYO7A mutations (c.1184G>A and c.4118C>T; P-iPSCs), the asymptomatic father of the patient (MYO7A c.1184G>A mutation; CF-iPSCs), and a normal donor (MYO7A(WT/WT); C-iPSCs). One of MYO7A mutation sites (c.4118C>T) in the P-iPSCs was corrected using CRISPR/Cas9. The corrected iPSCs (CP-iPSCs) retained cell pluripotency and normal karyotypes. Hair cell-like cells induced from CP-iPSCs showed restored organization of stereocilia-like protrusions; moreover, the electrophysiological function of these cells was similar to that of cells induced from C-iPSCs and CF-iPSCs. These results might facilitate the development of iPSC-based gene therapy for genetic disorders.

SIGNIFICANCE

Induced pluripotent stem cells (iPSCs) were generated from a deaf patient with compound heterozygous MYO7A mutations (c.1184G>A and c.4118C>T). One of the MYO7A mutation sites (c.4118C>T) in the iPSCs was corrected using CRISPR/Cas9. The genetic correction of MYO7A mutation resulted in morphologic and functional recovery of hair cell-like cells derived from iPSCs. These findings confirm the hypothesis that MYO7A plays an important role in the assembly of stereocilia into stereociliary bundles. Thus, the present study might provide further insight into the pathogenesis of sensorineural hearing loss and facilitate the development of therapeutic strategies against monogenic disease through the genetic repair of patient-specific iPSCs.

The prototoxin LYPD6B modulates heteromeric α3β4-containing nicotinic acetylcholine receptors, but not α7 homomers

Prototoxins are a diverse family of membrane-tethered molecules expressed in the nervous system that modulate nicotinic cholinergic signaling, but their functions and specificity have yet to be completely explored. We tested the selectivity and efficacy of leukocyte antigen, PLAUR (plasminogen activator, urokinase receptor) domain-containing (LYPD)-6B on α3β4-, α3α5β4-, and α7-containing nicotinic acetylcholine receptors (nAChRs). To constrain stoichiometry, fusion proteins encoding concatemers of human α3, β4, and α5 (D and N variants) subunits were expressed in Xenopus laevis oocytes and tested with or without LYPD6B. We used the 2-electrode voltage-clamp method to quantify responses to acetylcholine (ACh): agonist sensitivity (EC50), maximal agonist-induced current (Imax), and time constant (τ) of desensitization. For β4-α3-α3-β4-α3 and β4-α3-β4-α3-α3, LYPD6B decreased EC50 from 631 to 79 μM, reduced Imax by at least 59%, and decreased τ. For β4-α3-α5D-β4-α3 and β4-α3-β4-α-α5D, LYPD6B decreased Imax by 63 and 32%, respectively. Thus, LYPD6B acted only on (α3)3(β4)2 and (α3)2(α5D)(β4)2 and did not affect the properties of (α3)2(β4)3, α7, or (α3)2(α5N)(β4)2 nAChRs. Therefore, LYPD6B acts as a mixed modulator that enhances the sensitivity of (α3)3(β4)2 nAChRs to ACh while reducing ACh-induced whole-cell currents. LYPD6B also negatively modulates α3β4 nAChRs that include the α5D common human variant, but not the N variant associated with nicotine dependence.

The middle ear muscle reflex in the diagnosis of cochlear neuropathy

Cochlear neuropathy, i.e. the loss of auditory nerve fibers (ANFs) without loss of hair cells, may cause hearing deficits without affecting threshold sensitivity, particularly if the subset of ANFs with high thresholds and low spontaneous rates (SRs) is preferentially lost, as appears to be the case in both aging and noise-damaged cochleas. Because low-SR fibers may also be important drivers of the medial olivocochlear reflex (MOCR) and middle-ear muscle reflex (MEMR), these reflexes might be sensitive metrics of cochlear neuropathy. To test this hypothesis, we measured reflex strength and reflex threshold in mice with noise-induced neuropathy, as documented by confocal analysis of immunostained cochlear whole-mounts. To assay the MOCR, we measured contra-noise modulation of ipsilateral distortion-product otoacoustic emissions (DPOAEs) before and after the administration of curare to block the MEMR or curare + strychnine to also block the MOCR. The modulation of DPOAEs was 1) dominated by the MEMR in anesthetized mice, with a smaller contribution from the MOCR, and 2) significantly attenuated in neuropathic mice, but only when the MEMR was intact. We then measured MEMR growth functions by monitoring contra-noise induced changes in the wideband reflectance of chirps presented to the ipsilateral ear. We found 1) that the changes in wideband reflectance were mediated by the MEMR alone, and 2) that MEMR threshold was elevated and its maximum amplitude was attenuated in neuropathic mice. These data suggest that the MEMR may be valuable in the early detection of cochlear neuropathy.

Nicotinic acetylcholine receptors: upregulation, age-related effects and associations with drug use

Nicotinic acetylcholine receptors are ligand-gated ion channels that exogenously bind nicotine. Nicotine produces rewarding effects by interacting with these receptors in the brain's reward system. Unlike other receptors, chronic stimulation by an agonist induces an upregulation of receptor number that is not due to increased gene expression in adults; while upregulation also occurs during development and adolescence there have been some opposing findings regarding a change in corresponding gene expression. These receptors have also been well studied with regard to human genetic associations and, based on evidence suggesting shared genetic liabilities between substance use disorders, numerous studies have pointed to a role for this system in comorbid drug use. This review will focus on upregulation of these receptors in adulthood, adolescence and development, as well as the findings from human genetic association studies which point to different roles for these receptors in risk for initiation and continuation of drug use.

Cochlear hair cells: The sound-sensing machines

The sensory epithelium of the mammalian inner ear contains two types of mechanosensory cells: inner (IHC) and outer hair cells (OHC). They both transduce mechanical force generated by sound waves into electrical signals. In their apical end, these cells possess a set of stereocilia representing the mechanosensing organelles. IHC are responsible for detecting sounds and transmitting the acoustic information to the brain by converting graded depolarization into trains of action potentials in auditory nerve fibers. OHC are responsible for the active mechanical amplification process that leads to the fine tuning and high sensitivity of the mammalian inner ear. This active amplification is the consequence of the ability of OHC to alter their cell length in response to changes in membrane potential, and is controlled by an efferent inhibitory innervation. Medial olivocochlear efferent fibers, originating in the brainstem, synapse directly at the base of OHC and release acetylcholine. A very special type of nicotinic receptor, assembled by α9α10 subunits, participates in this synapse. Here we review recent knowledge and the role of both afferent and efferent synapse in the inner ear.

Corticofugal modulation of peripheral auditory responses

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

The mammalian efferent vestibular system plays a crucial role in the high-frequency response and short-term adaptation of the vestibuloocular reflex

Although anatomically well described, the functional role of the mammalian efferent vestibular system (EVS) remains unclear. Unlike in fish and reptiles, the mammalian EVS does not seem to play a role in modulation of primary afferent activity in anticipation of active head movements. However, it could play a role in modulating long-term mechanisms requiring plasticity such as vestibular adaptation. We measured the efficacy of vestibuloocular reflex (VOR) adaptation in α9-knockout mice. These mice carry a missense mutation of the gene encoding the α9 nicotinic acetylcholine receptor (nAChR) subunit. The α9 nAChR subunit is expressed in the vestibular and auditory periphery, and its loss of function could compromise peripheral input from the predominantly cholinergic EVS. We measured the VOR gain (eye velocity/head velocity) in 26 α9-knockout mice and 27 cba129 control mice. Mice were randomly assigned to one of three groups: gain-increase adaptation (1.5×), gain-decrease adaptation (0.5×), or no adaptation (baseline, 1×). After adaptation training (horizontal rotations at 0.5 Hz with peak velocity 20°/s), we measured the sinusoidal (0.2-10 Hz, 20-100°/s) and transient (1,500-6,000°/s(2)) VOR in complete darkness. α9-Knockout mice had significantly lower baseline gains compared with control mice. This difference increased with stimulus frequency (∼ 5% <1 Hz to ∼ 25% >1 Hz). Moreover, vestibular adaptation (difference in VOR gain of gain-increase and gain-decrease adaptation groups as % of gain increase) was significantly reduced in α9-knockout mice (17%) compared with control mice (53%), a reduction of ∼ 70%. Our results show that the loss of α9 nAChRs moderately affects the VOR but severely affects VOR adaptation, suggesting that the EVS plays a crucial role in vestibular plasticity.

Conotoxin Interactions with α9α10-nAChRs: Is the α9α10-Nicotinic Acetylcholine Receptor an Important Therapeutic Target for Pain Management?

The α9α10-nicotinic acetylcholine receptor (nAChR) has been implicated in pain and has been proposed to be a novel target for analgesics. However, the evidence to support the involvement of the α9α10-nAChR in pain is conflicted. This receptor was first implicated in pain with the characterisation of conotoxin Vc1.1, which is highly selective for α9α10-nAChRs and is an efficacious analgesic in chronic pain models with restorative capacities and no reported side effects. Numerous other analgesic conotoxin and non-conotoxin molecules have been subsequently characterised that also inhibit α9α10-nAChRs. However, there is evidence that α9α10-nAChR inhibition is neither necessary nor sufficient for analgesia. α9α10-nAChR-inhibiting analogues of Vc1.1 have no analgesic effects. Genetically-modified α9-nAChR knockout mice have a phenotype that is markedly different from the analgesic profile of Vc1.1 and similar conotoxins, suggesting that the conotoxin effects are largely independent of α9α10-nAChRs. Furthermore, an alternative mechanism of analgesia by Vc1.1 and other similar conotoxins involving non-canonical coupling of GABAB receptors to voltage-gated calcium channels is known. Additional incongruities regarding α9α10-nAChRs in analgesia are discussed. A more comprehensive characterisation of the role of α9α10-nAChRs in pain is crucial for understanding the analgesic action of conotoxins and for improved drug design.

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.

Pharmacologically distinct nicotinic acetylcholine receptors drive efferent-mediated excitation in calyx-bearing vestibular afferents

Electrical stimulation of vestibular efferent neurons rapidly excites the resting discharge of calyx/dimorphic (CD) afferents. In turtle, this excitation arises when acetylcholine (ACh), released from efferent terminals, directly depolarizes calyceal endings by activating nicotinic ACh receptors (nAChRs). Although molecular biological data from the peripheral vestibular system implicate most of the known nAChR subunits, specific information about those contributing to efferent-mediated excitation of CD afferents is lacking. We sought to identify the nAChR subunits that underlie the rapid excitation of CD afferents and whether they differ from α9α10 nAChRs on type II hair cells that drive efferent-mediated inhibition in adjacent bouton afferents. We recorded from CD and bouton afferents innervating the turtle posterior crista during electrical stimulation of vestibular efferents while applying several subtype-selective nAChR agonists and antagonists. The α9α10 nAChR antagonists, α-bungarotoxin and α-conotoxin RgIA, blocked efferent-mediated inhibition in bouton afferents while leaving efferent-mediated excitation in CD units largely intact. Conversely, 5-iodo-A-85380, sazetidine-A, varenicline, α-conotoxin MII, and bPiDDB (N,N-dodecane-1,12-diyl-bis-3-picolinium dibromide) blocked efferent-mediated excitation in CD afferents without affecting efferent-mediated inhibition in bouton afferents. This pharmacological profile suggested that calyceal nAChRs contain α6 and β2, but not α9, nAChR subunits. Selective blockade of efferent-mediated excitation in CD afferents distinguished dimorphic from calyx afferents by revealing type II hair cell input. Dimorphic afferents differed in having higher mean discharge rates and a mean efferent-mediated excitation that was smaller in amplitude yet longer in duration. Molecular biological data demonstrated the expression of α9 in turtle hair cells and α4 and β2 in associated vestibular ganglia.

αS-conotoxin GVIIIB potently and selectively blocks α9α10 nicotinic acetylcholine receptors

Although acetylcholine is widely utilized in vertebrate nervous systems, nicotinic acetylcholine receptors (nAChRs), including the α9α10 subtype, also are expressed in a wide variety of non-neuronal cells. These cell types include cochlear hair cells, adrenal chromaffin cells and immune cells. α9α10 nAChRs present in these cells may respectively play roles in protection from noise-induced hearing loss, response to stress and neuroprotection. Despite these critical functions, there are few available selective ligands to confirm mechanistic hypothesis regarding the role of α9α10 nAChRs. Conus, has been a rich source of ligands for receptors and ion channels. Here, we identified Conus geographus venom as a lead source for a novel α9α10 antagonist. The active component was isolated and the encoding gene cloned. The peptide signal sequence and cysteine arrangement had the signature of the σ-conotoxin superfamily. Previously isolated σ-conotoxin GVIIIA, also from Conus geographus, targets the 5-HT3 receptor. In contrast, αS-GVIIIB blocked the α9α10 nAChR with an IC50 of 9.8 nM, yet was inactive at the 5-HT3 receptor. Pharmacological characterization of αS-GVIIIB shows that it is over 100-fold selective for the α9α10 nAChR compared to other nAChR subtypes. Thus, the S-superfamily represents a novel conotoxin scaffold for flexibly targeting a variety of receptor subtypes. Functional competition studies utilized distinct off-rate kinetics of conotoxins to identify the α10/α9 nAChR interface as the site of αS-GVIIIB binding; this adds to the importance of the (+) face of the α10 rather than the (+) face of the α9 nAChR subunit as critical to binding of α9α10-targeted conotoxins.

Characterization of the transcriptome of nascent hair cells and identification of direct targets of the Atoh1 transcription factor

Hair cells are sensory receptors for the auditory and vestibular system in vertebrates. The transcription factor Atoh1 is both necessary and sufficient for the differentiation of hair cells, and is strongly upregulated during hair-cell regeneration in nonmammalian vertebrates. To identify genes involved in hair cell development and function, we performed RNA-seq profiling of purified Atoh1-expressing hair cells from the neonatal mouse cochlea. We identified >600 enriched transcripts in cochlear hair cells, of which 90% have not been previously shown to be expressed in hair cells. We identified 233 of these hair cell genes as candidates to be directly regulated by Atoh1 based on the presence of Atoh1 binding sites in their regulatory regions and by analyzing Atoh1 ChIP-seq datasets from the cerebellum and small intestine. We confirmed 10 of these genes as being direct Atoh1 targets in the cochlea by ChIP-PCR. The identification of candidate Atoh1 target genes is a first step in identifying gene regulatory networks for hair-cell development and may inform future studies on the potential role of Atoh1 in mammalian hair cell regeneration.

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.

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.

Activity-dependent regulation of prestin expression in mouse outer hair cells

Prestin is a membrane protein necessary for outer hair cell (OHC) electromotility and normal hearing. Its regulatory mechanisms are unknown. Several mouse models of hearing loss demonstrate increased prestin, inspiring us to investigate how hearing loss might feedback onto OHCs. To test whether centrally mediated feedback regulates prestin, we developed a novel model of inner hair cell loss. Injection of diphtheria toxin (DT) into adult CBA mice produced significant loss of inner hair cells without affecting OHCs. Thus, DT-injected mice were deaf because they had no afferent auditory input despite OHCs continuing to receive normal auditory mechanical stimulation and having normal function. Patch-clamp experiments demonstrated no change in OHC prestin, indicating that loss of information transfer centrally did not alter prestin expression. To test whether local mechanical feedback regulates prestin, we used Tecta(C1509G) mice, where the tectorial membrane is malformed and only some OHCs are stimulated. OHCs connected to the tectorial membrane had normal prestin levels, whereas OHCs not connected to the tectorial membrane had elevated prestin levels, supporting an activity-dependent model. To test whether the endocochlear potential was necessary for prestin regulation, we studied Tecta(C1509G) mice at different developmental ages. OHCs not connected to the tectorial membrane had lower than normal prestin levels before the onset of the endocochlear potential and higher than normal prestin levels after the onset of the endocochlear potential. Taken together, these data indicate that OHC prestin levels are regulated through local feedback that requires mechanoelectrical transduction currents. This adaptation may serve to compensate for variations in the local mechanical environment.

Acoustic input and efferent activity regulate the expression of molecules involved in cochlear micromechanics

Electromotile activity in auditory outer hair cells (OHCs) is essential for sound amplification. It relies on the highly specialized membrane motor protein prestin, and its interactions with the cytoskeleton. It is believed that the expression of prestin and related molecules involved in OHC electromotility may be dynamically regulated by signals from the acoustic environment. However little is known about the nature of such signals and how they affect the expression of molecules involved in electromotility in OHCs. We show evidence that prestin oligomerization is regulated, both at short and relatively long term, by acoustic input and descending efferent activity originating in the cortex, likely acting in concert. Unilateral removal of the middle ear ossicular chain reduces levels of trimeric prestin, particularly in the cochlea from the side of the lesion, whereas monomeric and dimeric forms are maintained or even increased in particular in the contralateral side, as shown in Western blots. Unilateral removal of the auditory cortex (AC), which likely causes an imbalance in descending efferent activity on the cochlea, also reduces levels of trimeric and tetrameric forms of prestin in the side ipsilateral to the lesion, whereas in the contralateral side prestin remains unaffected, or even increased in the case of trimeric and tetrameric forms. As far as efferent inputs are concerned, unilateral ablation of the AC up-regulates the expression of α10 nicotinic Ach receptor (nAChR) transcripts in the cochlea, as shown by RT-Quantitative real-time PCR (qPCR). This suggests that homeostatic synaptic scaling mechanisms may be involved in dynamically regulating OHC electromotility by medial olivocochlear efferents. Limited, unbalanced efferent activity after unilateral AC removal, also affects prestin and β-actin mRNA levels. These findings support that the concerted action of acoustic and efferent inputs to the cochlea is needed to regulate the expression of major molecules involved in OHC electromotility, both at the transcriptional and posttranscriptional levels.

Inhibition of nicotinic acetylcholine receptors, a novel facet in the pleiotropic activities of snake venom phospholipases A2

Phospholipases A2 represent the most abundant family of snake venom proteins. They manifest an array of biological activities, which is constantly expanding. We have recently shown that a protein bitanarin, isolated from the venom of the puff adder Bitis arietans and possessing high phospholipolytic activity, interacts with different types of nicotinic acetylcholine receptors and with the acetylcholine-binding protein. To check if this property is characteristic to all venom phospholipases A2, we have studied the capability of these enzymes from other snakes to block the responses of Lymnaea stagnalis neurons to acetylcholine or cytisine and to inhibit α-bungarotoxin binding to nicotinic acetylcholine receptors and acetylcholine-binding proteins. Here we present the evidence that phospholipases A2 from venoms of vipers Vipera ursinii and V. nikolskii, cobra Naja kaouthia, and krait Bungarus fasciatus from different snake families suppress the acetylcholine- or cytisine-elicited currents in L. stagnalis neurons and compete with α-bungarotoxin for binding to muscle- and neuronal α7-types of nicotinic acetylcholine receptor, as well as to acetylcholine-binding proteins. As the phospholipase A2 content in venoms is quite high, under some conditions the activity found may contribute to the deleterious venom effects. The results obtained suggest that the ability to interact with nicotinic acetylcholine receptors may be a general property of snake venom phospholipases A2, which add a new target to the numerous activities of these enzymes.

Characterization of transcriptomes of cochlear inner and outer hair cells

Inner hair cells (IHCs) and outer hair cells (OHCs) are the two types of sensory receptor cells that are critical for hearing in the mammalian cochlea. IHCs and OHCs have different morphology and function. The genetic mechanisms that define their morphological and functional specializations are essentially unknown. The transcriptome reflects the genes that are being actively expressed in a cell and holds the key to understanding the molecular mechanisms of the biological properties of the cell. Using DNA microarray, we examined the transcriptome of 2000 individually collected IHCs and OHCs from adult mouse cochleae. We show that 16,647 and 17,711 transcripts are expressed in IHCs and OHCs, respectively. Of those genes, ∼73% are known genes, 22% are uncharacterized sequences, and 5.0% are noncoding RNAs in both populations. A total of 16,117 transcripts are expressed in both populations. Uniquely and differentially expressed genes account for <15% of all genes in either cell type. The top 10 differentially expressed genes include Slc17a8, Dnajc5b, Slc1a3, Atp2a3, Osbpl6, Slc7a14, Bcl2, Bin1, Prkd1, and Map4k4 in IHCs and Slc26a5, C1ql1, Strc, Dnm3, Plbd1, Lbh, Olfm1, Plce1, Tectb, and Ankrd22 in OHCs. We analyzed commonly and differentially expressed genes with the focus on genes related to hair cell specializations in the apical, basolateral, and synaptic membranes. Eighty-three percent of the known deafness-related genes are expressed in hair cells. We also analyzed genes involved in cell-cycle regulation. Our dataset holds an extraordinary trove of information about the molecular mechanisms underlying hair cell morphology, function, pathology, and cell-cycle control.

Synaptic calcium regulation in hair cells of the chicken basilar papilla

Cholinergic inhibition of hair cells occurs by activation of calcium-dependent potassium channels. A near-membrane postsynaptic cistern has been proposed to serve as a store from which calcium is released to supplement influx through the ionotropic ACh receptor. However, the time and voltage dependence of acetylcholine (ACh)-evoked potassium currents reveal a more complex relationship between calcium entry and release from stores. The present work uses voltage steps to regulate calcium influx during the application of ACh to hair cells in the chicken basilar papilla. When calcium influx was terminated at positive membrane potential, the ACh-evoked potassium current decayed exponentially over ∼100 ms. However, at negative membrane potentials, this current exhibited a secondary rise in amplitude that could be eliminated by dihydropyridine block of the voltage-gated calcium channels of the hair cell. Calcium entering through voltage-gated channels may transit through the postsynaptic cistern, since ryanodine and sarcoendoplasmic reticulum calcium-ATPase blockers altered the time course and magnitude of this secondary, voltage-dependent contribution to ACh-evoked potassium current. Serial section electron microscopy showed that efferent and afferent synaptic structures are juxtaposed, supporting the possibility that voltage-gated influx at afferent ribbon synapses influences calcium homeostasis during long-lasting cholinergic inhibition. In contrast, spontaneous postsynaptic currents ("minis") resulting from stochastic efferent release of ACh were made briefer by ryanodine, supporting the hypothesis that the synaptic cistern serves primarily as a calcium barrier and sink during low-level synaptic activity. Hypolemmal cisterns such as that at the efferent synapse of the hair cell can play a dynamic role in segregating near-membrane calcium for short-term and long-term signaling.

Contribution of Variants in CHRNA5/A3/B4 Gene Cluster on Chromosome 15 to Tobacco Smoking: From Genetic Association to Mechanism

Cigarette smoking is the major cause of preventable death and morbidity throughout the world. Many compounds are present in tobacco, but nicotine is the primary addictive one. Nicotine exerts its physiological and pharmacological roles in the brain through neuronal nicotinic acetylcholine receptors (nAChRs), which are ligand-gated ion channels consisting of five membrane-spanning subunits that can modulate the release of neurotransmitters, such as dopamine, glutamate, and GABA and mediate fast signal transmission at synapses. Considering that there are 12 nAChR subunits, it is highly likely that subunits other than α4 and β2, which have been intensively investigated, also are involved in nicotine addiction. Consistent with this hypothesis, a number of genome-wide association studies (GWAS) and subsequent candidate gene-based associated studies investigating the genetic variants associated with nicotine dependence (ND) and smoking-related phenotypes have shed light on the CHRNA5/A3/B4 gene cluster on chromosome 15, which encodes the α5, α3, and β4 nAChR subunits, respectively. These studies demonstrate two groups of risk variants in this region. The first one is marked by single nucleotide polymorphism (SNP) rs16969968 in exon 5 of CHRNA5, which changes an aspartic acid residue into asparagine at position 398 (D398N) of the α5 subunit protein sequence, and it is tightly linked SNP rs1051730 in CHRNA3. The second one is SNP rs578776 in the 3'-untranslated region (UTR) of CHRNA3, which has a low correlation with rs16969968. Although the detailed molecular mechanisms underlying these associations remain to be further elucidated, recent findings have shown that α5* (where "*" indicates the presence of additional subunits) nAChRs located in the medial habenulo-interpeduncular nucleus (mHb-IPN) are involved in the control of nicotine self-administration in rodents. Disruption of α5* nAChR signaling diminishes the aversive effects of nicotine on the mHb-IPN pathway and thereby permits more nicotine consumption. To gain a better understanding of the function of the highly significant genetic variants identified in this region in controlling smoking-related behaviors, in this communication, we provide an up-to-date review of the progress of studies focusing on the CHRNA5/A3/B4 gene cluster and its role in ND.

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.

Crystal structures of free and antagonist-bound states of human α9 nicotinic receptor extracellular domain

We determined the X-ray crystal structures of the extracellular domain (ECD) of the monomeric state of human neuronal α9 nicotinic acetylcholine receptor (nAChR) and of its complexes with the antagonists methyllycaconitine and α-bungarotoxin at resolutions of 1.8 Å, 1.7 Å and 2.7 Å, respectively. The structure of the monomeric α9 ECD superimposed well with the structures of homologous proteins in pentameric assemblies, denoting native folding, despite the absence of a complementary subunit and transmembrane domain. The interaction motifs of both antagonists were similar to those in the complexes with homologous pentameric proteins, thus highlighting the major contribution of the principal side of α9 ECD to their binding. The structures revealed a functionally important β7-β10 strand interaction in α9-containing nAChRs, involving their unique Thr147, a hydration pocket similar to that of mouse α1 ECD and a membrane-facing network coordinated by the invariant Arg210.

α9-nicotinic acetylcholine receptors contribute to the maintenance of chronic mechanical hyperalgesia, but not thermal or mechanical allodynia

Background

The current pharmacological treatments for chronic pain are limited. The first analgesic drug approved for clinical use in decades that has a novel molecular target is the synthetic version of a naturally occurring conotoxin. Several conotoxins that target ion channels have progressed to clinical trials for the relief of pain. Vc1.1 and RgIA are analgesic α-conotoxins that target α9-subunit-containing nicotinic acetylcholine receptors (α9-nAChR) as well as GABA_B receptor mechanisms. However, the evidence for the involvement of α9-nAChRs in pain is controversial. In the present study, the role of the α9-nAChR in pain was assessed using a battery of behavioural pain tests and pain models in α9-nAChR knockout (KO) mice.

Results

α9-nAChR KO mice showed normal responses to acute noxious thermal and mechanical stimuli, and developed normal chronic cold and mechanical allodynia in inflammatory and nerve injury pain models. However, KO animals developed mechanical hyperalgesia to a lesser extent than their wild type (WT) counterparts in both inflammatory and neuropathic pain models. Chronic neuropathic pain is sustained in WT mice for at least 21 days post injury, while KO mice show significant recovery by 14 days post injury. KO sham mice were also resistant to the repeated-measures effect of the noxious pain test that caused a gradual onset of mild mechanical hyperalgesia in WT sham animals.

Conclusions

The α9-nAChR is not involved in acute pain perception or chronic thermal or mechanical allodynia or thermal hyperalgesia but does contribute to the intensity and duration of chronic mechanical hyperalgesia, suggesting that pain-relieving actions of antagonists that target this site may be restricted to high threshold mechanosensation. The α9-nAChR appears to be a valid target for pharmacological compounds that alleviate long-term mechanical hyperalgesia and may be of use as a prophylactic drug to prevent the development of some symptoms of chronic pain.

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.

A potentially novel nicotinic receptor in Aplysia neuroendocrine cells

Nicotinic receptors form a diverse group of ligand-gated ionotropic receptors with roles in both synaptic transmission and the control of excitability. In the bag cell neurons of Aplysia, acetylcholine activates an ionotropic receptor, which passes inward current to produce a long-lasting afterdischarge and hormone release, leading to reproduction. While testing the agonist profile of the cholinergic response, we observed a second current that appeared to be gated only by nicotine and not acetylcholine. The peak nicotine-evoked current was markedly smaller in magnitude than the acetylcholine-induced current, cooperative (Hill value of 2.7), had an EC50 near 500 μM, readily recovered from desensitization, showed Ca2+ permeability, and was blocked by mecamylamine, dihydro-β-erythroidine, or strychnine, but not by α-conotoxin ImI, methyllycaconitine, or hexamethonium. Aplysia transcriptome analysis followed by PCR yielded 20 full-length potential nicotinic receptor subunits. Sixteen of these were predicted to be cation selective, and real-time PCR suggested that 15 of the 16 subunits were expressed to varying degrees in the bag cell neurons. The acetylcholine-induced current, but not the nicotine current, was reduced by double-strand RNA treatment targeted to both subunits ApAChR-C and -E. Conversely, the nicotine-evoked current, but not the acetylcholine current, was lessened by targeting both subunits ApAChR-H and -P. To the best of our knowledge, this is the first report suggesting that a nicotinic receptor is not gated by acetylcholine. Separate receptors may serve as a means to differentially trigger plasticity or safeguard propagation by assuring that only acetylcholine, the endogenous agonist, initiates large enough responses to trigger reproduction.

The precise temporal pattern of prehearing spontaneous activity is necessary for tonotopic map refinement

Patterned spontaneous activity is a hallmark of developing sensory systems. In the auditory system, rhythmic bursts of spontaneous activity are generated in cochlear hair cells and propagated along central auditory pathways. The role of these activity patterns in the development of central auditory circuits has remained speculative. Here we demonstrate that blocking efferent cholinergic neurotransmission to developing hair cells in mice that lack the α9 subunit of nicotinic acetylcholine receptors (α9 KO mice) altered the temporal fine structure of spontaneous activity without changing activity levels. KO mice showed a severe impairment in the functional and structural sharpening of an inhibitory tonotopic map, as evidenced by deficits in synaptic strengthening and silencing of connections and an absence in axonal pruning. These results provide evidence that the precise temporal pattern of spontaneous activity before hearing onset is crucial for the establishment of precise tonotopy, the major organizing principle of central auditory pathways.

α-conotoxin RgIA protects against the development of nerve injury-induced chronic pain and prevents both neuronal and glial derangement

Neuropathic pain affects millions of people worldwide, causing substantial disability and greatly impairing quality of life. Commonly used analgesics or antihyperalgesic compounds are generally characterized by limited therapeutic outcomes. Thus, there is a compelling need for novel therapeutic strategies able to prevent nervous tissue alterations responsible for chronic pain. The α9α10 nicotinic acetylcholine receptor antagonist α-conotoxin RgIA (RgIA), a peptide isolated from the venom of a carnivorous cone snail, induces relief in both acute and chronic pain models. To evaluate potential disease-modifying effects of RgIA, the compound was given to rats following chronic constriction injury (CCI) of the sciatic nerve. Two or 10 nmol RgIA injected intramuscularly once a day for 14 days reduced the painful response to suprathreshold stimulation, increased pain threshold to nonnoxious stimuli, and normalized alterations in hind limb weight bearing. Histological analysis of the sciatic nerve revealed that RgIA prevented CCI-induced decreases of axonal compactness and diameter, loss of myelin sheath, and decreases in the fiber number. Moreover, RgIA significantly reduced edema and inflammatory infiltrate, including a decrease of CD86(+) macrophages. In L4-L5 dorsal root ganglia, RgIA prevented morphometric changes and reduced the inflammatory infiltrate consistent with a disease-modifying effect. In the dorsal horn of the spinal cord, RgIA prevented CCI-induced activation of microglia and astrocytes. These data suggest that RgIA-like compounds may represent a novel class of therapeutics for neuropathic pain that protects peripheral nervous tissues as well as prevents central maladaptive plasticity by inhibiting glial cell activation.

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.

Presence of multiple binding sites on α9α10 nAChR receptors alludes to stoichiometric-dependent action of the α-conotoxin, Vc1.1

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels involved in fast synaptic transmission. nAChRs are pentameric receptors formed from a combination of different or similar subunits to produce heteromeric or homomeric channels. The heteromeric, α9α10 nAChR subtype is well-known for its role in the auditory system, being expressed in cochlear hair cells. These nAChRs have also been shown to be involved in immune-modulation. Antagonists of α9α10 nAChRs, like the α-conotoxin Vc1.1, have analgesic effects in neuropathic pain. Unlike other nAChR subtypes there is no evidence that functional receptor stoichiometries of α9α10 exist. By using 2-electrode voltage clamp methods and maintaining a constant intracellular Ca(2+) concentration, we observed a biphasic activation curve for ACh that is dependent on receptor stoichiometry. Vc1.1, but not the α9α10 antagonists RgIA or atropine, inhibits ACh-evoked currents in a biphasic manner. Characteristics of the ACh and Vc1.1 activation and inhibition curves can be altered by varying the ratio of α9 and α10 mRNA injected into oocytes, changing the curves from biphasic to monophasic when an excess of α10 mRNA is used. These results highlight the difference in the pharmacological profiles of at least two different α9α10 nAChR stoichiometries, possibly (α9)₃(α10)₂ and (α9)₂(α10)₃. As a result, we infer that there is an additional binding site for ACh and Vc1.1 at the α9-α9 interface on the hypothesized (α9)₃(α10)₂ nAChR, in addition to the α10-α9 and or α9-α10 interfaces that are common to both stoichiometries. This study provides further evidence that receptor stoichiometry contributes another layer of complexity in understanding Cys-loop receptors.

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.

Caenorhabditis elegans nicotinic acetylcholine receptors are required for nociception

Polymodal nociceptors sense and integrate information on injurious mechanical, thermal, and chemical stimuli. Chemical signals either activate nociceptors or modulate their responses to other stimuli. One chemical known to activate or modulate responses of nociceptors is acetylcholine (ACh). Across evolution nociceptors express subunits of the nicotinic acetylcholine receptor (nAChR) family, a family of ACh-gated ion channels. The roles of ACh and nAChRs in nociceptor function are, however, poorly understood. Caenorhabditis elegans polymodal nociceptors, PVD, express nAChR subunits on their sensory arbor. Here we show that mutations reducing ACh synthesis and mutations in nAChR subunits lead to defects in PVD function and morphology. A likely cause for these defects is a reduction in cytosolic calcium measured in ACh and nAChR mutants. Indeed, overexpression of a calcium pump in PVD mimics defects in PVD function and morphology found in nAChR mutants. Our results demonstrate, for the first time, a central role for nAChRs and ACh in nociceptor function and suggest that calcium permeating via nAChRs facilitates activity of several signaling pathways within this neuron.

Effects of ginsenoside Rg3 on α9α10 nicotinic acetylcholine receptor-mediated ion currents

Ginsenosides is a low molecular weight substance found in ginseng as one of the active ingredients. Ginsenosides, like other herbal medicines, has a wide range of neuropharmacological actions including neuroprotective effects. The α9α10 nicotinic acetylcholine receptor is one of numerous nicotinic acetylcholine receptors that exists as a heteropentameric form in auditory hair cells of the cochlea. In this study, we report the effects of ginsenosides on rat α9α10 nicotinic acetylcholine receptor-mediated ion currents using the two-electrode voltage clamp technique. Treatment with acetylcholine evoked inward currents (IACh) in oocytes heterologously expressing the α9α10 nicotinic acetylcholine receptor. Ginsenosides blocked IACh in order of potency of Rg3> Rb2> CK>Re = Rg2> Rf>Rc> Rb1> Rg1 with reversible manners, and the blocking effect of Rg3 on IACh was same after pre-application than co-application of Rg3. The half maximal inhibitory concentration (IC50) of Rg3 was 39.6 ± 4.9 µm. Rg3-induced IACh inhibition was not affected by acetylcholine concentration and was independent of membrane holding potential. Although the inhibitory effect of Rg3 on IACh was abolished in oocytes expressing α9 subunit alone, indicating that the presence of α10 subunit might be required for Rg3-induced regulations of α9α10 nicotinic acetylcholine receptor channel activity. These results indicate that α10 subunit of α9α10 nicotinic acetylcholine receptor might play an important role in Rg3-induced regulation of the α9α10 nicotinic acetylcholine receptor.

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.

K(+) channels: function-structural overview

Potassium channels are particularly important in determining the shape and duration of the action potential, controlling the membrane potential, modulating hormone secretion, epithelial function and, in the case of those K(+) channels activated by Ca(2+), damping excitatory signals. The multiplicity of roles played by K(+) channels is only possible to their mammoth diversity that includes at present 70 K(+) channels encoding genes in mammals. Today, thanks to the use of cloning, mutagenesis, and the more recent structural studies using x-ray crystallography, we are in a unique position to understand the origins of the enormous diversity of this superfamily of ion channels, the roles they play in different cell types, and the relations that exist between structure and function. With the exception of two-pore K(+) channels that are dimers, voltage-dependent K(+) channels are tetrameric assemblies and share an extremely well conserved pore region, in which the ion-selectivity filter resides. In the present overview, we discuss in the function, localization, and the relations between function and structure of the five different subfamilies of K(+) channels: (a) inward rectifiers, Kir; (b) four transmembrane segments-2 pores, K2P; (c) voltage-gated, Kv; (d) the Slo family; and (e) Ca(2+)-activated SK family, SKCa.

Role of α5* nicotinic acetylcholine receptors in the effects of acute and chronic nicotine treatment on brain reward function in mice

OBJECTIVE

Allelic variation in the α5 nicotinic acetylcholine receptor (nAChR) subunit gene, CHRNA5, increases vulnerability to tobacco addiction. Here, we investigated the role of α5* nAChRs in the effects of nicotine on brain reward systems.

MATERIALS AND METHODS

Effects of acute (0.03125-0.5 mg/kg SC) or chronic (24 mg/kg per day; osmotic minipump) nicotine and mecamylamine-precipitated withdrawal on intracranial self-stimulation (ICSS) thresholds were assessed in wild-type and α5 nAChR subunit knockout mice. Noxious effects of nicotine were further investigated using a conditioned taste aversion procedure.

RESULTS

Lower nicotine doses (0.03125-0.125 mg/kg) decreased ICSS thresholds in wild-type and α5 knockout mice. At higher doses (0.25-0.5 mg/kg), threshold-lowering effects of nicotine were diminished in wild-type mice, whereas nicotine lowered thresholds across all doses tested in α5 knockout mice. Nicotine (1.5 mg/kg) conditioned a taste aversion to saccharine equally in both genotypes. Mecamylamine (5 mg/kg) elevated ICSS thresholds by a similar magnitude in wild-type and α5 knockout mice prepared with minipumps delivering nicotine. Unexpectedly, mecamylamine also elevated thresholds in saline-treated α5 knockout mice.

CONCLUSION

α5* nAChRs are not involved in reward-enhancing effects of lower nicotine doses, the reward-inhibiting effects of nicotine withdrawal, or the general noxious effects of higher nicotine doses. Instead, α5* nAChRs regulate the reward-inhibiting effects nicotine doses that oppose the reward-facilitating effects of the drug. These data suggest that disruption of α5* nAChR signaling greatly expands the range of nicotine doses that facilitate brain reward activity, which may help explain the increased tobacco addiction vulnerability associated with CHRNA5 risk alleles.

Protection from noise-induced hearing loss by Kv2.2 potassium currents in the central medial olivocochlear system

The central auditory brainstem provides an efferent projection known as the medial olivocochlear (MOC) system, which regulates the cochlear amplifier and mediates protection on exposure to loud sound. It arises from neurons of the ventral nucleus of the trapezoid body (VNTB), so control of neuronal excitability in this pathway has profound effects on hearing. The VNTB and the medial nucleus of the trapezoid body are the only sites of expression for the Kv2.2 voltage-gated potassium channel in the auditory brainstem, consistent with a specialized function of these channels. In the absence of unambiguous antagonists, we used recombinant and transgenic methods to examine how Kv2.2 contributes to MOC efferent function. Viral gene transfer of dominant-negative Kv2.2 in wild-type mice suppressed outward K(+) currents, increasing action potential (AP) half-width and reducing repetitive firing. Similarly, VNTB neurons from Kv2.2 knock-out mice (Kv2.2KO) also showed increased AP duration. Control experiments established that Kv2.2 was not expressed in the cochlea, so any changes in auditory function in the Kv2.2KO mouse must be of central origin. Further, in vivo recordings of auditory brainstem responses revealed that these Kv2.2KO mice were more susceptible to noise-induced hearing loss. We conclude that Kv2.2 regulates neuronal excitability in these brainstem nuclei by maintaining short APs and enhancing high-frequency firing. This safeguards efferent MOC firing during high-intensity sounds and is crucial in the mediation of protection after auditory overexposure.

Functional expression of human α9* nicotinic acetylcholine receptors in X. laevis oocytes is dependent on the α9 subunit 5' UTR

Nicotinic acetylcholine receptors (nAChRs) containing the α9 subunit are expressed in a wide variety of non-neuronal tissues ranging from immune cells to breast carcinomas. The α9 subunit is able to assemble into a functional homomeric nAChR and also co-assemble with the α10 subunit into functional heteromeric nAChRs. Despite the increasing awareness of the important roles of this subunit in vertebrates, the study of human α9-containing nAChRs has been severely limited by difficulties in its expression in heterologous systems. In Xenopus laevis oocytes, functional expression of human α9α10 nAChRs is very low compared to that of rat α9α10 nAChRs. When oocytes were co-injected with cRNA of α9 and α10 subunits of human versus those of rat, oocytes with the rat α9 human α10 combination had an ∼-fold higher level of acetylcholine-gated currents (I_ACh) than those with the human α9 rat α10 combination, suggesting difficulties with human α9 expression. When the ratio of injected human α9 cRNA to human α10 cRNA was increased from 1∶1 to 5∶1, I_ACh increased 36-fold (from 142±23 nA to 5171±748 nA). Functional expression of human α9-containing receptors in oocytes was markedly improved by appending the 5′-untranslated region of alfalfa mosaic virus RNA4 to the 5′-leader sequence of the α9 subunit cRNA. This increased the functional expression of homomeric human α9 receptors by 70-fold (from 7±1 nA to 475±158 nA) and of human α9α10 heteromeric receptors by 80-fold (from 113±62 nA to 9192±1137 nA). These findings indicate the importance of the composition of the 5′ untranslated leader sequence for expression of α9-containing nAChRs.

Pyrrole- and dihydropyrrole-fused neonicotinoids: design, synthesis, and insecticidal evaluation

Versatile pyrrole- and dihydropyrrole-fused neonicotinoids were obtained from cyclic and non-cyclic nitroeneamines. Anhydrous aluminum chloride (AlCl₃) exhibited high catalytic selectivity for the synthesis of the titled etherified compounds at room temperature and the eliminated products under reflux conditions. The target molecules have been identified on the basis of satisfactory analytical and spectral [¹H and ¹³C nuclear magnetic resonance (NMR), high-resolution mass spectrometry (HRMS), and X-ray] data. All synthesized compounds have been screened for insecticidal activity. The preliminary insecticidal activity results showed that some of the aimed compounds displayed excellent insecticidal activity against cowpea aphids (Aphis craccivora).

Multiple cholinergic signaling pathways in pituitary gonadotrophs

Acetylcholine (ACh) has been established as a paracrine factor in the anterior pituitary gland, but the receptors mediating ACh action and the cell types bearing these receptors have not been identified. Our results showed that the expression of the nicotinic subunits mRNAs followed the order β2 > β1 = α9 > α4 in cultured rat pituitary cells. The expression of the subunits in immortalized LβT2 mouse gonadotrophs followed the order β2 > α4 = α1. M4 > M3 muscarinic receptor mRNA were also identified in pituitary and LβT2 cells. The treatment of cultured pituitary cells with GnRH down-regulated the expression of α9 and α4 mRNAs, without affecting the expression of M3 and M4 receptor mRNAs, and ACh did not alter the expression of GnRH receptor mRNA. We also performed double immunostaining to show the expression of β2-subunit and M4 receptor proteins in gonadotrophs. Functional nicotinic channels capable of generating an inward current, facilitation of electrical activity, and Ca(2+) influx were identified in single gonadotrophs and LβT2 cells. In both cell types, the M3 receptor-mediated, phospholipase C-dependent Ca(2+) mobilization activated an outward apamin-sensitive K(+) current and caused hyperpolarization. The activation of M4 receptors by ACh inhibited cAMP production and GnRH-induced LH release in a pertussis toxin-sensitive manner. We concluded that multiple cholinergic receptors are expressed in gonadotrophs and that the main secretory action of ACh is inhibitory through M4 receptor-mediated down-regulation of cAMP production. The expression of nicotinic receptors in vitro compensates for the lack of regular GnRH stimulation of gonadotrophs.

Development of novel pharmacotherapeutics for tobacco dependence: progress and future directions

INTRODUCTION

The vast majority of tobacco smokers seeking to quit will relapse within the first month of abstinence. Currently available smoking cessation agents have limited utility in increasing rates of smoking cessation and in some cases there are notable safety concerns related to their use. Hence, there is a pressing need to develop safer and more efficacious smoking cessation medications.

METHODS

Here, we provide an overview of current efforts to develop new pharmacotherapeutic agents to facilitate smoking cessation, identified from ongoing clinical trials and published reports.

RESULTS

Nicotine is considered the major addictive agent in tobacco smoke, and the vast majority of currently available smoking cessation agents act by modulating nicotinic acetylcholine receptor (nAChR) signaling. Accordingly, there is much effort directed toward developing novel small molecule therapeutics and biological agents such as nicotine vaccines for smoking cessation that act by modulating nAChR activity. Our increasing knowledge of the neurobiology of nicotine addiction has revealed new targets for novel smoking cessation therapeutics. Indeed, we highlight many examples of novel small molecule drug development around non-nAChR targets. Finally, there is a growing appreciation that medications already approved for other disease indications could show promise as smoking cessation agents, and we consider examples of such repurposing efforts.

CONCLUSION

Ongoing clinical assessment of potential smoking cessation agents offers the promise of new effective medications. Nevertheless, much of our current knowledge of molecular mechanisms of nicotine addiction derived from preclinical studies has not yet been leveraged for medications development.