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

Key role of the TM2-TM3 loop in calcium potentiation of the α9α10 nicotinic acetylcholine receptor

The α9α10 nicotinic cholinergic receptor (nAChR) is a ligand-gated pentameric cation-permeable ion channel that mediates synaptic transmission between descending efferent neurons and mechanosensory inner ear hair cells. When expressed in heterologous systems, α9 and α10 subunits can assemble into functional homomeric α9 and heteromeric α9α10 receptors. One of the differential properties between these nAChRs is the modulation of their ACh-evoked responses by extracellular calcium (Ca2+). While α9 nAChRs responses are blocked by Ca2+, ACh-evoked currents through α9α10 nAChRs are potentiated by Ca2+ in the micromolar range and blocked at millimolar concentrations. Using chimeric and mutant subunits, together with electrophysiological recordings under two-electrode voltage-clamp, we show that the TM2-TM3 loop of the rat α10 subunit contains key structural determinants responsible for the potentiation of the α9α10 nAChR by extracellular Ca2+. Moreover, molecular dynamics simulations reveal that the TM2-TM3 loop of α10 does not contribute to the Ca2+ potentiation phenotype through the formation of novel Ca2+ binding sites not present in the α9 receptor. These results suggest that the TM2-TM3 loop of α10 might act as a control element that facilitates the intramolecular rearrangements that follow ACh-evoked α9α10 nAChRs gating in response to local and transient changes of extracellular Ca2+ concentration. This finding might pave the way for the future rational design of drugs that target α9α10 nAChRs as otoprotectants.

N-Glycosylation Deficiency in Transgene α7 nAChR and RIC3 Expressing CHO Cells Without NACHO

The human neuronal nicotinic acetylcholine receptor α7 (nAChR) is an important target implicated in diseases like Alzheimer's or Parkinson's, as well as a validated target for drug discovery. For α7 nAChR model systems, correct folding and ion influx functions are essential. Two chaperones, resistance to inhibitors of cholinesterase 3 (RIC3) and novel nAChR regulator (NACHO), enhance the assembly and function of α7 nAChR. This study investigates the consequence of NACHO absence on α7 nAChR expression and function. Therefore, the sequences of human α7 nAChR and human RIC3 were transduced in Chinese hamster ovary (CHO) cells. Protein expression and function of α7 nAChR were confirmed by Western blot and voltage clamp, respectively. Cellular viability was assessed by cell proliferation and lactate dehydrogenase assays. Intracellular and extracellular expression were determined by in/on-cell Western, compared with another nAChR subtype by novel cluster fluorescence-linked immunosorbent assay, and N-glycosylation efficiency was assessed by glycosylation digest. The transgene CHO cell line showed expected protein expression and function for α7 nAChR and cell viability was barely influenced by overexpression. While intracellular levels of α7 nAChR were as anticipated, plasma membrane insertion was low. The glycosylation digest revealed no appreciable N-glycosylation product. This study demonstrates a stable and functional cell line expressing α7 nAChR, whose protein expression, function, and viability are not affected by the absence of NACHO. The reduced plasma membrane insertion of α7 nAChR, combined with incorrect matured N-glycosylation at the Golgi apparatus, suggests a loss of recognition signal for lectin sorting.

Cholinergic modulation in the vertebrate auditory pathway

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

αO-Conotoxin GeXIVA[1,2] Suppresses In Vivo Tumor Growth of Triple-Negative Breast Cancer by Inhibiting AKT-mTOR, STAT3 and NF-κB Signaling Mediated Proliferation and Inducing Apoptosis

Breast cancer is one of the leading causes of cancer mortality worldwide, and triple-negative breast cancer (TNBC) is the most problematic subtype. There is an urgent need to develop novel drug candidates for TNBC. Marine toxins are a valuable source for drug discovery. We previously identified αO-conotoxin GeXIVA[1,2] from Conus generalis, which is a selective antagonist of α9 nicotinic acetylcholine receptors (nAChRs). Recent studies indicated that α9 nAChR expression is positively correlated with breast cancer development; thus, α9 nAChR could serve as a therapeutic target for breast cancer. In this study, we aimed to investigate the in vivo antitumor effects of GeXIVA[1,2] on TNBC and to elucidate its underlying anticancer mechanism. Our data showed that GeXIVA[1,2] effectively suppressed 4T1 tumor growth in vivo at a very low dose of 0.1 nmol per mouse. Our results uncovered that the antitumor mechanism of GeXIVA[1,2] simultaneously induced apoptosis and blocked proliferation. Further investigations revealed that GeXIVA[1,2]-induced Caspase-3-dependent apoptosis was achieved through regulating Bax/Bcl-2 balance, and GeXIVA[1,2]-inhibited proliferation was mediated by the downregulation of the AKT-mTOR, STAT3 and NF-κB signaling pathways. Our study provides valuable arguments to demonstrate the potential of GeXIVA[1,2] as a novel marine-derived anticancer drug candidate for the treatment of TNBC.

Molecular Specializations Underlying Phenotypic Differences in Inner Ear Hair Cells of Zebrafish and Mice

Hair cells (HCs) are the sensory receptors of the auditory and vestibular systems in the inner ears of vertebrates that selectively transduce mechanical stimuli into electrical activity. Although all HCs have the hallmark stereocilia bundle for mechanotransduction, HCs in non-mammals and mammals differ in their molecular specialization in the apical, basolateral and synaptic membranes. HCs of non-mammals, such as zebrafish (zHCs), are electrically tuned to specific frequencies and possess an active process in the stereocilia bundle to amplify sound signals. Mammalian cochlear HCs, in contrast, are not electrically tuned and achieve amplification by somatic motility of outer HCs (OHCs). To understand the genetic mechanisms underlying differences among adult zebrafish and mammalian cochlear HCs, we compared their RNA-seq-characterized transcriptomes, focusing on protein-coding orthologous genes related to HC specialization. There was considerable shared expression of gene orthologs among the HCs, including those genes associated with mechanotransduction, ion transport/channels, and synaptic signaling. For example, both zebrafish and mouse HCs express Tmc1, Lhfpl5, Tmie, Cib2, Cacna1d, Cacnb2, Otof, Pclo and Slc17a8. However, there were some notable differences in expression among zHCs, OHCs, and inner HCs (IHCs), which likely underlie the distinctive physiological properties of each cell type. Tmc2 and Cib3 were not detected in adult mouse HCs but tmc2a and b and cib3 were highly expressed in zHCs. Mouse HCs express Kcna10, Kcnj13, Kcnj16, and Kcnq4, which were not detected in zHCs. Chrna9 and Chrna10 were expressed in mouse HCs. In contrast, chrna10 was not detected in zHCs. OHCs highly express Slc26a5 which encodes the motor protein prestin that contributes to OHC electromotility. However, zHCs have only weak expression of slc26a5, and subsequently showed no voltage dependent electromotility when measured. Notably, the zHCs expressed more paralogous genes including those associated with HC-specific functions and transcriptional activity, though it is unknown whether they have functions similar to their mammalian counterparts. There was overlap in the expressed genes associated with a known hearing phenotype. Our analyses unveil substantial differences in gene expression patterns that may explain phenotypic specialization of zebrafish and mouse HCs. This dataset also includes several protein-coding genes to further the functional characterization of HCs and study of HC evolution from non-mammals to mammals.

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

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

Genetic tools for studying cochlear inhibition

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

Structural mechanisms of α7 nicotinic receptor allosteric modulation and activation

The α7 nicotinic acetylcholine receptor is a pentameric ligand-gated ion channel that plays an important role in cholinergic signaling throughout the nervous system. Its unique physiological characteristics and implications in neurological disorders and inflammation make it a promising but challenging therapeutic target. Positive allosteric modulators overcome limitations of traditional α7 agonists, but their potentiation mechanisms remain unclear. Here, we present high-resolution structures of α7-modulator complexes, revealing partially overlapping binding sites but varying conformational states. Structure-guided functional and computational tests suggest that differences in modulator activity arise from the stable rotation of a channel gating residue out of the pore. We extend the study using a time-resolved cryoelectron microscopy (cryo-EM) approach to reveal asymmetric state transitions for this homomeric channel and also find that a modulator with allosteric agonist activity exploits a distinct channel-gating mechanism. These results define mechanisms of α7 allosteric modulation and activation with implications across the pentameric receptor superfamily.

New Alpha9 nAChR Ligands Based on a 5-(Quinuclidin-3-ylmethyl)-1,2,4-oxadiazole Scaffold

Several lines of evidence have indicated that nicotinic acetylcholine receptors (nAChR) that contain α9 subunits, probably in combination with α10 subunits, may be valuable targets for the management of pain associated with inflammatory diseases through a cholinergic anti-inflammatory system (CAS), which has also been associated with α7 nAChR. Both α7- and α9-containing neuronal nAChR can be pharmacologically distinguished from the high-affinity nicotinic receptors of the brain by their sensitivity to α-bungarotoxin, but in other ways, they have quite distinct pharmacological profiles. The early association of α7 with CAS led to the development of numerous new ligands, variously characterized as α7 agonists, partial agonists, or silent agonists that desensitized α7 receptors without activation. Subsequent reinvestigation of one such family of α7 ligands based on an N,N-diethyl-N'-phenylpiperazine scaffold led to the identification of potent agonists and antagonists for α9. In this paper, we characterize the α9/α10 activity of a series of compounds based on a 5-(quinuclidin-3-ylmethyl)-1,2,4-oxadiazole (QMO) scaffold and identify two new potent ligands of α9, QMO-28, an agonist, and QMO-17, an antagonist. We separated the stereoisomers of these compounds to identify the most potent agonist and discovered that only the 3R isomer of QMO-17 was an α9 antagonist, permitting an in silico model of α9 antagonism to be developed. The α9 activity of these compounds was confirmed to be potentially useful for CAS management of inflammatory pain in cell-based assays of cytokine release.

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

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

Maintained Spatial Learning and Memory Functions in Middle-Aged α9 Nicotinic Receptor Subunit Knock-Out Mice

Age-related hearing loss is linked to cognitive impairment, but the mechanisms that relate to these conditions remain unclear. Evidence shows that the activation of medial olivocochlear (MOC) neurons delays cochlear aging and hearing loss. Consequently, the loss of MOC function may be related to cognitive impairment. The α9/α10 nicotinic receptor is the main target of cholinergic synapses between the MOC neurons and cochlear outer hair cells. Here, we explored spatial learning and memory performance in middle-aged wild-type (WT) and α9-nAChR subunit knock-out (KO) mice using the Barnes maze and measured auditory brainstem response (ABR) thresholds and the number of cochlear hair cells as a proxy of cochlear aging. Our results show non-significant spatial learning differences between WT and KO mice, but KO mice had a trend of increased latency to enter the escape box and freezing time. To test a possible reactivity to the escape box, we evaluated the novelty-induced behavior using an open field and found a tendency towards more freezing time in KO mice. There were no differences in memory, ABR threshold, or the number of cochlear hair cells. We suggest that the lack of α9-nAChR subunit alters novelty-induced behavior, but not spatial learning in middle-aged mice, by a non-cochlear mechanism.

The Remarkable Outer Hair Cell: Proceedings of a Symposium in Honour of W. E. Brownell

In 1985, Bill Brownell and colleagues published the remarkable observation that cochlear outer hair cells (OHCs) express voltage-driven mechanical motion: electromotility. They proposed OHC electromotility as the mechanism for the elusive "cochlear amplifier" required to explain the sensitivity of mammalian hearing. The finding and hypothesis stimulated an explosion of experiments that have transformed our understanding of cochlear mechanics and physiology, the evolution of hair cell structure and function, and audiology. Here, we bring together examples of current research that illustrate the continuing impact of the discovery of OHC electromotility.

Nicotinic acetylcholine receptors: Therapeutic targets for novel ligands to treat pain and inflammation

Nicotinic acetylcholine receptors (nAChRs) have been historically defined as ligand-gated ion channels and function as such in the central and peripheral nervous systems. Recently, however, non-ionic signaling mechanisms via nAChRs have been demonstrated in immune cells. Furthermore, the signaling pathways where nAChRs are expressed can be activated by endogenous ligands other than the canonical agonists acetylcholine and choline. In this review, we discuss the involvement of a subset of nAChRs containing α7, α9, and/or α10 subunits in the modulation of pain and inflammation via the cholinergic anti-inflammatory pathway. Additionally, we review the most recent advances in the development of novel ligands and their potential as therapeutics.

The α9α10 acetylcholine receptor: a non-neuronal nicotinic receptor

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

A Mixture of Artemisia argyi and Saururus chinensis Improves PM2.5-Induced Cognitive Dysfunction by Regulating Oxidative Stress and Inflammatory Response in the Lung and Brain

This study was performed to investigate the improving effect of a mixture of Artemisia argyi and Saururus chinensis (AASC) on cognitive dysfunction in mice with long-term exposure to fine particles (particulate matter smaller than 2.5 µm: PM_2.5). The main compounds of AASC were identified as dicaffeoylquinic acid isomers of A. argyi and a quercetin-3-glucoside of S. chinesis. As a result of behavioral tests for the evaluation of cognitive function, it was confirmed that cognitive dysfunction was induced in the PM_2.5 exposure group, and a tendency to improve in the AASC group was confirmed. Increased oxidative stress and inflammatory response and mitochondrial dysfunction were observed in the brain and lung tissues of the PM group. Damage to the brain and lung affected the accumulation of amyloid beta (Aβ) in the brain. It increased Aβ and induced the cholinergic dysfunction, hyperphosphorylation of the tau protein, and activation of apoptosis, leading to cognitive impairment. However, AASC suppressed brain and lung oxidative stress and inflammation, thereby suppressing brain Aβ expression. Consequently, this study shows the potential that a steady intake of plant resources with antioxidant and anti-inflammatory activity could prevent cognitive impairment caused by PM_2.5.

Signaling of nicotinic acetylcholine receptors in mononuclear phagocytes

Nicotinic acetylcholine receptors are not only expressed by the nervous system and at the neuro-muscular junction but also by mononuclear phagocytes, which belong to the innate immune system. Mononuclear phagocyte is an umbrella term for monocytes, macrophages, and dendritic cells. These cells play pivotal roles in host defense against infection but also in numerous often debilitating diseases that are characterized by exuberant inflammation. Nicotinic acetylcholine receptors of the neuronal type dominate in these cells, and their stimulation is mainly associated with anti-inflammatory effects. Although the cholinergic modulation of mononuclear phagocytes is of eminent clinical relevance for the prevention and treatment of inflammatory diseases and neuropathic pain, we are only beginning to understand the underlying mechanisms on the molecular level. The purpose of this review is to report and critically discuss the current knowledge on signal transduction mechanisms elicited by nicotinic acetylcholine receptors in mononuclear phagocytes.

Functional impact of subunit composition and compensation on Drosophila melanogaster nicotinic receptors-targets of neonicotinoids

Neonicotinoid insecticides target insect nicotinic acetylcholine receptors (nAChRs) and their adverse effects on non-target insects are of serious concern. We recently found that cofactor TMX3 enables robust functional expression of insect nAChRs in Xenopus laevis oocytes and showed that neonicotinoids (imidacloprid, thiacloprid, and clothianidin) exhibited agonist actions on some nAChRs of the fruit fly (Drosophila melanogaster), honeybee (Apis mellifera) and bumblebee (Bombus terrestris) with more potent actions on the pollinator nAChRs. However, other subunits from the nAChR family remain to be explored. We show that the Dα3 subunit co-exists with Dα1, Dα2, Dβ1, and Dβ2 subunits in the same neurons of adult D. melanogaster, thereby expanding the possible nAChR subtypes in these cells alone from 4 to 12. The presence of Dα1 and Dα2 subunits reduced the affinity of imidacloprid, thiacloprid, and clothianidin for nAChRs expressed in Xenopus laevis oocytes, whereas the Dα3 subunit enhanced it. RNAi targeting Dα1, Dα2 or Dα3 in adults reduced expression of targeted subunits but commonly enhanced Dβ3 expression. Also, Dα1 RNAi enhanced Dα7 expression, Dα2 RNAi reduced Dα1, Dα6, and Dα7 expression and Dα3 RNAi reduced Dα1 expression while enhancing Dα2 expression, respectively. In most cases, RNAi treatment of either Dα1 or Dα2 reduced neonicotinoid toxicity in larvae, but Dα2 RNAi enhanced neonicotinoid sensitivity in adults reflecting the affinity-reducing effect of Dα2. Substituting each of Dα1, Dα2, and Dα3 subunits by Dα4 or Dβ3 subunit mostly increased neonicotinoid affinity and reduced efficacy. These results are important because they indicate that neonicotinoid actions involve the integrated activity of multiple nAChR subunit combinations and counsel caution in interpreting neonicotinoid actions simply in terms of toxicity.

The evolutionary tuning of hearing

After the transition to life on land, tympanic middle ears emerged separately in different groups of tetrapods, facilitating the efficient detection of airborne sounds and paving the way for high frequency sensitivity. The processes that brought about high-frequency hearing in mammals are tightly linked to the accumulation of coding sequence changes in inner ear genes; many of which were selected during evolution. These include proteins involved in hair bundle morphology, mechanotransduction and high endolymphatic potential, somatic electromotility for sound amplification, ribbon synapses for high-fidelity transmission of sound stimuli, and efferent synapses for the modulation of sound amplification. Here, we review the molecular evolutionary processes behind auditory functional innovation. Overall, the evidence to date supports the hypothesis that changes in inner ear proteins were central to the fine tuning of mammalian hearing.

RC-4BC cells express nicotinic and muscarinic acetylcholine receptors

Acetylcholine is one of the most important endogenous neurotransmitters in a range of organisms spanning different animal phyla. Within pituitary gland it acts as autocrine and paracrine signal. In a current study we assessed expression profile of the different subunits of nicotinic as well as muscarinic acetylcholine receptors in RC-4BC cells, which are derived from rat pituitary gland tumor. Our findings indicate that β2, δ, and M2 subunits are expressed by the cells with the lowest Ct values compared to other tested subunits. The detected Ct values were 26.6±0.16, 27.95±0.5, and 28.8±0.25 for β2, δ, and M2 subunits, respectively.

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

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

Smoking-mediated nicotinic acetylcholine receptors (nAChRs) for predicting outcomes for head and neck squamous cell carcinomas

Background

As a human tumor disease, head and neck squamous cell carcinoma (HNSCC) is associated with a high mortality rate worldwide. Nicotinic acetylcholine receptors (nAChRs) are transmembrane receptor proteins and exert their biological effects following activation by nicotine. We aimed to construct a prognostic signature based on the expression of nAChRs among smokers with HNSCC.

Methods

The transcriptome profile of nAChRs was obtained from The Cancer Genome Atlas (TCGA). Following the integration of survival information, univariate Cox regression and least absolute shrinkage and selection operator (LASSO) analyses were performed to screen the prognosis-related nAChRs and construct a prognostic signature. Kaplan–Meier (KM), receiver operating characteristic (ROC), principal component analysis (PCA), and independent prognostic analysis were utilized to verify the predictive power of the nAChR-associated prognostic signature. The expression of α5 nAChR in clinical samples was verified by quantitative reverse transcriptase PCR.

Results

Subunits α2, α5, α9, and β4 were related to the prognosis. The prognostic signature comprised the expression of subunits α5, α9, and β4. The nAChR-associated signature showed high sensitivity and specificity for prognostic prediction and was an independent factor for overall survival. Based on the clinical variables and expression of nAChRs, a nomogram was constructed for predicting the outcomes of HNSCC patients who were smokers in the clinical settings. In clinical specimens, α5 nAChR showed high expression in HNSCC tissues, especially among smokers.

Conclusions

The nAChR-associated signature constructed in this study may provide a better system for the classification of HNSCC patients and facilitate personalized treatment according to their smoking habits.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-022-10161-x.

Alkaloid ligands enable function of homomeric human α10 nicotinic acetylcholine receptors

In the nervous system, nicotinic acetylcholine receptors (nAChRs) rapidly transduce a chemical signal into one that is electrical via ligand-gated ion flux through the central channel of the receptor. However, some nAChR subunits are expressed by non-excitable cells where signal transduction apparently occurs through non-ionic mechanisms. One such nAChR subunit, α10, is present in a discreet subset of immune cells and has been implicated in pathologies including cancer, neuropathic pain, and chronic inflammation. Longstanding convention holds that human α10 subunits require co-assembly with α9 subunits for function. Here we assessed whether cholinergic ligands can enable or uncover ionic functions from homomeric α10 nAChRs. Xenopus laevis oocytes expressing human α10 subunits were exposed to a panel of ligands and examined for receptor activation using voltage-clamp electrophysiology. Functional expression of human α10 nAChRs was achieved by exposing the oocytes to the alkaloids strychnine, brucine, or methyllycaconitine. Furthermore, acute exposure to the alkaloid ligands significantly enhanced ionic responses. Acetylcholine-gated currents mediated by α10 nAChRs were potently inhibited by the snake toxins α-bungarotoxin and α-cobratoxin but not by α-conotoxins that target α9 and α9α10 nAChRs. Our findings indicate that human α10 homomers are expressed in oocytes and exposure to certain ligands can enable ionic functions. To our knowledge, this is the first demonstration that human α10 subunits can assemble as functional homomeric nAChRs. These findings have potential implications for receptor regulatory-mechanisms and will enable structural, functional, and further pharmacological characterization of human α10 nAChRs.

Association of Polymorphism CHRNA5 and CHRNA3 Gene in People Addicted to Nicotine

Smoking is a chronic and relapsing addictive trait that harms public health. Among the many identified genetic variants of nicotine dependence, the variants in the CHRNA5/A3/B4 gene cluster on chromosome 15 that encode the α5, α3, and β4 subunits have recently received a lot of attention. Importantly, variants in this gene cluster have been associated with nicotine addiction. Among the many significant variants in this cluster, the polymorphism SNP rs16969968 seems to be the most interesting factor in nicotine addiction. This polymorphism causes an amino acid change from aspartate to asparagine at position 398 of the α5 nicotinic receptor protein sequence. Our study aimed to analyze three polymorphic variants: the rs16969968 located in the CHRNA5 gene, the rs578776 and rs1051730 located in the CHRNA3 gene in nicotine-addicted subjects, and in controls. Our study encompasses an association analysis of genotypes and haplotypes. A group of 401 volunteers was recruited for the study and divided into two groups: the study group consisted of addicted smokers and a control group of 200 unrelated non-smokers who were not dependent on any substance and healthy. A statistically significant difference was observed in the frequency of genotypes of the rs1051730 polymorphism of the CHRNA3 gene (χ² = 6.704 p = 0.035). The T/T genotype was statistically significantly more frequent in the group of nicotine-dependent subjects. The haplotypes rs16969968, rs578776, and rs1051730 were distinguished, of which the G-T-T and G-C-T haplotypes were present only in the study group. With differences in frequencies, statistical significance was noted-for the G-T-T haplotype p = 0.01284 and the G-C-T haplotype p = 0.00775. The research stated that novel haplotypes G-T-T and G-C-T, though with very low-frequency variants in CHRNA3, were associated with nicotine addiction.

Neurotransmitters: Potential Targets in Glioblastoma

Simple Summary

Aiming to discover potential treatments for GBM, this review connects emerging research on the roles of neurotransmitters in the normal neural and the GBM microenvironments and sheds light on the prospects of their application in the neuropharmacology of GBM. Conventional therapy is blamed for its poor effect, especially in inhibiting tumor recurrence and invasion. Facing this dilemma, we focus on neurotransmitters that modulate GBM initiation, progression and invasion, hoping to provide novel therapy targeting GBM. By analyzing research concerning GBM therapy systematically and scientifically, we discover increasing insights into the regulatory effects of neurotransmitters, some of which have already shown great potential in research in vivo or in vitro. After that, we further summarize the potential drugs in correlation with previously published research. In summary, it is worth expecting that targeting neurotransmitters could be a promising novel pharmacological approach for GBM treatment.

Abstract

For decades, glioblastoma multiforme (GBM), a type of the most lethal brain tumor, has remained a formidable challenge in terms of its treatment. Recently, many novel discoveries have underlined the regulatory roles of neurotransmitters in the microenvironment both physiologically and pathologically. By targeting the receptors synaptically or non-synaptically, neurotransmitters activate multiple signaling pathways. Significantly, many ligands acting on neurotransmitter receptors have shown great potential for inhibiting GBM growth and development, requiring further research. Here, we provide an overview of the most novel advances concerning the role of neurotransmitters in the normal neural and the GBM microenvironments, and discuss potential targeted drugs used for GBM treatment.

Oligo-basic amino acids, potential nicotinic acetylcholine receptor inhibitors

Oligo-basic amino acids have been extensively studied in molecular biology and pharmacology, but the inhibitory activity on nicotinic acetylcholine receptors (nAChRs) was unknown. In this study, the inhibitory activity of 8 oligopeptides, including both basic and acidic amino acids, was evaluated on 9 nAChR subtypes by a two-electrode voltage clamp (TEVC). Among them, the oligo-lysine K9, K12, d-K9, d-K9F, and oligo-arginine R9 showed nanomolar inhibitory activity on various nAChRs, especially for α7 and α9α10 nAChRs. d-K9 containing N-Fmoc protecting group (d-K9F) has an enhanced inhibitory activity on most of the nAChRs, including 47-fold promotion on α1β1δε nAChR. However, H9 and H12 only showed weak inhibitory activity on α9α10 and α1β1δε nAChRs, and the acidic oligopeptide D9 has no inhibitory activity on nAChRs. Flexible docking of K9 in α10(+) α9(-) and α7(+) α7(-) binding pockets showed particularly strong dipole-dipole interactions, which may be responsible for the inhibition of nAChRs. These results demonstrated that oligo-basic amino acids have the potential to be the lead compounds as selective nAChR subtype inhibitors, and oligo-lysines deserved to be modified for further exploitation and utilization. On the other hand, the toxicity and side effects of these nAChR inhibitory peptides should be contemplated in the application.

From 2-Triethylammonium Ethyl Ether of 4-Stilbenol (MG624) to Selective Small-Molecule Antagonists of Human α9α10 Nicotinic Receptor by Modifications at the Ammonium Ethyl Residue

Nicotinic acetylcholine receptors containing α9 subunits (α9*-nAChRs) are potential druggable targets arousing great interest for pain treatment alternative to opioids. Nonpeptidic small molecules selectively acting as α9*-nAChRs antagonists still remain an unattained goal. Here, through modifications of the cationic head and the ethylene linker, we have converted the 2-triethylammonium ethyl ether of 4-stilbenol (MG624), a well-known α7- and α9*-nAChRs antagonist, into some selective antagonists of human α9*-nAChR. Among these, the compound with cyclohexyldimethylammonium head (7) stands out for having no α7-nAChR agonist or antagonist effect along with very low affinity at both α7- and α3β4-nAChRs. At supra-micromolar concentrations, 7 and the other selective α9* antagonists behaved as partial agonists at α9*-nAChRs with a very brief response, followed by rebound current once the application is stopped and the channel is disengaged. The small or null postapplication activity of ACh seems to be related to the slow recovery of the rebound current.

Corticofugal and Brainstem Functions Associated With Medial Olivocochlear Cholinergic Transmission

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

Strychnine and its mono- and dimeric analogues: a pharmaco-chemical perspective

Covering: up to November 2021Since its isolation in 1818, strychnine has attracted the attention of a plethora of chemists and pharmacologists who have established its structure, developed total syntheses, and examined its complex pharmacology. While numerous reviews on structure elucidation and total synthesis of strychnine are available, reports on structure-activity relationships (SARs) of this fascinating alkaloid are rare. In this review, we present and discuss structures, synthetic approaches, metabolic transformations, and the diverse pharmacological actions of strychnine and its mono- and dimeric analogues. Particular attention is given to its SARs at glycine receptors (GlyRs) in light of recently published high-resolution structures of strychnine-GlyR complexes. Other pharmacological actions of strychnine and its derivatives, such as their antagonistic properties at nicotinic acetylcholine receptors (nAChRs), allosteric modulation of muscarinic acetylcholine receptors as well as anti-cancer and anti-plasmodial effects are also critically reviewed, and possible future developments in the field are discussed.

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

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

Grxcr1 regulates hair bundle morphogenesis and is required for normal mechanoelectrical transduction in mouse cochlear hair cells

Tasmanian devil (tde) mice are deaf and exhibit circling behaviour. Sensory hair cells of mutants show disorganised hair bundles with abnormally thin stereocilia. The origin of this mutation is the insertion of a transgene which disrupts expression of the Grxcr1 (glutaredoxin cysteine rich 1) gene. We report here that Grxcr1 exons and transcript sequences are not affected by the transgene insertion in tde homozygous (tde/tde) mice. Furthermore, 5'RACE PCR experiments showed the presence of two different transcripts of the Grxcr1 gene, expressed in both tde/tde and in wild-type controls. However, quantitative analysis of Grxcr1 transcripts revealed a significantly decreased mRNA level in tde/tde mice. The key stereociliary proteins ESPN, MYO7A, EPS8 and PTPRQ were distributed in hair bundles of homozygous tde mutants in a similar pattern compared with control mice. We found that the abnormal morphology of the stereociliary bundle was associated with a reduction in the size and Ca2+-sensitivity of the mechanoelectrical transducer (MET) current. We propose that GRXCR1 is key for the normal growth of the stereociliary bundle prior to the onset of hearing, and in its absence hair cells are unable to mature into fully functional sensory receptors.

Selective Agonists and Antagonists of α9 Versus α7 Nicotinic Acetylcholine Receptors

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

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

INTRODUCTION

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

AREAS COVERED

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

EXPERT OPINION

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

A Once-Daily High Dose of Intraperitoneal Ascorbate Improves Vestibulo-ocular Reflex Compensation After Unilateral Labyrinthectomy in the Mouse

Ascorbate potentiates the response of nicotinic-acetylcholine-receptors containing α9 and α10 subunits found predominantly in the efferent systems of the inner ear, such as the efferent vestibular system (EVS). Prior mouse studies have shown that an attenuated EVS results in reduced vestibulo-ocular reflex (VOR) gain (=eye_velocity/head_velocity) plasticity in intact (VOR adaptation) and surgically-lesioned (VOR compensation) mice. We sought to determine whether ascorbate-treatment could improve VOR recovery after vestibular organ injury, possibly through potentiation of the EVS pathway. We tested 10 cba129 mice, 5 received ascorbate-treatment and 5 did not, but otherwise experienced the same conditions. Ascorbate-treatment comprised a once-daily intraperitoneal injection of L-form reduced ascorbate (4 g/kg) in 0.2 ml saline starting 1 week before, and ending 4 weeks after, unilateral labyrinthectomy surgery. These were deliberately high doses to determine the ascorbate effects on recovery. Baseline, acute, and chronic sinusoidal VOR gains (frequency and velocity ranges: 0.2-10 Hz, 20-100 deg/s) were measured 3-5 days before, 3-5 days after, and 28-31 days after labyrinthectomy. Mice treated with ascorbate had acute ipsilesional VOR gains 12 % higher compared to control mice (+45.2 ± 14.9 % from baseline versus +33.7 ± 15.4 %, P < 0.001). Similarly, chronic ipsilesional and contralesional VOR gains were respectively 16 % (+74.3 ± 16.3 % from baseline versus +58.1 ± 15.8 %, P < 0.001) and 13 % (+78.6 ± 16.0 % versus +65.6 ± 10.9 %, P < 0.001) higher compared to control mice. These data suggest ascorbate-treatment had a prophylactic effect reducing acute loss, and helped recovery during acute to chronic stages of compensation. One possible mechanism is that an ascorbate-enhanced EVS drives an increase in the number and sensitivity of irregular-discharging primary vestibular afferents, important for VOR plasticity.

α9-Containing Nicotinic Receptors in Cancer

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

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

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

Interaction of α9α10 Nicotinic Receptors With Peptides and Proteins From Animal Venoms

Unlike most neuronal nicotinic acetylcholine receptor (nAChR) subunits, α7, α9, and α10 subunits are able to form functional homo- or heteromeric receptors without any β subunits. While the α7 subtype is widely distributed in the mammalian brain and several peripheral tissues, α9 and α9α10 nAChRs are mainly found in the cochlea and immune cells. α-Conotoxins that specifically block the α9α10 receptor showed anti-nociceptive and anti-hyperalgesic effects in animal models. Hence, this subtype is considered a drug target for analgesics. In contrast to the α9α10-selective α-conotoxins, the three-finger toxin α-bungarotoxin inhibits muscle-type and α7 nAChRs in addition to α9α10 nAChRs. However, the selectivity of α-neurotoxins at the α9α10 subtype was less intensively investigated. Here, we compared the potencies of α-conotoxins and α-neurotoxins at the human α9α10 nAChR by two-electrode voltage clamp analysis upon expression in Xenopus oocytes. In addition, we analyzed effects of several α9α10-selective α-conotoxins on mouse granulocytes from bone marrow to identify possible physiological functions of the α9α10 nAChR subtype in these cells. The α-conotoxin-induced IL-10 release was measured upon LPS-stimulation. We found that α-conotoxins RgIA, PeIA, and Vc1.1 enhance the IL-10 expression in granulocytes which might explain the known anti-inflammatory and associated analgesic activities of α9α10-selective α-conotoxins. Furthermore, we show that two long-chain α-neurotoxins from the cobra Naja melanoleuca venom that were earlier shown to bind to muscle-type and α7 nAChRs, also inhibit the α9α10 subtype at nanomolar concentrations with one of them showing a significantly slower dissociation from this receptor than α-bungarotoxin.

Functional and ultrastructural analysis of reafferent mechanosensation in larval zebrafish

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

Neurobiological Mechanisms of Nicotine Reward and Aversion

Neuronal nicotinic acetylcholine receptors (nAChRs) regulate the rewarding actions of nicotine contained in tobacco that establish and maintain the smoking habit. nAChRs also regulate the aversive properties of nicotine, sensitivity to which decreases tobacco use and protects against tobacco use disorder. These opposing behavioral actions of nicotine reflect nAChR expression in brain reward and aversion circuits. nAChRs containing α4 and β2 subunits are responsible for the high-affinity nicotine binding sites in the brain and are densely expressed by reward-relevant neurons, most notably dopaminergic, GABAergic, and glutamatergic neurons in the ventral tegmental area. High-affinity nAChRs can incorporate additional subunits, including β3, α6, or α5 subunits, with the resulting nAChR subtypes playing discrete and dissociable roles in the stimulatory actions of nicotine on brain dopamine transmission. nAChRs in brain dopamine circuits also participate in aversive reactions to nicotine and the negative affective state experienced during nicotine withdrawal. nAChRs containing α3 and β4 subunits are responsible for the low-affinity nicotine binding sites in the brain and are enriched in brain sites involved in aversion, including the medial habenula, interpeduncular nucleus, and nucleus of the solitary tract, brain sites in which α5 nAChR subunits are also expressed. These aversion-related brain sites regulate nicotine avoidance behaviors, and genetic variation that modifies the function of nAChRs in these sites increases vulnerability to tobacco dependence and smoking-related diseases. Here, we review the molecular, cellular, and circuit-level mechanisms through which nicotine elicits reward and aversion and the adaptations in these processes that drive the development of nicotine dependence. SIGNIFICANCE STATEMENT: Tobacco use disorder in the form of habitual cigarette smoking or regular use of other tobacco-related products is a major cause of death and disease worldwide. This article reviews the actions of nicotine in the brain that contribute to tobacco use disorder.

Universal nature of cholinergic regulation demonstrated with nicotinic acetylcholine receptors

Mammalian nicotinic acetylcholine receptors (nAChRs) were initially discovered as ligand-gated ion channels mediating fast synaptic transmission in the neuro-muscular junctions and autonomic ganglia. They were further found to be involved in a wide range of basic biological processes within the brain and in non-excitable tissues. The present review summarizes the data obtained in our laboratory during last two decades. Investigation of autonomic ganglia with the nAChR subunit-specific antibodies was followed by identification of nAChRs in B lymphocytes, discovery of mitochondrial nAChRs and their role in mitochondrial apoptosis pathway, and revealing the role of α7 nAChRs and α7-specific antibodies in neuroinflammation-related Alzheimer disease and COVID-19. The data obtained demonstrate the involvement of nAChRs in cell survival, proliferation, cell-to-cell communication and inflammatory reaction. Together with the ability of nAChRs to function in both ionotropic and metabotropic way, these data illustrate the universal nature of cholinergic regulation mediated by nAChRs.

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

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

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

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

The Cholinergic Lateral Line Efferent Synapse: Structural, Functional and Molecular Similarities With Those of the Cochlea

Vertebrate hair cell (HC) systems are innervated by efferent fibers that modulate their response to external stimuli. In mammals, the best studied efferent-HC synapse, the cholinergic medial olivocochlear (MOC) efferent system, makes direct synaptic contacts with HCs. The net effect of MOC activity is to hyperpolarize HCs through the activation of α9α10 nicotinic cholinergic receptors (nAChRs) and the subsequent activation of Ca²⁺-dependent SK2 potassium channels. A serious obstacle in research on many mammalian sensory systems in their native context is that their constituent neurons are difficult to access even in newborn animals, hampering circuit observation, mapping, or controlled manipulation. By contrast, fishes and amphibians have a superficial and accessible mechanosensory system, the lateral line (LL), which circumvents many of these problems. LL responsiveness is modulated by efferent neurons which aid to distinguish between external and self-generated stimuli. One component of the LL efferent system is cholinergic and its activation inhibits LL afferent activity, similar to what has been described for MOC efferents. The zebrafish (Danio rerio) has emerged as a powerful model system for studying human hearing and balance disorders, since LL HC are structurally and functionally analogous to cochlear HCs, but are optically and pharmacologically accessible within an intact specimen. Complementing mammalian studies, zebrafish have been used to gain significant insights into many facets of HC biology, including mechanotransduction and synaptic physiology as well as mechanisms of both hereditary and acquired HC dysfunction. With the rise of the zebrafish LL as a model in which to study auditory system function and disease, there has been an increased interest in studying its efferent system and evaluate the similarity between mammalian and piscine efferent synapses. Advances derived from studies in zebrafish include understanding the effect of the LL efferent system on HC and afferent activity, and revealing that an α9-containing nAChR, functionally coupled to SK channels, operates at the LL efferent synapse. In this review, we discuss the tools and findings of these recent investigations into zebrafish efferent-HC synapse, their commonalities with the mammalian counterpart and discuss several emerging areas for future studies.

Evidence of a dual mechanism of action underlying the anti-proliferative and cytotoxic effects of ammonium-alkyloxy-stilbene-based α7- and α9-nicotinic ligands on glioblastoma cells

Glioblastomas (GBMs), the most frequent brain tumours, are highly invasive and their prognosis is still poor despite the use of combination treatment. MG624 is a 4-oxystilbene derivative that is active on α7- and α9-containing neuronal nicotinic acetylcholine receptor (nAChR) subtypes. Hybridisation of MG624 with a non-nicotinic resveratrol-derived pro-oxidant mitocan has led to two novel compounds (StN-4 and StN-8) that are more potent than MG624 in reducing the viability of GBM cells, but less potent in reducing the viability of mouse astrocytes. Functional analysis of their activity on α7 receptors showed that StN-4 is a silent agonist, whereas StN-8 is a full antagonist, and neither alters intracellular [Ca²⁺] levels when acutely applied to U87MG cells. After 72 h of exposure, both compounds decreased U87MG cell proliferation, and pAKT and oxphos ATP levels, but only StN-4 led to a significant accumulation of cells in phase G₁/G₀ and increased apoptosis. One hour of exposure to either compound also decreased the mitochondrial and cytoplasmic ATP production of U87MG cells, and this was not paralleled by any increase in the production of reactive oxygen species. Knocking down the α9 subunit (which is expressed at relatively high levels in U87MG cells) decreased the potency of the effects of both compounds on cell viability, but cell proliferation, ATP production, pAKT levels were unaffected by the presence of the noncell-permeable α7/α9-selective antagonist αBungarotoxin. These last findings suggest that the anti-tumoral effects of StN-4 and StN-8 on GBM cells are not only due to their action on nAChRs, but also to other non-nicotinic mechanisms.

Synaptic Contributions to Cochlear Outer Hair Cell Ca2+ Dynamics

For normal cochlear function, outer hair cells (OHCs) require a precise control of intracellular Ca2+ levels. In the absence of regulatory elements such as proteinaceous buffers or extrusion pumps, OHCs degenerate, leading to profound hearing impairment. Influx of Ca2+ occurs both at the stereocilia tips and the basolateral membrane. In this latter compartment, two different origins for Ca2+ influx have been poorly explored: voltage-gated L-type Ca2+ channels (VGCCs) at synapses with Type II afferent neurons, and α9α10 cholinergic nicotinic receptors at synapses with medio-olivochlear complex (MOC) neurons. Using functional imaging in mouse OHCs, we dissected Ca2+ influx individually through each of these sources, either by applying step depolarizations to activate VGCC, or stimulating MOC axons. Ca2+ ions originated in MOC synapses, but not by VGCC activation, was confined by Ca2+-ATPases most likely present in nearby synaptic cisterns. Although Ca2+ currents in OHCs are small, VGCC Ca2+ signals were comparable in size to those elicited by α9α10 receptors, and were potentiated by ryanodine receptors (RyRs). In contrast, no evidence of potentiation by RyRs was found for MOC Ca2+ signals over a wide range of presynaptic stimulation strengths. Our study shows that despite the fact that these two Ca2+ entry sites are closely positioned, they differ in their regulation by intracellular cisterns and/or organelles, suggesting the existence of well-tuned mechanisms to separate the two different OHC synaptic functions.SIGNIFICANCE STATEMENT Outer hair cells (OHCs) are sensory cells in the inner ear operating under very special constraints. Acoustic stimulation leads to fast changes both in membrane potential and in the intracellular concentration of metabolites such as Ca2+ Tight mechanisms for Ca2+ control in OHCs have been reported. Interestingly, Ca2+ is crucial for two important synaptic processes: inhibition by efferent cholinergic neurons, and glutamate release onto Type II afferent fibers. In the current study we functionally imaged Ca2+ at these two different synapses, showing close positioning within the basolateral compartment of OHCs. In addition, we show differential regulation of these two Ca2+ sources by synaptic cisterns and/or organelles, which could result crucial for functional segregation during normal hearing.

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

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

Therapeutic Targeting of α7 Nicotinic Acetylcholine Receptors

The α7-type nicotinic acetylcholine receptor is one of the most unique and interesting of all the members of the cys-loop superfamily of ligand-gated ion channels. Since it was first identified initially as a binding site for α-bungarotoxin in mammalian brain and later as a functional homomeric receptor with relatively high calcium permeability, it has been pursued as a potential therapeutic target for numerous indications, from Alzheimer disease to asthma. In this review, we discuss the history and state of the art for targeting α7 receptors, beginning with subtype-selective agonists and the basic pharmacophore for the selective activation of α7 receptors. A key feature of α7 receptors is their rapid desensitization by standard "orthosteric" agonist, and we discuss insights into the conformational landscape of α7 receptors that has been gained by the development of ligands binding to allosteric sites. Some of these sites are targeted by positive allosteric modulators that have a wide range of effects on the activation profile of the receptors. Other sites are targeted by direct allosteric agonist or antagonists. We include a perspective on the potential importance of α7 receptors for metabotropic as well as ionotropic signaling. We outline the challenges that exist for future development of drugs to target this important receptor and approaches that may be considered to address those challenges. SIGNIFICANCE STATEMENT: The α7-type nicotinic acetylcholine receptor (nAChR) is acknowledged as a potentially important therapeutic target with functional properties associated with both ionotropic and metabotropic signaling. The functional properties of α7 nAChR can be regulated in diverse ways with the variety of orthosteric and allosteric ligands described in this review.

The Cation Channel TMEM63B Is an Osmosensor Required for Hearing

Hypotonic stress causes the activation of swelling-activated nonselective cation channels (NSCCs), which leads to Ca²⁺-dependent regulatory volume decrease (RVD) and adaptive maintenance of the cell volume; however, the molecular identities of the osmosensitive NSCCs remain unclear. Here, we identified TMEM63B as an osmosensitive NSCC activated by hypotonic stress. TMEM63B is enriched in the inner ear sensory hair cells. Genetic deletion of TMEM63B results in necroptosis of outer hair cells (OHCs) and progressive hearing loss. Mechanistically, the TMEM63B channel mediates hypo-osmolarity-induced Ca²⁺ influx, which activates Ca²⁺-dependent K⁺ channels required for the maintenance of OHC morphology. These findings demonstrate that TMEM63B is an osmosensor of the mammalian inner ear and the long-sought cation channel mediating Ca²⁺-dependent RVD.

Diverse identities and sites of action of cochlear neurotransmitters

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

Nicotinic acetylcholine receptors in chemotherapeutic drugs resistance: An emerging targeting candidate

There is no definitive cure for cancer, and most of the current chemotherapy drugs have limited effects due to the development of drug resistance and toxicity at high doses. Therefore, there is an ongoing need for identifying the causes of chemotherapeutic resistance, and it will be possible to develop innovative treatment approaches based on these novel targeting candidates. Cigarette smoking is known to be one of the main causes of resistance to chemotherapeutic agents. Nicotine as a component of cigarette smoke is an exogenous activator of nicotinic acetylcholine receptors (nAChRs). It can inhibit apoptosis, increase cell proliferation and cell survival, reducing the cytotoxic effects of chemotherapy drugs and cause a reduced therapeutic response. Recent studies have demonstrated that nAChRs and their downstream signaling pathways have considerable implications in different cancer's initiation, progression, and chemoresistance. In some previous studies, nAChRs have been targeted to obtain better efficacies for chemotherapeutics. Besides, nAChRs-based therapies have been used in combination with chemotherapy drugs to reduce the side effects. This strategy requires lower doses of chemotherapy drugs compared to the conditions that must be used alone. Here, we discussed the experimental and clinical studies that show the nAChRs involvement in response to chemotherapy agents. Also, controversies relating to the effects of nAChR on chemotherapy-induced apoptosis are in our focus in this review article. Delineating the complex influences of nAChRs would be of great interest in establishing new effective chemotherapy regimens.

Efferent feedback controls bilateral auditory spontaneous activity

In the developing auditory system, spontaneous activity generated in the cochleae propagates into the central nervous system to promote circuit formation. The effects of peripheral firing patterns on spontaneous activity in the central auditory system are not well understood. Here, we describe wide-spread bilateral coupling of spontaneous activity that coincides with the period of transient efferent modulation of inner hair cells from the brainstem medial olivocochlear system. Knocking out α9/α10 nicotinic acetylcholine receptors, a requisite part of the efferent pathway, profoundly reduces bilateral correlations. Pharmacological and chemogenetic experiments confirm that the efferent system is necessary for normal bilateral coupling. Moreover, auditory sensitivity at hearing onset is reduced in the absence of pre-hearing efferent modulation. Together, these results demonstrate how afferent and efferent pathways collectively shape spontaneous activity patterns and reveal the important role of efferents in coordinating bilateral spontaneous activity and the emergence of functional responses during the prehearing period.