Acetylcholine receptor: an allosteric protein

SR9883 is a novel small-molecule enhancer of α4β2* nicotinic acetylcholine receptor signaling that decreases intravenous nicotine self-administration in rats

Background

Most smokers attempting to quit will quickly relapse to tobacco use even when treated with the most efficacious smoking cessation agents currently available. This highlights the need to develop effective new smoking cessation medications. Evidence suggests that positive allosteric modulators (PAM) and other enhancers of nicotinic acetylcholine receptor (nAChR) signaling could have therapeutic utility as smoking cessation agents.

Methods

3-[3-(3-pyridyl)-1,2,4-oxadiazol-5-yl]benzonitrile (NS9283) was used as a starting point for medical chemistry efforts to develop novel small molecule enhancers of α4β2* nAChR stoichiometries containing a low-affinity agonist binding site at the interface of α4/α4 and α4/α5 subunits.

Results

The NS9283 derivative SR9883 enhanced the effect of nicotine on α4β2* nAChR stoichiometries containing low-affinity agonist binding sites, with EC50 values from 0.2-0.4 μM. SR9883 had no effect on α3β2* or α3β4* nAChRs. SR9883 was bioavailable after intravenous (1 mg kg-1) and oral (10-20 mg kg-1) administration and penetrated into the brain. When administered alone, SR9883 (5-10 mg kg-1) had no effect on locomotor activity or intracranial self-stimulation (ICSS) thresholds in mice. When co-administered with nicotine, SR9883 enhanced locomotor suppression and elevations of ICSS thresholds induced by nicotine. SR9883 (5 and 10 mg kg-1) decreased responding for intravenous nicotine infusions (0.03 mg kg-1 per infusion) but had no effect on responding for food rewards in rats.

Conclusions

These data suggest that SR9883 is useful for investigating behavioral processes regulated by certain α4β2* nAChR stoichiometries. SR9883 and related compounds with favorable drug-like physiochemical and pharmacological properties hold promise as novel treatments of tobacco use disorder.

Protein dynamics underlying allosteric regulation

Allostery is the mechanism by which information and control are propagated in biomolecules. It regulates ligand binding, chemical reactions, and conformational changes. An increasing level of experimental resolution and control over allosteric mechanisms promises a deeper understanding of the molecular basis for life and powerful new therapeutics. In this review, we survey the literature for an up-to-date biological and theoretical understanding of protein allostery. By delineating five ways in which the energy landscape or the kinetics of a system may change to give rise to allostery, we aim to help the reader grasp its physical origins. To illustrate this framework, we examine three systems that display these forms of allostery: allosteric inhibitors of beta-lactamases, thermosensation of TRP channels, and the role of kinetic allostery in the function of kinases. Finally, we summarize the growing power of computational tools available to investigate the different forms of allostery presented in this review.

Calcium Homeostasis, Transporters, and Blockers in Health and Diseases of the Cardiovascular System

Calcium is a highly positively charged ionic species. It regulates all cell types' functions and is an important second messenger that controls and triggers several mechanisms, including membrane stabilization, permeability, contraction, secretion, mitosis, intercellular communications, and in the activation of kinases and gene expression. Therefore, controlling calcium transport and its intracellular homeostasis in physiology leads to the healthy functioning of the biological system. However, abnormal extracellular and intracellular calcium homeostasis leads to cardiovascular, skeletal, immune, secretory diseases, and cancer. Therefore, the pharmacological control of calcium influx directly via calcium channels and exchangers and its outflow via calcium pumps and uptake by the ER/SR are crucial in treating calcium transport remodeling in pathology. Here, we mainly focused on selective calcium transporters and blockers in the cardiovascular system.

The EGL-30 pathway regulates experience-dependent aversive behavior of Caenorhabditis elegans to the pathogenic bacterium Pseudomonas aeruginosa

Avoidance of harmful substances is survival strategy used cross invertebrates and vertebrates. For example, the nematode Caenorhabditis elegans evolves a sufficient avoidance response to pathogenic bacteria. Despite G protein has been found to exert neural plasticity for avoidance behaviours in C. elegans, the function of Gi/o and Gq subunit signalling in experience-dependent aversive behaviour remains unclear. In this study, we show that EGL-30/Gq coupled with EGL-8/UNC-13 regulates aversive behaviour of C. elegans to pathogenic bacterium Pseudomonas aeruginosa PA01 via acetylcholine and its receptor nAChR. Pyocyanin, a toxin secreted from P. aeruginosa, acts as a signal molecule to trigger aversive behaviour. ODR-3 and ODR-7 in AWA and AWC neurons function as upstream of EGL-30 to induce experience-dependent aversive behaviour to P. aeruginosa, respectively. These results suggested that a novel signalling pathway to regulate a behavioural response.

Charge-voltage curves of Shaker potassium channel are not hysteretic at steady state

Charge-voltage curves of many voltage-gated ion channels exhibit hysteresis but such curves are also a direct measure of free energy of channel gating and, hence, should be path-independent. Here, we identify conditions to measure steady-state charge-voltage curves and show that these are curves are not hysteretic. Charged residues in transmembrane segments of voltage-gated ion channels (VGICs) sense and respond to changes in the electric field. The movement of these gating charges underpins voltage-dependent activation and is also a direct metric of the net free-energy of channel activation. However, for most voltage-gated ion channels, the charge-voltage (Q-V) curves appear to be dependent on initial conditions. For instance, Q-V curves of Shaker potassium channel obtained by hyperpolarizing from 0 mV is left-shifted compared to those obtained by depolarizing from a holding potential of -80 mV. This hysteresis in Q-V curves is a common feature of channels in the VGIC superfamily and raises profound questions about channel energetics because the net free-energy of channel gating is a state function and should be path independent. Due to technical limitations, conventional gating current protocols are limited to test pulse durations of <500 ms, which raises the possibility that the dependence of Q-V on initial conditions reflects a lack of equilibration. Others have suggested that the hysteresis is fundamental thermodynamic property of voltage-gated ion channels and reflects energy dissipation due to measurements under non-equilibrium conditions inherent to rapid voltage jumps (Villalba-Galea. 2017. Channels. https://doi.org/10.1080/19336950.2016.1243190). Using an improved gating current and voltage-clamp fluorometry protocols, we show that the gating hysteresis arising from different initial conditions in Shaker potassium channel is eliminated with ultra-long (18-25 s) test pulses. Our study identifies a modified gating current recording protocol to obtain steady-state Q-V curves of a voltage-gated ion channel. Above all, these findings demonstrate that the gating hysteresis in Shaker channel is a kinetic phenomenon rather than a true thermodynamic property of the channel and the charge-voltage curve is a true measure of the net-free energy of channel gating.

Anticholinergics and falls in older adults

INTRODUCTION

The use of medications with anticholinergic (ACh) properties is associated with numerous adverse effects especially in older adults. Emerging evidence suggests the presence of long-term effects with ACh use.

AREAS COVERED

Our article presents an overview of ACh effects and falls in older individuals including examination of emerging evidence on ACh use and cumulative exposure on short-term and long-term falls risk. The databases CINAHL, MEDLINE, EMBASE, and Web of Science were searched for articles published from January 2002 to December 2021.

EXPERT OPINION

Anticholinergic side effects include muscle weakness, blurred vision, and mental confusion which are likely to lead to increased falls risk. Many commonly used medications such as beta-blockers, calcium-channel blockers, and antihistamines are now known to have mild ACh properties. With polypharmacy now considered unavoidable in older patients, the cumulative effects of the use of multiple drugs with mild ACh properties may also lead to increased falls risk. The relationship between ACh drugs and falls may also be irreversible as ACh effects may extend beyond the period of drug use, due to cognitive and physical deconditioning following the initial exposure to ACh drugs. Future long-term studies with accurate quantification of cumulative ACh exposure and measurement of actual falls outcomes are urgently required.

Molecular dynamics simulations suggest possible activation and deactivation pathways in the hERG channel

The elusive activation/deactivation mechanism of hERG is investigated, a voltage-gated potassium channel involved in severe inherited and drug-induced cardiac channelopathies, including the Long QT Syndrome. Firstly, the available structural data are integrated by providing a homology model for the closed state of the channel. Secondly, molecular dynamics combined with a network analysis revealed two distinct pathways coupling the voltage sensor domain with the pore domain. Interestingly, some LQTS-related mutations known to impair the activation/deactivation mechanism are distributed along the identified pathways, which thus suggests a microscopic interpretation of their role. Split channels simulations clarify a surprising feature of this channel, which is still able to gate when a cut is introduced between the voltage sensor domain and the neighboring helix S5. In summary, the presented results suggest possible activation/deactivation mechanisms of non-domain-swapped potassium channels that may aid in biomedical applications.

Costa et al. present the electro-mechanical coupling between the voltage sensor and pore domain of the hERG channel using a combination of molecular dynamics simulations and theoretical network analyses.

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.

Exploring K v 1.2 Channel Inactivation Through MD Simulations and Network Analysis

The KCNA2 gene encodes the K v 1.2 channel, a mammalian Shaker-like voltage-gated K+ channel, whose defections are linked to neuronal deficiency and childhood epilepsy. Despite the important role in the kinetic behavior of the channel, the inactivation remained hereby elusive. Here, we studied the K v 1.2 inactivation via a combined simulation/network theoretical approach that revealed two distinct pathways coupling the Voltage Sensor Domain and the Pore Domain to the Selectivity Filter. Additionally, we mutated some residues implicated in these paths and we explained microscopically their function in the inactivation mechanism by computing a contact map. Interestingly, some pathological residues shown to impair the inactivation lay on the paths. In summary, the presented results suggest two pathways as the possible molecular basis of the inactivation mechanism in the K v 1.2 channel. These pathways are consistent with earlier mutational studies and known mutations involved in neuronal channelopathies.

Alpha6-Containing Nicotinic Acetylcholine Receptors Mediate Nicotine-Induced Structural Plasticity in Mouse and Human iPSC-Derived Dopaminergic Neurons

Midbrain dopamine (DA) neurons are considered a critical substrate for the reinforcing and sensitizing effects of nicotine and tobacco dependence. While the role of the α4 and β2 subunit containing nicotinic acetylcholine receptors (α4β2^∗nAChRs) in mediating nicotine effects on DA release and DA neuron activity has been widely explored, less information is available on their role in the morphological adaptation of the DA system to nicotine, eventually leading to dysfunctional behaviors observed in nicotine dependence. In particular, no information is available on the role of α6^∗nAChRs in nicotine-induced structural plasticity in rodents and no direct evidence exists regarding the occurrence of structural plasticity in human DA neurons exposed to nicotine. To approach this problem, we used two parallel in vitro systems, mouse primary DA neuron cultures from E12.5 embryos and human DA neurons differentiated from induced pluripotent stem cells (iPSCs) of healthy donors, identified using TH⁺ immunoreactivity. In both systems, nicotine 1–10 μM produced a dose-dependent increase of maximal dendrite length, number of primary dendrites, and soma size when measured after 3 days in culture. These effects were blocked by pretreatments with the α6^∗nAChR antagonists α-conotoxin MII and α-conotoxin PIA, as well as by the α4β2nAChR antagonist dihydro-β-erythroidine (DHβE) in both mouse and human DA neurons. Nicotine was also ineffective when the primary DA neurons were obtained from null mutant mice for either the α6 subunit or both the α4 and α6 subunits of nAChR. When pregnant mice were exposed to nicotine from gestational day 15, structural plasticity was also observed in the midbrain DA neurons of postnatal day 1 offspring only in wild-type mice and not in both null mutant mice. This study confirmed the critical role of α4α6^∗nAChRs in mediating nicotine-induced structural plasticity in both mouse and human DA neurons, supporting the translational relevance of neurons differentiated from human iPSCs for pharmacological studies.

The Intracellular Loop of the Glycine Receptor: It's not all about the Size

The family of Cys-loop receptors (CLRs) shares a high degree of homology and sequence identity. The overall structural elements are highly conserved with a large extracellular domain (ECD) harboring an α-helix and 10 β-sheets. Following the ECD, four transmembrane domains (TMD) are connected by intracellular and extracellular loop structures. Except the TM3–4 loop, their length comprises 7–14 residues. The TM3–4 loop forms the largest part of the intracellular domain (ICD) and exhibits the most variable region between all CLRs. The ICD is defined by the TM3–4 loop together with the TM1–2 loop preceding the ion channel pore. During the last decade, crystallization approaches were successful for some members of the CLR family. To allow crystallization, the intracellular loop was in most structures replaced by a short linker present in prokaryotic CLRs. Therefore, no structural information about the large TM3–4 loop of CLRs including the glycine receptors (GlyRs) is available except for some basic stretches close to TM3 and TM4. The intracellular loop has been intensively studied with regard to functional aspects including desensitization, modulation of channel physiology by pharmacological substances, posttranslational modifications, and motifs important for trafficking. Furthermore, the ICD interacts with scaffold proteins enabling inhibitory synapse formation. This review focuses on attempts to define structural and functional elements within the ICD of GlyRs discussed with the background of protein-protein interactions and functional channel formation in the absence of the TM3–4 loop.

FGFR1-5-HT1A Heteroreceptor Complexes: Implications for Understanding and Treating Major Depression

The serotonin and neurotrophic factor hypotheses of depression are well known. The discovery of brain fibroblast growth factor receptor 1 (FGFR1)-5 hydroxytryptamine receptor 1A (5-HT1A) heteroreceptor complexes, and their enhancement of neuroplasticity, offers an integration of these hypotheses at the molecular level. They were first described in the hippocampus and later in midbrain 5-HT neurons, where these heterocomplexes are enriched in 5-HT1A autoreceptors. Combined FGF2 and 5-HT1A agonist treatment increased the formation of these heterocomplexes and the facilitatory allosteric receptor-receptor interactions within them led to the enhancement of FGFR1 signaling and was associated with the development of antidepressant effects. We discuss these findings with regard to a theory of motifs critically involved in these interactions and suggest that these complexes represent novel targets for antidepressants.

Single-molecule bioelectronics

Experimental techniques that interface single biomolecules directly with microelectronic systems are increasingly being used in a wide range of powerful applications, from fundamental studies of biomolecules to ultra-sensitive assays. In this study, we review several technologies that can perform electronic measurements of single molecules in solution: ion channels, nanopore sensors, carbon nanotube field-effect transistors, electron tunneling gaps, and redox cycling. We discuss the shared features among these techniques that enable them to resolve individual molecules, and discuss their limitations. Recordings from each of these methods all rely on similar electronic instrumentation, and we discuss the relevant circuit implementations and potential for scaling these single-molecule bioelectronic interfaces to high-throughput arrayed sensing platforms.

Characterization of RO5126946, a Novel α7 nicotinic acetylcholine receptor-positive allosteric modulator

Both preclinical evidence and clinical evidence suggest that α7 nicotinic acetylcholine receptor activation (α7nAChR) improves cognitive function, the decline of which is associated with conditions such as Alzheimer's disease and schizophrenia. Moreover, allosteric modulation of α7nAChR is an emerging therapeutic strategy in an attempt to avoid the rapid desensitization properties associated with the α7nAChR after orthosteric activation. We used a calcium assay to screen for positive allosteric modulators (PAMs) of α7nAChR and report on the pharmacologic characterization of the novel compound RO5126946 (5-chloro-N-[(1S,3R)-2,2-dimethyl-3-(4-sulfamoyl-phenyl)-cyclopropyl]-2-methoxy-benzamide), which allosterically modulates α7nAChR activity. RO5126946 increased acetylcholine-evoked peak current and delayed current decay but did not affect the recovery of α7nAChRs from desensitization. In addition, RO5126946's effects were absent when nicotine-evoked currents were completely blocked by coapplication of the α7nAChR-selective antagonist methyl-lycaconitine. RO5126946 enhanced α7nAChR synaptic transmission and positively modulated GABAergic responses. The absence of RO5126946 effects at human α4β2nAChR and 5-hydroxytryptamine 3 receptors, among others, indicated selectivity for α7nAChRs. In vivo, RO5126946 is orally bioavailable and brain-penetrant and improves associative learning in a scopolamine-induced deficit model of fear conditioning in rats. In addition, procognitive effects of RO5126946 were investigated in the presence of nicotine to address potential pharmacologic interactions on behavior. RO5126946 potentiated nicotine's effects on fear memory when both compounds were administered at subthreshold doses and did not interfere with procognitive effects observed when both compounds were administered at effective doses. Overall, RO5126946 is a novel α7nAChR PAM with cognitive-enhancing properties.

Merging old and new perspectives on nicotinic acetylcholine receptors

This review covers history underlying the discovery of the molecular mediators of nicotine's effects in the brain and the diversity of the nicotinic acetylcholine receptor (nAChR) subtypes. Models are presented for both their structure and their function as mediators of signal transduction, with special consideration of the differences between the two main subtypes: heteromeric receptors, which are specialized for rapid electrochemical signal transduction, and homomeric α7 receptors, which have come to be implicated in both ionotropic and metabotropic signaling. This review presents perspectives on the pharmacology and therapeutic targeting of nAChRs for the treatment of nicotine dependence or disease.

Diverse strategies targeting α7 homomeric and α6β2* heteromeric nicotinic acetylcholine receptors for smoking cessation

Preclinical studies suggest that a diversity of nicotinic acetylcholine receptors (nAChRs) with different sensitivities to nicotine may contribute to tobacco addiction. Using rodent intravenous nicotine self-administration as a preclinical model with good predictive validity for therapeutic efficacy for tobacco cessation, investigators have identified heteromeric α6β2* and homomeric α7 nAChRs as promising novel therapeutic targets to promote smoking abstinence (*denotes possible assembly with other subunits). The data suggest that diverse strategies that target these subclasses of nAChRs, namely inhibition of α6β2* nAChRs and stimulation of α7 nAChRs, will support tobacco cessation. α6β2* nAChRs, members of the high-affinity family of β2* nAChRs, function similarly to α4β2* nAChRs, the primary target of the FDA-approved drug varenicline, but have a much more selective neuroanatomical pattern of expression in catecholaminergic nuclei. Although activation of β2* nAChRs facilitates nicotine self-administration, stimulation of α7 nAChRs appears to negatively modulate both nicotine reinforcement and β2* nAChR function in the mesolimbic dopamine system. Although challenges and caveats must be considered in the development of therapeutics that target these nAChR subpopulations, an accumulation of data suggests that α7 nAChR agonists, partial agonists, or positive allosteric modulators and α6β2* nAChR antagonists, partial agonists, or negative allosteric modulators may prove to be effective therapeutics for tobacco cessation.

Asymmetric perturbations of signalling oligomers

This review focuses on rapid and reversible noncovalent interactions for symmetric oligomers of signalling proteins. Symmetry mismatch, transient symmetry breaking and asymmetric perturbations via chemical (ligand binding) and physical (electric or mechanic) effects can initiate the signalling events. Advanced biophysical methods can reveal not only structural symmetries of stable membrane-bound signalling proteins but also asymmetric functional transition states. Relevant techniques amenable to distinguish between symmetric and asymmetric architectures are discussed including those with the capability of capturing low-populated transient conformational states. Typical examples of signalling proteins are overviewed for symmetry breaking in dimers (GPCRs, growth factor receptors, transcription factors); trimers (acid-sensing ion channels); tetramers (voltage-gated cation channels, ionotropic glutamate receptor, CNG and CHN channels); pentameric ligand-gated and mechanosensitive channels; higher order oligomers (gap junction channel, chaperonins, proteasome, virus capsid); as well as primary and secondary transporters. In conclusion, asymmetric perturbations seem to play important functional roles in a broad range of communicating networks.

Nicotine aversion: Neurobiological mechanisms and relevance to tobacco dependence vulnerability

Nicotine stimulates brain reward circuitries, most prominently the mesocorticolimbic dopamine system, and this action plays a critical in establishing and maintaining the tobacco smoking habit. Compounds that attenuate nicotine reward are considered promising therapeutic candidates for tobacco dependence, but many of these agents have other actions that limit their potential utility. Nicotine is also highly noxious, particularly at higher doses, and aversive reactions to nicotine after initial exposure can decrease the likelihood of developing a tobacco habit in many first time smokers. Nevertheless, relatively little is known about the mechanisms of nicotine aversion. The purpose of this review is to present recent new insights into the neurobiological mechanisms that regulate avoidance of nicotine. First, the role of the mesocorticolimbic system, so often associated with nicotine reward, in regulating nicotine aversion is highlighted. Second, genetic variation that modifies noxious responses to nicotine and thereby influences vulnerability to tobacco dependence, in particular variation in the CHRNA5-CHRNA3-CHRNB4 nicotinic acetylcholine receptor (nAChR) subunit gene cluster, will be discussed. Third, the role of the habenular complex in nicotine aversion, primarily medial habenular projections to the interpeduncular nucleus (IPN) but also lateral habenular projections to rostromedial tegmental nucleus (RMTg) and ventral tegmental area (VTA) are reviewed. Forth, brain circuits that are enriched in nAChRs, but whose role in nicotine avoidance has not yet been assessed, will be identified. Finally, the feasibility of developing novel therapeutic agents for tobacco dependence that act not by blocking nicotine reward but by enhancing nicotine avoidance will be considered. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

PKC and PKA regulate AChR dynamics at the neuromuscular junction of living mice

The steady state of the acetylcholine receptor (AChR) density at the neuromuscular junction (NMJ) is critical for efficient and reliable synaptic transmission. However, little is known about signaling molecules involved in regulating the equilibrium between the removal and insertion of AChRs that establishes a stable postsynaptic receptor density over time. In this work, we tested the effect of activities of two serine/threonine kinases, PKC and PKA, on the removal rate of AChRs from and the re-insertion rate of internalized recycled AChRs into synaptic sites of innervated and denervated NMJs of living mice. Using an in vivo time-lapse imaging approach and various pharmacological agents, we showed that PKC and PKA activities have antagonistic effects on the removal and recycling of AChRs. Inhibition of PKC activity or activation of PKA largely prevents the removal of pre-existing AChRs and promotes the recycling of internalized AChRs into the postsynaptic membrane. In contrast, stimulation of PKC or inactivation of PKA significantly accelerates the removal of postsynaptic AChRs and depresses AChR recycling. These results indicate that a balance between PKA and PKC activities may be critical for the maintenance of the postsynaptic receptor density.

A gating mechanism of pentameric ligand-gated ion channels

Pentameric ligand-gated ion channels (pLGICs) play a central role in intercellular communication in the nervous system and are involved in fundamental processes such as attention, learning, and memory. They are oligomeric protein assemblies that convert a chemical signal into an ion flux through the postsynaptic membrane, but the molecular mechanism of gating ions has remained elusive. Here, we present atomistic molecular dynamics simulations of the prokaryotic channels from Gloeobacter violaceus (GLIC) and Erwinia chrysanthemi (ELIC), whose crystal structures are thought to represent the active and the resting states of pLGICs, respectively, and of the eukaryotic glutamate-gated chloride channel from Caenorhabditis elegans (GluCl), whose open-channel structure was determined complexed with the positive allosteric modulator ivermectin. Structural observables extracted from the trajectories of GLIC and ELIC are used as progress variables to analyze the time evolution of GluCl, which was simulated in the absence of ivermectin starting from the structure with bound ivermectin. The trajectory of GluCl with ivermectin removed shows a sequence of structural events that couple agonist unbinding from the extracellular domain to ion-pore closing in the transmembrane domain. Based on these results, we propose a structural mechanism for the allosteric communication leading to deactivation/activation of the GluCl channel. This model of gating emphasizes the coupling between the quaternary twisting and the opening/closing of the ion pore and is likely to apply to other members of the pLGIC family.

Endogenous cholinergic inputs and local circuit mechanisms govern the phasic mesolimbic dopamine response to nicotine

Nicotine exerts its reinforcing action by stimulating nicotinic acetylcholine receptors (nAChRs) and boosting dopamine (DA) output from the ventral tegmental area (VTA). Recent data have led to a debate about the principal pathway of nicotine action: direct stimulation of the DAergic cells through nAChR activation, or disinhibition mediated through desensitization of nAChRs on GABAergic interneurons. We use a computational model of the VTA circuitry and nAChR function to shed light on this issue. Our model illustrates that the α4β2-containing nAChRs either on DA or GABA cells can mediate the acute effects of nicotine. We account for in vitro as well as in vivo data, and predict the conditions necessary for either direct stimulation or disinhibition to be at the origin of DA activity increases. We propose key experiments to disentangle the contribution of both mechanisms. We show that the rate of endogenous acetylcholine input crucially determines the evoked DA response for both mechanisms. Together our results delineate the mechanisms by which the VTA mediates the acute rewarding properties of nicotine and suggest an acetylcholine dependence hypothesis for nicotine reinforcement.

Nicotine is the major addictive substance in tobacco smoke. Nicotine exerts its control over neural circuits through nicotinic acetylcholine receptors that normally respond to endogenous acetylcholine. Activation of dopamine neurons in the mesolimbic dopaminergic circuits, which signal motivational properties of actions and stimuli, is at the heart of mediating nicotine reward and dependence. However, major questions have remained unsettled over the precise mechanisms by which nicotine usurps dopaminergic signaling: through receptor activation on dopamine neurons or through receptor desensitization on local inhibitory interneurons. Here we reconcile this debate by showing that both mechanisms are possible. Most notably we present a novel hypothesis suggesting that the mechanisms for nicotine action are state-dependent; they are controlled by the rate of the endogenous cholinergic input to the dopaminergic circuits.

Animal models of nicotine exposure: relevance to second-hand smoking, electronic cigarette use, and compulsive smoking

Much evidence indicates that individuals use tobacco primarily to experience the psychopharmacological properties of nicotine and that a large proportion of smokers eventually become dependent on nicotine. In humans, nicotine acutely produces positive reinforcing effects, including mild euphoria, whereas a nicotine abstinence syndrome with both somatic and affective components is observed after chronic nicotine exposure. Animal models of nicotine self-administration and chronic exposure to nicotine have been critical in unveiling the neurobiological substrates that mediate the acute reinforcing effects of nicotine and emergence of a withdrawal syndrome during abstinence. However, important aspects of the transition from nicotine abuse to nicotine dependence, such as the emergence of increased motivation and compulsive nicotine intake following repeated exposure to the drug, have only recently begun to be modeled in animals. Thus, the neurobiological mechanisms that are involved in these important aspects of nicotine addiction remain largely unknown. In this review, we describe the different animal models available to date and discuss recent advances in animal models of nicotine exposure and nicotine dependence. This review demonstrates that novel animal models of nicotine vapor exposure and escalation of nicotine intake provide a unique opportunity to investigate the neurobiological effects of second-hand nicotine exposure, electronic cigarette use, and the mechanisms that underlie the transition from nicotine use to compulsive nicotine intake.

Integrin triplets of marine sponges in the murine and human MHCI-CD8 interface and in the interface of human neural receptor heteromers and subunits

Based on our theory, main triplets of amino acid residues have been discovered in cell-adhesion receptors (integrins) of marine sponges, which participate as homologies in the interface between two major immune molecules, MHC class I (MHCI) and CD8αβ. They appear as homologies also in several human neural receptor heteromers and subunits. The obtained results probably mean that neural and immune receptors also utilize these structural integrin triplets to form heteromers and ion channels, which are required for a tuned and integrated intracellular and intercellular communication and a communication between cells and the extracellular matrix with an origin in sponges, the oldest multicellular animals.

An integrated catch-and-hold mechanism activates nicotinic acetylcholine receptors

In neuromuscular acetylcholine (ACh) receptor channels (AChRs), agonist molecules bind with a low affinity (LA) to two sites that can switch to high affinity (HA) and increase the probability of channel opening. We measured (by using single-channel kinetic analysis) the rate and equilibrium constants for LA binding and channel gating for several different agonists of adult-type mouse AChRs. Almost all of the variation in the equilibrium constants for LA binding was from differences in the association rate constants. These were consistently below the limit set by diffusion and were substantially different even though the agonists had similar sizes and the same charge. This suggests that binding to resting receptors is not by diffusion alone and, hence, that each binding site can undergo two conformational changes ("catch" and "hold") that connect three different structures (apo-, LA-bound, and HA-bound). Analyses of ACh-binding protein structures suggest that this binding site, too, may adopt three discrete structures having different degrees of loop C displacement ("capping"). For the agonists we tested, the logarithms of the equilibrium constants for LA binding and LA↔HA gating were correlated. Although agonist binding and channel gating have long been considered to be separate processes in the activation of ligand-gated ion channels, this correlation implies that the catch-and-hold conformational changes are energetically linked and together comprise an integrated process having a common structural basis. We propose that loop C capping mainly reflects agonist binding, with its two stages corresponding to the formation of the LA and HA complexes. The catch-and-hold reaction coordinate is discussed in terms of preopening states and thermodynamic cycles of activation.

Crystal structure of Lymnaea stagnalis AChBP complexed with the potent nAChR antagonist DHβE suggests a unique mode of antagonism

Nicotinic acetylcholine receptors (nAChRs) are pentameric ligand-gated ion channels that belong to the Cys-loop receptor superfamily. These receptors are allosteric proteins that exist in different conformational states, including resting (closed), activated (open), and desensitized (closed) states. The acetylcholine binding protein (AChBP) is a structural homologue of the extracellular ligand-binding domain of nAChRs. In previous studies, the degree of the C-loop radial extension of AChBP has been assigned to different conformational states of nAChRs. It has been suggested that a closed C-loop is preferred for the active conformation of nAChRs in complex with agonists whereas an open C-loop reflects an antagonist-bound (closed) state. In this work, we have determined the crystal structure of AChBP from the water snail Lymnaea stagnalis (Ls) in complex with dihydro-β-erythroidine (DHβE), which is a potent competitive antagonist of nAChRs. The structure reveals that binding of DHβE to AChBP imposes closure of the C-loop as agonists, but also a shift perpendicular to previously observed C-loop movements. These observations suggest that DHβE may antagonize the receptor via a different mechanism compared to prototypical antagonists and toxins.

Preclinical evidence that activation of mesolimbic alpha 6 subunit containing nicotinic acetylcholine receptors supports nicotine addiction phenotype

INTRODUCTION

Nicotine is a major psychoactive ingredient in tobacco yet very few individuals quit smoking with the aid of nicotine replacement therapy. Targeted therapies with more selective action at nicotinic acetylcholine receptors (nAChRs) that contain a β2 subunit (β2*nAChRs; *denotes assembly with other subunits) have enjoyed significantly greater success, but exhibit potential for unwanted cardiac, gastrointestinal, and emotive side effects.

DISCUSSION

This literature review focuses on the preclinical evidence that suggests that subclasses of β2*nAChRs that assemble with the α6 subunit may provide an effective target for tobacco cessation. α6β2*nAChRs have a highly selective pattern of neuroanatomical expression in catecholaminergic nuclei including the ventral tegmental area and its projection regions. α6β2*nAChRs promote dopamine (DA) neuron activity and DA release in the mesolimbic dopamine system, a brain circuitry that is well-studied for its contributions to addiction behavior. A combination of genetic and pharmacological studies indicates that activation of α6β2*nAChRs is necessary and sufficient for nicotine psychostimulant effects and nicotine self-administration. α6β2*nAChRs support maintenance of nicotine use, support the conditioned reinforcing effects of drug-associated cues, and regulate nicotine withdrawal.

CONCLUSIONS

These data suggest that α6β2*nAChRs represent a critical pool of high affinity β2*nAChRs that regulates nicotine dependence phenotype and suggest that inhibition of these receptors may provide an effective strategy for tobacco cessation therapy.

Low concentrations of nicotine differentially desensitize nicotinic acetylcholine receptors that include α5 or α6 subunits and that mediate synaptosomal neurotransmitter release

Desensitization is a complex property of nicotinic acetylcholine receptors (nAChR). Several subtypes of nAChR have high sensitivity to nicotine and mediate effects of nicotine at concentrations found in blood of tobacco smokers. Desensitization of some of these receptor subtypes has been studied in model systems, however, other subtypes have been difficult to express heterologously in native forms. In addition, model systems may not have the same accessory molecules and post-translational modifications found in native populations. We have used wild-type and subunit null mutant mice to study desensitization properties of the high sensitivity α4β2-nAChRs including those that have α5 subunits at both GABAergic and dopaminergic nerve terminals. In addition, we have studied the desensitization of one subtype of α6β2-nAChRs at dopaminergic terminals using α4 subunit null mutant mice. Exposure to low nicotine concentrations, leads to rapid, but partial desensitization of activity mediated by these receptors. α4β2-nAChRs including α5 subunits show faster rates of recovery from desensitization than α4β2-nAChRs without α5. Inclusion of the α5 subunit significantly shifts the concentration response for desensitization to higher values, indicating that receptors with α5 subunits are less desensitized by a 10-min exposure to low concentrations of nicotine. Receptors with α6 subunits appear to desensitize to a lesser degree than those with α4 subunits, indicating that α6β2-nAChRs are somewhat resistant to desensitization by nicotine. These results highlight the importance of studying various receptor subtypes in native systems and how they may differentially respond to nicotine and to nicotinic drugs.

Muscarinic and nicotinic acetylcholine receptor agonists and allosteric modulators for the treatment of schizophrenia

Muscarinic and nicotinic acetylcholine (ACh) receptors (mAChRs and nAChRs) are emerging as important targets for the development of novel treatments for the symptoms associated with schizophrenia. Preclinical and early proof-of-concept clinical studies have provided strong evidence that activators of specific mAChR (M(1) and M(4)) and nAChR (α(7) and α(2)β(4)) subtypes are effective in animal models of antipsychotic-like activity and/or cognitive enhancement, and in the treatment of positive and cognitive symptoms in patients with schizophrenia. While early attempts to develop selective mAChR and nAChR agonists provided important preliminary findings, these compounds have ultimately failed in clinical development due to a lack of true subtype selectivity and subsequent dose-limiting adverse effects. In recent years, there have been major advances in the discovery of highly selective activators for the different mAChR and nAChR subtypes with suitable properties for optimization as potential candidates for clinical trials. One novel strategy has been to identify ligands that activate a specific receptor subtype through actions at sites that are distinct from the highly conserved ACh-binding site, termed allosteric sites. These allosteric activators, both allosteric agonists and positive allosteric modulators, of mAChR and nAChR subtypes demonstrate unique mechanisms of action and high selectivity in vivo, and may provide innovative treatment strategies for schizophrenia.

Neurophysiology of Nicotine Addiction

Tobacco use is a major health problem, and nicotine is the main addictive component. Nicotine binds to nicotinic acetylcholine receptors (nAChR) to produce its initial effects. The nAChRs subtypes are composed of five subunits that can form in numerous combinations with varied functional and pharmacological characteristics. Diverse psychopharmacological effects contribute to the overall process of nicotine addiction, but two general neural systems are emerging as critical for the initiation and maintenance of tobacco use. Mesocorticolimbic circuitry that includes the dopaminergic pathway originating in the ventral tegmental area and projecting to the nucleus accumbens is recognized as vital for reinforcing behaviors during the initiation of nicotine addiction. In this neural system β2, α4, and α6 are the most important nAChR subunits underlying the rewarding aspects of nicotine and nicotine self-administration. On the other hand, the epithalamic habenular complex and the interpeduncular nucleus, which are connected via the fasciculus retroflexus, are critical contributors regulating nicotine dosing and withdrawal symptoms. In this case, the α5 and β4 nAChR subunits have critical roles in combination with other subunits. In both of these neural systems, particular nAChR subtypes have roles that contribute to the overall nicotine addiction process.

Reward, addiction, withdrawal to nicotine

Nicotine is the principal addictive component that drives continued tobacco use despite users' knowledge of the harmful consequences. The initiation of addiction involves the mesocorticolimbic dopamine system, which contributes to the processing of rewarding sensory stimuli during the overall shaping of successful behaviors. Acting mainly through nicotinic receptors containing the α4 and β2 subunits, often in combination with the α6 subunit, nicotine increases the firing rate and the phasic bursts by midbrain dopamine neurons. Neuroadaptations arise during chronic exposure to nicotine, producing an altered brain condition that requires the continued presence of nicotine to be maintained. When nicotine is removed, a withdrawal syndrome develops. The expression of somatic withdrawal symptoms depends mainly on the α5, α2, and β4 (and likely α3) nicotinic subunits involving the epithalamic habenular complex and its targets. Thus, nicotine taps into diverse neural systems and an array of nicotinic acetylcholine receptor (nAChR) subtypes to influence reward, addiction, and withdrawal.

Increased nicotinic receptor desensitization in hypoglossal motor neurons following chronic developmental nicotine exposure

Neuronal nicotinic acetylcholine receptors (nAChRs) are expressed on hypoglossal motor neurons (XII MNs) that innervate muscles of the tongue. Activation of XII MN nAChRs evokes depolarizing currents, which are important for regulating the size and stiffness of the upper airway. Although data show that chronic developmental nicotine exposure (DNE) blunts cholinergic neurotransmission in the XII motor nucleus, it is unclear how nAChRs are involved. Therefore, XII MN nAChR desensitization and recovery were examined in tissues from DNE or control pups using a medullary slice preparation and tight-seal whole cell patch-clamp recordings. nAChR-mediated inward currents were evoked by brief pressure pulses of nicotine or the α4β2 nAChR agonist RJR-2403. We found that, regardless of treatment, activatable nAChRs underwent desensitization, but, following DNE, nAChRs exhibited increased desensitization and delayed recovery. Similar results were produced using RJR-2403, showing that DNE influences primarily the α4β2 nAChR subtype. These results show that while some nAChRs preserve their responsiveness to acute nicotine following DNE, they more readily desensitize and recover more slowly from the desensitized state. These data provide new evidence that chronic DNE modulates XII MN nAChR function, and suggests an explanation for the association between DNE and the incidence of central and obstructive apneas.

Desensitization of alpha7 nicotinic receptor is governed by coupling strength relative to gate tightness

Binding of a neurotransmitter to its membrane receptor opens an integral ion conducting pore. However, prolonged exposure to the neurotransmitter drives the receptor to a refractory state termed desensitization, which plays an important role in shaping synaptic transmission. Despite intensive research in the past, the structural mechanism of desensitization is still elusive. Using mutagenesis and voltage clamp in an oocyte expression system, we provide several lines of evidence supporting a novel hypothesis that uncoupling between binding and gating machinery is the underlying mechanism for α7 nicotinic receptor (nAChR) desensitization. First, the decrease in gate tightness was highly correlated to the reduced desensitization. Second, nonfunctional mutants in three important coupling loops (loop 2, loop 7, and the M2-M3 linker) could be rescued by a gating mutant. Furthermore, the decrease in coupling strength in these rescued coupling loop mutants reversed the gating effect on desensitization. Finally, coupling between M1 and hinge region of the M2-M3 linker also influenced the receptor desensitization. Thus, the uncoupling between N-terminal domain and transmembrane domain, governed by the balance of coupling strength and gate tightness, underlies the mechanism of desensitization for the α7 nAChR.

Antibody effector mechanisms in myasthenia gravis-pathogenesis at the neuromuscular junction

Myasthenia gravis (MG) is an autoimmune disorder caused by autoantibodies that are either directed to the muscle nicotinic acetylcholine receptor (AChR) or to the muscle-specific tyrosine kinase (MuSK). These autoantibodies define two distinct subforms of the disease-AChR-MG and MuSK-MG. Both AChR and MuSK are expressed on the postsynaptic membrane of the neuromuscular junction (NMJ), which is a highly specialized region of the muscle dedicated to receive and process signals from the motor nerve. Autoantibody binding to proteins of the postsynaptic membrane leads to impaired neuromuscular transmission and muscle weakness. Pro-inflammatory antibodies of the human IgG1 and IgG3 subclass modulate the AChR, cause complement activation, and attract lymphocytes; together acting to decrease levels of the AChR and AChR-associated proteins and to reduce postsynaptic folding. In patients with anti-MuSK antibodies, there is no evidence of loss of junctional folds and no apparent loss of AChR density. Anti-MuSK antibodies are predominantly of the IgG4 isotype, which functionally differs from other IgG subclasses in its anti-inflammatory activity. Moreover, IgG4 undergoes a posttranslational modification termed Fab arm exchange that prevents cross-linking of antigens. These findings suggest that MuSK-MG may be different in etiological and pathological mechanisms from AChR-MG. The effector functions of IgG subclasses on synapse structure and function are discussed in this review.

Linking the acetylcholine receptor-channel agonist-binding sites with the gate

The gating isomerization of neuromuscular acetylcholine receptors links the rearrangements of atoms at two transmitter-binding sites with those at a distant gate region in the pore. To explore the mechanism of this reversible process, we estimated the gating rate and equilibrium constants for receptors with point mutations of alpha-subunit residues located between the binding sites and the membrane domain (N95, A96, Y127, and I49). The maximum energy change caused by a side-chain substitution at alphaA96 was huge (approximately 8.6 kcal/mol, the largest value measured so far for any alpha-subunit amino acid). A Phi-value analysis suggests that alphaA96 experiences its change in energy (structure) approximately synchronously with residues alphaY127 and alphaI49, but after the agonist molecule and other residues in loop A. Double mutant-cycle experiments show that the energy changes at alphaA96 are strongly coupled with those of alphaY127 and alphaI49. We identify a column of mutation-sensitive residues in the alpha-subunit that may be a pathway for energy transfer through the extracellular domain in the gating isomerization.

The structural basis of function in Cys-loop receptors

Cys-loop receptors are membrane-spanning neurotransmitter-gated ion channels that are responsible for fast excitatory and inhibitory transmission in the peripheral and central nervous systems. The best studied members of the Cys-loop family are nACh, 5-HT3, GABAA and glycine receptors. All these receptors share a common structure of five subunits, pseudo-symmetrically arranged to form a rosette with a central ion-conducting pore. Some are cation selective (e.g. nACh and 5-HT3) and some are anion selective (e.g. GABAA and glycine). Each receptor has an extracellular domain (ECD) that contains the ligand-binding sites, a transmembrane domain (TMD) that allows ions to pass across the membrane, and an intracellular domain (ICD) that plays a role in channel conductance and receptor modulation. Cys-loop receptors are the targets for many currently used clinically relevant drugs (e.g. benzodiazepines and anaesthetics). Understanding the molecular mechanisms of these receptors could therefore provide the catalyst for further development in this field, as well as promoting the development of experimental techniques for other areas of neuroscience.In this review, we present our current understanding of Cys-loop receptor structure and function. The ECD has been extensively studied. Research in this area has been stimulated in recent years by the publication of high-resolution structures of nACh receptors and related proteins, which have permitted the creation of many Cys loop receptor homology models of this region. Here, using the 5-HT3 receptor as a typical member of the family, we describe how homology modelling and ligand docking can provide useful but not definitive information about ligand interactions. We briefly consider some of the many Cys-loop receptors modulators. We discuss the current understanding of the structure of the TMD, and how this links to the ECD to allow channel gating, and consider the roles of the ICD, whose structure is poorly understood. We also describe some of the current methods that are beginning to reveal the differences between different receptor states, and may ultimately show structural details of transitions between them.

New therapeutic potential for psychoactive natural products

Much of our knowledge in neuroscience was discovered through the study of mind-altering natural products. However, although much has been learned about human physiology and basic biological processes, the underlying causes of CNS disorders and other disease states are still elusive. Based on its main past successes, the continued study of mind-altering compounds promises to yield novel agents that may be developed into medications and to identify new targets for the treatment of diseases. This Highlight describes the history of investigations into several classes of mind-altering natural products and relates recent and potential therapeutic uses for these agents.

{alpha}7 nicotinic acetylcholine receptor regulates airway epithelium differentiation by controlling basal cell proliferation

Airway epithelial basal cells are known to be critical for regenerating injured epithelium and maintaining tissue homeostasis. Recent evidence suggests that the alpha7 nicotinic acetylcholine receptor (nAChR), which is highly permeable to Ca(2+), is involved in lung morphogenesis. Here, we have investigated the potential role of the alpha7 nAChR in the regulation of airway epithelial basal cell proliferation and the differentiation of the human airway epithelium. In vivo during fetal development and in vitro during the regeneration of the human airway epithelium, alpha7 nAChR expression coincides with epithelium differentiation. Inactivating alpha7 nAChR function in vitro increases cell proliferation during the initial steps of the epithelium regeneration, leading to epithelial alterations such as basal cell hyperplasia and squamous metaplasia, remodeling observed in many bronchopulmonary diseases. The regeneration of the airway epithelium after injury in alpha7(-/-) mice is delayed and characterized by a transient hyperplasia of basal cells. Moreover, 1-year-old alpha7(-/-) mice more frequently present basal cells hyperplasia. Modulating nAChR function or expression shows that only alpha7 nAChR, as opposed to heteropentameric alpha(x)beta(y) nAChRs, controls the proliferation of human airway epithelial basal cells. These findings suggest that alpha7 nAChR is a key regulator of the plasticity of the human airway epithelium by controlling basal cell proliferation and differentiation pathway and is involved in airway remodeling during bronchopulmonary diseases.

Review. Neurobiology of nicotine dependence

Nicotine is a psychoactive ingredient in tobacco that significantly contributes to the harmful tobacco smoking habit. Nicotine dependence is more prevalent than dependence on any other substance. Preclinical research in animal models of the various aspects of nicotine dependence suggests a critical role of glutamate, gamma-aminobutyric acid (GABA), cholinergic and dopamine neurotransmitter interactions in the ventral tegmental area and possibly other brain sites, such as the central nucleus of the amygdala and the prefrontal cortex, in the effects of nicotine. Specifically, decreasing glutamate transmission or increasing GABA transmission with pharmacological manipulations decreased the rewarding effects of nicotine and cue-induced reinstatement of nicotine seeking. Furthermore, early nicotine withdrawal is characterized by decreased function of presynaptic inhibitory metabotropic glutamate 2/3 receptors and increased expression of postsynaptic glutamate receptor subunits in limbic and frontal brain sites, while protracted abstinence may be associated with increased glutamate response to stimuli associated with nicotine administration. Finally, adaptations in nicotinic acetylcholine receptor function are also involved in nicotine dependence. These neuroadaptations probably develop to counteract the decreased glutamate and cholinergic transmission that is hypothesized to characterize early nicotine withdrawal. In conclusion, glutamate, GABA and cholinergic transmission in limbic and frontal brain sites are critically involved in nicotine dependence.

Effects of dichlorobenzene on acetylcholine receptors in human neuroblastoma SH-SY5Y cells

para-Dichlorobenzene (DCB), a deodorant and an industrial chemical, is a highly volatile compound and is known to be an indoor air contaminant. Because of its widespread use and volatility, the toxicity of DCB presents a concern to industrial workers and public. Some toxic aspects of DCB have already been focused but its effects on neuronal signal transduction have been hitherto unknown. The effects of DCB on the cytosolic calcium homeostasis are investigated in human neuroblastoma SH-SY5Y cells in this study. DCB, above 200 microM, was found to induce a rise in cytosolic calcium concentration that could not be counteracted by nicotinic acetylcholine receptor (nAChR) and muscarinic acetylcholine receptor (mAChR) antagonists but was partially inhibited by thapsigargin. To understand the actions of DCB on the acetylcholine receptors, we investigated its effects on the changes of cytosolic calcium concentration following nicotinic AChR stimulation with epibatidine and muscarinic AChR stimulation with methacholine in human neuroblastoma SH-SY5Y cells. DCB inhibited the cytosolic calcium concentration rise induced by epibatidine and methacholine with respective IC(50)s of 34 and 294 microM. The inhibitions of DCB were not the same as thapsigargin's inhibition. In the electrophysiological observations, DCB blocked the influx currents induced by epibatidine. Our findings suggest that DCB interferes with the functional activities of AChR, including its coupling influx currents and cytosolic calcium elevations.

Role of negatively charged amino acids in beta 4 F-loop in activation and desensitization of alpha 3 beta 4 rat neuronal nicotinic receptors

The role of negatively charged amino acids in the F-loop of the beta 4 subunit in channel activation and desensitization was studied using the patch-clamp technique. The selected amino acids were changed to their neutral analogs via point mutations. Whole-cell currents were recorded in COS cells transiently transfected with the alpha 3 beta 4 nicotinic acetylcholine receptor. The application of acetylcholine (ACh), nicotine (Nic), cytisine (Cyt), carbamylcholine (CCh) and epibatidine (Epi) to cells clamped at -40 mV produced inward currents which displayed biphasic desensitization. The EC50 of Epi and Nic were increased by a factor of 3-6 due to mutations D191N or D192N. Only Epi remained an agonist in the double-mutated receptors with EC50 increased 17-fold. The interaction of the receptors with the competitive antagonist (+)tubocurarine (TC) was weakened almost 3-fold in the double-mutated receptors. The mutations increased the proportion of the slower desensitization component and increased the response plateau, resulting in decreased receptor desensitization. The double mutation substantially accelerated the return from long-term desensitization induced by Epi.