The human alpha7 nicotinic acetylcholine receptor is a host target for the rabies virus glycoprotein

The rabies virus enters the nervous system by interacting with several molecular targets on host cells to modify behavior and trigger receptor-mediated endocytosis of the virion by poorly understood mechanisms. The rabies virus glycoprotein (RVG) interacts with the muscle acetylcholine receptor and the neuronal α4β2 subtype of the nicotinic acetylcholine receptor (nAChR) family by the putative neurotoxin-like motif. Given that the neurotoxin-like motif is highly homologous to the α7 nAChR subtype selective snake toxin α-bungarotoxin (αBTX), other nAChR subtypes are likely involved. The purpose of this study is to determine the activity of the RVG neurotoxin-like motif on nAChR subtypes that are expressed in brain regions involved in rabid animal behavior. nAChRs were expressed in Xenopus laevis oocytes, and two-electrode voltage clamp electrophysiology was used to collect concentration-response data to measure the functional effects. The RVG peptide preferentially and completely inhibits α7 nAChR ACh-induced currents by a competitive antagonist mechanism. Tested heteromeric nAChRs are also inhibited, but to a lesser extent than the α7 subtype. Residues of the RVG peptide with high sequence homology to αBTX and other neurotoxins were substituted with alanine. Altered RVG neurotoxin-like peptides showed that residues phenylalanine 192, arginine 196, and arginine 199 are important determinants of RVG peptide apparent potency on α7 nAChRs, while serine 195 is not. The evaluation of the rabies ectodomain reaffirmed the observations made with the RVG peptide, illustrating a significant inhibitory impact on α7 nAChR with potency in the nanomolar range. In a mammalian cell culture model of neurons, we confirm that the RVG peptide binds preferentially to cells expressing the α7 nAChR. Defining the activity of the RVG peptide on nAChRs expands our understanding of basic mechanisms in host-pathogen interactions that result in neurological disorders.

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.

Advances in smoking cessation pharmacotherapy: Non-nicotinic approaches in animal models

The landscape of worldwide tobacco use is changing, with a decrease in traditional smoking and an exponential rise in electronic cigarette use. No new nicotine cessation pharmacotherapies have come to market in the last 10 years. The current therapies that have been approved by the United States Food and Drug Administration for nicotine cessation include nicotine replacement therapy, varenicline, a nicotinic acetylcholine receptor partial agonist, and the atypical antidepressant bupropion. Nicotine replacement therapy and varenicline both act on nicotinic acetylcholine receptors. Bupropion inhibits the dopamine transporter, the norepinephrine transporter, and the nicotinic acetylcholine receptors to inhibit smoking behavior. Notwithstanding these treatments, rates of successful nicotine cessation in clinical trials remain low. Recent pharmacological approaches to improve nicotine cessation rates in animal models have turned their focus away from activating nicotinic acetylcholine receptors. The present review focuses on such pharmacological approaches, including nicotine vaccines, anti-nicotine antibodies, nicotine-degrading enzymes, cannabinoids, and metformin. Both immunopharmacological and enzymatic approaches rely on restricting and degrading nicotine within the periphery, thus preventing psychoactive effects of nicotine on the central nervous system. In contrast, pharmacologic inhibition of the enzymes which degrade nicotine could affect smoking behavior. Cannabinoid receptor agonists and antagonists interact with the dopamine reward pathway and show efficacy in reducing nicotine addiction-like behaviors in preclinical studies. Metformin is currently approved by the Food and Drug Administration for the treatment of diabetes. It activates specific intracellular kinases that may protect against the lower metabolism, higher oxidation, and inflammation that are associated with nicotine withdrawal. Further studies are needed to investigate non-nicotinic targets to improve the treatment of tobacco use disorder. This article is part of the special issue on 'Contemporary Advances in Nicotine Neuropharmacology'.

α4β2 nicotinic acetylcholine receptors intrinsically influence body weight in mice

Desensitization of the nicotinic acetylcholine receptor (nAChR) containing the β2 subunit is a potentially critical mechanism underlying the body weight (BW) reducing effects of nicotine. The purpose of this study was a) to determine the α subunit(s) that partners with the β2 subunit to form the nAChR subtype that endogenously regulates energy balance and b) to probe the extent to which nAChR desensitization could be involved in the regulation of BW. We demonstrate that deletion of either the α4 or the β2, but not the α5, subunit of the nAChR suppresses weight gain in a sex-dependent manner. Furthermore, chronic treatment with the β2-selective nAChR competitive antagonist dihydro-β-erythroidine (DHβE) in mice fed a high-fat diet suppresses weight gain. These results indicate that heteromeric α4β2 nAChRs play a role as intrinsic regulators of energy balance and that desensitizing or inhibiting this nAChR is likely a relevant mechanism and thus could be a strategy for weight loss.

Distinctive single-channel properties of α4β2-nicotinic acetylcholine receptor isoforms

Central nervous system nicotinic acetylcholine receptors (nAChR) are predominantly of the α4β2 subtype. Two isoforms exist, with high or low agonist sensitivity (HS-(α4β2)2β2- and LS-(α4β2)2α4-nAChR). Both isoforms exhibit similar macroscopic potency and efficacy values at low acetylcholine (ACh) concentrations, mediated by a common pair of high-affinity α4(+)/(-)β2 subunit binding interfaces. However LS-(α4β2)2α4-nAChR also respond to higher concentrations of ACh, acting at a third α4(+)/(-)α4 subunit interface. To probe isoform functional differences further, HS- and LS-α4β2-nAChR were expressed in Xenopus laevis oocytes and single-channel responses were assessed using cell-attached patch-clamp. In the presence of a low ACh concentration, both isoforms produce low-bursting function. HS-(α4β2)2β2-nAChR exhibit a single conductance state, whereas LS-(α4β2)2α4-nAChR display two distinctive conductance states. A higher ACh concentration did not preferentially recruit either conductance state, but did result in increased LS-(α4β2)2α4-nAChR bursting and reduced closed times. Introduction of an α4(+)/(-)α4-interface loss-of-function α4W182A mutation abolished these changes, confirming this site's role in mediating LS-(α4β2)2α4-nAChR responses. Small or large amplitude openings are highly-correlated within individual LS-(α4β2)2α4-nAChR bursts, suggesting that they arise from distinct intermediate states, each of which is stabilized by α4(+)/(-)α4 site ACh binding. These findings are consistent with α4(+)/(-)α4 subunit interface occupation resulting in allosteric potentiation of agonist actions at α4(+)/(-)β2 subunit interfaces, rather than independent induction of high conductance channel openings.

Cigarette smoke exposure effects on the brainstem expression of nicotinic acetylcholine receptors (nAChRs), and on cardiac, respiratory and sleep physiologies

Cigarette smoking during pregnancy is the largest modifiable risk factor for adverse outcomes in the infant. Investigations have focused on the psychoactive component of cigarettes, nicotine. One proposed mechanism leading to adverse effects is the interaction between nicotine and its nicotinic acetylcholine receptors (nAChRs). Much data has been generated over the past three decades on the effects of cigarette smoke exposure (CSE) on the expression of the nAChRs in the brainstem and physiological parameters related to cardiac, respiration and sleep, in the offspring of smoking mothers and animal models of nicotine exposure. This review summarises this data and discusses the main findings, highlighting that findings in animal models closely correlate with those from human studies, and that the major brainstem sites where the expression level for the nAChRs are consistently affected include those that play vital roles in cardiorespiration (hypoglossal nucleus, dorsal motor nucleus of the vagus, nucleus of the solitary tract), chemosensation (nucleus of the solitary tract, arcuate nucleus) and arousal (rostral mesopontine sites such as the locus coeruleus and nucleus pontis oralis). These findings provide evidence for the adverse effects of CSE during and after pregnancy to the infant and the need to continue with the health campaign advising against CSE.

Reduced α4 subunit expression in α4+- and α4+- /β2+- nicotinic acetylcholine receptors alters α4β2 subtype up-regulation following chronic nicotine treatment

BACKGROUND AND PURPOSE

Genomic analysis has shown many variants in both CHRNA4 and CHRNB2, genes which encode the α4 and β2 subunits of nicotinic ACh receptors (nAChR) respectively. Some variants influence receptor expression, raising the possibility that CHRNA4 variants may affect response to tobacco use in humans. Chronic exposure to nicotine increases expression of nAChRs, particularly α4β2-nAChRs, in humans and laboratory animals. Here, we have evaluated whether the initial level of receptor expression affects the increase in expression.

EXPERIMENTAL APPROACH

Mice differing in expression of α4 and/or β2 nAChR subunits were chronically treated with saline, 0.25, 1.0 or 4.0 mg·kg-1 ·h-1 nicotine. Brain preparations were analysed autoradiographically by [125 I]-epibatidine binding, immunoprecipitation and Western blotting.

KEY RESULTS

Immunochemical studies confirmed that most of the [3 H]-epibatidine binding corresponds to α4β2*-nAChR and that increases in binding correspond to increases in α4 and β2 proteins. Consistent with previous reports, the dose-dependent increase in nAChR in wild-type mice following chronic nicotine treatment, measured with any of the methods, reached a maximum. Although receptor expression was reduced by approximately 50% in β2+- mice, the pattern of response to chronic treatment resembled that of wild-type mice. In contrast, both α4+- and α4+- /β2+- exhibited relatively greater up-regulation. Consistent with previous reports, α4β2α5-nAChR did not increase in response to nicotine.

CONCLUSIONS AND IMPLICATIONS

These results indicate that mice with reduced expression of the α4 nAChR subunit have a more robust response to chronic nicotine than mice with normal expression of this subunit.

LINKED ARTICLES

This article is part of a themed section on Nicotinic Acetylcholine Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.11/issuetoc.

Molecular and cellular characterization of nicotinic acetylcholine receptor subtypes in the arcuate nucleus of the mouse hypothalamus

Nicotine, acting through nicotinic acetylcholine receptors (nAChRs), increases the firing rate of both orexigenic agouti-related peptide (AgRP) and anorexigenic pro-opiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus (ARC), yet nicotine and other nAChR agonists decrease food intake in mice. Viral-mediated knockdown of the β4 nAChR subunit in all neuronal cell types in the ARC prevents the nicotinic agonist cytisine from decreasing food intake, but it is not known whether the β4 subunit is selectively expressed in anorexigenic neurons or how other nAChR subtypes are distributed in this nucleus. Using translating ribosome affinity purification (TRAP) on ARC tissue from mice with ribosomes tagged in either AgRP or POMC cells, we examined nAChR subunit mRNA levels using real-time PCR. Both AgRP and POMC cells express a comparable panel of nAChR subunits with differences in α7 mRNA levels and a trend for difference in α4 levels, but no differences in β4 expression. Immunoprecipitation of assembled nAChRs revealed that the β4 subunit forms assembled channels with α3, β2 and α4, but not other subunits found in the ARC. Finally, using cell type-selective, virally delivered small hairpin RNAs targeting either the β4 or α7 subunit, we examined the contribution of each subunit in either AgRP or POMC cells to the behavioural response to nicotine, refining the understanding of nicotinic regulation of this feeding circuit. These experiments identify a more complex set of nAChRs expressed in ARC than in other hypothalamic regions. Thus, the ARC appears to be a particular target of nicotinic modulation.

Stoichiometry of the Heteromeric Nicotinic Receptors of the Renshaw Cell

Neuronal nicotinic acetylcholine receptors (nAChRs) are pentamers built from a variety of subunits. Some are homomeric assemblies of α subunits, others heteromeric assemblies of α and β subunits which can adopt two stoichiometries (2α:3β or 3α:2β). There is evidence for the presence of heteromeric nAChRs with the two stoichiometries in the CNS, but it has not yet been possible to identify them at a given synapse. The 2α:3β receptors are highly sensitive to agonists, whereas the 3α:2β stoichiometric variants, initially described as low sensitivity receptors, are indeed activated by low and high concentrations of ACh. We have taken advantage of the discovery that two compounds (NS9283 and Zn) potentiate selectively the 3α:2β nAChRs to establish (in mice of either sex) the presence of these variants at the motoneuron-Renshaw cell (MN-RC) synapse. NS9283 prolonged the decay of the two-component EPSC mediated by heteromeric nAChRs. NS9283 and Zn also prolonged spontaneous EPSCs involving heteromeric nAChRs, and one could rule out prolongations resulting from AChE inhibition by NS9283. These results establish the presence of 3α:2β nAChRs at the MN-RC synapse. At the functional level, we had previously explained the duality of the EPSC by assuming that high ACh concentrations in the synaptic cleft account for the fast component and that spillover of ACh accounts for the slow component. The dual ACh sensitivity of 3α:2β nAChRs now allows to attribute to these receptors both components of the EPSC.SIGNIFICANCE STATEMENT Heteromeric nicotinic receptors assemble α and β subunits in pentameric structures, which can adopt two stoichiometries: 3α:2β or 2α:3β. Both stoichiometric variants are present in the CNS, but they have never been located and characterized functionally at the level of an identified synapse. Our data indicate that 3α:2β receptors are present at the spinal cord synapses between motoneurons and Renshaw cells, where their dual mode of activation (by high concentrations of ACh for synaptic receptors, by low concentrations of ACh for extrasynaptic receptors) likely accounts for the biphasic character of the synaptic current. More generally, 3α:2β nicotinic receptors appear unique by their capacity to operate both in the cleft of classical synapses and at extrasynaptic locations.

Designing selective modulators for the nicotinic receptor subtypes: challenges and opportunities

Nicotinic receptors are membrane proteins involved in several physiological processes. They are considered suitable drug targets for various CNS disorders or conditions, as shown by the large number of compounds which have entered clinical trials. In recent years, nonconventional agonists have been discovered: positive allosteric modulators, allosteric agonists, site-specific agonists and silent desensitizers are compounds able to modulate the receptor interacting at sites different from the orthodox one, or to desensitize the receptor without prior opening. While these new findings can further complicate the pharmacology of these proteins and the design and optimization of ligands, they undoubtedly offer new opportunities to find drugs for the many therapeutic indications involving nicotinic receptors.

Mutation Linked to Autosomal Dominant Nocturnal Frontal Lobe Epilepsy Reduces Low-Sensitivity α4β2, and Increases α5α4β2, Nicotinic Receptor Surface Expression

A number of mutations in α4β2-containing (α4β2*) nicotinic acetylcholine (ACh) receptors (nAChRs) are linked to autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE), including one in the β2 subunit called β2V287L. Two α4β2* subtypes with different subunit stoichiometries and ACh sensitivities co-exist in the brain, a high-sensitivity subtype with (α4)₂(β2)₃ subunit stoichiometry and a low-sensitivity subtype with (α4)₃(β2)₂ stoichiometry. The α5 nicotinic subunit also co-assembles with α4β2 to form a high-sensitivity α5α4β2 nAChR. Previous studies suggest that the β2V287L mutation suppresses low-sensitivity α4β2* nAChR expression in a knock-in mouse model and also that α5 co-expression improves the surface expression of ADNFLE mutant nAChRs in a cell line. To test these hypotheses further, we expressed mutant and wild-type (WT) nAChRs in oocytes and mammalian cell lines, and measured the effects of the β2V287L mutation on surface receptor expression and the ACh response using electrophysiology, a voltage-sensitive fluorescent dye, and superecliptic pHluorin (SEP). The β2V287L mutation reduced the EC₅₀ values of high- and low-sensitivity α4β2 nAChRs expressed in Xenopus oocytes for ACh by a similar factor and suppressed low-sensitivity α4β2 expression. In contrast, it did not affect the EC₅₀ of α5α4β2 nAChRs for ACh. Measurements of the ACh responses of WT and mutant nAChRs expressed in mammalian cell lines using a voltage-sensitive fluorescent dye and whole-cell patch-clamping confirm the oocyte data. They also show that, despite reducing the maximum response, β2V287L increased the α4β2 response to a sub-saturating ACh concentration (1 μM). Finally, imaging SEP-tagged α5, α4, β2, and β2V287L subunits showed that β2V287L reduced total α4β2 nAChR surface expression, increased the number of β2 subunits per α4β2 receptor, and increased surface α5α4β2 nAChR expression. Thus, the β2V287L mutation alters the subunit composition and sensitivity of α4β2 nAChRs, and increases α5α4β2 surface expression.

How can we Improve on Modeling Nicotine Addiction to Develop Better Smoking Cessation Treatments?

Clinically effective smoking cessation treatments are few in number, mainly varenicline, bupropion, and nicotine replacement therapy being prescribed by health organizations. Of the many compounds tested for smoking cessation, a good proportion fail in human trials despite positive findings in rodents. This chapter aims to cover the uses and some pit falls of current methodologies employed to discover clinical treatments in the laboratory. Complicating factors include the complex nature of genetics in tobacco smoking and the comorbidity associated with other psychiatric disorders, which has not been addressed fully in the rodent laboratory. This chapter reviews the evidence from intravenous nicotine self-administration studies and proposes modifications on how we can improve the validity of the animal models by incorporating clinically relevant factors considered to be critical in tobacco smoking. For example, choice procedures that incorporate alternative reinforcers, use of reinstatement models, and second-order schedules of reinforcement are proposed to have better scientific validity that may lead to better clinical outcomes. Furthermore, improved experimental methods will also improve our chances of discovering effective treatments that ultimately may mitigate the effects of tobacco smoking with regard to health worldwide.

Antagonism of the Prokineticin System Prevents and Reverses Allodynia and Inflammation in a Mouse Model of Diabetes

Neuropathic pain is a severe diabetes complication and its treatment is not satisfactory. It is associated with neuroinflammation-related events that participate in pain generation and chronicization. Prokineticins are a new family of chemokines that has emerged as critical players in immune system, inflammation and pain. We investigated the role of prokineticins and their receptors as modulators of neuropathic pain and inflammatory responses in experimental diabetes. In streptozotocin-induced-diabetes in mice, the time course expression of prokineticin and its receptors was evaluated in spinal cord and sciatic nerves, and correlated with mechanical allodynia. Spinal cord and sciatic nerve pro- and anti-inflammatory cytokines were measured as protein and mRNA, and spinal cord GluR subunits expression studied. The effect of preventive and therapeutic treatment with the prokineticin receptor antagonist PC1 on behavioural and biochemical parameters was evaluated. Peripheral immune activation was assessed measuring macrophage and T-helper cytokine production. An up-regulation of the Prokineticin system was present in spinal cord and nerves of diabetic mice, and correlated with allodynia. Therapeutic PC1 reversed allodynia while preventive treatment blocked its development. PC1 normalized prokineticin levels and prevented the up-regulation of GluN2B subunits in the spinal cord. The antagonist restored the pro-/anti-inflammatory cytokine balance altered in spinal cord and nerves and also reduced peripheral immune system activation in diabetic mice, decreasing macrophage proinflammatory cytokines and the T-helper 1 phenotype. The prokineticin system contributes to altered sensitivity in diabetic neuropathy and its inhibition blocked both allodynia and inflammatory events underlying disease.

Differential α4(+)/(-)β2 Agonist-binding Site Contributions to α4β2 Nicotinic Acetylcholine Receptor Function within and between Isoforms

Two α4β2 nicotinic acetylcholine receptor (α4β2-nAChR) isoforms exist with (α4)2(β2)3 and (α4)3(β2)2 subunit stoichiometries and high versus low agonist sensitivities (HS and LS), respectively. Both isoforms contain a pair of α4(+)/(-)β2 agonist-binding sites. The LS isoform also contains a unique α4(+)/(-)α4 site with lower agonist affinity than the α4(+)/(-)β2 sites. However, the relative roles of the conserved α4(+)/(-)β2 agonist-binding sites in and between the isoforms have not been studied. We used a fully linked subunit concatemeric nAChR approach to express pure populations of HS or LS isoform α4β2*-nAChR. This approach also allowed us to mutate individual subunit interfaces, or combinations thereof, on each isoform background. We used this approach to systematically mutate a triplet of β2 subunit (-)-face E-loop residues to their non-conserved α4 subunit counterparts or vice versa (β2HQT and α4VFL, respectively). Mutant-nAChR constructs (and unmodified controls) were expressed in Xenopus oocytes. Acetylcholine concentration-response curves and maximum function were measured using two-electrode voltage clamp electrophysiology. Surface expression was measured with (125)I-mAb 295 binding and was used to define function/nAChR. If the α4(+)/(-)β2 sites contribute equally to function, making identical β2HQT substitutions at either site should produce similar functional outcomes. Instead, highly differential outcomes within the HS isoform, and between the two isoforms, were observed. In contrast, α4VFL mutation effects were very similar in all positions of both isoforms. Our results indicate that the identity of subunits neighboring the otherwise equivalent α4(+)/(-)β2 agonist sites modifies their contributions to nAChR activation and that E-loop residues are an important contributor to this neighbor effect.

Distinctive effects of nicotinic receptor intracellular-loop mutations associated with nocturnal frontal lobe epilepsy

Previously characterized nicotinic acetylcholine receptor (nAChR) autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE)-associated mutations are found in α2, α4 and β2 subunit transmembrane (TM) domains. They predominantly increase ACh potency and, for β2-subunit mutants, increase macroscopic currents. Two recently-identified mutations, α4(R336H) and β2(V337G), located in the intracellular cytoplasmic loop (C2) have been associated with non-familial NFLE. Effects of these mutations on α4β2-nAChR function and expression were studied for the first time, using two-electrode voltage clamp recordings in Xenopus laevis oocytes. Biased-ratio preparations elucidated the mutations' effects at alternate isoforms: high-sensitivity [HS; (α4)2(β2)3] or low-sensitivity [LS; (α4)3(β2)2] via 1:10 or 30:1 [α4:β2] cRNA injection ratios, respectively. An unbiased (1:1 [α4:β2] cRNA) injection ratio was also used to study potential shifts in isoform expression. α4(R336H)-containing receptors showed significant increases in maximal ACh-induced currents (Imax) in all preparations (140% increase compared to wild type control). β2(V337G)-containing receptors significantly increased Imax in the LS-favoring preparation (20% increase compared to control). Expression of either mutation consistently produced enrichment of HS-isoform expression in all preparations. α4β2-nAChR harboring either NFLE mutant subunit showed unchanged ACh, sazetidine-A, nicotine, cytisine and mecamylamine potency. However, both mutant subunits enhanced partial agonist efficacies in the LS-biased preparation. Using β2-subunit-specific [(125)I]mAb 295 immunolabeling, nAChR cell-surface expression was determined. Antibody binding studies revealed that the β2(V337G) mutation tended to reduce cell-surface expression, and function per receptor was significantly increased by either NFLE mutant subunit in HS-favoring preparations. These findings identify both common and differing features between TM- and C2-domain AD/NFLE-associated mutations. As we discuss, the shared features may be particularly salient to AD/NFLE etiology.

Unravelling the mechanism of action of NS9283, a positive allosteric modulator of (α4)3(β2)2 nicotinic ACh receptors

BACKGROUND AND PURPOSE

Strong implications in major neurological diseases make the neuronal α4β2 nicotinic ACh receptor (nAChR) a highly interesting drug target. In this study, we present a detailed electrophysiological characterization of NS9283, a potent positive allosteric modulator acting selectively at 3α:2β stoichiometry of α2* and α4* nAChRs.

EXPERIMENTAL APPROACH

The whole-cell patch-clamp technique equipped with an ultra-fast drug application system was used to perform electrophysiological characterization of NS9283 modulatory actions on human α4β2 nAChRs stably expressed in HEK293 cells (HEK293-hα4β2).

KEY RESULTS

NS9283 was demonstrated to increase the potency of ACh-evoked currents in HEK293-hα4β2 cells by left-shifting the concentration-response curve ~60-fold. Interestingly, this modulation did not significantly alter maximal efficacy levels of ACh. Further, NS9283 did not affect the rate of desensitization of ACh-evoked currents, was incapable of reactivating desensitized receptors and only moderately slowed recovery from desensitization. However, NS9283 strongly decreased the rate of deactivation kinetics and also modestly decreased the rate of activation. This resulted in a left-shift of the ACh window current of (α4)3(β2)2 nAChRs in the presence of NS9283.

CONCLUSIONS AND IMPLICATIONS

This study demonstrates that NS9283 increases responsiveness of human (α4)3(β2)2 nAChR to ACh with no change in maximum efficacy. We propose that this potentiation is due to a significant slowing of deactivation kinetics. In summary, the mechanism of action of NS9283 bears high resemblance to that of benzodiazepines at the GABAA receptor and to our knowledge, NS9283 constitutes the first nAChR compound of this class.

Desformylflustrabromine Modulates α4β2 Neuronal Nicotinic Acetylcholine Receptor High- and Low-Sensitivity Isoforms at Allosteric Clefts Containing the β2 Subunit

Alterations in expression patterns of α4β2 nicotinic acetylcholine receptors have been demonstrated to alter cholinergic neurotransmission and are implicated in neurologic disorders, including autism, nicotine addiction, Alzheimer's disease, and Parkinson's disease. Positive allosteric modulators (PAMs) represent promising new leads in the development of therapeutic agents for the treatment of these disorders. This study investigates the involvement of the β2-containing subunit interfaces of α4β2 receptors in the modulation of acetylcholine (ACh)-induced responses by the PAM desformylflustrabromine (dFBr). Eight amino acids on the principal face of the β2 subunit were mutated to alanine to explore the involvement of this region in the potentiation of ACh-induced currents by dFBr. ACh-induced responses obtained from wild-type and mutant α4β2 receptors expressed in Xenopus laevis oocytes were recorded in the presence and absence of dFBr using two-electrode voltage clamp electrophysiology. Wild-type and mutant receptors were expressed in both high and low ACh sensitivity isoforms by using biased injection ratios of 1:5 or 5:1 α4 to β2 complementary RNA. Mutations were made in the B, C, and A loops of the principal face of the β2 subunit, which are regions not involved in the binding of ACh. Mutant β2(Y120A) significantly eliminated dFBr potency in both isoform preparations. Several other mutations altered dFBr potentiation levels in both preparations. Our findings support the involvement of the principal face of the β2 subunit in dFBr modulation of ACh-induced responses. Findings from this study will aid in the improved design of dFBr-like PAMs for potential therapeutic use.

Structure of neuronal nicotinic receptors

Nicotinic acetylcholine receptors (nAChRs) are a critical component of the brain's cholinergic neurotransmission system that modulates important physiological processes and whose dysfunction has been observed in patients with neurodegenerative diseases and mental illness. nAChRs are a heterogeneous family of receptor subtypes consisting of pentameric combinations of α and β subunits, and are widely expressed throughout the central and peripheral nervous system. nAChR subtypesnAChR subtypes share a common basic structure but their biophysical and pharmacological properties depend on their subunit compositionSubunit composition , which is therefore central to understanding receptor function in the nervous system and discovering new subtype-selective drugs. We briefly review some recent findings concerning the structure and function of nAChRs, particularly the native subtypes.

The additional ACh binding site at the α4(+)/α4(-) interface of the (α4β2)2α4 nicotinic ACh receptor contributes to desensitization

BACKGROUND AND PURPOSE

Nicotinic ACh (α4β2)2α4 receptors are highly prone to desensitization by prolonged exposure to low concentrations of agonist. Here, we report on the sensitivity of the three agonist sites of the (α4β2)2α4 to desensitization induced by prolonged exposure to ACh. We present electrophysiological data that show that the agonist sites of the (α4β2)2α4 receptor have different sensitivity to desensitization and that full receptor occupation decreases sensitivity to desensitization.

EXPERIMENTAL APPROACH

Two-electrode voltage-clamp electrophysiology was used to study the desensitization of concatenated (α4β2)2α4 receptors expressed heterologously in Xenopus oocytes. Desensitization was assessed by measuring the degree of functional inhibition caused by prolonged exposure to ACh, as measured under equilibrium conditions. We used the single-point mutation α4W182A to measure the contribution of individual agonist sites to desensitization.

KEY RESULTS

(α4β2)2α4 receptors are less sensitive to activation and desensitization by ACh than (α4β2)2β2 receptors. Incorporation of α4W182A into any of the agonist sites of concatenated (α4β2)2α4 receptors decreased sensitivity to activation and desensitization but the effects were more pronounced when the mutation was introduced into the α4(+)/α4(-) interface.

CONCLUSIONS AND IMPLICATIONS

The findings suggest that the agonist sites in (α4β2)2α4 receptors are not functionally equivalent. The agonist site at the α4(+)/α4(-) interface defines the sensitivity of (α4β2)2α4 receptors to agonist-induced activation and desensitization. Functional differences between (α4β2)2α4 and (α4β2)2β2 receptors might shape the physiological and behavioural responses to nicotinic ligands when the receptors are exposed to nicotinic ligands for prolonged periods of times.

Non-equivalent ligand selectivity of agonist sites in (α4β2)2α4 nicotinic acetylcholine receptors: a key determinant of agonist efficacy

The α4β2 nicotinic acetylcholine receptor (nAChR) is the most abundant nAChR type in the brain, and this receptor type exists in alternate (α4β2)2α4 and (α4β2)2β2 forms, which are activated by agonists with strikingly differing efficacies. Recent breakthroughs have identified an additional operational agonist binding site in the (α4β2)2α4 nAChR that is responsible for the signature sensitivity of this receptor to activation by agonists, yet the structural mechanisms determining agonist efficacy at this receptor type are not yet fully understood. In this study, we characterized the ligand selectivity of the individual agonist sites of the (α4β2)2α4 nAChR to determine whether differences in agonist selectivity influence agonist efficacy. Applying the substituted cysteine accessibility method to individual agonist sites in concatenated (α4β2)2α4 receptors, we determined the agonist selectivity of the agonist sites of the (α4β2)2α4 receptor. We show that (a) accessibility of substituted cysteines to covalent modification by methanesulfonate reagent depends on the agonist site at which the modification occurs and (b) that agonists such as sazetidine-A and TC-2559 are excluded from the site at the α4/α4 interface. Given that additional binding to the agonist site in the α4/α4 interface increases acetylcholine efficacy and that agonists excluded from the agonist site at the α4/α4 interface behave as partial agonists, we conclude that the ability to engage all agonist sites in (α4β2)2α4 nAChRs is a key determinant of agonist efficacy. The findings add another level of complexity to the structural mechanisms that govern agonist efficacy in heteromeric nAChRs and related ligand-gated ion channels.

Chronic sazetidine-A maintains anxiolytic effects and slower weight gain following chronic nicotine without maintaining increased density of nicotinic receptors in rodent brain

Chronic nicotine administration increases the density of brain α4β2* nicotinic acetylcholine receptors (nAChRs), which may contribute to nicotine addiction by exacerbating withdrawal symptoms associated with smoking cessation. Varenicline, a smoking cessation drug, also increases these receptors in rodent brain. The maintenance of this increase by varenicline as well as nicotine replacement may contribute to the high rate of relapse during the first year after smoking cessation. Recently, we found that sazetidine-A (saz-A), a potent partial agonist that desensitizes α4β2* nAChRs, does not increase the density of these receptors in brain at doses that decrease nicotine self-administration, increase attention in rats, and produce anxiolytic effects in mice. Here, we investigated whether chronic saz-A and varenicline maintain the density of nAChRs after their up-regulation by nicotine. In addition, we examined the effects of these drugs on a measure of anxiety in mice and weight gain in rats. After increasing nAChRs in the rodent brain with chronic nicotine, replacing nicotine with chronic varenicline maintained the increased nAChR binding, as well as the α4β2 subunit proteins measured by western blots. In contrast, replacing nicotine treatments with chronic saz-A resulted in the return of the density of nAChRs to the levels seen in saline controls. Nicotine, saz-A and varenicline each demonstrated anxiolytic effects in mice, but only saz-A and nicotine attenuated the gain of weight over a 6-week period in rats. These findings suggest that apart from its modest anxiolytic and weight control effects, saz-A, or drugs like it, may be useful in achieving long-term abstinence from smoking.

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

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

Rare human nicotinic acetylcholine receptor α4 subunit (CHRNA4) variants affect expression and function of high-affinity nicotinic acetylcholine receptors

Nicotine, the primary psychoactive component in tobacco smoke, produces its behavioral effects through interactions with neuronal nicotinic acetylcholine receptors (nAChRs). α4β2 nAChRs are the most abundant in mammalian brain, and converging evidence shows that this subtype mediates the rewarding and reinforcing effects of nicotine. A number of rare variants in the CHRNA4 gene that encode the α4 nAChR subunit have been identified in human subjects and appear to be underrepresented in a cohort of smokers. We compared three of these variants (α4R336C, α4P451L, and α4R487Q) to the common variant to determine their effects on α4β2 nAChR pharmacology. We examined [(3)H]epibatidine binding, interacting proteins, and phosphorylation of the α4 nAChR subunit with liquid chromatography and tandem mass spectrometry (LC-MS/MS) in HEK 293 cells and voltage-clamp electrophysiology in Xenopus laevis oocytes. We observed significant effects of the α4 variants on nAChR expression, subcellular distribution, and sensitivity to nicotine-induced receptor upregulation. Proteomic analysis of immunopurified α4β2 nAChRs incorporating the rare variants identified considerable differences in the intracellular interactomes due to these single amino acid substitutions. Electrophysiological characterization in X. laevis oocytes revealed alterations in the functional parameters of activation by nAChR agonists conferred by these α4 rare variants, as well as shifts in receptor function after incubation with nicotine. Taken together, these experiments suggest that genetic variation at CHRNA4 alters the assembly and expression of human α4β2 nAChRs, resulting in receptors that are more sensitive to nicotine exposure than those assembled with the common α4 variant. The changes in nAChR pharmacology could contribute to differences in responses to smoked nicotine in individuals harboring these rare variants.

Acute activation, desensitization and smoldering activation of human acetylcholine receptors

The behavioral effects of nicotine and other nicotinic agonists are mediated by AChRs in the brain. The relative contribution of acute activation versus chronic desensitization of AChRs is unknown. Sustained “smoldering activation” occurs over a range of agonist concentrations at which activated and desensitized AChRs are present in equilibrium. We used a fluorescent dye sensitive to changes in membrane potential to examine the effects of acute activation and chronic desensitization by nicotinic AChR agonists on cell lines expressing human α4β2, α3β4 and α7 AChRs. We examined the effects of acute and prolonged application of nicotine and the partial agonists varenicline, cytisine and sazetidine-A on these AChRs. The range of concentrations over which nicotine causes smoldering activation of α4β2 AChRs was centered at 0.13 µM, a level found in smokers. However, nicotine produced smoldering activation of α3β4 and α7 AChRs at concentrations well above levels found in smokers. The α4β2 expressing cell line contains a mixture of two stoichiometries, namely (α4β2)₂β2 and (α4β2)₂α4. The (α4β2)₂β2 stoichiometry is more sensitive to activation by nicotine. Sazetidine-A activates and desensitizes only this stoichiometry. Varenicline, cytisine and sazetidine-A were partial agonists on this mixture of α4β2 AChRs, but full agonists on α3β4 and α7 AChRs. It has been reported that cytisine and varenicline are most efficacious on the (α4β2)₂α4 stoichiometry. In this study, we distinguish the dual effects of activation and desensitization of AChRs by these nicotinic agonists and define the range of concentrations over which smoldering activation can be sustained.

The unique α4+/-α4 agonist binding site in (α4)3(β2)2 subtype nicotinic acetylcholine receptors permits differential agonist desensitization pharmacology versus the (α4)2(β2)3 subtype

Selected nicotinic agonists were used to activate and desensitize high-sensitivity (HS) (α4)2(β2)3) or low-sensitivity (LS) (α4)3(β2)2) isoforms of human α4β2-nicotinic acetylcholine receptors (nAChRs). Function was assessed using (86)Rb(+) efflux in a stably transfected SH-EP1-hα4β2 human epithelial cell line, and two-electrode voltage-clamp electrophysiology in Xenopus laevis oocytes expressing concatenated pentameric HS or LS α4β2-nAChR constructs (HSP and LSP). Unlike previously studied agonists, desensitization by the highly selective agonists A-85380 [3-(2(S)-azetidinylmethoxy)pyridine] and sazetidine-A (Saz-A) preferentially reduced α4β2-nAChR HS-phase versus LS-phase responses. The concatenated-nAChR experiments confirmed that approximately 20% of LS-isoform acetylcholine-induced function occurs in an HS-like phase, which is abolished by Saz-A preincubation. Six mutant LSPs were generated, each targeting a conserved agonist binding residue within the LS-isoform-only α4(+)/(-)α4 interface agonist binding site. Every mutation reduced the percentage of LS-phase function, demonstrating that this site underpins LS-phase function. Oocyte-surface expression of the HSP and each of the LSP constructs was statistically indistinguishable, as measured using β2-subunit-specific [(125)I]mAb295 labeling. However, maximum function is approximately five times greater on a "per-receptor" basis for unmodified LSP versus HSP α4β2-nAChRs. Thus, recruitment of the α4(+)/(-)α4 site at higher agonist concentrations appears to augment otherwise-similar function mediated by the pair of α4(+)/(-)β2 sites shared by both isoforms. These studies elucidate the receptor-level differences underlying the differential pharmacology of the two α4β2-nAChR isoforms, and demonstrate that HS versus LS α4β2-nAChR activity can be selectively manipulated using pharmacological approaches. Since α4β2 nAChRs are the predominant neuronal subtype, these discoveries likely have significant functional implications, and may provide important insights for drug discovery and development.

Selective potentiation of (α4)3(β2)2 nicotinic acetylcholine receptors augments amplitudes of prefrontal acetylcholine- and nicotine-evoked glutamatergic transients in rats

Prefrontal glutamate release evoked through activation of α4β2* nicotinic acetylcholine receptors (nAChRs) situated on thalamic glutamatergic afferents mediates cue detection processes and thus contributes to attentional performance. However, little is known about the respective contributions of the high sensitivity and low sensitivity (LS) stoichiometries of the α4β2 nAChR, (α4)2(β2)3 and (α4)3(β2)2, to these processes. In the present study we employed glutamate-sensitive microelectrodes and the (α4)3(β2)2-selective positive allosteric modulator (PAM) NS9283 to investigate the importance of the LS α4β2 nAChR for glutamate release in the rat medial prefrontal cortex (mPFC). Firstly, the signaling evoked by physiologically relevant ACh concentrations through the (α4)3(β2)2 nAChR in HEK293 cells was potentiated by NS9283, consistent with the classification of NS9283 as a PAM. In urethane-anesthetized rats, intra-prefrontal pressure ejections of NS9283 evoked glutamatergic transients. Importantly, this glutamate release was attenuated by removal of cholinergic projections to the recording area. This finding indicates that the effects of NS9283 depend on endogenous ACh, again consistent with effects of a PAM. We then conducted microdialysis to demonstrate the presence of extracellular ACh in urethane-anesthetized control rats. While detectable, those levels were significantly lower than in awake rats. Finally, the amplitudes of glutamatergic transients evoked by local pressure ejections of a low concentration of nicotine were significantly augmented following systemic administration of NS9283 (3.0mg/kg). In conclusion, our results indicate that a LS α4β2 nAChR PAM such as NS9283 may enhance the cholinergic modulation of glutamatergic neurotransmission in the cortex, thereby perhaps alleviating the attentional impairments common to a range of brain disorders.

Exploring the nicotinic acetylcholine receptor-associated proteome with iTRAQ and transgenic mice

Neuronal nicotinic acetylcholine receptors (nAChRs) containing α4 and β2 subunits are the principal receptors in the mammalian central nervous system that bind nicotine with high affinity. These nAChRs are involved in nicotine dependence, mood disorders, neurodegeneration and neuroprotection. However, our understanding of the interactions between α4β2-containing (α4β2(∗)) nAChRs and other proteins remains limited. In this study, we identified proteins that interact with α4β2(∗) nAChRs in a genedose dependent pattern by immunopurifying β2(∗) nAChRs from mice that differ in α4 and β2 subunit expression and performing proteomic analysis using isobaric tags for relative and absolute quantitation (iTRAQ). Reduced expression of either the α4 or the β2 subunit results in a correlated decline in the expression of a number of putative interacting proteins. We identified 208 proteins co-immunoprecipitated with these nAChRs. Furthermore, stratified linear regression analysis indicated that levels of 17 proteins was correlated significantly with expression of α4β2 nAChRs, including proteins involved in cytoskeletal rearrangement and calcium signaling. These findings represent the first application of quantitative proteomics to produce a β2(∗) nAChR interactome and describe a novel technique used to discover potential targets for pharmacological manipulation of α4β2 nAChRs and their downstream signaling mechanisms.

Genetic matters: thirty years of progress using mouse models in nicotinic research

This research update summarizes thirty years of studies on genetic influences on responses to the acute or chronic administration of nicotine. Early studies established that various inbred mice are differentially sensitive to the effects of the drug. Classical genetic analyses confirmed that nicotine effects on locomotion, body temperature and seizures are heritable. A significant inverse correlation between the locomotor and hypothermic effects and the density of nicotine binding sites suggested that differential expression α4β2-neuronal nicotinic acetylcholine receptor (nAChR) mediated some of this genetic variability. Subsequent studies with α4 and β2 nAChR null (decreased sensitivity) and gain of function mutants (increased sensitivity) supports the role of the α4β2*nAChR subtype. However, null mutant mice still respond to nicotine, indicating that other nAChR subtypes also mediate these responses. Mice differing in initial sensitivity to nicotine also differ in tolerance development following chronic treatment: those mice that are initially more sensitive to nicotine develop tolerance at lower treatment doses than less sensitive mice, indicating that tolerance is an adaptive response to the effects of nicotine. In contrast, the sensitivity of mice to pre-pulse inhibition of acoustic startle response is correlated with the expression of α7-nAChR. While genetic variability in nAChR expression and function is an important factor contributing to differences in response to nicotine, the observations that altered activity of opioid, glutamate, and cannabinoid receptors among others also change nicotine sensitivity reinforces the proposal that the genetics of nicotine response is more complex than differences in nAChRs.

Neuronal nicotinic receptor agonists improve gait and balance in olivocerebellar ataxia

Clinical studies have reported that the nicotinic receptor agonist varenicline improves balance and coordination in patients with several types of ataxia, but confirmation in an animal model has not been demonstrated. This study investigated whether varenicline and nicotine could attenuate the ataxia induced in rats following destruction of the olivocerebellar pathway by the neurotoxin 3-acetylpyridine (3-AP). The administration of 3-AP (70 mg/kg followed by 300 mg niacinamide/kg; i.p.) led to an 85% loss of inferior olivary neurons within one week without evidence of recovery, and was accompanied by a 72% decrease in rotorod activity, a 3-fold increase in the time to traverse a stationary beam, a 19% decrease in velocity and 31% decrease in distance moved in the open field, and alterations in gait parameters, with a 19% increase in hindpaw stride width. The daily administration of nicotine (0.33 mg free base/kg) for one week improved rotorod performance by 50% and normalized the increased hindpaw stride width, effects that were prevented by the daily preadministration of the nicotinic antagonist mecamylamine (0.8 mg free base/kg). Varenicline (1 and 3 mg free base/kg daily) also improved rotorod performance by approximately 50% following one week of administration, and although it did not alter the time to traverse the beam, it did improve the ability to maintain balance on the beam. Neither varenicline nor nicotine, at doses that improved balance, affected impaired locomotor activity in the open field. Results provide evidence that nicotinic agonists are of benefit for alleviating some of the behavioral deficits in olivocerebellar ataxia and warrant further studies to elucidate the specific mechanism(s) involved.

Allosteric modulators of α4β2 nicotinic acetylcholine receptors: a new direction for antidepressant drug discovery

Allosteric modulation of ligand-gated ion channels has been intensively studied in the past three decades and is now an established strategy to control receptor function in numerous disease states. Allosteric sites on the GABA(A) receptor are targets for widely prescribed drugs that are used for a variety of pathophysiological states including insomnia and epilepsy. Modulators might be especially valuable to control receptors for which the design of selective orthosteric drugs has proven difficult due to safety issues (e.g., α4β2 nicotinic acetylcholine receptors and might have several advantages over orthosteric ligands. Modulators influence the action of the endogenous agonist but generally have no effect of their own on the unoccupied receptor. Moreover, the higher subtype selectivity exerted by modulators and that the effects of modulators depend on the simultaneous presence of agonist help to overcome safety problems by preventing over-dosage compared with the administration of orthosteric drugs.

Neonicotinic analogues: selective antagonists for α4β2 nicotinic acetylcholine receptors

Nicotine is an agonist of nicotinic acetylcholine receptors (nAChRs) that has been extensively used as a template for the synthesis of α4β2-preferring nAChRs. Here, we used the N-methyl-pyrrolidine moiety of nicotine to design and synthesise novel α4β2-preferring neonicotinic ligands. We increased the distance between the basic nitrogen and aromatic group of nicotine by introducing an ester functionality that also mimics acetylcholine (Fig. 2). Additionally, we introduced a benzyloxy group linked to the benzoyl moiety. Although the neonicotinic compounds fully inhibited binding of both [α-(125)I]bungarotoxin to human α7 nAChRs and [(3)H]cytisine to human α4β2 nAChRs, they were markedly more potent at displacing radioligand binding to human α4β2 nAChRs than to α7 nAChRs. Functional assays showed that the neonicotinic compounds behave as antagonists at α4β2 and α4β2α5 nAChRs. Substitutions on the aromatic ring of the compounds produced compounds that displayed marked selectivity for α4β2 or α4β2α5 nAChRs. Docking of the compounds on homology models of the agonist binding site at the α4/β2 subunit interfaces of α4β2 nAChRs suggested the compounds inhibit function of this nAChR type by binding the agonist binding site.

Augmentation of cognitive function by NS9283, a stoichiometry-dependent positive allosteric modulator of α2- and α4-containing nicotinic acetylcholine receptors

BACKGROUND AND PURPOSE

Positive allosteric modulation of α4β2 nicotinic acetylcholine (nACh) receptors could add a new dimension to the pharmacology and therapeutic approach to these receptors. The novel modulator NS9283 was therefore tested extensively.

EXPERIMENTAL APPROACH

Effects of NS9283 were evaluated in vitro using fluorescence-based Ca(2+) imaging and electrophysiological voltage clamp experiments in Xenopus oocytes, mammalian cells and thalamocortical neurons. In vivo the compound was tested in models covering a range of cognitive domains in mice and rats.

KEY RESULTS

NS9283 was shown to increase agonist-evoked response amplitude of (α4)(3) (β2)(2) nACh receptors in electrophysiology paradigms. (α2)(3) (β2)(2) , (α2)(3) (β4)(2) and (α4)(3) (β4)(2) were modulated to comparable extents, but no effects were detected at α3-containing or any 2α : 3β stoichiometry nACh receptors. Native nACh receptors in thalamocortical neurons similarly displayed DHβE-sensitive currents that were receptive to modulation. NS9283 had favourable effects on sensory information processing, as shown by reversal of PCP-disrupted pre-pulse inhibition. NS9283 further improved performance in a rat model of episodic memory (social recognition), a rat model of sustained attention (five-choice serial reaction time task) and a rat model of reference memory (Morris water maze). Importantly, the effects in the Morris water maze could be fully reversed with mecamylamine, a blocker of nACh receptors.

CONCLUSIONS AND IMPLICATIONS

These results provide compelling evidence that positive allosteric modulators acting at the (α4)(3) (β2)(2) nACh receptors can augment activity across a broad range of cognitive domains, and that α4β2 nACh receptor allosteric modulation therefore constitutes a promising therapeutic approach to symptomatic treatment of cognitive impairment.

Mice expressing the ADNFLE valine 287 leucine mutation of the Β2 nicotinic acetylcholine receptor subunit display increased sensitivity to acute nicotine administration and altered presynaptic nicotinic receptor function

Several mutations in α4 or β2 nicotinic receptor subunits are linked to autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE). One such missense mutation in the gene encoding the β2 neuronal nicotinic acetylcholine receptor (nAChR) subunit (CHRNB2) is a valine-to-leucine substitution in the second transmembrane domain at position 287 (β2VL). Previous studies indicated that the β2VL mutation in mice alters circadian rhythm consistent with sleep alterations observed in ADNFLE patients (Xu et al., 2011). The current study investigates changes in nicotinic receptor function and expression that may explain the behavioral phenotype of β2VL mice. No differences in β2 mRNA expression were found between wild-type (WT) and heterozygous (HT) or homozygous mutant (MT) mice. However, antibody and ligand binding indicated that the mutation resulted in a reduction in receptor protein. Functional consequences of the β2VL mutation were assessed biochemically using crude synaptosomes. A gene-dose dependent increase in sensitivity to activation by acetylcholine and decrease in maximal nAChR-mediated [(3)H]-dopamine release and (86)Rb efflux were observed. Maximal nAChR-mediated [(3)H]-GABA release in the cortex was also decreased in the MT, but maximal [(3)H]-GABA release was retained in the hippocampus. Behaviorally both HT and MT mice demonstrated increased sensitivity to nicotine-induced hypolocomotion and hypothermia. Furthermore, WT mice display only a tonic-clonic seizure (EEG recordable) 3 min after injection of a high dose of nicotine, while MT mice also display a dystonic arousal complex (non-EEG recordable) event 30s after nicotine injection. Data indicate decreases in maximal response for certain measures are larger than expected given the decrease in receptor expression.

Structure-activity studies of 7-heteroaryl-3-azabicyclo[3.3.1]non-6-enes: a novel class of highly potent nicotinic receptor ligands

The potential for nicotinic ligands with affinity for the α4β2 or α7 subtypes to treat such diverse diseases as nicotine addiction, neuropathic pain, and neurodegenerative and cognitive disorders has been exhibited clinically for several compounds while preclinical activity in relevant in vivo models has been demonstrated for many more. For several therapeutic programs, we sought nicotinic ligands with various combinations of affinity and function across both subtypes, with an emphasis on dual α4β2-α7 ligands, to explore the possibility of synergistic effects. We report here the structure-activity relationships (SAR) for a novel series of 7-heteroaryl-3-azabicyclo[3.3.1]non-6-enes and characterize many of the analogues for activity at multiple nicotinic subtypes.

Allosteric modulation of alpha4beta2 nicotinic acetylcholine receptors by HEPES

A number of new positive allosteric modulators (PAMs) have been reported that enhance responses of neuronal alpha7 and alpha4beta2 nicotinic acetylcholine receptor subtypes to orthosteric ligands. PAMs represent promising new leads for the development of therapeutic agents for disorders involving alterations in nicotinic neurotransmission including Autism, Alzheimer's and Parkinson's disease. During our recent studies of alpha4beta2 PAMs, we identified a novel effect of 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES). The effects of HEPES were evaluated in a phosphate buffered recording solution using two-electrode voltage clamp techniques and alpha4beta2 and alpha7 nicotinic acetylcholine receptor subtypes expressed in Xenopus laevis oocytes. Acetylcholine induced responses of high-sensitivity alpha4beta2 receptors were potentiated 190% by co-exposure to HEPES. Responses were inhibited at higher concentrations (bell-shaped concentration/response curve). Coincidentally, at concentrations of HEPES typically used in oocyte recording (5-10mM), the potentiating effects of HEPES are matched by its inhibitory effects, thus producing no net effect. Mutagenesis results suggest HEPES potentiates the high-sensitivity stoichiometry of the alpha4beta2 receptors through action at the beta2+/beta2- interface and is dependent on residue beta2D218. HEPES did not potentiate low-sensitivity alpha4beta2 receptors and did not produce any observable effect on acetylcholine induced responses on alpha7 nicotinic acetylcholine receptors.

Selective down-regulation of α4β2 neuronal nicotinic acetylcholine receptors in the brain of uremic rats with cognitive impairment

Cognitive impairment is common in patients with chronic kidney disease. Brain nicotinic acetylcholine receptors modulate cognitive functions, such as learning and memory. Pharmacological cholinergic enhancement is useful in patients with cognitive dysfunction. The major nicotinic acetylcholine receptor subtypes in the brain are heteromeric α4β2 and homomeric α7 receptors. To study the involvement of neuronal acetylcholine receptors in cognitive impairment in uremic rats, bilateral nephrectomy was performed. 24 weeks after nephrectomy, memory was assessed using the one trial step-down inhibitory avoidance test. Neuronal nicotinic acetylcholine receptors in the brain were studied by radioligand binding, immunoprecipitation, Western blot and sucrose gradient experiments. We demonstrated that rats with severe renal failure show disorders of short term memory. Long term memory was not altered in these rats. The number of functional α4β2 heteromeric neuronal nicotinic receptors was decreased in the brains of rats with severe renal failure. There was a significant correlation between the degree of renal impairment and the number of heteromeric nicotinic acetylcholine receptors in the brain. The down-regulation of functional α4β2 receptors in the brains of rats with severe renal failure was not due to a reduction of α4 or β2 subunit proteins. The number of α7 homomeric neuronal nicotinic acetylcholine receptors was not altered. These findings may have important clinical significance for the management of cognitive impairment in patients with chronic kidney disease.

Nicotinic α5 subunits drive developmental changes in the activation and morphology of prefrontal cortex layer VI neurons

BACKGROUND

Nicotinic signaling in prefrontal layer VI pyramidal neurons is important to the function of mature attention systems. The normal incorporation of α5 subunits into α4β2* nicotinic acetylcholine receptors augments nicotinic signaling in these neurons and is required for normal attention performance in adult mice. However, the role of α5 subunits in the development of the prefrontal cortex is not known.

METHODS

We sought to answer this question by examining nicotinic currents and neuronal morphology in layer VI neurons of medial prefrontal cortex of wild-type and α5 subunit knockout (α5(-/-)) mice during postnatal development and in adulthood.

RESULTS

In wild-type but not in α5(-/-) mice, there is a developmental peak in nicotinic acetylcholine currents in the third postnatal week. At this juvenile time period, the majority of neurons in all mice have long apical dendrites extending into cortical layer I. Yet, by early adulthood, wild-type but not α5(-/-) mice show a pronounced shift toward shorter apical dendrites. This cellular difference occurs in the absence of genotype differences in overall cortical morphology.

CONCLUSIONS

Normal developmental changes in nicotinic signaling and dendritic morphology in prefrontal cortex depend on α5-comprising nicotinic acetylcholine receptors. It appears that these receptors mediate a specific developmental retraction of apical dendrites in layer VI neurons. This finding provides novel insight into the cellular mechanisms underlying the known attention deficits in α5(-/-) mice and potentially also into the pathophysiology of developmental neuropsychiatric disorders such as attention-deficit disorder and autism.

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.

Characterizing low affinity epibatidine binding to α4β2 nicotinic acetylcholine receptors with ligand depletion and nonspecific binding

Background

Along with high affinity binding of epibatidine (K_d1≈10 pM) to α4β2 nicotinic acetylcholine receptor (nAChR), low affinity binding of epibatidine (K_d2≈1-10 nM) to an independent binding site has been reported. Studying this low affinity binding is important because it might contribute understanding about the structure and synthesis of α4β2 nAChR. The binding behavior of epibatidine and α4β2 AChR raises a question about interpreting binding data from two independent sites with ligand depletion and nonspecific binding, both of which can affect equilibrium binding of [³H]epibatidine and α4β2 nAChR. If modeled incorrectly, ligand depletion and nonspecific binding lead to inaccurate estimates of binding constants. Fitting total equilibrium binding as a function of total ligand accurately characterizes a single site with ligand depletion and nonspecific binding. The goal of this study was to determine whether this approach is sufficient with two independent high and low affinity sites.

Results

Computer simulations of binding revealed complexities beyond fitting total binding for characterizing the second, low affinity site of α4β2 nAChR. First, distinguishing low-affinity specific binding from nonspecific binding was a potential problem with saturation data. Varying the maximum concentration of [³H]epibatidine, simultaneously fitting independently measured nonspecific binding, and varying α4β2 nAChR concentration were effective remedies. Second, ligand depletion helped identify the low affinity site when nonspecific binding was significant in saturation or competition data, contrary to a common belief that ligand depletion always is detrimental. Third, measuring nonspecific binding without α4β2 nAChR distinguished better between nonspecific binding and low-affinity specific binding under some circumstances of competitive binding than did presuming nonspecific binding to be residual [³H]epibatidine binding after adding a large concentration of cold competitor. Fourth, nonspecific binding of a heterologous competitor changed estimates of high and low inhibition constants but did not change the ratio of those estimates.

Conclusions

Investigating the low affinity site of α4β2 nAChR with equilibrium binding when ligand depletion and nonspecific binding are present likely needs special attention to experimental design and data interpretation beyond fitting total binding data. Manipulation of maximum ligand and receptor concentrations and intentionally increasing ligand depletion are potentially helpful approaches.

An autoradiographic survey of mouse brain nicotinic acetylcholine receptors defined by null mutants

Nine nicotinic receptor subunits are expressed in the central nervous system indicating that a variety of nicotinic acetylcholine receptors (nAChR) may be assembled. A useful method with which to identify putative nAChR is radioligand binding. In the current study the binding of [(125)I]α-bungarotoxin, [(125)I]α-conotoxinMII, 5[(125)I]-3-((2S)-azetidinylmethoxy)pyridine (A-85380), and [(125)I]epibatidine has been measured autoradiographically to provide data on many nAChR binding sites. Each binding site was evaluated semi-quantitatively for samples prepared from wild-type and α2, α4, α6, α7, β2, β4, α5 and β3 null mutant mice. Deletion of the α7 subunit completely and selectively eliminated [(125)I]α-bungarotoxin binding. The binding of [(125)I]α-conotoxinMII was eliminated in most brain regions by deletion of either the α6 or β2 subunit and is reduced by deletion of either the α4 or β3 subunit. The binding of 5[(125)I]A-85380 was completely eliminated by deletion of the β2 subunit and significantly reduced by deletion of the α4 subunit. Most, but not all, α4-independent sites require expression of the α6 subunit. The effect of gene deletion on total [(125)I]epibatidine binding was very similar to that on [(125)I]A-85380 binding. [(125)I]Epibatidine also labels β4* nAChR, which was readily apparent for incubations conducted in the presence of 100nM cytisine. The effects of α3 gene deletion could not be evaluated, but persistence of residual sites implies the expression of α3* nAChR. Taken together these results confirm and extend previously published evaluations of the effect of nAChR gene deletion and help to define the nAChR subtypes measurable by ligand binding.

Cortico-thalamic connectivity is vulnerable to nicotine exposure during early postnatal development through α4/β2/α5 nicotinic acetylcholine receptors

Tobacco smoke exposure during development can result in lasting alterations in sensory processing and attention. This suggests that some constituent of smoke, such as the primary addictive component, nicotine, alters neurodevelopment. Although many effects of developmental nicotine exposure have been identified in humans and animal models, very few mechanistic studies have identified the molecular and anatomical basis for a defined behavioral consequence of developmental exposure. We show in this study that a mouse model of developmental nicotine exposure results in hypersensitive passive avoidance in adulthood. We have used transgenic mice in which β2 subunit containing nicotinic acetylcholine receptors (β2* nAChRs) are expressed exclusively on corticothalamic neurons (β2 tr(CT) mice) to identify the receptor subtypes involved and also to define the circuit level site of action responsible for this persistent, nicotine-induced behavioral phenotype. Further characterization of the native nAChRs expressed in this circuit indicates that both (α4)(2)(β2)(3) and (α4)(2)(β2)(2)α5 nAChR subtypes are present in corticothalamic projections. Consistent with a role for (α4)(2)(β2)(2)α5 nAChRs in mediating the effect of developmental nicotine exposure on adult passive avoidance behavior, constitutive deletion of the α5 nAChR subunit also alters this behavior. A critical period for this developmental consequence of nicotine exposure was defined by limiting exposure to the early post-natal period. Taken together, these studies identify a novel consequence of developmental nicotine exposure in the mouse, define the nAChR subtypes and neural circuit involved in this behavioral change and delimit the neurodevelopmental period critical for vulnerability to a behavioral alteration that persists into adulthood.

86Rb+ efflux mediated by alpha4beta2*-nicotinic acetylcholine receptors with high and low-sensitivity to stimulation by acetylcholine display similar agonist-induced desensitization

The nicotinic acetylcholine receptors (nAChR) assembled from alpha4 and beta2 subunits are the most densely expressed subtype in the brain. Concentration-effect curves for agonist activation of alpha4beta2*-nAChR are biphasic. This biphasic agonist sensitivity is ascribed to differences in subunit stoichiometry. The studies described here evaluated desensitization elicited by low concentrations of epibatidine, nicotine, cytisine or methylcarbachol of brain alpha4beta2-nAChR function measured with acetylcholine-stimulated (86)Rb(+) efflux from mouse thalamic synaptosomes. Each agonist elicited concentration-dependent desensitization. The agonists differed in potency. However, IC(50) values for each agonist for desensitization of (86)Rb(+) efflux both with high (EC(50) approximately 3 microM) and low (EC(50) approximately 150 microM) acetylcholine sensitivity were not significantly different. Concentrations required to elicit desensitization were higher that their respective K(D) values for receptor binding. Even though the two components of alpha4beta2*-nAChR-mediated (86)Rb(+) efflux from mouse brain differ markedly in EC(50) values for agonist activation, they are equally sensitive to desensitization by exposure to low agonist concentrations. Mice were also chronically treated with nicotine by continuous infusion of 0, 0.5 or 4.0mg/kg/h and desensitization induced by nicotine was evaluated. Consistent with previous results, chronic nicotine treatment increased the density of epibatidine binding sites. Acute exposure to nicotine also elicited concentration-dependent desensitization of both high-sensitivity and low-sensitivity acetylcholine-stimulated (86)Rb(+) efflux from cortical and thalamic synaptosomes. Although chronic nicotine treatment reduced maximal (86)Rb(+) efflux from thalamus, IC(50) values in both brain regions were unaffected by chronic nicotine treatment.

Alpha-conotoxin AuIB isomers exhibit distinct inhibitory mechanisms and differential sensitivity to stoichiometry of alpha3beta4 nicotinic acetylcholine receptors

Non-native disulfide isomers of alpha-conotoxins are generally inactive although some unexpectedly demonstrate comparable or enhanced bioactivity. The actions of "globular" and "ribbon" isomers of alpha-conotoxin AuIB have been characterized on alpha3beta4 nicotinic acetylcholine receptors (nAChRs) heterologously expressed in Xenopus oocytes. Using two-electrode voltage clamp recording, we showed that the inhibitory efficacy of the ribbon isomer of AuIB is limited to approximately 50%. The maximal inhibition was stoichiometry-dependent because altering alpha3:beta4 RNA injection ratios either increased AuIB(ribbon) efficacy (10alpha:1beta) or completely abolished blockade (1alpha:10beta). In contrast, inhibition by AuIB(globular) was independent of injection ratios. ACh-evoked current amplitude was largest for 1:10 injected oocytes and smallest for the 10:1 ratio. ACh concentration-response curves revealed high (HS, 1:10) and low (LS, 10:1) sensitivity alpha3beta4 nAChRs with corresponding EC(50) values of 22.6 and 176.9 microM, respectively. Increasing the agonist concentration antagonized the inhibition of LS alpha3beta4 nAChRs by AuIB(ribbon), whereas inhibition of HS and LS alpha3beta4 nAChRs by AuIB(globular) was unaffected. Inhibition of LS and HS alpha3beta4 nAChRs by AuIB(globular) was insurmountable and independent of membrane potential. Molecular docking simulation suggested that AuIB(globular) is likely to bind to both alpha3beta4 nAChR stoichiometries outside of the ACh-binding pocket, whereas AuIB(ribbon) binds to the classical agonist-binding site of the LS alpha3beta4 nAChR only. In conclusion, the two isomers of AuIB differ in their inhibitory mechanisms such that AuIB(ribbon) inhibits only LS alpha3beta4 nAChRs competitively, whereas AuIB(globular) inhibits alpha3beta4 nAChRs irrespective of receptor stoichiometry, primarily by a non-competitive mechanism.

Chrna4 A529 knock-in mice exhibit altered nicotine sensitivity

The reasons why people smoke are varied, but research has shown that genetic influences on various aspects of nicotine addiction are a major factor. There also is a strong genetic influence on measures of nicotine sensitivity in mice. Despite the established contribution of genetics to nicotine sensitivity in mice and humans, no naturally occurring genetic variation has been identified that demonstrably alters sensitivity to nicotine in either species. However, one genetic variant has been implicated in altering nicotine sensitivity in mice is a T529A polymorphism in Chrna4, the gene that encodes the nicotinic receptor (nAChR) alpha4 subunit. The Chrna4 T529A polymorphism leads to a threonine to alanine substitution at position 529 of the alpha4 subunit. To more definitively address whether the Chrna4 T529A polymorphism does, in fact, influence sensitivity to nicotine, knock-in mice were generated in which the threonine codon at position 529 was mutated to an alanine codon. Compared with Chrna4 T529 littermate controls, the Chrna4 A529 knock-in mice exhibited greater sensitivity to the hypothermic effects of nicotine, reduced oral nicotine consumption and did not develop conditioned place preference to nicotine. The Chrna4 A529 knock-in mice also differed from T529 littermates for two parameters of acetylcholine-stimulated Rb+ efflux in midbrain: maximal efflux and the percentage of alpha4beta2* receptors with high sensitivity to activation by agonists. Results indicate that the polymorphism affects the function of midbrain alpha4beta2* nAChRs and contributes to individual differences in several behavioral and physiological responses to nicotine thought to be modulated by midbrain alpha4beta2* nAChRs.

Nicotinic receptor-based therapeutics and candidates for smoking cessation

Tobacco dependence is the most preventable cause of death and is a chronic, relapsing disorder in which compulsive tobacco use persists despite known negative health consequences. All currently available cessation agents (nicotine, varenicline and bupropion) have limited efficacy and are associated with high relapse rates, revealing a need for more efficacious, alternative pharmacotherapies. The major alkaloid in tobacco, nicotine, activates nicotinic receptors (nAChRs) which increase brain extracellular dopamine producing nicotine reward leading to addiction. nAChRs are located primarily presynaptically and modulate synaptic activity by regulating neurotransmitter release. Subtype-selective nAChR antagonists that block reward-relevant mesocorticolimbic and nigrostriatal dopamine release induced by nicotine may offer advantages over current therapies. An innovative approach is to provide pharmacotherapies which are antagonists at nAChR subtypes mediating nicotine evoked dopamine release. In addition, providing multiple medications with a wider array of targets and mechanisms should provide more treatment options for individuals who are not responsive to the currently available pharmacotherapies. This review summarizes the currently available smoking cessation therapies and discusses emerging potential therapeutic approaches employing pharmacological agents which act as antagonists at nicotinic receptors.

Mouse striatal dopamine nerve terminals express alpha4alpha5beta2 and two stoichiometric forms of alpha4beta2*-nicotinic acetylcholine receptors

Wild-type and alpha5 null mutant mice were used to identify nicotinic cholinergic receptors (nAChRs) that mediate alpha-conotoxin MII (alpha-CtxMII)-resistant dopamine (DA) release from striatal synaptosomes. Concentration-effect curves for ACh-stimulated release (20 s) were monophasic when wild-type synaptosomes were assayed but biphasic with synaptosomes from the alpha5 null mutant. Deleting the alpha5 gene also resulted in decreased maximal ACh-stimulated alpha-CtxMII-resistant DA release. When a shorter perfusion time (5 s) was used, biphasic curves were detected in both wild-type and alpha5 null mutants, indicative of high- and low-sensitivity (HS and LS) activity. In addition, DHbetaE-sensitive (HS) and DHbetaE-resistant (LS) components were found in both genotypes. These results indicate that alpha-CtxMII-resistant DA release is mediated by alpha4alpha5beta2, (alpha4)(2)(beta2)(3) (HS), and (alpha4)(3)(beta2)(2) (LS) nAChRs.