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

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.

Analogs of α-conotoxin PnIC selectively inhibit α7β2- over α7-only subtype nicotinic acetylcholine receptors via a novel allosteric mechanism

This study was undertaken to identify and characterize the first ligands capable of selectively identifying nicotinic acetylcholine receptors containing α7 and β2 subunits (α7β2-nAChR subtype). Basal forebrain cholinergic neurons express α7β2-nAChR. Here, they appear to mediate neuronal dysfunction induced by the elevated levels of oligomeric amyloid-β associated with early Alzheimer's disease. Additional work indicates that α7β2-nAChR are expressed across several further critically important cholinergic and GABAergic neuronal circuits within the central nervous system. Further studies, however, are significantly hindered by the inability of currently available ligands to distinguish heteromeric α7β2-nAChR from the closely related and more widespread homomeric α7-only-nAChR subtype. Functional screening using two-electrode voltage-clamp electrophysiology identified a family of α7β2-nAChR-selective analogs of α-conotoxin PnIC (α-CtxPnIC). A combined electrophysiology, functional kinetics, site-directed mutagenesis, and molecular dynamics approach was used to further characterize the α7β2-nAChR selectivity and site of action of these α-CtxPnIC analogs. We determined that α7β2-nAChR selectivity of α-CtxPnIC analogs arises from interactions at a site distinct from the orthosteric agonist-binding site shared between α7β2- and α7-only-nAChR. As numerous previously identified α-Ctx ligands are competitive antagonists of orthosteric agonist-binding sites, this study profoundly expands the scope of use of α-Ctx ligands (which have already provided important nAChR research and translational breakthroughs). More immediately, analogs of α-CtxPnIC promise to enable, for the first time, both comprehensive mapping of the distribution of α7β2-nAChR and detailed investigations of their physiological roles.

A potential cost of evolving epibatidine resistance in poison frogs

BACKGROUND

Some dendrobatid poison frogs sequester the toxin epibatidine as a defense against predators. We previously identified an amino acid substitution (S108C) at a highly conserved site in a nicotinic acetylcholine receptor β2 subunit of dendrobatid frogs that decreases sensitivity to epibatidine in the brain-expressing α4β2 receptor. Introduction of S108C to the orthologous high-sensitivity human receptor similarly decreased sensitivity to epibatidine but also decreased sensitivity to acetylcholine, a potential cost if this were to occur in dendrobatids. This decrease in the acetylcholine sensitivity manifested as a biphasic acetylcholine concentration-response curve consistent with the addition of low-sensitivity receptors. Surprisingly, the addition of the β2 S108C into the α4β2 receptor of the dendrobatid Epipedobates anthonyi did not change acetylcholine sensitivity, appearing cost-free. We proposed that toxin-bearing dendrobatids may have additional amino acid substitutions protecting their receptors from alterations in acetylcholine sensitivity. To test this, in the current study, we compared the dendrobatid receptor to its homologs from two non-dendrobatid frogs.

RESULTS

The introduction of S108C into the α4β2 receptors of two non-dendrobatid frogs also does not affect acetylcholine sensitivity suggesting no additional dendrobatid-specific substitutions. However, S108C decreased the magnitude of neurotransmitter-induced currents in Epipedobates and the non-dendrobatid frogs. We confirmed that decreased current resulted from fewer receptors in the plasma membrane in Epipedobates using radiolabeled antibodies against the receptors. To test whether S108C alteration of acetylcholine sensitivity in the human receptor was due to (1) adding low-sensitivity binding sites by changing stoichiometry or (2) converting existing high- to low-sensitivity binding sites with no stoichiometric alteration, we made concatenated α4β2 receptors in stoichiometry with only high-sensitivity sites. S108C substitutions decreased maximal current and number of immunolabeled receptors but no longer altered acetylcholine sensitivity.

CONCLUSIONS

The most parsimonious explanation of our current and previous work is that the S108C substitution renders the β2 subunit less efficient in assembling/trafficking, thereby decreasing the number of receptors in the plasma membrane. Thus, while β2 S108C protects dendrobatids against sequestered epibatidine, it incurs a potential physiological cost of disrupted α4β2 receptor function.

Insights Into the Differential Desensitization of α4β2 Nicotinic Acetylcholine Receptor Isoforms Obtained With Positive Allosteric Modulation of Mutant Receptors

The development of highly efficacious positive allosteric modulators (PAMs) of α7 nicotinic acetylcholine receptors (nAChR) has proven useful in defining the ligand dependence of the conformational dynamics of α7 receptors. No such effective modulators are known to exist for the α4β2 nAChR of the brain, limiting our ability to understand the importance of desensitization for the activity profile of specific ligands. In this study, we used mutant β2 subunits that allowed the use of the α7 PAM 3a,4,5,9b-tetrahydro-4-(1-naphthalenyl)-3H-cyclopentan[c]quinoline-8-sulfonamide (TQS) to probe the desensitizing effects of nicotinic ligands on the two forms of α4β2 receptors; high sensitivity (HS) (two α4 and three β2 subunits) and low sensitivity (LS) (three α4 and two β2 subunits). A total of 28 different ligands of 8 different categories, based on activity and selectivity, were tested for their ability to induce TQS-sensitive desensitization of HS and LS α4β2 receptors. Results confirm that HS α4β2 receptor responses are strongly limited by desensitization, by at least an order of magnitude more so than the responses of LS receptors. The activation of α4β2 receptors by the smoking-cessation drugs cytisine and varenicline is strongly limited by desensitization, as is the activation of LS receptors by the HS-selective agonists 6-[5-[(2S)-2-Azetidinylmethoxy]-3-pyridinyl]-5-hexyn-1-ol dihydrochloride and 4-(5-ethoxy-3-pyridinyl)-N-methyl-(3E)-3-buten-1-amine difumarate. The evaluation of drugs previously identified as α7-selective agonists revealed varying patterns of α4β2 cross-desensitization that were predictive of the effects of these drugs on the activation of wild-type α4β2 receptors by acetylcholine, supporting the utility of TQS-sensitive receptors for the development of focused therapeutics. SIGNIFICANCE STATEMENT: To varying degrees, ligands regulate the balance of active and desensitized states of the two forms of the primary nAChR subtypes in brain. Using mutant beta subunits, an allosteric modulator can reverse ligand-induced desensitization, revealing the differential desensitization of the receptors by specific ligands. This study shows that drugs believed to be selective for therapeutic targets may cross-desensitize other targets and that, within a class of drugs, improved specificity can be achieved by using agents that reduce such cross-desensitization.

A potential cost of evolving epibatidine resistance in poison frogs

Background

Some poison arrow frogs sequester the toxin epibatidine as a defense against predators. We previously identified a single amino acid substitution (S108C) at a highly conserved site in a neuronal nicotinic acetylcholine receptor (nAChR) ß2 subunit that prevents epibatidine from binding to this receptor. When placed in a homologous mammalian nAChR this substitution minimized epibatidine binding but also perturbed acetylcholine binding, a clear cost. However, in the nAChRs of poison arrow frogs, this substitution appeared to have no detrimental effect on acetylcholine binding and, thus, appeared cost-free.

Results

The introduction of S108C into the α4β2 nAChRs of non-dendrobatid frogs also does not affect ACh sensitivity, when these receptors are expressed in Xenopus laevis oocytes. However, α4β2 nAChRs with C108 had a decreased magnitude of neurotransmitter-induced currents in all species tested ( Epipedobates anthonyi , non-dendrobatid frogs, as well as human), compared with α4β2 nAChRs with the conserved S108. Immunolabeling of frog or human α4β2 nAChRs in the plasma membrane using radiolabeled antibody against the β2 nAChR subunit shows that C108 significantly decreased the number of cell-surface α4β2 nAChRs, compared with S108.

Conclusions

While S108C protects these species against sequestered epibatidine, it incurs a potential physiological cost of disrupted α4β2 nAChR function. These results may explain the high conservation of a serine at this site in vertebrates, as well as provide an example of a tradeoff between beneficial and deleterious effects of an evolutionary change. They also provide important clues for future work on assembly and trafficking of this important neurotransmitter receptor.

Acute effects of the imidacloprid metabolite desnitro-imidacloprid on human nACh receptors relevant for neuronal signaling

Several neonicotinoids have recently been shown to activate the nicotinic acetylcholine receptor (nAChR) on human neurons. Moreover, imidacloprid (IMI) and other members of this pesticide family form a set of diverse metabolites within crops. Among these, desnitro-imidacloprid (DN-IMI) is of special toxicological interest, as there is evidence (i) for human dietary exposure to this metabolite, (ii) and that DN-IMI is a strong trigger of mammalian nicotinic responses. We set out here to quantify responses of human nAChRs to DN-IMI and an alternative metabolite, IMI-olefin. To evaluate toxicological hazards, these data were then compared to those of IMI and nicotine. Ca²⁺-imaging experiments on human neurons showed that DN-IMI exhibits an agonistic effect on nAChRs at sub-micromolar concentrations (equipotent with nicotine) while IMI-olefin activated the receptors less potently (in a similar range as IMI). Direct experimental data on the interaction with defined receptor subtypes were obtained by heterologous expression of various human nAChR subtypes in Xenopus laevis oocytes and measurement of the transmembrane currents evoked by exposure to putative ligands. DN-IMI acted on the physiologically important human nAChR subtypes α7, α3β4, and α4β2 (high-sensitivity variant) with similar potency as nicotine. IMI and IMI-olefin were confirmed as nAChR agonists, although with 2–3 orders of magnitude lower potency. Molecular docking studies, using receptor models for the α7 and α4β2 nAChR subtypes supported an activity of DN-IMI similar to that of nicotine. In summary, these data suggest that DN-IMI functionally affects human neurons similar to the well-established neurotoxicant nicotine by triggering α7 and several non-α7 nAChRs.

Addiction-related neuroadaptations following chronic nicotine exposure

The addiction-relevant molecular, cellular, and behavioral actions of nicotine are derived from its stimulatory effects on neuronal nicotinic acetylcholine receptors (nAChRs) in the central nervous system. nAChRs expressed by dopamine-containing neurons in the ventral midbrain, most notably in the ventral tegmental area (VTA), contribute to the reward-enhancing properties of nicotine that motivate the use of tobacco products. nAChRs are also expressed by neurons in brain circuits that regulate aversion. In particular, nAChRs expressed by neurons in the medial habenula (mHb) and the interpeduncular nucleus (IPn) to which the mHb almost exclusively projects regulate the "set-point" for nicotine aversion and control nicotine intake. Different nAChR subtypes are expressed in brain reward and aversion circuits and nicotine intake is titrated to maximally engage reward-enhancing nAChRs while minimizing the recruitment of aversion-promoting nAChRs. With repeated exposure to nicotine, reward- and aversion-related nAChRs and the brain circuits in which they are expressed undergo adaptations that influence whether tobacco use will transition from occasional to habitual. Genetic variation that influences the sensitivity of addiction-relevant brain circuits to the actions of nicotine also influence the propensity to develop habitual tobacco use. Here, we review some of the key advances in our understanding of the mechanisms by which nicotine acts on brain reward and aversion circuits and the adaptations that occur in these circuits that may drive addiction to nicotine-containing tobacco products.

Functional alterations by a subgroup of neonicotinoid pesticides in human dopaminergic neurons

Neonicotinoid pesticides, originally developed to target the insect nervous system, have been reported to interact with human receptors and to activate rodent neurons. Therefore, we evaluated in how far these compounds may trigger signaling in human neurons, and thus, affect the human adult or developing nervous system. We used SH-SY5Y neuroblastoma cells as established model of nicotinic acetylcholine receptor (nAChR) signaling. In parallel, we profiled dopaminergic neurons, generated from LUHMES neuronal precursor cells, as novel system to study nAChR activation in human post-mitotic neurons. Changes of the free intracellular Ca²⁺ concentration ([Ca²⁺]_i) were used as readout, and key findings were confirmed by patch clamp recordings. Nicotine triggered typical neuronal signaling responses that were blocked by antagonists, such as tubocurarine and mecamylamine. Pharmacological approaches suggested a functional expression of α7 and non-α7 nAChRs on LUHMES cells. In this novel test system, the neonicotinoids acetamiprid, imidacloprid, clothianidin and thiacloprid, but not thiamethoxam and dinotefuran, triggered [Ca²⁺]_i signaling at 10-100 µM. Strong synergy of the active neonicotinoids (at low micromolar concentrations) with the α7 nAChR-positive allosteric modulator PNU-120596 was observed in LUHMES and SH-SY5Y cells, and specific antagonists fully inhibited such signaling. To provide a third line of evidence for neonicotinoid signaling via nAChR, we studied cross-desensitization: pretreatment of LUHMES and SH-SY5Y cells with active neonicotinoids (at 1-10 µM) blunted the signaling response of nicotine. The pesticides (at 3-30 µM) also blunted the response to the non-α7 agonist ABT 594 in LUHMES cells. These data show that human neuronal cells are functionally affected by low micromolar concentrations of several neonicotinoids. An effect of such signals on nervous system development is a toxicological concern.

Sleep-related hypermotor epilepsy associated mutations uncover important kinetic roles of α4β2- nicotinic acetylcholine receptor intracellular structures

Sleep-related hypermotor epilepsy (SHE) is a group of seizure disorders prominently associated with mutations in nicotinic acetylcholine receptors (nAChR). The most prevalent central nervous system nAChR subtype contains α4 and β2 subunits, in two ratios. (α4β2)₂β2-nAChR have high agonist sensitivity (HS-isoform), whereas (α4β2)₂α4-nAChR agonist responses exhibit a small high-sensitivity, and a predominant low-sensitivity, phase of function (LS-isoform). Multiple non-synonymous mutations in the second and third transmembrane domains of α4 and β2 subunits are associated with SHE. We recently demonstrated that two additional, SHE-associated, missense mutations in the major cytoplasmic loops of these subunits [α4(R336H) and β2(V337G)] cause increased macroscopic function-per receptor. Here, we use single-channel patch-clamp electrophysiology to show that these mutations influence single-channel amplitudes and open- and closed-state kinetics. Pure populations of HS- or LS-isoform α4β2-nAChR were expressed by injecting either 1:10 or 30:1 α4:β2 cRNA ratios, respectively, into Xenopus laevis oocytes. Functional properties of the resulting mutant α4β2-nAChR isoforms were compared to their wildtype counterparts. α4(R336H) subunit incorporation minimally affected single-channel amplitudes, whereas β2(V337G) subunit incorporation reduced them significantly in both isoforms. However, for both mutant subunits, increased function-per-receptor was predominantly caused by altered single channel kinetics. The α4(R336H) mutation primarily destabilizes desensitized states between openings. By contrast, the β2(V337G) mutation principally stabilizes receptor open states. The use of naturally-occurring and physiologically-impactful mutations has allowed us to define valuable new insights regarding the functional roles of nAChR intracellular domains. Further mechanistic context is provided by intracellular-domain structures recently published for other members of the Cys-loop receptor superfamily (α3β4-nAChR and 5-HT_3AR).

Concatemers to re-investigate the role of α5 in α4β2 nicotinic receptors

Nicotinic acetylcholine receptors (nAChRs) are pentameric ion channels expressed in the central nervous systems. nAChRs containing the α4, β2 and α5 subunits are specifically involved in addictive processes, but their functional architecture is poorly understood due to the intricacy of assembly of these subunits. Here we constrained the subunit assembly by designing fully concatenated human α4β2 and α4β2α5 receptors and characterized their properties by two-electrodes voltage-clamp electrophysiology in Xenopus oocytes. We found that α5-containing nAChRs are irreversibly blocked by methanethiosulfonate (MTS) reagents through a covalent reaction with a cysteine present only in α5. MTS-block experiments establish that the concatemers are expressed in intact form at the oocyte surface, but that reconstitution of nAChRs from loose subunits show inefficient and highly variable assembly of α5 with α4 and β2. Mutational analysis shows that the concatemers assemble both in clockwise and anticlockwise orientations, and that α5 does not contribute to ACh binding from its principal (+) site. Reinvestigation of suspected α5-ligands such as galantamine show no specific effect on α5-containing concatemers. Analysis of the α5-D398N mutation that is linked to smoking and lung cancer shows no significant effect on the electrophysiological function, suggesting that its effect might arise from alteration of other cellular processes. The concatemeric strategy provides a well-characterized platform for mechanistic analysis and screening of human α5-specific ligands.

Implications of Oligomeric Amyloid-Beta (oAβ42) Signaling through α7β2-Nicotinic Acetylcholine Receptors (nAChRs) on Basal Forebrain Cholinergic Neuronal Intrinsic Excitability and Cognitive Decline

Neuronal and network-level hyperexcitability is commonly associated with increased levels of amyloid-β (Aβ) and contribute to cognitive deficits associated with Alzheimer's disease (AD). However, the mechanistic complexity underlying the selective loss of basal forebrain cholinergic neurons (BFCNs), a well-recognized characteristic of AD, remains poorly understood. In this study, we tested the hypothesis that the oligomeric form of amyloid-β (oAβ₄₂), interacting with α7-containing nicotinic acetylcholine receptor (nAChR) subtypes, leads to subnucleus-specific alterations in BFCN excitability and impaired cognition. We used single-channel electrophysiology to show that oAβ₄₂ activates both homomeric α7- and heteromeric α7β2-nAChR subtypes while preferentially enhancing α7β2-nAChR open-dwell times. Organotypic slice cultures were prepared from male and female ChAT-EGFP mice, and current-clamp recordings obtained from BFCNs chronically exposed to pathophysiologically relevant level of oAβ₄₂ showed enhanced neuronal intrinsic excitability and action potential firing rates. These resulted from a reduction in action potential afterhyperpolarization and alterations in the maximal rates of voltage change during spike depolarization and repolarization. These effects were observed in BFCNs from the medial septum diagonal band and horizontal diagonal band, but not the nucleus basalis. Last, aged male and female APP/PS1 transgenic mice, genetically null for the β2 nAChR subunit gene, showed improved spatial reference memory compared with APP/PS1 aged-matched littermates. Combined, these data provide a molecular mechanism supporting a role for α7β2-nAChR in mediating the effects of oAβ₄₂ on excitability of specific populations of cholinergic neurons and provide a framework for understanding the role of α7β2-nAChR in oAβ₄₂-induced cognitive decline.

Advances in the In vitro and In vivo pharmacology of Alpha4beta2 nicotinic receptor positive allosteric modulators

Receptors containing α4 and β2 subunits are a major neuronal nicotinic acetylcholine receptor (nAChR) subtype in the brain. This receptor plays a critical role in nicotine addiction, with potential smoking cessation therapeutics producing modulation of α4β2 nAChR. In addition, compounds that act as agonists at α4β2 nAChR may be useful for the treatment of pathological pain. Further, as the α4β2 nAChR has been implicated in cognition, therapeutics that act as α4β2 nAChR agonists are also being examined as treatments for cognitive disorders and neurological diseases that impact cognitive function, such as Alzheimer's disease and schizophrenia. This review will cover the molecular in vitro evidence that allosteric modulators of the α4β2 neuronal nAChR provide several advantages over traditional α4β2 nAChR orthosteric ligands. Specifically, we explore the concept that nAChR allosteric modulators allow for greater pharmacological selectivity, while minimizing potential deleterious off-target effects. Further, here we discuss the development and preclinical in vivo behavioral assessment of allosteric modulators at the α4β2 neuronal nAChR as therapeutics for smoking cessation, pathological pain, as well as cognitive disorders and neurological diseases that impact cognitive function. This article is part of the special issue on 'Contemporary Advances in Nicotine Neuropharmacology'.

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.

Potentiation of a neuronal nicotinic receptor via pseudo-agonist site

Neuronal nicotinic receptors containing α4 and β2 subunits assemble in two pentameric stoichiometries, (α4)3(β2)2 and (α4)2(β2)3, each with distinct pharmacological signatures; (α4)3(β2)2 receptors are strongly potentiated by the drug NS9283, whereas (α4)2(β2)3 receptors are unaffected. Despite this stoichiometry-selective pharmacology, the molecular identity of the target for NS9283 remains elusive. Here, studying (α4)3(β2)2 receptors, we show that mutations at either the principal face of the β2 subunit or the complementary face of the α4 subunit prevent NS9283 potentiation of ACh-elicited single-channel currents, suggesting the drug targets the β2-α4 pseudo-agonist sites, the α4-α4 agonist site, or both sites. To distinguish among these possibilities, we generated concatemeric receptors with mutations at specified subunit interfaces, and monitored the ability of NS9283 to potentiate ACh-elicited single-channel currents. We find that a mutation at the principal face of the β2 subunit at either β2-α4 pseudo-agonist site suppresses potentiation, whereas mutation at the complementary face of the α4 subunit at the α4-α4 agonist site allows a significant potentiation. Thus, monitoring potentiation of single concatemeric receptor channels reveals that the β2-α4 pseudo-agonist sites are required for stoichiometry-selective drug action. Together with the recently determined structure of the (α4)3(β2)2 receptor, the findings have implications for structure-guided drug design.

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.

The nicotinic receptor drug sazetidine-A reduces alcohol consumption in mice without affecting concurrent nicotine consumption

Alcohol and nicotine addiction are frequently co-morbid. The nicotinic acetylcholine receptors (nAChRs) are critical for both alcohol and nicotine addiction mechanisms, since nAChR drugs that reduce nicotine consumption have been shown to also reduce alcohol consumption. Sazetidine-A, a pre-clinical nAChR drug with agonist and desensitizing effects at α4β2 and α7 nAChRs, has been reported to reduce alcohol consumption and nicotine self-administration in rats when administered at high doses. However, this effect has not been replicated in mice. In this study, we examined the effect of sazetidine-A on alcohol and nicotine consumption in male and female mice utilizing voluntary oral consumption procedures previously developed in our lab. We found that sazetidine-A (1 mg/kg, i.p) reduced overnight alcohol consumption, but did not affect nicotine consumption when presented either alone or concurrently with alcohol. Sazetidine-A did not reduce water or saccharin consumption at any dose tested. In a chronic co-consumption experiment in which either alcohol or nicotine was re-introduced after one week of forced abstinence, sazetidine-A attenuated post-abstinence consumption of alcohol but not nicotine. Sazetidine-A also significantly reduced alcohol consumption in an acute, binge drinking-in-the-dark procedure. Finally, we tested the effect of sazetidine-A on alcohol withdrawal, and found that sazetidine-A significantly reduced handling-induced convulsions during alcohol withdrawal. Collectively, these data suggest a novel role for the nAChR targets of sazetidine-A in specifically mediating alcohol consumption, separate from the involvement of nAChRs in mediating nicotine consumption. Delineation of this pathway may provide insight into novel therapies for the treatment of alcohol use disorders.

Native System and Cultured Cell Electrophysiology for Investigating Anesthetic Mechanisms

Anesthetic agents interact with a variety of ion channels and membrane-bound receptors, often at agent-specific binding sites of a single protein. These molecular-level interactions are ultimately responsible for producing the clinically anesthetized state. Between these two scales of effect, anesthetic agents can be studied in terms of how they impact the physiology of neuronal circuits, individual neurons, and cells expressing individual receptor types. The acutely dissected hippocampal slice is one of the most extensively studied and characterized preparations of intact neural tissue and serves as a highly useful experimental model system to test hypotheses of anesthetic mechanisms. Specific agent-receptor interactions and their effect on excitable membranes can further be defined with molecular precision in cell-based expression systems. We highlight several approaches in these respective systems that we have used and that also have been used by many investigators worldwide. We emphasize economy and quality control, to allow an experimenter to carry out these types of studies in a rigorous and efficient manner.

Cognitive Effects of Nicotine: Recent Progress

BACKGROUND

Cigarette smoking is the main cause of preventable death in developed countries. While the direct positive behavioral reinforcing effect of nicotine has historically been considered the primary mechanism driving the development of TUD, accumulating contemporary research suggests that the cognitive-enhancing effects of nicotine may also significantly contribute to the initiation and maintenance of TUD, especially in individuals with pre-existing cognitive deficits.

METHODS

We provide a selective overview of recent advances in understanding nicotine's effects on cognitive function, a discussion of the role of cognitive function in vulnerability to TUD, followed by an overview of the neurobiological mechanisms underlying the cognitive effects of nicotine.

RESULTS

Preclinical models and human studies have demonstrated that nicotine has cognitiveenhancing effects. Attention, working memory, fine motor skills and episodic memory functions are particularly sensitive to nicotine's effects. Recent studies have demonstrated that the α4, β2, and α7 subunits of the nicotinic acetylcholine receptor (nAChR) participate in the cognitive-enhancing effects of nicotine. Imaging studies have been instrumental in identifying brain regions where nicotine is active, and research on the dynamics of large-scale networks after activation by, or withdrawal from, nicotine hold promise for improved understanding of the complex actions of nicotine on human cognition.

CONCLUSION

Because poor cognitive performance at baseline predicts relapse among smokers who are attempting to quit smoking, studies examining the potential efficacy of cognitive-enhancement as strategy for the treatment of TUD may lead to the development of more efficacious interventions.

Organelle-specific single-molecule imaging of α4β2 nicotinic receptors reveals the effect of nicotine on receptor assembly and cell-surface trafficking

Nicotinic acetylcholine receptors (nAChRs) assemble in the endoplasmic reticulum (ER) and traffic to the cell surface as pentamers composed of α and β subunits. Many nAChR subtypes can assemble with varying subunit ratios, giving rise to multiple stoichiometries exhibiting different subcellular localization and functional properties. In addition to the endogenous neurotransmitter acetylcholine, nicotine also binds and activates nAChRs and influences their trafficking and expression on the cell surface. Currently, no available technique can specifically elucidate the stoichiometry of nAChRs in the ER versus those in the plasma membrane. Here, we report a method involving single-molecule fluorescence measurements to determine the structural properties of these membrane proteins after isolation in nanoscale vesicles derived from specific organelles. These cell-derived nanovesicles allowed us to separate single membrane receptors while maintaining them in their physiological environment. Sorting the vesicles according to the organelle of origin enabled us to determine localized differences in receptor structural properties, structural influence on transport between organelles, and changes in receptor assembly within intracellular organelles. These organelle-specific nanovesicles revealed that one structural isoform of the α4β2 nAChR was preferentially trafficked to the cell surface. Moreover, nicotine altered nAChR assembly in the ER, resulting in increased production of the receptor isoform that traffics more efficiently to the cell surface. We conclude that the combined effects of the increased assembly of one nAChR stoichiometry and its preferential trafficking likely drive the up-regulation of nAChRs on the cell surface upon nicotine exposure.

Nicotinic acetylcholine receptors in neuropathic and inflammatory pain

Nicotinic acetylcholine receptors (nAChRs) are actively being investigated as therapeutic targets for the treatment of pain and inflammation, but despite more than 30 years of research, there are currently no FDA-approved analgesics that are specific for these receptors. Much of the initial research effort focused on the α4β2 nAChR subtype, but more recently, additional subtypes have been identified as promising new leads and include α6β4, α7, and α9-containing nAChRs. This Review will focus on the distribution of these nAChRs in the cell types involved in neuropathic pain and inflammation and the activity of currently available nicotinic ligands.

The (α4)3(β2)2 Stoichiometry of the Nicotinic Acetylcholine Receptor Predominates in the Rat Motor Cortex

The α4β2 nicotinic acetylcholine receptor (nAChR) is important in central nervous system physiology and in mediating several of the pharmacological effects of nicotine on cognition, attention, and affective states. It is also the likely receptor that mediates nicotine addiction. This receptor assembles in two distinct stoichiometries: (α4)₂(β2)₃ and (α4)₃(β2)₂, which are referred to as high-sensitivity (HS) and low-sensitivity (LS) nAChRs, respectively, based on a difference in the potency of acetylcholine to activate them. The physiologic and pharmacological differences between these two receptor subtypes have been described in heterologous expression systems. However, the presence of each stoichiometry in native tissue currently remains unknown. In this study, different ratios of rat α4 and β2 subunit cDNA were transfected into human embryonic kidney 293 cells to create a novel model system of HS and LS α4β2 nAChRs expressed in a mammalian cell line. The HS and LS nAChRs were characterized through pharmacological and biochemical methods. Isolation of surface proteins revealed higher amounts of α4 or β2 subunits in the LS or HS nAChR populations, respectively. In addition, sazetidine-A displayed different efficacies in activating these two receptor stoichiometries. Using this model system, a neurophysiological "two-concentration" acetylcholine or carbachol paradigm was developed and validated to determine α4/β2 subunit stoichiometry. This paradigm was then used in layers I-IV of slices of the rat motor cortex to determine the percent contribution of HS and LS α4β2 receptors in this brain region. We report that the majority of α4β2 nAChRs in this brain region possess a stoichiometry of the (α4)₃(β2)₂ LS subtype.

Unraveling amino acid residues critical for allosteric potentiation of (α4)3(β2)2-type nicotinic acetylcholine receptor responses

Neuronal nicotinic acetylcholine receptors (nAChRs) are promising drug targets to manage several neurological disorders and nicotine addiction. Growing evidence indicates that positive allosteric modulators of nAChRs improve pharmacological specificity by binding to unique sites present only in a subpopulation of nAChRs. Furthermore, nAChR positive allosteric modulators such as NS9283 and CMPI have been shown to potentiate responses of (α4)3(β2)2 but not (α4)2(β2)3 nAChR isoforms. This selective potentiation underlines that the α4:α4 interface, which is present only in the (α4)3(β2)2 nAChR, is an important and promising drug target. In this report we used site-directed mutagenesis to substitute specific amino acid residues and computational analyses to elucidate CMPI's binding mode at the α4:α4 subunit extracellular interface and identified a unique set of amino acid residues that determined its affinity. We found that amino acid residues α4Gly-41, α4Lys-64, and α4Thr-66 were critical for (α4)3(β2)2 nAChR potentiation by CMPI, but not by NS9283, whereas amino acid substitution at α4His-116, a known determinant of NS9283 and of agonist binding at the α4:α4 subunit interface, did not reduce CMPI potentiation. In contrast, substitutions at α4Gln-124 and α4Thr-126 reduced potentiation by CMPI and NS9283, indicating that their binding sites partially overlap. These results delineate the role of amino acid residues contributing to the α4:α4 subunit extracellular interface in nAChR potentiation. These findings also provide structural information that will facilitate the structure-based design of novel therapeutics that target selectively the (α4)3(β2)2 nAChR.

Orthosteric and allosteric potentiation of heteromeric neuronal nicotinic acetylcholine receptors

Heteromeric nicotinic ACh receptors (nAChRs) were thought to have two orthodox agonist-binding sites at two α/β subunit interfaces. Highly selective ligands are hard to develop by targeting orthodox agonist sites because of high sequence similarity of this binding pocket among different subunits. Recently, unorthodox ACh-binding sites have been discovered at some α/α and β/α subunit interfaces, such as α4/α4, α5/α4 and β3/α4. Targeting unorthodox sites may yield subtype-selective ligands, such as those for (α4β2)₂ α5, (α4β2)₂ β3 and (α6β2)₂ β3 nAChRs. The unorthodox sites have unique pharmacology. Agonist binding at one unorthodox site is not sufficient to activate nAChRs, but it increases activation from the orthodox sites. NS9283, a selective agonist for the unorthodox α4/α4 site, was initially thought to be a positive allosteric modulator (PAM). NS9283 activates nAChRs with three engineered α4/α4 sites. PAMs, on the other hand, act at allosteric sites where ACh cannot bind. Known PAM sites include the ACh-homologous non-canonical site (e.g. morantel at β/α), the C-terminus (e.g. Br-PBTC and 17β-estradiol), a transmembrane domain (e.g. LY2087101) or extracellular and transmembrane domain interfaces (e.g. NS206). Some of these PAMs, such as Br-PBTC and 17β-estradiol, require only one subunit to potentiate activation of nAChRs. In this review, we will discuss differences between activation from orthosteric and allosteric sites, their selective ligands and clinical implications. These studies have advanced understanding of the structure, assembly and pharmacology of heteromeric neuronal nAChRs.

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.

Isoform-specific mechanisms of α3β4*-nicotinic acetylcholine receptor modulation by the prototoxin lynx1

This study investigates-for the first time to our knowledge-the existence and mechanisms of functional interactions between the endogenous mammalian prototoxin, lynx1, and α3- and β4-subunit-containing human nicotinic acetylcholine receptors (α3β4*-nAChRs). Concatenated gene constructs were used to express precisely defined α3β4*-nAChR isoforms (α3β4)₂β4-, (α3β4)₂α3-, (α3β4)₂α5(398D)-, and (α3β4)₂α5(398N)-nAChR in Xenopus oocytes. In the presence or absence of lynx1, α3β4*-nAChR agonist responses were recorded by using 2-electrode voltage clamp and single-channel electrophysiology, whereas radioimmunolabeling measured cell-surface expression. Lynx1 reduced (α3β4)₂β4-nAChR function principally by lowering cell-surface expression, whereas single-channel effects were primarily responsible for reducing (α3β4)₂α3-nAChR function [decreased unitary conductance (≥50%), altered burst proportions (3-fold reduction in the proportion of long bursts), and enhanced closed dwell times (3- to 6-fold increase)]. Alterations in both cell-surface expression and single-channel properties accounted for the reduction in (α3β4)₂α5-nAChR function that was mediated by lynx1. No effects were observed when α3β4*-nAChRs were coexpressed with mutated lynx1 (control). Lynx1 is expressed in the habenulopeduncular tract, where α3β4*-α5*-nAChR subtypes are critical contributors to the balance between nicotine aversion and reward. This gives our findings a high likelihood of physiologic significance. The exquisite isoform selectivity of lynx1 interactions provides new insights into the mechanisms and allosteric sites [α(-)-interface containing] by which prototoxins can modulate nAChR function.-George, A. A., Bloy, A., Miwa, J. M., Lindstrom, J. M., Lukas, R. J., Whiteaker, P. Isoform-specific mechanisms of α3β4*-nicotinic acetylcholine receptor modulation by the prototoxin lynx1.

Unorthodox Acetylcholine Binding Sites Formed by α5 and β3 Accessory Subunits in α4β2* Nicotinic Acetylcholine Receptors

All nicotinic acetylcholine receptors (nAChRs) evolved from homomeric nAChRs in which all five subunits are involved in forming acetylcholine (ACh) binding sites at their interfaces. Heteromeric α4β2* nAChRs typically have two ACh binding sites at α4/β2 interfaces and a fifth accessory subunit surrounding the central cation channel. β2 accessory subunits do not form ACh binding sites, but α4 accessory subunits do at the α4/α4 interface in (α4β2)₂α4 nAChRs. α5 and β3 are closely related subunits that had been thought to act only as accessory subunits and not take part in forming ACh binding sites. The effect of agonists at various subunit interfaces was determined by blocking homologous sites at these interfaces using the thioreactive agent 2-((trimethylammonium)ethyl) methanethiosulfonate (MTSET). We found that α5/α4 and β3/α4 interfaces formed ACh binding sites in (α4β2)₂α5 and (α4β2)₂β3 nAChRs. The α4/α5 interface in (β2α4)₂α5 nAChRs also formed an ACh binding site. Blocking of these sites with MTSET reduced the maximal ACh evoked responses of these nAChRs by 30-50%. However, site-selective agonists NS9283 (for the α4/α4 site) and sazetidine-A (for the α4/β2 site) did not act on the ACh sites formed by the α5/α4 or β3/α4 interfaces. This suggests that unorthodox sites formed by α5 and β3 subunits have unique ligand selectivity. Agonists or antagonists for these unorthodox sites might be selective and effective drugs for modulating nAChR function to treat nicotine addiction and other disorders.

Crystal structure of a human neuronal nAChR extracellular domain in pentameric assembly: Ligand-bound α2 homopentamer

In this study we report the X-ray crystal structure of the extracellular domain (ECD) of the human neuronal α2 nicotinic acetylcholine receptor (nAChR) subunit in complex with the agonist epibatidine at 3.2 Å. Interestingly, α2 was crystallized as a pentamer, revealing the intersubunit interactions in a wild type neuronal nAChR ECD and the full ligand binding pocket conferred by two adjacent α subunits. The pentameric assembly presents the conserved structural scaffold observed in homologous proteins, as well as distinctive features, providing unique structural information of the binding site between principal and complementary faces. Structure-guided mutagenesis and electrophysiological data confirmed the presence of the α2(+)/α2(-) binding site on the heteromeric low sensitivity α2β2 nAChR and validated the functional importance of specific residues in α2 and β2 nAChR subunits. Given the pathological importance of the α2 nAChR subunit and the high sequence identity with α4 (78%) and other neuronal nAChR subunits, our findings offer valuable information for modeling several nAChRs and ultimately for structure-based design of subtype specific drugs against the nAChR associated diseases.

Synthesis and Behavioral Studies of Chiral Cyclopropanes as Selective α4β2-Nicotinic Acetylcholine Receptor Partial Agonists Exhibiting an Antidepressant Profile. Part III

We report the synthesis and biological characterization of novel derivatives of 3-[(1-methyl-2(S)-pyrrolidinyl)methoxy]-5-cyclopropylpyridine (4a-f and 5) as potent and highly selective α4β2-nicotinic acetylcholine receptor (nAChR) full or partial agonists. A systematic structure-activity study was carried out on the previously described compound 3b, particularly concerning its (2-methoxyethyl)cyclopropyl side-chain, in an effort to improve its metabolic stability while maintaining receptor selectivity. Compound 4d exhibited very similar subnanomolar binding affinity for α4β2- and α4β2*-nAChRs compared to 3b, and it showed excellent potency in activating high-sensitivity (HS) α4β2-nAChRs with an EC50 value of 8.2 nM. Testing of 4d in the SmartCube assay revealed that the compound has a combined antidepressant plus antipsychotic signature. In the forced swim test at a dose of 30 mg/kg given intraperitoneally, 4d was found to be as efficacious as sertraline, thus providing evidence of the potential use of the compound as an antidepressant. Additional promise for use of 4d in humans comes from pharmacokinetic studies in mice indicating brain penetration, and additional assays show compound stability in the presence of human microsomes and hepatocytes. Thus, 4d has a very favorable preclinical drug profile.

Analysis of neuronal nicotinic acetylcholine receptor α4β2 activation at the single-channel level

The neuronal nicotinic acetylcholine receptor α4β2 forms pentameric proteins with two alternate stoichiometries. The high-sensitivity receptor is related to (α4)2(β2)3 stoichiometry while the low-sensitivity receptor to (α4)3(β2)2 stoichiometry. Both subtypes share two binding sites at the α4((+))/β2((-)) interface with high affinity for agonists. (α4)3(β2)2 has an additional binding site at the α4((+))/α4((-)) interface with low affinity for agonists. We investigated activation kinetics of both receptor subtypes by patch-clamp recordings of single-channel activity in the presence of several concentrations of acetylcholine (0.5 to 300μM). We used kinetic software to fit these data with kinetic models. We found that the high-sensitivity subtype correlates with the low-conductance channel (g-70=29pS) and does not activate with high efficacy. On the contrary, the low-sensitivity subtype correlated with a high-conductance channel (g-70=44pS) and exhibited higher activation efficacy. Opening events of individual nAChRs at high agonist concentrations occurred in clusters, which allowed us to determine kinetic constants for the activation of the triliganded receptor. Our kinetic modeling identified an intermediate state, between resting and open conformation of the receptor. Binding of the third molecule increases the efficacy of receptor activation by favoring the transition between resting and intermediate state around 18 times. The low rate for this transition in the diliganded receptor explains the action of acetylcholine as partial agonist when it binds to the high-affinity sites. The presence of the third binding site emerges as a potent modulator of nicotinic receptor α4β2 activation which may display different functions depending on agonist concentration.

Heteromeric α7β2 Nicotinic Acetylcholine Receptors in the Brain

The α7 nicotinic acetylcholine receptor (α7 nAChR) is highly expressed in the brain, where it maintains various neuronal functions including (but not limited to) learning and memory. In addition, the protein expression levels of α7 nAChRs are altered in various brain disorders. The classic rule governing α7 nAChR assembly in the mammalian brain was that it was assembled from five α7 subunits to form a homomeric receptor pentamer. However, emerging evidence demonstrates the presence of heteromeric α7 nAChRs in heterologously expressed systems and naturally in brain neurons, where α7 subunits are co-assembled with β2 subunits to form a novel type of α7β2 nAChR. Interestingly, the α7β2 nAChR exhibits distinctive function and pharmacology from traditional homomeric α7 nAChRs. We review recent advances in probing the distribution, function, pharmacology, pathophysiology, and stoichiometry of the heteromeric α7β2 nAChR, which have provided new insights into the understanding of a novel target of cholinergic signaling.

Effects of varenicline on alpha4-containing nicotinic acetylcholine receptor expression and cognitive performance in mice

Nicotinic acetylcholine receptor (nAChR) subtypes containing the α4 subunit, particularly α4β2 nAChRs, play an important role in cognitive functioning. The impact of the smoking cessation aid varenicline, a selective partial α4β2 nAChR agonist, on (1) changes of central protein and mRNA expression of this receptor and (2) on memory deficits in a mouse model of cognitive impairment was investigated. Protein and mRNA expression of both the α4 and β2 receptor subunits in mouse brain endothelial and hippocampal cells as well as hippocampus and neocortex tissues were determined by western blot and realtime PCR, respectively. The β2 antibody showed low specificity, though. Tissues were examined following a 2-week oral treatment with various doses of varenicline (0.01, 0.1, 1, 3 mg/kg/day) or vehicle. In addition, episodic memory of mice was assessed following this treatment with an object recognition task using (1) normal mice and (2) animals with anticholinergic-induced memory impairment (i.p. injection of 0.5 mg/kg scopolamine). Varenicline dose-dependently increased protein expression of both the α4 and β2 subunit in cell cultures and brain tissues, respectively, but had no effect on mRNA expression of both subunits. Scopolamine injection induced a significant reduction of object memory in vehicle-treated mice. By contrast, cognitive performance was not altered by scopolamine in varenicline-treated mice. In conclusion, a 2-week oral treatment with varenicline prevented memory impairment in the scopolamine mouse model. In parallel, protein, but not mRNA expression was upregulated, suggesting a posttranscriptional mechanism. Our findings suggest a beneficial effect of varenicline on cognitive dysfunction.

Photolabeling a Nicotinic Acetylcholine Receptor (nAChR) with an (α4)3(β2)2 nAChR-Selective Positive Allosteric Modulator

Positive allosteric modulators (PAMs) of nicotinic acetylcholine (ACh) receptors (nAChRs) have potential clinical applications in the treatment of nicotine dependence and many neuropsychiatric conditions associated with decreased brain cholinergic activity, and 3-(2-chlorophenyl)-5-(5-methyl-1-(piperidin-4-yl)-1H-pyrrazol-4-yl)isoxazole (CMPI) has been identified as a PAM selective for neuronal nAChRs containing theα4 subunit. In this report, we compare CMPI interactions with low-sensitivity (α4)3(β2)2 and high-sensitivity (α4)2(β2)3 nAChRs, and with muscle-type nAChRs. In addition, we use the intrinsic reactivity of [(3)H]CMPI upon photolysis at 312 nm to identify its binding sites inTorpedonAChRs. Recording fromXenopusoocytes, we found that CMPI potentiated maximally the responses of (α4)3(β2)2nAChR to 10μM ACh (EC10) by 400% and with anEC50of ∼1µM. CMPI produced a left shift of the ACh concentration-response curve without altering ACh efficacy. In contrast, CMPI inhibited (∼35% at 10µM) ACh responses of (α4)2(β2)3nAChRs and fully inhibited human muscle andTorpedonAChRs with IC50values of ∼0.5µM. Upon irradiation at 312 nm, [(3)H]CMPI photoincorporated into eachTorpedo[(α1)2β1γδ] nAChR subunit. Sequencing of peptide fragments isolated from [(3)H]CMPI-photolabeled nAChR subunits established photolabeling of amino acids contributing to the ACh binding sites (αTyr(190),αTyr(198),γTrp(55),γTyr(111),γTyr(117),δTrp(57)) that was fully inhibitable by agonist and lower-efficiency, state-dependent [(3)H]CMPI photolabeling within the ion channel. Our results establish that CMPI is a potent potentiator of nAChRs containing anα4:α4 subunit interface, and that its intrinsic photoreactivy makes it of potential use to identify its binding sites in the (α4)3(β2)2nAChR.

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

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

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.

Functional Upregulation of α4* Nicotinic Acetylcholine Receptors in VTA GABAergic Neurons Increases Sensitivity to Nicotine Reward

Chronic nicotine exposure increases sensitivity to nicotine reward during a withdrawal period, which may facilitate relapse in abstinent smokers, yet the molecular neuroadaptation(s) that contribute to this phenomenon are unknown. Interestingly, chronic nicotine use induces functional upregulation of nicotinic acetylcholine receptors (nAChRs) in the mesocorticolimbic reward pathway potentially linking upregulation to increased drug sensitivity. In the ventral tegmental area (VTA), functional upregulation of nAChRs containing the α4 subunit (α4* nAChRs) is restricted to GABAergic neurons. To test the hypothesis that increased functional expression of α4* nAChRs in these neurons modulates nicotine reward behaviors, we engineered a Cre recombinase-dependent gene expression system to selectively express α4 nAChR subunits harboring a "gain-of-function" mutation [a leucine mutated to a serine residue at the 9' position (Leu9'Ser)] in VTA GABAergic neurons of adult mice. In mice expressing Leu9'Ser α4 nAChR subunits in VTA GABAergic neurons (Gad2(VTA):Leu9'Ser mice), subreward threshold doses of nicotine were sufficient to selectively activate VTA GABAergic neurons and elicit acute hypolocomotion, with subsequent nicotine exposures eliciting tolerance to this effect, compared to control animals. In the conditioned place preference procedure, nicotine was sufficient to condition a significant place preference in Gad2(VTA):Leu9'Ser mice at low nicotine doses that failed to condition control animals. Together, these data indicate that functional upregulation of α4* nAChRs in VTA GABAergic neurons confers increased sensitivity to nicotine reward and points to nAChR subtypes specifically expressed in GABAergic VTA neurons as molecular targets for smoking cessation therapeutics.

Allosteric modulation of nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are receptors for the neurotransmitter acetylcholine and are members of the 'Cys-loop' family of pentameric ligand-gated ion channels (LGICs). Acetylcholine binds in the receptor extracellular domain at the interface between two subunits and research has identified a large number of nAChR-selective ligands, including agonists and competitive antagonists, that bind at the same site as acetylcholine (commonly referred to as the orthosteric binding site). In addition, more recent research has identified ligands that are able to modulate nAChR function by binding to sites that are distinct from the binding site for acetylcholine, including sites located in the transmembrane domain. These include positive allosteric modulators (PAMs), negative allosteric modulators (NAMs), silent allosteric modulators (SAMs) and compounds that are able to activate nAChRs via an allosteric binding site (allosteric agonists). Our aim in this article is to review important aspects of the pharmacological diversity of nAChR allosteric modulators and to describe recent evidence aimed at identifying binding sites for allosteric modulators on nAChRs.

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.

An Accessory Agonist Binding Site Promotes Activation of α4β2* Nicotinic Acetylcholine Receptors

Neuronal nicotinic acetylcholine receptors containing α4, β2, and sometimes other subunits (α4β2* nAChRs) regulate addictive and other behavioral effects of nicotine. These nAChRs exist in several stoichiometries, typically with two high affinity acetylcholine (ACh) binding sites at the interface of α4 and β2 subunits and a fifth accessory subunit. A third low affinity ACh binding site is formed when this accessory subunit is α4 but not if it is β2. Agonists selective for the accessory ACh site, such as 3-[3-(3-pyridyl)-1,2,4-oxadiazol-5-yl]benzonitrile (NS9283), cannot alone activate a nAChR but can facilitate more efficient activation in combination with agonists at the canonical α4β2 sites. We therefore suggest categorizing agonists according to their site selectivity. NS9283 binds to the accessory ACh binding site; thus it is termed an accessory site-selective agonist. We expressed (α4β2)2 concatamers in Xenopus oocytes with free accessory subunits to obtain defined nAChR stoichiometries and α4/accessory subunit interfaces. We show that α2, α3, α4, and α6 accessory subunits can form binding sites for ACh and NS9283 at interfaces with α4 subunits, but β2 and β4 accessory subunits cannot. To permit selective blockage of the accessory site, α4 threonine 126 located on the minus side of α4 that contributes to the accessory site, but not the α4β2 sites, was mutated to cysteine. Alkylation of this cysteine with a thioreactive reagent blocked activity of ACh and NS9283 at the accessory site. Accessory agonist binding sites are promising drug targets.

Roles of nicotinic acetylcholine receptor β subunit cytoplasmic loops in acute desensitization and single-channel features

To evaluate physiological roles of the large, second cytoplasmic loops (C2) situated between the M3 and M4 transmembrane domains of nicotinic acetylcholine receptor (nAChR) subunits. We have constructed chimeric β2 (β2χ) and β4 (β4χ) subunits in which the "nested" C2 domains (but not the "proximal" sequences of ∼14 residues immediately adjacent to the M3 or M4 domains) of these β subunits were replaced by the corresponding sequence from the serotonin 5-HT3A receptor subunit. We previously reported that heterologously expressed nAChR containing α4 and β2χ subunits displayed a faster whole-cell current decay in its agonist response compared to responses of all-wild-type α4β2-nAChR. This suggests an unexpected, functional role for the C2 domain of the β2 subunit in α4β2-nAChR acute desensitization. Here we report that there also is faster desensitization of α4β4χ-nAChR relative to α4β4-nAChR stably and heterologously expressed in the human SH-EP1 cell-line. In addition, cell-attached, single-channel recording shows that both acetylcholine-activated α4β2χ- and α4β4χ-nAChR have a significantly lower mean open probability, shorter mean open-time, and a longer mean closed-time than their fully wild-type counterparts while not having different conductance amplitudes. These findings reveal microscopic bases for the faster desensitization of α4(∗)-nAChR containing chimeric instead of wild-type β subunits. Our findings also remain consistent with novel and unexpected roles of β subunit-nested C2 domains in modulation of α4(∗)-nAChR function.

Inside-out neuropharmacology of nicotinic drugs

Upregulation of neuronal nicotinic acetylcholine receptors (AChRs) is a venerable result of chronic exposure to nicotine; but it is one of several consequences of pharmacological chaperoning by nicotine and by some other nicotinic ligands, especially agonists. Nicotinic ligands permeate through cell membranes, bind to immature AChR oligomers, elicit incompletely understood conformational reorganizations, increase the interaction between adjacent AChR subunits, and enhance the maturation process toward stable AChR pentamers. These changes and stabilizations in turn lead to increases in both anterograde and retrograde traffic within the early secretory pathway. In addition to the eventual upregulation of AChRs at the plasma membrane, other effects of pharmacological chaperoning include modifications to endoplasmic reticulum stress and to the unfolded protein response. Because these processes depend on pharmacological chaperoning within intracellular organelles, we group them as "inside-out pharmacology". This term contrasts with the better-known, acute, "outside-in" effects of activating and desensitizing plasma membrane AChRs. We review current knowledge concerning the mechanisms and consequences of inside-out pharmacology. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

Enantiopure Cyclopropane-Bearing Pyridyldiazabicyclo[3.3.0]octanes as Selective α4β2-nAChR Ligands

We report the synthesis and characterization of a series of enantiopure 5-cyclopropane-bearing pyridyldiazabicyclo[3.3.0]octanes that display low nanomolar binding affinities and act as functional agonists at α4β2-nicotinic acetylcholine receptor (nAChR) subtype. Structure-activity relationship studies revealed that incorporation of a cyclopropane-containing side chain at the 5-position of the pyridine ring provides ligands with improved subtype selectivity for nAChR β2 subunit-containing nAChR subtypes (β2*-nAChRs) over β4*-nAChRs compared to the parent compound 4. Compound 15 exhibited subnanomolar binding affinity for α4β2- and α4β2*-nAChRs with negligible interaction. Functional assays confirm selectivity for α4β2-nAChRs. Furthermore, using the SmartCube assay system, this ligand showed antidepressant, anxiolytic, and antipsychotic features, while mouse forced-swim assay further confirm the antidepressant-like property of 15.

Roles for N-terminal extracellular domains of nicotinic acetylcholine receptor (nAChR) β3 subunits in enhanced functional expression of mouse α6β2β3- and α6β4β3-nAChRs

Functional heterologous expression of naturally expressed mouse α6*-nicotinic acetylcholine receptors (mα6*-nAChRs; where "*" indicates the presence of additional subunits) has been difficult. Here we expressed and characterized wild-type (WT), gain-of-function, chimeric, or gain-of-function chimeric nAChR subunits, sometimes as hybrid nAChRs containing both human (h) and mouse (m) subunits, in Xenopus oocytes. Hybrid mα6mβ4hβ3- (∼ 5-8-fold) or WT mα6mβ4mβ3-nAChRs (∼ 2-fold) yielded higher function than mα6mβ4-nAChRs. Function was not detected when mα6 and mβ2 subunits were expressed together or in the additional presence of hβ3 or mβ3 subunits. However, function emerged upon expression of mα6mβ2mβ3(V9'S)-nAChRs containing β3 subunits having gain-of-function V9'S (valine to serine at the 9'-position) mutations in transmembrane domain II and was further elevated 9-fold when hβ3(V9'S) subunits were substituted for mβ3(V9'S) subunits. Studies involving WT or gain-of-function chimeric mouse/human β3 subunits narrowed the search for domains that influence functional expression of mα6*-nAChRs. Using hβ3 subunits as templates for site-directed mutagenesis studies, substitution with mβ3 subunit residues in extracellular N-terminal domain loops "C" (Glu(221) and Phe(223)), "E" (Ser(144) and Ser(148)), and "β2-β3" (Gln(94) and Glu(101)) increased function of mα6mβ2*- (∼ 2-3-fold) or mα6mβ4* (∼ 2-4-fold)-nAChRs. EC50 values for nicotine acting at mα6mβ4*-nAChR were unaffected by β3 subunit residue substitutions in loop C or E. Thus, amino acid residues located in primary (loop C) or complementary (loops β2-β3 and E) interfaces of β3 subunits are some of the molecular impediments for functional expression of mα6mβ2β3- or mα6mβ4β3-nAChRs.

The novel α7β2-nicotinic acetylcholine receptor subtype is expressed in mouse and human basal forebrain: biochemical and pharmacological characterization

We examined α7β2-nicotinic acetylcholine receptor (α7β2-nAChR) expression in mammalian brain and compared pharmacological profiles of homomeric α7-nAChRs and α7β2-nAChRs. α-Bungarotoxin affinity purification or immunoprecipitation with anti-α7 subunit antibodies (Abs) was used to isolate nAChRs containing α7 subunits from mouse or human brain samples. α7β2-nAChRs were detected in forebrain, but not other tested regions, from both species, based on Western blot analysis of isolates using β2 subunit-specific Abs. Ab specificity was confirmed in control studies using subunit-null mutant mice or cell lines heterologously expressing specific human nAChR subtypes and subunits. Functional expression in Xenopus oocytes of concatenated pentameric (α7)5-, (α7)4(β2)1-, and (α7)3(β2)2-nAChRs was confirmed using two-electrode voltage clamp recording of responses to nicotinic ligands. Importantly, pharmacological profiles were indistinguishable for concatenated (α7)5-nAChRs or for homomeric α7-nAChRs constituted from unlinked α7 subunits. Pharmacological profiles were similar for (α7)5-, (α7)4(β2)1-, and (α7)3(β2)2-nAChRs except for diminished efficacy of nicotine (normalized to acetylcholine efficacy) at α7β2- versus α7-nAChRs. This study represents the first direct confirmation of α7β2-nAChR expression in human and mouse forebrain, supporting previous mouse studies that suggested relevance of α7β2-nAChRs in Alzheimer disease etiopathogenesis. These data also indicate that α7β2-nAChR subunit isoforms with different α7/β2 subunit ratios have similar pharmacological profiles to each other and to α7 homopentameric nAChRs. This supports the hypothesis that α7β2-nAChR agonist activation predominantly or entirely reflects binding to α7/α7 subunit interface sites.

Selective ligand behaviors provide new insights into agonist activation of nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors are a diverse set of ion channels that are essential to everyday brain function. Contemporary research studies selective activation of individual subtypes of receptors, with the hope of increasing our understanding of behavioral responses and neurodegenerative diseases. Here, we aim to expand current binding models to help explain the specificity seen among three activators of α4β2 receptors: sazetidine-A, cytisine, and NS9283. Through mutational analysis, we can interchange the activation profiles of the stoichiometry-selective compounds sazetidine-A and cytisine. In addition, mutations render NS9283--currently identified as a positive allosteric modulator--into an agonist. These results lead to two conclusions: (1) occupation at each primary face of an α subunit is needed to activate the channel and (2) the complementary face of the adjacent subunit dictates the binding ability of the agonist.

Sazetidine-A Activates and Desensitizes Native α7 Nicotinic Acetylcholine Receptors

The aim of this study was to investigate the ability of sazetidine-A, a novel partial agonist at α4β2 nicotinic acetylcholine receptors (nAChRs), to affect the function of native α7 nAChRs in SH-SY5Y cells and primary cortical cultures. The α7-selective positive allosteric modulator PNU-120596 was used to reveal receptor activation, measured as an increase in intracellular calcium using fluorescent indicators. In the absence of PNU-120596, sazetidine-A elicited mecamylamine-sensitive increases in fluorescence in SH-SY5Y cells (EC₅₀ 4.2 µM) but no responses from primary cortical neurons. In the presence on PNU-120596, an additional response to sazetidine-A was observed in SH-SY5Y cells (EC₅₀ 0.4 µM) and robust responses were recorded in 14 % of cortical neurons. These PNU-120596-dependent responses were blocked by methyllycaconitine, consistent with the activation of α7 nAChRs. Preincubtion with sazetidine-A concentration-dependently attenuated subsequent responses to the α7-selective agonist PNU-282987 in SH-SY5Y cells (IC₅₀ 476 nM) and cortical cultures. These findings support the ability of sazetidine-A to interact with α7 nAChRs, which may contribute to sazetidine-A’s actions in complex physiological systems.

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

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