Nitrogen substitution modifies the activity of cytisine on neuronal nicotinic receptor subtypes

Cytisine and cytisine derivatives. More than smoking cessation aids

Cytisine, a natural bioactive compound that is mainly isolated from plants of the Leguminosae family (especially the seeds of Laburnum anagyroides), has been marketed in central and eastern Europe as an aid in the clinical management of smoking cessation for more than 50 years. Its main targets are neuronal nicotinic acetylcholine receptors (nAChRs), and pre-clinical studies have shown that its interactions with various nAChR subtypes located in different areas of the central and peripheral nervous systems are neuroprotective, have a wide range of biological effects on nicotine and alcohol addiction, regulate mood, food intake and motor activity, and influence the autonomic and cardiovascular systems. Its relatively rigid conformation makes it an attractive template for research of new derivatives. Recent studies of structurally modified cytisine have led to the development of new compounds and for some of them the biological activities are mediated by still unidentified targets other than nAChRs, whose mechanisms of action are still being investigated. The aim of this review is to describe and discuss: 1) the most recent pre-clinical results obtained with cytisine in the fields of neurological and non-neurological diseases; 2) the effects and possible mechanisms of action of the most recent cytisine derivatives; and 3) the main areas warranting further research.

Phytochemical Information and Biological Activities of Quinolizidine Alkaloids in Sophora: A Comprehensive Review

Quinolizidine alkaloids, a main form of alkaloids found in the genus Sophora, have been shown to have many pharmacological effects. This review aims to summarize the photochemical reports and biological activities of quinolizidine alkaloids in Sophora. The collected information suggested that a total of 99 quinolizidine alkaloids were isolated and detected from different parts of Sophora plants, represented by lupinine-type, cytisine-type, sparteine-type, and matrine-type. However, quality control needs to be monitored because it could provide basic information for the reasonable and efficient use of quinolizidine alkaloids as medicines and raw materials. The nonmedicinal parts may be promising to be used as a source of quinolizidine alkaloid raw materials and to reduce the waste of resources and environmental pollution. In addition, the diversity of chemical compounds based on the alkaloid scaffold to make a biological compound library needs to be extended, which may reduce toxicity and find new bioactivities of quinolizidine alkaloids. The bioactivities most reported are in the fields of antitumor activity along with the effects on the cardiovascular system. However, those studies rely on theoretical research, and novel drugs based on quinolizidine alkaloids are expected.

Critical needs in drug discovery for cessation of alcohol and nicotine polysubstance abuse

Polysubstance abuse of alcohol and nicotine has been overlooked in our understanding of the neurobiology of addiction and especially in the development of novel therapeutics for its treatment. Estimates show that as many as 92% of people with alcohol use disorders also smoke tobacco. The health risks associated with both excessive alcohol consumption and tobacco smoking create an urgent biomedical need for the discovery of effective cessation treatments, as opposed to current approaches that attempt to independently treat each abused agent. The lack of treatment approaches for alcohol and nicotine abuse/dependence mirrors a similar lack of research in the neurobiology of polysubstance abuse. This review discusses three critical needs in medications development for alcohol and nicotine co-abuse: (1) the need for a better understanding of the clinical condition (i.e. alcohol and nicotine polysubstance abuse), (2) the need to better understand how these drugs interact in order to identify new targets for therapeutic development and (3) the need for animal models that better mimic this human condition. Current and emerging treatments available for the cessation of each drug and their mechanisms of action are discussed within this context followed by what is known about the pharmacological interactions of alcohol and nicotine. Much has been and will continue to be gained from studying comorbid alcohol and nicotine exposure.

The cytisine derivatives, CC4 and CC26, reduce nicotine-induced conditioned place preference in zebrafish by acting on heteromeric neuronal nicotinic acetylcholine receptors

RATIONALE

Cigarette smoking is one of the most serious health problems worldwide and people trying to stop smoking have high rates of relapse. Zebrafish (Danio rerio), by combining pharmacological and behavioral assays, is a promising animal model for rapidly screening new compounds to induce smoking cessation.

OBJECTIVES

This study aims to identify possible acetylcholine nicotinic receptors (nAChRs) involved in mediating nicotine (NIC)-induced conditioned place preference (CPP) in zebrafish and investigate the effect of the CC4 and CC26 cytisine derivatives in reducing NIC-induced CPP.

METHODS

CPP was evaluated using a two-compartment chamber, and the zebrafish were given CC4 (0.001-5 mg/kg), CC26 (0.001-1 mg/kg), cytisine (0.1-2.5 mg/kg), and varenicline (1-10 mg/kg) alone or with NIC (0.001 mg/kg). Swimming activity was evaluated using a square observational chamber. The affinity of the nicotinic ligands for native zebrafish brain nAChRs was evaluated by binding studies using [(3)H]-Epibatidine (Epi) and [(125)I]-αBungarotoxin (αBgtx) radioligands, and their subtype specificity was determined by means of electrophysiological assay of oocyte-expressed α4β2 and α7 subtypes.

RESULTS

CC4 and CC26 induced CPP with an inverted U-shaped dose-response curve similar to that of NIC. However, when co-administered with NIC, they blocked its reinforcing or slightly aversive effect. Binding and electrophysiological studies showed that this effect was due to binding to high-affinity heteromeric but not α7-containing receptors.

CONCLUSIONS

We have further characterized CC4 and identified a new compound (CC26) that may be active in inducing smoking cessation. Zebrafish is a very useful model for screening new compounds that can affect the rewarding properties of NIC.

Role of neuronal nicotinic acetylcholine receptors (nAChRs) on learning and memory in zebrafish

RATIONALE

Neuronal nicotinic acetylcholine receptors (nAChRs) play a modulatory role in cognition, and zebrafish provide a preclinical model to study learning and memory.

OBJECTIVES

We investigated the effect of nicotine (NIC) and some new cytisine-derived partial agonists (CC4 and CC26) on spatial memory in zebrafish using a rapid assay on T-maze task. The role of α4/α6β2 and the α7 nAChRs in NIC-induced memory enhancement was evaluated using selective nAChR antagonists.

RESULTS

Low and high doses of NIC, cytisine (CYT), CC4 and CC26 respectively improved and worsened the mean running time, showing an inverted U dose-response function. The effective dose (ED50) (×10⁻⁵ mg/kg) was 0.4 for CC4, 4.5 for CYT, 140 for NIC and 200 for CC26. NIC-induced cognitive enhancement was reduced by the selective nAChR subtype antagonists: methyllycaconitine (MLA) for α7, α-conotoxin (MII) for α6β2, dihydro-β-erythroidine (DhβE) for α4β2, the nonselective antagonist mecamylamine (MEC) and the muscarinic antagonist scopolamine (SCOP), with DhβE being more active than MLA or MII. All the partial agonists blocked the cognitive enhancement. The improvement with the maximal active dose of each partial agonist was blocked by low doses of DhβE (0.001 mg/kg) and MII (0.01 mg/kg). MLA reduced the effects of CC26 and CC4 at doses of 0.01 and 1 mg/kg, respectively, but did not antagonize CYT-induced memory improvement at any of the tested dose. No change in swimming activity was observed.

CONCLUSIONS

Our findings demonstrate that zebrafish make a useful model for the rapid screening of the effect of new α4β2 nAChR compounds on spatial memory.

Nicotine-modulated subunit stoichiometry affects stability and trafficking of α3β4 nicotinic receptor

Heteromeric nAChRs are pentameric cation channels, composed of combinations of two or three α and three or two β subunits, which play key physiological roles in the central and peripheral nervous systems. The prototypical agonist nicotine acts intracellularly to upregulate many nAChR subtypes, a phenomenon that is thought to contribute to the nicotine dependence of cigarette smokers. The α3β4 subtype has recently been genetically linked to nicotine dependence and lung cancer; however, the mode of action of nicotine on this receptor subtype has been incompletely investigated. Here, using transfected mammalian cells as model system, we characterized the response of the human α3β4 receptor subtype to nicotine and the mechanism of action of the drug. Nicotine, when present at 1 mm concentration, elicited a ∼5-fold increase of cell surface α3β4 and showed a more modest upregulatory effect also at concentrations as low as 10 μM. Upregulation was obtained if nicotine was present during, but not after, pentamer assembly and was caused by increased stability and trafficking of receptors assembled in the presence of the drug. Experimental determinations as well as computational studies of subunit stoichiometry showed that nicotine favors assembly of pentamers with (α3)2(β4)3 stoichiometry; these are less prone than (α3)3(β4)2 receptors to proteasomal degradation and, because of the presence in the β subunit of an endoplasmic reticulum export motif, more efficiently transported to the plasma membrane. Our findings uncover a novel mechanism of nicotine-induced α3β4 nAChR upregulation that may be relevant also for other nAChR subtypes.

Cytisine modulates chronic voluntary ethanol consumption and ethanol-induced striatal up-regulation of ΔFosB in mice

Chronic administration of ethanol induces persistent accumulation of ΔFosB, an important transcription factor, in the midbrain dopamine system. This process underlies the progression to addiction. Previously, we have shown that cytisine, a neuronal nicotinic acetylcholine receptor (nAChR) partial agonist, reduces various ethanol-drinking behaviors and ethanol-induced striatal dopamine function. However, the effects of cytisine on chronic ethanol drinking and ethanol-induced up-regulation of striatal ΔFosB are not known. Therefore, we examined the effects of cytisine on chronic voluntary ethanol consumption and associated striatal ΔFosB up-regulation in C57BL/6J mice using behavioral and biochemical methods. Following the chronic voluntary consumption of 15% (v/v) ethanol under a 24-h two-bottle choice intermittent access (IA; 3 sessions/week) or continuous access (CA; 24 h/d and 7 d/week) paradigm, mice received repeated intraperitoneal injections of saline or cytisine (0.5 or 3.0 mg/kg). Ethanol and water intake were monitored for 24 h post-treatment. Pretreatment with cytisine (0.5 or 1.5 mg/kg) significantly reduced ethanol consumption and preference in both paradigms at 2 h and 24 h post-treatment. The ΔFosB levels in the ventral and dorsal striatum were determined by Western blotting 18-24 h after the last point of ethanol access. In addition, cytisine (0.5 mg/kg) significantly attenuated up-regulation of ΔFosB in the ventral and dorsal striatum following chronic ethanol consumption in IA and CA paradigms. The results indicate that cytisine modulates chronic voluntary ethanol consumption and reduces ethanol-induced up-regulation of striatal ΔFosB. Further, the data suggest a critical role of nAChRs in chronic ethanol-induced neurochemical adaptations associated with ethanol addiction.

(+)-Laburnamine, a natural selective ligand and partial agonist for the α4β2 nicotinic receptor subtype

(+)-Laburnamine (1), a rare alkaloid extracted from Laburnum anagyroides seeds (∼4 mg from 1 kg), was shown to bind with high affinity (Ki, 293 nM) to the α4β2 nicotinic receptor subtype, which is, respectively, 126 and 136 times higher than to the α3β4 (Ki 37 μM) and α7 subtypes (Ki 40 μM). When its ability to release [(3)H]-dopamine from striatal slices was tested in a functional assay, compound 1 behaved as a partial agonist with an EC50 of 5.8 μM and an Emax that was 43% that of nicotine. When incubated with nicotine in the same assay, 1 prevented a maximal effect from being reached.

CC4, a dimer of cytisine, is a selective partial agonist at α4β2/α6β2 nAChR with improved selectivity for tobacco smoking cessation

BACKGROUND AND PURPOSE

Many of the addictive and rewarding effects of nicotine are due to its actions on the neuronal nicotinic ACh receptor (nAChR) subtypes expressed in dopaminergic mesocorticolimbic cells. The partial agonists, cytisine and varenicline, are helpful smoking cessation aids. These drugs have a number of side effects that limit their usefulness. The aim of this study was to investigate the preclinical pharmacology of the cytisine dimer1,2-bisN-cytisinylethane (CC4).

EXPERIMENTAL APPROACH

The effects of CC4 on nAChRs were investigated using in vitro assays and animal behaviours.

KEY RESULTS

When electrophysiologically tested using heterologously expressed human subtypes, CC4 was less efficacious than cytisine on neuronal α4β2, α3β4, α7 and muscle-type receptors, and had no effect on 5-hydroxytryptamine3 receptors. Acting through α4β2 and α6β2 nAChRs, CC4 is a partial agonist of nAChR-mediated striatal dopamine release and, when co-incubated with nicotine, prevented nicotine's maximal effect on this response. In addition, it had low affinity for, and was less efficacious than nicotine and cytisine on the α3β4 and α7-nAChR subtypes. Like cytisine and nicotine, CC4-induced conditioned place preference (CPP), and its self-administration shows an inverted-U dose-response curve. Pretreatment with non-reinforcing doses of CC4 significantly reduced nicotine-induced self-administration and CPP without affecting motor functions.

CONCLUSION AND IMPLICATIONS

Our in vitro and in vivo findings reveal that CC4 selectively reduces behaviours associated with nicotine addiction consistent with the partial agonist selectivity of CC4 for β2-nAChRs. The results support the possible development of CC4 or its derivatives as a promising drug for tobacco smoking cessation.

Structural characterization of binding mode of smoking cessation drugs to nicotinic acetylcholine receptors through study of ligand complexes with acetylcholine-binding protein

Smoking cessation is an important aim in public health worldwide as tobacco smoking causes many preventable deaths. Addiction to tobacco smoking results from the binding of nicotine to nicotinic acetylcholine receptors (nAChRs) in the brain, in particular the α4β2 receptor. One way to aid smoking cessation is by the use of nicotine replacement therapies or partial nAChR agonists like cytisine or varenicline. Here we present the co-crystal structures of cytisine and varenicline in complex with Aplysia californica acetylcholine-binding protein and use these as models to investigate binding of these ligands binding to nAChRs. This analysis of the binding properties of these two partial agonists provides insight into differences with nicotine binding to nAChRs. A mutational analysis reveals that the residues conveying subtype selectivity in nAChRs reside on the binding site complementary face and include features extending beyond the first shell of contacting residues.

Unichiral 2-(2'-pyrrolidinyl)-1,4-benzodioxanes: the 2R,2'S diastereomer of the N-methyl-7-hydroxy analogue is a potent α4β2- and α6β2-nicotinic acetylcholine receptor partial agonist

A series of unichiral 7-substituted 2-(1'-methyl-2'-pyrrolidinyl)-1,4-benzodioxanes were synthesized and tested for the affinity for the α4β2 and α7 central nicotinic receptors; the 2R,2'S diastereomer of the 7-OH analogue [(R,S)-7], unique in the series, has a high α4β2 affinity (12nM K(i)). N-Demethylation and configuration inversion of the stereocenters greatly weaken its α4β2 affinity, confirming that such a rigid molecule can be considered a new template for α4β2 ligands. Docking analysis showed how (R,S)-7 is capable of strongly and specifically interacting with the amino acidic counterpart of the α4β2 receptor binding site. Further pharmacological characterization demonstrated that (R,S)-7 also has a high affinity for the α6β2 receptor, and in vitro functional tests indicated that it is a potent α4β2 and α6β2 partial agonist, with modest affinity and potency for the α3β4 receptor. Comparison with varenicline, a well-known nicotinic partial agonist used as a smoking cessation aid, interestingly reveals similar nicotinoid profiles.

Electron impact mass spectral study of halogenated N-acetyl- and N-propionylcytisines

(-)-Cytisine and its derivatives, characterised by high affinity to neuronal nicotinic acetylcholine receptors with specificity for the α4β2 subtype, have been shown to be important probes in central nervous system (CNS) research. Electron impact mass spectral (EI-MS) fragmentations of halogenated derivatives of N-acetylcytisine and N-propionylcytisine have been investigated. Detailed fragmentation pathways have been identified for all significant ions including a few characteristic fragment ions. The principal mass spectral fragmentation routes of iodine and bromine compounds have been determined on the basis of low (EI), high resolution (HRD) and B(2)/E linked scan mass spectra as well as linked scans at constant B/E.

First total synthesis of (+/-)-3-hydroxy-11-norcytisine: structure confirmation and biological characterization

The first total synthesis of the natural product 3-hydroxy-11-norcytisine (1), structurally related to cytisine (2), a benchmark ligand at neuronal nicotinic acetylcholine receptors (NNRs), has been achieved. The synthesis permits the unambiguous confirmation of the structure originally proposed for 1 and has enabled initial biological characterization of 1 and its related compounds against NNRs.

Nicotinic receptor ligands reduce ethanol intake by high alcohol-drinking HAD-2 rats

Neuronal nicotinic acetylcholine receptors (nAChRs) are implicated in the reinforcing effects of many drugs of abuse, including ethanol. The present study examined the efficacy of cytisine, a nAChR partial agonist, and lobeline, a putative nAChR antagonist, on the maintenance of ethanol drinking by HAD-2 rats. Adult male HAD-2 rats were given access to ethanol (15 and 30%, with ad libitum access to water and food) 22 h/day for 12 weeks, beginning at 60 days of age, after which cytisine (0.0, 0.5, and 1.5 mg/kg) was tested for 3 consecutive days. The rats were given an 18-day washout period and were then tested with lobeline (0.0, 1.0, and 5.0 mg/kg) for 3 consecutive days. Ethanol intake was measured at 1, 4, and 22 h postinjection. Rats were injected intraperitoneally just before lights out (1200 h). There was a significant main effect of cytisine treatment on the second test day, with the 1.5 mg/kg dose significantly reducing ethanol intake at the 1- and 4-h time-points, relative to saline, and the 0.5 mg/kg dose inducing a significant reduction at the 4-h time-point. Conversely, lobeline treatment resulted in significant main effects of treatment for all three time-points within each test day, with the 5.0 mg/kg dose significantly reducing ethanol intake, relative to saline, at each time-point within each test day. These findings provide further evidence that activity at the nAChR influences ethanol intake and is a promising target for pharmacotherapy development for the treatment of alcohol dependence and relapse.

Novel acetylcholine and carbamoylcholine analogues: development of a functionally selective alpha4beta2 nicotinic acetylcholine receptor agonist

A series of carbamoylcholine and acetylcholine analogues were synthesized and characterized pharmacologically at neuronal nicotinic acetylcholine receptors (nAChRs). Several of the compounds displayed low nanomolar binding affinities to the alpha4beta2 nAChR and pronounced selectivity for this subtype over alpha3beta4, alpha4beta4, and alpha7 nAChRs. The high nAChR activity of carbamoylcholine analogue 5d was found to reside in its R-enantiomer, a characteristic most likely true for all other compounds in the series. Interestingly, the pronounced alpha4beta2 selectivities exhibited by some of the compounds in the binding assays translated into functional selectivity. Compound 5a was a fairly potent partial alpha4beta2 nAChR agonist with negligible activities at the alpha3beta4 and alpha7 subtypes, thus being one of the few truly functionally selective alpha4beta2 nAChR agonists published to date. Ligand-protein docking experiments using homology models of the amino-terminal domains of alpha4beta2 and alpha3beta4 nAChRs identified residues Val111(beta2)/Ile113(beta4), Phe119(beta2)/Gln121(beta4), and Thr155(alpha4)/Ser150(alpha3) as possible key determinants of the alpha4beta2/alpha3beta4-selectivity displayed by the analogues.

5-(2-Pyrrolidinyl)oxazolidinones and 2-(2-pyrrolidinyl)benzodioxanes: synthesis of all the stereoisomers and alpha4beta2 nicotinic affinity

The four stereoisomers of 2-oxazolidinone 5-substituted with 1-methyl-2-pyrrolidinyl (1), of 1,4-benzodioxane 2-substituted with the same residue (2) and of the nor-methyl analogue of this latter (2a) were synthesized as candidate nicotinoids. Of the 12 compounds, two N-methylated pyrrolidinyl-benzodioxane stereoisomers, namely those with the same relative configuration at the pyrrolidine stereocentre as (S)-nicotine, bind at alpha4beta2 nicotinic acetylcholine receptor with submicromolar affinity. Consistently with the biological data, docking analysis enlightens significant differences in binding site interactions not only between 1 and 2, but also between 2 and 2a and between the stereoisomers of 2 accounting for the critical role played, in the case of the pyrrolidinyl-benzodioxanes, by the chirality of both the stereolabile and stereostable stereogenic atoms, namely the protonated tertiary nitrogen and the two asymmetric carbons.

Design, Synthesis, and Preliminary Pharmacological Evaluation of New Quinoline Derivatives as Nicotinic Ligands

A series of nicotinic ligands, carrying a quinoline nucleus, and characterized by a pharmacophoric distance between the quinoline nitrogen (H-bond acceptor) and the cationic nitrogen atoms higher than that proposed in the classical pharmacophoric models, have been synthesized and tested for their affinity for the central nicotinic receptor. The enantiomers of the nicotine analogue 1-methyl-2-pyrrolidinyl-6-quinoline and of its methiodide display enantioselectivity in binding studies, but not when tested in vivo; on alpha7* nicotinic receptor enantioselectivity is inverted with respect to the alpha4beta2* subtype. N,N,N-Trimethyl-4-(quinolin-6-yl)but-3-yn-1-ammonium iodide (3c) and trans-N,N,N-trimethyl-4-(quinolin-6-yl)but-3-en-1-ammonium iodide (4c), showing pharmacophoric distances in the range 8.5-10.4 A, interact with the alpha4beta2* nicotinic receptor with Ki in the microM range; compound 3c shows preference for the alpha7* subtype.

Regulation of neuronal nicotinic receptor traffic and expression

Neuronal nicotinic acetylcholine receptors (nAChRs) are a family of cation channels widely distributed in the brain, whose subunit composition and biophysical properties vary depending on the subtype and the area of the brain in which they are found. Brain nAChRs are also the target of nicotine, the most widespread drug of abuse. Chronic nicotine exposure differentially affects the number, subunit composition, stoichiometry and functional state of some nAChR subtypes, leaving others substantially unaffected. In this review, we will summarise recent data concerning the nAChR subtypes expressed in the CNS, and how they are regulated by means of chronic nicotine and/or nicotinic drugs. We will particularly focus on the possible mechanisms involved in the up-regulation of nAChRs.

IBU-octyl-cytisine, a novel bifunctional compound eliciting anti-inflammatory and cholinergic activity, ameliorates CNS inflammation by inhibition of T-cell activity

Experimental autoimmune encephalomyelitis (EAE) is a central nervous system (CNS) inflammatory model in which MOG-specific T-cells initiate an autoimmune attack leading to demyelinization and consequently, neurological damage and morbidity. As EAE pathogenesis results from the involvement of immune cells, CNS resident-cells and inflammatory mediators, our treatment strategy was to use a bifunctional compound with dual anti-inflammatory properties: a non-steroidal anti-inflammatory moiety and a nicotinic agonist moiety, intended to interact with the alpha7 nicotinic receptor present on immune cells. We used IBU-Octyl-Cytisine, with an ibuprofen (IBU) moiety and Cytisine, as the nicotinic agonist. The two moieties are attached by an eight carbon (octyl) spacer. Treatment of EAE with IBU-Octyl-Cytisine (2.5 mg/kg/day, i.p.) reduced significantly (by 70%) disease severity and inflammatory infiltrates in the spinal cord. An equivalent dose of IBU was ineffective, whereas Cytisine was significantly toxic. Treatment with IBU-Octyl-Cytisine inhibited the T-cell response toward the encephalitogenic epitope of myelin oligodendrocyte glycoprotein (MOG). In addition, expression of CCR5 by CD4(+)T-cells was lower, indicating a reduced migratory capacity following treatment. IBU-Octyl-Cytisine reduced Th(1) but not Th(2) cytokine production. This reduction was accompanied by a drop in the level of T-bet mRNA, a transcription factor pivotal to Th(1) lineage differentiation. Thus, IBU-Octyl-Cytisine is an effective treatment for EAE, influencing T-cell responses in several stages of disease pathogenesis. This bifunctional compound was more efficient than IBU or Cytisine separately, as well as than both moieties unconjugated. Thus, it seems that this strategy may be applicable in wider context.

Electron ionization mass spectral study of selected N-amide and N-alkyl derivatives of cytisine

(-)-Cytisine and its derivatives are promising alkaloids in the development of new drugs for the treatment of disorders of the central nervous system (CNS). Electron ionization (EI) mass spectral fragmentations of cytisine (1), N-methylcytisine (2), N-ethylcytisine (3), N-acetylcytisine (4), N-propionylcytisine (5) and N-benzoylcytisine (6) have been investigated. Detailed fragmentation pathways have been identified for all significant ions, including a few characteristic fragment ions. The principal fragmentation routes of compounds 1-6 have been determined on the basis of EI low-resolution, high-resolution and B2/E linked scans as well as linked scans at constant B/E.

Long-term exposure to the new nicotinic antagonist 1,2-bisN-cytisinylethane upregulates nicotinic receptor subtypes of SH-SY5Y human neuroblastoma cells

Nicotinic drug treatment can affect the expression of neuronal nicotinic acetylcholine receptors (nAChR) both in vivo and in vitro through molecular mechanisms not fully understood. The present study investigated the effect of the novel cytisine dimer 1,2-bisN-cytisinylethane (CC4) on nAChR natively expressed by SH-SY5Y neuroblastoma cells in culture. CC4 lacked the agonist properties of cytisine and was a potent antagonist (IC50=220 nM) on nAChRs. Chronic treatment of SH-SY5Y cells with 1 mM CC4 for 48 h increased the expression of 3H-epibatidine (3H-Epi; 3-4-fold) or 125I-alpha-bungarotoxin (125I-alphaBgtx; 1.2-fold) sensitive receptors present on the cell membrane and in the intracellular pool. Comparable data were obtained with nicotine or cytisine, but not with carbamylcholine, d-tubocurarine, di-hydro-beta-erythroidine or hexametonium. Immunoprecipitation and immunopurification studies showed that the increase in 3H-Epi-binding receptors was due to the enhanced expression of alpha3beta2 and alpha3beta2beta4 subtypes without changes in subunit mRNA transcription or receptor half-life. The upregulation was not dependent on agonist/antagonist properties of the drugs, and did not concern muscarinic or serotonin receptors. Whole-cell patch clamp analysis of CC4-treated cells demonstrated larger nicotine-evoked inward currents with augmented sensitivity to the blockers alpha-conotoxin MII or methyllycaconitine. In conclusion, chronic treatment with CC4 increased the number of nAChRs containing beta2 and alpha7 subunits on the plasma membrane, where they were functionally active. In the case of beta2-containing receptors, we propose that CC4, by binding to intracellular receptors, triggered a conformational reorganisation of intracellular subunits that stimulated preferential assembly and membrane-directed trafficking of beta2-containing receptor subtypes..

Synthesis and alpha4beta2 nicotinic affinity of unichiral 5-(2-pyrrolidinyl)oxazolidinones and 2-(2-pyrrolidinyl)benzodioxanes

The RS and SR enantiomers of 2-oxazolidinone and 1,4-benzodioxane bearing a 2-pyrrolidinyl substituent at the 5- and 2-position, respectively, were synthesized as candidate nicotinoids. One of the two benzodioxane stereoisomers reasonably fits the pharmacophore elements of (S)-nicotine and binds at alpha4beta2 nicotinic acetylcholine receptor with submicromolar affinity. Interestingly, both the synthesized pyrrolidinylbenzodioxanes exhibit analogous affinity at alpha(2) adrenergic receptor resembling the behaviour of some known alpha(2)-AR ligands recently proved to possess neuronal nicotinic affinity.

Varenicline is a partial agonist at alpha4beta2 and a full agonist at alpha7 neuronal nicotinic receptors

Varenicline, a new nicotinic ligand based on the structure of cytisine, has recently been approved by the U.S. Food and Drug Administration for use as a smoking cessation aid. Varenicline has been shown to be a partial agonist of alpha4beta2 receptors, and in equilibrium binding assays, it is highly selective for the alpha4beta2 receptor. In this study, we have examined the functional activity of varenicline at a variety of rat neuronal nicotinic receptors expressed in Xenopus laevis oocytes and assayed under two-electrode voltage clamp. We also find that varenicline is a potent, partial agonist at alpha4beta2 receptors, with an EC50 of 2.3 +/- 0.3 microM and an efficacy (relative to acetylcholine) of 13.4 +/- 0.4%. Varenicline has lower potency and higher efficacy at alpha3beta4 receptors, with an EC50 of 55 +/- 8 microM and an efficacy of 75 +/- 6%. Varenicline also seems to be a weak partial agonist at alpha3beta2 and alpha6-containing receptors, with an efficacy <10%. It is remarkable that varenicline is a potent, full agonist at alpha7 receptors with an EC50 of 18 +/- 6 microM and an efficacy of 93 +/- 7% (relative to acetylcholine). Thus, whereas varenicline is a partial agonist at some heteromeric neuronal nicotinic receptors, it is a full agonist at the homomeric alpha7 receptor. Some combination of these actions may be involved in the mechanism of varenicline as a smoking cessation aid.

C3-halogenation of cytisine generates potent and efficacious nicotinic receptor agonists

Neuronal nicotinic acetylcholine receptors subserve predominantly modulatory roles in the brain, making them attractive therapeutic targets. Natural products provide key leads in the quest for nicotinic receptor subtype-selective compounds. Cytisine, found in Leguminosae spp., binds with high affinity to alpha4beta2* nicotinic receptors. We have compared the effect of C3 and C5 halogenation of cytisine and methylcytisine (MCy) on their interaction with native rat nicotinic receptors. 3-Bromocytisine (3-BrCy) and 3-iodocytisine (3-ICy) exhibited increased binding affinity (especially at alpha7 nicotinic receptors; Ki approximately 0.1 microM) and functional potency, whereas C5-halogenation was detrimental. 3-BrCy and 3-ICy were more potent than cytisine at evoking [3H]dopamine release from striatal slices (EC50 approximately 11 nM), [3H]noradrenaline release from hippocampal slices (EC50 approximately 250 nM), increases in intracellular Ca2+ in PC12 cells and inward currents in Xenopus oocytes expressing human alpha3beta4 nicotinic receptor (EC50 approximately 2 microM). These compounds were also more efficacious than cytisine. C3-halogenation of cytisine is proposed to stabilize the open conformation of the nicotinic receptor but does not enhance subtype selectivity.

Nicotinic Agonists, Antagonists, and Modulators From Natural Sources

1. Acetylcholine receptors were initially defined as nicotinic or muscarinic, based on selective activation by two natural products, nicotine and muscarine. Several further nicotinic agonists have been discovered from natural sources, including cytisine, anatoxin, ferruginine, anabaseine, epibatidine, and epiquinamide. These have provided lead structures for the design of a wide range of synthetic agents. 2. Natural sources have also provided competitive nicotinic antagonists, such as the Erythrina alkaloids, the tubocurarines, and methyllycaconitine. Noncompetitive antagonists, such as the histrionicotoxins, various izidines, decahydroquinolines, spiropyrrolizidine oximes, pseudophrynamines, ibogaine, strychnine, cocaine, and sparteine have come from natural sources. Finally, galanthamine, codeine, and ivermectin represent positive modulators of nicotinic function, derived from natural sources. 3. Clearly, research on acetylcholine receptors and functions has been dependent on key natural products and the synthetic agents that they inspired.

Norchloro-fluoro-homoepibatidine: specificity to neuronal nicotinic acetylcholine receptor subtypes in vitro

The subtype-specificity of newly synthesised epibatidine-related compounds, norchloro-fluoro-homoepibatidine (NCFHEB) and derivatives, to neuronal nicotinic acetylcholine receptors (nAChR) has been investigated. NCFHEBs were assayed in competitive binding assays to (+/-)-[(3)H]epibatidine-labelled rat thalamic nAChRs and human alpha4beta2, alpha3beta4, and alpha7 nAChRs, expressed in stably transfected HEK-293 and SH-SY5Y cells. The binding affinity of (+)-NCFHEB (K(i): 0.064 nM) and (-)-NCFHEB (K(i): 0.112 nM) to human alpha4beta2 nAChR is in the same order of magnitude as that of epibatidine (K(i): 0.014 nM). However, because the affinity of both NCFHEB-enantiomers to human alpha3beta4 nAChR is up to 65 times lower than that of epibatidine, the alpha4beta2 subtype-specificity of NCFHEB is increased up to 1,400% compared to epibatidine.