Relations between structure and nicotine-like activity: X-ray crystal structure analysis of (-)-cytisine and (-)-lobeline hydrochloride and a comparison with (-)-nicotine and other nicotine-like compounds

Quinolizidine Alkaloids and Isoflavones from the Herb of Thermopsis lupinoides and Their Antiviral, Antifungal, and Insecticidal Activities

As part of our ongoing investigation of natural bioactive substances from the genus Thermopsis of the tribe Fabaceae for agricultural protection, the chemical constituents of the herb Thermopsis lupinoides were systematically investigated, which led to the isolation of 39 quinolizidine alkaloids (QAs) (1-39), including 14 new QAs (1-14) and 14 isoflavones (40-53). Their structures were elucidated through comprehensive spectroscopic data analysis (IR, UV, NMR, HRESIMS), ECD calculations, and X-ray crystallography. The antitomato spotted wilt virus (TSWV) and antifungal (against Botrytis cinerea, Gibberella zeae, Phytophythora capsica, and Alternaria alternata) and insecticidal (against Aphis fabae and Tetranychus urticae) activities of the isolated compounds were screened using the lesion counting method, mycelial inhibition assay, and spray method, respectively. The bioassay results showed that 34 exhibited excellent protective activity against TSWV, with an EC50 value of 36.04 μg/mL, which was better than that of the positive control, ningnanmycin (86.03 μg/mL). The preliminary mechanistic exploration illustrated that 34 induced systemic acquired resistance in the host plant by acting on the salicylic acid signaling pathway. Moreover, 1 showed significant antifungal activity against B. cinerea (EC50 value of 20.83 μg/mL), while 2 exhibited good insecticidal activity against A. fabae (LC50 value of 24.97 μg/mL). This research is promising for the invention of novel pesticides from QAs with high efficiency and satisfactory ecological compatibility.

Lobeline: A multifunctional alkaloid modulates cholinergic and glutamatergic activities

Developing drugs for Alzheimer's disease (AD) is an extremely challenging task due to its devastating pathology. Previous studies have indicated that natural compounds play a crucial role as lead molecules in the development of drugs. Even though, there are remarkable technological advancements in the isolation and synthesis of natural compounds, the targets for many of them are still unknown. In the present study, lobeline, a piperidine alkaloid has been identified as a cholinesterase inhibitor through chemical similarity assisted target fishing method. The structural similarities between lobeline and donepezil, a known acetylcholinesterase (AChE) inhibitor encouraged us to hypothesize that lobeline may also exhibit AChE inhibitory properties. It was further confirmed by in silico, in vitro and biophysical studies that lobeline could inhibit cholinesterase. The binding profiles indicated that lobeline has a higher affinity for AChE than BChE. Since excitotoxicity is one of the major pathological events associated with AD progression, we also investigated the neuroprotective potential of lobeline against glutamate mediated excitotoxicity in rat primary cortical neurons. The cell based NMDA receptor (NMDAR) assay with lobeline suggested that neuroprotective potential of lobeline is mediated through the blockade of NMDAR activity.

The Nicotinic Agonist Cytisine: The Role of the NH···N Interaction

We report a detailed structural study of cytisine, an alkaloid used to help with smoking cessation, looking forward to unveiling its role as a nicotinic agonist. High-resolution rotational spectroscopy has allowed us to characterize two different conformers exhibiting axial and equatorial arrangements of the piperidinic NH group. Unexpectedly, the axial form has been found as the predominant configuration, in contrast to that observed for related molecules, such as piperidine. This anomalous behavior has been justified in terms of an intramolecular NH···N hydrogen bond. Moreover, this interaction justifies the overstabilization of the axial conformer over the equatorial one and is crucial for the mechanism of action of cytisine over the nicotinic receptor, further rationalizing its behavior as a nicotinic agonist.

Spectral and photophysical properties of cytisine in acetonitrile - Theory and experiment

Spectral and photophysical properties of (-)-cytisine that is used as a smoking cessation aid, and which derivatives are promising tools in a treatment of neurological diseases, were investigated in acetonitrile, non-specifically interacting solvent with a polarity similar to water. The two chair conformers of cytisine were found the most stable in the ground state S₀ and the lowest excited singlet state S₁(π,π*), wherein axial one was characterized by a significantly larger abundance, fluorescence lifetime 0.15 ns and fluorescence quantum yield 0.008. The S₁(π,π*) excited state of both cytisine conformers deactivated almost exclusively via internal conversion.

Synthesis, structure, antioxidant activity, and water solubility of trolox ion conjugates

The interaction of trolox with ammonia, alkylamines of different classes, and amino derivatives of heterocyclic compounds, including nitroxyl radicals and alkaloids, led to the production of ammonium salts called ion conjugates (ICs). Five ICs were characterised by X-ray diffraction. This is the first time a wide range of ICs were made from trolox with amines, and ESI-MS data demonstrated they have the potential to generate pseudomolecular [(A⁻B⁺) + H]⁺ ions. For all obtained trolox ICs, a significant increase (1–3 orders of magnitude) in water solubility was achieved while retaining high antioxidant activity. ICs synthesised from two biologically active fragments may be used to create polyfunctional agents with varying solubility and bioavailability.

Spectral and Photophysical Behavior of Cytisine in n-Hexane. Experimental Evidence for the S1(n,π*) → S0 Fluorescence

Spectral and photophysical properties of (-)-cytisine (the compound used as a smoking cessation aid and a potential drug in Alzheimer's and Parkinson's diseases) were investigated. The two conformers of cytisine, whose presence in the S₀ state has been earlier proved by the NMR and IR methods as well as in theoretical calculation, in nonpolar n-hexane show a rarely observed prompt fluorescence from the S₁(n,π*) excited state. This observation is unambiguously evidenced by very small radiative rate constants of these two emitting conformers, k_F = 7.4 × 10⁵ and 3.0 × 10⁵ s^-1. Their lifetimes in the S₁(n,π*) state are relatively long, τ_S1 = 1.9 and 6.7 ns; therefore, their fluorescence quantum yield is relatively high ϕ_F ∼ 10^-3. The long-wavelength band in the cytisine absorption originates from the excitation to the S₂(π,π*) state, while the S₁(n,π*) state is not observed in this spectrum. Thus, the excited state S₂(π,π*) is manifested only in the absorption spectrum, while the excited state S₁(n,π*) is seen only in the fluorescence spectrum, so cytisine in n-hexane is characterized by close lying (n,π*) and (π,π*) excited singlet states.

Cytisine basicity, solvation, logP, and logD theoretical determination as tool for bioavailability prediction

Cytisine, an α4β2 nicotinic receptor partial agonist, is a plant alkaloid widely used as a smoking cessation agent. Despite long history of use, knowledge on pharmacokinetics of cytisine still demands an extension. This work is aimed at theoretical determination of physicochemical parameters that affect the bioavailability of cytisine. The acidic dissociation constant, Gibbs free energy of solvation in water and n-octanol as well as n-octanol/water partition coefficient and n-octanol/water distribution coefficient of cytisine were calculated as quantities corresponding to its solubility and permeability. Cytisine structure was optimized with several quantum chemical methods-ab initio: HF and MP2, and DFT functionals (B3LYP, B3LYP-D3, CAM-B3LYP, M06-2X, TPSS, VSXC) with 6-311++G(d,p) basis set. Solvation of cytisine in water and n-octanol was determined with the SMD continuum model. It was shown that lipophilicity of cytisine depends on the pH of an environment. Protonated cytisine, the most populated state under acidic conditions, is characterized by enhanced hydrophilicity. Then neutral cytisine, dominating in a basic environment, demonstrates more lipophilic character. It appears that cytisine is very well soluble in the gastrointestinal (GI) tract fluids. Then the distribution of cytisine ought to occur very rapidly. However, permeability of cytisine through the mucous membrane of the GI tract may be limited, leading to the diminished bioavailability.

The vesicular monoamine transporter-2: an important pharmacological target for the discovery of novel therapeutics to treat methamphetamine abuse

Methamphetamine abuse escalates, but no approved therapeutics are available to treat addicted individuals. Methamphetamine increases extracellular dopamine in reward-relevant pathways by interacting at vesicular monoamine transporter-2 (VMAT2) to inhibit dopamine uptake and promote dopamine release from synaptic vesicles, increasing cytosolic dopamine available for reverse transport by the dopamine transporter (DAT). VMAT2 is the target of our iterative drug discovery efforts to identify pharmacotherapeutics for methamphetamine addiction. Lobeline, the major alkaloid in Lobelia inflata, potently inhibited VMAT2, methamphetamine-evoked striatal dopamine release, and methamphetamine self-administration in rats but exhibited high affinity for nicotinic acetylcholine receptors (nAChRs). Defunctionalized, unsaturated lobeline analog, meso-transdiene (MTD), exhibited lobeline-like in vitro pharmacology, lacked nAChR affinity, but exhibited high affinity for DAT, suggesting potential abuse liability. The 2,4-dicholorophenyl MTD analog, UKMH-106, exhibited selectivity for VMAT2 over DAT, inhibited methamphetamine-evoked dopamine release, but required a difficult synthetic approach. Lobelane, a saturated, defunctionalized lobeline analog, inhibited the neurochemical and behavioral effects of methamphetamine; tolerance developed to the lobelane-induced decrease in methamphetamine self-administration. Improved drug-likeness was afforded by the incorporation of a chiral N-1,2-dihydroxypropyl moiety into lobelane to afford GZ-793A, which inhibited the neurochemical and behavioral effects of methamphetamine, without tolerance. From a series of 2,5-disubstituted pyrrolidine analogs, AV-2-192 emerged as a lead, exhibiting high affinity for VMAT2 and inhibiting methamphetamine-evoked dopamine release. Current results support the hypothesis that potent, selective VMAT2 inhibitors provide the requisite preclinical behavioral profile for evaluation as pharmacotherapeutics for methamphetamine abuse and emphasize selectivity for VMAT2 relative to DAT as a criterion for reducing abuse liability of the therapeutic.

Lobeline shows protective effects against MPTP-induced dopaminergic neuron death and attenuates behavior deficits in animals

We previously demonstrated that lobeline effectively inhibited dopamine transporter (DAT)-mediated dopamine (DA) transportation. Therefore, the present study aimed to investigate whether lobeline shows protective effects against neurotoxin-induced cell death in vivo. Mice were administered 30 mg/kg 1-methyl-4-phenyl-1,2,3,6-tetra-hydropyridine (MPTP) and treated with 80 mg/kg L-dopa, 10 mg/kg GBR12935 or 1 or 3 mg/kg lobeline, respectively, via injection. Rotarod and swim tests as well as tyrosine hydroxylase (TH) immunohistochemistry were carried out to evaluate the effects of these drugs. Compared with L-DA and GBR12935, lobeline (3 mg/kg administered via intraperitoneal injection) on behavior and dopaminergic neurons. Compared with L-DA and GBR12935, lobeline (3 mg/kg injected subcutaneously) significantly reduced MPTP induced locomotive deficits detected in behavioral tests. In addition, TH immunostaining showed that lobeline (3 mg/kg) markedly decreased the neurotoxin-induced immunoreactivity loss in the substantia nigra and striatum. Lobeline may be useful in the protection of dopaminergic neurons and may alleviate the symptoms of Parkinson's disease.

End-plate acetylcholine receptor: structure, mechanism, pharmacology, and disease

The synapse is a localized neurohumoral contact between a neuron and an effector cell and may be considered the quantum of fast intercellular communication. Analogously, the postsynaptic neurotransmitter receptor may be considered the quantum of fast chemical to electrical transduction. Our understanding of postsynaptic receptors began to develop about a hundred years ago with the demonstration that electrical stimulation of the vagus nerve released acetylcholine and slowed the heart beat. During the past 50 years, advances in understanding postsynaptic receptors increased at a rapid pace, owing largely to studies of the acetylcholine receptor (AChR) at the motor endplate. The endplate AChR belongs to a large superfamily of neurotransmitter receptors, called Cys-loop receptors, and has served as an exemplar receptor for probing fundamental structures and mechanisms that underlie fast synaptic transmission in the central and peripheral nervous systems. Recent studies provide an increasingly detailed picture of the structure of the AChR and the symphony of molecular motions that underpin its remarkably fast and efficient chemoelectrical transduction.

Design, synthesis and interaction at the vesicular monoamine transporter-2 of lobeline analogs: potential pharmacotherapies for the treatment of psychostimulant abuse

The vesicular monoamine transporter-2 (VMAT2) is considered as a new target for the development of novel therapeutics to treat psychostimulant abuse. Current information on the structure, function and role of VMAT2 in psychostimulant abuse are presented. Lobeline, the major alkaloidal constituent of Lobelia inflata, interacts with nicotinic receptors and with VMAT2. Numerous studies have shown that lobeline inhibits both the neurochemical and behavioral effects of amphetamine in rodents, and behavioral studies demonstrate that lobeline has potential as a pharmacotherapy for psychostimulant abuse. Systematic structural modification of the lobeline molecule is described with the aim of improving selectivity and affinity for VMAT2 over neuronal nicotinic acetylcholine receptors and other neurotransmitter transporters. This has led to the discovery of more potent and selective ligands for VMAT2. In addition, a computational neural network analysis of the affinity of these lobeline analogs for VMAT2 has been carried out, which provides computational models that have predictive value in the rational design of VMAT2 ligands and is also useful in identifying drug candidates from virtual libraries for subsequent synthesis and evaluation.

Effects of lobeline, a nicotinic receptor ligand, on the cloned Kv1.5

The goal of the present study was to examine the effects of lobeline, an agonist at nicotinic receptors, on cloned Kv channels, Kv1.5, Kv3.1, Kv4.3, and human ether-a-gogo-related gene (HERG), which are stably expressed in Chinese hamster ovary (CHO) or human embryonic kidney 293 (HEK293) cells. Whole-cell patch-clamp experiments revealed that lobeline accelerated the decay rate of Kv1.5 inactivation, decreasing the current amplitude at the end of the pulse in a concentration-dependent manner with a half-maximal inhibitory concentration (IC(50)) value of 15.1 μM. Using a time constant for the time course of drug-channel interaction, the apparent association (k(+1)), and dissociation rate (k(-1)) constants were 2.4 μΜ(-1) s(-1) and 40.9 s(-1), respectively. The calculated K(D) was 17.0 μΜ. Lobeline slowed the decay rate of the tail current, resulting in a tail crossover phenomenon. The inhibition of Kv1.5 by lobeline steeply increased at potentials between -10 and +10 mV, which corresponds to the voltage range of channel activation. At more depolarized potentials a weaker voltage dependence was observed (δ=0.26). The voltage dependence of the steady-state activation curve was not affected by lobeline, but lobeline shifted the steady-state inactivation curve of Kv1.5 in the hyperpolarizing direction. Lobeline produced use-dependent inhibition of Kv1.5 at frequencies of 1 and 2 Hz, and slowed the recovery from inactivation. Lobeline also inhibited Kv3.1, Kv4.3, and HERG in a concentration-dependent manner, with IC(50) values of 21.7, 28.2, and 0.34 μM, respectively. These results indicate that lobeline produces a concentration-, time-, voltage-, and use-dependent inhibition of Kv1.5, which can be interpreted as an open-channel block mechanism.

The Exceptional Hydrogen-Bond Properties of Neutral and Protonated Lobeline

The X-ray diffraction structure of (-)-lobeline, a high affinity nicotinic ligand, has been determined. A comparison with its hydrobromide and hydrochloride salts shows the great flexibility of the two lateral chains of the N-methylpiperidine ring. Infrared studies carried out on the same species, in the solid state and in solution, reveal the propensity of this molecular framework to accommodate very specific hydrogen bonds (HBs) depending on the state-neutral or protonated-of the molecule. In solution, a strong internal HB between the hydroxyl group and the piperidine nitrogen gives an exceptionally high HB affinity to the hydroxyl oxygen of the lobeline base. In the ionic form, both NH+ and OH groups of the molecule cooperate as HB donors to chelate the counterion. These interactions provide very stable structures and indicate that protonated lobeline can also act as a strong HB donor.

Central Nicotinic Receptors: Structure, Function, Ligands, and Therapeutic Potential

The growing interest in nicotinic receptors, because of their wide expression in neuronal and non-neuronal tissues and their involvement in several important CNS pathologies, has stimulated the synthesis of a high number of ligands able to modulate their function. These membrane proteins appear to be highly heterogeneous, and still only incomplete information is available on their structure, subunit composition, and stoichiometry. This is due to the lack of selective ligands to study the role of nAChR under physiological or pathological conditions; so far, only compounds showing selectivity between alpha4beta2 and alpha7 receptors have been obtained. The nicotinic receptor ligands have been designed starting from lead compounds from natural sources such as nicotine, cytisine, or epibatidine, and, more recently, through the high-throughput screening of chemical libraries. This review focuses on the structure of the new agonists, antagonists, and allosteric ligands of nicotinic receptors, it highlights the current knowledge on the binding site models as a molecular modeling approach to design new compounds, and it discusses the nAChR modulators which have entered clinical trials.

Hydrogen-Bond Interactions of Nicotine and Acetylcholine Salts: A Combined Crystallographic, Spectroscopic, Thermodynamic and Theoretical Study

The hydrogen-bond (HB) interactions of the monocharged active forms of nicotine and acetylcholine (ACh) have been compared theoretically by using density functional theory (DFT) calculations and experimentally on the basis of crystallographic observations and the measurement of equilibrium constants in solution. The 2,4,6-trinitrophenolate (picrate) counterion was used to determine the experimental HB basicity of the cations despite its potential multisite HB acceptor properties. The preferred HB interaction site of the ammonium picrate salts was determined from a survey of crystallographic data found in the Cambridge Structural Database (CSD) and is supported by theoretical calculations. Two distinct classes of ammonium groups were characterised depending on the absence (quaternary ammonium) or presence (tertiary, secondary and primary ammoniums) of an N(+)HO hydrogen bond linking the two ions. The crystal structure of nicotinium picrate was determined and compared with that of ACh. This analysis revealed the peculiar behaviour of the ammonium moiety of nicotinic acetylcholine receptor (nAChR) ligands towards the picrate anion. Dedicated methods have been developed to separate the individual contributions of the anion and cation accepting sites to the overall HB basicity of the ion pairs measured in solution. The HB basicities of the picrate anions associated with the two different ammonium classes were determined in dichloromethane solution by using several model ion pairs with non-basic ammonium cations. The experimental and theoretical studies performed on the nicotine and ACh cations consistently show the significant HB ability of the acceptor site of nAChR agonists in their charged form. Both the greater HB basicity of the pyridinic nitrogen over the carbonyl oxygen and the greater HB acidity of the N(+)H unit relative to N(+)CH could contribute to the higher affinity for nAChRs of nicotine-like ligands relative to ACh-like ligands.

Vesicular monoamine transporter 2: role as a novel target for drug development

In the central nervous system, vesicular monoamine transporter 2 (VMAT2) is the only transporter that moves cytoplasmic dopamine (DA) into synaptic vesicles for storage and subsequent exocytotic release. Pharmacologically enhancing DA sequestration by VMAT2, and thus preventing the oxidation of DA in the cytoplasm, may be a strategy for treating diseases such as Parkinson's disease. VMAT2 may also be a novel target for the development of treatments for psychostimulant abuse. This review summarizes the possible role of VMAT2 as a therapeutic target, VMAT2 ligands reported in the literature, and the structure-activity relationship of these ligands, including tetrabenazine analogs, ketanserin analogs, lobeline analogs, and 3-amine-2-phenylpropene analogs. The molecular structure of VMAT2 and its relevance to ligand binding are briefly discussed.

Computational evidence for the ligand selectivity to the alpha4beta2 and alpha3beta4 nicotinic acetylcholine receptors

The homology models of the alpha4beta2 and alpha3beta4 nicotinic acetylcholine receptors (nAChRs) suggest that the two nAChR subtypes are different in their ligand-binding pockets due to the non-conserved residues in the beta-subunits. The docking of nicotine, epibatidine, A-84543, and the two analogs of A-84543 ligands 1 and 2 to the homology models of alpha4beta2 and alpha3beta4 is presented. It is found that the protonated amino groups of these ligands bind to the alpha-subunits, whereas the remaining parts of the ligands bind to the beta-subunits. The two non-conserved amino acids Lys77 and Phe117 in the beta2-subunit corresponding to Ile77 and Gln117 in the beta4-subunit are identified to be the key players determining the binding modes of the ligands. We demonstrate how the increase in the number of the atoms connecting the pyrrolidine and pyridine rings in A-84543, 1, and 2, and an introduction of the alkynyl substituent in the pyridine ring affect the binding and shift the selectivity of these ligands toward the beta2-containing receptors. Further improvement in affinity and selectivity in this and other series of the ligands may be achieved by designing molecules that would specifically target the non-conserved regions in nAChRs.

Synthesis and binding studies of epibatidine analogues as ligands for the nicotinic acetylcholine receptors

Neuronal nicotinic acetylcholine receptors (nAChRs) are transmembrane ligand-gated ion channels. Recent research demonstrated that selective nAChR ligands may have therapeutic potential in a number of CNS diseases and disorders. The alkaloid epibatidine is a highly potent non-opioid analgesic and nAChR agonist, but too toxic to be a useful ligand. To develop ligands selective for distinct nAChR subtypes and with reduced toxicity, a series of epibatidine and homoepibatidine analogues were synthesized. (+/-)-8-Methyl-3-(pyridin-3-yl)-8-azabicyclo[3,2,1]oct-2-ene, showed high affinity towards alpha4beta2 (Ki=2 nM), subtype selectivity (alpha4beta2/alpha7 affinity ratio>100) and relatively low toxicity in mice and can be labeled with 11C and 18F as positron emission tomography (PET) tracers for imaging of nAChRs.

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.

Novel approaches to plant drug discovery based on high throughput pharmacological screening and genetic manipulation

Plants are potentially important for novel therapeutic drug leads, but the slowness of conventional methods for investigation of plants limits enthusiasm in the pharmaceutical industry. To overcome some of the drawbacks, we have applied high throughput pharmacological screening (HTPS) to crude plant extracts. Using a "differential smart screen", (DSS) the spectrum of activity contained in a crude extract is measured at several closely related receptor subtypes. This spectrum is then compared to that of known compounds. A unique spectrum suggests that the extract merits further investigation. Evaluation of species and environmental libraries of whole plants has demonstrated the value of this approach for rapid prioritization of plants for investigation. In addition, genomic and genetic manipulation of plants and plant cell cultures can increase the value of DSS. For example, the whole genomic potential of a plant species for biodiversity can be accessed by using gain of function mutations to generate a "functional genomics library" of mutant clonal cultures, and the bioactivity of these cultures tested by DSS. Clones that overproduce activity differing from the wild-type plant can be identified in this way. This "Natural Products Genomics" (NPG) strategy is limited by the massive numbers of clonal cultures that are required to cover all possible gain-of-function mutations. The rapidity and efficiency of this process can be improved by using transgenic plants expressing appropriate mammalian proteins. These may be designed to make the plant cell resemble a human cell for a specific form of toxicity. Now, "unnatural selection" of resistant mutant clones can be used to provide a sub-population potentially enriched in useful compounds. Alternatively, transgenic plant cells can be used for "in situ screening" in which a mammalian receptor protein, linked to a reporter construct, such as green fluorescent protein, is expressed. Clonal cultures that produce ligands for this receptor can now be rapidly identified visually in an ultra-HTPS. Overall, our aim is to use pharmacological screening, together with functional genomic approaches, to make plant drug discovery competitive with combinatorial chemistry.

Molecular modeling of mono- and bis-quaternary ammonium salts as ligands at the alpha4beta2 nicotinic acetylcholine receptor subtype using nonlinear techniques

The neuronal nicotinic acetylcholine receptor (nAChR) has been a target for drug development studies for over a decade. A series of mono- and bis-quaternary ammonium salts, known to be antagonists at nAChRs, were separated into 3 structural classes and evaluated using both self-organizing map (SOM) and genetic functional approximation (GFA) algorithm models. Descriptors from these compounds were used to create several nonlinear quantitative structure-activity relationships (QSARs). The SOM methodology was effective in appropriately grouping these compounds with diverse structures and activities. The GFA models were also able to predict the activities of these molecules. Charge distribution and the hydrophobic free energies were found to be important indicators of bioactivity for this particular class of molecules. These QSAR approaches may be a useful to screen and select in silico new drug candidates from larger compound libraries to be further evaluated in in vitro biological assays.

Structures of Aplysia AChBP complexes with nicotinic agonists and antagonists reveal distinctive binding interfaces and conformations

Upon ligand binding at the subunit interfaces, the extracellular domain of the nicotinic acetylcholine receptor undergoes conformational changes, and agonist binding allosterically triggers opening of the ion channel. The soluble acetylcholine-binding protein (AChBP) from snail has been shown to be a structural and functional surrogate of the ligand-binding domain (LBD) of the receptor. Yet, individual AChBP species display disparate affinities for nicotinic ligands. The crystal structure of AChBP from Aplysia californica in the apo form reveals a more open loop C and distinctive positions for other surface loops, compared with previous structures. Analysis of Aplysia AChBP complexes with nicotinic ligands shows that loop C, which does not significantly change conformation upon binding of the antagonist, methyllycaconitine, further opens to accommodate the peptidic antagonist, alpha-conotoxin ImI, but wraps around the agonists lobeline and epibatidine. The structures also reveal extended and nonoverlapping interaction surfaces for the two antagonists, outside the binding loci for agonists. This comprehensive set of structures reflects a dynamic template for delineating further conformational changes of the LBD of the nicotinic receptor.

Synthesis and α4β2 nicotinic affinity of 2-pyrrolidinylmethoxyimines and prolinal oxime ethers

Homochiral E and Z isomers of N-methylprolinal O-isopropyloxime and (1-methyl-2-pyrrolidinyl)methoxyimines were synthesized as candidate bioisosteres of nicotine and its isoxazolic analogue ABT 418. Two of them, namely (S)-2-isopropylideneaminooxymethyl- and (Z)-(S)-2-ethylideneaminooxymethyl-1-methylpyrrolidine, proved to bind at alpha4beta2 nicotinic acetylcholine receptor with submicromolar affinity and remarkable selectivity over alpha7 and muscarinic receptors thus supporting the hypothesized bioisosteric relationship between their methyloxyimino group and the aromatic heterocycles of the reference ligands.

α4β2 nACh Receptor pharmacophore models

Progress towards the development of alpha4beta2 nicotinic acetylcholinergic receptor pharmacophores is reviewed from the early Beers and Riech model to the newer vector models.

Up-regulation of brain nicotinic acetylcholine receptors in the rat during long-term self-administration of nicotine: disproportionate increase of the alpha6 subunit

In male rats continually self-administering nicotine (approximately 1.5 mg free base/kg/day), we found a significant increase of nicotinic acetylcholine receptors (nAChRs) labeled by epibatidine (Epb) in 11 brain areas. A large increase of high-affinity Epb binding sites was apparent in the ventral tegmentum/substantia nigra, nucleus tractus solitarii, nucleus accumbens, thalamus/subthalamus, parietal cortex, hypothalamus, and amygdala. A smaller but significant up-regulation of high-affinity Epb sites was seen in the piriform cortex, hippocampus, caudate/putamen, and cerebellar cortex. The up-regulation of nAChRs, shown by immunoadsorption and Western blotting, involved alpha4, alpha6, and beta2 subunits. As a consequence of long-term self-administration of nicotine, the alpha6 immunoreactive (IR) binding of either labeled Epb or 125I-alpha-conotoxin MII increased to a much greater extent than did alpha4 or beta2 IR binding of Epb. In addition, the beta2 IR binding of Epb was consistently enhanced to a greater extent than was alpha4. These findings may reflect a larger surface membrane retention of alpha6-containing and, to some degree, beta2-containing nAChRs compared with alpha4-containing nAChRs during long-term self-administration of nicotine.

Rigidified acetylcholine mimics: conformational requirements for binding to neuronal nicotinic receptors

Rigidified derivatives have been designed and synthesized assuming the g+t conformer of acetylcholine (N-C-C-O=+60 degrees, C-C-O-C=180 degrees ) as active conformation for binding to cytisine sensitive neuronal nicotinic receptors. The SAR of the compounds evaluated, along with those of more flexible analogues, support the g+t conformer hypothesis and highlight the stringent steric limitation of this nicotinic receptor sub-type. Compound 3e has low microM affinity for cytisine sensitive nicotinic receptor binding sites while being selective with regard to the alpha-bungarotoxin sensitive subclass. We also report few compounds with microM affinity for the alpha-bungarotoxin sensitive subclass.

A structural model of agonist binding to the alpha3beta4 neuronal nicotinic receptor

(alpha3)2(beta4)3 is the most abundant type of neuronal nicotinic ACh receptor (nAChR) mediating cholinergic actions on the autonomic nervous system. Studies to refine or devise drugs selectively acting on (alpha3)2(beta4)3 receptors would benefit from a detailed description of the hitherto unclear agonist-binding domain. The present study reports a three-dimensional model for the ligand-binding domain (LBD) of this receptor either in its unoccupied or agonist-bound conformation. The receptor model was based on the structure of the acetylcholine-binding protein (AChBP), and was obtained using molecular modelling techniques. ACh, nicotine and cytisine (full agonists), muscarine (a selective agonist of muscarinic ACh receptors) and the allosteric modulator eserine were docked into the binding pockets of the receptor model. Simulated agonist-receptor complexes were compared with the agonist-binding complex of the AChBP, as well as of the (alpha4)2(beta2)3 type of nAChR, which is the commonest in the brain. Agonist docking identified discrete amino-acid residues of the beta subunits important for pharmacological selectivity of nAChRs. The predicted affinity of muscarine for the nAChR was low, suggesting the present model to be suitable for effective discrimination of nicotinic agonist binding versus nonselective cholinergic binding. Furthermore, the current model outlined a potential binding site for the allosteric modulator eserine, the site of action of which has remained elusive. The present LBD model of the receptor in its free state provides a novel structural framework to interpret experimental observations and a useful template for future investigations to develop (alpha3)2(beta4)3-selective ligands.

Cytisine derivatives as ligands for neuronal nicotine receptors and with various pharmacological activities

Neuronal nicotinic acetylcholine receptors (nAChRs) form a family of ACh-gated cation channels made up of different subtypes. They are widely distributed in peripheral and central nervous systems and are involved in complex cerebral processes as learning, memory, nociception, movement, etc. The possibility that subtype-selective ligands be used in the treatment of CNS disorders promoted the synthesis of a large number of structural analogues of nicotine and epibatidine, two very potent nAChR agonists. Pursuing our long standing research on the structural modification of quinolizidine alkaloids, we devoted our attention to cytisine, another very potent ligand for many nAChR subtypes. Thus a systematic structural modification of cytisine was undertaken in order to obtain compounds of potential therapeutic interest at peripheral as well as central level, with a particular concern for achieving nAChR subtype selective ligands. Up to the present more than 80 cytisine derivatives, mainly of N-substitution and a few by modifying the pyridone ring, have been prepared. The biological results, which concern so far about an half of the prepared compounds, indicate that the introduction of a nitro group in position 3 of the pyridone nucleus further enhances the high affinity of cytisine, while the introduction of substituents on the basic nitrogen, though reducing in different degrees the affinity, gives rise to compounds with a higher selectivity for central (alpha(4)beta(2)) versus gangliar (alpha(3)-containing) receptor subtype. On the other hand, the analgesic, antihypertensive and inotropic activities found in some N-substituted cytisines, represent an attractive starting point for the development of more active compounds.

Evaluation of radioiodinated 5-iodo-3-(2(S)-azetidinylmethoxy)pyridine as a ligand for SPECT investigations of brain nicotinic acetylcholine receptors

5-Iodo-3-(2(S)-azetidinylmethoxy)pyridine (5IA), an A-85380 analog iodinated at the 5-position of the pyridine ring, was evaluated as a radiopharmaceutical for investigating brain nicotinic acethylcholine receptors (nAChRs) by single photon emission computed tomography (SPECT). [123/125I]5IA was synthesized by the iododestannylation reaction under no-carrier-added conditions and purified by high-performance liquid chromatography (HPLC) with high radiochemical yield (50%), high radiochemical purity (> 98%), and high specific radioactivity (> 55 GBq/micromol). The binding affinity of 5IA for brain nAChRs was measured in terms of displacement of [3H]cytisine and [125I]5IA from binding sites in rat cortical membranes. The binding data revealed that the affinity of 5IA was the same as that of A-85380 and more than seven fold higher than that of (-)-nicotine, and that 5IA bound selectively to the alpha4beta2 nAChR subtype. Biodistribution studies in rats indicated that the brain uptake of [125I]51A was rapid and profound. Regional cerebral distribution studies in rats demonstrated that the accumulation of [125I]5IA was consistent with the density of high affinity nAChRs with highest uptake observed in the nAChR-rich thalamus, moderate uptake in the cortex and lowest uptake in the cerebellum. Administration of the nAChR agonists (-)-cytisine and (-)-nicotine reduced the uptake of [125I]5IA in all regions studied with most pronounced reduction in the thalamus, and resulted in similar levels of radioactivity throughout the brain. [125I]5IA binding sites were shown to be saturable with unlabeled 5IA. Behavioral studies in mice demonstrated that 5IA did not show signs of behavioral toxicity. Furthermore, SPECT studies with [123I]5IA in the common marmoset demonstrated appropriate brain uptake and regional localization for a high-affinity nAChR imaging radiopharmaceutical. These results suggested that [123I]5IA is a promising radiopharmaceutical for SPECT studies of central nAChRs in human subjects.

A novel mechanism of action and potential use for lobeline as a treatment for psychostimulant abuse

Lobeline, an alkaloidal constituent of Lobelia inflata LINN., has a long history of therapeutic usage ranging from emetic and respiratory stimulant to tobacco smoking cessation agent. Although classified as both an agonist and an antagonist at nicotinic receptors, lobeline has no structural resemblance to nicotine, and structure--function relationships do not suggest a common pharmacophore. Lobeline inhibits nicotine-evoked dopamine release and [3H]nicotine binding, thus acting as a potent antagonist at both alpha3beta2(*) and alpha4beta2(*) neuronal nicotinic receptor subtypes. However, lobeline does not release dopamine from its presynaptic terminal, but appears to induce the metabolism of dopamine intraneuronally. Reevaluation of the mechanism by which lobeline alters dopamine function reveals that its primary mechanism is inhibition of dopamine uptake and promotion of dopamine release from the storage vesicles within the presynaptic terminal, via an interaction with the tetrabenazine-binding site on the vesicular monoamine transporter (VMAT2). Thus, lobeline appears to perturb the fundamental mechanisms of dopamine storage and release. Based on its neurochemical mechanism, the ability of lobeline to functionally antagonize the neurochemical and behavioral effects of the psychostimulants amphetamine and methamphetamine was examined. Lobeline was found to inhibit the amphetamine-induced release of dopamine in vitro, and amphetamine-induced hyperactivity, drug discrimination, and self-administration. However, lobeline does not support self-administration in rats, suggesting a lack of addiction liability. Thus, lobeline may reduce the abuse liability of these psychostimulants. The development of lobeline and lobeline analogs with targeted selectivity at VMAT2 represents a novel class of therapeutic agents having good potential as efficacious treatments for methamphetamine abuse.

Neuronal nicotinic receptor agonists: a multi-approach development of the pharmacophore

Based on the results obtained with different automated computational approaches as applied to the study of eleven high-affinity agonists of the neuronal nicotine acetylcholine receptor (nAChR), belonging to different chemical classes, new relevant features were detected which complement the existing pharmacophores. Convergent results from DISCO (Distance Comparison), QXP (Quick Explore), Catalyst/HipHop, and MIPSIM (Molecular Interaction Potential Similarity) allowed us to identify and locate, in a well defined spatial arrangement, three geometrically independent key structural features: (i) a positively charged nitrogen atom for ionic or hydrogen bond interactions, (ii) a lone pair of the pyridine nitrogen or a specific lone pair of a carbonyl oxygen, as a hydrogen bond acceptor, and (iii) a centre of a hydrophobic area generally occupied by aliphatic cycles. The pharmacophore presented herein, along with predictive 2D and 3D QSAR models recently developed in our group, could represent valuable computational tools for the design of new nAChR agonists having therapeutical potential.

Lobeline and structurally simplified analogs exhibit differential agonist activity and sensitivity to antagonist blockade when compared to nicotine

In the present study, lobeline and two structurally simplified analogs were evaluated for activity in muscarinic and nicotinic binding assays, a functional assay for nicotinic receptor activation (86Rb+ efflux from striatal synaptosomes) and an acetylcholinesterase (AChE) assay. Lobeline displaced [3H]cytisine binding to rat cortical membranes with a mean inhibition constant (KI) value of 16.0 nM, while the lobeline analogs CRM-I-13-1 and CRM-I-32-1 exhibited values of 15.0 and 5.4 microM, respectively. [3H]methylscopolamine was displaced by lobeline with a mean KI value of 37.0 microM while CRM-I-13-1 and CRM-I-32-1 exhibited values of 55.0 and 16.0 microM, respectively. While nicotine stimulated 86Rb+ efflux from striatal synaptosomes in a mecamylamine reversible manner at each concentration tested, lobeline slightly increased 86Rb+ efflux at lower concentrations and reduced efflux at higher concentrations. Further, none of the lobeline effects were reversed with mecamylamine. Although less potent, the two lobeline analogs exhibited a similar pattern of activity. These data may suggest that lobeline and structurally similar compounds bind with different subtype selectivity than nicotine, or exert their agonists effects through non-nicotinic mechanisms. All of the compounds tested were at least several hundred times less potent than physostigmine as AChE inhibitors. While some differences were apparent between the lobeline analog which contained the 2-keto-ethyl portion of lobeline and the analog which contained the phenyl 2-hydroxy-ethyl moiety, each compound was much less active than lobeline in most parameters assessed.

Lobeline inhibits nicotine-evoked [(3)H]dopamine overflow from rat striatal slices and nicotine-evoked (86)Rb(+) efflux from thalamic synaptosomes

The present study evaluated the interaction of lobeline with neuronal nicotinic acetylcholine receptors using two in vitro assays, [(3)H] overflow from [(3)H]dopamine ([(3)H]DA)-preloaded rat striatal slices and (86)Rb(+) efflux from rat thalamic synaptosomes. To assess agonist interactions, the effect of lobeline was determined and compared to S(-)-nicotine. To assess antagonist interactions, the ability of lobeline to inhibit the effect of S(-)-nicotine was determined. Both S(-)-nicotine (0.1-1 microM) and lobeline (>1.0 microM) evoked [(3)H] overflow from superfused [(3)H]DA-preloaded striatal slices. However, lobeline-evoked [(3)H] overflow is mecamylamine-insensitive, indicating that this response is not mediated by nicotinic receptors. Moreover, at concentrations (<1.0 microM) which did not evoke [(3)H] overflow, lobeline inhibited S(-)-nicotine (0.1-10 microM)-evoked [(3)H] overflow, shifting the S(-)-nicotine concentration-response curve to the right. S(-)-Nicotine (30 nM-300 microM) increased (EC(50) value=0.2 microM) (86)Rb(+) efflux from thalamic synaptosomes. In contrast, lobeline (1 nM-10 microM) did not evoke (86)Rb(+) efflux, and the lack of intrinsic activity indicates that lobeline is not an agonist at this nicotinic receptor subtype. Lobeline completely inhibited (IC(50) value=0.7 microM) (86)Rb(+) efflux evoked by 1 microM S(-)-nicotine, a concentration which maximally stimulated (86)Rb(+) efflux. Thus, the results of these in vitro experiments demonstrate that lobeline inhibits the effects of S(-)-nicotine, and suggest that lobeline acts as a nicotinic receptor antagonist.

Mixed nicotinic-muscarinic properties of the alpha9 nicotinic cholinergic receptor

The rat alpha9 nicotinic acetylcholine receptor (nAChR) was expressed in Xenopus laevis oocytes and tested for its sensitivity to a wide variety of cholinergic compounds. Acetylcholine (ACh), carbachol, choline and methylcarbachol elicited agonist-evoked currents, giving maximal or near maximal responses. Both the nicotinic agonist suberyldicholine as well as the muscarinic agonists McN-A-343 and methylfurtrethonium behaved as weak partial agonists of the receptor. Most classical cholinergic compounds tested, being either nicotinic (nicotine, epibatidine, cytisine, methyllycaconitine, mecamylamine, dihydro-beta-erythroidine), or muscarinic (muscarine, atropine, gallamine, pilocarpine, bethanechol) agonists and antagonists, blocked the recombinant alpha9 receptor. Block by nicotine, epibatidine, cytisine, methyllycaconitine and atropine was overcome at high ACh concentrations, suggesting a competitive type of block. The present results indicate that alpha9 displays mixed nicotinic-muscarinic features that resemble the ones described for the cholinergic receptor of cochlear outer hair cells (OHCs). We suggest that alpha9 contains the structural determinants responsible for the pharmacological properties of the native receptor.