Multispectroscopic and Theoretical Investigation on the Binding Interaction of a Neurodegenerative Drug, Lobeline with Human Serum Albumin: Perturbation in Protein Conformation and Hydrophobic-Hydrophilic Surface.

Lobeline (LOB), a naturally occurring alkaloid, has a broad spectrum of pharmacological activities and therapeutic potential, including applications in central nervous system disorders, drug misuse, multidrug resistance, smoking cessation, depression, and epilepsy. LOB represents a promising compound for developing treatments in various medical fields. However, despite extensive pharmacological profiling, the biophysical interaction between the LOB and proteins remains largely unexplored. In the current article, a range of complementary photophysical and cheminformatics methodologies were applied to study the interaction mechanism between LOB and the carrier protein HSA. Steady-state fluorescence and fluorescence lifetime experiments confirmed the static-quenching mechanisms in the HSA-LOB system. "K" (binding constant) of the HSA-LOB system was determined to be 105 M-1, with a single preferable binding site in HSA. The forces governing the HSA-LOB stable complex were analyzed by thermodynamic parameters and electrostatic contribution. The research also investigated how various metal ions affect complex binding. Site-specific binding studies depict Site I as probable binding in HSA by LOB. We conducted synchronous fluorescence, 3D fluorescence, and circular dichroism studies to explore the structural alteration occurring in the microenvironment of amino acids. To understand the robustness of the HSA-LOB complex, we used theoretical approaches, including molecular docking and MD simulations, and analyzed the principal component analysis and free energy landscape. These comprehensive studies of the structural features of biomolecules in ligand binding are of paramount importance for designing targeted drugs and delivery systems.

Varenicline and GZ-793A differentially decrease methamphetamine self-administration under a multiple schedule of reinforcement in rats.

Methamphetamine is a potent psychostimulant with high abuse rates. Currently, there is no Food and Drug Administration-approved pharmacotherapy for methamphetamine addiction. Ideally, a pharmacotherapy should selectively decrease methamphetamine self-administration without affecting responding for other reinforcers. One way to test this is with the use of a multiple schedule of reinforcement, in which drug and food are available in alternating components within a session. The present study evaluated GZ-793A, a vesicular monoamine transporter-2 inhibitor, and varenicline, a partial agonist at α4ß2 and full agonist at α7 nicotinic acetylcholine receptors, for their ability to decrease methamphetamine and food self-administration using a multiple schedule of reinforcement. Male Sprague-Dawley rats self-administered methamphetamine (0.03 mg/kg/intravenous infusion) and food pellets under a multiple schedule of reinforcement. GZ-793A or varenicline was administered before multiple schedule sessions. GZ-793A (5 and 20 mg/kg) significantly decreased methamphetamine intake compared with saline and did not alter food-maintained responding. In contrast, varenicline decreased methamphetamine intake less specifically across time. The results suggest that vesicular monoamine transporter-2 inhibition may be a viable pharmacological target for the treatment of methamphetamine-use disorders.

Pyrrolidine analogs of GZ-793A: synthesis and evaluation as inhibitors of the vesicular monoamine transporter-2 (VMAT2).

Central heterocyclic ring size reduction from piperidinyl to pyrrolidinyl in the vesicular monoamine transporter-2 (VMAT2) inhibitor GZ-793A and its analogs resulted in novel N-propane-1,2(R)-diol analogs 11a-i. These compounds were evaluated for their affinity for the dihydrotetrabenazine (DTBZ) binding site on VMAT2 and for their ability to inhibit vesicular dopamine (DA) uptake. The 4-difluoromethoxyphenethyl analog 11f was the most potent inhibitor of [(3)H]-DTBZ binding (Ki=560 nM), with 15-fold greater affinity for this site than GZ-793A (Ki=8.29 µM). Analog 11f also showed similar potency of inhibition of [(3)H]-DA uptake into vesicles (Ki=45 nM) compared to that for GZ-793A (Ki=29 nM). Thus, 11f represents a new water-soluble inhibitor of VMAT function.

Synthesis and evaluation of novel azetidine analogs as potent inhibitors of vesicular [3H]dopamine uptake.

Lobelane analogs that incorporate a central piperidine or pyrrolidine moiety have previously been reported by our group as potent inhibitors of VMAT2 function. Further central ring size reduction of the piperidine moiety in lobelane to a four-membered heterocyclic ring has been carried out in the current study to afford novel cis-and trans-azetidine analogs. These azetidine analogs (15a-15c and 22a-22c) potently inhibited [(3)H]dopamine (DA) uptake into isolated synaptic vesicles (Ki⩽66nM). The cis-4-methoxy analog 22b was the most potent inhibitor (Ki=24nM), and was twofold more potent that either lobelane (2a, Ki=45nM) or norlobelane (2b, Ki=43nM). The trans-methylenedioxy analog, 15c (Ki=31nM), was equipotent with the cis-analog, 22b, in this assay. Thus, cis- and trans-azetidine analogs 22b and 15c represent potential leads in the discovery of new clinical candidates for the treatment of methamphetamine abuse.

Phenyl ring-substituted lobelane analogs: inhibition of [³H]dopamine uptake at the vesicular monoamine transporter-2.

Lobeline attenuates the behavioral effects of methamphetamine via inhibition of the vesicular monoamine transporter (VMAT2). To increase selectivity for VMAT2, chemically defunctionalized lobeline analogs, including lobelane, were designed to eliminate nicotinic acetylcholine receptor affinity. The current study evaluated the ability of lobelane analogs to inhibit [³H]dihydrotetrabenazine (DTBZ) binding to VMAT2 and [³H]dopamine (DA) uptake into isolated synaptic vesicles and determined the mechanism of inhibition. Introduction of aromatic substituents in lobelane maintained analog affinity for the [³H]DTBZ binding site on VMAT2 and inhibitory potency in the [³H]DA uptake assay assessing VMAT2 function. The most potent (K(i) = 13-16 nM) analogs in the series included para-methoxyphenyl nor-lobelane (GZ-252B), para-methoxyphenyl lobelane (GZ-252C), and 2,4-dichlorphenyl lobelane (GZ-260C). Affinity of the analogs for the [³H]DTBZ binding site did not correlate with inhibitory potency in the [³H]DA uptake assay. It is noteworthy that the N-benzylindole-, biphenyl-, and indole-bearing meso-analogs 2,6-bis[2-(1-benzyl-1H-indole-3-yl)ethyl]-1-methylpiperidine hemifumarate (AV-1-292C), 2,6-bis(2-(biphenyl-4-yl)ethyl)piperidine hydrochloride (GZ-272B), and 2,6-bis[2-(1H-indole-3-yl)ethyl]-1-methylpiperidine monofumarate (AV-1-294), respectively] inhibited VMAT2 function (K(i) = 73, 127, and 2130 nM, respectively), yet had little to no affinity for the [³H]DTBZ binding site. These results suggest that the analogs interact at an alternate site to DTBZ on VMAT2. Kinetic analyses of [³H]DA uptake revealed a competitive mechanism for 2,6-bis(2-(4-methoxyphenyl)ethyl)piperidine hydrochloride (GZ-252B), 2,6-bis(2-(4-methoxyphenyl)ethyl)-1-methylpiperidine hydrochloride (GZ-252C), 2,6-bis(2-(2,4-dichlorophenyl)ethyl)piperidine hydrochloride (GZ-260C), and GZ-272B. Similar to methamphetamine, these analogs released [³H]DA from the vesicles, but with higher potency. In contrast to methamphetamine, these analogs had higher potency (>100-fold) at VMAT2 than DAT, predicting low abuse liability. Thus, modification of the lobelane molecule affords potent, selective inhibitors of VMAT2 function and reveals two distinct pharmacological targets on VMAT2.

The novel pyrrolidine nor-lobelane analog UKCP-110 [cis-2,5-di-(2-phenethyl)-pyrrolidine hydrochloride] inhibits VMAT2 function, methamphetamine-evoked dopamine release, and methamphetamine self-administration in rats.

Both lobeline and lobelane attenuate methamphetamine self-administration in rats by decreasing methamphetamine-induced dopamine release via interaction with vesicular monoamine transporter-2 (VMAT2). A novel derivative of nor-lobelane, cis-2,5-di-(2-phenethyl)-pyrrolidine hydrochloride (UKCP-110), and its trans-isomers, (2R,5R)-trans-di-(2-phenethyl)-pyrrolidine hydrochloride (UKCP-111) and (2S,5S)-trans-di-(2-phenethyl)-pyrrolidine hydrochloride (UKCP-112), were evaluated for inhibition of [(3)H]dihydrotetrabenazine binding and [(3)H]dopamine uptake by using a rat synaptic vesicle preparation to assess VMAT2 interaction. Compounds were evaluated for inhibition of [(3)H]nicotine and [(3)H]methyllycaconitine binding to assess interaction with the major nicotinic receptor subtypes. In addition, compounds were evaluated for inhibition of methamphetamine-evoked endogenous dopamine release by using striatal slices. The most promising compound, UKCP-110, was evaluated for its ability to decrease methamphetamine self-administration and methamphetamine discriminative stimulus cues and for its effect on food-maintained operant responding. UKCP-110, UKCP-111, and UKCP-112 inhibited [(3)H]dihydrotetrabenazine binding (K(i) = 2.66 ± 0.37, 1.05 ± 0.10, and 3.80 ± 0.31 µM, respectively) and had high potency inhibiting [(3)H]dopamine uptake (K(i) = 0.028 ± 0.001, 0.046 ± 0.008, 0.043 ± 0.004 µM, respectively), but lacked affinity at nicotinic receptors. Although the trans-isomers did not alter methamphetamine-evoked dopamine release, UKCP-110 inhibited (IC(50) = 1.8 ± 0.2 µM; I(max) = 67.18 ± 6.11 µM) methamphetamine-evoked dopamine release. At high concentrations, UKCP-110 also increased extracellular dihydroxyphenylacetic acid. It is noteworthy that UKCP-110 decreased the number of methamphetamine self-infusions, while having no effect on food-reinforced behavior or the methamphetamine stimulus cue. Thus, UKCP-110 represents a new lead in the development of novel pharmacotherapies for the treatment of methamphetamine abuse.

Nicotinic ligands modulate ethanol-induced dopamine function in mice.

The aim of the present study was to examine the effects of two neuronal nicotinic acetylcholine receptor ligands on acute ethanol-induced dopamine (DA) function in the C57BL/6J mouse ventral striatumusing an ex vivo assay and high-performance liquid chromatography coupled with electrochemicaldetection. Acute systemic injection of ethanol (2.5 g/kg) significantly increased the DA and dihydroxyphenylacetic acid (DOPAC) content in the ventral striatum. Pretreatment with lobeline (1 or 10 mg/kg) inhibited the ethanol-induced increase in the tissue DA and DOPAC content in the ventral striatum. Similarly, pretreatment with cytisine (0.5 or 3 mg/kg) also reduced the ethanol-induced increase in the tissue DA and DOPAC content in the ventral striatum. However, when given alone lobeline or cytisine did not produce significant effect on the DA or DOPAC content in the ventral striatum compared with controls. These findings provide evidence that lobeline and cytisine modulate ethanol-induced DA function by targeting nicotinic acetylcholine receptors in the ventral striatum, a reward-relevant brain region implicated in ethanol dependence.

GZ-793A inhibits the neurochemical effects of methamphetamine via a selective interaction with the vesicular monoamine transporter-2.

Lobeline and lobelane inhibit the behavioral and neurochemical effects of methamphetamine via an interaction with the vesicular monoamine transporter-2 (VMAT2). However, lobeline has high affinity for nicotinic receptors, and tolerance develops to the behavioral effects of lobelane. A water-soluble analog of lobelane, R-N-(1,2-dihydroxypropyl)-2,6-cis-di-(4-methoxyphenethyl)piperidine hydrochloride (GZ-793A), also interacts selectively with VMAT2 to inhibit the effects of methamphetamine, but does not produce behavioral tolerance. The current study further evaluated the mechanism underlying the GZ-793A-mediated inhibition of the neurochemical effects of methamphetamine. In contrast to lobeline, GZ-793A does not interact with the agonist recognition site on α4ß2* and α7* nicotinic receptors. GZ-793A (0.3-100µM) inhibited methamphetamine (5µM)-evoked fractional dopamine release from rat striatal slices, and did not evoke dopamine release in the absence of methamphetamine. Furthermore, GZ-793A (1-100µM) inhibited neither nicotine (30µM)-evoked nor electrical field-stimulation-evoked (100Hz/1min) fractional dopamine release. Unfortunately, GZ-793A inhibited [3H]dofetilide binding to human-ether-a-go-go related gene channels expressed on human embryonic kidney cells, and further, prolonged action potentials in rabbit cardiac Purkinje fibers, suggesting the potential for GZ-793A to induce ventricular arrhythmias. Thus, GZ-793A selectively inhibits the neurochemical effects of methamphetamine and lacks nicotinic receptor interactions; however, development as a pharmacotherapy for methamphetamine use disorders will not be pursued due to its potential cardiac liabilities.

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.

The comparative binding characteristics of nicotinic ligands and their pharmacology.

Five drugs [(-)- and (+)-nicotine, (-)-lobeline, (-)-anabasine and (-)-cytisine] were infused IV into the urethane-pentobarbital anesthetized rat. Changes in heart rate, blood pressure, respiratory rate, minute and tidal volume, which appeared to be largely centrally mediated, were studied. Each of these compounds produced different pharmacologic profiles. The nature of these dissimilarities is not readily explained on the basis of pharmacokinetic considerations suggesting that the drugs have different mechanisms of action. Binding data obtained with these compounds using the rat brain P2 preparation also show differences. (-)-Lobeline and (-)-anabasine, like the nicotinic antagonists mecamylamine and hexamethonium, bind predominantly to low affinity sites with KDs in the micromolar range whereas (-)-cytisine binds only to a single high affinity site with a KD in the nanomolar range. Further, the binding patterns of these drugs are different from (-)- and (+)-nicotine which bind to both high and low affinity sites but differ from each other in binding characteristics. Thus the binding data are consistent with the pharmacologic data in suggesting that the drugs have different modes of action and support the concept that the low affinity site has an important role in the central nervous system action of these compounds.

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.

Neuronal nicotinic receptor ligands modulate chronic nicotine-induced ethanol consumption in C57BL/6J mice.

Alcohol and nicotine are commonly abused drugs in humans and evidence suggests that neuronal nicotinic acetylcholine receptors (nAChRs) in the midbrain dopamine system are common targets for the neurobehavioral interactions between alcohol (ethanol) and nicotine. The present study examined the efficacy of nAChR ligands with different pharmacological profiles such as cytisine, lobeline and dihydro-ß-erythroidine (DHßE) to modulate chronic nicotine-induced increase in ethanol intake by C57BL/6J mice, using a two-bottle choice procedure. After establishment of baseline ethanol preference (10%, v/v), animals received daily subcutaneous injections of saline, nicotine (0.4 mg/kg) or different doses of cytisine, lobeline or DHßE 15 min prior to nicotine, for 10 days. Ethanol and water were presented immediately after the last (saline or nicotine) injection and fluid levels were monitored for post 1 h and 2 h treatment. Compared to control, nicotine injection significantly increased mean ethanol intake over 10 days, at both post 1 h and 2 h. Pretreatment with cytisine (0.5, 1.5 or 3.0 mg/kg) or lobeline (4.0 or 10.0 mg/kg) significantly reduced nicotine-induced increase in ethanol intake post 1 h and 2 h, without affecting water consumption. DHßE (0.5 or 2.0 mg/kg) failed to suppress nicotine-induced ethanol intake across 2 h post injection. These results indicate that nAChRmediated signaling is critical in regulating nicotine-induced ethanol drinking behaviors.

A Drosophila gustatory receptor essential for aversive taste and inhibiting male-to-male courtship.

Contact chemosensation is required for several behaviors that promote insect survival. These include evasive behaviors such as suppression of feeding on repellent compounds, known as antifeedants, and inhibition of male-to-male courtship. However, the gustatory receptors (GRs) required for responding to nonvolatile avoidance chemicals are largely unknown. Exceptions include Drosophila GR66a and GR93a, which are required to prevent ingestion of caffeine, and GR32a, which is necessary for inhibiting male-to-male courtship. However, GR32a is dispensable for normal taste. Thus, distinct GRs may function in sensing avoidance pheromones and antifeedants. Here, we describe the requirements for GR33a, which is expressed widely in gustatory receptor neurons (GRNs) that respond to aversive chemicals. Gr33a mutant flies were impaired in avoiding all nonvolatile repellents tested, ranging from quinine to denatonium, lobeline, and caffeine. Gr33a mutant males also displayed increased male-to-male courtship, implying that it functioned in the detection of a repulsive male pheromone. In contrast to the broadly required olfactory receptor (OR) OR83b, which is essential for trafficking other ORs, GR66a and GR93a are localized normally in Gr33a mutant GRNs. Thus, rather than regulating GR trafficking, GR33a may be a coreceptor required for sensing all nonvolatile repulsive chemicals, including tastants and pheromones.

Computational neural network analysis of the affinity of lobeline and tetrabenazine analogs for the vesicular monoamine transporter-2.

Back-propagation artificial neural networks (ANNs) were trained on a dataset of 104 VMAT2 ligands with experimentally measured log(1/K(i)) values. A set of related descriptors, including topological, geometrical, GETAWAY, aromaticity, and WHIM descriptors, was selected to build nonlinear quantitative structure-activity relationships. A partial least squares (PLS) regression model was also developed for comparison. The nonlinearity of the relationship between molecular descriptors and VMAT2 ligand activity was demonstrated. The obtained neural network model outperformed the PLS model in both the fitting and predictive ability. ANN analysis indicated that the computed activities were in excellent agreement with the experimentally observed values (r(2)=0.91, rmsd=0.225; predictive q(2)=0.82, loormsd=0.316). The generated models were further tested by use of an external prediction set of 15 molecules. The nonlinear ANN model has r(2)=0.93 and root-mean-square errors of 0.282 compared with the experimentally measured activity of the test set. The stability test of the model with regard to data division was found to be positive, indicating that the generated model is predictive. The modeling study also reflected the important role of atomic distribution in the molecules, size, and steric structure of the molecules when they interact with the target, VMAT2. The developed models are expected to be useful in the rational design of new chemical entities as ligands of VMAT2 and for directing synthesis of new molecules in the future.

Regulation of brain nicotinic receptors by chronic agonist infusion.

Several studies have demonstrated that chronic treatment with nicotine elicits an increase in the number of brain nicotinic receptors. To determine whether this effect is elicited by other nicotinic agonists found in tobacco, the effects of chronic infusion with nicotine on brain nicotinic receptors were compared with those after anabasine and lobeline. C57BL/6 mice were infused with saline or equimolar doses (18.5 mumol/kg/h) of nicotine, anabasine, or lobeline for 8 days. Nicotinic receptors, quantified by the binding of [3H]nicotine and [125I]iodo-alpha-bungarotoxin (alpha-[125I]BTX), and muscarinic receptors, quantified by the binding of [3H]quinuclidinyl benzilate ([3H]QNB), were then assayed in eight brain regions. An increase in [3H]nicotine binding was observed in all regions except cerebellum following chronic infusion with nicotine and anabasine, whereas lobeline did not alter the number or affinity of these binding sites. This increase was due to changes in Bmax and not in the affinity of the receptor for the ligand (KD). A slight increase in alpha-[125I]BTX binding was observed in cortex following chronic anabasine infusion. [3H]QNB binding sites were largely unaltered following chronic infusion with any of the nicotinic analogs. The levels of the agonists in the brain were also determined after chronic treatment, and the amounts of lobeline and anabasine were found to be higher than that of nicotine. Thus, the failure of lobeline to elicit changes in nicotine binding is not due to reduced brain concentrations.

Effects of SKF 525-A, phenobarbital and 3-methylcholanthrene on the toxicity of lobeline sulfate.

Pretreatment of mice with the microsomal enzyme inducers and inhibitors modified the toxicity of lobeline sulfate. The intraperitoneal LD50 of lobeline sulfate following SKF 525-A (75 mg/kg), phenobarbital (PB), and 3-methylcholanthrene (3-MC) were 18.3, 85.5 and 82.1 mg/kg, respectively, as compared to that of saline treated controls, 55.3 mg/kg. Pretreatment of mice with disulfiram (DSF), diethylmaleate (DEM), or ethoxyquin hydrochloride (EQ-HC1) exhibited a very slight effect on the toxicity of lobeline sulfate. These results suggest that the hepatic microsomal monooxygenase system, but not glutathione (GSH), is involved in the detoxication of lobeline sulfate.