Pre-clinical properties of the alpha4beta2 nicotinic acetylcholine receptor partial agonists varenicline, cytisine and dianicline translate to clinical efficacy for nicotine dependence

Nicotinic receptors in airway disease

With continued smoking of tobacco products and expanded use of nicotine delivery devices worldwide, understanding the impact of smoking and vaping on respiratory health remains a major global unmet need. Although multiple studies have shown a strong association between smoking and asthma, there is a relative paucity of mechanistic understanding of how elements in cigarette smoke impact the airway. Recognizing that nicotine is a major component in both smoking and vaping products, it is critical to understand the mechanisms by which nicotine impacts airways and promotes lung diseases such as asthma. There is now increasing evidence that α7 nicotinic acetylcholine receptors (α7nAChRs) are critical players in nicotine effects on airways, but the mechanisms by which α7nAChR influences different airway cell types have not been widely explored. In this review, we highlight and integrate the current state of knowledge regarding nicotine and α7nAChR in the context of asthma and identify potential approaches to alleviate the impact of smoking and vaping on the lungs.

Varenicline enhances recognition memory via α7 nicotinic acetylcholine receptors in the medial prefrontal cortex in male mice

Previous studies postulated that chronic administration of varenicline, a partial and full agonist at α4β2 and α7 nicotinic acetylcholine receptors (nAChRs), respectively, enhances recognition memory. However, whether its acute administration is effective, on which brain region(s) it acts, and in what signaling it is involved, remain unknown. To address these issues, we conducted a novel object recognition test using male C57BL/6J mice, focusing on the medial prefrontal cortex (mPFC), a brain region associated with nicotine-induced enhancement of recognition memory. Systemic administration of varenicline before the training dose-dependently enhanced recognition memory. Intra-mPFC varenicline infusion also enhanced recognition memory, and this enhancement was blocked by intra-mPFC co-infusion of a selective α7, but not α4β2, nAChR antagonist. Consistent with this, intra-mPFC infusion of a selective α7 nAChR agonist augmented object recognition memory. Furthermore, intra-mPFC co-infusion of U-73122, a phospholipase C (PLC) inhibitor, or 2-aminoethoxydiphenylborane (2-APB), an inositol trisphosphate (IP3) receptor inhibitor, suppressed the varenicline-induced memory enhancement, suggesting that α7 nAChRs may also act as Gq-coupled metabotropic receptors. Additionally, whole-cell recordings from mPFC layer V pyramidal neurons in vitro revealed that varenicline significantly increased the summation of evoked excitatory postsynaptic potentials, and this effect was suppressed by U-73122 or 2-APB. These findings suggest that varenicline might acutely enhance recognition memory via mPFC α7 nAChR stimulation, followed by mPFC neuronal excitation, which is mediated by the activation of PLC and IP3 receptor signaling. Our study provides evidence supporting the potential repositioning of varenicline as a treatment for cognitive impairment.

Evaluation of novel epibatidine analogs in the rat nicotine drug discrimination assay and in the rat chronic constriction injury neuropathic pain model

Nicotine is the primary psychoactive component responsible for maintaining tobacco dependence in humans. Chronic pain is often a consequence of tobacco-related pathologies, and the development of a dual therapeutic that could treat chronic pain and tobacco dependence would be advantageous. Epibatidine reliably substitutes for nicotine in the drug discrimination assay, and is a potent analgesic, but has a side-effect profile that limits its therapeutic potential. Thus, considerable efforts to produce epibatidine derivatives are underway. Here we tested three epibatidine derivatives, 2'-fluoro-3'-(4-nitrophenyl)deschloroepibatidine (RTI-7527-102; i.e., RTI-102), 2'-fluorodeschloroepibatidine (RTI-7527-36; i.e., RTI-36), and 3'-(3″-dimethylaminophenyl)-epibatidine (RTI-7527-76; i.e., RTI-76) in both the rat nicotine drug discrimination assay as well as in the rat chronic constriction injury (CCI) of the sciatic nerve neuropathic pain model. Male and female Sprague-Dawley rats were trained on a fixed-ratio 10 schedule to discriminate nicotine (0.32 mg/kg base) from vehicle. All compounds dose-dependently substituted for nicotine, without significant decreases in response rates. In the discrimination assay the rank order potency was RTI-36 > nicotine > RTI-102 > RTI-76. Evidence suggests the α4β2* subtype is particularly important to nicotine-related abuse potential. Thus, here we utilized the antagonist dihydro-β-erythroidine (DHβE) to examine relative β2 subunit contribution. DHβE (3.2 mg/kg, s.c.) antagonized the discriminative stimulus effects of nicotine. However, relative to antagonism of nicotine, DHβE produced less antagonism of RTI-102 and RTI-76 and greater antagonism of RTI-36. It is likely that at nicotinic receptor subunits RTI-102, RTI-76 and RTI-36 possess differing activity. To confirm that the full discriminative stimulus of these compounds was due to nAChR activity beyond the β2 subunit, we examined these compounds in the presence of the non-selective nicotinic receptor antagonist mecamylamine. Mecamylamine (0.56 mg/kg, s.c.) pretreatment abolished nicotine-paired lever responding for all compounds. In a separate cohort, male and female Sprague-Dawley rats underwent CCI surgery and tested for CCI-induced mechanical allodynia via the von Frey assay. Each compound produced CCI-induced mechanical allodynia reversal. RTI-36 displayed higher potency than either RTI-102 or RTI-76. These novel epibatidine analogs may prove to be useful tools in the fight against nicotine dependence as well as novel neuropathic pain analgesics.

Reduction of nicotine and ethanol intake in alcohol-preferring (UChB) female rats by the α4β2 nicotinic acetylcholine receptor partial agonists 5-bromocytisine and cytisine

RATIONALE

Neuronal nicotinic acetylcholine receptors (nAChRs) are implicated in the reinforcing effects of nicotine and ethanol. Previous studies have shown that cytisine and its 5-bromo derivative are partial agonists at the α4β2 nAChRs and that the parent molecule cytisine is effective in reducing both nicotine- and ethanol-self-administration in rats. However, whether 5-bromocytisine affects nicotine or ethanol self-administration was unknown.

OBJECTIVES

The present study compared the effects of 5-bromocytisine and cytisine on nicotine self-administration and further assessed the effect of daily drug injection on voluntary ethanol consumption in alcohol-preferring female rats. Animals were administered a 1.5mg/kg i.p. dose of 5-bromocytisine or cytisine every day for 15-16 days.

RESULTS

The initial efficacy of 5-bromocytisine and cytisine in reducing nicotine intake was similar (-80%) while for voluntary ethanol intake 5-bromocytisine was a superior inhibitor over cytisine (-78% and -40% respectively). The efficacy of cytisine began to diminish after 10 days of daily administration, which was attributed to tolerance development to its inhibitory effects both on nicotine and ethanol self-administration. Tolerance did not develop for 5-bromocytisine.

CONCLUSION

5-Bromocytisine, a weaker α4β2 nAChR partial agonist than cytisine, also produces a sustained inhibition of both nicotine and ethanol self-administration, and unlike cytisine, it does not develop tolerance.

Nicotine and Cytisine Embryotoxicity in the Experimental Zebrafish Model

Tobacco smoking is one of the most serious health problems. Potentially lethal effects of nicotine for adults can occur with as little as 30 to 60 mg, although severe symptoms can arise with lower doses. Furthermore, the route of administration also influences the toxicity. Cytisine is one of the most popular medications in nicotinism treatment. Like nicotine, cytisine is a plant alkaloid, signaling through nicotinic acetylcholine receptors. Our study evaluated the effects of cytisine in nicotine-induced embryotoxic effects using zebrafish larvae. We examined the teratogenicity of nicotine and cytisine alone or in combination. Nicotine increased mortality and delayed hatching of zebrafish larvae in a dose-dependent manner. Cytisine did not affect mortality in a wide range of concentrations, and hatching delay was observed only at the highest concentrations, above 2 mM. Administering compounds together partially reduced the adverse teratogenic effect induced by nicotine alone. The protective effect of cytisine against the nicotine effect, observed in zebrafish, will contribute to future studies or treatments related to nicotine addiction or prenatal nicotine exposure in humans.

Behavioral Study of 3- and 5-Halocytisine Derivatives in Zebrafish Using the Novel Tank Diving Test (NTT)

Anxiety is a serious mental disorder, and recent statistics have determined that 35.12% of the global population had an anxiety disorder during the COVID-19 pandemic. A mechanism associated with anxiolytic effects is related to nicotinic acetylcholine receptor (nAChR) agonists, principally acting on the α4β2 nAChR subtype. nAChRs are present in different animal models, including murine and teleosteos ones. Zebrafish has become an ideal animal model due to its high human genetic similarities (70%), giving it high versatility in different areas of study, among them in behavioral studies related to anxiety. The novel tank diving test (NTT) is one of the many paradigms used for studies on new drugs related to their anxiolytic effect. In this work, an adult zebrafish was used to determine the behavioral effects of 3- and 5-halocytisine derivatives, using the NTT at different doses. Our results show that substitution at position 3 by chlorine or bromine decreases the time spent by the fish at the bottom compared to the control. However, the 3-chloro derivative at higher doses increases the bottom dwelling time. In contrast, substitution at the 5 position increases bottom dwelling at all concentrations showing no anxiolytic effects in this model. Unexpected results were observed with the 5-chlorocytisine derivative, which at a concentration of 10 mg/L produced a significant decrease in bottom dwelling and showed high times of freezing. In conclusion, the 3-chloro and 3-bromo derivatives show an anxiolytic effect, the 3-chlorocytisine derivative being more potent than the 3-bromo derivative, with the lowest time at the bottom of the tank at 1mg/L. On the other hand, chlorine, and bromine at position 5 produce an opposite effect.

The role of nicotinic receptors in alcohol consumption

The use of alcohol causes significant morbidity and mortality across the globe. Alcohol use disorder (AUD) is defined by the excessive use of this drug despite a negative impact on the individual's life. While there are currently medications available to treat AUD, they have limited efficacy and several side effects. As such, it is essential to continue to look for novel therapeutics. One target for novel therapeutics is nicotinic acetylcholine receptors (nAChRs). Here we systematically review the literature on the involvement of nAChRs in alcohol consumption. Data from both genetic and pharmacology studies provide evidence that nAChRs modulate alcohol intake. Interestingly, pharmacological modulation of all nAChR subtypes examined can decrease alcohol consumption. The reviewed literature demonstrates that nAChRs should continue to be investigated as novel therapeutics for AUD.

Characterization of Ultrapotent Chemogenetic Ligands for Research Applications in Nonhuman Primates

Chemogenetics is a technique for obtaining selective pharmacological control over a cell population by expressing an engineered receptor that is selectively activated by an exogenously administered ligand. A promising approach for neuronal modulation involves the use of "Pharmacologically Selective Actuator Modules" (PSAMs); these chemogenetic receptors are selectively activated by ultrapotent "Pharmacologically Selective Effector Molecules" (uPSEMs). To extend the use of PSAM/PSEMs to studies in nonhuman primates, it is necessary to thoroughly characterize the efficacy and safety of these tools. We describe the time course and brain penetrance in rhesus monkeys of two compounds with promising binding specificity and efficacy profiles in in vitro studies, uPSEM792 and uPSEM817, after systemic administration. Rhesus monkeys received subcutaneous (s.c.) or intravenous (i.v.) administration of uPSEM817 (0.064 mg/kg) or uPSEM792 (0.87 mg/kg), and plasma and cerebrospinal fluid samples were collected over 48 h. Both compounds exhibited good brain penetrance, relatively slow washout, and negligible conversion to potential metabolites─varenicline or hydroxyvarenicline. In addition, we found that neither of these uPSEMs significantly altered the heart rate or sleep. Our results indicate that both compounds are suitable candidates for neuroscience studies using PSAMs in nonhuman primates.

UFR2709, an Antagonist of Nicotinic Acetylcholine Receptors, Delays the Acquisition and Reduces Long-Term Ethanol Intake in Alcohol-Preferring UChB Bibulous Rats

Alcoholism is a worldwide public health problem with high economic cost and which affects health and social behavior. It is estimated that alcoholism kills 3 million people globally, while in Chile it is responsible for around 9 thousand deaths per year. Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels expressed in the central nervous system, and they were suggested to modulate the ethanol mechanism involved in abuse and dependence. Previous work demonstrated a short-term treatment with UFR2709, a nAChRs antagonist, which reduced ethanol intake using a two-bottle free-choice paradigm in University of Chile bibulous (UChB) rats. Here, we present evidence of the UFR2709 efficacy in reducing the acquisition and long-term ethanol consumption. Our results show that UFR2709 (2.5 mg/kg i.p.) reduces the seek behavior and ethanol intake, even when the drug administration was stopped, and induced a reduction in the overall ethanol intake by around 55%. Using naïve UChB bibulous rats, we demonstrate that UFR2709 could delay and reduce the genetically adaptive impulse to seek and drink ethanol and prevent its excessive intake.

Fluorescence activation mechanism and imaging of drug permeation with new sensors for smoking-cessation ligands

Nicotinic partial agonists provide an accepted aid for smoking cessation and thus contribute to decreasing tobacco-related disease. Improved drugs constitute a continued area of study. However, there remains no reductionist method to examine the cellular and subcellular pharmacokinetic properties of these compounds in living cells. Here, we developed new intensity-based drug-sensing fluorescent reporters (iDrugSnFRs) for the nicotinic partial agonists dianicline, cytisine, and two cytisine derivatives - 10-fluorocytisine and 9-bromo-10-ethylcytisine. We report the first atomic-scale structures of liganded periplasmic binding protein-based biosensors, accelerating development of iDrugSnFRs and also explaining the activation mechanism. The nicotinic iDrugSnFRs detect their drug partners in solution, as well as at the plasma membrane (PM) and in the endoplasmic reticulum (ER) of cell lines and mouse hippocampal neurons. At the PM, the speed of solution changes limits the growth and decay rates of the fluorescence response in almost all cases. In contrast, we found that rates of membrane crossing differ among these nicotinic drugs by >30-fold. The new nicotinic iDrugSnFRs provide insight into the real-time pharmacokinetic properties of nicotinic agonists and provide a methodology whereby iDrugSnFRs can inform both pharmaceutical neuroscience and addiction neuroscience.

Neurobiological Mechanisms of Nicotine Reward and Aversion

Neuronal nicotinic acetylcholine receptors (nAChRs) regulate the rewarding actions of nicotine contained in tobacco that establish and maintain the smoking habit. nAChRs also regulate the aversive properties of nicotine, sensitivity to which decreases tobacco use and protects against tobacco use disorder. These opposing behavioral actions of nicotine reflect nAChR expression in brain reward and aversion circuits. nAChRs containing α4 and β2 subunits are responsible for the high-affinity nicotine binding sites in the brain and are densely expressed by reward-relevant neurons, most notably dopaminergic, GABAergic, and glutamatergic neurons in the ventral tegmental area. High-affinity nAChRs can incorporate additional subunits, including β3, α6, or α5 subunits, with the resulting nAChR subtypes playing discrete and dissociable roles in the stimulatory actions of nicotine on brain dopamine transmission. nAChRs in brain dopamine circuits also participate in aversive reactions to nicotine and the negative affective state experienced during nicotine withdrawal. nAChRs containing α3 and β4 subunits are responsible for the low-affinity nicotine binding sites in the brain and are enriched in brain sites involved in aversion, including the medial habenula, interpeduncular nucleus, and nucleus of the solitary tract, brain sites in which α5 nAChR subunits are also expressed. These aversion-related brain sites regulate nicotine avoidance behaviors, and genetic variation that modifies the function of nAChRs in these sites increases vulnerability to tobacco dependence and smoking-related diseases. Here, we review the molecular, cellular, and circuit-level mechanisms through which nicotine elicits reward and aversion and the adaptations in these processes that drive the development of nicotine dependence. SIGNIFICANCE STATEMENT: Tobacco use disorder in the form of habitual cigarette smoking or regular use of other tobacco-related products is a major cause of death and disease worldwide. This article reviews the actions of nicotine in the brain that contribute to tobacco use disorder.

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

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

A Phase I, Open-label, Randomized, 2-Way Crossover Study to Evaluate the Relative Bioavailability of Intranasal and Oral Varenicline

PURPOSE

To estimate the systemic bioavailability of OC-01 (varenicline) nasal spray, an investigational treatment for dry eye disease, relative to oral varenicline approved for smoking cessation.

METHODS

The Study to Evaluate the Relative Bioavailability of Varenicline Administered as OC-01 (Varenicline) Nasal Spray as Compared to Varenicline Administered Orally as Chantix (ZEN study) was a Phase I, open-label, randomized, single-center, 2-way crossover study. On day 1, 22 healthy participants were randomized 1:1 to a single intranasal dose of varenicline 0.12 mg in OC-01 nasal spray or a single oral dose of varenicline 1 mg. On day 15, all participants crossed over to receive a single dose of the alternate treatment. Plasma samples were collected for 6 days after each dose, and pharmacokinetic parameters were estimated using noncompartmental analysis. Tolerability was monitored throughout.

FINDINGS

After a single dose of intranasal varenicline 0.12 mg in OC-01 nasal spray, peak systemic exposure (mean plasma C_max) was 0.34 ng/mL, which occurred at a median T_max of 2.0 hours. In comparison, mean plasma C_max after oral varenicline 1 mg was 4.63 ng/mL at a median T_max of 3.0 hours. On the basis of geometric mean ratio point estimates, peak exposure (C_max) and total exposure (AUC_0-∞) after intranasal varenicline 0.12 mg were 7.0% and 7.5%, respectively, of the systemic exposure associated with oral varenicline 1 mg. Dose-normalized C_max and AUC_0-∞ for intranasal varenicline remained 39% and 33% lower versus oral varenicline, respectively. No new or unexpected tolerability signals were detected.

IMPLICATIONS

At its highest intended single dose in OC-01 nasal spray, intranasal varenicline delivered less drug to the systemic circulation than oral varenicline at its highest approved single dose. ClinicalTrials.gov identifier: NCT04072146.

Attenuating nicotine's effects with high affinity human anti-nicotine monoclonal antibodies

Use of nicotine-specific monoclonal antibodies (mAbs) to sequester and reduce nicotine distribution to brain has been proposed as a therapeutic approach to treat nicotine addiction (the basis of tobacco use disorder). A series of monoclonal antibodies with high affinity for nicotine (nic•mAbs) was isolated from B-cells of vaccinated smokers. Genes encoding 32 unique nicotine binding antibodies were cloned, and the mAbs expressed and tested by surface plasmon resonance to determine their affinity for S-(–)-nicotine. The highest affinity nic•mAbs had binding affinity constants (K_D) between 5 and 67 nM. The 4 highest affinity nic•mAbs were selected to undergo additional secondary screening for antigen-specificity, protein properties (including aggregation and stability), and functional in vivo studies to evaluate their capacity for reducing nicotine distribution to brain in rats. The 2 most potent nic•mAbs in single-dose nicotine pharmacokinetic experiments were further tested in a dose-response in vivo study. The most potent lead, ATI-1013, was selected as the lead candidate based on the results of these studies. Pretreatment with 40 and 80 mg/kg ATI-1013 reduced brain nicotine levels by 56 and 95%, respectively, in a repeated nicotine dosing experiment simulating very heavy smoking. Nicotine self-administration was also significantly reduced in rats treated with ATI-1013. A pilot rat 30-day repeat-dose toxicology study (4x200mg/kg ATI-1013) in the presence of nicotine indicated no drug-related safety concerns. These data provide evidence that ATI-1013 could be a potential therapy for the treatment of nicotine addiction.

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.

Translational PET applications for brain circuit mapping with transgenic neuromodulation tools

Transgenic neuromodulation tools have transformed the field of neuroscience over the past two decades by enabling targeted manipulation of neuronal populations and circuits with unprecedented specificity. Chemogenetic and optogenetic neuromodulation systems are among the most widely used and allow targeted control of neuronal activity through the administration of a selective compound or light, respectively. Innovative genetic targeting strategies are utilized to transduce specific cells to express transgenic receptors and opsins capable of manipulating neuronal activity. These allow mapping of neuroanatomical projection sites and link cellular manipulations with brain circuit functions and behavior. As these tools continue to expand knowledge of the nervous system in preclinical models, developing translational applications for human therapies is becoming increasingly possible. However, new strategies for implementing and monitoring transgenic tools are needed for safe and effective use in translational research and potential clinical applications. A major challenge for such applications is the need to track the location and function of chemogenetic receptors and opsins in vivo, and new developments in positron emission tomography (PET) imaging techniques offer promising solutions. The goal of this review is to summarize current research combining transgenic tools with PET for in vivo mapping and manipulation of brain circuits and to propose future directions for translational applications.

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

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

Enhancement of alcohol aversion by the nicotinic acetylcholine receptor drug sazetidine-A

The prevalence of alcohol use disorders (AUDs) has steadily increased in the United States over the last 30 years. Alcohol acts on multiple receptor systems including the nicotinic acetylcholine receptors (nAChRs), which are known to mediate alcohol consumption and reward. We previously reported that the preclinical drug sazetidine-A, a nAChR agonist and desensitizer, reduces alcohol consumption without affecting nicotine consumption in C57BL/6J mice. Here, we found that sazetidine-A enhances the expression of alcohol aversion without affecting the expression or acquisition of conditioned alcohol reward in C57BL/6J mice. Microinjection of sazetidine-A into the ventral midbrain targeting the ventral tegmental area (VTA) reduced binge alcohol consumption, implicating this region in mediating the effects of sazetidine-A. Furthermore, the sazetidine-A-induced reduction in alcohol consumption was mediated by non-α4 containing nAChRs, as sazetidine-A reduced binge alcohol consumption in both α4 knock-out and wild-type mice. Finally, we found that in mice pretreated with sazetidine-A, alcohol induced Fos transcript in Th-, but not Gad2-expressing neurons in the VTA as measured by increased Fos transcript expression. In summary, we find that sazetidine-A enhances the expression of alcohol aversion, which may underlie the reduction in alcohol consumption induced by sazetidine-A. Elucidating the identity of non-α4 nAChRs in alcohol aversion mechanisms will provide a better understanding the complex role of nAChRs in alcohol addiction and potentially reveal novel drug targets to treat AUDs.

Varenicline rescues nicotine-induced decrease in motivation for sucrose reinforcement

Varenicline is one of the top medications used for smoking cessation and is often prescribed before termination of nicotine use. The effect of this combined nicotine and varenicline use on the reward system and motivation for primary reinforcement is underexplored. The goal of this study was to assess the effects of nicotine and varenicline on motivation for a food reinforcer. In Experiment 1, we first assessed the responding for sucrose after pretreatment with nicotine (0, 0.1, or 0.4 mg/kg) and varenicline (0.0, 0.1, 1.0 mg/kg) using a behavioral economics approach. The responding for sucrose was then assessed using a progressive ratio schedule of reinforcement after pretreatment with all possible combinations of nicotine and varenicline doses. In Experiment 2, rats were assessed for the consumption of sucrose in home cages after pretreatment with nicotine and varenicline. We found that (a) nicotine decreased economic demand for sucrose, (b) varenicline rescued nicotine-induced reduction in economic demand for sucrose, and (c) history of varenicline treatment predicted responding for sucrose on a progressive ratio schedule of reinforcement where rats with a history of varenicline treatment responded significantly lower for sucrose across nicotine doses than rats that had not been exposed to varenicline. The results of Experiment 2 largely confirmed that nicotine decreases motivation for sucrose using a passive consumption protocol and that varenicline rescues this effect. Overall, these findings suggest that varenicline interacts with the effects of nicotine by restoring nicotine-induced reduction in motivation for appetitive rewards.

Determination of Cytisine and N-Methylcytisine from Selected Plant Extracts by High-Performance Liquid Chromatography and Comparison of Their Cytotoxic Activity

Quinolizidine alkaloids exhibit various forms of biological activity. A lot of them were found in the Leguminosae family, including Laburnum and Genista. The aim of the study was the optimization of a chromatographic system for the analysis of cytisine and N-methylcytisine in various plant extracts as well as an investigation of the cytotoxic activities of selected alkaloids and plant extracts obtained from Laburnum anagyroides, Laburnum anagyroides L. quercifolium, Laburnum alpinum, Laburnum watereri, Genista germanica, and Genista tinctoria against various cancer cell lines. The determination of investigated compounds was performed by High Performance Liquid Chromatography with Diode Array Detection (HPLC-DAD), while High Performance Liquid Chromatography coupled with Quadrupole Time-of-Flight–Mass Spectrometry (HPLC-QTOF-MS) was applied for the qualitative analysis of plant extracts. The retention, separation selectivity, peaks shape, and systems efficiency obtained for cytisine and N-methylcytisine in different chromatographic systems were compared. The application of columns with alkylbonded and phenyl stationary phases led to a very weak retention of cytisine and N-methylcytisine, even when the mobile phases containing only 5% of organic modifiers were used. The strongest retention was observed when hydrophilic interaction chromatography (HILIC) or especially when ion exchange chromatography (IEC) were applied. The most optimal system in terms of alkaloid retention, peak shape, and system efficiency containing an strong cation exchange (SCX) stationary phase and a mobile phase consisted of 25% acetonitrile and formic buffer at pH 4.0 was applied for investigating alkaloids analysis in plant extracts. Cytotoxic properties of the investigated plant extracts as well as cytisine and N-methylcytisine were examined using human tongue squamous carcinoma cells (SCC-25), human pharyngeal squamous carcinoma cells (FaDu), human triple-negative breast adenocarcinoma cell line (MDA-MB-231), and human breast adenocarcinoma cell line (MCF-7). The highest cytotoxic activity against FaDu, MCF-7, and MDA-MB cancer cell lines was observed after applying the Genista germanica leaves extract. In contrast, the highest cytotoxic activity against SCC-25 cell line was obtained after treating with the seed extract of Laburnum watereri. The investigated plant extracts exhibit significant cytotoxicity against the tested human cancer cell lines and seem to be promising for further research on its anticancer activity.

An Integrated Genomic Strategy to Identify CHRNB4 as a Diagnostic/Prognostic Biomarker for Targeted Therapy in Head and Neck Cancer

Although many studies have shown the association between smoking and the increased incidence and adverse prognosis of head and neck squamous cell carcinoma (HNSCC), the mechanisms and pharmaceutical targets involved remain unclear. Here, we integrated gene expression signatures, genetic alterations, and survival analyses to identify prognostic indicators and therapeutic targets for smoking HNSCC patients, and we discovered that the FDA-approved drug varenicline inhibits the target for cancer cell migration/invasion. We first identified 18 smoking-related and prognostic genes for HNSCC by using RNA-Seq and clinical follow-up data. One of these genes, CHRNB4 (neuronal acetylcholine receptor subunit beta-4), increased the risk of death by approximately threefold in CHRNB4-high expression smokers compared to CHRNB4-low expression smokers (log rank, p = 0.00042; hazard ratio, 2.82; 95% CI, 1.55–5.14), former smokers, and non-smokers. Furthermore, we examined the functional enrichment of co-regulated genes of CHRNB4 and its 246 frequently occurring copy number alterations (CNAs). We found that these genes were involved in promoting angiogenesis, resisting cell death, and sustaining proliferation, and contributed to much worse outcomes for CHRNB4-high patients. Finally, we performed CHRNB4 gene editing and drug inhibition assays, and the results validate these observations. In summary, our study suggests that CHRNB4 is a prognostic indicator for smoking HNSCC patients and provides a potential new therapeutic drug to prevent recurrence or distant metastasis.

Unequal interactions between alcohol and nicotine co-consumption: suppression and enhancement of concurrent drug intake

RATIONALE

Alcohol and nicotine addiction are prevalent conditions that co-occur. Despite the prevalence of co-use, factors that influence the suppression and enhancement of concurrent alcohol and nicotine intake are largely unknown.

OBJECTIVES

Our goals were to assess how nicotine abstinence and availability influenced concurrent alcohol consumption and to determine the impact of quinine adulteration of alcohol on aversion-resistant alcohol consumption and concurrent nicotine consumption.

METHODS

Male and female C57BL/6J mice voluntarily consumed unsweetened alcohol, nicotine, and water in a chronic 3-bottle choice procedure. In experiment 1, nicotine access was removed for 1 week and re-introduced the following week, while the alcohol and water bottles remained available at all times. In experiment 2, quinine (100-1000 μM) was added to the 20% alcohol bottle, while the nicotine and water bottles remained unaltered.

RESULTS

In experiment 1, we found that alcohol consumption and preference were unaffected by the presence or absence of nicotine access in both male and female mice. In experiment 2a, we found that quinine temporarily suppressed alcohol intake and enhanced concurrent nicotine, but not water, preference in both male and female mice. In experiment 2b, chronic quinine suppression of alcohol intake increased nicotine consumption and preference in female mice without affecting water preference, whereas it increased water and nicotine preference in male mice.

CONCLUSIONS

Quinine suppression of alcohol consumption enhanced the preference for concurrent nicotine preference in male and female mice, suggesting that mice compensate for the quinine adulteration of alcohol by increasing their nicotine preference.

Expression of α3β2β4 nicotinic acetylcholine receptors by rat adrenal chromaffin cells determined using novel conopeptide antagonists

Adrenal chromaffin cells release neurotransmitters in response to stress and may be involved in conditions such as post-traumatic stress and anxiety disorders. Neurotransmitter release is triggered, in part, by activation of nicotinic acetylcholine receptors (nAChRs). However, despite decades of use as a model system for studying exocytosis, the nAChR subtypes involved have not been pharmacologically identified. Quantitative real-time PCR of rat adrenal medulla revealed an abundance of mRNAs for α3, α7, β2, and β4 subunits. Whole-cell patch-clamp electrophysiology of chromaffin cells and subtype-selective ligands were used to probe for nAChRs derived from the mRNAs found in adrenal medulla. A novel conopeptide antagonist, PeIA-5469, was created that is highly selective for α3β2 over other nAChR subtypes heterologously expressed in Xenopus laevis oocytes. Experiments using PeIA-5469 and the α3β4-selective α-conotoxin TxID revealed that rat adrenal medulla contain two populations of chromaffin cells that express either α3β4 nAChRs alone or α3β4 together with the α3β2β4 subtype. Conclusions were derived from observations that acetylcholine-gated currents in some cells were sensitive to inhibition by PeIA-5469 and TxID, while in other cells, currents were sensitive only to TxID. Expression of functional α7 nAChRs was determined using three α7-selective ligands: the agonist PNU282987, the positive allosteric modulator PNU120596, and the antagonist α-conotoxin [V11L,V16D]ArIB. The results of these studies identify for the first time the expression of α3β2β4 nAChRs as well as functional α7 nAChRs by rat adrenal chromaffin cells.

Randomized Clinical Trials Investigating Innovative Interventions for Smoking Cessation in the Last Decade

Every year, billions of dollars are spent treating smoking and related conditions, yet smoking-related morbidity and mortality continue to rise. There are currently only three FDA-approved medications for smoking cessation: nicotine replacement therapy, bupropion, and varenicline. Although these medications increase abstinence rates, most individuals relapse following treatment. This chapter reviews clinical trials published within the past 10 years investigating novel smoking cessation pharmacotherapies. Among these pharmacotherapies, some showed promising results, such as cytisine and endocannabinoid modulators, whereas others failed to produce significant effects. More research is needed to develop drugs that produce higher rates of long-term abstinence and to determine which subgroups of patients benefit from a given treatment.

UFR2709, a Nicotinic Acetylcholine Receptor Antagonist, Decreases Ethanol Intake in Alcohol-Preferring Rats

Brain nicotinic acetylcholine receptors (nAChRs), a heterogeneous family of pentameric acetylcholine-gated cation channels, have been suggested as molecular targets for the treatment of alcohol abuse and dependence. Here, we examined the effect of the competitive nAChR antagonist UFR2709 on the alcohol consumption of high-alcohol-drinking UChB rats. UChB rats were given free access to ethanol for 24-h periods in a two-bottle free choice paradigm and their ethanol and water intake were measured. The animals were i.p. injected daily for 17 days with a 10, 5, 2.5, or 1 mg/kg dose of UFR2709. Potential confounding motor effects of UFR2709 were assessed by examining the locomotor activity of animals administered the highest dose of UR2709 tested (10 mg/kg i.p.). UFR2709 reduced ethanol consumption and ethanol preference and increased water consumption in a dose-dependent manner. The most effective dose of UFR2709 was 2.5 mg/kg, which induced a 56% reduction in alcohol consumption. Administration of UFR2709 did not affect the weight or locomotor activity of the rats, suggesting that its effects on alcohol consumption and preference were mediated by specific nAChRs.

Radioiodinated 9-fluorenone derivatives for imaging α7-nicotinic acetylcholine receptors

A series of 9H-fluoren-9-one substituents were synthesized and evaluated for imaging cerebral α7-nAChRs. Meta-iodine substituted 9-fluorenone 5 with high binding affinity (K _i = 9.3 nM) and selectivity was radiolabeled with ¹²⁵I. Fully in vitro and in vivo studies of [¹²⁵I]5 have been performed. [¹²⁵I]5 exhibited well brain uptake with a peak concentration of 7.5 ± 0.9% ID/g in mice brains. Moreover, ex vivo autoradiography studies and micro single-photon emission computed tomography (micro-SPECT/CT) dynamic imaging in mice confirmed its in vivo imaging properties. Besides, molecular docking and MD studies were also performed to interpret the binding mechanisms of the two series of ligands towards α7-nAChRs. To conclude, the meta-iodine substituted 9-fluorenone [¹²⁵I]5 could be a promising tracer for imaging α7-nAChRs.

Cytisine for nicotine addiction treatment: a review of pharmacology, therapeutics and an update of clinical trial evidence for smoking cessation

AIMS

To review cytisine's history of use, pre-clinical evidence, clinical pharmacokinetics, efficacy, adverse reactions (ARs) and safety for smoking cessation.

METHODS

A synoptic review of the use of cytisine as a smoking cessation medication, mechanism of action, pharmacokinetics and safety. Relevant literature on data included in these sections were identified through a search of 11 databases with additional literature obtained from reports and monographs. Three databases (PubMed, EMBASE and www.elibrary.ru) were systematically searched for studies published from 2012 to August 2018 in any language to provide an updated meta-analysis of cytisine's efficacy and ARs for smoking cessation compared with placebo. We pooled the relative risks (RR) of abstinence in the efficacy analysis and RR of ARs, either reported by the authors or calculated from the reports.

RESULTS

Cytisine has been in use since 1964 and is currently marketed in 18 countries. Systemic bioavailability from oral ingestion is high and clearance is primarily renal, with minimal or no metabolism. Brain uptake in animal models is moderate. The plasma half-life averages 4.8 hours. Eight studies were included for meta-analysis of efficacy. With heterogeneous results, the overall RR versus placebo of successful continuous abstinence at the longest follow-up was 1.74 [95% confidence interval (CI) = 1.38-2.19]. Nausea, vomiting, dyspepsia, upper abdominal pain and dry mouth that were mild or moderate were the most common ARs, with RR versus placebo 1.10 (95% CI = 0.95-1.28). The cost of cytisine in eastern and central Europe is several-fold less than that of other smoking cessation medications.

CONCLUSIONS

Cytisine is a low-cost medication found to increase the likelihood of smoking cessation. The most frequently reported ARs of cytisine involve gastrointestinal symptoms that are mostly reported as either mild or moderate in severity.

NAChR α4β2 Subtype and their Relation with Nicotine Addiction, Cognition, Depression and Hyperactivity Disorder

BACKGROUND

Neuronal α4β2 nAChRs are receptors involved in the role of neurotransmitters regulation and release, and this ionic channel participates in biological process of memory, learning and attention. This work aims to review the structure and functioning of the α4β2 nAChR emphasizing its role in the treatment of associated diseases like nicotine addiction and underlying pathologies such as cognition, depression and attention-deficit hyperactivity disorder.

METHODS

The authors realized extensive bibliographic research using the descriptors "Nicotine Receptor α4β2" and "cognition", "depression", "attention-deficit hyperactivity disorder", besides cross-references of the selected articles and after analysis of references in the specific literature.

RESULTS

As results, it was that found 179 relevant articles presenting the main molecules with affinity to nAChR α4β2 related to the cited diseases. The α4β2 nAChR subtype is a remarkable therapeutic target since this is the most abundant receptor in the central nervous system.

CONCLUSION

In summary, this review presents perspectives on the pharmacology and therapeutic targeting of α4β2 nAChRs for the treatment of cognition and diseases like nicotine dependence, depression and attention-deficit hyperactivity disorder.

Determining the pharmacokinetics of nicotinic drugs in the endoplasmic reticulum using biosensors

Nicotine dependence is thought to arise in part because nicotine permeates into the endoplasmic reticulum (ER), where it binds to nicotinic receptors (nAChRs) and begins an "inside-out" pathway that leads to up-regulation of nAChRs on the plasma membrane. However, the dynamics of nicotine entry into the ER are unquantified. Here, we develop a family of genetically encoded fluorescent biosensors for nicotine, termed iNicSnFRs. The iNicSnFRs are fusions between two proteins: a circularly permutated GFP and a periplasmic choline-/betaine-binding protein engineered to bind nicotine. The biosensors iNicSnFR3a and iNicSnFR3b respond to nicotine by increasing fluorescence at [nicotine] <1 µM, the concentration in the plasma and cerebrospinal fluid of a smoker. We target iNicSnFR3 biosensors either to the plasma membrane or to the ER and measure nicotine kinetics in HeLa, SH-SY5Y, N2a, and HEK293 cell lines, as well as mouse hippocampal neurons and human stem cell-derived dopaminergic neurons. In all cell types, we find that nicotine equilibrates in the ER within 10 s (possibly within 1 s) of extracellular application and leaves as rapidly after removal from the extracellular solution. The [nicotine] in the ER is within twofold of the extracellular value. We use these data to run combined pharmacokinetic and pharmacodynamic simulations of human smoking. In the ER, the inside-out pathway begins when nicotine becomes a stabilizing pharmacological chaperone for some nAChR subtypes, even at concentrations as low as ∼10 nM. Such concentrations would persist during the 12 h of a typical smoker's day, continually activating the inside-out pathway by >75%. Reducing nicotine intake by 10-fold decreases activation to ∼20%. iNicSnFR3a and iNicSnFR3b also sense the smoking cessation drug varenicline, revealing that varenicline also permeates into the ER within seconds. Our iNicSnFRs enable optical subcellular pharmacokinetics for nicotine and varenicline during an early event in the inside-out pathway.

Designer receptor technology for the treatment of epilepsy

Epilepsy remains refractory to medical treatment in ~30% of patients despite decades of new drug development. Neurosurgery to remove or disconnect the seizure focus is often curative but frequently contraindicated by risks of irreversible impairment to brain function. Novel therapies are therefore required that better balance seizure suppression against the risks of side effects. Among experimental gene therapies, chemogenetics has the major advantage that the action on the epileptogenic zone can be modulated on demand. Two broad approaches are to use a designer G-protein-coupled receptor or a modified ligand gated ion channel, targeted to specific neurons in the epileptogenic zone using viral vectors and cell-type selective promoters. The receptor can be activated on demand by either an exogenous compound or by pathological levels of extracellular glutamate that occur in epileptogenic tissue. We review the principal designer receptor technologies and their modes of action. We compare the drawbacks and benefits of each designer receptor with particular focus on the drug activators and the potential for clinical translation in epilepsy.

•

Inhibitory designer receptors (DRs) allow on-demand suppression of seizures upon activation by exogenous drugs or endogenous neurotransmitters.

•

DRs include modified G-protein coupled receptors, chimaeric ligand-gated ion channels, and mutated non-mammalian channels.

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Identification of drug activators of DRs that are already approved for use in humans significantly accelerates clinical translation.

Prolonged exposure of rats to varenicline increases anxiety and alters serotonergic system, but has no effect on memory

Varenicline is a drug used for smoking addiction cessation treatment and acts as a partial agonist of nicotinic cholinergic receptors. Recent clinical trial data support use of varenicline for treatment of conditions/addictions that are not related to smoking cessation. Considering the importance of this issue and the need for new studies on its effects, especially on behavior, more studies using animal models are necessary. Thus, the aim of this study was to evaluate the effects of prolonged exposure to varenicline in anxiety-like behavior and memory, as well as in cerebral neurochemistry of rats. Male rats received three different doses of varenicline: 0.03 (therapeutic dose for humans), 0.1 and 0.3 mg/kg orally (gavage) for 30 days. Animal behavior was analyzed through open field, elevated plus maze, light/dark box, social interaction, Barnes maze and novel object recognition tests. Neurotransmitter levels and their metabolites in different brain structures (hippocampus, striatum and frontal cortex) were measured. Results showed that prolonged exposure of rats to varenicline: 1) did not interfere in motor activity, but caused an anxiogenic effect on elevated plus maze, light/dark box and social interaction testes; 2) did not alter memory; and 3) promoted alterations on serotoninergic system in the striatum and frontal cortex. In conclusion, compilation of the data indicates that prolonged exposure of rats to varenicline promoted anxiogenic effects and alteration in serotonergic system, which corroborated behavioral findings.

Ultrapotent chemogenetics for research and potential clinical applications

Chemogenetics enables noninvasive chemical control over cell populations in behaving animals. However, existing small-molecule agonists show insufficient potency or selectivity. There is also a need for chemogenetic systems compatible with both research and human therapeutic applications. We developed a new ion channel-based platform for cell activation and silencing that is controlled by low doses of the smoking cessation drug varenicline. We then synthesized subnanomolar-potency agonists, called uPSEMs, with high selectivity for the chemogenetic receptors. uPSEMs and their receptors were characterized in brains of mice and a rhesus monkey by in vivo electrophysiology, calcium imaging, positron emission tomography, behavioral efficacy testing, and receptor counterscreening. This platform of receptors and selective ultrapotent agonists enables potential research and clinical applications of chemogenetics.

The contribution of agonist and antagonist activities of α4β2* nAChR ligands to smoking cessation efficacy: a quantitative analysis of literature data

RATIONALE AND OBJECTIVE

Two mechanisms underlie smoking cessation efficacies of α4β2* nicotinic acetylcholine receptor (nAChR) agonists: a "nicotine-like" agonist activity reduces craving by substituting for nicotine during a quit attempt, and a "nicotine-blocking" antagonist activity attenuates reinforcement by competing with inhaled nicotine during a relapse. To evaluate the contribution of each mechanism to clinical efficacy, we estimated the degree of agonist and antagonist activities of nicotine replacement therapy (NRT), varenicline, cytisine, and the discontinued nAChR agonists dianicline, ABT-418, ABT-089, CP-601927, and CP-601932, relative to the functional effects of nicotine from smoking.

METHODS

Functional activities that occur in vivo with clinical doses were predicted from literature data on binding and functional potencies at the target α4β2 nAChR, as well as at α6β2* nAChRs, and from estimates of free drug exposures in human brain. Agonist activity is comprised of nAChR activation and desensitization, which were expressed as percentages of desensitization and activation by nicotine from smoking. Antagonist activity was expressed as the reduction in nAChR occupancy by nicotine during smoking in the presence of an agonist.

RESULTS

Comparisons with odds ratios at end of treatment suggest that extensive α4β2 and α6β2* nAChR desensitization combined with α6β2* nAChR activation at similar levels as nicotine from smoking is associated with clinical efficacy (NRT, varenicline, cytisine, ABT-418). Effective competition with inhaled nicotine for α4β2 and α6β2* nAChRs further improves clinical efficacy (varenicline). Other discontinued nAChR agonists have lower agonist and antagonist activities at α4β2 nAChRs and are inactive or less efficacious than NRT (dianicline, ABT-089, CP-601927, CP-601932).

CONCLUSION

Three pharmacological effects appear to be key factors underlying smoking cessation efficacy: the degree of activation of α6β2* nAChRs, desensitization of α4β2 and α6β2* nAChRs (agonist activity), and the reduction of nicotine occupancy at α4β2 and α6β2* nAChRs (antagonist activity). No single activity is dominant, and the level of smoking cessation efficacy depends on the profile of these activities achieved at clinical doses. While adequate agonist activity alone seems sufficient for a clinical effect (e.g., NRT, cytisine), clinical efficacy is improved with substantial competitive antagonism of α4β2 nAChRs, i.e., if the drug has a dual agonist-antagonist mechanism of action (e.g., varenicline).

Involvement of Nicotinic Receptor Subtypes in the Behavioral Effects of Nicotinic Drugs in Squirrel Monkeys

Evidence suggests that the α4β2, but not the α7, subtype of the nicotinic acetylcholine receptor (nAChR) plays a key role in mediating the behavioral effects of nicotine and related drugs. However, the importance of other nAChR subtypes remains unclear. The present studies were conducted to examine the involvement of nAChR subtypes by determining the effects of selected nicotinic agonists and antagonists in squirrel monkeys either 1) responding for food reinforcement or 2) discriminating the nicotinic agonist (+)-epibatidine (0.001 mg/kg) from vehicle. In food-reinforcement studies, nicotine, (+)-epibatidine, varenicline and cytisine all produced dose-dependent decreases in rates of food-maintained responding. The rate-decreasing effects of nicotine were antagonized by mecamylamine (nonselective), not appreciably altered by dihydro-β-erythroidine (α4β2 selective), and exacerbated by the nicotinic partial agonists, varenicline and cytisine. Results from discrimination studies show that non-nicotinic drugs did not substitute for (+)-epibatidine, and that except for lobeline, the nicotinic agonists produced either full [(+)-epibatidine, (-)-epibatidine, and nicotine] or partial (varenicline, cytisine, anabaseine, and isoarecolone) substitution for (+)-epibatidine. In interaction studies with antagonists differing in selectivity, (+)-epibatidine discrimination was substantively antagonized by mecamylamine, slightly attenuated by hexamethonium (peripherally restricted) or dihydro-β-erythroidine, and not altered by methyllycaconitine (α7 selective). Varenicline and cytisine enhanced (+)-epibatidine's discriminative-stimulus effects. Correlational analysis revealed a close correspondence between relative behavioral potencies of nicotinic agonists in both studies and their published relative binding affinities at α4β2 and α3β4, but not α7 nAChR, subtypes. Collectively, these results are consistent with the idea that the α4β2 and α3β4, but not α7 nAChR subtypes play a role in the behavioral effects of nicotinic agonists.

Absolute Configuration and Pharmacology of the Poison Frog Alkaloid Phantasmidine

Phantasmidine, a rigid congener of the well-known nicotinic acetylcholine receptor agonist epibatidine, is found in the same species of poison frog ( Epipedobates anthonyi). Natural phantasmidine was found to be a 4:1 scalemic mixture, enriched in the (2a R,4a S,9a S) enantiomer by chiral-phase LC-MS comparison to the synthetic enantiomers whose absolute configurations were previously established by Mosher's amide analysis. The major enantiomer has the opposite S configuration at the benzylic carbon to natural epibatidine, whose benzylic carbon is R. Pharmacological characterization of the synthetic racemate and separated enantiomers established that phantasmidine is ∼10-fold less potent than epibatidine, but ∼100-fold more potent than nicotine in most receptors tested. Unlike epibatidine, phantasmidine is sharply enantioselective in its activity and the major natural enantiomer whose benzylic carbon has the 4a S configuration is more active. The stereoselective pharmacology of phantasmidine is ascribed to its rigid and asymmetric shape as compared to the nearly symmetric conformations previously suggested for epibatidine enantiomers. While phantasmidine itself is too toxic for direct therapeutic use, we believe it is a useful platform for the development of potent and selective nicotinic agonists, which may have value as pharmacological tools.

Reinforcement enhancement by nicotine in adult rats: behavioral selectivity and relation to mode of delivery and blood nicotine levels

RATIONALE

Reinforcement-enhancing effects of nicotine occur in human subjects and laboratory rats. However, the doses used in animal studies typically exceed smoking-associated levels of exposure, and generalized behavioral activation by nicotine can potentially confound data interpretation.

METHODS

During daily 60-min sessions, male adult rats pressed an "active" lever to illuminate a brief cue light. Pressing on either the active or inactive lever retracted both levers for 60 s. Nicotine (0.025-0.2 mg/kg) was given either by continuous intravenous (IV) infusion, or spaced IV pulses (3-s or 30-s/pulse), or pre-session subcutaneous (SC) injection.

RESULTS

Almost all rats responded preferentially for the cue light for several weeks. After several home-cage nicotine injections, reinforcement enhancement occurred even within the first nicotine test session. Nicotine increased active lever responding without altering inactive lever responding, with effects reliably observed at doses as low as 0.1 mg/kg SC or 0.1 mg/kg/session IV. Within the session, the 0.1 mg/kg dose maximally increased active lever responding by 2-3-fold, coinciding with serum levels of 25 ng/ml. Intravenous nicotine (tested at 0.1 mg/kg/60-min session) was equally effective whether delivered by continuous infusion or in a series of equally spaced 0.003 mg/kg pulses each of 3-s or 30-s duration.

CONCLUSIONS

Low doses of nicotine can potentiate responding for a primary sensory reinforcer without producing a generalized increase in lever pressing. Reinforcer enhancement by nicotine generalized to several modes of drug delivery, appeared to track circulating levels of drug, and occurred even at serum levels within the daytime range of moderate cigarette smokers.

(−)-Cytisine: Access to a stereochemically defined and functionally flexible piperidine scaffold

Cytisine undergoes ready fragmentation to provide a highly flexible (and “privileged”) piperidine scaffold capable of exploring a diversity of chemical space.

TC299423, a Novel Agonist for Nicotinic Acetylcholine Receptors

(E)-5-(Pyrimidin-5-yl)-1,2,3,4,7,8-hexahydroazocine (TC299423) is a novel agonist for nicotinic acetylcholine receptors (nAChRs). We examined its efficacy, affinity, and potency for α6β2^∗ (α6β2-containing), α4β2^∗, and α3β4^∗ nAChRs, using [¹²⁵I]-epibatidine binding, whole-cell patch-clamp recordings, synaptosomal ⁸⁶Rb⁺ efflux, [³H]-dopamine release, and [³H]-acetylcholine release. TC299423 displayed an EC₅₀ of 30–60 nM for α6β2^∗ nAChRs in patch-clamp recordings and [³H]-dopamine release assays. Its potency for α6β2^∗ in these assays was 2.5-fold greater than that for α4β2^∗, and much greater than that for α3β4^∗-mediated [³H]-acetylcholine release. We observed no major off-target binding on 70 diverse molecular targets. TC299423 was bioavailable after intraperitoneal or oral administration. Locomotor assays, measured with gain-of-function, mutant α6 (α6L9′S) nAChR mice, show that TC299423 elicits α6β2^∗ nAChR-mediated responses at low doses. Conditioned place preference assays show that low-dose TC299423 also produces significant reward in α6L9′S mice, and modest reward in WT mice, through a mechanism that probably involves α6(non-α4)β2^∗ nAChRs. However, TC299423 did not suppress nicotine self-administration in rats, indicating that it did not block nicotine reinforcement in the dosage range that was tested. In a hot-plate test, TC299423 evoked antinociceptive responses in mice similar to those of nicotine. TC299423 and nicotine similarly inhibited mouse marble burying as a measure of anxiolytic effects. Taken together, our data suggest that TC299423 will be a useful small-molecule agonist for future in vitro and in vivo studies of nAChR function and physiology.

Selective and regulated trapping of nicotinic receptor weak base ligands and relevance to smoking cessation

To better understand smoking cessation, we examined the actions of varenicline (Chantix) during long-term nicotine exposure. Varenicline reduced nicotine upregulation of α4β2-type nicotinic receptors (α4β2Rs) in live cells and neurons, but not for membrane preparations. Effects on upregulation depended on intracellular pH homeostasis and were not observed if acidic pH in intracellular compartments was neutralized. Varenicline was trapped as a weak base in acidic compartments and slowly released, blocking ¹²⁵I-epibatidine binding and desensitizing α4β2Rs. Epibatidine itself was trapped; ¹²⁵I-epibatidine slow release from acidic vesicles was directly measured and required the presence of α4β2Rs. Nicotine exposure increased epibatidine trapping by increasing the numbers of acidic vesicles containing α4β2Rs. We conclude that varenicline as a smoking cessation agent differs from nicotine through trapping in α4β2R-containing acidic vesicles that is selective and nicotine-regulated. Our results provide a new paradigm for how smoking cessation occurs and suggest how more effective smoking cessation reagents can be designed.

DOI:http://dx.doi.org/10.7554/eLife.25651.001

Tobacco continues to be widely used worldwide, primarily via cigarette smoking, and is a leading cause of preventable deaths. Stopping smoking is difficult because the nicotine in tobacco is highly addictive, and so several drugs have been developed to help people break their addiction. Varenicline (also known by the trade name Chantix) is a commonly prescribed anti-smoking drug, but it is not fully understood how it works.

Nicotine affects the brain by binding to proteins called nicotinic acetylcholine receptors (nAChRs) that sit on the surface of neurons. This binding releases a number of chemical signals, including some that produce feelings of pleasure. Over time, the receptors become less sensitive to nicotine and produce more “high-affinity” binding sites for nicotine to bind to. This adaptation is one reason why stopping smoking can produce strong feelings of withdrawal.

Previously, varenicline was thought to partially activate nAChRs, preventing nicotine from binding to the receptors. However, this can only explain how varenicline counteracts the rapid-acting effects of nicotine, not nicotine’s longer-term effects. Furthermore, it was not known how nAChR signaling responds to long-term exposure to a combination of both drugs (as occurs when people try to quit smoking with the aid of varenicline).

Now, Govind et al. reveal how varenicline reverses the effect of long-term nicotine exposure on nAChR signaling. Both varenicline and nicotine accumulate in acidic compartments – called vesicles – inside cells, where they become charged and less able to move through the cell membrane. When the vesicles also contain high-affinity nAChRs, varenicline becomes trapped inside them and is only slowly released. By contrast, nicotine is not trapped because it exits the vesicles more rapidly. Long-term exposure to nicotine greatly increased the number of vesicles that contained high-affinity nAChRs, thereby trapping more varenicline.

One consequence of trapping varenicline was that the activity of the nAChRs on the surface of the neuron was diminished, apparently through the slow release of the trapped varenicline from the acidic vesicles. This slow release causes the receptors to enter a “desensitized” state in which they do not signal.

Understanding how varenicline counteracts the long-term effects of nicotine on nAChR signaling will help us to design more effective anti-smoking drugs. Govind et al. also found that compounds similar to varenicline become trapped in vesicles, but it is not clear how the degree of trapping of a compound correlates with how effectively it combats nicotine addiction. The results may also help us to understand and treat addictions to other drugs of abuse, such as opioids, amphetamines and cocaine, which have chemical properties that mean they might also be selectively trapped in acidic vesicles.

DOI:http://dx.doi.org/10.7554/eLife.25651.002

Involvement of Proton-Coupled Organic Cation Antiporter in Varenicline Transport at Blood-Brain Barrier of Rats and in Human Brain Capillary Endothelial Cells

Varenicline is a selective partial α₄β₂ nicotinic acetylcholine receptor agonist, which is used to help achieve smoking cessation. Here, we investigated varenicline transport at the blood-brain barrier by means of in vivo microdialysis, in situ brain perfusion, and brain efflux index measurements in rats, and in vitro uptake studies in human brain capillary endothelial cells. Microdialysis demonstrated that varenicline is actively transported from blood to brain in rats. Blood-to-brain uptake transport of varenicline, as measured by the in situ brain perfusion technique, was strongly inhibited by diphenhydramine, a potent inhibitor of proton-coupled organic cation (H⁺/OC) antiporter. However, brain efflux index study showed that brain-to-blood efflux transport of varenicline was not inhibited by diphenhydramine. In human brain capillary endothelial cells, varenicline was taken up time- and concentration-dependently. The uptake was dependent on an oppositely directed proton gradient, but was independent of extracellular sodium and membrane potential. The uptake was inhibited by a metabolic inhibitor, and by substrates of H⁺/OC antiporter, but not by substrates or inhibitors of OCTs, OCTNs, PMAT, and MATE1, which are known organic cation transporters. The present results suggest that the H⁺/OC antiporter contributes predominantly to varenicline uptake at the blood-brain barrier.

PET imaging of α7 nicotinic acetylcholine receptors: a comparative study of [18F]ASEM and [18F]DBT-10 in nonhuman primates, and further evaluation of [18F]ASEM in humans

PURPOSE

The α₇ nicotinic acetylcholine receptor (nAChR) is implicated in many neuropsychiatric disorders, making it an important target for positron emission tomography (PET) imaging. The first aim of this work was to compare two α₇ nAChRs PET radioligands, [¹⁸F]ASEM (3-(1,4-diazabicyclo[3.2.2]nonan-4-yl)-6-([¹⁸F]fluorodibenzo[b,d]thiophene 5,5-dioxide) and [¹⁸F]DBT-10 (7-(1,4-diazabicyclo[3.2.2]nonan-4-yl)-2-([¹⁸F]fluorodibenzo[b,d]thiophene 5,5-dioxide), in nonhuman primates. The second aim was to assess further the quantification and test-retest variability of [¹⁸F]ASEM in humans.

METHODS

PET scans with high specific activity [¹⁸F]ASEM or [¹⁸F]DBT-10 were acquired in three rhesus monkeys (one male, two female), and the kinetic properties of these radiotracers were compared. Additional [¹⁸F]ASEM PET scans with blocking doses of nicotine, varenicline, and cold ASEM were acquired separately in two animals. Next, six human subjects (five male, one female) were imaged with [¹⁸F]ASEM PET for 180 min, and arterial sampling was used to measure the parent input function. Different modeling approaches were compared to identify the optimal analysis method and scan duration for quantification of [¹⁸F]ASEM distribution volume (V _T). In addition, retest scans were acquired in four subjects (three male, one female), and the test-retest variability of V _T was assessed.

RESULTS

In the rhesus monkey brain [¹⁸F]ASEM and [¹⁸F]DBT-10 exhibited highly similar kinetic profiles. Dose-dependent blockade of [¹⁸F]ASEM binding was observed, while administration of either nicotine or varenicline did not change [¹⁸F]ASEM V _T. [¹⁸F]ASEM was selected for further validation because it has been used in humans. Accurate quantification of [¹⁸F]ASEM V _T in humans was achieved using multilinear analysis with at least 90 min of data acquisition, resulting in V _T values ranging from 19.6 ± 2.5 mL/cm³ in cerebellum to 25.9 ± 2.9 mL/cm³ in thalamus. Test-retest variability of V _T was 11.7 ± 9.8%.

CONCLUSIONS

These results confirm [¹⁸F]ASEM as a suitable radiotracer for the imaging and quantification of α₇ nAChRs in humans.

Therapeutic concentrations of varenicline in the presence of nicotine increase action potential firing in human adrenal chromaffin cells

Varenicline is a nicotinic acetylcholine receptor (nAChR) agonist used to treat nicotine addiction, but a live debate persists concerning its mechanism of action in reducing nicotine consumption. Although initially reported as α4β2 selective, varenicline was subsequently shown to activate other nAChR subtypes implicated in nicotine addiction including α3β4. However, it remains unclear whether activation of α3β4 nAChRs by therapeutically relevant concentrations of varenicline is sufficient to affect the behavior of cells that express this subtype. We used patch-clamp electrophysiology to assess the effects of varenicline on native α3β4* nAChRs (asterisk denotes the possible presence of other subunits) expressed in human adrenal chromaffin cells and compared its effects to those of nicotine. Varenicline and nicotine activated α3β4* nAChRs with EC₅₀ values of 1.8 (1.2-2.7) μM and 19.4 (11.1-33.9) μM, respectively. Stimulation of adrenal chromaffin cells with 10 ms pulses of 300 μM acetylcholine (ACh) in current-clamp mode evoked sodium channel-dependent action potentials (APs). Under these conditions, perfusion of 50 or 100 nM varenicline showed very little effect on AP firing compared to control conditions (ACh stimulation alone), but at higher concentrations (250 nM) varenicline increased the number of APs fired up to 436 ± 150%. These results demonstrate that therapeutic concentrations of varenicline are unlikely to alter AP firing in chromaffin cells. In contrast, nicotine showed no effect on AP firing at any of the concentrations tested (50, 100, 250, and 500 nM). However, perfusion of 50 nM nicotine simultaneously with 100 nM varenicline increased AP firing by 290 ± 104% indicating that exposure to varenicline and nicotine concurrently may alter cellular behavior such as excitability and neurotransmitter release.

Selection criteria limit generalizability of smoking pharmacotherapy studies differentially across clinical trials and laboratory studies: A systematic review on varenicline

BACKGROUND

The selection criteria used in clinical trials for smoking cessation and in laboratory studies that seek to understand mechanisms responsible for treatment outcomes may limit their generalizability to one another and to the general population.

METHODS

We reviewed studies on varenicline versus placebo and compared eligibility criteria and participant characteristics of clinical trials (N=23) and laboratory studies (N=22) across study type and to nationally representative survey data on adult, daily USA smokers (2014 National Health Interview Survey; 2014 National Survey on Drug Use and Health).

RESULTS

Relative to laboratory studies, clinical trials more commonly reported excluding smokers who were unmotivated to quit and for specific medical conditions (e.g., cardiovascular disease, COPD), although both study types frequently reported excluding for general medical or psychiatric reasons. Laboratory versus clinical samples smoked less, had lower nicotine dependence, were younger, and more homogeneous with respect to smoking level and nicotine dependence. Application of common eligibility criteria to national survey data resulted in considerable elimination of the daily-smoking population for both clinical trials (≥47%) and laboratory studies (≥39%). Relative to the target population, studies in this review recruited participants who smoked considerably more and had a later smoking onset age, and were under-representative of Caucasians.

CONCLUSIONS

Results suggest that selection criteria of varenicline studies limit generalizability in meaningful ways, and differences in criteria across study type may undermine efforts at translational research. Recommendations for improvements in participant selection and reporting standards are discussed.

Pharmacokinetic Drug Interactions with Tobacco, Cannabinoids and Smoking Cessation Products

Tobacco smoke contains a large number of compounds in the form of metals, volatile gases and insoluble particles, as well as nicotine, a highly addictive alkaloid. Marijuana is the most widely used illicit drug of abuse in the world, with a significant increase in the USA due to the increasing number of states that allow medical and recreational use. Of the over 70 phytocannabinoids in marijuana, Δ9-tetrahydrocannabinol (Δ9THC), cannabidiol (CBD) and cannibinol are the three main constituents. Both marijuana and tobacco smoking induce cytochrome P450 (CYP) 1A2 through activation of the aromatic hydrocarbon receptor, and the induction effect between the two products is additive. Smoking cessation is associated with rapid downregulation of CYP1A enzymes. On the basis of the estimated half-life of CYP1A2, dose reduction of CYP1A drugs may be necessary as early as the first few days after smoking cessation to prevent toxicity, especially for drugs with a narrow therapeutic index. Nicotine is a substrate of CYP2A6, which is induced by oestrogen, resulting in lower concentrations of nicotine in females than in males, especially in females taking oral contraceptives. The significant effects of CYP3A4 inducers and inhibitors on the pharmacokinetics of Δ9THC/CBD oromucosal spray suggest that CYP3A4 is the primary enzyme responsible for the metabolism of Δ9THC and CBD. Limited data also suggest that CBD may significantly inhibit CYP2C19. With the increasing use of marijuana and cannabis products, clinical studies are needed in order to determine the effects of other drugs on pharmacokinetics and pharmacodynamics.

Effects of varenicline on sympatho-vagal balance and cue reactivity during smoking withdrawal: a randomised placebo-controlled trial

Background

Varenicline is an effective smoking cessation medication. Some concern has been raised that its use may precipitate adverse cardiovascular events although no patho-physiological mechanism potentially underlying such an effect has been reported. The aim of this study was to test the hypothesis that varenicline impacts on sympatho-vagal balance during smoking withdrawal.

Methods

In this randomised, placebo-controlled trial, muscle sympathetic nerve activity (MSNA), baroreflex sensitivity (BRS), heart rate, and blood pressure were assessed in 17 smokers four weeks before a quit attempt (baseline) and again on the third day of that quit attempt (acute smoking withdrawal).

Results

Regarding the primary endpoint of our study, we did not find a significant effect of varenicline compared to placebo on changes in MSNA burst incidence between baseline and acute smoking withdrawal (−3.0 ± 3.3 vs.−3.9 ± 5.0 bursts/100 heart beats; p = 0.308). However, heart rate and systolic blood pressure significantly decreased in the placebo group only, while no significant changes in these parameters were observed in the varenicline group. Exposure to smoking cues during acute withdrawal lead to a significant increase of heart rate in the placebo group, while heart rate decreased in the varenicline group, and the difference in these changes was significant between groups (+2.7 ± 1.0 vs.−1.8 ± 0.5 1/min; p = 0.002). In all 17 participants combined, a significant increase in heart rate during smoking cue exposure was detected in subjects who relapsed in the course of six weeks after the quit date compared to those who stayed abstinent (+2.5 ± 1.2 vs.−1.1 ± 0.7; p = 0.018). Six-week abstinence rates were higher in the varenicline group compared to placebo (88 vs. 22 % p = 0.015).

Conclusion

We did not find evidence of adverse effects of varenicline on sympatho-vagal balance. Varenicline probably blunts the heart rate response to smoking cues, which may be linked to improved cessation outcome.

Nicotinic effects of tobacco smoke constituents in nonhuman primates

RATIONALE

Recent studies in rodents suggest that non-nicotine constituents of tobacco smoke (e.g., minor tobacco alkaloids) may promote tobacco consumption-either through their own pharmacological effects or by augmenting the effects of nicotine. However, there is scant information on the behavioral pharmacology of minor tobacco alkaloids in primate species.

OBJECTIVE

The present studies were conducted to determine whether the minor tobacco alkaloids nornicotine, anabasine, anatabine, myosmine, and cotinine exhibit nicotine-like behavioral effects in squirrel monkeys.

METHODS

Initial experiments were conducted to determine the effects of nicotine (0.032-1.0 mg/kg) and the minor tobacco alkaloids nornicotine (1-1.8 mg/kg), anabasine (0.1-1.0 mg/kg), anatabine (10-32 mg/kg), myosmine (0.32-1.8 mg/kg), and cotinine (10-180 mg/kg) on food-maintained performance (n = 4). Next, the ability of tobacco alkaloids to substitute for the α4β2-selective nicotinic agonist (+)-epibatidine in drug discrimination experiments was evaluated in a separate group of monkeys (n = 4).

RESULTS

Results show that nicotine and each minor tobacco alkaloid except cotinine (a) produced dose-related decreases in food-maintained responding; (b) substituted for (+)-epibatidine and, in additional experiments, produced additive effects when combined with nicotine; (c) induced emesis or tremor at doses that reduced food-maintained responding and had (+)-epibatidine-like discriminative-stimulus effects; and (d) based on correlation with reported receptor binding affinities, likely produced their behavioral effects through α4β2 receptor mechanisms.

CONCLUSION

Selected minor tobacco alkaloids have nicotinic-like effects that may contribute to tobacco consumption and addiction.

Neuronal Nicotinic Acetylcholine Receptor Modulators Reduce Sugar Intake

Excess sugar consumption has been shown to contribute directly to weight gain, thus contributing to the growing worldwide obesity epidemic. Interestingly, increased sugar consumption has been shown to repeatedly elevate dopamine levels in the nucleus accumbens (NAc), in the mesolimbic reward pathway of the brain similar to many drugs of abuse. We report that varenicline, an FDA-approved nicotinic acetylcholine receptor (nAChR) partial agonist that modulates dopamine in the mesolimbic reward pathway of the brain, significantly reduces sucrose consumption, especially in a long-term consumption paradigm. Similar results were observed with other nAChR drugs, namely mecamylamine and cytisine. Furthermore, we show that long-term sucrose consumption increases α4β2 * and decreases α6β2* nAChRs in the nucleus accumbens, a key brain region associated with reward. Taken together, our results suggest that nAChR drugs such as varenicline may represent a novel treatment strategy for reducing sugar consumption.